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Injection Molding
The basic concept of injection molding revolves around the ability of a thermoplastic material to be softened by heat and to harden when cooled .In most operations ,granular material (the plastic resin) is fed into one end of the cylinder (usually through a feeding device known as a hopper ),heated, and softened(plasticized or plasticated),forced out the other end of the cylinder,while it is still in the form of a melt,through a nozzle into a relatively cool mold held closed under pressure.Here,the melt cools and hardens until fully set-up.The mold is then opened,the piece ejected,and the sequence repeated.
Thus,the significant elements of an injection molding machine become :1)the way in which the melt is plasticized (softened) and forced into the mold (called the injection unit); 2)the system for opening the mold and closing it under pressure (called the clamping unit);3)the type of mold used;4)the machine controls.
The part of an injection-molding machine,which converts a plastic material from a sold phase to homogeneous seni-liguid phase by raising its temperature .This unit maintains the material at a present temperature and force it through the injection unit nozzle into a mold .The plunger is a combination of the injection and plasticizing device in which a heating chamber is mounted between the plunger and mold. This chamber heats the plastic material by conduction .The plunger,on each storke; pushes unmelted plastic material into the chamber ,which in turn forces plastic melt at the front of the chamber out through the nozzle
The part of an injection molding machine in which the mold is mounted,and which provides the motion and force to open and close the mold and to hold the mold close with force during injection .This unit can also provide other features necessary for the effective functioning of the molding operation .Moving plate is the member of the clamping unit,which is moved toward a stationary member.the moving section of the mold is bolted to this moving plate .This member usually includes the ejector holes and moldmounting pattern of blot holes or “T” slots .Stationary plate is the fixed member of the clamping unit on which the stationary section of the mold is bolted .This member usually includes a mold-mounting pattern of boles or “T” slots.Tie rods are member of the clamping force actuating mechanism that serve as the tension member of the clamp when it is holding the mold closed.They also serve as a gutde member for the movable plate .Ejector is a provision in the clamping unit that actuates a mechanism within the mold to eject the molded part(s) from the mold .The ejection actuating force may be applied hydraulically or pneumatically by a cylinder(s) attached to the moving plate ,or mechanically by the opening storke of the moving plate.
Methods of melting and injecting the plastic differ from one machine to another and are constantly being improred .couventional machines use a cylinder and piston to do both jobs .This method simplifies machine construction but makes control of injection temperatures and pressures an inherently difficult problem .Other machines use a plastcating extruder to melt the plastic and piston to inject it while some hare been designed to use a screw for both jobs :Nowadays,sixty percent of the machines use a reciprocating screw,35% a plunger (concentrated in the smaller machine size),and 5%a screw pot.
Many of the problems connected with in jection molding arises because the densities of polymers change so markedly with temperature and pressure.Athigh temperatures,the density of a polymer is considerably cower than at room temperature,provided the pressure is the same.Therefore,if modls were filled at atmospheric pressure, “shrinkage”would make the molding deviate form the shape of the mold.
To compensate for this poor effect, molds are filled at high pressure.The pressure compresses the polymer and allows more materials to flow into the mold,shrinkage is reduced and better quality moldings are produced.Cludes a mold-mounting pattern of bolt holes or “T” slots.Tie rods are members of the clamping force actuating machanism that serve as the tension members of clamp when it is holding the mold closed.Ejector is a provision in the claming unit that actuates a mechanism within the mold to eject themolded part(s) form the mold.The ejection actuating force may be applied hydraulically or pneumatically by a cylinder(s) attached to the moving plate,or mechanically by the opening stroke of the moving plate.
The function of a mold is twofold :imparting the desired shape to the plasticized polymer and cooling the injection molded part.It is basically made up of two sets of components :the cavities and cores and the base in which the cavities and cores are mounted. The mold ,which contains one or more cavities,consists of two basic parts :(1) a stationary molds half one the side where the plastic is injected,(2)Amoving half on the closing or ejector side of the machine. The separation between the two mold halves is called the parting line.In some cases the cavity is partly in the stationary and partly in the moving section.The size and weight of the molded parts limit the number of cavities in the mold and also determine the machinery capacity required.The mold components and their functions are as following :
(1) Mold Base-Hold cavity(cavities) in fixed ,correct position relative to machine nozzle .
(2) Guide Pins-Maintain Proper alignment of entry into mold intrior .
(3) Sprue Bushing(sprue)-Provide means of entry into mold interior .
(4) Runners-Conrey molten plastic from sprue to cavities .
(5) Gates-Control flow into cavities.
(6) Cavity(female) and Force(male)-Contorl the size,shape and surface of mold article.
(7) Water Channels-Control the temperature of mold surfaces to chill plastic to rigid state.
(8) Side (actuated by came,gears or hydraulic cylinders)-Form side holes,slots,undercuts and threaded sections.
