A Guide toPolyolefin Injection Molding

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A Guide to Polyolefin Injection Molding

Table of ContentsIntroduction ....................................................................................................... 2Polyolefins are derived from petrochemicals .................................................... 2Molecular structure and composition affect properties and processability ......... 2

Chain branching .......................................................................................... 3 Density ......................................................................................................... 3

Molecular weight ........................................................................................ 4Molecular weight distribution ...................................................................... 4Copolymers ................................................................................................ 5Modifiers and additives .............................................................................. 5

Working closely with molders ............................................................................ 5How polyolefins are made .................................................................................. 5

Low density polyethylene (LDPE) ............................................................... 6High density polyethylene (HDPE) ............................................................. 6Linear low density polyethylene (LLDPE) .................................................... 7Polypropylene ............................................................................................. 7

Shipping and handling polyolefin resins ............................................................. 7Material handling ........................................................................................ 7How to solve material handling problems ................................................... 9Other material handling practices ............................................................... 10

The injection molding process ........................................................................... 11Injection units .............................................................................................. 11Plasticator specifications ............................................................................ 13Screw designs ............................................................................................. 13Nozzles ...................................................................................................... 14Clamp mechanisms ................................................................................... 15Clamp specifications ................................................................................... 16Injection molds ........................................................................................... 17Types of mold ............................................................................................ 17Sprues and runners ................................................................................... 17Mold venting ............................................................................................... 19Gating ........................................................................................................ 20Mold cooling .............................................................................................. 20Ejection devices ........................................................................................ 21Spiral flow measurement ........................................................................... 21

General injection molding operating procedures .............................................. 22General safety ........................................................................................... 22Heat ........................................................................................................... 23Electricity ................................................................................................... 23Machinery motion ...................................................................................... 23

The injection molding process and its effect on part performance .................... 23The molding cycle ..................................................................................... 23Shrinkage .................................................................................................. 29Warpage .................................................................................................... 29Color dispersion and air entrapment ......................................................... 30Part ejection and mold release .................................................................. 31Clarity ........................................................................................................ 32Gloss ......................................................................................................... 32Polypropylene integral hinges .................................................................... 32

Appendices1. Injection Molding Terms ........................................................................ 332. Metric Conversion Guide ....................................................................... 383. Abbreviations ........................................................................................ 404. ASTM test methods applicable to polyolefins ........................................ 415. Injection molding problems, causes and solutions ................................ 426. ASTM and ISO sample preparation and test procedures ..................... 467. Compression and injection molded sample preparation for HDPE ........ 47

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A Guide To Polyolefin Injection Molding

IntroductionPolyolefins are the most widelyused plastics for injectionmolding. This manual, A Guide toPolyolefin Injection Molding,contains general informationconcerning materials, methodsand equipment for producing highquality, injection molded,polyolefin products at optimumproduction rates.

Polyolefins that can beinjection molded include:

Low density polyethylene(LDPE)Linear low density polyethylene(LLDPE)High density polyethylene(HDPE)Ethylene copolymers, such asethylene vinyl acetate (EVA)Polypropylene and propylenecopolymers (PP)Thermoplastic olefins (TPO)

In general, the advantages ofinjection molded polyolefins com-pared with other plastics are:

LightweightOutstanding chemicalresistanceGood toughness at lowertemperaturesExcellent dielectric propertiesNon-hygroscopic

The basic properties of polyolefinscan be modified with a broadrange of fillers, reinforcementsand chemical modifiers.Furthermore, polyolefins areconsidered to be relatively easy toinjection mold.

Major application areas for poly-olefin injection molding are:

AppliancesAutomotive productsConsumer products

FurnitureHousewaresIndustrial containersMaterials handling equipmentPackagingSporting goodsToys and novelties

This manual contains extensive information on the injection mold-ing of polyolefins. However, it makes no specific recommendations for the processing of LyondellBasell resins for specific applications. For more detailed information please contact your LyondellBasell polyolefins sales or technical service representative.

Polyolefins arederived frompetrochemials

Polyolefins are plastic resins polymerized from petroleum-based gases. The two principal gases are ethylene and propylene. Ethylene is the principal raw material for mak-ing polyethylene (PE and ethylene copolymer resins; propylene is the main ingredient for making polypropylene (PP) and propylene copolymer resins.Polyolefin resins are classified as thermoplastics, which means that they can be melted, solidified and melted again. This contrasts with thermoset resins, such as phenolics, which, once solidified, can not be reprocessed.

Most polyolefin resins for injection molding are used in pellet form. The pellets are about 1/8 inch long and 1/8 inch in diameter and usual-ly somewhat translucent to white in color. Many polyolefin resins con-tain additives, such as thermal stabi-lizers. They also can be

compounded with colorants, flameretardants, blowing agents, fillers,reinforce-ments, and otherfunctional addi-tives such asantistatic agents and lubricants.

Molecular structure andcomposition affectproperties andprocessabilityFour basic molecular propertiesaffect most of the resincharacteris-tics essential toinjection molding high qualitypolyolefin parts. These molecularproperties are:

Chain branchingCrystallinity or densityAverage molecular weightMolecular weight distribution

The materials and processes usedto produce the polyolefinsdetermine these molecularproperties.

The basic building blocks for thegases from which polyolefins arederived are hydrogen and carbonatoms. For polyethylene, theseatoms are combined to form theethylene monomer, C2H4.

In the polymerization process, thedouble bond connecting the carbonatoms is broken. Under the rightconditions, these bonds reformwith other ethylene molecules toform long molecular chains.

The resulting product is polyethyl-ene resin.

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For polypropylene, the hydrogenand carbon atoms are combinedto form the propylene monomer,CH3CH:CH2.

The third carbon atom forms aside branch which causes thebackbone chain to take on a spiralshape.

Ethylene copolymers, such asethylene vinyl acetate (EVA), aremade by the polymerization ofethylene units with randomlydistributed vinyl acetate (VA)comonomer groups.

Chain branchingPolymer chains may be fairlylinear, as in high densitypolyethylene, or highly branchedas in low density polyethylene. Forevery 100-ethylene units in thepolyethylene molecular chain,there can be one to ten short orlong branches that radiate three-dimensionally (Figure 1). Thedegree and type of branching arecontrolled by the process (reactor),catalyst, and/or any comonomersused.

Chain branching affects many ofthe properties of polyethylenesincluding density, hardness,flexibility and transparency, toname a few. Chain branches alsobecome points in the molecularstructure where oxidation mayoccur. If excessively hightemperatures are reached duringprocessing, oxidation can occurwhich may adversely affect thepolymer’s properties. This oxidation

or degradation may cause cross-linking in polyethylenes and chainscission in polypropylenes.

Polypropylene, on the other hand,can be described as being linear(no branching) or very highlybranched. Although the suspendedcarbon forms a short branch onevery repeat unit, it is alsoresponsible for the unique spiraland linear configuration of thepolypropylene molecule.

DensityPolyolefins are semi-crystallinepolymers which means they arecomposed of molecules which arearranged in a very orderly(crystalline) structure andmolecules which are randomlyoriented (amorphous). This mixtureof crystalline and amorphousregions (Figure 2) is essential inproviding the desired properties toinjection molded parts. A totallyamorphous polyolefin would begrease-like and have poor physicalproperties. A totally crystalline poly-olefin would be very hard andbrittle.

HDPE resins have linear molecularchains with comparatively few sidechain branches. Therefore, thechains are packed more closelytogether (Figure 3). The result iscrystallinity up to 95 percent. LDPEresins generally have crystallinityfrom 60 percent to 75 percent.LLDPE resins have crystallinityfrom 60 percent to 85 percent. PPresins are highly crystalline, butthey are not very dense. PP resinshave a nominal specific gravity

range of 0.895 to 0.905 g/cm3,which is the lowest for acommodity thermo-plastic anddoes not vary appreciably frommanufacturer to manufacturer.

For polyethylene, the density andcrystallinity are directly related, thehigher the degree of crystallinity,the higher the resin density. Higherdensity, in turn, influencesnumerous properties. As densityincreases, heat softening point,resistance to gas and moisturevapor permeation and stiffnessincrease. However, increaseddensity generally results in areduction of stress crackingresistance and low temperaturetoughness.

LDPE resins have densitiesranging from 0.910 to 0.930grams per cubic centimeter(g/cm3)LLDPE resins range from0.915 to 0.940 g/cm3HDPE resins range from>0.940 to >0.960 g/cm3

As can be seen, all naturalpolyolefin resins, i.e, those withoutany fillers or reinforcements, havedensities less than 1.00 g/cm3. Thislight weight is one of the key

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advantages for parts injectionmolded from polyolefins. A generalguide to the effects of density onthe properties for various types ofpolyethylene resins is shown inTable 1.

Molecular weightAtoms of different elements, suchas carbon, hydrogen, etc., havedifferent atomic weights. Forcarbon, the atomic weight is 12and for hydrogen it is one. Thus,the molecular weight of theethylene unit is the sum of theweight of its six atoms (two carbonatoms x 12 + four hydrogen x 1) or28.

