pet bottle manufacturing

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____________________ * Corresponding author: I. Lučića 5, HR-10000 Zagreb, Croatia, phone +38516168191, fax +38516150081, [email protected] PROCESS CAPABILITY ANALYSIS IN THE MANUFACTURING OF PET BOTTLES Maja Rujnić-Sokele * , Mladen Šercer, Damir Godec Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb ABSTRACT: Modern process control is based on the use of statistical methods to improve and ensure quality, and thus reduce the process cost due to waste and rejects. The paper deals with the process capability analysis in the manufacturing of poly(ethylene-terephthalate) bottles for packaging of table oil. PET bottles are manufactured by the procedure of injection stretch blow moulding. The preforms made by injection moulding are biaxially extruded in the blow mould, with blow parameters affecting the mechanical properties, the barrier properties and the transparency of the manufactured bottle. The performance of blow moulding machine was monitored by measuring the volumes of blown bottles. Based on the collected data, the capability analysis of the blow moulding process was conducted. KEYWORDS: poly(ethylene-terephthalate) bottles, process capability, injection stretch blow moulding 1 INTRODUCTION Poly(ethylene terephtalate) (PET) is a packaging material that is commonly used to package various beverages, mineral water, oil and vinegar, beer and more recently wine as well. PET bottles are manufactured by the procedure of injection stretch blow moulding. The preforms made by injection moulding are biaxially extruded in the blow mould, with blow parameters affecting the mechanical properties, the barrier properties and the transparency of the manufactured bottle. The statistical process control (SPC) can be applied to many different processes. The goals of SPC are to improve and ensure quality, and thus, reduce the process cost due to waste as result of rejects. While the aim of traditional product inspection is to detect defects, the aim of SPC is to prevent them. The process is considered capable if the actual product variability is less than the specification width. If the product variability is larger than the specification width, the process is considered not capable. [1] The experiment was carried out in an oil manufacturing factory, and the task was to determine the capability of the blow moulding procedure. The performance of the blow moulding machine was monitored during the period of four months by measuring the volumes of blown bottles. PET bottles can be filled with oil directly after blowing or after several days of storage in the silo. The productivity of the filling line is 11,000 bottles/h, and the productivity of the blow moulding machine is 6500 bottles/h. For this reason, all the bottles cannot be filled with oil directly after blowing, so a certain amount of bottles are stored in the silo. Since an empty PET bottle reaches its final dimensions after 72 hours, i.e. it shrinks during this period, the filling line consists of bottles of various volumes, depending on the storage time. 2 TESTING MATERIAL AND EQUIPMENT For the purposes of this work, the bottles that were used, were manufactured out of the preform of 26g mass, with PET properties presented in Table 1. Table 1: PET properties Property Value Intrinsic viscosity 0.81 0.02dl/g Acetaldehyde content < 1ppm Moisture content 0.2% Melting temperature 245±2ºC Density 1.41g/cm 3 The blow moulding machine, of type SBO 6/10, of the Sidel company, Octeville sur Mer, France, was used. The temperature regulation of mould cavities was carried out by the Vulcatherm device, manufactured by Vulcanic, France. The temperature regulation medium is water which can be heated to a maximum temperature of 90ºC. The water cycle consists of the cycle with cold water for bottleneck and mould bottom cooling and a cycle with warm water for heating the mould body. For this type of product the temperature of the cold water is 10 to 12ºC, and of warm water about 50ºC. DOI 10.1007/s12289-010-0824- © Springer-Verlag France 2010 y Int J Mater Form (2010) Vol. 3 Suppl 1:531 534

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Page 1: Pet Bottle Manufacturing

____________________ * Corresponding author: I. Lučića 5, HR-10000 Zagreb, Croatia, phone +38516168191, fax +38516150081, [email protected]

PROCESS CAPABILITY ANALYSIS IN THE MANUFACTURING OF PET BOTTLES

Maja Rujnić-Sokele*, Mladen Šercer, Damir Godec

Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb ABSTRACT: Modern process control is based on the use of statistical methods to improve and ensure quality, and thus reduce the process cost due to waste and rejects. The paper deals with the process capability analysis in the manufacturing of poly(ethylene-terephthalate) bottles for packaging of table oil. PET bottles are manufactured by the procedure of injection stretch blow moulding. The preforms made by injection moulding are biaxially extruded in the blow mould, with blow parameters affecting the mechanical properties, the barrier properties and the transparency of the manufactured bottle. The performance of blow moulding machine was monitored by measuring the volumes of blown bottles. Based on the collected data, the capability analysis of the blow moulding process was conducted.

