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Thepurposeofthisreportwastoexperimentallystudythewater-assistedinjectionmoldingprocessofpoly-butylene-terephthalate（PBT）composites.Experimentswerecarriedoutonan80-toninjection-moldingmachineequippedwithalabscalewaterinjectionsystem,whichincludedawaterpump,apressureaccumulator,awaterinjectionpin,awatertankequippedwithatemperatureregulator,andacontrolcircuit.ThematerialsincludedvirginPBTanda15%glassfiberfilledPBTcomposite,andaplatecavitywitharibacrosscenterwasused.Variousprocessingvariableswereexaminedintermsoftheirinfluenceonthelengthofwaterpenetrationinmoldedparts,andmechanicalpropertytestswereperformedontheseparts.X-raydiffraction（XRD）wasalsousedtoidentifythematerialandstructuralparameters.Finally,acomparisonwasmadebetweenwater-assistedandgas-assistedinjectionmoldedparts.Itwasfoundthatthemeltfillpressure,melttemperature,andshortshotsizewerethedominantparametersaffectingwaterpenetrationbehavior.Materialatthemold-sideexhibitedahigherdegreeofcrystallinitythanthatatthewater-side.Partsmoldedbygasalsoshowedahigherdegreeofcrystallinitythanthosemoldedbywater.Furthermore,theglassfibersnearthesurfaceofmoldedpartswerefoundtobeorientedmostlyintheflowdirection,butorientedsubstantiallymoreperpendiculartotheflowdirectionwithincreasingdistancefromtheskinsurface.

Keywords:

Waterassistedinjectionmolding;

Glassfiberreinforcedpoly-butylene-terephthalate（PBT）composites;

Processingparameters;

B.Mechanicalproperties;

Crystallinity;

A.Polymermatrixcomposites;

1.Introduction

Water-assistedinjectionmoldingtechnology[1]hasproveditselfabreakthroughinthemanufactureofplasticpartsduetoitslightweight,fastercycletime,andrelativelylowerresincostperpart.Inthewater-assistedinjectionmoldingprocess,themoldcavityispartiallyfilledwiththepolymermeltfollowedbytheinjectionofwaterintothecoreofthepolymermelt.Aschematicdiagramofthewater-assistedinjectionmoldingprocessisillustratedinFig.1.Water-assistedinjectionmoldingcanproducepartsincorporatingboththickandthinsectionswithlessshrink-ageandwarpageandwithabettersurfacefinish,butwithashortercycletime.Thewater-assistedinjectionmoldingprocesscanalsoenablegreaterfreedomofdesign,materialsavings,weightreduction,andcostsavingsintermsoftoolingandpresscapacityrequirements[2–4].Typicalapplicationsincluderodsandtubes,andlargesheet-likestructuralpartswithabuilt-inwaterchannelnetwork.Ontheotherhand,despitetheadvantagesassociatedwiththeprocess,themoldingwindowandprocesscontrolaremorecriticalanddifficultsinceadditionalprocessingparametersareinvolved.Watermayalsocorrodethesteelmold,andsomematerialsincludingthermoplasticcompositesaredifficulttomoldsuccessfully.Theremovalofwateraftermoldingisalsoachallengeforthisnoveltechnology.Table1liststheadvantagesandlimitationsofwater-assistedinjectionmoldingtechnology.

Fig.1.Schematicdiagramofwater-assistedinjectionmoldingprocess.

Waterassistedinjectionmoldinghasadvantagesoveritsbetterknowncompetitorprocess,gasassistedinjectionmolding[5],becauseitincorporatesashortercycletimetosuccessfullymoldapartduetothehighercoolingcapacityofwaterduringthemoldingprocess.Theincompressibility,

lowcost,andeaseofrecyclingthewatermakesitanidealmediumfortheprocess.Sincewaterdoesnotdissolveanddiffuseintothepolymermeltsduringthemoldingprocess,theinternalfoamingphenomenon[6]thatusuallyoccursingas-assistedinjectionmoldedpartscanbeeliminated.Inaddition,waterassistedinjectionmoldingprovidesabettercapabilityofmoldinglargerpartswithasmallresidualwallthickness.Table2listsacomparisonofwaterandgasassistedinjectionmolding.Withincreasingdemandsformaterialswithimprovedperformance,whichmaybecharacterizedbythecriteriaoflowerweight,higherstrength,andafasterandcheaperproductioncycletime,theengineeringofplasticsisaprocessthatcannotbeignored.Theseplasticsincludethermoplasticandthermosetpolymers.Ingeneral,thermoplasticpolymershaveanadvantageoverthermosetpolymersinpopularmaterialsinstructuralapplications.Poly-butylene-terephthalate（PBT）isoneofthemostfrequentlyusedengineeringthermoplasticmaterials,whichisformedbypolymerizing1.4butyleneglycolandDMTtogether.Fiber-reinforcedcompositematerialshavebeenadaptedtoimprovethemechanicalpropertiesofneatplasticmaterials.Today,shortglassfiberreinforcedPBTiswidelyusedinelectronic,communicationandautomobileapplications.Therefore,theinvestigationoftheprocessingoffiber-reinforcedPBTisbecomingincreasinglyimportant[7–10].Thisreportwasmadetoexperimentallystudythewaterassistedinjectionmoldingprocessofpoly-butylene-terephthalate（PBT）materials.Experimentswerecarriedoutonan80-toninjection-moldingmachineequippedwithalabscalewaterinjectionsystem,whichincludedawaterpump,apressureaccumulator,awaterinjectionpin,awatertankequippedwithatemperatureregulator,andacontrolcircuit.ThematerialsincludedavirginPBTanda15%glassfiberfilledPBTcomposite,andaplatecavitywitharibacrosscenterwasused.Variousprocessingvariableswereexaminedintermsoftheirinfluenceonthelengthofwaterpenetrationinmoldedparts,whichincludedmelttemperature,moldtemperature,meltfillingspeed,short-shotsize,waterpressure,watertemperature,waterholdandwaterinjectiondelaytime.Mechanicalpropertytestswerealsoperformedonthesemoldedparts,andXRDwasusedtoidentifythematerialandstructuralparameters.Finally,acomparisonwasmadebetweenwater-assistedandgas-assistedinjectionmoldedparts.