Unlike simple compounds, likeethylene or propylene, everypolyolefin resin consists of amixture of large and small chains,i.e., chains of high and lowmolecular weights. The molecularweight of the polymer chaingenerally is in the thousands andmay go up to over one million. Theaverage of these is called, quiteappropriately, the averagemolecular weight.

As average molecular weightincreases, resin toughnessincreases. The same holds true fortensile strength and environmentalstress crack resistance (ESCR) –cracking brought on when moldedparts are subjected to stresses inthe pres-ence of materials such assolvents, oils, detergents, etc.However, high-er molecular weightresults in an increase in meltviscosity and greater resistance toflow making injection molding moredifficult as the average molecularweight increases.

Melt flow rate (MFR) is a simplemeasure of a polymer’s meltviscosity under standard conditionsof temperature and static load(pressure). For polyethylenes, it isoften referred to as melt index (MI).MFR is the weight in grams of amelted resin that flows through astandard-sized orifice in 10

minutes (g/10 min). Melt flow rateis inversely related to the resin’saverage molecular weight: as theaverage molecular weightincreases, MFR decreases andvice versa.

Melt viscosity, or the resistance ofa resin to flow, is an extremelyimportant property since it affectsthe flow of the molten polymerfilling a mold cavity. Polyolefinswith higher melt flow rates requirelower injection molding processingpressures, temperatures andshorter molding cycles (less timeneeded for part cooling prior toejection from the mold). Resinswith high viscosities and, therefore,lower melt indices, require theopposite conditions for injectionmolding.

It should be remembered thatpressure influences flowproperties. Two resins may havethe same melt index, but differenthigh-pressure flow properties.Therefore, MFR or MI must beused in conjunction with othercharacteristics, such as molecular

weight distribution, to measure theflow and other properties of resins.Generally, injection molding resinsare characterized as havingmedium, high or very high flow.

For injection molding grades, theMFR (MI) values for polyethylenesare generally determined at 190°C(374°F) using a static load of 2,160g. MFR values for polypropylenesare determined at the same loadbut at a higher temperature 230°C(446°F). The MFR of otherthermoplastics may be determinedusing different combinations oftemperatures and static load. Forthis reason, the accurate predictionof the relative processability ofdifferent materials using MFR datais not possible.

Molecular weightdistributionDuring polymerization, a mixture ofmolecular chains of widely varyinglengths is produced. Some may beshort; others may be extremelylong containing several thousandmonomer units.

Table 1. General guide to the effects of polyethylene physical properties onprperties and processing

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The relative distribution of large,medium and small molecularchains in the polyolefin resin isimportant to its properties. Whenthe distribu-tion is made up ofchains close to the average length,the resin is said to have a “narrowmolecular weight distribution.”Polyolefins with “broad molecularweight distribution” are resins witha wider variety of chain lengths. Ingeneral, resins with narrowmolecular weight distributions havegood low-temperature impactstrength and low warpage. Resinswith broad molecular weightdistributions generally have greaterstress cracking resistance andgreater ease of processing (Figure4).

The type of catalyst and thepolymerization process used toproduce a polyolefin determines itsmolecular weight distribution. Themolecular weight distribution(MWD) of PP resins can also bealtered during production by con-trolled rheology additives thatselec-tively fracture long PPmolecular chains. This results in anarrower molecular weightdistribution and a higher melt flowrate.

CopolymersPolyolefins made with one basictype of monomer are calledhomopolymers. There are,however, many polyolefins, calledcopolymers, that are made of twoor more monomers. Many injectionmolding grades of LLDPE, LDPE,HDPE and PP are made withcomonomers that are used toprovide specific propertyimprovements.

The comonomers most often usedwith LLDPE and HDPE are calledalpha olefins. They includebutene, hexene and octene. Othercomonomers used with ethyleneto make injection molding gradesare ethyl acrylate to make the

copolymer ethylene ethyl acrylate(EEA) and vinyl acetate toproduce ethylene vinyl acetate(EVA).

Ethylene is used as a comonomerwith propylene to producepolypropylene randomcopolymers. Polypropylene can bemade more impact resistant byproducing a high ethylene-propylene copolymer in a secondreactor forming a finely dispersedsecondary phase of ethylene-propylene rubber. Products madein this manner are commonlyreferred to as impact copolymers.

Modifiers andadditivesNumerous chemical modifiers andadditives may be compounded withpolyolefin injection molding resins.In some grades, the chemicalmodifiers are added during resinmanufacture. Some of theseadditives include:

AntioxidantsAcid scavengersProcess stabilizersAnti-static agentsMold release additivesUltraviolet (UV) light stabilizers

NucleatorsClarifiersLubricants

Working closelywith moldersLyondellBasell offers a wide range of polyolefin resins for injection mold-ing, including Alathon® and Petrothene® HDPE,Petrothene® LDPE, LLDPE, and PP, Ultrathene® EVA copolymers and Flexathene® TPOs. These resins are tailored to meet the requirements of many areas of application.

Polyolefin resins with distinctly dif-ferent properties can be made by controlling the four basic molecular properties during resin production and by the use of modifiers and additives. Injection molders can work closely with their LyondellBasell polyolefins sales or technical service representative to determine the resin that best meets their needs.

LyondellBasell polyolefins technical service representatives are also available to assist injection molders and end-users by providing guidance for tool and part design and the development of specialty products to fulfill the requirements of new, demanding applications.

How polyolefinsare madeHigh-purity ethylene and propylene gases are the basic feedstocks for making polyolefins (Figure 5). These gases can be petroleum refinery by-products or they can be extracted from an ethane/propane liquified gas mix coming through pipelines from a gas field. High efficiency in the ethane/propane cracking and purification results in very pure ethylene and propylene, which are critical in the production of high quality polyolefins.

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LyondellBasell can produce polyolefins by more polymerization technologies and with a greater range of catalysts than any other supplier can. Two of LyondellBasell’s plants are pictured in Figure 6.

Low densitypolyethylene (LDPE)To make LDPE resins, LyondellBasell uses high pressure, high temperature tubular and autoclave polymerization reactors (Figures 7 and 8). Ethylene is pumped into the reactors and combined with a catalyst or initiator to make LDPE. The LDPE melt formed flows to a separator where unused gas is removed, recovered, and recycled back into the process. The LDPE is then fed to an extruder for pelletization. Additives, if required for specific applications, are incorporated at this point.

High densitypolyethylene (HDPE)There are a number of basic processes used by LyondellBasell for mak-ing HDPE for injection molding applications —including the solution process and the slurry process. In the multi-reactor slurry process used by LyondellBasell (Figure 9), ethylene and a comonomer (if used), together with an inert hydrocarbon carrier, are pumped into reactors where they are combined with a catalyst. However, in contrast to LDPE production, relatively low pressures and temperatures are used to produce HDPE. The granular polymer leaves the reactor system in a liquid slurry and is separated and dried. It is then conveyed to an extruder where additives are incorporated prior to pelletizing.

Figure 7. LDPE high temperature tubular process diagram

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LyondellBasell also utilizes a multi-reactor solution process for the production of HDPE (Figure 10). In this process, the HDPE formed is dissolved in the solvent carrier and then precipitated in a downstream process. An additional adsorption step results in a very clean product with virtually no catalyst residues.

Because both of these processes utilize multiple reactors, LyondellBasell has the capability of tailoring and optimizing the molecular weight distribution of the various product grades to provide a unique range of processability and physical properties.

Linear low densitypolyethylene (LLDPE)LyondellBasell uses a gas phase process for making LLDPE (Figure 11). This process is quite different from the LDPE process, but somewhat similar to the HDPE process. The major differences from the LDPE process are that relatively low pressure and low temperature polymerization reactors are used. Another difference is that the ethylene is copolymerized with butene or hexene comonomers in the reactor. Unlike HDPE, the polymer exits the reactor in a dry granular form, which is subsequently compounded with additives in an extruder.

With changes in catalysts and operating conditions, HDPE resins also can be produced in some of these LLDPE reactors.

PolypropyleneTo make PP, LyondellBasell uses a vertical, stirred, fluidized-bed, gas-phase process (Figure 12). LyondellBasell was the first polypropylene supplier in the United States to use gas-phase technology to produce PP. Impact copolymers are produced using two, fluidized bed, gas phase reactors operating in series.

LyondellBasell’s polyolefin production facilities are described in

Shipping andhandling ofpolyolefin resinsIt is of utmost importance to keep polyolefin resins clean. LyondellBasell ships polyolefin resins to molders in hopper cars, hopper trucks, corrugated boxes, and 50-pound plastic bags. Strict quality control throughout resin manufacture and subsequent handling, right through delivery to the molder, ensures the cleanliness of the products.

Figure 8. High temperature autoclave process diagram

Figure 9. HDPE parallel reactors — slurry process

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When bulk containers aredelivered, the molder must useappropriate procedures forunloading the resin. Maintenanceof the in-plant material handlingsystem is also essential. Whenbags and boxes are used, specialcare is necessary in opening thecontainers, as well as coveringthem, as they are unloaded.

Reground resin, whether used asa blend or as is, should also bestringently protected to keep itfree of contamination. Wheneverpossible, the regrind materialshould be used as it is generated.When this is not possible, thescrap should be collected in aclosed system and recycled withthe same precautions taken for

virgin resin. In all cases, the proportion of regrind used should be carefully controlled to assure consistency of processing and part performance.

