Injection Molding - 5.3

8/12/2019 Injection Molding - 5.3

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The development ofpowerful computer controls and programs hasgreatlyaccelerated integrating process variables withagoalofzerode-fectsat thelowest cost.Thecomputer simplifiesthefinetuningofmachinesettings with molding variables. Examples include establishing melt con-ditions formold filling and packing, which involves the simultaneousmeasurementandcontrolof two ormore critical variable parameters (Fig.2.10,page 131). It isduring this phaseofoperation that most variationsmake themselves evident and can be easily detected. Achange inmeltviscosityisreflected as achangein ram speed and can bedetected bymeasuring the ram position with respect to time.

Achangeinresin viscosityisreflected as achangein melt pressureand can bedetectedbymeasuring moldorcavity pressure with respecttotime.Other variations that similarly display themselves and can be de-tectedinclude melt temperature, hydraulic pressure, and oil temperature.COMPUTER-INTEGRATEDINJECTION MOLDINGThe ultimate result of computer-integrated injection molding (CIIM)insoftware packagesis totranslatethe results ofcomputer simulationof the molding of a specific part into machine settings for specificmicroprocessor-controlled machines (Figs 2.35and2.36). CIIMautomatesthe entryof a large number of set points in microprocessor-controlledmachinesandmaximizes their efficiency.Microprocessor control systemsMicroprocessor control systems (MCS) make it possible to completelyautomate an IMplant. They control machines, automatically, enablingthem to achieve high quality and zero defects. These systems readilyadapttoenhancingthe abilityofprocessing machines. Therearemanymoldings that would be difficult, if not impossible, to produce at thedesired quality level without thisfeature.

Onceprocessing variables are optimized through computer simulation,these values are entered in computer programs in the form of alargenumber of machine settings. Establishing the initial settings duringstartup isinherently complexandtime-consuming.Themanybenefitsofthesesystemsarewell recognizedandaccepted,but it isevident thatself-regulation of IM can beeffectiveonly whenthedesignof thepartand themoldareoptimized,andwhenthecorrect processing conditionsfor theoperationhave been predetermined. Otherwise,aself-regulating machineisconfusedand canprovideconflictinginstructions.Theresults couldbedisastrous,damagingthe machineor themold. Therefore,the efficientutilizationofmicroprocessor control systems dependson the successofutilizingcorrectandoptimum programs.

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ProcesssimulationsThesimulation approach replacesthetraditionaltrial-and-errormethod.Programsarepackagedfor thecomplete molding process, includingma-terials selection, molding and cost optimization, flow analysis, computer-ized shrinkage evaluation, and mold thermal analysis. The programs aremoldfilling,packing,and soforth,which accurately modeltheperform-anceofmicroprocessor-programmed injection.Major 3D CAD systemsfo rpartandmolddesign,aswellas forstructuraland flowanalysis,areintegratedwith these systems.

ImprovingperformanceMachinecontrol coordinatesfunctionsof themolding machine. Controlfunctions have evolved to advanced high-speed microprocessor-basedsystems.Surface-mountcontrol-board technology is being used to reducethesizeofmachine control systems.

Tocomplement the new controls, sophisticated hydraulics have beenintroduced. Servo control valves offer increasedflexibility andaccuracy,as well as shortened machine function response time. Microprocessorcontrolsandservo proportional hydraulics provide dynamicresponsetoachieve true closed-loop systems. Closed-loop systems maintain long-term repeatabilityofmachine velocitiesand pressures independent ofcomponent wearand factors suchas oiltemperature, ambient tempera-ture, and variations that occur in material viscosity.

MOLDING VARIABLESVERSUSPERFORMANCEMeltflowbehaviorTherearevariables during molding thatinfluencepart performance suchasmachine settings (Fig. 2.37andTable2.4 onpage 144).Theinformationpresented here shows how melt flow variables behave toinfluenceprod-uct properties. A flow analysis can be made to aid designers andmoldmakersinobtainingagood mold.Ofparamount importanceiscon-trolling the fill pattern of the molding so that parts can be producedreliablyandeconomically.Agood fillpatternfor amoldingis onethatisusually unidirectional in nature, thus producing a unidirectional andconsistentmolecular orientationin themolded product. This approachhelpstoavoid warpage problems causedby adifferential orientation,aneffectbest demonstratedby thewarpage that occursinthin center-gateddisks. In this case all the radials are oriented parallel to the flow direction,with thecircumferencestransverseto the flowdirection.Thedifferenceintheamountsofshrinkage manifestsitself intermsofwarpageof thedisk.


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thebest balanceofgate locationsfo rcosmetic impactandmolding con-siderations. Figures 2.38to2.44 show variousflowpatterns,orientationpatterns,and property performances.Inthepractical worldofmold design therearemany instance wheredesign trade-offs mustbemadeinordertoachievea successful overalldesign. Although naturally balanced runner systemsarecertainly desir-able,theymayleadtoproblemsinmold coolingorincreased costdue to

Figure 2.38 Cavity melt flow: looking at apart'sthickness (fountainflow).

