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PAGE\*MERGEFORMAT#TheInjectionMolding1、TheinjectionmoldingInjectionmoldingisprincipallyusedfortheproductionofthethermoplasticparts,althoughsomeprogresshasbeenmadeindevelopingamethodforinjectionmoldingsomethermosettingmaterials.Theproblemofinjectionamethodplasticintoamoldcavityfromareservoirofmeltedmaterialhasbeenextremelydifficulttosolveforthermosettingplasticwhichcureandhardenundersuchconditionswithinafewminutes.Theprincipleofinjectionmoldingisquitesimilartothatofdie-casting.Theprocessconsistsoffeedingaplasticcompoundinpoweredorgranularformfromahopperthroughmeteringandmeltingstagesandtheninjectingitintoamold.Afterabriefcoolingperiod,themoldisopenedandthesolidifiedpartejected.Injection-moldingmachineoperation.Theadvantageofinjectionmoldingare:(i)ahighmoldingspeedadapterformassproductionispossible；(ii)thereisawidechoiceofthermoplasticmaterialsprovidingavarietyofusefulproperties；(iii)itispossibletomoldthreads,undercuts,sideholes,andlargethinsection.2、Theinjection-moldingmachineSeveralmethodsareusedtoforceorinjectthemeltedplasticintothemold.Themostcommonlyusedsysteminthelargermachinesisthein-linereciprocatingscrew,asshowninFigure2-1.Thescrewactsasacombinationinjectionandplasticizingunit.Astheplasticisfedtotherotatingscrew,itpassesthroughthreezonesasshown:feed,compression,andmetering.Afterthefeedzone,thescrew-flightdepthisgraduallyreduced,forcetheplastictocompress.Theworkisconvertedtoheatbyconductionfromthebarrelsurface.Asthechamberinfrontofthescrewbecomesfilled,itforcesthescrewback,trippingalimitswitchthatactivatesahydrauliccylinderthatforcesthescrewforwardandinjectsthefluidplasticintotheclosedmold.Anantiflowbackvalvepresentsplasticunderpressurefromescapingbackintothescrewflight.Theclampingforcethatamachineiscapableofexertingispartofthesizedesignationandismeasuredintons.Arule-of-thumbcanbeusedtodeterminethetonnagerequiredforaparticularjob.Itisbasedontwotonsofclampforcepersquareinchofprojectedarea.Iftheflowpatternisdifficultandthepartsarethin,thismayhavetogotothreeorfourtons.Manyreciprocating-screwmachinesarecapableofhandingthermosettingplasticmaterials.Previouslythesematerialswerehandledbycompressionortransfermolding.Thermosettingmaterialscureorpolymerizeinthemoldandareejectedhotintherangeof375°C〜410°C.Thermosettingpartsmustbeallowedtocoolinthemoldinorderorremovethemwithoutdistortion.Thusthermosettingcyclescanbefaster.Ofcoursethemoldmustbeheatedratherthanchilled,aswiththermoplastics.3、BasicUnderfeedMouldAsimplemouldofthistypeisshowninFigure3-1,andthedescriptionofthedesignandtheopeningsequencefollows.Themouldconsistsofthreebasicparts,namely：themovinghalf,thefloatingcavityplateandthefeedplaterespectively.ThemovinghalfconsistsofThemovingmouldplateassembly,supportblock,backingplate,ejectorassemblyandthepinejectionsystem.Thusthemovinghalfinthisdesignisidenticalwiththemovinghalfofbasicmoulds.Thefloatingcavityplate,whichmaybeoftheintegerorinsert-bolsterdesign,islocatedonsubstantialguidepillars(notshown)fittedinthefeedplate.Theseguidepillarsmustbeofsufficientlengthtosupportthefloatingcavityplateoveritsfullmovementandstillprojecttoperformthefunctionofalignmentbetweenthecavityandcorewhenthemouldisbeingclosed.Guidebushesarefittedintothemovingmouldplateandthefloatingcavityplaterespectively.Themaximummovementofthefloatingcavityplateiscontrolledbystoporsimilardevice.Themovingmouldplateissuitablyboredtoprovideaclearanceforthestopboltassembly.Thestopboltsmustbelongenoughtoprovidesufficientspacebetweenthefeedplateandthefloatingcavityplateforeasyremovalofthefeedsystem.Theminimumspaceprovideforshouldbe65mmjustsufficientforanoperatortoremovethefeedsystembyhandifnecessary.Thedesireoperatingsequenceisforthefirstdaylighttooccurbetweenthefloatingcavityplate.Thisensuresthesprueispulledfromthespruebushimmediatelythemouldisopened.Toachievethissequence,springsmaybeincorporatedbetweenthefeedplateandthefloatingcavityplate.Thespringsshouldbestrongenoughtogiveaninitialimpetustothefloatingcavityplatetoensureitmovesawaywiththemovinghalf.Itisnormalpracticetomountthespringsontheguidepillars(Figure3-2)andaccommodatetheminsuitablepocketinthecavityplate.Themajorpartofthefeedsystem(runnerandsprue)isaccommodatedinthefeedplatetofacilitateautomaticoperation,therunnershouldbeofatrapezoidalformsothatonceitispulledfromthefeedplateiscaneasilybeextracted.Notethatifaroundrunnerisused,halftherunnerisformedinthefloatingcavityplate,whereitwouldremain,andbepreventedfromfallingorbeingwipedclearwhenthemouldisopened.Nowthatwehaveconsideredthemouldassemblyinthesomedetail,welookatthecycleofoperationforthistypeofmould.Theimpressionsarefilledviathefeedsystem(Figure3-1(a))andafterasuitabledwellperiod,themachineplatenscommencetoopen.Aforceisimmediatelyexertedbythecompressionsprings,whichcausethefloatingcavityplatetomoveawaywiththemovinghalfaspreviouslydiscussed.Thesprueispulledfromthespruebushbythespruepuller.Afterthefloatingcavityplatehasmovedapredetermineddistance,itisarrestedbythestopbolts.Themovinghalfcontinuestomovebackandthemoldings,havingshrunkontothecores,arewithdrawnfromthecavities.Thepingatebreaksatitsjunctionwiththerunner(Figure3-1(b)).Thespruepuller,beingattachedtothemovinghalf,ispulledthroughthefloatingcavityplateandtherebyreleasethefeedsystemwhichisthenfreetofallbetweenthefloatingcavityplateandthefeedplate.Themovinghalfcontinuestomovebackuntiltheejectorsystemisoperatedandthemoldingsareejected(Figure3-1(c)).Whenthemouldisclosed,therespectiveplatesarereturnedtotheirmoldingpositionandthecycleisrepeated.4、FeedSystemItisnecessarytoprovideaflow-wayintheinjectionmouldtoconnectthenozzle(oftheinjectionmachine)toeachimpression.Thisflow-wayistermedthefeedsystem.Normallythefeedsystemcomprisesasprue,runnerandgate.Thesetermsapplyequallytotheflow-wayitself,andtothemoldedmaterialwhichisremovefromtheflow-wayitselfintheprocessofextractedthemolding.Atypicalfeedsystemforafour-impression,twoplate-typemouldisshowninFigure4-1.Itisseenthatthematerialpassesthroughthesprue,mainrunner,branchrunnerandgatebeforeenteringtheimpression.Asthetemperatureofmoltenplasticisloweredwhichgoingthroughthesprueandrunner,theviscositywillrise;however,theviscosityisloweredbyshearheatgeneratedwhengoingthroughthegatetofillthecavity.Itisdesirabletokeepthedistancethatthematerialhastotraveldowntoaminimumtoreducepressureandheatlosses.Itisforthisreasonthatcarefulconsiderationmustbegiventotheimpressionlayoutgate'sdesign.4.1.SprueAsprueisachannelthroughwhichtotransfermoltenplasticinjectedfromthenozzleoftheinjectorintothemold.Itisapartofspruebush,whichisaseparatepartfromthemold.RunnerArunnerisachannelthatguidesmoltenplasticintothecavityofamold.GateAgateisanentrancethroughwhichmoltenplasticentersthecavity.Thegatehasthefollowingfunction：restrictstheflowandthedirectionofmoltenplastic；simplifiescuttingofarunnerandmoldingstosimplifyfinishingofparts；quicklycoolsandsolidifiestoavoidbackflowaftermoltenplastichasfilledupinthecavity.ColdslugwellThepurposeofthecoldslugwell,shownoppositethesprue,istheoreticallytoreceivethematerialthathaschilledatthefrontofnozzleduringthecoolingandejectionphase.Perhapsofgreaterimportanceisthefactthatitprovidespositionmeanswherebythespruebushforejectionpurposes.Thesprue,therunnerandthegatewillbediscardedafterapartiscomplete.However,therunnerandthegateareimportantitemsthataffectthequalityorthecostofparts.5、EjectionAmoldingisformedinmouldbyinjectingaplasticmelt,underpressure,intoanimpressionviaafeedsystem.Itmustthereforeberemovedmanually.Furthermore,allthermoplasticmaterialscontractastheysolidify,whichmeansthatthemoldingwillshrinkontothecorewhichformsit.Thisshrinkagemakesthemoldingdifficulttoremove.Facilitiesareprovidedontheinjectionmachineforautomaticactuationofanejectorsystem,andthisissituatedbehindthemovingplaten.Becauseofthis,themould'sejectorsystemwillbemosteffectivelyoperatedifplacedinthemovinghalfofthemould,i.e.thehalfattachedtothemovingplaten.Wehavestatedpreviouslythatweneedtoejectthemoldingfromthecoreanditthereforefollowsthatthecore,too,willmostsatisfactorilybelocatedinthemovinghalf.Theejectorsysteminamouldwillbediscussedunderthreeheadings,namely：(i)theejectorgrid;(ii)theejectorplateassembly;and(iii)themethodofejection.5.1、EjectorgridTheejectorgrid(Figure5-1)isthatpartofthemouldwhichsupportsthemouldplateandprovidesaspaceintowhichtheejectorplateassemblycanbefittedandoperated.Thegridnormallyconsistsofabackplateontowhichismountedanumberofconvenientlyshaped“supportblocks”.Theejectorplateassemblyisthatpartofthemouldtowhichtheejectorelementisattached.Theassemblyiscontainedinapocket,formedbytheejectorgrid,directlybehindthemouldplate.Theassembly(Figure5-2)consistsofanejectorplate,aretainingplateandanejectorrod.Oneendofthislattermemberisthreadedanditisscrewedintotheejectorplate.Inthisparticulardesigntheejectorrodfunctionnotonlyasanactuatingmemberbutalsoasamethodofguidingtheassembly.Notethattheparallelportionoftheejectorrodpassesthroughanejectorrodbushfittedinthebackplateofthemould.5.2、EjectiontechniquesWhenamoldingcools,itcontractsbyanamountdependingonthematerialbeingprocessed.Foramoldingwhichhasnointernalform,forexample,asolidrectangularblock,themoldingwillshrinkawayfromthecavitywalls,therebypermittingasimpleejectiontechniquetobeadopted.However,whenthemoldinghasinternalform,themolding,asitcools,willshrinkontothecoreandsomepositivetypeofejectionisnecessary.Thedesignerhasseveralejectiontechniquesfromwhichtochoose，butingeneral,thechoicewillberestricteddependingupontheshapeofthemolding.Thebasicejectiontechniquesareasfollows：(i)pinejection(ii)sleeveejection(iii)stripperplateejectionand(W)airejection.Figure2-1aFigure2-1b0AAFigure3-1Figure3-2Figure4-1bn10PAGE\*MERGEFORMAT#Figure5-1Figure5-2注塑模1、注塑模尽管成型某些热固性材料的方法取得了一定的进步，但注塑模主要（还是）用来生产热塑性塑件。（这主要是因为）热固性塑料熔体在很短的时间内就固化和硬化，在从料斗想模具型腔注入热固性塑料熔体的过程中，也会出现这种情况，这个问题一直非常难解决，注塑成型原理和铸造十分相似。注塑成型的工艺包括：首先把料斗中的粉状或粒状的塑料混合物依次输送发哦计量区和溶化区，然后再注射到模具的型腔中，经过短时冷却后，开模，推出成型塑件，注塑机分为手动、半自动及全自动操作。注塑模具有以下优点：（i）较高的成型速度使大批量生产成为可能；（ii）为成型具有不同使用性能的热塑性材料提供了较宽的选择；（iii）可以成型带有螺纹的塑件、侧向凹陷的塑件、带有侧孔的塑件以及较大的薄壁件。2、注塑机熔融塑料进入模具中通常有几种方式。在大型注塑机上常带采用螺杆式的注入方式，如图2-1所示。螺杆同时具有注射和塑化的功能。树脂原料进入旋转的螺杆时，要经过图示的三个区域：喂入区、压实区和计量区。经过喂入区后，为压实树脂原料，螺杆螺旋部分的深度逐渐降低，同时传递树脂原料间因剪切作用而产生的热量，使原料呈半流动状态。在计量区，螺缸表面的加热装置对熔体进一步加热。当熔体充满螺杆前部区域时，螺杆在熔体压力的作用下后退，触动限位开关使液压缸工作，在液压力的作用下推动螺杆向前运动，将熔融塑料注射到闭合的模具型腔中。防倒流阀能够阻止受压熔体倒流进螺杆的螺旋区。注塑机的锁模系统做提供的锁模力由（塑件在分型面的投影）尺寸决定，锁模力以吨位单位。