Material handlingLyondellBasell utilizes material handling systems and inspection procedures that are designed to prevent external contamination and product cross-contamination during production, storage, loading and shipment.

Since polyolefin resins are non-hygroscopic (do not absorb water) they do not require drying prior to being molded. However, under certain conditions, condensation may form on the pellet surfaces.

When cartons of resin aremoved from a cold warehouse environment to a warmmolding area or when transferring cold pellets from a silo to an indoor storagesystem, the temperature of the material should be allowed to equilibrate, for up to eighthours to drive off any condensation` before molding.

Figure 10. HDPE solution process

Figure 11. LLDPE fluidized bed process

BAYPORT, TX

Table 2. LYB polyolefinproduction facilities

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The best way to improve resinutilization is to eliminatecontaminants from transfersystems. If bulk handling systemsare not dedicated to one materialor are not adequately purged,there is always the possibility ofcontamination resulting fromremnants of materials previouslytransferred.

Occasionally, clumps of “angelhair” or “streamers” mayaccumulate in a silo and plug theexit port. Contaminants of this typecan also cause plugging of transfersystem filters and/or problems thataffect the molding machine. All ofthese problems can result inmolding machine downtime,excessive scrap and the time andcosts of cleaning silos, transferlines and filters. Polyolefin dust,fines, streamers and angel haircontamination may be generatedduring the transfer of polymerthrough smoothbore piping. Thesetransfer systems also may containlong radius bends to convey theresin from a hopper car to the siloor holding bin. A polyolefin pellet

Figure 12. PP dual reactors – gas-phase process

conveyed through a transfer linetravels at a very high velocity. Asthe pellet contacts the smooth pipewall, it slides and friction isgenerated. The friction, in turn,creates sufficient heat to raise thetemperature of the pellet surface tothe resin’s softening point. As thishappens, a small amount of moltenpolyolefin is deposited on the pipewall and freezes almost instantly.Over time, this results in depositsdescribed as angel hair orstreamers.

As the pellets meet the pipe wall,along the interior surface of a longradius bend, the deposits becomealmost continuous and streamersare formed. Eventually, the angelhair and streamers are dislodgedfrom the pipe wall and find theirway into the molding process, thestorage silo or the transfer filters.The amount of streamers formedincreases with increased transferair temperature and velocity.

Other good practices of materialhandling include control (cooling)of the transfer air temperature tominimize softening and melting of

the pellets. Proper design of the transfer lines is also critical in terms of utilizing the optimum bend radii, blind tees, and proper angles. Consult your LyondellBasell technical service engineer for guidance in this area.

How to solve materialhandling problemsSince smooth piping is a leading contributor to angel hair and streamers, one solution is to roughen the interior wall of the piping. This causes the pellets to tumble instead of sliding along the pipe, minimizing streamer formation. However, as the rapidly moving polyolefin pellets contact an extremely rough surface, small particles may be broken off the pellets creating fines or dust.

Two pipe finishes, in particular, have proven to be effective in minimizing buildup and giving the longest life in transfer systems. One is a sand-blasted finish of 600 to 700 RMS roughness. This finish is probably the easiest to obtain. However, due to its sharp edges, it will initially create dust and fines until the edges become rounded.

The other finish is achieved with shot blasting using a #55 shot with 55-60 Rockwell hardness toproduce a 900 RMS roughness.Variations of this finish arecommonly known as “hammer-finished” surfaces. The shotblasting allows deeper penetrationand increases hardness, which inturn leads to longer surface life.

The rounded edges obtained minimize the initial problems encountered with dust andfines. They also reduce metal contamination possibly associated with thesandblasted finish.

Whenever a new transfersystem is installed or when a portion of an existing system is replaced, the interior surfaces should be treated by either

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sand or shot blasting. Theinitial cost of having this doneis far outweighed by theprevention of future problems.

Elimination of long-radiusbends where possible is alsoimportant as they are probablythe leading contributor tostreamer formation. When thistype of bend is used, it iscritical that the interior surfaceshould be either sand- or shot-blasted.

The use of self-cleaning,stainless steel “tees” in placeof long bends prevents theformation of streamers alongthe curvature of the bend,causing the resin to tumbleinstead of slide (Figure 13).However, there is a loss ofefficiency within the transfersystem when this method isused. Precautions should betaken to ensure that sufficientblower capacity is available toprevent clogging of the transferlines and maintain the requiredtransfer rate.

To extend the life of the transferpiping, it should be rotated 90° atperiodic intervals. Resin pelletstend to wear grooves in thebottom of the piping as they aretransferred which not onlycontributes to fines and streamerformation but also acceleratedwear due to non-uniform abrasion.

Figure 13. Eliminate long-radiusbends where possible. The useof stainless steel "tees"prevents the formation ofstreamers along the curvatureof the bend.

Regardless of the type ofequipment used or the materialstransferred, a transfer systemshould be maintained and keptclean in the same manner as anyother piece of productionequipment. Periodic washing anddrying of silos and holding binsreduces the problem of fines anddust build-up due to staticcharges.

Other steps to eliminatecontamination include:

Inspect the entire transfersystem on a regular basisClean all filters in the transfersystem periodicallyEnsure that the suction line isnot lying on the ground duringstorage or when the system isstarted to prevent debris fromentering the systemPlace air filters over hopper carhatches and bottom valvesduring unloading to preventdebris or moisture fromcontaminating the materialPurge the lines with air andthen with a small amount ofproduct prior to filling storagesilos or binsAllow blowers to run forseveral minutes afterunloading to clear the lines andreduce the chance of cross-contamination of product.

Information regarding transfer systems and types of interior finishes available can be obtained from most suppliers of materials handling equipment or by consulting your LyondellBasell technical service engineer. Complete systems can be supplied which, when properly maintained, efficiently convey contamination-free product.

Other materialhandling practicesBeside-the-press vacuum loadersare used to feed many injectionmolding machines. These unitsdraw resin pellets from drums orcartons placed beside themachine. In some set-ups, thevacuum loaders draw from multiplesources and directly feed thehopper with resin, regrind,colorants and other concentrateadditives. Good housekeepingprocedures are particularly impor-tant when working with beside-the-press loaders since contaminantscan easily get into the materialcontainers.

Blending with colorants, additivesand other materials is done usingon-the-machine blending unitsconsisting of multiple hoppersfeeding different resin compoundingredients. Colorants, additives,regrind and base resin arecombined using either volumetricor, the more accurate, weight-lossfeeding (gravimetric) techniques.Microprocessor controls monitorand control the amount of materialfed into a mixing chamber belowthe hoppers. Recipe data can bestored in the control unit for instantretrieval.

Central blending units can also beused especially when much higheroverall volumes are required. Acentral vacuum loading systemtransfers the finished blend to theindividual molding machines.

The injectionmolding processThe injection molding processbegins with the gravity feeding ofpolyolefin pellets from a hopperinto the plasticating/injection unit ofthe molding machine. Heat andpressure are applied to thepolyolefin resin, causing it to meltand flow. The melt is injectedunder high pressure into the mold.

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Pressure is maintained on thematerial in the cavity until it coolsand solidifies. When the parttemperatures have been reducedsufficiently below the material'sdistortion temperature, the moldopens and the part is ejected.

The complete process is called amolding cycle. The period betweenthe start of the injection of the meltinto the mold cavity and theopening of the mold is called theclamp close time. The totalinjection cycle time consists of theclamp close time plus the timerequired to open the mold, ejectthe part, and close the mold again.

There are four basic componentsto an injection molding machine:

1. Injection unit/plasticator2. Clamp unit3. Injection mold4. Control system

Injection unitsPlunger injection units (Figure 14)were the first types used forinjection molding, but their usetoday is quite limited.

Figure 14. Schematic cross-section of a typical plunger (or ram or piston) injection molding system

The reciprocating screw injectionmolder is the most commonmolding machine in use today formold-ing polyolefins. The injectionunit (Figure 15) mixes, plasticatesand injects a thermoplastic meltinto a closed mold. Thereciprocating screw accomplishesthis in the following manner:

1. The injection cycle starts withthescrew in the forwardposition.

2. Initially, the screw begins torotate in the heated barrel.Resin pellets are forced by thisaction to move forward throughthe channels of the screw.

3. As the pellets move forward,they are tumbled, mixed andgradually compressed togetheras the screw channels becomeshallower. The section of thescrew nearest the hopper iscalled the feed section, inwhich no compression takesplace.

4. As the pellets travel down thebarrel, they are heated byfriction and the heat conductedfrom the external electric

heater bands. The friction iscaused by the pellets slidingagainst themselves and theinner wall of the barrel and thescrew surface. The heat fromthe friction and conductioncause the pellets to melt. Themajority of the melting occursin the transition section of thescrew, where compression ofthe polymer is taking place asthe root diameter of the screwis increased.

5. Next, the melted polymer isfurther mixed andhomogenized in the meteringsection of the screw. In themetering section of the screw,the root diameter has reachedits maximum, and no furthercompression takes place.