S H E R T H I N N I N G L Y E R

P L U G F L O W F S T F I L L

O R I E N T T IO I S L O W F I L L

Figure 2.39 The effects of different fillrates.


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F i l l p o i n tF l o w p a t h v i t y

Figure2.40Duringthecompensation phase, plastic melt doesnot flowuniform lythroughthediaphragmof theplate mold (a),butspreadsin abranching pattern(b).

Tensi le

Notched Izod impactFlexura l

Figure2.41Test specimens with differentwaysofgating produce different flowdirectionsandproperties.


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S T R E S S P R L L E L T O O R I E N T A T I O N

S T R E S S P E R P E N D I C U L A R T O O R I E N T A T I O NFigure 2.42Orientation affects strength: the highest tensile strength is in thedirection parallel to the orientation.

Figure2.43Flow linesorweld linesin atelephone handset:thegatewaslocatedatthetop-centerof thehandle.


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Molding variables versus performance 185


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Figure 2.44 Locating agate to obtain the required performance of a retainerproduct that is subject tobeing flexed inservice:(a) retaineredge gated;(b)retainer centergated;and (c)left and centerretainers(betweenfingers) thatwereedgegated, withthefailedretaineron therightw hichwascentergated.

excessive runner-to-part weights.Andtherearemany cases, suchaspartsrequiring multiple gates or family molds, in which balanced runnerscannot be used. Flow analysis tools allow successful designs of runners tobalanceforpressure,temperature,or acombinationofboth.

PartinglinesTheIM processing parting line technique controls the process by using themovement between the two halves as the plastic isinjectedinto the moldas the feedbackvariable. This movement across the mold parting line isusedtoinitiatethetransferfrominjectiontoholdingpressure;itthereforeperforms as atransfer point controller (TPC).TPCs have been aroundforsome timeand are acommon componentofmost process control pack-agesfor IM.Four strategiesareincludedin theusual commercial transferpoint packages; parting lineaddsafifth. Parting linehas amajor advan-


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tageinthatitssensor issimply added to the outsideof themold. Thistechnique adds little or no machining cost. It may be an add-on to oldermachines without fullcontrol packages.

BackpressureIMback pressure indicates resistanceto thebackward movementof thescrew during preparation for a subsequent melt shot. This pressure isexerted by the plastic on the screw while it isbeing fedinto the shotchamber(forwardend of thebarrel,infrontof thescrew).During rotationof the screw and the material under pressure, thorough mixingof theplastic is achieved, and some temperature increase also occurs. In dealingwith heat-sensitiveandshear rate insensitive plastics, care mustbetakentokeep this value within prescribed limits. The action reviewed concernsaconventional screw where back pressure is used to improve the meltingcharacteristicsof anotherwise marginally performing screwfor theplasticbeing processed.

Withatwo-stage screw,thefirststageishydraulically isolatedfromthesecond-stage screw by the unfilled devolatization zone. Consequently,backpressure cannotbeused to affect melting. Applying back pressureaffectsthesecond stage only,andservestoincreasethereverse pressureflowcomponent. This will necessitatealongerfilledlengthof thesecondstage to produce adequate conveying,so the lengthofunfilled channelwill be reduced and devolatilization impaired. In an extreme case,backfilling canprogressto thevent port andvent bleed will occur.Theonly practical advantage lies in the additional mixing it induces in thesecond stage. However, the additional length of a two-stage screw isalmostalwayssufficienttoensure adequate mixing without applicationofbackpressure.

ScrewbridgingWhen an empty hopper is not the causeoffailure, plastic might havestoppedflowingthroughthefeed throat.Anoverheated feed throat,orstartupfollowedwith a long delay, could build up sticky plastics and stopflowinthehopper throat. Plasticscanalso stickto thescrewat the feedthroat or just forward from it. When this happens, plastic just turnsaround with the screw, effectively sealingoff the screw channel frommovingplasticforward.As aresult,thescrewissaidto bebridgedand itstopsfeedingthescrew.

Thecommon cure is to use a rod to break up the sticky plastic or to pushitdown through the hopper and into the screw, where its flight may takeapieceof the rod andforceitforward.Thetypeof rod fedintothescrew


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should be made of the plastics being processed. Other rods used could beofrelativelysoftmaterial suchascopper.Weld/meldlinesProblems can develop when molding parts include openings and/ormultiple gating (Fig.2.45).In theprocessoffillingacavitythe hotmeltisobstructedby thecore,and by themeetingof two ormore melt streams.Witha core the melt splits and surrounds the core. The split stream thenreunitesand continuesflowinguntilthecavityisfilled. Therejoiningofthesplit streamsformsa weld line that lacks the strength properties in anareawithoutaweld line; thisisbecausethe flowingmaterial tendstowipeair, moisture,andlubricant intothearea wherethejoiningof thestreamtakesplaceand introducesforeign substances intothewelding surface.Furthermore, since the plastic material has lost some of its heat, thetemperatureforself-weldingis not conduciveto themostfavorablere-sults.Asurface thatis to besubjectedtoload-bearing shouldnotcontainweld lines.Ifthisis notpossible,theallowable working stress shouldbereducedby atleast15 forunreinforced plasticsand 40-60 with RPs.Themeld lineissimilarto aweld line exceptthe flowfrontsmove parallelrather than meeting head on.

