in molding - mcgill universitydigitool.library.mcgill.ca/thesisfile61108.pdf · 1 r ii resume la...

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The Measurement and Control of Nozzle Melt Temperature

in Injection Molding

by

Gino Ruscitti

A Thesis submitted ta the Faculty of Graduate Studies

and Research in partial fulfillment of the

requirements of the degree of

Master of Engineering

Department of Chemical Engineering McGiII University Montreal. Canada (c) Gino Ruscitti. 1992 March 1992

ABSTRACT

The temperature of the poIymer melt plays an important role in injection

moIding, directly affecting the quality of the molded part.

A new OOrrel, nozzle, and saew tip were constructed for an injection

moIdirig machine to allow the installation a Vanzetti infrared optical fiber

temperature sensor and several exposed junction thermocouples. These sensors

were compared for the measurement of poIymer melt temperature.

Open Ioop pseudo-random binary sequence experiments were performed

to obtain dynamic temperature models for nozzle melt temperature when the band

heater power was varied. The open Ioop response of the nozzle melt temperature

was found to be second arder overdamped with a dead time.

Process control simulations were done with the PlO, Dahlin, a second order

modified Dahlin, and Smith predictor control algorithms in order to determine the

optimum parameters for the controllers and to compare their responses.

Experiments using the PlO and the Dahlin controller were performed, with step

changes in set points and disturbances introduced into the system.

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RESUME

La température du polymère fondu est une variable importante dans le

moulage par injection, celle-ci affecte directement la qualité de la pièce moulue.

Un nouveau cylindre, ajutage, et point de vis ont été construits pour une

machine de moulage par injection afin de permettre l'installation d'un indicateur de

température Vanzetti à l'infrarouge et plusieurs thermocouples à jonction exposé.

Ces indicateurs ont été comparés pour mesurer la température du polymère

fondu.

Des expériences de séquences binaires pseudo·aléatoires en boucle

owerte ont été réalisées de façon à obtenir des modèles de températures

dynamiqua,. pour la température du polymère fondu dans l'ajutage quand la

puissance du chauffage a été variée. La réponse de la température du polymère

fondu était du deuxième ordre suramortisé avec le temps de délai.

Des simulations sur le contrÔle de procédé ont été faites avec les stratégies

de contrOle PlO, Dahlin, un Oahlin modifié aux deuxième ordre et prédicteur de

Smith pour déterminer les meilleurs paramètres des contrOleurs et pour comparer

leurs réponses. Des expériences utilisant les contrôleurs PlO et Dahlin ont été

réalisées avec des changements dans les points de consignl) et de dérangements

introduits dans le système.

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ACKNOWLEDGEMENTS

1 wish to express my sincere gratitude to my Research Directors, Professor

W. lan P8tterson and Professor Musa R. Kamal for their advice and guidance

throughout the course of this work and for their critical review of this thesis.

1 would like to express my deepest gratitude to my wife, Rosie, for her love,

support and encouragement throughout the duration of this work.

1 want to thank Mr. A. Krish for his helpful comments and advice.

especially want to thank Mr. A. Gagnon for his care, interest, and helpfulness for

the construction and commissioning of the new parts for the I,t)(perimental

apparatus. 1 also thank Mr. L. Cusmich. Mr. J. Dumont, Mr. E. Siliauskas, Mr. W.

Greenland, and Mr. N. Habib for their contributions in modifying the experimental

apparatus. 1 thank Mr. A. Kallos for helping me with the use of the VAX computer

and Mr. M. Parker for providing the zao cross-assembler.

1 wish to thank my coIleagues in the polymer group, D. Abu Fara for ail his

help with the injection moIding machine, G. Lohfink for his useful suggestions and

discussions, M. Samara, R. DiRaddo, B. Nelson, T. Broadhead, T. Samurkas, O.

Kennache, E. Chu, and P. Singh for their help and assistance.

1 wish to thank Mr. J. Nanigian for his time, advice, and useful suggestions.

1 thank Le Fonds pour la Formation de Chercheurs et ,'Aide à la Recherche

(Le Fonds FCAR) and the Department of Chemical Engineering for financial

support.

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TABLE OF CONTENTS

ABSTRACT ............................................. .

RESUrllE .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ii

ACKNOWLEDGEMENTS .................................... iii

TABLE OF CONTENTS .................................... iv

UST OF FIGURES .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. viii

UST OF TABLES . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. xii

1 INTRODUCnON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

1.1 BACKGROUND AND SCOPE .......................... 3

1.2 OBJECTIVES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 8

2 EXPERIMENTAL APPARATUS AND MODIFICAnONS . . . . . . • . . . . .. 9

2.1 POLYMER MELT TEMPERATURE MEASUREMENT .......... 9

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2.1.1 TEMPERATURE MEASUREMENT PROBLEM . . . . . . . . . . . . .. 9

2.1.2 SENSOR EVALUATION . . • . . . • • . . . . . . • . . . . • • . . . .• 10

2.1.3 SENSOR SELECTION . • • • . . • • • • . • • . • • • . . • . • • • • •• 15

2.2 THE INJECTION MOLDING MACHINE. . .. . . . . . . . . • . . . . .. 16

2.3 BARREL CONSTRUCTION .. . . . . . . . . . . . . . . . . . . . . . . . .. 17

2.4 BAND HEATERS AND BARREL TEMPERATURE

MEASUREMENT .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 22

2.5 LOCATION OF THE TEMPERATURE SEN SORS . . . . . . . . . . .. 23

2.6 SCREW TlP THERMOCOUPLE . . . . . . . . . . . . . . . . . • • . . . .. 28

2.7 DATA ACQUISITION SYSTEM . . . . . . . . . . . . . . . . . . . . . . . .. 28

2.8 EXPERIMENTAL PROCEDURES. . . . . . . . . . . . . ... . . . . . .. 33

2.9 MATERIAL USED AND PROPERTIES . . . . . . . . . . . . . . . . . . .. 37

3 TEMPERATURE SENSOR EVALUAnON .... . . . . . . . . . . . . . . . . .. 38

3.1 NOZZLE BAND HEATER .........•............•..... 38

3.2 TYPICAL TEMPERATURE PROFILES. . . . . . . . . . . . . • . . . . .. 39

3.3 IMMERSED RIBBON THERMOCOUPLE . . . . . . . . . . . . . . . . .. 42

3.4 VANZETTIINFRARED TEMPERATURE SENSOR •.......... 42

3.5 SCREW TIP THERMOCOUPLE ............. . . . . . • . . . .. 45

3.6 FLUSH MOUNTED RIBBON THERMOCOUPLES ........... 46

3.7 AOIABATIC COMPRESSION HEATING ......•.....•....• 48

3.8 AIR SHOTS . . . • . . . . . • . . . . . . . . . • • . . . . . . • . . . . • • . . .. 48

3.9 SUMMARY ...•.....•.................•....••..•• 51

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3.10 PROCESS IDENTIFICATION AND CONTROL . . . . . . . . . . . .. 52

3.10.1 CONTROLLED VARIABlE (Sensor) . . . . . . . . . . . . . . . .. 53

3.10.2 MANIPULATED VARIABLE (Final Control Element) . . . . .. 54

4 PROCESS IDENTIFICATION .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 56

4.1 MODELLING TECHNIQUES AND PROCEDURES . . . . . . . . . .. 56

4.1.1 STEP TESTS ..... . . . . . . . . . . . . . . . . . . . . . . . . . .. 57

4.1.2 IDENTIFICATION USING PRBS TESTS . . . . . . . . . . . . . . .. 58

4.2 IDENTIFICATION EXPERIMENTS ...................... 61

4.2.1 PRBS EXPERIMENTS .......................... 61

4.2.2 SELECTION OF THE MANIPULATED AND CONTROLLED

VARIABLES ................................ 61

4.3 SUMMARY ...................................... 64

5 CONTROL OF NOZZLE MELT TEMPERATURE . . . . . . . . . . . . . . . .. 66

5.1 CONTROLLER DESIGN AND EVALUATION .............. , 67

5.1.1 PlO CONTHOUER ............................ 68

5.1.2 DAHUN CONTROLLER . . . • • . . . • . . . . . . . . . . . . . . . .. 70

5.1.3 SMITH PREDICTOR . . . . . . . . . . . . . . . . . . . . . . . . . . .. 75

5.1.4 TUNING CRITERIA ............................ 76

5.2 CONTROL SIMULATIONS AND RESULTS ................ 78

5.2.1 PlO CONTROUER .•....•..................... 79

5.2.2 OAHUN CONTROLLER • • • • • • • • • • • • • • . . • • • . . . • • .. 86

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5.2.3 SMITH PREDICTOR • • . . . . . . . . . • • . . . . . . . . . . . . • .. 89

5.3 SUMMARY ..........•........................... 92

1 CONCLUSIONS AND RECOMMENDAOONS . . . . . . . . . . . . . . . . . .. 95

6.1 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 95

6.2 RECOMMENDA TIONS ... • . . . . . . . . . . . . . . . . . . . . . . . . .. pt)

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . •. 101

APPENDIX A:

APPENDIX 8:

APPENDIX C:

APPENDIX D:

Material of Construction for the Barrel and Nozzle . •. 104

Specifications of the Vanzeiu Infrared Optical Fiber

Temperature Sensor . . . . . . . . . . . . . . . . . . . . . . .. 105

Calibration of the Thermocouples and the Cold

Junction Compensation and Amplification Circuit ... 106

Glossary of Injection Moiding Terrns ............ 109

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UST OF FIGURES

Number Caption Page

Figure 1.1: Schematic diagram of an injection molding machine. ....... 1

Figure 2.1: Construction of a typical plastic melt thermocouple. ....... 11

Figure 2.2: Schematic diagram of the ribbon thermocouple [7]. ....... 13

Figure 2.3: Dimensions of the original barrel and nozzle of the injection

molding machine. Ail dimensions are in mm. . . . . . . . . . . .. 18

Figure 2.4: Design and dimensions of the new barrel. .............. 19

Figure 2.5: Design and dimensions of the nozzle. .. . . . . . . . . . . . . . .. 20

Figure 2.6: Comparison of the dimensions of the old and new barrel~,. .. 21

Figure 2.7: D!mensions of the Vanzetti infrared sensor. ............. 24

Figure 2.8: Dimensions of the ribbon and metal thermocouples. . . . . . .. 26

Figure 2.9: Location of the sensors on the nozzle. . . . . . . . . . . . . . . . .. 27

Figure 2.10: Dimensions of the screw tip and screw tip thermocouple. ... 29

Figure 2.11: Microcomputer configuration for machine sequencing and

data acquisition and control. ... . . . . . . . . . . . . . . . . . . . .. 32

Figure 3.1: Typical results from the temperature sensors during an

injection molding cycle. The temperature profile along the

barrel was 141, 163, 172, 180 oC. . . . . . . . . . . . . . . . . . . .. 40

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Figure 3.2: Response of the immersed thermocouple and the Vanzetti to

different injection speeds and cushion sizes. The screw

position is shown in Figure 3.2d. . . . . . . . . . . . . . . . . . . . .. 43

Figure 3.2: Response of the screw tip thermocouple to different inje~~;Jn

velocities and cushion sizes. The screw position is in this

figure applies alsa to the data shown in Figure 3.2a,b. ..... 44

Figure 3.3: Comparison of sensors in axial position 2. The flush mounted

ribbon thermocouple and the metal thermocouple measure

the same temperature. .. . . . . . . . . . . . . . . . . . . . . . . . . .. 46

Figure 3.4: The sprue was plugged to prevent flowing of the melt into the

mold. The temperature rise is due to heat generated from

compression of the molt. .......................... 49

Figure 3.5: Temperature profiles for air shots. This shows the effed of

pressura on heat generation during in je di on and plastication.

The temperature profile across the barrel was 140, 156, 163,

178°C. ....................................... 50

Figure 3.6: Block diagram of the feedback process controlloop. ...... 53

Figure 4.1 : Generation of a pseudo random binary sequence (PRBS) and

how it is applied in an ident,fir..ation experiment. ... . . . . . .. 58

Figure 4.2: Exemple of a PRBS identification experiment. The

manipulated variable was band heaters 3 and 4. ......... 62

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Figure 5.1 : Block diagram of the feedback control loop. The dynamics of

the sensar and the final control eJement are incorporated in

the process dynamics, Gp. . . . . . . . . . . . . . . . . . . . . . . . .. 67

Figure 5.2 Block diagram of the Smith predictor control algorithm for

dead time compensation. .. . . . . . . . . . . . . . . . . . . . . . . .. 75

Figure 5.3: Results of the computer simulation ta determine the optimum

PlO parameters, according ta the IAE criteria, for a step

change in set point. ............ . . . . . . . . . . . . . . ... , 80

Figure 5.4: Experimental results for the implementation of the PlO

parameters whose optimization is shawn in Figure 5.3. ..... 81

Figure 5.5: Results of the computer simulations ta determine the optimum

PlO parameters (IAE criteria) for a disturbsnce and a change

in set point. Heater power was restricted to increase the

stability of the controller. . . . . . . . . . . . . . . . . . . . . . . . . . .. 83


parameters whose optimization is shawn in FitJure 5.5 ..... , 84


parameters whose optimization is shawn in Figure 5.5. A fan

blowing over the band heaters was used ta introduce a

disturbance to the process. ........................ 85

Figure 5.8: Fit of a first arder plus dead time model ta the data for the

immersed thermocouple and band heater 3. ............ 87

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parameters (IAE criteria) for the 08hlin controller for a

disturbance and a change in set point. ................ 88

Figure 5.10: Experimental results of the first arder Oehlin controller ta

~ in set point.. .......................................................... 9()


parameters (IAE criteria) for the second arder Dahlin controller

for a disturbance and a change in set point. ............ 91


parameters (IAE criteria) for the Smith dead time compansator

using a PlO controller for a disturbance and a change in set

poim. ............................................................................... 93

Figure C.1 : Circuit for the A0594A used for thermocouple coId Junction

compensation and amplification. ........... . . . . • . . .. 108

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UST OF TABLES

Number Caption Page

Table 2.1: Specifications of the injection molding machine. .......... 17

Table 2.2: lime and servovalve openings for each phase of the injection

molding cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 35

Table 2.3: PhYGical properties of the poIymer. ................... 37

Table 4.1: Fitted models of the PRBS experiments. Each band heater

power was 500 W. A bit is the unit value of a byte. ....... 63

Table 5.1: Controllsr parameters determined trom computer simulations.

A bit is the unit value of a byte. . . . . . . . . . . . . . . . . . . . . .. 92

Table 8.1: Specifications of the Vanzetti infrared optical fiber temperature

senIOr. ........... .. .. .. . .. . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . .. .... 105

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1 INTRODUCTION

ThermopIastic injection molding is a cyclic process where pellets of solid

polymerie material are fed into a heated metering screw, conveyed, melted, and

injected into a coId moId to produce a thr. dimensional part of almost any shape.

A schematic diagram of the main elements of an injection molding machine is

shawn in Figure 1.1. The polymerie material is melted trom band heaters on the

exterior of the barrel with sorne contribution through viscous heating from the work

of the screw. A typical cycle begins with the injection of the melted polymer into

the moId by acJvancing the screw, that is not rotating, and maintaining pressure on

WATER COOLED MOl.D

MOl.DED PART

SCREW

BAND HEATERS HYDRAULIC MOTOR AND PISTON

figure 1.1: Schematic diagram of an injection moIcIing machine.

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the screw until the material in the gate freezes. The screw then rotates. working

and melting the polymer. preparing it for the next cycle. The screw retracts in the

barrel as it rotates. pumping molten poIymer ahead of itself and feeding fresh

poIymer pellets at the back of the screw. The screw stops rotating when it has

retumed to its initial position. After a predetermined amount of time. the part

(assumed ta be frozen) is ejected by opening the maki. The mold is then c!osed

and a new cycle begins.

The temperature of the polymer melt plays a very important role in injection

molding. The most important poIymer property affected by temperature is

viscosity. The most ir'1f"'Ortant machine and process variables affected by melt

temperature are mold temperature. noule and mold pressure. the ability to control

mold pressure. cycle time, and injection time [1]. These parameters directlyaffect

the quality of the molded part.

Ideally. melt temperature should be measured in the mold since it is here

that the processing parameters ultimately determine the part quality. However.

accurate determination of the temperature of the melt in the cavity is difficult

because of the complex and highly transient temperature profiles during filling and

because of the large temperature gradients between the cold mold wall and the

poIymer melt [2]. In addition. the temperature sensing device may affect the flow

field and leave its imprint on the part.

The melt temperature in the mold can be adequately controlled by

manipulating the melt temperature in the nozzle. The barrel heater settJng is not

an adequate indication of the melt temperature is because the melt worked by the

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screw is hotter than the barref [3]. The use of sheathed thermocouples to directly

measure the meIt temperature Is unreliable because a) flush mounted

thermocouples are more responsive to metaI temperature than to melt temperature

and b) immersed thermocouples are slow responding and are subject ta

conduction errors through the stem because they must have a thick metal sheath

ta be able ta withstand the pressures and shearing forces that exist when the

poIyrner meIt is quickly injected Into the maki.