注射成型
注射成型的基本概念是使熱塑性材料在受熱時(shí)熔融，冷卻時(shí)硬化，在大部分加工中，粒狀材料（即塑料樹脂）從料筒的一端（通常通過一個(gè)叫做“料斗”的進(jìn)料裝置）送進(jìn)，受熱并熔融（即塑化或增塑），然后當(dāng)材料還是溶體時(shí)，通過一個(gè)噴嘴從料筒的另一端擠到一個(gè)相對(duì)較冷的壓和封閉的模子里。溶里在模腔里冷卻，硬化，直至完全固化。然后將模子打開，推出工件，并重復(fù)以上工序。
因此，注塑成型機(jī)的重要組成部分是：1）將溶體塑化并擠進(jìn)進(jìn)模腔（稱做“注塑單元”）；2）開模和加壓合模統(tǒng)稱為“合模系統(tǒng)”）；3）使用上下模具；4）機(jī)器控制。
注射裝置通過家熱將固態(tài)塑料轉(zhuǎn)化成均勻的半液態(tài)塑體。它保持塑料當(dāng)前溫度并加壓使之通過噴嘴進(jìn)入模具。柱塞集塑化和注射為一體。在柱塞與模具之間沒有加熱室，幾熱室通過熱傳導(dǎo)加熱塑料。每次行程，柱塞將待熔化塑料推入加熱室，依次將熔化的塑料擠到加熱室前端并通過噴嘴注射出去（注射到模具中去）
合模裝置是注射成型機(jī)的一個(gè)部分，該裝置上安裝有模具。合模裝置提供運(yùn)動(dòng)和動(dòng)力以開模幾合模，并且在注射過程中提供壓力保持模具緊閉同時(shí)，該裝置還提供成型工藝有效運(yùn)動(dòng)所必需的其它特性。動(dòng)模板是合模裝置的一部分，在合模時(shí)向定模板移動(dòng)。型腔的運(yùn)動(dòng)部分用螺栓固定在動(dòng)模板上，動(dòng)模板通常動(dòng)模板通常含頂出桿，螺栓空或T型槽的安裝模具的型板。拉桿是合模力驅(qū)動(dòng)機(jī)構(gòu)中的一個(gè)構(gòu)件，當(dāng)夾具保持動(dòng)模閉合時(shí)，拉桿起夾具的拉力構(gòu)件的作用。頂出器是合模裝置的一個(gè)部件，它驅(qū)動(dòng)模具中的執(zhí)行機(jī)構(gòu)從模具中頂出成型制品。頂出力可由液壓或氣動(dòng)裝置通過裝在動(dòng)模板上的油（氣）缸來施加，或者通過動(dòng)模板開模行程來施加。
塑料熔融與塑料注射的方式隨機(jī)器的不同而不同,并不斷得以改善。常規(guī)的機(jī)器采用一個(gè)料筒和一個(gè)活塞來進(jìn)行熔融和注射。雖然這種方式簡(jiǎn)化了機(jī)器的結(jié)構(gòu)，但卻使注射的溫度和壓力控制成了一個(gè)固有的難題。有的機(jī)器用一個(gè)塑化擠壓器來熔化塑料，用一個(gè)活塞進(jìn)行注射。另外，還有一些已設(shè)計(jì)出來的機(jī)器采用一個(gè)螺桿來完成熔融和注射。目前，百分之六十的機(jī)器采用往復(fù)式螺桿，百分之三十五的機(jī)器采用柱塞；百分之六十的機(jī)器采用螺罐。
聚合物的密度在不同的溫度和壓力下變化很大，并由此產(chǎn)生了注射成型的許多問題。在壓力不變的情況下，在高溫下的聚合物的密度比在低溫下的要低得多，因此，如果在大氣壓下向模子充模，成型件回因“收縮”而與模子的形狀形成偏差。
為了補(bǔ)償這一偏差，模子充料在高壓下進(jìn)行。高壓壓縮了聚合物并能使更多的材料流入模腔。這樣，“收縮”就減少了，生產(chǎn)的成型件的質(zhì)量就得以提高。
模具的功能有兩方面：一是使可塑聚合物按希望的形狀成型，而是使其冷卻定型。注射模具基本由兩套部件組成：型腔和型芯，安裝型腔和型芯的模板。具有一個(gè)型腔或多個(gè)型腔的模具由兩個(gè)基本部分組成：（1）注射塑料一側(cè)的定模（2）閉合及頂出機(jī)器一側(cè)的動(dòng)模。兩哈夫?qū)﹂_的定模、動(dòng)模分界線叫做分型面。有時(shí)，型腔一部分在定模中，一部分在動(dòng)模中。塑料成型制品的尺寸和重量限制了一副模具中型腔的數(shù)目，并且取得了所需的注射機(jī)床生產(chǎn)能力。模具的零部件及其功能如下：
（1） 模板—保持型腔相對(duì)注射機(jī)床的噴嘴具有固定、正確的位置。
（2） 導(dǎo)柱—保持定模、動(dòng)模哈夫在合模時(shí)具有正確的相對(duì)位置。
（3） 主流道襯套—提供熔融塑料流入模具內(nèi)部的通道。
（4） 流道—將容忍塑料從澆注口傳送到各型腔。
（5） 澆口—控制熔融塑料流入型腔。
（6） 凹模（陰模）和凸模（陽(yáng)模）—控制成型塑料制品的尺寸、形狀及表面紋理。
（7） 冷卻水道—控制模具表面溫度，使塑料制品冷固成型。
（8） 側(cè)板（由凸輪、齒輪或液壓油缸驅(qū)動(dòng)）—形成制品的側(cè)孔、側(cè)槽、側(cè)凹螺紋部分。
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