W e l d

M e l d

Figure2.45Flow paths are determined by part shape and gate location. Flowfronts that meet head-on will weld together, forming a weld line. Butparallelfronts tendtoblend, potentially producingaless distinct weld linebut astrongerbond calledameld line).


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TOLERANCESAND SHRINKAGESCertain IMpartscan bemoldedtoextremely close tolerancesofless thanathousandthof aninch(25.4Jim),ordownto0.0 , particularly whenTPsarefilledwith additivesor TSs areused (Chapter1). Toeliminate shrinkand to provide a very smooth and aesthetic surface, one should use asmall amount ofchemical blowing agent (


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Table2.11 ContinuedAverage rate perA S T M D 955

Material 0.125in. 3.18mm) 0.250in. 6 .35mm)PolyetherimideU nreinforced 0.005 0.00730 glassfiber 0.002 0.004Polyphenyleneoxide/PSalloyU nreinforced 0.005 0.00830 glassfiber 0.001 0.002Polyphenylene sulfideUnreinforced 0.011 0.004

40 glassfiber 0.002 NAPolypropylene, homoUnreinforced 0.015 0.02530 glass fiber 0.0035 0.004PolystyreneU nreinforced 0.004 0.00630 glassfiber 0.0005 0.001

There arevarious methodsofestimating shrinkages.Aneasy methodfo restimating shrink allowanceis asfollows:

M = 1 S)LwhereM =molddimension,S =plastic shrinkage (in./in.ormm/mm),and L =part dimension.

Ifpartsaresmallandhave thin walls, this estimateis thebest guide.Ifparts are larger(>10in.,0.25m)and/oruse rather high-shrink plastics,consider using

LM=L/(1-L)whereLM =largest mold dimension.

MOLDINGTECHNIQUESInadditionto theconventionalIMreviewed, specialized techniquesareusedtomeetspecificproduct requirements that generate cost reductionsandreducecycletime; coupled with thisare thenecessary molding capa-bilities to produce specific products. They include gas-assisted IM,coinjection,liquidIM,injection-compression molding (coining), continu-ous IM, fusible-core molding, multilive feed molding, reaction IM


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(Chapter11), reactiveIM(Chapter3),tandemIM,metaland ceramicIM,two-colorIM,foammolding (Chapter9),expandable polystyrene (Chap-ter 9),structural sandwich molding, partsconsolidation molding, offsetmolding, jet molding, oscillatory molding, molding with rotation(stretch/orientationthat differs frominjectionstretch blow molding;seeChapter4), and others[1,9].Someofthese methodsare now reviewed.

Gas-assisted IMAsignificantdevelopment ininjectionmolding technologyhasbeentheintroduction of gas assist. Nitrogen,an inexpensive inert gas, isintro-duced to the plastic melt through theinjectionnozzle, the mold runner, ordirectlyinto the mold cavity. The gasdoesnot mix with the plastic, buttakesthelineofleast resistance throughthelessviscouspartsof themelt.Theplastic is pushed against the mold and leaves hollow channels withinthepart.

Alongwith the ability to produce hollow parts, parts with heavy ribsandbosses can be achieved with low in-moldstresses,reduced part warp-age, and the elimination ofsinks. Along with the lowering of inmoldstresses, gas-assisted injection offersmaterial savings (sincegasdisplacesresin and less plasticisused), lower clamp tonnage requirements, andreduced cooling/cycle times. The gaspressureis maintained through thecoolingcycle.In effect,the gaspackstheplastic intothemold withoutasecond-stage high-pressure packing in the cycle as used in IM,whichrequires high tonnagetomold large parts[1,14,69].

CoinjectionCoinjection means that two or more different plastics are 'laminated'together.These plastics couldbe thesame exceptforcolor. Whendifferentplastics areused,they must be compatible in that they provide properadhesion(ifrequired), meltatapproximatelythesame temperature, andsoon. Two ormoreinjectionunitsarerequired, with each material havingits own injection unit.The materialscan be injected into specially de-signed molds: rotary, shuttletable,etc.[I].

Theterm Coinjection can denote different products, suchas sandwichconstruction, double-shot injection, multiple-shot injection, structuralfoamconstruction, two-color molding,andinmolding. Whateverits de-signation,a 'sandwich' configuration has been made in which two ormoreplasticsarelaminated together totake advantageof the differentpropertieseach plastic contributesto thestructure.

This form of injection has been in use since the early 1940s. Manydifferentadvantages exist:(1) itcombinesthe performance ofmaterials;(2)itpermitsthe use of alow-cost plastic suchas aregrind;(3) itprovides


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adecorative'thin'surfaceof anexpensive plastic;(4) itincludes reinforce-ments; (5) itpermits the use ofbarrier plastics (Chapter4).Coinjectionmoldingisbeing redefined todayinlightof theapproachesnowavailablefo rmolding multicomponent parts suchasautomotive taillights, contain-ers,andbusiness machine housings.