1.1 BACKGROUND AND SCOPE

Work done previously in the department of Chemical Engineering at McGiII

University for nozzJe malt temperature measurement was by Gomes [4]. A 3.2

mm diameter thermocouple was .nstalled in the screw tip of the injection molding

machine and was allowed ta protrude 7 mm into the melt. Control of the nozzle

melt temperature was achieved by using cIosed Ioop feedback control. The melt

temperature was measured with the saew tip thermocouple and the manipulatect

variable was the front zone barrel heater.

Temporal axial ternperature profiles during injection have been reported

[5,6,7] indicating that spatial axial and/or radial temperature profiles exist in the

nozzIe of injection moIding machines. The equipment did not provide the means

ta correctly measure temperature profiles. The only malt temperature

measurement is the screw tip thermocouple (installed by Gomes [4]) and it cannat

measure these profiles due ta its slow response time and its disadvantageous

position.

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Several studies were performed to investigate the effect of the following

machine parameters on the nozzle melt temperature:

- barrel heater temperature

- back pressure

- screw speed

- cycle time

- cushion size

- shot size.

Peter [8] found that melt temperature was affected by barrel heater

temperature and by back pressure but not by the screw spead. Peter explained

that the screw speed does nct affect the melt temperature because the same

amount of work must be performed per cycle. Flush mounted thermocouples were

used in the nozzle, screw ana screw tip and il was found that the screw tip

thttrmtœuple measurement lagged behind the temperature of the melt injected

into a cup by several cycles.

Border [9], while attempting to control the melt viscosity, found that the

steady-state relationship between the change in melt temperature to a step change

in back pressure was

where Pb is the average back pressure. After the step input to the back pressure,

a change in ~emperature was noted only after the second shot (due to the cushion

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remaining tram the previous shot) and the new steady state temperature was

attained 14-20 shots later. This Iength of time was required because a new

temperature distribution had to be established alang the length of the barrel before

a steady state temperature reading could be obtained from the nozzle

thermocouple.

SchonewaId [10J found that the melt temperature was affected by barrel

temperature, shot size, and cycle time but it was insensitive to screw speed and

back pressure. The barrel heater power wa~ not kept constant during the

experiments but was set to maintain a constant temperature in the barrel 50 that

8I1y smali change in melt temperature was easily compensated for. Schonewald

used 8 thermoc.'lUpie that was immersed 3/8 inches into the melt and was

mounted in a temperature controlled housing. The housing was kept at the same

temperature as the thermocouple tip in order to r&duce conduction errors. The

thermocouple used was an open-tipped iron-constantan type used by Tychesen

and Georgi [11J.

Amano [5J has performed an extensive study on the effect of machine

parameters on melt temperature. A 1.6 mm in diameter thermocouple was

immersed to the center of the channel that connect&d the accumulated polymer

in the nozzIe to a capillary rheometer. Very low injection rates of 1 cm/sec were

used which enabIed the thin thermocouple to withstand the injection forces and to

detect temperature profiles during injection. Amano has shawn that barrel

temperature, bar)!. pressure, screw speed, and cycle time affect the melt

temperature to varying degrees. It was found that temperature increases of 5 to

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13°C accur during injection. Amano [12] also showed that three dimensional

temperature profiles exist in the poIymer melt.

Peischl [13] used a ring-bar device to study the thermal melt homogeneity

in injection ITlOlding. The poIymer melt ftowed through a ring that held a bar upon

which seven 0.8 mm thermocouples were installed and each thermocouple

extended approximately 1.6 mm upstream trom the bar into the melt. Slow air

shots (1520 s) were used because of limitations in the response time of the

thermocouples. Melt homogeneity was defined as the difference between the

highest and lowest reading of the seven thermocouples where the smaller the

difference. the better the melt homogeneity. Peischl found that the screw channel

volume to shot size ratio is the most important parameter for melt homogeneity:

the larger the ratio the better the homogeneity. Flatter barrel temperature profiles.

slower screw speed. and increasad back pressures also increase the melt

homogeneity.

The use of fast response temperature sensing devices showed that large

temperature gradients exist in the nozzle of injection molding machines

[6.14.15.16]. Amano [5] and Peischl [13] have also shown this using

sheathed thermocouples but with very low injection rates.

Orroth found temperature changes of 6°C during injection using a Vanzetti

infrared probe installed in the noule.

M8her, also using a Vanzetti probe but installed in the barrel next to the

nozzle. also observed temperature increases during injection. He reported that

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there existed shot to shot variations in temperature and that the temperature varied

approxlnately 11°C within a shot.

Sukanek reported temperature variations of up to 14°C using a similar

infr.-ad opticaI fiber probe.

Using ribbon thermocouples installed flush in the nozzle, Nanjgian observed

variations in temperature of goC.

These temperature profiles present new problems for melt temperature

control. If malt temperature control is to be accurate to within SoC for example,

what temperature is to be taken as the actuel melt temperature? Once a

convention for the actual meIt temperature has been found, how will the

temperature sensing device be used to indicate actual melt temperature j.e. where

should the sensor be pIaced in the nozzle and et what time during the injection

cycle should the temperature be taken? Before these questions can be answered,

a suitable temperature sensor or temperature measuring technique must be

developed.

SpecificalIy, this work has objectives given in the next section, designed to

partially answer thjs problem.

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1.2 OBJEC, ryVES

1- Evaluate alternative techniques for the rneasurernent of the nozzle melt

temperature and obtain spatial radial and axial temperature profiles in the

nozzle.

2- Obtain dynamic temperature responStlS suitable for control purposes,

modifying the injection rnoIding machine as necessary.

3- Evaluate different algorithms for the control of poIymer melt temperature at

the nozzle.

Secondary Objective

a) Examine temperature variations during the injection molding cycle.

ln chapter 2, the experimental considerations are presented leading to the

selection of the temperature sensars. The necessary equipment modifications are

aise discussed. A qualitative evaluation of the temperature sensors based on

experimental results is presented in chapter 3. In chapter 4, the modelling

techniques and results are shawn for the temperature response of the system.

Chapter 5 presents the design and Implementation of various control techniques

for the control of nozzle melt temperature.

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2 EXPERIMENTAL APPARATUS AND MODIFICATIONS

The selection and implementation of the tamperature sensors is discussed

in this chapter. The new sansors oouId not be installed on the original nozzle and

barrel and therefore a new nozzIe and barrel were constructed. The injeCtion

moIding machine was computer controlled, however another computer was added

for temperature data acquisition and control because the first was overloaded.

2.1 POL YMER MELT TEMPERATURE MEASUREMENT

2.1.1 TEMPERATURE MEASUREMENT PROBlEM

The measurement of poIymer meIt temperature has always been a difficult

task due to the extrema environments encountered in plastics processing. The

problems are compounded in injection moIding where the process is cyclic with

ensuing cyclic variations in temperature and pressure, and much higher pressures

are attained in injeCtion moIding compared to extrusion. The accu rate

measurement of these variations was of interest in this work.

For the measurement of melt temperature in injection molding, the sensing

ctevice must satisfy the foIlowing requirements:

• The sensor must be able to operate to temperatures of 350°C and

pressures of 100 MPa (15,000 psi).

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If the sensor is immersed in the melt, it must withstand the shearing forces

of the highly viscous flowinp poIyrner melt as it is injected into the moId.

- The sensor must have a fast response time (i.e. small thermal inertia) sc as

to detect temperature variations within each injection cycle.

- The sensor and its leads must be compact enough to satisfy the limited

space available for installatioa in th, equipment.

- The sensor must be able to witl1stand the corrosive effects of the polymer

melt.

The following section describes the various sensor types considered for this work

and the reasons for those chosen.

2.1.2 SENSOR EVAlUATION

o Sheathed Thermocouples (Groundecl and Ungrounded): A typical plastic melt

thermocouple is shown in Figure 2.1 where the thermocouple jundion and lead

wires are encased in 8 protective metal sheath. The jundion is in contact with the

sheath for grounded thermocouples and is electrically isolated tram the sheath for

ungrounded thermocouples which causes slower response times. There are three

major drawbacks with sheathed thermocouples:

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• Stem Effect: heat conducted from the metaI walls causes taise readings of

the melt temperature. This error can be as large as the temperature

differenœ between the matai wall and the bulk polymer, and its effects are

quantified by Steward [17].

• Shear Heating: the probe itself is heated due to viscous heating when

immersed in a ftowing melt stream. It has been shown that this can cause

apparent temperature increases of 11 0 C [17].

25.4mm, 12.7 OR FLUSH 0.8 IMMERSION

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HEX

1/2"-20UNF

..... _ THERMOCOUPL,"--,-", CONDUCTOR

.. SHEATH--....

GROUNDED JUNCTION UNGROUNDED JUNCTION

Flgur. 2.1: Construction of a typicaI plastic melt thermocouple.

--

12

Slow Response Time: for an immersed probe, the thick metal sheath

needed to withstand the shearing forces causes a substantial thermal lag

due to the relatively large thermai capacity of the sheath coupled with its

high thermal conductivity compared to that of polymers.

ij) Exgosed Junctjon Thermocouples and the Rjbbon Thermocouple: Typical

exposed junction thermocouples are very fragile and cannet be used in plastics

processing to measure malt temperatures. Nanmac Corporation has developed

an exposed junction thermocouple that is extremely robust and has almost no

conduction errors and very fast response times [7]. The thermal elements are flat

ribbons instead of round wires and the ribbons are insulated from the

thermocouple weil and from each other by a high temperature, low thermal

conductivity ceramic insulator. At the sensing tip, the ribbons are 180° apart and

are folded aeross the center, overlapped and spot welded to form the thermal

junction (see Figure 2.2). For tests in water, the ribbon thermocouple has a time

constant of 18 msec. For molten solder the time constant is 3 ms.

Siower response times are expected when measuring polymer melts

because the thermal conductivity of water is approximately 2.5 times that of the

melt. Some conduction errors are a!5O expected iJecause the thermal conductivity

of the melt is approximately 10% of that of the ceramic insulator of the ribbon

thermocouple. Nevertheless, an exposed junction thermocouple with a ceramie

insulator will achieve much faster response times and reduced temperature errors

due to stem effects than an industry standard sheathed thermocouple.

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13

iiD Rasistance Temperature Detectors (BTQ): RTOs exhibit similar characteristics

for measuring poIymer meIt temperatufes as sheathed thermocouples because the

probes are constructed the same way (SM Figure 2.1). The RTD is insulated from

the sheath similarly to an ungrounded thermocouple but the sensing element is

longer and this causes the RTD to be stem sensitive. This increases the

temperature errors due to heat conducted by to the stem.

Insulaflng rod and cyllnder

Thermal lunctlon

Body (.heath)

figure 2.2: Schematic diagram of the ribbon thermocouple [7].

14

ix) Infrsred Temperature Measurement: An optical fiber coupled to a quartz Ions

is used to transmit the melt infrared radiation to s transducer which produces a

Hnearized OC voltage output. The quartz Ions acts as an interface between the

poIymer and the fiber and is installed flush with the barrel wall. The characteristics

of this sensor are:

• Non-contaà with the media to be measured.

• The sensor is insensitive to conduction haat lasses through the stem and

to shear heating.

• The sensor is ftush mounted and therefore doss not suffer tram shear

heating.

- The respanse time is 10 ms.

- The -average- temperature of a volume of polymer sean by the quartz lens

is measured.

Vanzetti's infrared system 8verages poIymer temperature to depths between

5 and 20 mm into the melt pool, according to the manufacturer's specifications.

It was found in a study by Galskoy [18] that. for ABS and poIypropylene, the

sensor is sensitive ta depths of 8 mm.

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15

Ircon has deveIoped an infrarld therrnometer similar to the Vanzetti's,

however, Ircon had not commenced production of its sensor at the time this work

was begun.

Many other infrared thermometers are available, however only the Va'1Zetti

and the Ircon thermometers .. e equipped with the quartz lens that is required for

installation on a plastics processing machine for malt temperature measurement.

xl Ultrason je Measuremem: The sanie velocity in a liquid is depandent on the bulk

modulus, and hence on the temperature, pressure, and physical and chemical

structure of the substance as described by Herbertz [19]. He also showed that

et a constant pressure the etrective temperature (not true temperature) of a

poIymer melt can be correlated to the sonie velocity.

The use of ultrasonic techniques to measure melt temperature would require

a substantial developmental effort to implement the system. This is beyond the

scope of this work and is not considered further.

2.1.3 SENSOR SELECTION

The temperature sensors that were selected for use in this work were the

Vanzetti thermometer and the ribbon thermocouples. The Vanzetti meets ail the

criteria and will provide the best overall temperature response. It is the sensor that

respJnds the fastest of those considered and is the only one that will read the bulk

--

16

poIymer melt temperature and not disturb the fIow field and not be affected by

conduction errors. Onty one Vanzetti thermometer was used due to its high cost.

Ribbon thermocouples suffer trom the seme drawbacks as sheathed

thermocouples but to a much Iesser extent because the junctions are exposed and

they are insulated trom the thermocouple weil. Ribbon thermocouples were used

to provide simultaneous temperature rneasurements at different locations in the

nozzle. Their relatively Iow cost aliowed for the installation of one immersed in the

meIt. three that were flush mounted. and one in the saew tip. Type E (chrome)-

constantan) ribbon thermocouples were used because il is the metal combination

that is most resistant to corrosion affects trom molten plastics. This type al50 has

the largest temperature coefficient of the standard thermocouples.

A detailed description of the locations of the sensors is given in section 2.5.

2.2 THE INJECTION MOLDING MACHINE

The experimental work was carried out on a Danson-Metamec reciprocating

screw injection molding machine with the specifications given in Table 2.1.

The dimensions of the original barrel and nozzle are shown in Figure 2.3 (ail

dimensions are specified in mm). The barrel was made of a hardened metaJ

(possibly nitrided) and coulet not be bored for the installation of additional sensors.

The nozzle section had sufficient space to install two transducers. a pressure

transducer and second sensor with a non-standard bore. The senser locations

were st the smallest point of the tapered section in the nozzle and therefore the

bulk temperature of the malt could not be measured.

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17

A new barrel and nozzle were designed and built to allow the installation of

multiple ternperature sensors et various locations in the nozzle. These sensors

couId be flush mounted or immersed into the malt and used to compare the

performance of temperature sensors with different operating principles and

conditions.

2.3 BARREL CONSTRUCTION

The constraints for the construction of the new barrel were:

The McGili Department of Chemical Engineering machine shop was limited

to boring a Iength of no greater than 26 cm.

The length and internai geometry of the new barrel and nozzle was to be

the seme as the original barrel and nozzle.

Madel

C8pacity

Screw diameter

Screw LlO

Screw rotation speed

Clamping force

Hydraulic pump

Pump capacity

Electric mater

ServovaIve

Danson Metamec eo-SR

2113 oz. (66.1 g)

35 mm

15:1

40-150 RPM

53386 kN (60T)

Sperry-Vlckers vane pump

1.82 m3/hr et 13.8 MPa

14.92 kW. 3 phase. 50 Hz

Moog A076-103 • 2.28 m3/hr et 8.9 MPa pressure drop

Tible 2.1: Specifications of the injection moIding machine.

-•

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18

The metaI used to construct the barrel was to be soft enough to machine

with conventional tools and be a type that rould later be hardened, if

necessary.

The material chosen to construct the barrel was 420 stainless steel (see

Appendix A). The barrel was made of three sections that were screwad together

(see Figure 2.4) in order to respect the limitations of the machine shop. The

spacer was added to correct a design error. The new nozzle was longer (110.4

mm in length) to allow more space for the temperature sensors at the nozzle inlet

52.0

BARREL

66.0

55.0

MOZZlE

35.0.

Flgur. 2.3: Dimensions of the original barrel and nozzle of the injection moIding machine. Ail dimensions are in mm.

~ ~

14 NPT 2" - 10 THREAD L "-..}USHING

Pzz~m--1 70.0. 35.0. .,

œ4 ZZZZZZZZ???_, ,!&i=E::;Zi2ZZizZ~Z>Z2ZZZzi!2ZZiz",,~

,~ 217.8 ~ 1 ~ 241.5 ~ 1 r- 248.0 --.:.t r- 271.8--.:.t

FIgure 2.4: Design and dimensions of the new barrel.

38.1°1 Ü 13.5

=2J F*??24&" 6.0

35.0

SPACER

]65.0. 145.0.

~ 19

-

20

section (see Figure 2.5). The length of the new barrel was shorter to maintain the

same totallength of the nozzle and barrel sections. The nozzle and barrel section

were honed as one piece to ensure that there were no steps seross the sections

that were screwed together. A comparison of the cid and new barrels 15 shawn

in Figure 2.6.

The first operation with the new barrel caused a screw seizure resulting in

scoring a section of the barrel. This section was rebuilt and the nozzle and barrel

were hardened by heat treatment and honed again to avoid another screw seizure.

30.2

3.0 •

J 45.7. ~ J 65.... ~

84.3 •

Flgur. 2.5: Design and dimensions of the nozzle.

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21

This was unsuccessful as the next try et operation of the injection moIding

machine with the hardened barrel, the screw again seized in the barrel but with

minimal damage. To ensure that no further damage occurred to the barrel, the

interior of the barrel (excluding the spacer) and nozzJe were honed (0.019 mm

removed), flash chrome plated with 0.025 mm of chrome and honed again

removing 0.006 mm of chrome. The thr. sections of the barrel had been chrome

pIated as a single piece and, henœ, can never ba separated. Only the cylindrical

section of the nozzle was chrome plated. Ail honing was performed with the

nozzIe and ail the sections of barrel, with the exception of the spacer, screwed

together.

It was found upon installation of the chrome plated barrel that the screw was

slightly bant et the section that was inserted into the chuck of the injection moIding

ORIGINAL BARREL

117.5 Bandheat.r

683.6 NEW BARREL

FIgure 2.1: Comparison of the dimensions of the oId and new barrels.

.-

22

machine. Approximately 0.03 mm was removed from the out of round section .-1d

thls eliminated ail scouring between the saew and the barrel. It was also found

that ,,:-. boIts to fasten the barrel anto the machine must be torqued equally and

very carefully because the bracket that the barrel is mounted to is sphericaIIy

convexe

The seizure of the screw in the unhardened barrel was most likely inevitable

because 8 hardened screw that is rotating and reciprocating under great pressures

is certain to contact and damage the barrel. Operation with a hardened barrel may

be possible as the above mentioned failure was likely due to improperly torqU8d

rnounting boIts. The oparating life of a hardened barrel was judged satisfactory

for a machine that is used infrequently for experimental work. Truly problem frae

operation of 8 one piece barrel raquires additional hardening with procedures such

as nitriding or chrome plating.

2.4 BAND HEATERS AND BARREL TEMPERATURE MEASUREMENT

The two original band heaters and two surface mounted thermocouples

were replaced with four band heaters and four ungrounded bayonet

thermocouples. The new arrangement is shown in Figure 2.6 where the

numbering system for the band heaters is noted. Barrel temperature profites will

be repolted in that order trom the happer side where the coId pellets are

introduced into the barrel to the nozzle side where the poIymer is moIten. The use

of only two heater zones limited the ability to control the temperature profile &long

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the barrel. Wrth four band heaters. there are four temperature zones and a proper

temperature profile can be estabIished.

The new band heaters were 500 W each. Three were conventional mica

ba1d heaters instaIIed on the barrel before the nozzle is installed and the fourth

W8S hinged 50 that it can be instaIled and removed while the nozzJe is in place.

This was necess.-y because the fourth heater encased bath the barrel and nozzIe.

and must be removed during installation and removal of the nozzle.

The bayonet thermocouples were type E. consistent with the type of the

thermocouples used in the nozzIe. The barrel was designed with thermocouple

wells that were 12.3 mm deep into the barrel wall metaI (5.2 mm fram the inside

of the barrel wall) in arder ta minimize tert1fl8"ature gradient errors and ta obtain

a measurement that was doser to the inner wall temperature than to that of the

band heater [17]. The bayonet thermocouples were ungrounded because the data

acquisition system had single ended inputs.

2.5 LOCATION OF THE TEMPERATURE SENSORS

The constraints for selecting the locations of the temperature sensors were:

An immersed sensor must be placed in the nozzle 50 that screw

interference was avoided.

A sensor that was pIaced in a section that the screw traversed must be

flush mounted.

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24

111e sensors used were:

i) a Vanzetti infrared temperature detector

ii) an immersed ribbon thermocouple

iii) thr. flush mounted ribbon thermocouples

iv) a ribbon thermocouple mounted in the saew tip

v) a -metaiM thermocouple.

The dimensions of the Vanzetti infrared temperature sensor are shown in

Figure 2.7 and the specifications are given in Appendix B. The dimensions of the

38.1

38.1

12.1 OPTICAL FIBER 9.6 R

---4-40 )( 1/8" SETSCREW

1/4"-28 UNF

QUARTZ LENS

Flgur. 2.7: Dimensions of the Vanzetti infrared sensor.

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25

ribbon thermocouples are shawn in Figure 2.8. The metal thermocouple was

ungrounded with a housing of approximately the same dimensions as the ribbon

thermocouple. The thermocouple junction was approximately 11.3 mm deep into

the housiI'Ig, rneasured trom the tip. The saew tip thermocouple is discussed in

the next section.

The strategy for sensor placement was as follows:

The Vanzetti, an immersed ribbon thermocouple, and a flush mounted

ribbon thermocouple were pIaced at the same axial location in the nollle and were

pIaced as close as possible to the screw without danger of collision (see axial

position 2 in Figure 2.9). The bulk temperature of the polymer melt could thus be

estimated with two different types of temperature sensors. The flush mounted

sensor was used to compare the barrel-melt temperature interface with the bulk

temperature.

A flush mounted ribbon thermocouple was placed st the furthest possible

downstream location in the nozzle (see axial position 1 in Figure 2.9). This was

opposite to the pressure transducer which was placed in the same position in the

new nozzle as in the original nozzle.

A flush mounted ribbon thermocouple was pIaced at the furthest possible

upstream location in the nozzle section. The hinged band heater had a

2.5 x 2.5cm section removed to accommodate the sensor. The reading was used

to compare the barrel-malt interface temperature with the temperature reading of

the SCl'ew-tip thermocouple, which rneasured bulk rnelt temperature. This ftush

mounted thermocouple was also used to compare the temperature of the heated

6.4. 0.5" HEX BOLT ..

3/8"-24 NFT .. B c

-

4.7. J L. , D

Dimensions (mm) Thermocouple

A B C

Immersed 6.3 9.9 16.0

FMRT-NOZ 6.3 9.9 16.2

FMRT-MID 0.0 10.9 16.0

FMRT-SCR 0.0 9.8 15.8

t.4etal 0.0 5.8 19.7

'ï Figur. 2.8: Di"'l8nsions of the ribbon and metaI thermocouples.

26

0

19.0

4.8

4.7

4.8

4.8

(~

Axial position 1

SIDE VIEW

FM'" • Flulh Mount" .... Thermocouple

NOZ • Noale ... "D • Middle SCIt • Scnaw ...

J.7 NESSUIIE SENSOI

TOP VIEW

tJ.4

2

Flgur. 2.8: Location of the sensors on the nozzle.

27

3

.......

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part of the nozzle with the unheated pwt.

2.6 SCREW TlP THERMOCOUPLE

The screw tip thermocouple that was installed by Gomes [4] was found to

be damaged when the original barrel and screw were removed. The sheathed

thermocouple was 3 mm in diameter which, when installed, protruded 7 mm trom

the screw tip. Examination revealed that it had baen pushed approximately 1 cm

into the screw tip. The sheathed thermocouple was replaced by a ribbon

thermocouple and the screw tip was redesigned to accommodate the new sensor

as shawn ln Figure 2.10. The screw tlp surfaces (indicated as "FLASH CHROME

PLA TED" ln Figure 2.10) were chrome plated sinee there was evidence of a high

rate of wear by the check valve. The screw and slip ring assembly was not

modified and the reader is referred to Gomes [4] for the details.

2.7 DATA ACQUISITION SYSTEM

The injection molding machine was interfaced to a microcomputer in 1982

[20]. It was a Cromenco Single Card Computer (zao bPsed with 17K of RAM)

and was used for:

machine sequencing

data acquisition for the pressure, linear displacement, velocity 1 and screw

tip temperature sensors

control of pressure [21,22,23].

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70.6

32.9

10.9

2.6

4.0

9.S

12.0

16.2

19.0. ~a-""'-12. 7 •

Thermocouple

9.6 •

3/8"-24 Nn

Flalh Chrome Pla'ed 13.8. 19.0 •

Weld

14.9.

14mm-16 UN Lef' Handed Thread

~--6.4.

Screw tlp moterlal: 4140 Carbon St.el

~ur. 2.10: Dimensions of the screw tip and screw tip thermocouple.

29

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The department's VAX·780 computer was used for permanent data storage

because the Cromenco did not have either a tape or disk drive. In this work, the

Cromenco was used for control of machine sequencing and data acquisition of the

SCfew position measurement.

Another zao microcomputer which used an IMSAI chassis was

commissioned for temperature data acquisition and control because the Cromenco

was already being used to its maximum capacity. The IMSAI had the following

specifications:

zao microprocessor

48KRAM

space for 16K ROM

12 bit analog to digital converter with 16 single ended 0-5 volt inputs

seriai p.;rt for a terminal

seriai port for data retrieval and storage with the VAX

parallel port for reœMng screw position data trom the Cromenco (interrupt

driven with a signal trom the IMSAI)

parallel port for drMng 4 solid state relays that independently controlled the

band heaters.

An integrated circuit (Analog Deviees AD594A) was used for cc!d junction

compensation and amplification. The calibration of the thermocouples and the

coId junction compensation and amplification circuit is discussed in Appendix C.

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31

For computer program deveIopment. a zao cross-assembler was acquirad

trom the McGiI University Electrical Engineering department and was installed on

the VAX. Programs for the IMSAI and Cromenco were edited on the VAX in zao

assembler language and stored in a file. The file was run through the

cross-assembler which creatad another file containing the machine language

version cf the program in hexadecirnal format. The hexadecimal file was then

downIoaded ta the Cromenco or the IMSAI. AIl experimental parameters were in

hexadecimal format either in one byte or two bytes (a word) depending an the

requirement. The band heater powers will be expressed as a byte value (0 - FF

hex or 0 - 255 decimal corresponding ta 0 - 100 " heater power) because these

were the units used in the computer. Math was performed in 32 bit fixed point

arithmetic for the control experiments.

A program ta emulete the VTS2 terminal was developed for the IMSAI and

stored in an EPROM. Included in the emuletor were programs to:

upload experimental data tram the IMSAI ta the VAX for permanent starage,

and

download machine language programs fram the VAX to the IMSAI that were

used for data acquisition and control.

This allowed the IMSAI to be used as a terminal for the VAX with a full screen

editor ta develop programs and facilltate the transfer of programs and data ta and

from the VAX. The amount of RAM in the IMSAI was insufficient for long

duration experirnents. Included in the data acquisition program were subroutines

that transferred data ta the VAX as the experiment was being perfarmed.

32

The set up of the microcomputer system is summarized in Figure 2.11.

Time proportioning was used ta regulate the electrical power ta the band

heaters. The IMSAI did not have an onboard circuit ta produce periodic interruptS

and therefore the interrupt signal fram the Cromenco was used ta interrupt the

IMSAI f!N8ry 0.01 s. An interrupt oriven subroutine was used ta switch the power

on and off based on a cycle of 2.56 seconds. For exemple, if 50% band heater

power was desired, the number 128 (80u~) was stored in the appropriate memory

TEMPERATURE DATA ACQUISITION AND CON:'ROL

MACHINE SEQUENCING. DATA ACQUISITION F'OR SCREW POSITION

D D ...5/Lfi.~~~§g\a~~~IPA~lEL PORT!.!/.gz:fl~Z:~~~~\~~~-.

~-'" ... -.;;'"" CROMENCO SINGlE IMSAI CHA SIS ,... •••••• ---. Z80 CPU

1············· .. •·······••

®J I-t-~~I

III .1 1 1

~~ YAX-780

SERIAL COMMUNICATION

zao CROSS ASSEMEl.ER DATA ~ PROGRAM STORAGE

INTERRUPTS CARO COMPUTER r-''''-_-Z-80-C-PU----' 1

--_ ••••••• _-_ ••••• 1

INJECTION MOLDING MACHINE

Flgur. 2.11: Microcomputer configuration for machine sequencing and data acquisition and control.

(

location. ,~t each interrupt, a counter was incremented and when the counter

reached 128, the power was switched off. When the coumer reached 255 (FF1e>. the power was swltched on, the counter reset ta zero and the time proportioning

cycle restarted.

2.8 EXPERIMENTAL PROCEDURES

1he injection rnoIding machine was a standard thermoplastic machine. The

reciprocating saew pushed the poIymer malt into a cooIed mold and maintained

the pressure for a set amount of time. 1he screw then rotated until it retracted ta

a set position which plasticated new poIymer and moved the molten poIymer

ehead of the screw. After a set amount of time for cooling the part had elapsecl.

the part was ejected and the ~cIe repeated. The carriage (heId the barrel.

happer. hydraulic rnotors that rotated and pushed the screw) was stationary and

this maintained the nozzIe constantly press8d against the mold.

1he function of the Cromenco computer was :0 perform machine

sequencing. The fill, hold. and cool times were specified in hundredths of a second

aIong with servo-valve openings that set the open Ioop fill and hoId pressures.

Plastication was performed to a set screw position and a servovalve opening was

specified ta create a back pressure which affected the shearing work performed

on the melt. The Cromenco was also used to acquire data for screw position trom

the linear displacement transducer which was transmitted ta the Imsai via a parallel

port.

--------- ------- - -- - ----

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,

34

The Imsai was used for temperature data acquisition and control. For the

identification and control experiments, the data was sent ta the Vax as it was

acquired because there was insufficient memory on the Imsai ta store the rire

experiment. The Imsai was aIso used as a buffer in the avent of communication

problems. The data tram the foIlowing instruments was recorded

• Vanzetti

• immersed ribbon thermocouple

·3 flush mounted ribbon thermocouples (SCR, MID, NOZ)

• screw tip thermocouple

• matai temperature

• 4 band heater temparatures

• 4 band heater powers

• screw position trom the Cromenco

It was important ta establish an experimental procedure that had a smooth

transition trom no molding ta molding conditions ta reduce the amount of lime

required to attain steady state conditions because of the large process time

constants and poor reliability of the injection molding machine. It was also

important to estabIish a tempe rature profile seross the four band heaters in order

ta have stable operating conditions. Initial test runs were perforrned ta estabIish

a starting point and the experimental procedure was developed as the temperature

sensor avaluation experiments and sorne trial step tests were performed. The

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35

procedure was finalized for the identification experiments and maintained for the

control experiments.

The injection moIding machine was operated with the conditions shawn in

Table 2.2. PIastication was terminated when the saew position was approximately

0.01 meters from Ils fully retracted position. The screw rotation !'Ipeed was set with

a manuel hand valve .-Kt was fixed at 55 RPM for ail experiments.

The band heater temperature profile found to be nlOSt suitable for stable

machine performance with the poIymer used (588 MCtion ~.9) was 140,160, 170,

180°C. The heater powers used during steady state conditions were 75.3,42.7,

17.8, 15.7 percent of full power.

The startup procedure for the injection moIding machine when it was coId

was as foIlows. The band heaters were set to full power until the band heater

Machine Time Servovalve Opening sequence (s) (%)

Fdl 2.00 38

Hoid 2.00 25

Cool 12.87 -Plasticate To fixed position 50

MisceIlaneous -3.5

Total cycle time -20.4

Ta"" 2.2: Time and servovalve openings for each phase of the injection rnoIding cycle.

-

ft -

36

zone set point minus 20° C was attained at which the power was turned off. This

startup phase was terminated when ail four band heaters ~ been turned off. A

type of bandwidth control was then used until moIding was started. The heater

powers used to maintain the set point during no moIding conditions were 20.0.

15.7.9.8. 20.0 percent of full power. For each zone, if the temperature was 2°C

above the set point the heater power was halved and if the temperature was 2°C

beIow the set point the heater power was c:Ioubled. The calibration of the Vanzetti

was verifieeS and modified as necessary when stable temperatures were obtain (see

section 3.4). Thirty seconds before rnoIding started. the band heater power setting

were switched to the moIding settings to avoid a decrease in the band heater

temperatures. When steady state temperatures were attained. data acquisition

was started before the experirnent comrnenced to obtain the initial state.

It was discovered during the course of the experiments that the haat

exchanger for the hydraulic system was not functioning. The symptom was that

the injection moIding machine became sluggish and the screw rotational speed

decreased after approximately 10 minutes of operation. In was found that the

cooIing water lines to the heat exchanger were not operational and the heat

exchanger had become filled with sludge and rust. The condition was corrected

but the heat exchanger was only fully effective when the identification experiments

were performed.

{

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37

2.9 MATERIAL USEO AND PROPERTIES

The poIymer used for this work was SCLAIR 2908 which Is a hlgh dens'ty

polyethylene manufactured by DuPont of Canada. The physical properties of the

poIymer.8 shawn in Table 2.3 [24].

Type of resin HOPE

Molecular weight (MW) 74500 kg/krnole

Molecular number (MN) 22300

MW/MN 3.33

Density 962 kg/cu.m

Malt index 7.40 g/10 min

Meiting range 113-146 oC

Average specifie heat 2540 J/kg.K (soIid) 2450 J/kg.K (meIt)

Average thermal conductivity 0.345 J/m.s.K (soIid) 0.261 J/m.s.K (melt)

Power law index, n 0.822

OE/R 2167.4 K

A 13.99 kg.S(n-2) /m

Rheological clwacterization n = A exp(OE/RT) ~(n.1)

Table 2.3: Physical properties of the poIymer.

--

38

3 TEMPERATURE SENSOR EVALUA nON

A series of experiments were performed ta qualitatively evaluate the general

respanse characteristics of the various temperature sensors. The results for the

Vanzetti and the ribbon thermocouples were compared and the location of the

ribbon thermocouples were evaluated. Finally, the sensors that best reflected the

poIymer bulk temperature were chosen for process identification and control.