通常靠经验来决定塑件所需要的锁模力总吨数，一般在塑件投影面积上每平方英寸需要作用两吨锁模力。如果熔体流动困难或塑件较薄，锁模力应提高到三到四吨。许多往复螺杆式注塑机能生产热固性塑料。以前，热固性塑料由挤出模具或传递模具生产。热固性塑料熔体在模具内固化或发生聚合反应，并在温度357°C〜410°C范围内推出。热塑性塑料熔体必须在模具内冷却成型，以保证推出时不发生变形，这种热固性循环速度很快。当然，生产热塑性塑料时，模具必须被加热，而不是冷却。3、模具基本结构这种简单的模具结构如图3-1所示，模具设计和开模顺序将在以下的篇幅中论述。模具由三个基本部分组成，分别是动模部分、浮动型腔板和定模板。动模部分包括动模板组件、支撑块、支撑板，推出机构和顶杆推出系统。这样设计的动模部分与最基本的模具结构中的动模部分相同。浮动型腔板通过导柱（图中没有示出）固定在定模板上，设计形式可以是整体式，也可以是镶拼式。导柱必须有足够的长度，（以便磨具工作时）支撑浮动型腔板完成开合动作，并在模具合模时完成对型腔和型芯的找正作用。导套分别安装在动模板和浮动型腔板上。浮动型腔板的最大运动距离由限位钉或类似装置控制。在动模板的适当位置钻孔，以便安装限位钉。限位钉必须有足够的长度为定模板和浮动型腔板之间提供足够的空间，从而使浇注系统的凝料顺利脱落。如果必须使用手动方式取出凝料，开模时取出凝料所需的最小距离应为65mm。模具的开模顺序是：浮动型腔板和定模板先分开。确保模具打开时浇注系统凝料立即从浇口套脱出。为实现这样的顺序，定模板和浮动型腔板之间装有弹簧。弹簧的弹力必须足够大，确保在初始推力的作用下浮动型腔板跟随动模部分一起运动。弹簧套在导柱上，一同装在浮动型腔板上相应的弹簧座处（见图5-2），这种形式在弹簧装配中十分常见。浇注系统的主要部分（分流道和主流道）开设在定模板上，为便于自动操作，分流道应采用梯形截面，以便于脱出浇注系统凝料。注意，如果采用圆形分流道，分流道的一半开设在浮动型腔板上，开模时，分流道可能留在浮动型腔板，将阻碍凝料的脱落或去除。我们已经详细讨论模具装配，再看这类模具的生产循环过程。熔体经过浇注系统充满型腔后（见图3-1（a）），经过适当的保压过程，注塑机带动模具开模。压缩弹簧的弹力立即释放出来，推动浮动型腔板和动模部分一起移动，如同前面论述的一样。主流道凝料被拉料杆从浇口套中拉出。浮动型腔板移动预定距离后，被限位钉限位。动模部分继续后移，塑件由于收缩作用包紧在型芯上从型腔中脱出（跟随动模一同移动）。点浇口在结合处拉断，与分流道脱离（见图3-1（b））。安装在动模部分的拉料杆脱离浮动型腔板，浇注系统凝料由此脱下，然后在浮动型腔板和定模板之间自由落下。动模部分继续后退，直到推出机构开始运动，推出塑件（见图3-1（c））。和模时，模具的各模板回到成型位置，重复下一个注塑循环。4、浇注系统在注塑模中，连接（注塑机）喷嘴和各个型腔的流动通道是十分必要的，这种进料通道称为浇注系统。通常，浇注系统由主流道、分流道和浇口组成。这些术语应用在相应的进料通道本身，以及取出塑料时从进料通道中一同取出的浇注系统凝料。图4-1所示为典型的两版式四腔浇注系统。从图中可以看出原料通过主流道、第一分流道、第二分流道和浇口注入道型腔中。熔融塑料通过主流道和分流道时温度降低而使熔体黏度升高，然而，熔体通过浇口填充型腔时，由于剪切作用产生的热量又使黏度降低。浇注系统要保持适当的长度，使熔体的压力减少和热量损失降到最低。因此，设计时必须充分考虑型腔布局和浇口形式。4.1、主流道主流道是将熔融塑料从注塑机喷嘴传递到模具型腔的通道。主流道是浇口套的一部分，浇口套是独立于模具的单独零件。4.2、分流道分流道是引导熔体塑料进入模具型腔的通道。4.3、浇口浇口是熔融塑料进入型腔的入口。浇口有以下作用：约束熔融塑料流动，应道熔融塑料的流动方向；使分流道和塑件末端易于分离；快速冷却固化，防止熔融塑料充满型腔后倒流。4.4、冷料井冷料井正对着主流道。理论上，冷料井的作用是用来储存在（塑件）冷却和推出过程中注塑机喷嘴处所形成的熔体前锋冷料。也许冷料井更重要的作用是（开模时）帮助浇道凝料推出浇口套。塑料成型后，主流道、分流道和浇口部分凝料将被废弃。但是，分流道和浇口对塑件质量和成本有重要影响。5、推出机构塑料熔体在压力作用下进过浇注系统进入模具型腔形成塑件，因此（塑件成型后）必须手动取出。此外，所有热塑性熔体凝固时都会收缩，这意味这着塑件将包紧在型芯上。收缩作用使塑件脱模（存在）困难。注塑机为推出系统提高了自动推出力，其推出动力装置安装在注塑机移动板的后面。因此，模具的推出系统安装在动模部分将获得最大效率，例如，这部分（可以）安装在移动板上。我们在前面论述了（开模时）需要从型芯上推下塑件，因此，塑件必须跟随型芯（一同移动），推出系统装在动模部分最为合适。模具的推出系统将在以下三个标题中讨论，即：（i）推出支架；（ii）推板装配机构；（iii）推出方式。5.1、推出支架推出支架（见图5-1）是模具的一部分，用来支撑模板，为推板的装配和运动提供空间。推出支架通常由定模座板和几块支撑块组成，支撑块安装在动模座板上，且便于加工成型。推板装配机构也是模具的一部分，用来安装推出零件。推板装配机构安装在推板支撑架内，直接装在模板后面。装配机构（见图5-2）由推板、推板固定板和注塑机顶出杆组成。注塑机顶出杆一端带有螺纹，通过螺纹紧固在推板上。在这种结构中，顶出杆的作用不仅是推出零件，而且为推板装配机构起导向作用。注意，顶出杆上直接相等的部分通过顶出杆套筒，套筒安装在模具座板上。5.2、推出方式塑件冷却后，其收缩状态决定于塑料熔体的成型过程。对于内部不带有其他形状的塑件，例如，实芯矩形件，塑件的收缩作用使其脱离型腔壁，因此，可以采用简单的脱模方式（推出塑件）。然而，若塑件具有内部形状，冷却收缩时塑件将包紧在型芯上，因此，有必要采用一些适当的方法推出塑件。设计推出机构时有几种方式可供选择，但是，总的来说，选择哪种推出方式取决于塑件的形状。基本的推出方式有以下几种：（i）推杆脱模；（ii）推管脱模;（iii）推件板脱模；（iv）气动脱模。