LiquidIMLiquid IM(LIM)hasbeenin uselonger than reactionIM(RIM),but theprocesses are practically similar. The advantages it offers in the auto-mated low-pressure processingof(usually) thermoset resins- fastcycles,low labor cost,lowcapital investment, energy saving,andspace saving-maymakeLIMcompetitivetopotting, encapsulating, compression trans-fer,and injectionmolding, particularly when insert moldingisrequired[I].Differentresinscan beused,suchaspolyester, silicones, polyurethanes,nylon,andacrylic.Amajorapplicationfor LIMwith siliconesisencapsu-lationofelectricalandelectronic devices.

LIMemploystwo ormore pumpstomovethecomponentsof theliquidsystem(suchascatalystandresin)to amixing headbeforetheyareforcedinto a heated mold cavity. Screws or static mixers are used in somesystems. Only a single pump is required for a one-part resin, but systemshavingtwo ormore partsarenormally used. Equipmentisavailabletoprocessalltypesofresin systems, with unsophisticated or sophisticatedcontrolsystems.Avery critical control involves precision mixing.Ifvoidsorgaseous by-products develop, vacuum is used in the mold.

Injection-compression molding coining)Coining, also calledinjectionstampingandmoreoften injection-compres-sion molding,is avariantofinjection molding (Figs2.46and 2.47).Theessentialdifferenceliesin themannerinwhichthethermal contractionofthe molding during cooling (shrinkage)is compensated. With conven-tionalinjection molding,thereductioninmaterial volumein thecavitydue tothermal contractioniscompensatedbyforcinginmore plastic meltduringthepressure-holding phase.

Bycontrast with injection-compression molding, the meltis injectedintoacavity thathas arelatively short shotin acompression mold (maleplug fitsintoafemalemold) rather thantheusual flatsurfacematchingmold halvesfor injection molding. Themelt injected intothe cavityisliterallystress-free;itworks without a holding pressure phase, and thetransportofplastic melt that accompanies this action avoids stressesin thepart, particularlyin thegate area(s).The ICMprocessforthermoplasticshasbeen usedforpartsofdifferent sizes, particularly thick-walled parts


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Figure2.46Coining combines injectionmoldingandcompression molding.

with tight dimensional requirements, such as optical lenses. When meltentersthemold,it is notcompletely closed.Theshort-shot melt literallyflowsunrestrictedin thecavityand isbasically stress-free.A fter injectionis completed, the mold is closed, with the pressure on the melt veryuniform.ContinuousIMIMMshave been used tomold continuous all-plastic productsorstrips.Anexample is theVelcro strips that uses rotating mold halves with aconstant flow ofplastic meltfromtheinjectionunitto themold[I].Thereare systems where metal fiber, etc., are continuously fed through amulticavitymold and precision plastic parts molded around the metal.


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Tem perature - control led ma ni fold,2 2 0 - 2 4 O 0 F 1 0 4 - 1 1 60 C )

Mater ial

Mater ial distr ibut ion

Runne r c u t o f fPressure sensor

Movable mold hal fStat ionary mold half

Mold cavity,3 4 00F 17 10C)Opened be tween 0.10in.a nd 0.20 in.2 .5mm a n d5 m m )Figure2.47Close-upof acoining mold.

Figure2.48 shows six copper wires being directed through the open moldhalves. The IMM is on a movable platform; moving in a rectangularmotion.Thewires have gone throughsqueeze rolls,toproducethe de-sireddiameter,and move at a constant speed. With the mold closed, meltisinjected intothemulticavity mold (20cavities around each wire).The mold has recesses toaccurately retain the wires. During moldfilling,themoldand the IMMmoveat theconstant speedof thewires.Whenthe mold opens, the IMM moves sideways to reposition the moldawayfrom thewires that havetheplastic 'buttons7.Thewires continuetraveling whilethe IMMreturns to the starting position. Theplatformmoves sideways(backto itsoriginal position)and themold closes,so it isready for the nextinjection shot. These buttons areaccurately molded(diameter and thickness) and accurately spaced about lin.(25.4mm)apart. Theaccurate spacing is kept from shot to shot. Tolerancesforalldimensionsare inthousandths of aninch (tensofmicrometers).Theproduct was used inhigh-frequency electrical lines. Figure 2.48 showsthe buttons around the wire exitingthe IMM.The runners were coldrunners,one of thethreemajortypes.Eachrunnerfeedsmelt aroundtwowires.


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Figure 2.48 Coaxial cable cores produced by continuous IM using polysty


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Fusible-coremoldingTheuse offusible-core technology(FMCT),aswellassoluble-core tech-nology(SCT),toinjectionmold parts with cavities that couldnotother-wise beformedor released has been known in the plastics industry sinceatleastthe1940s,but notfrequentlyused (sinceit wasmoreof amysteryin the past). Other forms, types, or terms include lost-core technology(LCT), soluble salt-core technology (SSCT), lost ice-core technology(LICT),andceramic-core technology(CCT).LCT hasbeenthemostpopu-larterm, used since the1940sand possibly earlier; it also pertains to all theother terms.

Morerecently,fusiblecores and soluble cores have beenused.Automo-bile engine intake manifolds molded ofglassfiber reinforced nylon ap-pearto beeconomicalandtechnologically interesting.Use of afusiblecoretomoldthecomplex, curved part produced thesought-afterpropertiesofhigh quality and a smooth interior surface.