3.1 NOZZLE BAND HEATER

There was a driving force ta cool the noale because it was unheatoo,

uninsulated, and the nozzle tip was in constant contact with the cold mokt Initial

results showed that the heat Ioss ta the mold was substantial as the difference

between the readings of FMRT-NOZ and FMRT-SCR was approximately 25°C.

ft was net possible ta install a nozzle heater on the new nozzle or ta

construct another nozzle that could accommodate a heater within the time frame

of this work. An insulator made of Transite1 of approximately 2.5 cm thickness

was placed between the nozzle and the mold. This reduced the temperature

difference between the readings of the FMRT-NOZ and FMRT-SCR ta

approximately 12°C.

,0 1. Tr.,site is a compacted asbestes material.

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39

The exterior of the nozzIe had 8 highly poIished finish and, therefore,

radiation heat lasses were negligible. The nozzJe was surrounded by the stationary

pIaten (approximately 8.5 cm deep) while it was in contact with the moId and this

reduced naturel convection heat lasses because the air ftow was restricted.

AIthough conduction, convection, and radiation loss6s were reduced, the

temperature gradient between the FMRT-NOZ and FMRT-SCR thermocouples was

not eliminated.

3.2 TYPICAl TEMPERATURE PROFILES

Figure 3.1 shows the temperatures during 8 typical injection moIding cycle.

Generally, the temperature increased during filling and decreased after the injection

pressure was released.

The temperature increase during 8 shot is explained by 8 combination of the

foIlowing:

The movement of the hotter melt during injection from the heated section

of the barrel to the unheated nozzle section. Also, the melt is at 8 higher

temperature as it leaves the screw, after having been plastic3ted.

Heat is generated due to the shearing action caused by the melt fIowing

through the barrel.

The temperature rises when the material is compressed.

The foIlowing experiments were performed to qualitatively evaluate the response

charecteristics of the sensors and the usefulness of the positions of the sensors:

.;n.

<!J#

n, ':J

200~------~--------------------~------~ Â Vanzetti ~ .... RT-NOZ

• Imm .... " o 'NRT-MID

• Sere. Tlp o 'NRT-SCR 195~,,·······,,···,,··,,············r ........ ,,· .... · ............ · .. · .... t ...... • .... • .... · .. · .. · ...... ·• .... ·t ........ · .. ·· .. • ............ · .... ··i· ............ · .............. ·· .... ·1

,.... (.) ......., LAI ct: :;)

i LAI CL. :2 LAI ....

165~------~----~~----~~----~------~ o 10 15 20 25

liME (sec)

Flgur. 3.1: Typical results from the temperature sensors during an injection molding cycle. The temperature profile alang the barrel was 141, 163, 172, 180 oC.

(

(

41

a) Effect of jrjection yeIocjty: servovaIve openings of 25% and 100% were

used, designated as SLOW and FAST injections respectiveIy.

b) Effect of cusbion sjze: the cavity was large enough to accept approximately

haIf the maximum shot size capacity of the machine and this aliowed the

use of Iwo cushlon sizes, designated as SMALL and LARGE. For the

SMALL cushion size the position of the screw at start of an injection was

selected 50 that the saew was as close as possible to its fully advanced

position at the end of Injection allowing for a small margin of errOl'. For the

LARGE cushion size the screw starting position for injection was set close

to its fully retracted position.

c) eompressjoo axpariments: the sprue was plugged and Injection rnoIding

cycles were performed where no fIow of material occurred and no parts

were moIded.

d) Air shots: the machine was allowed to cycle normally and the polymer

injected into the air. This gave the temperature response of the system

when the poIymer was not under pressure during injection and plastication.

The results for the effect of injection veIocity and cushion size are shawn only for

the Vanzetti and for the immersed and screw tip ribbon thermocouples because

these are the sensors that measured the bulk temperature of the malt. The results

-

-

42

are shawn in figure 3.2 as the temperature devIations trom the initial temperature

of the cycle. The saew position data shawn in Figure 3.2d appIies also to

Fegure 3.2a.b as the results for the COITesponding conditions of injection veIocity

and cushion size are trom the same experiment.

3.3 IMMERSED RIBBON THERMOCOUPLE

The immersed thermocouple recorded a 4 to 6°C increase in temperature

during injections and the effect of injection velocity and cushion size are shown in

Figure 3.2a. The response of the immersed sensor is essentially the same for the

different injection speeds and cushion sizes. The immersed sensor did not

differentiate between the injection speeds because its response is not sufficientIy

fast.

3.4 VANZEnlINFRARED TEMPERATURE SENSOR

The Vanzetti was calibrated by adjusting the emissivity compensation of the

instrument. The barrel was heated to operating temperatures and aliowed to

stabilize with no screw movement until the reading of the immersed sensor and the

FMRT-MID were identical. This ensured that the volume of polymer that the

Vanzetti saw was at a uniform temperature. The emissivity compensation was then

regulated so that the output of the Vanzetti was identical to that of the two

thermocouples. The calibration of the Vanzetti was verified et the beginning of

every experimental session.

(~

(~

43

(0) IMMERSED THERMOCOUPLE 10r-------.-------~------~------~~~~~

INJECTION CUSHION SPEED

... __ ..... _ ...... _- .. SlZE t---.....--SLOW ,.sr

LARGE T.. , c .... ":!.~_ ~~II ..... ·--......... • .. • • ......... _ ..... • ............ • .. t .... · ...... · .... · .. · .... · ........ ··

-2 .................................. .................................. .. ............. _ .................................................................................... ..

o 5 10 15 20 25 TlWE (MC)

(b) VANZml tG

INJECTION -g • :. .!.

CUSHION SPEED ._ ............. _...... .... . ................ _ .... -....... .. ............ "................... ". SlZE I--SL-t:III---,.-sr-

SWALL.

1 ,.

URGE ......................... __ .... .. ........ · ...... ·· .. · ........ · .... t ...... · .... ··· ................... ·· 1 ! 2

1 ._ .... _ ....... _ .... _ .............................. + ................................ ..

-2 _.............................. .................................. .. .. _............................. ._ ........ _ ........... _ ...... _.HM ......... _ ............. .

o 5 10 15 20 25 TlME (MC)

figure 3.2: Respanse of the immersed thermocouple and the Vanzetti ta different injection speeds and cushion sizes. The screw position is shawn in FlOure 3.2d.

-..,.,.

10

a--. {!. 1 ... . 1 1 )i

• i

1 !

1

•

0

(c) SCREW TIP THERMOCOUPLE

-_·· .. +--··--i--... --· ::~ ...... SL-:.OW_J:E_c:-r~-ST-· .. ·············· .. · .. ······ .... ·t ........ ····· .... ········ .......... ·t ........ ··· .... ··· ............ ·· .... ..

1 LARGE ' • .le .~ .......... _ ...................................................... · .... ·· .... · .. ·· .... _· .. · .. ···t .... ···_· ...... · ........ · .. ..

1

.................................. i-.................................... .................................. . .............................. ..

5 10 15 20 25 TM: (MC)

(d) SCREW POSITION

INJECTION CUSHIOM SPEED SIZE SLOW FAST

SWAU. • Â

LARGE 0 t::.

5 10 15 20 25 TIME (He)

Flgur. 3.2: Response of the SCl'ew tip thermoCouple to different injection veIocities and cushion sizes. The saew position is in this figure appIies also to the data shawn in Figure 3.2a,b.

(~

45

The response time of the Vanzetti was much faster than that of the

immersed sensor (see Figure 3.2) and it cIearty distinguished the increase and

decrease in temperature due to the different injection velocities. The Vanzetti's

response at the point of release of pressure after the hoId stage is 50 fast that one

may concIude that the Vanzetti is actuaIly recording the temperature increase that

occurs in tluids that have been compressed according ta Its pressure volume

temperature (PVT) relationship.

ft is concIuded that the screw did not come into the Vanzetti's field of view

when the cushion size was smali because Its response was not affected by the

cushion size. A marked difference in the response would have been expected had

the screw come into the Vanzetti's field of view because the poIymer melt and the

screw tip metaI have dlfferent emissivities.

3.5 SCREW TIP THERMOCOUPLE

The screw tip thermocouple provides a different vantage point in the

temperature measurement field because it is not stationary with respect ta the

barrel. Fresh poIymer does not tIow across the screw tip thermocouple during

injection and therefore it is expected ta sense temperature changes only fram heat

generation due ta compression. Polymer melt fIows across the screw tip during

pIastication and the sensor reads the temperature of the hotter poIymer which has

Just 18ft the screw 8fter having been sheared. These are the ~'1s that the screw

tip thermocouple should be able ta distinguish, however the results shawn in

-1 -

Figure 3.1 do not clearly demonstrate this. This Is attributed to the slow response

of the thermocouple. This is also be seen in Figure 3.2c.

3.6 FLUSH MOUNTED RIBBON THERMOCOUPLES

A ribbon thermocouple Installed flush with the barrel wall appears to be

more responsive to the matai temperature than to the bulk poIymer melt

temperature. This is seen in Figure 3.3 where the FMRT-MID and matai sensors

measure the same temperature. During injection. the FMRT -MID records slightly

185~------~------~------------------------~ .. Vanzetti

180

-o -L.J 175 ex :':) ~

~ L.J

~ 170 L.J ~

, , i i

• .ou .. i .... " ......... .

1 i

l··_·--1

1 i .............................. 1"" ........... ..

! ! ,

1 i

• Immersed

.... n.l ........... u ............ i .. , 1

! i ; , i

o rMRT-WID

o Wetol

160~------~------~------~~------~------~ o 5 10 15 20 25

TIWE (sec)

Flgur. 3.3: Comparison of sensors in axial position 2. The flush mounted ribbon thermocouple and the metaI thermocouple measure the seme temperature.

(

47

higher temperatures because It was in contact with the fresh hot meIt being

introduced into the nozzIe whereas the metaI sensor is imbedded in the metaI of

the nozzIe.

The ftush rnounted ribbon thermocouples are reading the poIymer melt

temperature that is nearest ta the metaI wall. Because of the very large thermal

conductivity of the barrel metaI compared te the poIymer melt. the temperature of

the poIymer malt that is very close to the wall will be the same as the wall

temperature. During injection, mixing and fIowing of the hot melt leaving ~

heated section of the barrel increases the bulk temperature of the meIt in the

nozzIe and inaeases the temperature of the meIt that is nex! to the wall as can be

seen in figure 3.1 where the FMRT -NOZ sensor measures a 4-5°C increase in

temperature. When the injection is complete. the temperature of the melt

decreases because there is no longer a supply of hot melt and the heat is

conducted from the melt by the cooIer metaI walls. For the flush mounted ribbon

thermocouples, the FMRT-NOZ is the most responsive to these variations because

It iS Iocated in the coIdest part of the nozzIe and is subjeded to the greatest

temperature variations.

The FMRT-SCR sensor was in the heated section of the barrel and,

therefore, recorded the highest temperature of the flush mounted ribbon

thermocouples. It measured very smali incre8S8S in temperature during injection

because the temperature difference between the bulk poIymer and the heated

metaI walis of the barrel was much smaller at that location than at the tip of the

nozzJe.

-

48

3.7 ADIABATIC COMPRESSION HEATING

Compression experiments were performed (Figure 3.4) ta investigate the

temperature rise of the poIymer meIt when suddenly subjectec:l to high pressures.

The Vanzetti measured an inct'ease in temperature of approximately 4.5° C

compared to 2°C for the immersed and screw tip thermocouples. The difference

in response times of the Vanzetti and the thermocoupies is again evident as the

measurements of the Vanzetti immediately rose to its maximum temperature

whereas the immersed sensor slowly increased in temperature. The increase and

decrease in temperature is a function of the pressure and is the response of the

melt to changes in pressure and volume according to its PVT relationship. It Is

interesting to note that thermodynamically. there is very little Iost work when the

meIt is compressed because the temperature before and after the compression is

identical.

3.8 AIR SHOTS

Air shots were performed to investigate the effect of pressure on heat

generation during injection and plastication. In the absence of pressure. the

temperature recorded by the screw tip thermocouple. the immersed thermocouple.

anc:t the Vanzetti are within 7°C of each other (sse Figure 3.5). This is in contrast

ta normal operation shown in Figure 3.1 where the temperature difference was

22° C showing that considerable heat is generated when shearing occurs under

pressure. It is also interesting ta note that the screw tip thermocouple and

FMRT -SCR measure essentially the seme temperature when air shots are

49

t .... ---..,.---...,..---.,...---,...---.... 200

T, • 0.2 •

~~,.....~~ ........................... .

- Vanzetti

- ImmerMd

- FURT-MID

tUo~--~--------------------~ta SM. a u TM: <MC)

.P----~----_.----~----~--~

i . 1 • .. .................................................... m ............. _ ........... _ ......... + ... _~ .................... _ ... t ............................. ..

Ê 1 1 • ~ Il!

1 6 --------+---~-.. --.. I--... _·-1 1 1

i 2 ~ .................... --........ .. ........................ " .... ,J. ............. " ............................. " ........ "........ .. ............................ ..

1 J \ i

oo .. ~-~S~-~=-...... ~ ...... ~----J tO tS 20 25

TM: <MC)

FIgure 3.4: The sprue was plugged to prevent tIowing of the meIt into the moki. The temperature rise is due to heat generated trom compression of the malt.

.....

.. """

o -

50

2~r-----~----------------------'-----~ Â Vanzetti ~ F'MRT-NOZ

• Imm .... ecI o F'MRT-MID

• Scre. Tlp o F'MRT-SCR 115~ ...... """'·""""'·""""!'''''''''"'''''''''''''''''''·''''+''·''.''''·''·'''' ................... + ........................................................................ 1

110

""""" (.) ~ .... a: :;)

i .... Q. ~ .... ~

170~ .. · ........................... 'lf .................. • .. • .......... ·+·" .. ·· ...... · .... · .......... ···· .. ·~· .. ·· .. · .... ·· .. · ...... · .... · .. ·· .. ·i .. · .. ···· ...... · .. · .. •· ........... ~ AIR SH PLASTICATION

1'5~----~~----~------~------~----~ o 5 10 15 20 25

liME (sec)

Flgur. 3.5: Temperature profiles for air shots. This shows the effect of pressure on heat generation during injeCtion and plastication. The temperature profile across the barrel was 140, 156, 163, 178°C.

(~

51

performed. The temperature gradient from the saew tip thermocouple ta the

FMRT -NOZ decreased fram 23° C (Figure 3.1) to 13° C.

3.9SUMMARY

The nozzle was unheated and in constant contact with the coId moId

causing an axial temperature gradient aIong the nozzIe metaI and poIymer. The

temperature differential was 25°C tom the FMRT-SCR to the FMRT-NOZ

thermocouple, and 12° C when an insulator was pIaced between the rnoId and the

nozzIe. A nozzle heater is required for applications where the temperature is

criticaI and for stabilizing the temperature of the malt that is being injected into the

cavity. Future work te measure temperature profiles in the bulk poIymer should

attempt ta have a constant metaI temperature across the nozzle section to ensure

that these profiles .e not due to a ooId nozzle.

The temperature of the poIymer meIt was greatest as it came off the screw

8fter having been plasticated and this was shawn by the tact that the screw tip

thermocouple measured the highest temperature of ail the sensors.

The response lime of the Vanzetti was essentially instantaneous. The

Vanzetti averaged the emitted infrared radiation over a volume as its readings were

always between those of the imrnersed and FMRT-MID sensors. When the

pressure on the malt was released 8fter the packing stage, the Vanzetti recorded

a rapid decrease in temperature. The poIymer rnelt is somewhat compressible and

therefore foIIows a PVT relationship in which there is a measurable temperature

change when the malt is compressed.

-

~\

52

The response of the immersed thermocouple was not sufficiently fast to

differentiate between the two injection speeds. The immersed thermocouple was

able ta detect the increase and decrease in temperature in an injection moIding

cycle.

The screw tip thermocouple readings were almast constant during a

moIding cycle because its velocity with respect to the melt was zero during

injection and it was in the heated section of the barrel where the tamperature

variation was the smallest.

The flush mounted ribbon thermocouples were more responsive ta the melt

temperature and were measuring the temperature of the cooled melt next to the

barrel wall. During injection. the FMRT-NOZ was able ta detect an increase in

temperature equivalent ta the Vanzetti and the immersed thermocouple. This was

due ta its being Iocated at the nozzle inlet. which is the section with the smallest

diameter in the barrel where it was subjected ta converging flow and the largest

melt velocities.

3.10 PROCESS IDENTIFICATION AND CONTROL

The black diagram of the feedback controlloop and the nomenclature used

is shawn in Figure 3.68 and the simplified form of the Ioop is shawn in Figure 3.6b.

The simplified Ioop incorporates the dynamics of the sensor and the final control

element into the dynamics of the process. The full model is used when the

dynamic model of the system is developed theoretically because each element of

the Ioop is modelled individually. The simplified modeI is used when the modeI is

cc

'f.' <

(

(

53

deveIoped empirically where identification experiments are perforrned on the

system (final control eIement, process, sensor) as a whoIe. It is, in fact, very

difficult to separate the elements of the system when the dynamic model Is

deveIoped experimentally.