6. The polymer melt flows in frontof the screw tip and thepressure produced by thebuild-up of polymer in front ofthe screw causes the screw tobe pushed backward in thebarrel as it continues to rotate.

7. The screw stops turning whenthe volume of melt producedahead of the screw tip is

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Figure 15. Schematic cross-section of a typical screw injection molding machine, showing the screwin the retracted (A) and forward (B) position

Figure 16. In this 2-stage injection molding machine, the screw-type preplasticizer is atop and parallel tothe horizontal plunger injection cylinder and chamber

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sufficient to completely fill themold cavity and runner system(the channels leading to themold cavity). This amount ofmaterial is called the shot sizeand the period during whichthe screw rotates is called thescrew recovery time.

8. The screw is then forcedforward, injecting the melt intothe mold. This is called theinjection stage.

In order to compensate for materialshrinkage in the cavity due to cool-ing, an excess amount of materialis generally held in front of thescrew at the end of the injectionstroke. This extra material is calledthe cushion and, during thepacking phase, some of thecushion material continues to beslowly injected into the cavity tocompensate for the volume lostdue to the shrinkage of thematerial in the mold and thecompressibility of the plastic.

Backpressure is the amount ofhydraulic pressure applied tothe back of the screw as itrotates. Varying the amount ofbackpressure alters thepressure exerted on thepolymer in front of the screw.Increasing backpressure alsochanges the amount of internalenergy transmitted to the meltby the shearing action of therotating screw. An increase inbackpressure raises the melttemperature without requiringan increase in heating cylindertemperatures and improvesmixing and plasticating.Unfortunately, increasingbackpressure also reducesscrew recovery rates and canadd unnecessary shear (heat)to the polymer which may leadto polymer degradation.Typically, backpressure is setat a minimum unless additionalmixing is required.

Two-stage systems, also calledscrew preplasticators, are available

(Figure 16) in which theplasticating unit feeds a separateinjection cylinder called anaccumulator. Melt is injected intothe mold using a ram in theaccumulator. Machines equippedwith accumulators can be used formolding parts requiring very largeshot sizes, for the high-speedinjection needed to fill long andnarrow mold cavities, and formolding parts requiring bettercontrol of shot size and injectionpressure.

PlasticatorspecificationsInjection capacity is defined as themaximum shot size in ounces (oz.)of general-purpose polystyrene(PS). In equating this topolyolefins, use approximately90% – 95% of the capacity statedfor PS. The plasti-cating rate isusually given in pounds/hour orounces/second for PS. Because ofdifferences in melting character-istics and different sensitivities toscrew design variables, it is notpossible to easily convert or apply

Figure 17. Screw type configurations used in injectionmolding machines

this value to polyolefins. Injectionrate is the maximum rate at whichthe plasticized material can beinjected through the nozzle in cubicinches/minute at a stated pressure.

Injection pressure is generallyexpressed as the hydraulicpressure in psi (pounds/squareinch) applied to the screw duringinjection. The maximum injectionpressure available varies and theactual pressure required dependson the resin, melt temperature,mold cooling, part design and molddesign. Most plasticating unitshave a chart which relates thehydraulic pressure to the pressureactually applied to the polymer.

Screw designsNumerous plasticating screwdesigns are available for injectionmolding polyolefins (Figure 17).However, since it is impossible tohave a screw designed for everymolding job, general-purposescrews are most commonly used.The shallower the screw channels,the smaller the resin volumeconveyed to the tip of the screw.

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On the other hand, while deepscrew channels accommodatelarger shot sizes more quickly, theydo not heat and plasticate the meltas efficiently as a screw withshallower channels. The threebasic screw sections are describedin Table 3.

There are a number of barrierscrew designs available whichoffer some benefits not provided bygeneral-purpose screws. Barrierscrews provide more efficientmixing without increasedbackpressure and, in some cases,recovery times may be decreased.These advantages are offset by theincreased risk of black speckformation. The deep flights in abarrier screw may have stagnantareas in which there is a reductionin the flow of the material. Themolten plastic tends to stay inthese areas and degrade,ultimately causing black specks inthe parts as the degraded materialflakes off the screw. Whenpurchasing barrier screws, it isrecommended that the molderwork closely with the screwdesigner to ensure that stagnantareas are avoided and that thescrew is properly designed for thematerial being used.

Plasticating screws forthermoplastics generally haveinterchangeable tips. The two mostcommonly used tips in the injectionmolding of polyolefins are slidingcheck ring and ball-check non-return valves. In the molding cycle,as the screw moves forward toinject material into the mold, thenon-return valves close to preventmaterial from flowing back over theflights of the screw. Typical slidingcheck ring and ball check valvesare shown in Figures 18 and 19.

Because of their tendency to wear,it is critical to periodically inspectthe condition of sliding ring shut-offtips. Excessive wear will result ininconsistencies in shot size andmelt temperature.

Figure 18. Typical sliding check rink showing injection stage(top) and retraction stage (bottom)

The typical length-to-diameter (L/D)ratio for polyolefin reciprocatingscrews is about 20-30:1, with acompression ratio of 2-3:1. Longerscrew lengths are generallypreferred as they provide betterhomogeneity of temperature andmelt quality.

NozzlesThe injection-unit nozzle isconnected to the barrel and directsthe flow of the melt into the mold.

The nozzle extends into the fixedplaten and mates to an indentationin the front of the mold called thesprue bushing.

The nozzle may have a positiveshut-off device or it may be openand rely on the freezing-off of themelt in the gate areas of the moldto keep the resin from flowing backinto the injection unit. Somenozzles may be connected to atemperature control device tocontrol the melt temperature.

Table 3. Functions of the three sections of an extrusion screw

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Figure 19. Typical ball check assembly showing injectionstage (top) and front discharge-retractionstage (bottom)

Clamp mechanismsThere are three basic types ofinjection moldingmachine clamps:mechanical, also called toggleunits, hydraulic and a combinationof these called hydromechanicalclamps.

Toggle clamps, which are lessexpensive to build, are most widelyused on small tonnage machines(typically, less than 500 tons). Thetoggle action can best be under-stood by looking at your arm whenit is bent at the elbow and thenwhen it is fully extended. In thetoggle clamp, a hydraulic cylindermoves the unit’s crossheadforward, extending the toggle linksand pushing the platen forward.The mechanical advantage is lowas the clamp opens or closes,which permits rapid clampmovement. This action slows andthe mechanical advantageincreases as the platen reachesthe mold-close position. The slowspeed is important for moldprotection.

Full clamp pressure is reachedwhen the linkage is fully extended.To adjust the toggle clamp todifferent mold heights, the entiretoggle mechanism and movingplaten assembly are moved alongtie rods. The position of the togglemechanism depends on where themold closes when the toggle is atfull extension. The toggle openswhen hydraulic pressure is appliedto the opposite side of the clampcylinder. See Figure 20.

Hydraulic clamps generally areused on injection moldingmachines in the 150 ton to 1,000+ton clamp tonnage range. In thistype of clamp, hydraulic oil is usedto move the platen through the fullclosing and opening strokes. Thefluid is metered into a reservoirbehind the main ram. At first quiterapid, the oil flow is slowed as theram reaches the mold-closeposition in order to protect the

Figure 20. Toggle clamping system

Figure 21. Hydraulic clamping system

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mold. An oil fill valve closes whenthe mold is closed. The areabehind the ram is then pressurizedto build full clamp tonnage. Toopen the mold, the oil valve is firstpartially opened to smoothly openthe mold. Once the mold halvesare separated, the clampaccelerates to a fast open speed(Figure 21).

Mold set-up is much easier with ahydraulic clamp than with a toggleclamp since hydraulic clamptonnage can be reachedanywhere along the clamp stroke.Mold set-up is accomplished bysetting the clamp position from themachine’s control center.

Hydromechanical clamps arecommonly used on very largeinjection molding machines, i.e.,over 1000 tons. In thehydromechanical clamp, ahydraulically actuated togglemechanism pushes the movingplaten at high speed to a pointwhere the mold halves are nearlyclosed. A mechanical locking plateor links prevent rearwardmovement during final build-up tofull clamp tonnage. Short-strokehydraulic cylinders are used tomove the platen the final shortclosing distance and develop fullclamp tonnage. See Figure 22.

Clamp specificationsKey clamp specifications toconsider in choosing an injectionmolding machine are:

Clamp strokeMinimum mold thicknessClearance between tie barsMaximum daylight openingPlaten sizeClamp tonnage

Clamp stroke is the maximum dis-tance (inches) the moving platencan travel. Clamp stroke is amajor factor in determining theminimum mold thickness that canbe used with the machine.

Figure 22 . Hydromechanical clamping system. Top view showsclamp open position of piston and ram. Bottom viewshows clamp closed, toggled and ram pressurized.

Generally, clamp specificationsalso state the mini-mum moldthickness for which the clamp candevelop its full tonnage.

Maximum daylight opening is thedistance (inches) between the twoplatens when the clamp is com-pletely open. This measurement isa major factor in determining theeffective maximum mold thicknesswhich takes into account the moldopening required for part ejectionor removal. Complicated moldsmay require more opening spaceand rigid mounting surfaces, sincehigh platen deflection under loadcould damage the mold. Anallowable deflection of 0.001 in/ftof span with full clamp load on thecenter of the platen is, generally,considered acceptable.