MultilivefeedIMThepatented Scorim process is a molding method to improve the strengthandstiffness ofpartsbyeliminating weld linesandcontrollingtheorien-tationoffibers.Aconventionalinjectionmolding machine usesaspecialhead thatsplitsthe meltflowinto the mold into two streams. During theholding stage, two hydraulic cylinders alternately actuate pistons aboveandbelowthehead, compressingthematerialin themoldin onedirection

Figure 2 49 Multifeedmolding,schematic.


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then the other. This action aligns the fibers, removes weld lines, andinduces orientationinliquidcrystal polymers(LCPs).Figure 2.49showstwopackingpistonsthat oscillate180out ofphase,two packingpistonsthatoscillateinphase,and twopackingpistonsthat compress melt withequal constant pressure.

COSTINGIMMSA major investment is the purchase ofIMMs.The costof an IMM,incombinationwiththecapabilityofthat machinetorepaytheinvestment,canmakethedifferencebetween successandfailureof abusiness.Manymolders make their purchasing decisions using empirical informationbased onhearsayor the performanceofanother machine they alreadyown. This approachhas itsmerits,but itcouldbedisastrousforthose withlittleknowledgeofmachines[1, 65, 69, 87, 88,9O].

Just likepeople, not all machines are created equal. Recognize thatidentical machine models, built and delivered with consecutive serialnumbers to the same site canperformsodifferently as to make somecompletely unacceptable. Therecan be significant differences betweenmachines,so themolder usuallyusesonemachineforcertain jobsandanotherforspecial precisionjobs.Differences are due tofactors suchashydraulicdesign, which affectslong-term pressuredrift.Theconsistencyofthemachine controlaffectsthemachine timing. Another areais thefinalcalibration ortuningof amolded product during startup.

One cannotdependon identical calibration or identical performancefrom many sources.Themachineto bepurchased needs toperformasrequired. Thereisalwaysnewtechnology thatcansuccessfullydifferenti-ategood machinesfromthose with poor expected performance.

Tocompare IMMs,youneedtohave done your homework;youneedtofindoutwhatyouneedtomonitorin themachineand how youdesireittooperate.Youalso needtoknowtherelative importanceofeachfactorfo rthepartsyouintendtomanufacture.Youneedto beabletocompareamachine under test conditionsto acommon yardstick,and youneedtoknow whereflawsexist that might inhibit productivity [9O].

Themonitoring system needs to relate to the molded part requirements.Thissetsup agoodset ofparameter guidesto bemonitored to definethe relationship between process deviation and part quality with thesoundnessof themachine designandconstructionof themachine. Factorstoanalyze include machine movements (clampingspeed,injection ramtime, back pressure holding capability,etc.),numberofwiresto the ma-chinesequence control using quick-disconnect clipsin an effort tosyn-chronize the measurements with the machine cycle, and locationofpressure transducer(s) connectingtheinjectionramcylindertoclampingspeed.


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Reviewingthese data will show whatcan andcannotbe met tooperatethe machine to a set ofstandardssuch as cycledeviation,clampingspeedlimitation, injection time, back pressure drifting, mold hold time, andplasticizing time. Some believe amachinerunoff should beconductedwithamold thatisrepresentativeof thetypeto beusedinproduction.Itisokaybut notnecessary.Asimple molding block withableed hole thatallows some materialtoescape duringinjection and hold willbe suffi-cient. Thus, the repeatability of the machine is measured rather than theperformanceof themold.Theplasticto beused,however, shouldbe thetype that will be used in production.

TROUBLESHOOTINGAlltypesofprocessing (IM, extrusion, etc.) have become more sophisti-cated, particularly with regard to process and power controls; so trouble-shooting requires a thorough, logical understanding of the completeprocess (Fig. 1.1, page2) and continuesto be avery important function.Problems are presented throughout this book, with suggested approachestosolutions.Onemust assemble informationofthis typeas thebasisfor atroubleshooting guide (Tables 2.12and2.13).Eachproblem will haveitsownsolutionorsolutions (Fig.2.50).Simplified guidestotroubleshootinggranulators, conveying equipment, metering/proportioning equipment,chillers,anddehumidifiersareavailable.

No twosimilar machines(fromone ormore suppliers) will operateinexactlythesame manner,and plasticsdo not meltor soften as perfectblends, but they do all operate within certain limits.

Asimplified approachtotroubleshootingis todevelopachecklist thatincorporates the basicrulesofproblemsolving:(1) have aplanand keep

Callyour Supervisor If anythingeven LOOKS wrong

Figure 2 50 Anticipateanyproblems.