3.10.1 CONTROUED VARIABLE (Sensor)

The Vanzetti, the immersed and the screw tip thermocouples are the three

sensors that produce the best measurement of melt temperature. These three

sensors were used in the ldentHication experiments te evaluate their usefulness for

process control purposes.

~ e .. Controll. m .. final Control .... ~ - Ge Eliment. Gf

~

Senaor Ga

a) Full model

~ ... Controll. m .... , - Ge -. ~

b) S/mpllflld model

y = Controlled variable Yap. Set palnt fGr' controlled variable m = Manipulai" variable

r-. Proel" Gp'

-

.. Comb/nid Proee .. , Gp

Gp = Gf Gp' Ga

G • Input-output model e • Error: dllf.enel bel ... mlaaur,m,nt of

eontrolled varlobl. and lit point

Flgur. 3.1: BIock diagram of the feeclback process controlloop.

.. - y

.. y

54

It was not found that the temperature during any part of the cycle was

decidedly indicative of the melt temperature. Henoe. due to the cyclic nature of the

temperature within an injection moIding cycle. the temperature was averaged over

the cycle. The thr. temperatures were considered as the controlled variables and

the final choice is discussed in section 4.2.2. The sampling time chosen for

identification was 20.4 seconds ta coïncide with the period of the injection molding

cycle. For control purposes, the sampling time was thus a multiple of 20.4

seconds.

3.10.2 MANIPULATED VARIABLE (Final Control Element)

The manipulated variable was the band heater power. It was not certain if

one band heater would provide sufficient power for heating or sufficient surface

area for cooling by natural convection. It was possible that band heater 4 was too

far downstream and in the area where a large melt pool existed (the cushion) to

have much affect on the final temperature of the polymer melt. Band heater 3

heated the shallow pool of melt in the metering section of the screw, however, its

effectiveness in control would probably have been nul/ified by the adjacent band

heaters whose control/ers would be attempting to maintain a constant temperature.

It follows that using two band heaters together as the manipulated variable should

be considered.

To r&duce the number of experirnents and ensure that the manipulated

variable action would be effective only two groups of two band heaters were

considered. Band haaters 3 and 4, and band heaters 2 and 3 were grouped

(~

55

together. This allowed the investigation of two possibilities: 1) faster response

could be achieved by manipulating band heaters 2 and 3 that were in the section

of the barrel where the shaliow malt pool existed (as a result of the saew) and 2)

verity the assumption that band heater 4 was too far downstream to be effective

for controlling the malt temperature.

56

4 PROCESS IDENTIFICATION

Effective process control is achieved by understanding the process and

using this knowledge ta develop an appropriate control strategy. An experimental

mathematical mode! was developed to quantify the dynamic temperature response

of the polymer melt to changes in heater power.

4.1 MODELLING TECHNIQUES AND PROCEDURES

Dynamic models for malt temperature response were determined

experimentally. Experimental models ara adequate for process contrai purposes

as the objective is to develop a suitable controller for the system with certain

desired response characteristics. If it \Vere desired to model ail the components

of a process controlloop (sensor, final control element, and the process itself)

fram first principles, the effort and time required to perform such a task would not

be justifiable as the results would not be, in ail likelihood, as good as the

experimental one. The experimental model incorporates into it ail the

unquantifiable parts of the controlloop such as imperfect contact with the heating

element. In addition, the theory for modelling certain areas of the system is not

sufficiently developed, such as poIymer ftow patterns around a rotating screw. or

may be too complex to be Justifiable for the objactives of the project. Hence. it is

sufficient to develop an -adequate- model to predict how a system will respond.

(

57

The disadvantage of experirnental models is that they do not easily generalize to

other machines performing the same process.

4.1.1 STEp TESTS

Step tests were performed, however they were done before the hydraulic

system heat exchanger had been repaired and therefore the results were

unreliable. Additionally, step tests are difficult to execute properly when dealing

with systems that have significa1t dead times or large process time constants

because step tests require 8 true initial steady state and no process disturbances

during the step. This demancIs long periods of time where the process must be

weil behaved. The results of a step test must be teken -as is· because it is not

possible to eliminate disturbances or drifts in the data and therefore replicates must

always be performed to ensure that the test was performed when the process did

not have unknown disturbances entering.

ln the process industries. it is very difficult to perform a successful step test

because one is usually not aliowed to disturb the process sufficiently to obtain a

significant response. A step test requires a large step to be able to separate the

response trom the normal noise of the process and this may be larger than can

be tolerated. In addition, ail the disturbances to the system must be eliminated

and this is norrnally not possible because of the complexity of the process or the

normal variability of the raw materials.

58

4.1.2 IDENTIFICATION USiNG PRBS TESTS

A short sequence PRBS test with a deterrninistic identification procedure

was used to obtain a dynamic model of the temperature response of the injection

moIding machine. A pseudo-random binary sequence (PRBS) is a random

sequence of ones and zeros calculated by a mathematical formula (hence called

pseudo) [25J. Its generation and implementation are shawn in Figure 4.1. A

sh(,r\ ;-;.tBS sequence of 4 bits (15 steps) was used because of reliability problems

PRBS COMPUTING FORMULA 4 BIT PRBS SHI" RIGHT .... -.. ~

NUMBER PRIS BIT

0001 1 0 0 0 1 ~ DISCARD 1000 0

4-PRBS BIT

0100 0 0010 1 1001 1 1100 0 0110 1 1011 0

1 0101 1 1010 1 110 t 1

XOR TABlE 1110 t liTS RESULT 1111 0

0111 0 0 0 0 00" 0 D 1 1 OOOt 1 1 0 1 1 1 0

PSEUDO RANDOM SIHARY SEQUENCE IMPLEMENTATION

PRBS START

---, r- --I .... ~ UPPER VALUE L--.J --II .... ~ lOWER VALUE

1101011110001

---t f4- TlME INTERVAL

PRIS PERIOD .1 PRBS REPEATS •

Flgur. 4.1: Generation of a pseudo random binary sequence (PRBS) and how it is appIied in an identification experiment.

(

(

59

with the injection rnoIding machine. The injection moIding machine was

susceptible te breakdown and was not expected ta run reliably for the 5 hours that

would have been required to run a conventionaI 6 bit (63 steps) PRBS test.

The deterrninistic identification results •• more reliable with a short PRBS

sequence than wIth step tests, however, it does not provide the versatility of a long

sequence PRBS test where a proper stochastic anaIysis can be performed.

The 4 bit PRBS sequence had a time interval consisting of 15 injection

moIding cycles. A moIding cycle was approximately 20.4 seconds and the period

was thus slightly more than 5 minutes. The PRBS sequence had 15 intervals and

lasted 75 minutes. The variation in magnitude of the rnanipulated variabl" {band

heater power) was ± 20%. The Iength of the PRBS time period and the magnitude

of variation W8re determlned from inspection of the results of the step tests and

also from results of Gomes [4].

From previous results [4], it was anticipated that the residual error in the

modeI would be white noise and that the model would be second order

overdamped with dead time. This madeI's continuous time Laplace transfer

function is:

0,(') • _Kp~exp(..;.....;....-8s_)_ ('t1' + 1)(tzS + 1)

wh«e ~ = process gain

8 = process dead time

t 1 = process time constant 1

(4.1)

-

eo

tz = process lime constant 2

s = Laplace damain independent variable.

The discrete-time pulse transfer function of the second arder overdamped rnodeI

for a computer controlled process (using a zero order hoId) is [28]

b Z-1 + baZ-2 HG (z) • z -Il _..:.'---=---

p 1 - .,Z-1 + 8tZ-1

where z = discrete-time independent variable

TI = sampling interval

8 k JI_

T. k = intager

-T ·1 · .xp( ---!.) 'f,

H = 1-exp(-T.s)

•

(4.2)

Using the Nelder and Mead search algorithm (available in the MA TLAB computer

program [26]), the four parameters (~, k, t1' 'fz) were estimated by the

(

61

minimization of the square of the error to fit HGp(z) to the data. The dead time,

8, W88 forcecf to be a multiple of the sampling interval.

4.2 IDENTIFICATION EXPERIMENTS

When performing PRas tests the power was kept constant to the band

heaters that were not being varied during the test. This ensured that the

icIentification results would not be affected by a cycling temperature controller

attempting to maintain a set temperature.

4.2.1 PRBS ExPEAIMENTS

Two n&1ipulated variables (band heaters 3-4 and 2-3) and three responses

(Vanzetti, immersed and saew tip thermocouples) were examined in the PRas

tests. The results of a typical PRas identification experiment are shown in

Figure 4.2 and the fitted modeIs of the two PRBS tests are shown in Table 4.1. Ail

the experimental results for the band heaters are expressed as a byte value (0 •

FF hex or 0 • 255 decimal corresponding to 0 • 100 " heater power) because

these were the actuaI units used in th8 computer.

4.2.2 SELECTION OF THE MANIPULATED AND CONTROLLED VARIABLES

The system responded faster to manipulations of band heaters 3 and 4 than

to band heaters 2 and 3 (compare the process time constants and dead times in

Table 4.1). Therefore, the combination of band heaters 3 and 4 was chosen to be

the manipulated variable.

.--

......

62

PRIS EXPERIMENT ~r---~----~----~----~--~~--~----~--~

195

1 ..... _ .... _.-t-... -+ _. ·~F .. · .. ··················f········ .. ········ ..

110

1750 10 20 30 40 50 70 10

T'ME (ml .. ) 250

f 2~ 0 .. 1 ~ 150

i 1'00 ! 50 ;

•••••••••••••••••••• 1" ........................................................................................................... ! .......................................... . 1 1 l' , I! 1 1 1 il! 1

-I-~,;;;:--· __ ·,·_-_·_;-_· ... _-+_·_-·· ... ··l····· .. ·· .. ···· .... · ... '

1 ,II! i ! 1 1 :: 1

···_--t--······t··_~_···_·-t-····_···+····_·····_·I·_·-. __ .j-........ _ ...... . IANOHEATEa 2 1 1 l'

........................................ .f ...................... ~ .................................................................. ~ ......................................... .

1 1 Il! 1 ... UftLI ... ·Jr... 1 1 : : 1

~--"I 1 1 ',' r::=+==~ 1 • .,. .......... ,..... .. .. ·t··· .. ·· .. ········ .. ··

°0 10 20 30 .tG 50 10 70 10 TIME (min)

Flgur. 4.2: Example of a PRBS identification experiment. The manipulated variable was band heaters 3 and 4 .

(

83

The screw tip thermocouple had the fastest response time of the sensors

when bMd heaters 3.-lei 4 were the manipulated variables. HOWf!N8r, this is not

the sole criterion for the selection cA a measurement sensor. The Vanzetti and the

immersed thermocouple were Iocated further downstream in the barrel and

provided 8 bett .. measurement of the temperature of the poIymer melt that was

being injected into the mokt There was aIways a cushion of poIymer meIt

between the screw tip thermocouple and the nozzle and, therefore, the screw tip

sensor did not measure the temperature of the meIt injected into the molet The

p....,... d the fltted modal Sumd

SenIor BMd ~ ""n r~) "1 (~

T althe (a) (~ Errar

l........t 3-4 0.8172 71.4 713.4 61.2 5.4

ttwmocoupIe 2-3 0.8882 127.3 1786.1 61.2 14.6

3-4 0.6781 174.7 734.2 20.4 12.8 Vanz_1

2-3 0.7738 206.4 2628.3 40.8 4.6

ScrIwTlp 3-4 0.4813 188.1 279.8 61.2 88.0

1hIrmocaupIe 2-3 0.8698 110.6 1205.3 102.0 115.3

ImInIIMd 1hIrmocaupIe

3-4 0.6260 764.8 - 122.4 11.2 (ftrIt arder madII)

T8"" 4.1: Fltted modeIs of the PRBS experirnents. Each band heater power was 500 W. A bit is the unit value of 8 byte.

n· .....

saew tip thermocouple signal was aIso noisier (888 Figure 4.2 and aIso note the

higher sum of squares in Table 4.1) than the imrnersed and Vanzetti sensors. The

screw tip thermocouple would have benefitted from filtering but it would have

increased its response time. For the above reasons the screw tip thermocouple

was eliminated as a control sensor.

The Vanzetti and the immersed thermocouple had similar overall re~ ,anse

characteristics, however the imrnersed thermocouple was chosen as the control

sensor because the Vanzetti was net weil established as a temperature sensor for

plastics processing. Also, a more reliable calibration method is needed because

the sensor required trequent calibration. At the start of every experimental session,

the Vanzetti calibration was verified and it was usually necessary to adjust the

emissivity compensation.

4.3 SUMMARY

Based on the results of the PRBS experiments, the manipulated variable

selected for the control of the melt temperature was the combination of band

heaters 3 and 4. The sensor selected to measure the melt temperature was the

immersed thermocouple. The dynamic model in the Laplace domain for the

response of the immersed sensor to changes in power to band heaters 3 and 4

is

G (s). 0.8172 exp( -81.28) P (71 .... + 1)(713.48 + 1)

(4.3)

65

( The discrete-time domain modal was used in the control simulations and it is

HG (z) • VeZ) • 0.002275%-4 + O.OO2048z -l (4.4) , m(z) 1 - 1.7233z-1 + O.7303z-J

(

c

--

o

66

5 CONTROL OF NOZZLE MELT TEMPERATURE

The parameter values for the controllers were obtained using a time-integral

performance criteria. The optimizations were performed using computer

simulations with the dynarnic model (Equation 4.4) developed in the previous

chapter. Nozzle melt temperature control experiments were then performed on the

injection moIding machine, implementing the various feedback controllers with the

tuning parameters found trom the simulations. Step changes in set points were

performed and a disturbance to the process was implemented by using a fan to

blow air over the barrel of the machine. Another 1 unplanned disturbance also

occurred when the computer failed during experiments.

The perfect controller does not exist as a controller must be a compromise

between regulatory (disturbance rejection) and servo (set point tracking) control

so that it is as versatile as possible. There are many different types of

disturbances and set point changes and each would require different optimum

tuning parameters. In addition, the desired response characteristics of the system

must be chosen by the designer.

(

(

(~

67

5.1 CONTROLLER DESIGN AND EVALUATION

A feedback controller can be designed (588 Fegure 5.1) after a dynamic

modeI of the system is deveIoped. 1lle foIlowing must be considered in the design

of a controIler:

i) Iy,ge of cgntroller: The PlO is the mast common controller and was used

in this work. In addition. the Dehlin controller and Smith predlctor W6fe

investigated for dead time compensation because the PRBS test results

(Table Table 4.1) showed that there is dead time in the temperature

response of the meIt in injection moIding.

-:..t.~ - ,.,. , Vap -•

... ControU .. m Prou •• - Gee.) - Gp(.)

~ - Gee.> Gf~) .p. - t + ac{ .. ·• .. p{.,

a) Laplace tran"" notation for eontlnuou. proee ••

• ControU .. Zero Ord .. Hoid H(z) D(z)

!!IL - Q(z~t~~ YiP{iJ - 1 +z z)

Proce •• Gp(z)

b) DI.er". Ume notatlan for comput .. controUed proce'lI

.. v

~-""_""' ____ ~ __ ~_~ ____________ '.""."<,<J,-~ figure 5.1: Black diagram of the feedback control Ioop. The dynamics' u~ ~;'18

sensor and the final control eIement are incorporated in the process dynamics. Gp.

n -

68

ii) Design or ggtimizing qit!-!ÏI: The desired response criteria of a control

aIgorithm must be chosen. 1hese are considerations such as whether the

controller is optimized for set point tracking or disturbance rejection and

how responsive the feedback loop must be. It was alsa expected that

saturation of the manipulated variable would be an important consideration.

Thr. time-integral performance criteria were considered.

5.1.1 PlO CONTROUER

The PlO control/er is the most widely used control/er in the process

industries. In practice, one needs only minimal knowledge of the prooess

dynamics of the system in order to tune the PlO controller. The three parameters

(P, l, and D) are often chosen by trial and error b9sed on the response of the

controlloop and experience. This procedure usually extends over a very long

period of time. Tuning the PlO controller can be done more accurately and

efficiently with a formai loop tuning procedure.

The velocity form of the PlO algorithm is [28]

where m = manipulated variable

e = error (set point - controllad variable)

T 8 = e:tmpling time

(

(~

n = samp!ing instant

P = controller gain

1 = controller integral or reset time

o = controller derivative time

and is implemented in a mathematically simplified form as

where t. D a • P (1 + -1 +-)

t.

o b = P (1 + 2-) ta

D C· P-

ta

69

(5.2)

A modified form of the PlO algorithm was used. The conventional form of

the PlO algorithm responds with a large action trom the derivative term when a

step change in set point accurs because the derivative of a step is infinitely large.

This undesirably large action can be eliminated by having the derivative term aet

on the change in the controlled variable r&ther than on the error. The modified PlO

controller, said to have set point kick compensation or velocity feedback [27] is

70

where Y is the controlled variable. It was implemented in a mathernatically

simplified form as

where a = P (1 + t.) 1

b = P

o c = P-t.