Table1

Advantagesanddisadvantagesofwater-assistedinjectionmolding

Advantages

Disadvantages

1.Shortcycletime

1.Corrosionofthesteelmoldduetowater

2.Lowassistingmediumcost（waterismuchcheaperandcanbeeasilyrecycled）

2.Largerorificesfortheinjectionpinrequired（easiertogetstuckbythepolymermelt）

3.Nointernalfoamingphenomenoninmoldedparts

3.Somematerialsaremoredifficulttomold（especiallyamorphousthermoplastics）

4.Removalofwateraftermoldingisrequired

2.Experimentalprocedure

2.1.Materials

ThematerialsusedincludedavirginPBT（Grade1111FB,Nan-YaPlastic,Taiwan）anda15%glassfiberfilledPBTcomposite（Grade1210G3,Nan-YaPlastic,Taiwan）.Table3liststhecharacteristicsofthecompositematerials.

2.2.Waterinjectionunit

Alabscalewaterinjectionunit,whichincludedawaterpump,apressureaccumulator,awaterinjectionpin,awatertankequippedwithatemperatureregulator,andacontrolcircuit,wasusedforallexperiments[3].Anorifice-typewaterinjectionpinwithtwoorifices（0.3mmindiameter）onthesideswasusedtomoldtheparts.Duringtheexperiments,thecontrolcircuitofthewaterinjectionunitreceivedasignalfromthemoldingmachineandcontrolledthetimeandpressureoftheinjectedwater.Beforeinjectionintothemoldcavity,thewaterwasstoredinatankwithatemperatureregulatorfor30mintosustainanisothermalwatertemperature.

2.3.Moldingmachineandmolds

Water-assistedinjectionmoldingexperimentswereconductedonan80-tonconventionalinjection-moldingmachinewithahighestinjectionrateof109cm3/s.Aplatecavitywithatrapezoidalwaterchannelacrossthecenterwasusedinthisstudy.Fig.2showsthedimensionsof

thecavity.Thetemperatureofthemoldwasregulatedbyawater-circulatingmoldtemperaturecontrolunit.Variousprocessingvariableswereexaminedintermsoftheirinfluenceonthelengthofwaterpenetrationinwaterchannelsofmoldedparts:

melttemperature,moldtemperature,melt

fillpressure,watertemperatureandpressure,waterinjectiondelaytimeandholdtime,andshortshotsizeofthepolymermelt.Table4liststheseprocessingvariablesaswellasthevaluesusedintheexperiments.

2.4.Gasinjectionunit

Inordertomakeacomparisonofwaterandgas-assistedinjectionmoldedparts,acommerciallyavailablegasinjectionunit（GasInjectionPPC-1000）wasusedforthegasassistedinjectionmoldingexperiments.DetailsofthegasinjectionunitsetupcanbefoundintheRefs.[11–15].Theprocessingconditionsusedforgas-assistedinjectionmoldingwerethesameasthatofwater-assistedinjectionmolding（termsinboldinTable4）,withtheexceptionofgastemperaturewhichwassetat25_C.

2.5.XRD

Inordertoanalyzethecrystalstructurewithinthewater-assistedinjection-moldedparts,wide-angleX-raydiffraction（XRD）with2DdetectoranalysesintransmissionmodewereperformedwithCuKaradiationat40kVand40mA.Morespecifically,themeasurementswereperformedonthemold-sideandwater-sidelayersofthewater-assistedinjection-moldedparts,withthe2hanglerangingfrom7_to40_.Thesamplesrequiredfortheseanalysesweretakenfromthecenterportionofthesemoldedparts.ToobtainthedesiredthicknessfortheXRDsamples,theexcesswasremovedbypolishingthe

Table3

Characteristicsoftheglass–fiberreinforcedPBTcomposite

Property

ASTMPBT15%G.F.PBT

Yieldstrength（kg/cm2）

D-6386001000

Bendingstress（kg/cm2）

D-5709001500

Hardness（R-scale）

D-785119120

Heatdistortiontemperature（_C）

（18.6kg/cm2）

D-64860200

Meltf