Platen size is given in horizontaland vertical measurements(inches) for the full platen. Sincethere are tie-bars running throughthe corners of the platens, mold-size limits are less than full platensize. A mold can extend betweenthe tie-bars in either the vertical or

horizontal direction but, generally,should not extend outside of theplatens.

Clearance between tie-bars isgiven for the distance (inches)between the top tie-bars(horizontal) and sidebars(vertical). Since the tie-bars arefixed on most injection moldingclamps, the distance betweenthem dictates the maximum sizeof a mold that can be placed inthe clamp.

Clamp tonnage is the maximumforce which the clamp candevelop. A clamping pressure offive-tons-per-square-inch of theprojected area of the molding(including the runner system) ismore than adequate forpolyolefins. However, wherepacking is not a major factor, thispressure may be as low as 2 tons/in2.An industry rule-of-thumb is that aclamp force of 2 to 3 tons per in2of the projected area of the part(s)and cold runner system isadequate for reciprocating-screw-

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type, injection-molding machines.Some thin-wall stack molds mayrequire 5 tons/in2 for optimum per-formance.

Injection moldsThere are many types of injectionmolds and tooling in use today,such as two-plate, three-plate andstack molds. Two and three-platemolds are more commonly usedfor heavy wall and non-packagingproducts. Both cold and hot-runnersystems are used for two andthree-plate molds. All stack moldsuse a hot manifold to convey themelt to the cavities. Each moldcomponent must be machined andfinished to exact dimensions withvery tight tolerances and must beheat-treated to be able to withstandvery high injection and clamppressures. Injection molds are themost expensive molds used inplastics processing with very longlead times required for their designand fabrication. Every mold mustbe tested and debugged to prove-out the ejection system, coolingand/or heating system andoperating components before it isplaced in production.

Types of moldsA two-plate mold (Figure 23) hasonly one parting line. If a runner isused, it is connected to the moldedproduct and requires manualremoval and separation after thepart is ejected. This type of mold isthe least expensive to make and iscommonly used for parts withrelatively simple geometries.

Three-plate molds (Figure 24)have two parting lines, one for therunner system and one for themolded product. When the moldopens, the runner is automaticallyseparated from the product toallow separate handling. Thiseliminates the need for manualseparation and removal and thesprue and runner system may be

Figure 23. Two-plate mold

Figure 24. Three-plate mold

fed directly to a recycling system.This type of mold is moreexpensive than the two-platemold.

Stack molds (Figures 25 and 26)can be two, three or four levels.The advantage of the stack mold isthat it can, generally, produce alarger number of products versus atwo or three-plate mold utilizing thesame machine clamp tonnage. Thedisadvantage is that it requires amolding press with much greaterdaylight opening to accommodatethe mold height. This type of mold

is much more expensive and takeslonger to build. Three level stackmolds are very new and four levelstack molds have been around forless than five years. The dairycontainer and lid industriescommonly use stack molds. Thefour-level is common for lids, andthe two-level is common forcontainers.

Sprues and runnersThe sprue and runner system isthe conduit that connects themachine nozzle to the cavities.

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Figure 25. 2 x 1 wash basin stack mold Photo courtesy of Tradesco Mold, Ltd.

Figure 26. 4 x 24 stack mold Photo courtesy of Tradesco Mold, Ltd.

During the injection phase of themolding cycle, the molten materialflows through the sprue andrunner to the cavities.

The sprue connects the machinenozzle to the runner and may beeither a cold or a hot sprue. In thehot sprue design, the sprue hasheating elements that maintain theplastic in a molten state eliminatingthe need for separation and recla-mation. Ideally, the sprue shouldbe as short as possible to minimizethe pressure loss during injection.A cold sprue is tapered for easyrelease from the mold.

There are three basic runner typesin use:

Cold RunnerInsulated RunnerHot Runner

Cold runners are commonly usedin two and three-plate molds, butnot in stack molds which requirethe use of a hot runner. The mostcom-monly used runner designsare full-round, half-round andtrapezoidal (Figure 27. The full-round is gener-ally preferred forease of machining and lowerpressure loss. For fast cycles afull-round is not recommendedsince the greater mass of materialtakes longer to cool and maycontrol the cycle time. The runnershould be polished for ease ofmold filling and part ejection.

The insulated runner (Figure 29) isthe precursor to the hot runner.The runner diameter is very largeand a thick skin is formed on theoutside of the runner. The moltenplastic flows in the center and, dueto external insulation and the lowthermal conductivity of thepolymer, remains molten during thecycle. This design eliminates theneed for removing and/or recyclingthe runner. The problem with thisdesign is that when the machine isdown for any extended period oftime the runner solidifies and hasto be physically removed before

Figure 27. Schematic showing typical runner designs found ininjection molds

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Figure 28. Insulated runner system

Figure 29. Hot-runner system

beginning the next molding cycle.As molders have become morecomfortable with hot runnertechnology, insulated runners arerapidly becoming obsolete and notmany molds are built today utilizingthis technology.

The externally heated hot-runnersystem (Figure 29) also maintainsthe plastic in a fluid state while themold is running with the pressureat each gate approximately thesame. Maintaining a uniformtemperature in the sprue bar andthe hot-runner manifold is verycritical to process and productconsistency. Start-up proceduresmust be carefully followed

according to the mold maker’sspecifications to prevent damageand material leakage in themanifold.

Mold ventingWhen molten plastic is injectedinto the mold, the air in the cavityhas to be displaced. To accomplishthis, vents are machined into theparting line to evacuate the air andare extremely important to theconsis-tent production of highquality products. In many cases,this is an area in mold design andconstruction that is oftenoverlooked.

Vents should be located at theextremities of the part and atlocations where melt flow frontscome together. Venting is alsoeasily achieved around ejector pinsand core slides provided that thereis sufficient clearance between thepin/slide and the mold. Typicalmold vents are channels cut fromthe cavity or runner straight to theedge of the mold. Closest to thepart, they are typically 0.0005-0.001 in. (0.013-0.025 mm) indepth and 0.063-0.5 in. (1.6-12.7mm) in width. The initial ventthickness should be maintained forabout 0.5 in. (12.7 mm) and thenthe depth can be increased toabout 0.003 in. (0.076 mm) to theedge of the mold. The vents shouldbe polished towards the edge ofthe mold to make them ‘self-cleaning.’ Build-up in the vents willeventually affect mold fillingresulting in non-uniform fill andunbalanced cavities. For thisreason, it is important that vents beinspected between production runsto ensure that they are clean andwithin specification. In some cases,reduction of the injection rate priorto final filling of the cavity willprevent burning and also preventthe mold from opening.

Continuous parting-line ventingmay be necessary in high speedmolding operations. Even thoughburning is not evident, the lack ofburn marks does not ensure thatthe molds are properly vented.Increasing vent areas may helpreduce cycle time. Proper ventingwill also aid mold filling bydecreasing the resistance due toair pressure on the flow front.

Mild sand blasting or vaporblasting of the mold cavity assistsin venting and part release.However, for high-gloss finishes,this blasting is not advisable. Vaporhoning may help alleviate aventing problem area but caremust be taken that honing is nottoo deep or wide to be noticeableon the finished part.

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GatingThe gate is the bridge betweenthe runner and the cavity.Depending on the specificmaterial and part design (wallthickness, geometry, etc.) thereare many different types of gateswhich can be used (Figure 30).

The type and size of gate are verycritical since they can affect manyfactors including mold-fill time,overall cycle, orientation,shrinkage, warpage, and partappearance.

Because it acts as a restriction tothe polymer flow, a high shear rateis created at the gate oftenresulting in a temperatureincrease. There is also a highpressure drop across the gatewhich needs to be overcome byincreased injection pressure orhigher temperatures. The pressuredrop can be reduced, to a certaindegree, by using shorter gate landlengths.

A large gate provides easy fillingwith relatively low shear rates andpressure drops. However, if it is toolarge, it will require an excessiveamount of time to cool, lengthen-ing the cycle. It is also possiblethat insufficient packing andsubsequent sinks or voids willoccur if either sections of the partor the sprue and runner systemfreeze off before the gate.

A gate which is too small willrequire higher pressures to injectthe material and may causeproblems in part filling. If the gatefreezes off before the part cools, itwill not be possible to developadequate packing, resulting invoids or sinks. With extremelysmall gates, jetting or melt fractureof the polymer flow will causesurface appearance defects,including delamination.

To ensure uniform fill, it is criticalthat the feed system (sprue,

Figure 30. Gating systemsrunners, and gates) be balanced.This depends on the size andlocation of the gates and is oftendetermined by experience.Advances in mold filling simulationsoftware have provided anadditional tool for analysis prior tothe manufacture of the tool. Fine-tuning may be required and isgenerally done by utilizing a seriesof short shots, observing the fillpattern, and making minoradjustments, as required. Formulti-cavity tools utilizing singlegates and a hot runner system,adjustment of the temperatures ofthe individual gates may be usedto balance the overall fill pattern.