5.1.2 DAHUN CONTROUER

Dahlin's method for tuning feedback controllers [28] derives a controller by

imposing a desired closed loop response on a system which has been subjected

to a unit step change in set point. The desired closed loop response to a unit step

change in set point is that of a first order system plus dead time having the

Laplace transfer fundion

Y(s) = exp( -8s) ! l'1S + 1 s

(5.5)

f

71

where 1'1 is the time constant of the desired response and is the parameter to be

optimized. The discrete-time form of the equation is

where

Vez) • z-lc dz -1 1 1 - CZ-1 1 - Z-1

-T d • 1 - exp(---.!)

1'1

-T c • exp(---!)

1'1

(S.8)

The design equation for the Dahlin controller (and any controller that

imposes a desired feedback response on the system given a certain input) is

obtalned by rearranging the characteristic equation of the feedback controlloop

(888 Figure 5.1b) giving

D(z) =

Vez) Y.(z)

1 _ Vez) Y.(z)

where D(z) is the controller equatiorl

1 (5.7) HG,(z)

bZ-k-1 HG,(z) = le for a first order model with dead time

.'P 1 _ 81-1

1 'f :: ---.. 1 _ Z-'

m, :: u..m • .. ILI'n ,,,- '7 ... ,,-. - f

d Va" -

~b

ad 'If:: -

K,b

(S.I)

When designing a Dahlin controfler. one must aJways be wary of "rlnglng"

(excessive movement of the manipulated variable). It was found that ringing can

be etiminated in the design stage by specifying a realistic response of the dosed

Ioop system to a unit step change in set point. For example. if the open Ioop

response of the system is second order, it is unrealistic to expect that the cIosed

loop response will be first order.

(

73

A controller was designed using Dehlin's basic method of specifying a

cIosed Ioop response to a unit step change in set point, but with the modification

that arder of the desired cIosed Ioop response W8S made equal to that of the open

Ioop dynamics. The dynamic response of the nozzIe malt temperature was second

arder overdamped, however the optimizatiot,s &lways converged to the result that

the best controller was critically darnped. This suggested that the desired

response should be second order unc:terdamped and the Laplace transfer to a unit

step change in set point is

V(s) • exp( -8s) 1 l'ail + 6s + 1 s

(5.1)

where "2 (time constant) and 6 (damping factor) are the parameters to be

optimized. The discrete-time form of the equation is

where

d Z-1 + 141~,-2 1 Vez) • z -k ---' --~------

1 - c,z -1 + ~ -1 1 - Z-1

d, • K, (8ÏIl(.) + • sin(.> cos(tT el - • 008(.) sin{tT J _ 2e cosCtT JJ J1-62

da • K, (.2 + • COI(+) IIn(.T J - • Iln(.) C08(tT J J J1-62

(5.10)

...,.. , i" .:» -

c, • 2e COI(tT J

a • • exp(-T-)

8't

74

Ca • el

•• {f:ii t

For a process that is second arder overdamped, the discrete-time pulse

transfer function using zero order hold to a unit step change in set point is given

in Equation 4.2. Applying Equation 5.7, the control equatian for a second arder

overdamped system with a dead time of one sampling period using a desired

feedback response th&" is second order underdamped is

3 5

mn = E Y~n~ - E u,mn-l 1-0 )-1

(5.11)

where

('

(~

f

75

&,ct bx fram Equation 4.2

Cxt d,c fram Equation 5.10

5.1.3 SMITH PREDICTOR

The Smith predictor [28] accomplishes dead time compensation using a

dynamic modal ta predict the response of the system (see Figure 5.2). When

action is taken by the controller, the model without dead time responds

® • ControUlr u -- Proc ... -.. D(z) - .... , sGpC.) ... y .. -

~b +~ Wod.1 .. HGp(z) - ri' HGp(z) + •

Figure 5.2 Block diagram of the Smith predictor control algorithm for dead time compensation.

;*6

immediately and later subtracts its own response as the process itseIf begins to

react. This 1ooIs· the controller by making it see only the process without dead

time. A major drawback to the Smith predictor is that if the model is not accurate,

the response of the control Ioop win deteriorate and may cause an unstable

feedbad{ response.

Any control algorithm can be used in the dead time compensator. The PlO

controller was used for this work.

5.1.4 TUNING CRITERIA

Many tuning methods have been developed for feedback controllers and the

most popular tUi1ing technique is the one-quarter decay ratio. The main drawback

with this method is that, in plant operation, the highly oscillatory nature of the

controller will cause ail the downstream processes and controllers to oscillate and

may lead to destabilization of the averall plant control.

The process readion curve methods developed by Cohen and Coon, and

by Ziegler and Nicols use semiempirical formulae to design a PlO controller.

These formulae can only be used for systems that have a ratio of process dead

time to process time constant of 0.1 to 1 [29], however, the main tuning criterion

is still the one-quarter decay ratio.

The Bode and Nyquist stability criteria can be used in frequency response

analysis. Ziegler and Nichols also developed an on-line tuning method based on

frequency response analysis, however, the main tuning criteria is again the

one-quarter decay ratio.

(~

(

(

77

The tuning method used in this work employed the time-Integral

performance aiteria [28]. Thr. criteria were investigated:

1) Integral of the square error (ISE):

• ISE • f ,I(t) dt (5.12)

o

li) Integral of the absolute value of the error (IAE):

-IAE-f Il(t) 1 dt (5.13) o

ili) Integral of the time-weighted absolute error (ITAE):

-ITAE = f t 1 I(t) 1 dt (5.14)

o

The Nelder and Mead [26] search algorithm was used to minimize the time-integral

criteria for the cIosed loop response of the system to a step change in set point

and an impulse change in disturbance. The advantage of using this tuning criteria

is that the entire response of the process is used and non-linearities, ego

saturation, can be simulated.

ln this wOrk, the IAE criteria was usect because it is the intermediate

between the ISE criteria which gives fast acting controllers that tend to be

78

oscillatory and the ITAE criteria which produce slow acting controllers that

emphasize long term accurate set point tracking.

5.2 CONTROL SIMULATIONS AND RESULTS

Simulations were performecl to determine the Iowest sampling time that

coulet be used that would not cause truncation e"ors. Truncation errors accur

because of the method that is used to store numbers in computers. A fixed

amount of space is reserved for each number which is usually trom two to eight

bytes depending on the precision specified or available, and if the number is an

integer, BCO, fixed point or floating point. Any digit that does not fit into the given

space is lost and rounding off does not accur at the final digit. In addition. ail

numbers are stored in binary format and there may be some lost precision simply

on the conversion of a decima! number to the binary format. If the problem is

severe, any computation can result in truncation errors. Usually the truncation is

reasonable, however 1 problems may still arise when many summations are

required. At every summation a part of the result is truncated and therefore lost,

and this accumulates with each summation.

It was found that with 32 bit fixed point arithmetic, the smallest sampling

time of 20.4 seconds could be used without eny degradation of the controller

action.

To reduce the amount of data that had to be stored, the sampling time for

the ail the temperature sensors was set to 2.04 seconds and, therefore, 10

(

79

readings of the immersed sensor were averaged to produce the signal that was

uaed for control of the nozzIe meIt temperature.

A step change in disturbance w. ImplementeeS with the use of a fan

bIowing air over the band heaters. This inaeased the convective heat Iosses and

could be simulating the opening of door in a processing plant causing a draft.

5.2.1 PID~CNUUER

Computer simulations were performed to determine the optimum PlO

parameters for the control of nozzle malt temperature. Initially, only a step test

was used and the results of the simulation are shown in Figure 5.3.

Figure 5.4 shows the experimental results for two step changes in set point

and disturbance, each. It can be seen that the controller was able to track the set

point for set point changes but was unable to eliminate the offset for the

disturbances. This was due to the use of only one step change in set point in the

simulation experirnents which determines the best proportional term for that set of

conditions while requiring littie use of the integral terrn.

Te ensure that a useful value was found for the integral terrn by the

optimization procedure, an impulse change in disturbance, which lastad for one

sampling interval, was used in addition te the step change in set point. Use of a

disturbance helps to ensure that a proper selection of the integral term occurs.

An Impulse change in disturbance was implemented as follows

'"' ~

80

.r-____ ~-----P~I-D~S~IW~U~LA~TI~O~N--~----~----~ rAt

, ~ 1

1 1 ~ ~ _ .. no.................... ......... . ................ t ............................. t.............................. . ............................ , ............................ ..

I ii - Temperature 1 1 Il - Set ,oint 1 1 : t 1 :

" 1 i ' 1 1 !Ir 2 ~ ......................................................... · .... · .. · .......... ·· .. ·· ............ · ........ · ........ · .. 1 ........................ · .... ,· .......................... .. :::t 1 1 1

I~ 1 1 1

1 1 1 Ji, i J ;!!

o~------~------~----~~----~-------4------~

-2"~----~------__ ------~------~------~----~ -10

ft ~

! ~ ~

1

~ ~

i 1 1

1Or-------~----~~----~~----~------~------~ ~ P • 9.27 1 1 : : 1 •

1 1 • 12.2 1 i l 10 _........................... '" .. · .. · .. · ...... · ........ l. .. ~ .. ·~ .. !~:·~!.·j .............. · .. ·· ........ ...II .... ····· ..................... ~ ........................... ..

i i 1 i ! , 1 1 i l' i .0 _ .................................... · ...... · ........ i .... · .. · .... · .... · .......... ·.f· ........ • ...... • .. • .............. · ......................... j ............................ .

1 l " 1 : 1 1

zo ~-... _.--.-.-......... -·-·-Ay-...... -·-·-·-r-·-·----11

: .... -.--.............. 1'-... --..... ---1 Y 1 i

o Vil -f 1 m jÎ il i i 1 ~ j i 1 z-2~ .. 10------~O------~1~O------~20~----~~------~.0------~~

TlME (min)

Figure 5.3: Results of the computer simulation to determine the optimum PlO parameters, according to the IAE criteria, for a step change in set point.

(~

<:

1H

114

"Z -e 1110

l'· 111

114

1820

140

-!120

e 0100

!!i ~IO w

PID EXPER'MENT

1 ............................... . ................................ 1 .................................. .1 ................................. . FAN ON 1 _._ .. -. -_ .. _-- --_·_ .... -Ti-_ .. _·-t"-.. · ---

..........

i 1 --r-. ............................ j ................................... j ................................... t ...... ·· ........................ l ............................. .. i i 1

.................................................................... ~ .................................. ~............... ........... .. ............................... .. 1 1 •

. __ . __ .. _;;.;~~ ... _._ .. __ ._ .. -l---- ._.- \ .......................... . • 1

20

1 1 F N OFF 40 60

T ... [ (min)

,. • t.27 1. IZ42 D. IZ.07

80 100

i ION· .. ,,· ... ·,

140 ~ 2O~·" .. , ........ · .. · ........ · .... _·+, .......... ·· .... · ........ · .. · .. li~ ... · ................................ + ........ ·· ..................... ·· .. ·I ... · .. · ........ · ........ · ............ ~ z

°O~--------------~--------~------~------~ 20 40 10 ~ ~ TI .. E (min)

81

Figur. 5.4: Experimental results for the implementation of the PlO parameters whose optimization is shown in Figure 5.3.

-

82

aV • brn + D (5.15)

where y = simulated temperature

m = simulated band heater power

8,b = model parameters

o = the disturbance

o is initially set 0, and changed to some constant for one sampfing interval to

simulate the disturbance.

The maximum heater power allowed was limited to increase the stability of

the temperature control Ioop. Saturation at zero percent heater power often

occurred and an excursion to a high heater power could not be quickly recovered

from. The maximum heater power allowed for band heater 3 was limited to 43.1

percent (byte value of 110) in order to balance the heating power of the band

heaters and the cooling of natural convection. This value was chosen because il

is approximately double the power required for steady-state operation.

Wlth the PlO parameters optimized, the simulated response of the system

when subjected to an impulse change in disturbance and step change in set point

is shown in Figure 5.5. The experimental results for changes in set point are

shown in Figure 5.6. Figure 5.7 shows the experimental resuits fC,r the

performance of the PlO controller when disturbances are introduced to the system.

The computer failures were unintentional but shows that the controller can recover

r't., ; ~ ......

83

PID SI"U~~ liON ·r---------------~--------~------~------~

- T .... rat.... i ~ 5 .. -.tPoiat , .................. ,. ..... _ ! ....................... ..

1 i i _û" 4 .... •• .. •• .. ·• .......... • .... •• .. ··f .. · ...... · .. · .... · .... · .......... ··i· .. · ....... ·· .. ········ .... · .. ,~ ...... · .. ··· .. · ......... ·· .... · .... i .... · ......... ······· ............. .

: : 1 • i ! 1 i

3 ~········ ............ · .. --···· .. ·t .. · ...... · .. · .. · .. ·· .. · .......... ··l...... .... .. .................. + ................................... ~ .................................. . 1 1 1

2 ~······ .. ·· .... · .......... ····· .. ·l ... '-'''-'''''-'-'r' _. _ ... __ .... + ... _-_ ........... _+ .................. _ .... __ . 1 0·_·· ...... --"-"1 ........ = ...... _ .. -'1-... ; ·_·_·_-_ .. ·j .. · .. _-_ .... _ .... _·_·t· ...... · ...... ·_· .... _ .......

o i V" l " 1 1: : - t ................................... , .................................... , ................... ·· .. ···· ........ 1 ...... ······· .. · .. · .. · .... ·· .. ··· .. ·, .. · ............ ···· .. ·· ......... ". . ! 1 !

1 1

1 i i l i

-!~~--------0--------~20~------~~--------~u------~~ TIME (min)

i 80 1 ! ! i Ji : ! i ! p. 13.07 R. 60 .. ······· .. · ........ · .... ·· .. ··· .. ·/-.. · .... · .... · .. ·· .. · .... · ........ ·1 .. ·· ............................... +-.................................. '1'. ~ .. 2::':5

1 Iii !il 40 .. ········· ................. ····· .. t .................................. +..... . · .... · .. ······ .... ·· .... t, "· .. · .... ··· ............ · .. · .. · .. ·1·· .............. · .. ··· .. · .... ·,, .. · ~ 1 Iii >' i ! 1 ! 20 .. ····· .. · .............. · .. ····· .. ,· .. · .. ·n .. ·· .. ··· .............. ·l...... .. ...... ········ .... · .. j ........ ····· .. · .......... · .. · .. · .. l· ............................... . i 0 ! ,,.... ! ,~ 1 i !te 'V! .. i ! JE i ! ! ! B -20 ~ .. ·······"'" .... · ...... · .. ···· .. i .............................. "j .............. · .... ········ .... · .. t .......... ··· ...... ·· .......... · .. ·']'· .. · .... ·· .... · .. ·· .. · ............ .

1 1 • 1 •

-~ ~ .................. " ........................ · .. · .......... · .. · .. ··~···· .. · ........ · ...... ······ .. · .... 1 .. "· .... ···· ....................... ~ ........................... " ..... . 1: :1 :1: a

1 i i 15 -60 .. ·· .. ··· .... · .... · .. "· .. · .. ···· .. ~ .. · ...... · ........ · .......... ,, .... t··· ........ · .. " ................... " ..................... " .. " ...... + ........... " ..................... . 1-10 i 1 1

-20 0 20 ~ 10 ~ TIME (min)

Flgur. 5.5: Results of the computer simulations to determine the optimum PlO parameters (IAE criteria) for 8 disturbance and a change in set point. Heater power Wl'S restricted to increase the stability of the controller.

c

C

111

1H

6 '14 -., i "2

tlO

111

'''0

PlO EXPERIWENT

- 1 •• ,.r.'.... i 1 i - Id Pola' . 1 1

........................... 1' ............................. ·· ...... ·· .. ···· .... · .. " .. ·t···· .. · .. ·· .. ······ .. ······ .. 1····· .. · .. · .................. 1" ........................... .

j 1 1 ........................... .,.......... ......... . ... l............................. .......... .... .. ....... ,j ............................. ~ ............................ .

1 1 Iii 1 1 1 1 i ", ........................... , .................................................... + ...................... t .. · .... · .... ··· .......... · .. ·t .... ·· ............ · .... ·· .. ·· 1 ! i 1

1 ! 1 . ..· .. ··· .. · .. t .... · .... · .. ···· .. ···· ...... ·l .... ·· .. ·· 1 - ••• 1 ...... • .. • ........ • ..........

l ' , fI 1 1

! 1 .-................................................... · .. · .. · .. ·· .. ··· .. ··· .... · .... f······ .. · .......... ····· .. ·· .. ~ ........ · .. ········· .. ···· .. t·· .. ··· .. ····· .. ··· .. · .. · .. ··

1 ..:

10 20

, 1 : . i 1

~ TlME (min)

!

~ 10

~'OO ................ " ............ + ................ ~I ....... , ... ~ ... " ....... " ........ "" ...... , ....... " .......... , ........ " ... + .. , .................. , .......... ~ ft • 13.07 1 - 295.5 D .15.25

.. .. e o 10 H .. · .... · .... ·· .. ···" .... ·-t.t· ........ , ...... 1

j

~ 10 "·I-·"d"'k·=~·1 i 140

Im~ ...... "" ............ " .... + ............................ ~! ............. _ ..... -, .. -i-." .... +·1 ...... · .... 1 ....... • .... • ........ • ...... • .. ; ...... • ...................... 1

°0~------,0------·~-------~~~~--------~------~ TlME (min)

84

FIgure 5.1: Experimental results for the implementation of the PlO parameters whose opt.i nization is ,t,own in Figure 5.5.