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G A T E

BEINGMOLDED

MOLD C A V I T Y

(H IGHLYE X A G G E R A T E DAFTEREJECT IONS COOL ING CROSSF L O WSHR INKAGE

FLOW DIRECTIONSHR INKAGEFigure2.51Directional shrinkage when processing crystallineTP.

updating it based on experiencegained;(2) watch the processing condi-tions;(3)changeonecondition/controlat atime;(4)allowsufficient timeforeach change, keepinganaccuratelog ofeach;(5)check housekeeping,storage areas, granulators, etc.;and (6)narrowtherangeofareasinwhichtheproblem belongs-machine,mold/dies,operating controls, material,part design,andmanagement.Toaccomplishthelast item, severalstepsmaybe taken:(a) Changetheresin.If theproblem remainsthesame,it isprobably not

the resin.(b ) Change the type of resin used, as that may pinpoint the problem.Figure 2.51 is an example where shrinkage of crystalline plastics(Chapter1) is not isotropic; even shrinkages in all directions occurswith amorphous plastics.(c) If thetrouble occursatrandom,it isprobably a function of the ma-chineor the heat control system. Change the mold/dieto anothermachine todetermineif it is themachine. Also consider changingtheoperator.


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Tr y remedies in descending orderChange gate locationClean mold facesClean ventsCheckfo r material contaminationCheck for uneven mold temperatureCheck mold faces fo rproper fitDry materialIncrease amountof materialIncrease back pressureIncrease clamp pressureIncrease cooling timeIncrease holding pressureIncrease injection hold timeIncreaseinjection pressureIncreaseinjection speed

Table 2 12 IMtroubleshoo ting guide (Co urtesyof RTPCo., W inona MN)Pr


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Table2.13 Common molding faults e f e tShortmoldings

Flashing atmoldpartinglines

Surfacesink marks

Voids

Possible causeInsufficient feedInsufficient pressureInadequate heatingInsufficientinjectiontimeColdmoldBackpressure due toentrapped airUnbalancedcavityin a

multicavitycavitymoldInsufficient lockingforceInjectionpressure to ohighMaterialtoo hotMold faces out oflineMold facescontaminatedFlow restrictedto oneormore cavities(inmulticavitymold)Materialtoo hotwhengate freezesInsufficient dwellplunger forward timeInsufficientmaterial shotinto cavity

Insufficient pressurePieceejectedtoo hotMaterialtoo hot gasformation)CondensationofmoistureonpolymergranulesCondensationofmoistureon themoldsurfaceInternalshrinkage aftercase-hardeningofouter layer

Suggested remedyAdjust feed settingIncreasepressureIncrease temperatureorlengthen cycleIncreaseinjection timeIncreasemold temperatureImprove ventingofmoldChecksizes ofcavities

IncreaselockingforceReduceinjection pressureReducecylinder temperatureRebedmold facesCleanmold facesCheckand remove restriction

Reducecylinder temperatureorenlarge gateIncreasedwell timeIncrease feedIncreasecylinder temp.Increasemold temp.IncreasepressureIncreasecooling timein themoldReducecylinder temp.Predrygranules

Increasemold temp.

IncreasepressureIncreasemold temp.EnlargesizeofgatesLengthendwell time


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Table2.13 Continued e f e tWeldLines

Distortionofmoldings

CrazingandblisteringSurface streaksBurnmarksBrittleness

Possible causeMaterialtoo coldMold too coldInjection pressure too lowGates wrongly located(includingtoo b ig adistancefrom gatetoweld joint) or designedEjection ofmo lding at toohigh a temperatureEjection pin working

unevenlyExistenceofmolded-instressesdue tom aterialtoocold,bad design,cavityoverpacked invicinityof gates

Excessivesurface strainbecauseof cold moldOverheatingofm aterialM oisture in granulesA ir trapped in moldcavitiesMaterialtoo coldMaterialhas degradedCo ntamination withother materialMold too cold

Suggested remedyIncrease cylinder tem p.Increase mold temp.Increase injection pressureRelocategatesand/orredesign

Increase mold cooling timeCorrectoradjust ejection

pinsIncreasecylinder tem p.Redesign moldingCheckfeed setting. Reduceinjection pressure andcylinder temperature.Reduce injection timeIncrease mold temperatureReducecylinder temperaturePredry granulesImprove mold ventingIncrease cylinder temp.Decrease cylinder temp.Checkthe materialfo rcontaminationCheck cylinder and hopperIncrease mold temperature

(d) If theproblem appears, disappears, orchanges from oneoperator toanother, observethe differences between their actions.

(e) If theproblem always appearsinaboutthesame positionof asingle-cavity mold, it is probably a function of the flow pattern due tounsatisfactorycooling, and requires readjustments.(f) If theproblem appearsin thesame cavityorcavitiesof amulticavitymold,it is in thecavityorgateandrunner system.

(g) If amachine operation malfunctions,check thehydraulicorelectriccircuits.As anexample,a pump makesoil flow, but there mustberesistance to flow to generate pressure. Determine where the fluid isgoing.Ifactuators fail to moveor move slowly, the fluid must be


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bypassing them or going somewhere else. Trace it by disconnectinglinesif necessary. Noflow,orlessthan normal flow in thesystem,willindicate that a pump orpumpdrive is atfault.Details on correctingmalfunctionsare in themachine instruction manual.