KEYWORDS: poly(ethylene-terephthalate) bottles, process capability, injection stretch blow moulding 1 INTRODUCTION

Poly(ethylene terephtalate) (PET) is a packaging material that is commonly used to package various beverages, mineral water, oil and vinegar, beer and more recently wine as well. PET bottles are manufactured by the procedure of injection stretch blow moulding. The preforms made by injection moulding are biaxially extruded in the blow mould, with blow parameters affecting the mechanical properties, the barrier properties and the transparency of the manufactured bottle. The statistical process control (SPC) can be applied to many different processes. The goals of SPC are to improve and ensure quality, and thus, reduce the process cost due to waste as result of rejects. While the aim of traditional product inspection is to detect defects, the aim of SPC is to prevent them. The process is considered capable if the actual product variability is less than the specification width. If the product variability is larger than the specification width, the process is considered not capable. [1] The experiment was carried out in an oil manufacturing factory, and the task was to determine the capability of the blow moulding procedure. The performance of the blow moulding machine was monitored during the period of four months by measuring the volumes of blown bottles. PET bottles can be filled with oil directly after blowing or after several days of storage in the silo. The productivity of the filling line is 11,000 bottles/h, and the productivity of the blow moulding machine is 6500 bottles/h. For this reason, all the bottles cannot be filled with oil directly after blowing, so a certain amount of bottles are stored in the silo.

Since an empty PET bottle reaches its final dimensions after 72 hours, i.e. it shrinks during this period, the filling line consists of bottles of various volumes, depending on the storage time. 2 TESTING MATERIAL AND

EQUIPMENT

For the purposes of this work, the bottles that were used, were manufactured out of the preform of 26g mass, with PET properties presented in Table 1.

Table 1: PET properties

Property Value Intrinsic viscosity 0.81 0.02dl/g Acetaldehyde content < 1ppm Moisture content 0.2% Melting temperature 245±2ºC Density 1.41g/cm3

The blow moulding machine, of type SBO 6/10, of the Sidel company, Octeville sur Mer, France, was used. The temperature regulation of mould cavities was carried out by the Vulcatherm device, manufactured by Vulcanic, France. The temperature regulation medium is water which can be heated to a maximum temperature of 90ºC. The water cycle consists of the cycle with cold water for bottleneck and mould bottom cooling and a cycle with warm water for heating the mould body. For this type of product the temperature of the cold water is 10 to 12ºC, and of warm water about 50ºC.

DOI 10.1007/s12289-010-0824-© Springer-Verlag France 2010

yInt J Mater Form (2010) Vol. 3 Suppl 1:531 534–

Page 2: Pet Bottle Manufacturing

3 EXPERIMENT

Over a period of four months the work of a blow moulding machine was monitored during the production of bottles for oil from the preforms of 26g mass. The monitoring was based on: a) production of bottles by stretch moulding, filling of bottles by table oil, and weighing of the filled bottles (measuring volume); b) production of bottles by stretch blow moulding, storing of empty bottles >72h (resulting in shrinking, i.e. volume reduction), filling of bottles by table oil, and weighing of filled bottles (measuring volume). The weighing, i.e. measuring of bottles volume was done two to four times during a single shift. Table 2 shows the values of arithmetic means and standard deviations.

Table 2: Values of arithmetic means and standard deviations in monitoring the volume of oil in PET bottles

Bottle (stored 0h) Bottle

(stored >72h) Number of measurements

261 406

Arithmetic mean, ml

1002.31 998.76

Standard deviation, ml

1.93 2.18

Range (max-min)

10.4 13.2

It can be noted that the requested average value of 1000ml was filled in the case when the bottles are filled immediately after blowing; however, when the bottles that had been stored for at least 72 hours were filled, the arithmetic mean of the values of volumes is lower than the required value. Figures 1 and 2 show the data about the volume of table oil in the bottles, in the case when these were filled immediately after blowing and after the bottles had been stored for at least 72 hours (average bottle storage time was 96 h).

Figure 1: Data on the volume of table oil in bottles immediately after blowing

Figure 2: Data on volume of table oil in bottles after storage >72h

The data on the volume of oil in the bottles were used to analyze the blowing process capabilities. The process is said to be capable if the output is approximately 100% (99.73%) of products within the limits of allowed deviations. The process analysis analyzes primarily the process dispersion, i.e. a relation is established between the determined dispersion and the limits of the allowed deviations. A capable process can output also rejects if not centred (the process centre is shifted in relation to the centre of the field of allowed deviations). The strongest argument of quality improvement is reflected in the constant reduction of dispersion [1]. The analysis of the process capability yields numerical amounts of a larger number of statistical parameters, so-called indices of process capability, and the indices based on the process dissipation have the highest “weight” [1]. Among the indices, indices Cp and Cpk have the highest importance and the widest application, and their amounts are obtained on the basis of the following expressions [1]:

sLUCp

6 (1)

3minzCpk (2)

zmin=min(zU, zL) (2a)

sxUzU

(2b)

sLxzL

(2c)

where U – upper limit of allowed deviations, L – bottom limit of allowed deviations, x - arithmetic mean of individual data set, s – estimate of standard deviation:

1

2i

n

xxs (3)

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Page 3: Pet Bottle Manufacturing

The value of index Cp shows whether the process is capable. The higher the index value, the lower the process dissipation. Theoretically, the process is capable if Cp is greater than or equals 1. However, as a rule, the minimum value of index Cp is required to be 1.33 (U-L=8s). The value of index Cpk shows whether rejects are produced during the process, i.e. not produced if the index value is greater than or equals 1. If the process is properly centred the values of indices Cp and Cpk are equal [1]. While Cp relates the spread of the process relative to the specification width, it does not address how well the process average x is centred to the target value. Cp is often referred to as process "potential". Cpk measures not only the process variation with respect to allowable specifications, it also considers the location of the process average. [2] In analyzing the blowing process based on the monitoring of data on the volumes of table oil in bottles there are several approaches, during which completely different results are obtained. In principle, the limits of allowed deviations have not been clearly defined, i.e. only the required nominal value of 1000ml is known. Depending on the set limits of allowed deviations, completely different values of Cpk. are obtained. The objective is the production of bottles of maximum possible volume (with the aim of fulfilling the requirements of average volume of 1000ml according to the Regulations on metrology requirements for packaging), i.e. minimum possible post-shrinkage in order to have bottles with minimal differences in volume on the filling line. 3.1 Example 1

Because of the mentioned reasons, the upper limit is set at 1005ml, and the bottom limit at 995ml. In this case the values of capability indices Cp and Cpk in filling the bottles directly after blowing are:

87.092.16

00.99500.1005p

C

47.092.13

31.100200.1005pUpk

CC

and in case of filling the bottles after a storage of at least 72h:

76.018.26

00.99500.1005p

C

57.018.23

00.99576.998pLpk

CC

3.2 Example 2

Another approach is also possible, in which only the bottom limit of allowed deviations, e.g. 995ml, is set.

Then only Cpk can be calculated, which in case of filling the bottles immediately after blowing amounts to:

27.192.13

00.99531.1002pLpk

CC

During the analysis of the process, based on the measuring of volumes of stored bottles, the capability index amounts to:

57.018.23

00.99576.998pLpk

CC

Histograms of spread of the bottle volume values measured immediately after blowing and after storage >72h are presented in Figures 3 and 4.

Figure 3: Analysis of the blowing process, volume measurements immediately after blowing

Figure 4: Analysis of the blowing process, volume measurements after storage of >72 h

3.3 Discussion about results of blowing capability analysis

According to the capability indices calculated on the basis of the set limits of allowed deviations at 995, i.e. 1005ml, the blowing process is not capable either in the case of measuring the volume of new or stored bottles. From the value Cpk, (which amounts to 0.47 when measuring the volume of new bottles, and 0.57 in measuring the volume of stored bottles) it could be

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Page 4: Pet Bottle Manufacturing

concluded that the more capable process is in case of filling the bottles after storage. However, bottles filled with more than 1005ml of oil do not represent in fact rejects, so that the more acceptable approach is the one mentioned in Example 2, i.e. when only the bottom limit of the permitted deviations is stipulated. In case of measuring the volume of bottles immediately after blowing, with the bottom limit of allowed deviations set at 995ml, Cpk is greater than 1, so that the blowing process may be considered capable. The analysis of the process based on the measuring of the volume of stored bottles, Cpk is 0.57, i.e. the process cannot be considered capable. 4 CONCLUSION

At the oil production factory the differences in the volume of oil in the filled bottles were noticed, depending on whether the bottles arrived to the filling line immediately after the blowing process or after having been stored for a certain time. An empty PET bottle acquires its final dimensions after 72 hours, during which period the bottle undergoes shrinkage so that the bottles arriving to the production line have different volumes depending on the storage period. The possible solution for resolving the problem would be acquisition of new blow molding machine, in which case the bottles could be filled directly after blowing, and the problem of post-shrinkage and filling of unequal oil quantities would disappear. The application of methods of statistical management of the process enables control of the quality of the PET bottles blowing process. The quality control shows the tendency towards the control directly on the production line; however, the majority of properties can be defined only after the production, in a testing laboratory. The work shows the implementation of statistical management of the process providing the example of PET bottle blowing. ACKNOWLEDGEMENT This work is part of the research included in the project Increasing Efficiency in Polymeric Products and Processing Development that is included in program Rapid Production – From Idea to Reality supported by the Ministry of Science, Education and Sports of the Republic of Croatia. The authors would like to thank the Ministry for the financing of this project. REFERENCES [1] Rauwendaal C: SPC – statistical process control in

extrusion. Carl Hanser Verlag, Munich Vienna New York Barcelona, 1995

[2] Mudronja V: Sposobnost procesa – teorija i praksa. In CIM'95 Proceedings, Zagreb, 10-17, 1995

[3] Crow K: Process Capability and Product Design. www.npd-solutions.com/proccap.html

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