•

.....

...,..

PID EXPERIMENT t'Or-----~----~------~----~----~------------~ ta. . ..................... !. .............. . - Te.peratu ...

- Set PoiDt

, .. -.s. 117

1 lM tl5

, ... tU

1120

t20

8 .. .. 100 e 0 .;; 10 :t

~

1 10

1 40

15 i 20

1 i : 1 ··· .... ·········1···.. ..·· .. ··········t .. ·· .. ········· .. ···;ÂN orF··· .. ·····r·· .... · .............. ·· .. ! .. · .. ·· ............ · ... .

1 i i 1 i i 1 i i . .............................. · .. ·· .. ···· ........ · ........ · ............ ·• ........ ·· .. ·· .. · .. · .. · .. r .. ········ ............... , ...................... ., ....................... .. i i i i ! i IIi . i i ! j t' ! 1 i ...................... -r ........................ 'f ......................... • .. • ................ • .... 1· .... · .................... , .. ····• .. ·· .... · .. · .. · .. ·.,·· ..................... .. • , 1 1 1 1 1 il. 1 i

10 20

10 20

30 4CI TIME (min)

30 40 TIME (min)

50

50

p. '3.07 1 • 295.5 D • 15.25

70

70

85

Flgur.5.7: Experimental resutts for the implementation of the PlO parameters whose optimization is shown in Figure 5.5. A fan blowing aver the band heaters was used to introduce a disturbance to the process.

li

•

(

(

86

when a complete Ioss of control occ.'V' 's. To recover from a oom~er failure, the

computer was reset and the program was down loaded from VAX-780 which

required about 45 seconds.

5.2.2 OAHUN CONTROLLER

The first controller was designed according to Dahlin's mçthod. A first order

plus dead time modeI was fitted ta the data for the immersed sansor and for band

heaters 3&4 and used in the design of a Dahlin controller. Use of the first arder

model ensured that ringing of the manipulated variable was reduced as a result of

a modeI-process mismatch (588 section 5.1.2). The first order model parameters

were deterrnined using the seme procedure described in section 4.1.2. The fit is

shawn in Figure 5.8 and the model parameters are given in Table 4.1. The

computer simulations and control 1er optimizations were carried out using the

second order model for the temperature response of the system even though the

first arder modeI was used in the design of the controller. The goal was to attain

a control 1er without ringing while using the actual model in the simulations.

The results of the optimization for the Dahlin parameter and the simulated

response are shown in Figure 5.9 where the same change in disturbance and set

point were used as for the PlO controller. The lcalculated valuew in Figure 5.9

refers to the untruncated value of the manipulated that was used in the Oahlin

controller equation. The truncated value used in the equation for the temperature

rnodeI is shawn by the lsimulated power ta the band heaterw• This must be done

because the Dehlin controller fails completely If truncation is performed in the

1 ........ -·"--..... ----~----~--------~ ! 1 ! - E.pwfmentol Dato

1.2 i i 1 - Flted M'" ................................. + .................................... j ................................... .j., ................................. " ............................. ..

1 Iii ................................. J.............. .. ........... 1 .................................... t ................................... J ................................. .. Iii

! 1 1 1 ................................. ; ............................... '" .......... · .................. · .. t .............. ·· ........ · ....... ; ................................. .. : i 1 i ................................... 1 ............................... ~ ................................ 1 .................................. .

i i Il! il·

. i ! .................................... ........................................ ... . .................... i ....... u .......................... .

1 1 1 Il!

............................... ~ ................................... ~ ................................... ~ ........... • ...... • ................ ·1· ................ • ................ .. i i i i i i i i

1100~------~5O---------1·00---------1~~-------2~00--------~2~

TlME (min>

il'! i ! 1 1 - 100 ~· .......... · ............ · ........ t .............. · ...... · .. · .. · ........ I .................... • ........ • .. • .. t .... ·· ............................ ·t .................... · ............ .. o Iii i ~ i 1 i : - : 1 1 1

e 1 Iii o 10 ~ .............. · .................. +-· .. ·• ................ · ............ ·i· .................................. !-................................... ! ................................. .. i i i 1 W 1 i 1

~ i i 1

j eo ~ ....... ,.. ............... IA_N_DH_EA_TE ... ··~ .. ~·I ................... .......... t...... .. ...... ;, ................................. .. Ë i! i

- 1 1 1

1 : i 1 40 ............................................................................................................................ 1. ................................. ..

'- ~ 1 ! ! .... i ! 1 e i . i i

~ 20 _ ................................. , .................................... + ................................... i .... · ............ ·· ................ + .. · .. · .. · ...... · .. · ........ · ...... · z 1 j 1 j

1 1 1 1 °o~----------· ... ----~--------~------~--------~ 50 too 150 200 2~

TlME (min>

87

Flgur. 5.8: Fit of a first order plus dead time model ta the data for the immersed thermocouple and ban,j heater 3.

(~

. L_

11t OROER DAHLIN SIWULATION .P---------------~--------~------~------~

- T •• peretue l' f\. 1 - 1ft PolDt ,

: 1 .. ! i 1 1 ~ ...... ··· .. ··· .. ·· .... · ...... ··· .. 1·· .. ·· ...... · .. · .......... ···· .... ··" ............ ·· .. · ...... · .... ·1· ...... ··· .... ··· .. · .. ····· .. · .. ···1· .. ·· .. · .. · .. ··· .................. .

I ii i 1 i: I~ j i

Col 1 1 1 j 2 ~ .................................... · ...... ·· .. · .... · ...... ·· .... t· .. · .......................... 1' ...................................................................... .

1

0r-----~;--=~~----t-----_t-----, 'iiV i

ii i

il il _2~ _________________________ i _________ i ________ ~

-20 0 20 .tG 60 10 rIME (min)

i ISO .... ---------------~------""""!"'-------,....------.., - SUA...ATEO POWER li .a ro IANOHEATER u • 40.21 !

, ,- CALCULATED VALUE 1 l ,

W 100 .... ·· .. · .. ··· .. ··· ...... ·· .... ··r··· .. · .................. · ........ ··,···· ·· .... · .... ·· .. ··· .. ··· .... ·'t .. ···· .... ··· ........ ··· .. · .. ···· .. t· .. ····· .......................... . ~ ! 1 ! !

~ 1 l' 1 1 li ! i i i SO I· .... • ...... • .................. • .. ·f·· .......... · .................. · .. j .. ·ft·, ........ · .... " ........ 1 ....... · ...... ·· ........ · .. · .. · .... · .... · .......... · .... · .. ·· .. · ...... · .. 3 il!

• 1 :

1 0 1 - 1 ~~-... l-----....-

~ IV Iii ~-SO~------------------------~----~ ~ 0 ~ .tG 60 10

TlME (min)

88

Figur. 5.1: Results of the computer simulation to determine the optimum parameters (IAE criteria) for the Dahlin controller for a disturbance and a change in set point .

89

control equation. This was also done in the experiments but the -calculated

values" were not available. The simulation predicts that a small amount of

ringing will occur. Rinr .ng was not completely eliminated because the ~ual

process was still second order. The implementation of the Oahlin controller for

melt temperature tontrol gave r8$'JIts that are similar to the simulation (588

Figure 5.10).

To completely eliminate ringing in the Dahlin controller, an underdamped

second order response was specified as the desired closed loop response. The

results of the optimization and the simulation are shown in Figure 5.11 where the

damping constant and time constant of the desired response were the optimized

parameters. The modified second order underdamped Dahlin (".ontroder (Equation

5.11) was not implemented because it was a prohib~ive task to program the 10

terms in assembler and the small improvement in closed loop performance over

the first order Dahlin controller (compare IAE errar in Table 5.1) did not justify the

experiment.

5.2.3 SMITH PREDICTOR

The Smith predictor dead time compensator with a PlO controller was

aptimized using the same method as for the regular PlO controller. The best PlO

parameters and the simulation are shown in Figure 5.12. The Smith predictor was

not implemented because it was found that, for this system, the dead time

compensater resulted in only a marginally better controller than the simple PlO

controller (compare IAE errer in Table 5.1). The small advantage of the Smith

{

(~

90

1 .. OROER DAHLIN EXPERIMENT t.4r-----------~----~----~----~----~----~

!ta

-tto 8

tl4

10 ao 30 40 50 10 70 TlME (min)

t 1 • i i ! 100 ""---i-'--'- -1---' --.. - .. +_ .. _-t::·~~~t---o 10 .. ---1 .. _ .. -· L-... -- - ... ~ .... ---...... I.--.---..j. .. -----.:J j i i i ~ i i i

~! 1 i , 10 .. · .. ··· .... ··t··· .. · .. ···.... .. · ...... ··· ...... ··t· ........ · ...... · .... · .. ·t· .... · .. · .. · ...... ·_ .. · i . 1 t 1 i 1 40 '-'''''' · .. -t--+-_·-+-· .... _· ...... -..... -.---+----~ 20 ....................... t,! ... · .. · .. · ................ II---..... J,-_ ... _-- .. -t·---·_· .. ·+--z 1 i

1 i 1 i oo~----~----------~-------A--~----~----~ ~ ~ 30 40 50 10 ~

TIME (min)

figure 5.10: Experimental results of the first order Dahlin controller ta changes in set point.

f

1

1

1

1

1

--

91

2nd OROER DAHLIN SIWULA TION .r-----------------~------~------~--------~ - T .... r.t .. ,. 1

-- ad.o_t !

1 1 1 =

1 i 1 6 .................................. " .................................. " ............... ·1\ ................ · .. ; .. · .. ···· .... ·· ........ · .. · ........ r ............ · ........ · .......... .. 1 1 1

! ! i 1 1 1 .-+...lI!C-=t---.f.-1 il! 1 i i :

4 • .......... · .. ··· ...... · ...... · .. t .... ··· .. ··· .. · .. · .. · ........ · .... i...... ..... · .. • ........ · ...... ; .... · .. ··· .... · .. · .... · .. ······· .. ·t .. ···· .... ··· .. · ................ .

I ii l' i i 1.. 1 1 1 1 n ! ! 1

2 1 • • 1 _ ........................................................................... ··· .. · ............ · .. t·· .. · ........ · .. · .. · .. ···· ...... ., ............ · .. · .......... · .... · ..

I l! ! 1 1 1

i ! J 1 i 0t------i~-;~=*~----1-------t------1

1 V .I.i 1 1 Il 1 il 1 l

-2~----------------~----------------~------~ -20 0 20 40 te) 10 TI .. E (min) -m

~ 1ft IR 1

~ ~ i -i ~ Col

• w

1

200r-----------------~------~------~~------~ - SlMWTED POWER i

TO IANDHEATER 1 i 1 150 _ CALCWTEO VALUE .. · .. ·1 .......... · .. · .............. · .. ·t .. ·· .... · .... · ................ · .... t .. · .... ······ ........ · .. · ....... ..

100 •••••.•• --.. - .. -1.-.-... --.-.... -..1.. . .. -....... -.... .1-.=-.; .. ;; .... 1. .----.. -...... . l i i 1

1 Il! 50 :·-·-·····-··-t:·----·t r=· .. ·l··-···· .. ····-··-t-·--·---····-· o ;l-: _'; ;

1 Il!

1 1 1

-50 • .... · .... · ........ ···· .......... ·t.. ..· ........ · ............ ····· .. t ...... · .. · .... · .......... · .. · .... ··j .. ·· .. ·· .. · .... ·· ................. + ................................ .. ~ i ! i 1

;""" i 1 1 1

1 1 l i -1~'~----------------------------------~------~ -20 0 20 40 te) 10

TIME (min)

Flgur. 5.11: Results of the computer simulation to determine the optimum parameters (IAE criteria) for the second arder Dahlin controller for a disturbanœ and a change in set point.

(

(

92

predictor will surely be Iost upon implementation because the dynamic mode. is

only an -adequate- model and therefore there will always be a discrepa~

between the model and the process.

5.3 SUMMARY

The full heating power of the band heaters 3 end 4 could not be used

because the process st steady state operated very close te saturation at .!ero

band heater power. This caused a severe nen-linearity and was rectified by

setting a limit for the highest band heatsr power at a byte value of 110 or at 43.1"

which was approximately double the power used du ring normal steady state

operation. It is not recommended that Iower wattage band heaters be used

ConIroIIer Tuning Errer P 1 0 Dahlln Camping Cft ..... (lAE) (bita / ·C) (8) (8) Time factor

Constant a t& (s)

PlO Set point ~.27 1242 82.07

PlO Set point 1561 13.0i 295.5 85.25 Dllturbllnce

Dahll., Set point 2149 40.21 (1. ortler) Dllturbance

o.hIln Set point 2000 49.78 .sq&3 (2nd arder) DlltlJbance

SmIth Set point 1467 118.5 50.35 25.34 Prmlctor DlltLlbance

TI"" 5.1: Controller parameters determined trom computer simulations. A bit is the unit value of a byte.

..,.. ~

-~ J u

! i w

~

i S Il 1

W ::t

~ w

i -

93

,~ ______ ~S~W~IT~H~P~R~E~DI~C.TO~R~SI~M~U~U~T~IO~N~~ ____ ~ - Temperature 1 ._.. Set Point ! ! ;

5 ................................... ,. ................................... ., ....... '.. . ......... -............. ..

4 .-.. -.-----1. .... ----....... --Li .. · ... -.. ---.-L-.. --...... .J-.... --.--......... ---! i · i ! ~ ! 1 i i 3 ·· .. ··· .. ············· .. · ....... · .. r······ .... · .... ········ .. ········T·····f···· ..................... !' .................................. 1' ................................. . ; i • ; 1 ! Il!

2 .................................... ;.. . ···· ............ ····· .... · .. ·~ .... ·f· .. · ...................... ~ .................................... ~ ................................. .. ! II! 1

Il! 1 1 1 · ........ ····· .. ·· ...... · .. · .. · .. ··T ... · .. · .... · .. ············· .. ·1 .. · .. t··· .......... · .. · ........ 1 .............. · .. ·· .. · ............ 1······· ...... · .. · ...... ··· ...... · ..

il; i i ot---------~!~~====~! .. IJ-------;~--------·~· ---------1 ... -1~--------~--------~--------~--------~--------~ -20 o 20 .0 60 10

TIME (min)

10 i :

10

40

20

! ......................................................................... i ............................................................................................................. .

1 ! ! , .'11.5 !

1 1 1 1. 50.35 ! · ...... ········ .. ·· .... · .. · ...... t······ .. ··· ........ ·········· .. · .. ·1 ...... lOlO • .. • .... ••• .. • .. ••• .. ···t· ...... ·· .... · .... ······ ...... · .... 1 .. ·· .. ·· .... · .... ··· .. ·········· .. ·· ! 1 ! D. 25.~ !

................................. .,,, ............................. -1........ . ................ t' ................................. ., ............................... .. i ! ! ! ! 1

0 : .

.1 1 !

................................. j ................................ ~........... .. .................. .; ................................. ; ................................. ..

~ i j ~ i ! ! !

................................................................ u ................................ ., ••••• ..., .......................................................................... .

i i i i : i i : .. · .. · .... ······ ...... · ........ ·r ............ · ........ ····· .... · .. ·r ........ · .......... · .. ·· .... ·· .... 1 .......... · .... ·· ........ ·· ........ ! .. · .. · .......... ·· .... ·· ...... · : , 1 : : 1 ~ :

o 20 40 60 10 TIME (min)

Figure 5.12: Results of the computer simulation to determine the optimum parameters (IAE criteria) for the Smith dead time compensator using a PlO controller for a disturbance and a change in set point.

(

94

because the full pciwer is required when the barrel is brought to normal operating

temperatures when starting up a cold in je di on molding machine.

The process time constant for the melt teMperature response was the

dominant time cons:ant and it was for this reason that the Dahlin and Smith

predictor dead time compensators did not provide a significant advantage over the

commonly used PlO controller.

The order of the process and that of the desired response must be the

same to prevemt ringing in a Dahlin controller. The second order underdamped

desired response resulted in no ringing at ail whereas the first order model w~h a

first order response still produced sorne ringing because the process Vias second

arder.

The derivative term of the PlO controll~r was toc large because there was

sorne oscillation in the temperature response as it was approaching the set point

and large controller adions occurred when the temperature was at the set point.

The derivative term is very sensitive to noise. There was no noise in the simulated

temperature and this allowed the optimization te; produce larger than desirable

derivative terms.

The set point kick compensation worked perfectly. A large and erratic

derlvative action did not occur when a set point change occurred.