In high-speed, thin-wall molding, itis common to provide coolingaround the gate to remove the heatproduced by the high shear rates.This may be supplemented by theuse of inserts fabricated of highconductivity alloys, such asberyllium-copper, in these criticalareas.

Mold coolingAlthough mold cooling isextremely critical to cycle time,warpage, molded-in stresses,mold-filling, etc., the sizing andlayout of the cooling pattern areoften over-looked and neglectedaspects in the initial stages of tooldesign.

The cooling layout should beconsidered in relationship to thethickness profile of the part and thegeneral filling pattern in order toprovide adequate cooling in criticalsections and not overcool otherswhich may cause part warpage. Inareas where coolant flow may berestricted due to part geometry i.e.,bosses, the use of insertsfabricated from high thermalconductivity alloys, such asberyllium-copper, should beconsidered.

In all cases, cooling channelsshould be sized in relation to theavailable coolant flow to ensure

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turbulent flow which is much moreeffective for heat removal thanlowering the temperature of thecoolant. Routine inspection andacid-cleaning of cooling channelsare recommended to maintain thecoolant flow velocity and minimizepressure drops. Ideally, thetemperature differential betweencoolant inlets and outlets should beabout 2°F. Jumpers betweencooling circuits should be avoidedin order to reduce temperaturedifferentials in the coolant.

The utilization of low pressure-dropmanifolds, valves, fittings, etc. andin-line flowmeters and temperatureindicators are also good practicesto provide information regardingthe efficiency and condition of thecooling system.

Ejection devicesThe ejection of injection moldedparts is most commonly accom-plished by air, vacuum, pins orstrip-per plates. Depending on partdesign, combinations of thesesystems are used for rapid positiveejection. Care should be taken inselecting ejection surfacesbecause of aesthetic andmoldability requirements.Wherever possible, the part shouldbe ejected off the core. For small,thin-walled moldings that mayshrink onto the core, air ejectionthrough the core is usuallyadequate for part removal. Onsome products with threaded orundercut features, collapsible,retractable or unscrewing coresare used.

Spiral flowmeasurementThe relative processability of aninjection molding resin is oftendetermined by its Melt Index (MI)or Melt Flow Rate (MFR). Thisinvolves measuring the relativeflow of the molten resin through aspecified capillary in a calibratedlaboratory instrument, while

maintaining the molten resin at 190°C (374°F) and 43.5 psifor Polyethylenes or at 230°C(446°F) for Polypropylenes.

Melt index is a good measure-ment of a resin’s relative flow properties at low shear rates, but only for resins of the same molecular weight distribution(MWD). Under actual injection molding conditions, differences in MWD will affect the resin’s melt viscosity (flow characteristics) at high shear rates. Temperatures, pressures and shear rates of actual molding do not conform to those of the MI or MFR test methods.

LyondellBasell has a number of unique manufacturing processes available which allow the control not only of the melt index and density, but also MWD. This capability results in a better overall balance in resin properties and processability. Because melt index and MWD play a key role in performance in actual end-use applications, LyondellBasell has utilized "Spiral Flow" (SF) as a more practical method of measuring and comparing a resin's performance using realistic processing conditions.

Figure 31. Broad MWD (left) andnarrow MWD (right)spiral flow

Figure 32. Effect of MWD on spiral flow of HDPE (all materialshave MFR - 5 gms/10 min)

Spiral flow measures the flowlength when molten resin isinjection molded into a long,0.0625" radius, half-round spiralchannel (Figure 31). The higherthe spiral flow number (SFN), theeasier the resin is to process. Themelt temperature is monitored andmaintained at 440°F (227°C) andinjection molding is conductedusing a constant pressure of 1,000psi (7,000 kPa). Spiral flow is amore realistic measurement thanmelt index because it is run at amuch higher shear rate allowingresins of similar MIs and differentMWD to be compared at realisticconditions. The broader MWDsresins exhibit lower melt viscosity(higher SFN) at higher shear ratesthan narrower MWD resins withsimilar melt indices (Figure 32).

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Since it does not take into account the effects of MWD, relying only on the melt index can be misleading. For example, ALATHON® ETP H 5057, a broad MWD, 57 melt index resin for thin-wall HDPE applications, exhibits flow properties similar to many narrow MWD resins in the 75 to 80 melt index range.

LyondellBasell has established the use of spiral flow as a specification for all high-flow (30 melt index and above) HDPE resins and has begun reporting the spiral flow number for each lot on the Certificate of Analysis (COA). This allows the molder to compare the spiral flow of an incoming lot of resin with the SFN of the lot on-hand and readily estimate how the new lot will process relative to current production. For example, if the current lot being run has a SFN of 20 in. and the new lot has a reported SFN of 22 in., the new lot can be processed at either lower temperatures and/or at a faster production rate. Only minor adjustments in either melt temperature and/or injection pressure may be required to compensate for SFN variability from lot to lot.

General injectionmolding operatingproceduresPrior to starting up the injection-molding machine, be sure to have the following available:

Safety glasses for all personnelassisting in the start-up.Loose fitting, heavy-dutyinsulat-ed work gloves.A large metal container orcardboard for collecting meltproduced during the start-upprocedure.Soft beryllium-copper, bronze,or aluminum tools for use inremoving any plastic from thenozzle area.

Always refer to the manufacturer’s operating manual for any specific start-up and shutdown procedures.

Refer to the LyondellBasell suggested resin startup conditions (Table 4) for general guidelines to use in starting up an injection molder on polyolefins.

General safetyAs with any process involving energy and mechanical motion, injection molding can be a hazardous operation if appropriate safety procedures aren’t well documented and followed. (Refer to the Manufacturer’s operating manual.)

Mechanical, electrical, and hydraulic interlocks are critical to

the safe operation of any piece ofprocessing equipment. In somecases, these interlocks may needto be bypassed while performingset-up and maintenance functions.Under no circumstance should thisbe done by non-qualifiedpersonnel. In order to assureutmost safety during normaloperation, interlocks should neverbe bypassed.

Keep all molding equipment andthe surrounding work areasclean and well maintained.

Hydraulic leaks should be repairedimmediately to eliminate safetyhazards. Hydraulic lines, valves,fittings and hoses should bechecked periodically per themanufacturer’s recommendations.

Table 4. Suggested start-up conditions (based on general purpose meltindex/flow rate products)

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Good housekeeping is essential.Loose pellets, tools, oil, etc. on andaround the molding machine cancause accidents, damage to theequipment, or contamination of theparts.

HeatHigh temperatures are necessaryin the injection molding process.Always use heat-resistant gloves,safety glasses and protective cloth-ing. Modern injection moldingmachines have warning signsidentifying specific hot areas onmolding machines; do not ignorethese signs. Keep the splashguardin place during purging and whenthe machine is operating.

Polymer left in the barrel or hotrunner system may often bepressurized. Care should be takenwhenever resin flow is interrupteddue to blockage or mechanicalproblems.

If the molding machine will be shutdown for an extended period oftime (30 minutes or longer), lowerthe heats, purge the machine orcycle it until the lower temperatureis reached before shutting it down,leaving the screw full of resin andin the retracted position.

ElectricityMolding machines utilize high elec-trical voltage and have warningsigns pointing out electricalhazards; do not ignore these signs.Keep water away from theseareas. A periodic inspection of allelectrical devices and connectionsfor wear, looseness, etc. is veryimportant.

Machinery motionThere is considerable mechanicalmotion during the injection moldingprocess. Neckties and loose fittingclothing should not be worn aroundmolding machines since these canbe caught by the equipmentmovement and lead to physical

injury. Do not reach around, under,through, or over guards while theequipment is operating.

Be sure all people working nearthe injection molding machineknow where the Emergency ShutOff but-on is located. Neverdisengage any of the safetymechanisms or inter-locks on theinjection molding machine.

Some machines store energy(hydraulic, pneumatic, electrical, orgravitational) which can be presenteven when the machine is turnedoff. Consult the manufacturer’soperating guide for methods toproperly de-energize theequipment. As with any piece ofpotentially hazardous equipment, asuitable lock-out/tag-out procedureshould be implemented andenforced.

The injectionmolding processand its effect onpart performance

The molding cycleAs detailed in the section on theInjection Molding Process, thereare several steps in theproduction of injection moldedparts. In most cases, the injectionmolding cycle begins with themold open, ejector pins/slidesretracted, and the screw/ ramready with the next shot ofmaterial. The cycle then proceedsas follows:

1. MOLD CLOSE: Mold closesand clamp develops full closingpres-sure

2. INJECTION: Material isinjected into the mold cavity

3. PACKING: Material is packedinto the mold to fill out the part

4. PLASTICATION: Screw beginsto rotate (or ram retracts) todevel-op the next shot ofmaterial

5. COOLING: Coinciding with thestart of plastication, the coolingcycle begins (Note: Sincecoolant continuously circulatesthrough the mold, coolingtechnically starts as soon asthe melt contacts the cavityduring injection)

6. MOLD OPEN: Mold opens,slides retract, ejector pinsactivate, part(s) are ejected.

The length of each of these stepswill depend on the complexity ofthe mold, the size of the machine,and the geometry and end-userequirements of the part. A typicalcycle for a four cavity, 16 oz.stadi-um cup can be found in(Figure 33) while one for a singlecavity, 12 lb. bumper fascia canbe found in (Figure 34).