(h) Checkforhydraulic contamination.Toolittle attentionispaidto thecleanliness requiredof the oilused. Dirtisresponsiblefor the major-ity of malfunctions, unsatisfactory component performance, andmachinedegradation, particularly withtheincreaseduse ofelectro-hydraulic servosystems. Injection pressure, holding pressure,plasticatingpressure, boostpressure,andboost cutoff areadverselyaffected by increased contamination levels in the fluid. Sourcesofcontaminationinclude new oil, a hydraulic system built with poorquality control,airfromtheenvironment, wearofhydraulic compo-nents, leaking orfaultyseals, and shop maintenance activity. Con-tamination controlis accomplished with the proper filters (suchas1OjIm)(see suppliers),andwith preventive maintenance proceduresthatare both correct and properly used.

(i) Set up a procedure to'breakin'the newmold/die.Theprocedureforsettingup amold/dieis asfollows.(1)Obtain samplesandmolding cycle informationif themoldwasusedbyothers.(2)Cleanaused mold. (3)Visually inspect the mold andmake correctionsif re-quired.(4)Check out,on abench,theactionsof themold/diecams,slides,unscrewing devices,and so on. (5)Installsafetydevices.(6)Operate themold/diein themachine,andmoveitvery slowly underlowpressure.(7)Openthemold/dieandinspectit. (8)Dry-cyclethemold withoutinject-ingmelt to check knockout stroke,speeds,cushions, and low-pressureclosing[54]. (9)A fter the moldis at operating heat, dry-cycleit again;expansionorcontractionof themold partsmay affect the fits.(10) Takeashot, using maximum mold lubrication and under conditions least likelytocause mold damage, usuallylowmeltfeedandpressure.(11)Buildupslowlyto operating conditions, and run the process until stabilized (usu-ally1-2h).Record operating information. (13) Takethe part toqualitycontrol forapproval. (14) Make required changes. (15) Repeattheprocessuntil it isapprovedby thecustomer.

Faultyor unacceptable parts usually result from problemsin one ormore of these areas: (1)premolding, material handling and storage(Chapter 16);(2)molding, conditions in the processing cycle;and (3)postmolding,parts handlingand finishingoperations (Chapter17).

Problemscaused in premolding and postmolding may include thoseinvolvingcontamination, color,the static dust collector,and so on. Inmolding (item 2) the molder is required to produce a good-quality meltbasedonvisual observationas it flowsfreely fromthenozzle.Eachmoldisunique andeach materialisunique, so onecannot generalize about


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what makesagood melt.Theexperienceof themolderand aknowledgeoftheprocess needsare thefinaldetermining factors.There are several ways to determine the efficiency of the melt. One

methodis toobservethescrew drivepressure;itshouldbeabout75 ofmaximum. If it isless than that, lowertherear-zone heat untilthedrivepressure starts to rise. With melt quality changing, raise the center zone torestore quality to what is required. Heat changes should be accomplishedin10-150Cincrements, with10-15min ofstabilization time allowed be-forethenext change.

Oncethe rear zone is set, one should lower thefrontzones to whateverlevelwill still give good molding conditions. With crystalline types, suchasnylon,PP, and PE, the operator must watchthescrew return.If thescrew ismoving backwardin ajerkymanner, thereisinsufficient heatinthe rear zone; the unmelted resin is jamming or plugging the screwcompression zone. The heat energy required to melt crystalline plastics isdifferent fromthat neededforamorphous plastics (Chapter1).

WearAllscrews,barrels, moldsordies,and anydevice that handles melt willwear,but hopefully by aninsignificant amount that does not influenceprocessability [54].The wear of screws (particularly on the flight OD)and barrelsis afunction of (1)the screw-barrel-drivealignment;(2) thestraightnessof thescrewandbarrel;(3) thescrew design;(4) the uniform-ity ofbarrel heating;(5) thematerial being processed; (6)abrasivefillers,reinforcingagents, pigments,and so on; (7) thescrewsurfacematerial;(8)thebarrel liner material;(9) acombinationof thescrewsurface and thebarrel liner; (10) improper support of thebarrel; (11) excessive loadsonthebarrel discharge end and heavy molds ordies;(12) corrosion causedbyadditives suchas flameretardants; (13) corrosion caused bycertainpolymer degradation;and (14) excessive back pressure on the injectionrecovery.

Screws are usually aligned properly by the supplier before shipment,but canbecome misaligned during shipment, duringinstallation, and byaccidentalimpactsandother aspectsoftheir use.An angular misalign-mentwill generally cause wearuniformly around the screwin a fairlylocalizedarea.Inthat vicinitythebarrel willbeworn aroundtheentireID.Ifthebarrelisbent,thescrew willbewornallaround nearthecenterandnearthe discharge, whereas the barrel is usually worn on one side near thecenter.Wearonscrewsandbarrels generallyfallsinto three categories.A b r a s i v e w e a riscausedbyabrasivefillerssuchascalcium carbonate, talc,

glassfibers,bariumsulfate(usedinmagnetic tapes,etc.),andeventhetitaniumdioxide pigments used in all white and pastel shades. Glass


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fiberstendtoabradetherootof thescrewat theleadingedge,and insevere casescanunderminethescrewflightcompletely, usually leavingno flight in thecompression-transitionzone. This action occurs exten-sively when partially melted or unmelted plastic pushes the glassagainst the screw or barrel.