-

95

6 CONCLUSIONS AND RECOMMENDATIONS

This study concerned the installation of the following sensors in the injection

molding machine nozzle: a) a flush mounted Vanzetti infrared optical fiber

temperaiure detector, b) an immersed ribbon thermocouple, c) three flush

mounted ribbon thermocouples, d) a metal thermocouple, and e) the original

pressure transducer in its origir.al location. A ribbon thermocouple was also

installed in the screw tip. Various measurements were performed and the following

conclusions made:

6.1 CONCLUSIONS

1- The injection molding machine was modified to enable various temperature

sensors to be studied. A new barrel, nozzle, and screw tip were

constructed and succesfully installed te achieve this objective.

2- The response time of the. Vanzetti is essentially instantaneous and it

detected temperature increases of aoc during injection of the melt. The

Vanzetti averages the temperature of the polymer over a volume because

the polymer melt is translucent. The Vanzetti is an ideal candidate senser

96

( for applications where temperature control is critical and the material is

highly viscous.

(~

(

3- Ribbon thermocouples measure the temperature of the melt and not the

barrel temperature. Flush mounted ribbon thermocouples appear to trace

the barrel temperature. However, the temperature of the melt next to the

barrel will be the seme as that of the barrel. Immersed ribbon

thermocouples are useful for providing point measurements of the melt at

any depth and are an inexpensive alternative to infrared optical fiber

measurements. It was found that the temperature of the bulk polymer was

significantly different than that of tha barrel metaI. The responM time of an

immersed ribbon thermocouple IS slow comparecl to the Vanzetti, however

it is much faster than sheathed thermocouples as it was able to detect

changes in temperature of 6° C during injection which is performance

comparable to the Vanzetti.

4- The screw tip ribbon thermocouple measured the temperature of the melt

as it came off the screw 8fter having been worked by the screw. The

readings of the screw tip thermocouple were almost constant during

injection because ils velocity with respect to the melt was zero. The screw

tip thermocouple does not provide 8 good reading of the temperature of the

meIt being injected into the moId because the melt that is next te the sensor

.......

97

may not actually gel injected into the mold for another one to thr. cycles

depending on the cushion size.

5- Open loop PRBS experiments were performed ta obtain dynamie

temperature responses suitable for control purposes and to determine

which of the sensors and which of the band heaters were best for control

purposes. The flush rnounted ribbon thermocouples were not considered

suitable for control purposes because they measured the temperature of the

melt next the barrel wall and not the temperature of the bulk polymer.

6- The open loop response of the nozzle melt temperature with the band

heater power as the manipulated variable was second arder overdamped

with dead time.

7- It was found that the combination of the two bant: heaters that were the

furthest downstream provided the œst response for control purposes. The

screw tip thermocouple had the fastest response time sinee it is the sensar

that is furthest upstream in the barrel and therefore was not suitable as a

control sensor. It was found that the Vanzetti required trequent rsc31ibration

and therefore could not be a reliable control sensar. The immersed ribbon

thermocouple was used as the senser for the control experirnents .

(

98

8· The PlO, Dahlin, and Smith predictor control a1gorithms were considered for

the cIosed Ioop feedback control of the nome melt temperature. The

parameters in the control equations were round by minimizing the integral

of the absolute error (IAE) in process control simulations using the dynamic

model found in the identification experiments. The simulation results

showed that the dead time oompensators did not provide a significant

advantage over the commonly used PlO controller. This was because the

dominant time constant in the system was the process time constant and

not the dead time.

9- Experiments using the PlO and the first order Oahlin controller were

performed and the results of the step test in set points were similar ta the

simulations. Oisturbances were a1so introduced ta the system and the

controllers responded satisfactorily.

10- The control band heater power was limited to 43.1" of its full range to add

stability to the control of the nozzle melt temperature. This was necessary

because the normal steady state operating point of the control band heaters

was approximately 21" and this equalized the cooling power of natural

convection and the heating power of the band heaters at the saturation

points of 0 and 43.1%, respectively, of band heater power.

..,....

-

· ..... _._. __ . __ ._--------------~

99

11- To eliminate ringing in Dahlin controllers, the order of the desired response

must be the same or greater than the arder of the open Ioop system. A

controller was designed using Dahlin's approach with the modification that

the desired response was second order underdamped with dead time. This

resulted in a controller with absolutely no ringing.

6.2 RECOMMENDATIONS

1- The volume of the mold cavity should be doubled and balanced (to help

eliminate flash) because only half the capacity of the injection moIding

machine is being used. wt1en the melt is being injected into the current

mold, a decrease in temperature of the melt after the initial increase was not

observed as has been reported by Amano [5,12]. The effects of melt

homogeneity, cushion size and the number of cycles the melt remains in the

cushion cannot be properly addressed with the current cavity size.

2- A nozzle band heater or cartridge heater should be used in future

temperature experiments to eliminate the variabilt'ty in temperature caused

by the nozzle being coIder than the barrel.

--------------------- -

(~

3-

100

A better understanding of the operation of the Vanzetti infrared optical fiber

temperature sensor is needed. In particular. the questions of pressure

effect. calibration, volume of poIymer seen. and response to different

poIymers should be addressed. The air purged model may provide more

reliable operation but it also requires more space for installation than is

currently availaNe.

4- The use of back pressure for fast temperature changes can be investigated

as an enhancement to using only band heaters for temperature control.

5- Installing ribbon thermocouples flush mounted may be severely limiting their

usefulness. There may exist a small but optimal depth of immersion

because poIymer tlow tends to be plug tIow except near the wall where a

large amount of shearing occurs.

1.

2.

3.

4.

5.

-- 6.

7.

8.

9.

10.

11.

12.

101

REFERENCES

Johannaber, loF., Schwittay, 1.0., Bayer, A.G., "Significance of Temperature Control for the Quality of Injection-Molded Parts", SPE Technical Papers, 26, 146 (1980).

Galskoy, A., Wang, K.K., "Measuring MeltTemperatures: Thermocouples or Pyrometers?", Piast. Eng., 34, 42 (1978).

Fritch, L. \'\., "Honing Mold Parameters by Measuring Flow Length", Piast. Eng., 42, 41 (1986).

Gomes, V.G., "The Dynamics and Control of Melt Temperature in Thermoplastic Injection Molding", M. Eng Thesis, McGiII University, Montreal (1985).

Amano, O., Utsugi, S., "Temperature Measurements of "olymer Melts in the Heating Barrel During Injection Molding. Part 1: Temperature Distribution Along the Screw Axis in the Reservoir", Polym. Eng. Sei., 28(23), 1565 (1988).

Maher, R.T., Plant, H.T., "Detection of Transient Melt Temperatures in Injection Molding", Mod. PI., 51 (5), 78 (1974).

Nanigian, J .• "Meet the Ribbon Thermocouple", SPE J., 27, 51 (1971).

W.Peter, J., "Injection Molding Procass Investigation for Automatic Control", SPE Technical Papers, 18,847 (1972).

Border, J., Suh, N.P., "Intelligent ln je di on Molding·, SPE Technical Papers, 28, 323 (1982).

Schonewald. R" Godley, P., "Melt Temperature Measurement Injection Molding Machines·, SPE Technical Papers, 18,675 (1972).

Tychesen, W., Georgi, W., MA Method for Measuring Melt Temperatures During Injection Molding·, Eighteenth Annual Technical Conference (1962).

Amano, O., Utsugi, S., "Temperature Measurements of Polymer Melts in the Heating Barrel During Injedion Molding. Part 2: Three-Dimensional Temperature Distribution in the Reservoir", Polym. Eng. Sei., 29(3), 171 (1989).

(

('

" tfl

'"

102

13. Peischl, G.C., Bruker, 1., ·Melt Homogeneity in Injection Molding: Application of a Ring-Bar DevieeM, Polym. Eng. Sei., 29, 202 (1989).

14. Nanigian, J., MSelecting the 'Right' Thermocouple: There Are More Choices Today·, Piast. Tech., 29(4), 259 (1982).

15. Orroth S.A.Jr., Schott, N.R., Flynn, R.H., Mlnfrared Fiber Optic Melt Temperature Measurements in the Injection Molding Nozzle", SPE Technical Papers, 20, 56 (1974).

16. Sukanek, P.C., Campbell, G.A., 'Transient Melt Temperatures in Screw Injection Molding", SPE Technical Papers, 36, 218 (1990).

17. Steward, E.L., ·Making the Most of Melt Temperature Measurement", Piast. Eng., 41 ,39(1985).

18. Galskoy, A., Wang, K.K., "Measuring MeltTemperatures:Thermocouplesor Pyrometers?", Piast. Eng., 34, 42 (1978).

19. Herbertz, T.J.M., "Better Control of Extrusion by Ultrasonic Measurement", Proc 3rd IFAC Conf on Inst & Aut in PRP Ind.,p.349 (1976).

20. Haber, A., MMicroprocessor Control System for the Injection Molding Process", M.Eng. Thesis, McGill Universitl, Montreal (1982).

21. Conley, N., M.Eng. Thesis,McGill University, Montreal, (1985).

22. Abu Fare, 0.1., "The Dynamics of Injection Hydraulics in Thermoplastics Injection Molding", M.Eng. Thesis, McGill University, Montreal (1983).

23. Abu Fara, 0.1., "Control of Nozzle and Cavity Pressure During Filling and Packing in Thermopla~ics Injection Molding·, Ph.D. Thesis, McGill University, Montreal (1988).

24. Moy, F., "Microstructure and the Distribution of Tensile Properties in Injection M~·.;oo Polyetnylene" Ph.O. Thesis, McGiII University, Montreal (1980).

25. Davies, W.D.T., ·System Identification for Self-Adaptive Control", Wiley-Interscience, London (1970).

26. Woods, D.J., "The Nelder-Mead Simplex Algorithm for Minimizing a NonUnear Function of Several Variables·, Report 85-5, Cept. Math. Sciences, Rice University, May, 1985.; quoted from PC-MATLAB Users' Guide, The MathWorks, lne, MA (1987).

-

103

27. Scharzenback, J., Gill, K.F., ·System Modelling and Control·, Edward Arnold Ltd., London (1984).

28. Stephanopoulos, G., ·Chemical Process Control: An Introduction tu Theory and Practice·, Prentice-Hall, Englewood Cliffs, N.J. (1984)

29. Smith, C.A., Corripio, A.B., ·Principles and Practice of Automatic Process Control·, John Wiley and Sons, New York, p.225. (1985).

104

(~

APPENDIXA: Material of Construction for the Barrel and Nozzle

MateriaJ 420 Stainless Steel (Thyroplast 2083)

Supplier Thyssen Marathon Canada Ltd.

Oensity 7.7 g/cm3

Thermal concJuctivity at 20°C 3OW/m'oC

Specifie heat at 20°C 0.46 J/g'OC

Thermal expansion between 20°C and 11.OX10~ m/m'oC 200°C

Modulus of elasticity at 2Q°C 220000 N/mma

Mechanical properties at 20°C, heat treatld to 50 HRC

( Tensile strength 1800 N/mmt

0.2% yield point min. 1480 N/mmt

Elongation (L-'1- SelO> " min, 10

(

1 [ ,

.~~ t

~

! .....

t ~

f ~

~

t t , ~ ~ t t

105

APPENDIX B: Specifications of the Vanzetti Infrared Optical Fiber

Temperature Sensor

The signal of the Vanzetti optical fiber sensor (ses Figure 2.7) is transformed

to a usable voltage output by a Unearized Thermal Detector (LTO). The LTO is a

self contained Thermal Monitoring System within an environmentally sealed

detector head. It supplies a linearized temperature signal and is housed in a

cylindrical unit that is 25 cm long by 10 cm in diameter. The specifications of the

sensor and L TD are given in Table 8.1. The manufacturer did not furnish the

technical basis for the functioning of the instrument.

Optical fiber assembly model number

LTD model number

l TD temperature range

LTO output

Emissivity control range

Aespanse time

Signal processing

1017-2015-2516-T1-7-2-2 SKB 10064

4035

120 - 370°C

0- 5 VDC (20mV;oC)

0.05 - 1.00

10 ms

none

T.ble 8.1: Specifications of the Vanzetti infrared optical fiber temperature sensor.

(

(~

APPENDIXC:

106

Calibration of the Thermocouples and the CoId Junction

Compensation and Amplification Circuit

The AnaIog Deviees AD594A was used for colet junction compensation and

.-nplification. Amplification was necessary because the data acquisition board had

inputs for a to 5 votts. The AD594A produces an output of 10 mV r C for a type

J Qron - constantan) thermocouple and this provides a measurement range of 0 -

500°C. The AD594Awas recalibrated for use with type E (chromel- constantan)

thermocouples and this was done as is shawn in Figure 3.1 where the

potentiometers P1, P2, and P3 W8re adjusted to match the type E output. The

recommended starting values for the resistances of the potentiometers were 280,

240, and 1.8 kg respectively. The resistances were changed as required for the

final calibration of the AD594A and the thermocouples which was neœsS8f)

because the error rating for the AD594A is ±3°C at 25°C with a gain error of

± 1.5% and the thermocouple error rating is ± 1.7° C for readings from a to 300°C.

The ,error rating for temperature measurement without the calibration of the

AD594A is thus ±7.7°C.

The strategy for the calibration of the AD594A and the thermocouples was

to assign a thermocouple to each AD594A a1d maintain this pairing throughout the

experiments. The combination of each thermocouple and AD594A was calibrated

together with the use of the thr. potentiometers P1, P2, P3.

1

• ..

107

P2 and P3 are the zero and gain, respectively for the temperature of the

coId jundion. The temperature of the AD594A chip would have to be varied and

measured to calibrate the cold junction temperature. Ta simplify the procedure.

it was assumed that the gain (P3) was correct and only the zero needed

adjustment. This assumption has a very small impad on the overall accuracy

because the rQOm temperature did not vary significantly.

P1 is the gain for the thermocouple temperature measurement and P2 was

used as the zero for the calibration. An ice bath was used ta set the zero (P2) for

the AD594A and boiling water was used to set the gain (P1). Each thermocouple

was placed in the ice bath ta adjusted P2 and then placed in boiling water to

adjust P1 and this was repeated until P1 Sind P2 required no further adjustment.

The output of the AD594A was set ~o be OJi2 V when the thermocouple was in the

ice bath and 1 V for the boiling VfatH (ta match the 10 mV rC rating of the

AD594A). An output of 0.02 V was chosen as the zero point because the data

acquisition board truncates the voltags reading at 0 V for negative voltages and

one could never be sure that at 0° C the output was adually 0 V and not some

negative voltage.

The output of the data acqui.;ition board was converted to a temperature as

follows:

1 - convert the computer value (CV) tram the 12 bit data acquisition

board ta an output voltage tram the AD594A

AD594A volts = 5 V (CV + 2048) ~bits

(

108

2 - convert the AD594A voltage to a thermocouple voltage in millivolts

(TCmV) knowing that at 0° C the output is 0 rnV and at 100° C the

output is 6.317 mV (trom the standard type E thermocouple tables)

TCrnV = 8.317 - 0 mV (AD594A volts - 0.020 V) 1.000 - 0.020 V

3 - Using the standard thermocouple tables, convert the thermocouple

output (TCrnV) to a ternperature.

P3

7 6 5 4 3 2

Pl

8 9 10 11 12 13 14

+5V

+ TO COMPUTER ADC BOARD

+ Thermocouple

P1=280k P2=2"Ok P3= 1.8k

Flgur. C.1: Circuit for the AD594A usecl for thermocouple cold junction compensation and amplification.

.,.,..

-~

109

APPENDIX D: Glossary of Injection Molding Terms

AIR SHOT The machine is operated such that the polymer is injected into

the air instead of into the mold.

BAND HEATER An electrical heating element that is fastened around thG

barrel and is used to melt the polymer.

BARREL A hardened cylindrical piece of metal within which the screw

operates to mix, convey and in je ct the polymer.

CAVITY The space in the mold that is fill9d with molten polymer during

injection and determines the shape of the final part.

CUSHION SIZE The amount of polymer melt that remains between the screw

and the nozzle after injection.

FILL The action of the screw moving forward ta fill the cavity with

molten polymer.

(~

GATE

HOLO

r-JOZZLE

PLASTICATION

SCREW

110

A restriction in the fIow channel between the sprue and the

cavity that allows for easy separation of the sprue from the

part 8fter it has been ejected.

The phase after injection where the screw maintsins pressure

on the soIidifying poIymer that is in the mold in order to

compensate for shrinkage.

The piace that is saewed to the front of the barrel that

provides a tapered channel ending in a hole (3 mm in

diameter for this work) through which the molten polymer

flows as it exits the barrel and enters the sprue.

The shearing. melting. and mbdrtg action that is produced as

the screw tums in the barrel to pump the polymer to the front

of the sasw.

The screw mixes. conveys and injects the poIymer into the

moIct.

SCREW POSmON The position of the saew measured relative to its most

retracted position (i.e. a lCI'ew position of zero).

SERVOVALVE

SHOTSIZE

SPRUE

..... 1

111

A hydraulic control valve whose opening is controlled by an

electrical signal such as a computer output and is used to

control the pressure on the screw during injection, holding,

and plastication.

The amount of poIymer that is injected into the mokJ.

The nozzle is seated against the sprue and the sprue contains

cl channel connecting the nozzle to the œvity through which

the n Jolten polymer nows .

in molding - mcgill universitydigitool.library.mcgill.ca/thesisfile61108.pdf · 1 r ii resume la...

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