Regardless of part size, weight, ormold complexity, nearly half ofevery injection cycle is spentcooling the part(s) to a temperaturesuffi-cient to allow ejection withoutpost-mold distortion. Factors thataffect the cooling rate of the part(s)will be covered later.

In almost all cases, part quality isthe result of steps 2-5. The highestquality parts begin with a homoge-neous plastic melt in terms of tem-perature and composition.Therefore, the quality of the nextpart to be produced is the result ofthe development of the shot duringthe current cycle. Shot size shouldbe sufficient to produce a cushionof material at the end of the step 3of 0.1-0.5 in. (2-13 mm). Thiscushion will keep the screw from‘bottoming out’ and help maintainplastic pressure within the cavity.

Achieving a homogeneous melt iscontrolled by many factors includ-ing: screw design, screw speed,screw and barrel wear, backpressure, shot to barrel capacityratio, soak time, and heater bandsettings.

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Figure 33. Injection molding cycle for a 16.0z. stadium cup (4cavity, HDPE, 31g each, 7.8 sec. cycle)

Screw design for polyolefinprocessing was covered in aprevious section. A worn barreland/or screw creates anincreased gap between the tworesulting in resin movingbackward or staying in placeinstead of being conveyedforward by the screw. A wornscrew and/or barrel can lead topoor melt consistency,degradation of the resin, andshot inconsistency. Screw speed

should be set so that the screwconsistently recovers 1-2 secondsbefore the mold opens.

Backpressure is typically set at theminimum level that delivers ahomogeneous melt (no unmeltedpellets leaving the nozzle). However,backpressure may need to beincreased to improve temperatureconsistency in the melt and to mini-mize or eliminate streaks due to poordispersion of colorant. Duringplastication, most of the energy

provided for melting the resinpellets comes from shear heatingdue to friction between the pellets,screw and barrel. As the backpres-sure is increased, the screwworks the material more in order toconvey it forward, thereby raisingthe temperature of the melt morequickly. The increased work by thescrew also increases the mixing ofthe molten plastic resulting inbetter temperature andhomogeneity of the melt. However,too much back pressure can resultin degradation of the plastic, anincrease in the screw recoverytime, increased energy costs, andmore wear on the screw andbarrel. Shear heating is alsodependent on the viscosity of theplastic, screw design, screwspeed, and back pressure. Thelatter two can be varied to someextent by the processor to controlthe shear heating and melttemperature.

The shot-to-barrel capacity ratio(SBCR) can also have an effect onmelt, and therefore part quality.The ideal range for the shot tobarrel capacity ratio is 30-60%. Ifthe SBCR is less than 30%, toomany shots of material reside inthe barrel under the influence ofheat from the heater bands andshear from the screw. This maylead to over-heating anddegradation of the resin. If theSBCR is greater than 60%, lessthan two shots of material are inthe barrel, which typically does notallow the melt temperature toequilibrate. A high SBCR will alsomean that the screw may recover(develop the next shot) just beforethe mold opens which can lead tocycle alarms due to inadequateshot size. A SBCR range of 30-60% will provide adequate time forthe melt temperature to equilibrate.

In order to achieve proper melthomogeneity, all of the pelletsshould be melted by the time theyreach the middle of the transitionzone on the screw. Figure 35

Figure 34. Injection molding cycle for automotive facia (12 lb. shot, 0.125 in.thickness, 107 sec. cycle)

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depicts the amount of energyneeded to process a polypropyleneimpact copolymer. In order for thepellets to be fully melted halfwaythrough the transition zone, 71% ofthe energy to reach the desiredmelt temperature (in this case,450°F) must be transferred to thepolymer.

The heater bands on the barrelprovide only a small amount of theenergy needed to melt the plastic.Most of the energy from the heaterbands maintains the barreltemperature during processing andraises the temperature of the solidpellets in the feed zone. There arefour typical temperature settingpatterns for injection molding on abarrel with five heater zones(Figure 36):

1. Increasing: This pattern hasthe lowest temperature settingat the feed throat and thehighest at the front of thescrew with a steady increase ofthe temperature settings inbetween. The nozzle istypically set at the sametemperature as the front zone.This pattern is the one mostcommonly used and isparticularly recommended forlower melting point materials(such as EVA or EMA) toprevent bridging at the feedthroat of the extruder. It is alsorecommended when the SBCRis low, typically 50% and screw recoveryand residence times (time fromresin entering the extruder toleaving the nozzle) are short.Sufficient feed throat coolingmust be provided to preventbridging. Otherwise, a low

Figure 35. Energy needed to bring PP to melt temperature

Figure 36. Various barrel temperature profiles

temperature set point shouldbe used at the feed throat. Inaddition, this profile canincrease the chance of airbeing entrapped in the meltinstead of venting backthrough the hopper.

3. Hump: This pattern has thehighest temperature settings(typically 20+ degrees higher

than desired melt temperature)in the middle of the screw tocorrespond with the transitionsection where the majority ofthe melting takes place.Settings near the feed throatare typically at the softeningpoint of the resin while thesettings at the front of thebarrel and the nozzle shouldbe at the desired melt

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temperature. This profile isrecommended when SBCR is25-50% and overall resi-dencetime is 2 to 4 minutes.

4. Flat: This profile uses thedesired melt temperature asthe settings for all of the barrelzones except for the feedthroat, which should be set ator below the softening point ofthe resin. This profile is typicalof processes where the SBCRis 20 to 40%.

In actual practice, the specificscrew design also plays animportant part in obtaining thedesired melt. It is possible thatdifferent screw designs mayrequire different profiles to achievesimilar melt characteristics even ifthey have the same or anequivalent SBCR.

By varying the screw speed andback pressure, shear heating is, forthe most part, an easily controlledsource of heat to the material. Thebest parts are typically producedwhen there is a balance of shearheating and heat from the heaterbands. Once a temperature profileis chosen, it is recommended thatthe processor monitor current flowinto the heater bands. The properbalance of shear heating andthermal heat is achieved when thecurrent cycles regularly (typicallyseveral times a minute). This is ofparticular importance in thetransition zone of the barrel; if thebarrel is divided into quarters alongthe length, the transition zone istypically the middle two quarters ofthe barrel. Typically the heaterbands in the feed zone will cycleregularly or be on nearly all thetime. The heater bands in themetering zone and nozzle shouldcycle regularly but less frequentlythan the feed zones. Becauseaccurate temperature control is soimportant, it is always a goodpractice to routinely check thecalibration of the controllers.

If a heater band is on all the time,either the set point is too high to bereached or there is a problem withthe thermocouple or heater band (itis working but not reading theactual temperature). If thethermocouple is fine and thedesired setting for the zone is notout of line, the processor canincrease the temperature settingsupstream (closer to the feed throat)of the zone in question. Should thisnot reduce the time that the heaterband is on, the temperature settingon the zone should be lowereduntil the band cycles regularly. Thiswill prolong the life of the heaterband and reduce the energyusage.

If a heater band does not drawcurrent, or does so infrequently,there are two possible problems.Either most of the heat going intothe plastic at this barrel zone is viauncontrolled shear heating or thethermocouple is broken, both ofwhich should be corrected. Abroken thermocouple will typicallyread out the maximum permissibletemperature. If all of the cavitiesare filling with acceptable cycletimes, the heater band set points inthe zones upstream of the zone inquestion should be reduced. If thisfails to get the heater band cycling,reset the upstream zone(s) to theiroriginal temperature(s) andincrease the temperature set point.

It may appear that the proceduresabove are only serving to increasethe overall melt temperature.While this is true to a small extent,the benefit is that they aid inproviding a more homogeneousand controllable melt temperaturethat will improve the molded parts.

Now that we have a homogeneousmelt stream in the barrel or accu-mulator, we need to examine theintroduction of the plastic into themold. The viscosity, or resistanceto flow, of the resin is affected bytem-perature and shear rate.Increasing the melt temperaturereduces the viscosity of the resin

making it easier to fill the mold.Increasing the pressure or injectionrate increases the shear rate,which decreases the viscositymaking it easier to fill the mold.Therefore, given a resin, machineand mold, there are three variablesthat can be used to fill out themold:

Injection/packing pressuresInjection rateMelt temperature

The curves in Figure 37 indicatethe relative temperature-pressurerelationships for PE resins ofvarying melt indices. The higherthe MFR or MI, the lower theinjection pressure and/or thetemperature required to fill a mold.Assuming the same mold fillingcharacteristics (fill speed and filltime), cycle time and injectiontemperature, a high flow resin:

1. Will allow pressures to bereduced about 25% when theresin MI or MFR is doubled.

2. Will allow a decrease in melttemperature of about 70°F(40°C) when the resin MI orMFR is doubled.

The effect of a higher flow PE resinon temperature and pressure canbe seen in Figure 38. Note that asthe MI or MFR of the resinincreases the possible reduction intemperature and/or pressure willbecome less.