A dh es ive wearorgallingiscausedbymetal-to-metal contact. Certain sensi-tivemetalscanmomentarily weld toeach other becauseofvery highlocalized heating.As thescrewrotates,theweldseparates,andmetalispulled from the screw to the barrel or vice versa. Proper clearanceusually eliminates this problem with proper alignment and hardness.With an improperly designed screw for a plastic operating at highoutput rates, an unmelted blockage will result,forcingthe screw againstthe barrelandcausing rapid adhesion wear.

Corros ion wear is caused by chemical attack in the melting of certainplastics, suchasPVC, ABS,PC, and PUR,aswellas flame-retardantcompounds, fiber-sizing agents, and so on. Materialsupplierscan iden-tifytheoffendingagents.Thewear usually showsapitted appearanceand is usually downstream, where it has a chanceto overheat anddegrade. This typeofwearcan becontrolledbyusing proper operatingprocedures;do not let themachine stayat theoperating heatfor anylength of time. Proper selection of the screw design and corrosion-resistantscrew/barrel materials can help. Nonreturn valves and screwtipsarealsosubjecttowear,so it isimportantto use thebest availablematerial.

Differentcoatings suchaschromeandnickel platingareusedtoprotectthescrewsurface. Dependingon the specificplastic being processed,aparticularcoating willbeavailable.Thewearsurfaces,primarilyof flightlands,areusually protectedbywelding special wear-resistant alloys overthesesurfaces. Themost popular and familiar alloysareStellite (trade-mark ofCabot Corp.) and Colmonoy (trademarkof Wall ColomonoyCorp.);othersare also used and areavailablefrom different suppliers.Different heat treatmentsare also used on the steels to increase wearresistance.

InspectionScrewsdo not have the same outside continuous diameter. Upon receiv-ing amachineor just a screw,it is a good idea to check its specifieddimensions (diameters versus locations, channel depths,concentricityand straightness, hardness, spline/attachment dimensions, etc.) andmakea proper visual inspection. Thisinformationshould be recorded sothat comparisons can bemade following a later inspection. Theinitialcheckalso guarantees proper delivery. Some special equipment should be


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Table2.14M anufa cturing tolerances o n screws[54]DiametersOutside diameterShank diameter

Injection registersClearance diametersLengthsO verall length (O A L)Transition zonesVentsectionsShank lengthsRingvalve locationConcentricityTIR of OD100in.HardnessBasem aterial 4140Flame-hardened flightsNitralloy3135M(o requivalent)Stellitebno . 6

Stellitebno. 12Finish

0.001in.0.005in.0.0005 in.+0.015in.iV^in. / 1 0 dia. V i o dia. V 3 2 i n . V3 2 in.

0.002in.0.004in.28-32Rc4 8 R c m i n .60-70Rc38-42Rc42-48Rc

ChanneldepthsDepth0.000-0.150in.0.151-0.350in.0.351-0.750in.Hollow bore length:Flight widths:0-0.500in.0.501-1.000in.>1.001in.

Hollow boretoshankInjection registersColmonoycno. 5Colmonoycno. 56Colmonoycno . 6Colmonoy84N-45dN-50dN-55 d

Tolerance0.002in.0.003in.+0.005in. V 40.010in.0.0515in.+0.020in.

0.015 in.0.001 in.36-40Rc46-50Rc50-55Rc36-42Rc40-44Rc44-48Rc46-50Rc

Unplated screws 16 RMSmax.Plated screwsRoot 8 R M SFlightsides,O D,and shank 16 RMSmax.max.a Trademark ofJoseph T,Ryerson&Son, Inc.b TrademarkofCabot Corp.c TrademarkofWall Colmonoy Corp.d TrademarkofMetallurgical IndustriesInc.used for inspection other than the usual methods (micrometer,etc.)toensure that the inspection is reproduced accurately. Such equipment isavailablefromsuppliers [54]andactually simplifies testingandtakes lesstime, particularly for roller and hardness testing. It is important thatscrewsaremanufacturedtocontrolled tolerances suchasthose giveninTable2.14.

Rebuildingscrews/barrelsInaproperly designed plasticator,themajorityofwearisconcentratedonthescrew because the screw can be replaced and built more easily than the


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barrel. The rebuilding ofinjection (alsoextrusion,blow molding,etc.)screws has become so common that the rebuilding businessbecame amajor segment of our industry. Onereasonfor the popularity ofscrewbuilding is that rebuilding is usually considerably less expensive thanreplacementof a newscrew. Rebuildingisusually done withhardfacingmaterials. With the proper choice ofhardfacing metallic material, therebuilt screwcanperformbetter thantheoriginal screw [54,63,65].

Itisconsiderably moredifficultandcostlytorepairaworn barrel thantorebuildaworn screw.If it is notgreater than about0.5mm(0.02in.), thewhole barrelcan behoned to alarger diameter and anoversized screwcanbeused.If thewear occurs nearthe end of thebarrel,asleevecan beplaced inside.Butdespite these remedies, aworn barrel generally needstobe replaced.

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Injection Molding - 5.3