However, the switch to a polyolefinwith higher flow characteristicsusually results in a loss of otherproperties such as resistance tostress cracking and impactstrength, especially at lowertemperatures.

Injection and/or packing pressuresare typically the first settingsadjusted by the processor becausethey have a quick response onmold fill. Increasing the pressureswill help fill out the mold correctingfor short shots and reducing oreliminating surface defects such assink marks and ripples near the

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gate. The downside of increasingpressure is the chance of trappingair in the cavity resulting in burnmarks or of increasing the flash onthe parting line due to the moldopening. Increasing injectionpressures also pack resin moretightly into the cavity, which mayreduce shrinkage, increase thegate temperature(s), and increasemolded-in stress. The reducedshrinkage can lead to a partsticking in the cavity and also post-mold dimensional differences.

Increasing the injection rate(s)reduces the viscosity of the resin,which may reduce the amount ofmolded-in stress in the part. Inaddition, an increased injectionrate may also yield a more uniformpart temperature (due to fasterintro-duction of material into themold) which can reduce differentialshrinkage (i.e. warpage) due totemperature variation. Increasingthe injection rate(s) without adecrease in injection pressure canlead to flashing of the part.Changing the injection rate(s) alsohas a fast effect on part qualityalthough it may take time to fine-tune the rate(s) for optimumquality. Excessive molded-in stresscan lead to an increase in warpageand a decrease in impact strengthand environmental stress crackingresistance.

Sometimes the injection rate andinjection pressure are notindependent variables; i.e. themachine is set with a maximumpressure and runs on injection ratesettings which are set on screwposition. This setup will allow themachine to vary the injectionpressure based on the pressureneeded to meet the rate set points.Conversely, the injection pressurecan be specified based on screwposition and the rate is allowed tovary. Some processors are nowutilizing pressure sensors withinthe cavity to control the operationof the machine via cavity pressure.This is a new approach that is

Figure 37. Temperature-pressure relationshps for polyethylene resinsof several melt indices

Figure 38. Effect of Melt index of polyethylene resin on injection temperature

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gaining acceptance for moldingparts with critical tolerances. It isalso applicable to molds (such assyringes) where core shift is ofconcern.

The final way to control theviscosity of the resin is to adjustthe melt temperature. An increasein temperature will decreaseviscosity. Changing temperaturesettings yields a slower responsethan pressure or injection rate.High resin temperature can lead todegradation and require longercooling time while lowtemperatures can lead to shotinconsistency, higher injectionpressures, and excessive wear/damage to the screw and barrel.

When setting an injection rate orinjection and packing pressureprofile, the aim of the processorshould be to provide a smoothdelivery of material into the mold. Amomentary slowing of the screwdue to either the transfer from onestep to another or too large of astep can result in a hesitation ofthe plastic flow front. Hesitation ofthe melt front can cause surfacedefects such as flow lines or tigerstripes, which may lead to poorweld and/or knitline strength.Therefore it is necessary to reducethe rates or pressures in aconsistent manner to preventflashing of the tool, potential coreshift(s), and bottoming out of thescrew.

In general, >95% of the partweight(s) should be deliveredduring the injection step. The finalpart weight is achieved via thepacking and holding step. Packingpressures are typically about halfthe level of the injection pressureand serve to achieve final partweight(s) and also to allow time forthe gates to freeze off beforeplastication can begin for the nextshot.

During the injection and packingsteps, coolant (typically a mixtureof ethylene glycol and deionizedwater if

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mold can also lead to a higheramount of molded-in stress withinthe part. Warmer moldtemperatures will increase thegloss level and may also improveresin flow by constricting the meltchannel less, at the expense ofincreased cycle times.The injection molder and the molddesign typically fix the amount oftime needed for the mold to open,eject the part and then close again.Mold open time can be reduced byusing only enough daylight to allowthe part(s) to fall freely, reducingthe amount of time required for airassists and also the number oftimes the ejector pins activate.

ShrinkageAmorphous resins such as ABS,Polycarbonate, and Polystyrenehave much lower shrinkage valuesthan the polyolefins. The highershrinkage of polyolefins is due tothe fact that, in their molten state,they take up more volume than inthe solid state because polyolefinresins are semi-crystalline. Whenthe resin solidifies, the chains inthe crystalline regions pack tightlytogether resulting in a reduction involume. In general, the polyolefinscan be ranked for shrinkage:

HDPE > LLDPE > LDPE > PP

Once a resin has been selected,shrinkage can be controlled, tosome extent, through mold designand processing conditions (Table5). Studies on a test mold in whichthe thickness and gate area of aflat plaque can be varied, indicatethe following:

Shrinkage is reduced as partthickness decreases. Theresponse to a thicknesschange is more pronouncedwith HDPE than PP.

Shrinkage is reduced when thegate area is reduced.

Since the degree of shrinkageis partly a result of cooling, itcan be reduced by molding atlower injection temperaturesand running a colder mold.Packing the part more willalso minimize shrinkage. Thisis done either by molding atmoderate temperatures andhigh pressures or by moldingat fairly high temperaturesand moderate pressures.However, excessivetemperature or pressure canresult in flash.

Another means of reducingshrinkage is the use of higherpressure and longer packingtime. This allows additionalresin to flow into the mold asthe material in the mold coolsand shrinks, packing out themold as much as possible butmay also increase cycle timeand higher molded-in stress.

Longer cooling time in themold before ejection isespecially useful whenever aninside dimension is critical. Asthe molded article cools andcontracts around the core, thecore will maintain the criticalinside dimension of the part.Generous draft, or tapering,will allow easier part ejection.A longer cooling time willmean an increase in cycletime, therefore many molderswill increase the mold coolingto reduce shrinkage.

Shrinkage is a time-dependentdependent function. Ingeneral, a polyolefin part hasachieved about 90% of itstotal shrinkage after 48 hours.Shrinkage can continue forseveral more days if the partsare packed hot and/or arestored in a warm warehouse.Parts that have shrunk afterpackaging typically exhibit‘nesting’ problems if the partsare stacked inside each other.

WarpageWarpage results from non-uniformshrinkage of the molded partcaused by non-uniform cooling.When a part warps after beingejected, it is assuming its ‘natural’shape by relieving the stressesforced upon it while being cooledin the mold. The problem, often adifficult one to solve, is to minimizethe ‘locked-in’ stresses, which thepart might later ‘remember’ andrelieve during cooling to room tem-perature. In cases where parts arefixtured after ejection, subsequentexposure to higher temperaturesmay cause relaxation andwarpage. Part designs incorporatingsignificant differences in cross-sectional thickness are more proneto warpage than those with a moreuniform thickness, due to higherresidual temperatures in the thickersections.

In addition to non-uniformcooling, locked-in stresses aregenerated in the mold by suchoperating conditions as excessivemolding pressures, slow fill times,low backpressure, or too low amelt temperature.

There is no single, clear-cutremedy for warpage. Adjustingmold conditions, redesigning thepart or the mold, switching to amaterial with a narrower MWD, ora combination of these may reducethe internal stresses. Generally, theleast warpage occurs when themelt temperature is set at themaximum, the mold temperature ishigh, injection pressure is aminimum and the injection time isshort (Table 5).

Molding at high temperaturesallows the stresses induced duringinjection to be reduced before thepart sets. Running a warm moldalso allows the stresses to relaxbefore the melt sets. Differentialcooling between the mold halves is

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often required to produce warp-free parts especially those havinglarge, flat sections.

Injection and packing pressuresshould be adequate to permit easyfill but should not be setexcessively high in order to allowsome of the molded-in stresses torelax before the part sets.

Increasing the injection rate willdecrease the injection time, whichwill allow the mold to fill fasterbefore the extremities can cool toomuch. This gives the entiremolding a chance to cool at a moreuniform rate, which reduces thewarpage.

Some of these remedies, such ashigh melt temperature or low injec-tion pressure can increase thecycle time. Switching to a higherMFR/MI resin can offset theincrease in time. Higher flow resins

will allow lower injection pressure,which can shorten the moldingcycle. In addition, higher flowresins typically exhibit less “elasticmemory” which can also reducewarpage. Lower density resins (forPE) are only slightly lesssusceptible to warpage than higherdensity resins.

Differences between flow andtransflow shrinkage can result inwarpage. HDPE is known to havea large difference between thesetwo while PP is more balancedbetween the flow and transflowshrinkage.

Because both shrinkage andwarpage are strongly influenced bythe mold cooling patterns and partgeometry (uniformity of thicknessesand flow patterns), it is very impor-tant that these be considered in theearly stages of part and molddesign.

Color dispersion andair entrapment

An effective means of improvingdispersion and preventing airbubbles from getting into the moldwith the melt is to use a breakerplate at the end of the barrelbetween the screw tip and thenozzle (Figure 40). Backpressureon the melt, in most cases,squeezes all the air out betweenthe melting pellets and producesbubble-free moldings. The breakerplate may be ¼ inch (6.5 mm)thick and must be large enough tofit into the opening of the nozzle.The plate is drilled with 20- to-40,small diameter (1/32 inch or 0.8mm) holes. Another option is toincrease the back pressure on thescrew making sure that it is not settoo high