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The Injection Molding1、The injection moldingInjection molding is principally used for the production of the thermoplastic parts,although some progress has been made in developing a method for injection molding some thermosetting materials.The problem of injection a method plastic into a mold cavity from a reservoir of melted material has been extremely difficult to solve for thermosetting plastic which cure and harden under such conditions within a few minutes.The principle of injection molding is quite similar to that of die-casting.The process consists of feeding a plastic compound in powered or granular form from a hopper through metering and melting stages and then injecting it into a mold.After a brief cooling period,the mold is opened and the solidified part ejected.Injection-molding machine operation.The advantage of injection molding are：（）a high molding speed adapter for mass production is possible；（）there is a wide choice of thermoplastic materials providing a variety of useful properties；（）it is possible to mold threads,undercuts,side holes,and large thin section.2、The injection-molding machineSeveral methods are used to force or inject the melted plastic into the mold.The most commonly used system in the larger machines is the in-line reciprocating screw,as shown in Figure 2-1.The screw acts as a combination injection and plasticizing unit.As the plastic is fed to the rotating screw,it passes through three zones as shown：feed,compression,and metering.After the feed zone,the screw-flight depth is gradually reduced,force the plastic to compress.The work is converted to heat by conduction from the barrel surface.As the chamber in front of the screw becomes filled,it forces the screw back,tripping a limit switch that activates a hydraulic cylinder that forces the screw forward and injects the fluid plastic into the closed mold.An antiflowback valve presents plastic under pressure from escaping back into the screw flight.The clamping force that a machine is capable of exerting is part of the size designation and is measured in tons.A rule-of-thumb can be used to determine the tonnage required for a particular job.It is based on two tons of clamp force per square inch of projected area.If the flow pattern is difficult and the parts are thin,this may have to go to three or four tons.Many reciprocating-screw machines are capable of handing thermosetting plastic materials.Previously these materials were handled by compression or transfer molding.Thermosetting materials cure or polymerize in the mold and are ejected hot in the range of 375C410C.Thermosetting parts must be allowed to cool in the mold in order or remove them without distortion. Thus thermosetting cycles can be faster.Of course the mold must be heated rather than chilled,as with thermoplastics.3、Basic Underfeed MouldA simple mould of this type is shown in Figure3-1,and the description of the design and the opening sequence follows.The mould consists of three basic parts,namely：the moving half,the floating cavity plate and the feed plate respectively.The moving half consists of The moving mould plate assembly,support block,backing plate,ejector assembly and the pin ejection system.Thus the moving half in this design is identical with the moving half of basic moulds.The floating cavity plate,which may be of the integer or insert-bolster design,is located on substantial guide pillars（not shown）fitted in the feed plate.These guide pillars must be of sufficient length to support the floating cavity plate over its full movement and still project to perform the function of alignment between the cavity and core when the mould is being closed.Guide bushes are fitted into the moving mould plate and the floating cavity plate respectively.The maximum movement of the floating cavity plate is controlled by stop or similar device.The moving mould plate is suitably bored to provide a clearance for the stop bolt assembly.The stop bolts must be long enough to provide sufficient space between the feed plate and the floating cavity plate for easy removal of the feed system.The minimum space provide for should be 65mm just sufficient for an operator to remove the feed system by hand if necessary.The desire operating sequence is for the first daylight to occur between the floating cavity plate.This ensures the sprue is pulled from the sprue bush immediately the mould is opened.To achieve this sequence,springs may be incorporated between the feed plate and the floating cavity plate.The springs should be strong enough to give an initial impetus to the floating cavity plate to ensure it moves away with the moving half.It is normal practice to mount the springs on the guide pillars（Figure3-2）and accommodate them in suitable pocket in the cavity plate.The major part of the feed system（runner and sprue）is accommodated in the feed plate to facilitate automatic operation,the runner should be of a trapezoidal form so that once it is pulled from the feed plate is can easily be extracted.Note that if a round runner is used,half the runner is formed in the floating cavity plate,where it would remain,and be prevented from falling or being wiped clear when the mould is opened.Now that we have considered the mould assembly in the some detail,we look at the cycle of operation for this type of mould.The impressions are filled via the feed system（Figure3-1（a）and after a suitable dwell period,the machine platens commence to open.A force is immediately exerted by the compression springs,which cause the floating cavity plate to move away with the moving half as previously discussed.The sprue is pulled from the sprue bush by the sprue puller.After the floating cavity plate has moved a predetermined distance,it is arrested by the stop bolts.The moving half continues to move back and the moldings,having shrunk on to the cores,are withdrawn from the cavities.The pin gate breaks at its junction with the runner（Figure3-1（b）.The sprue puller,being attached to the moving half,is pulled through the floating cavity plate and thereby release the feed system which is then free to fall between the floating cavity plate and the feed plate.The moving half continues to move back until the ejector system is operated and the moldings are ejected（Figure3-1（c）.When the mould is closed,the respective plates are returned to their molding position and the cycle is repeated.4、Feed SystemIt is necessary to provide a flow-way in the injection mould to connect the nozzle（of the injection machine）to each impression.This flow-way is termed the feed system.Normally the feed system comprises a sprue,runner and gate.These terms apply equally to the flow-way itself,and to the molded material which is remove from the flow-way itself in the process of extracted the molding.A typical feed system for a four-impression,two plate-type mould is shown in Figure4-1.It is seen that the material passes through the sprue,main runner,branch runner and gate before entering the impression.As the temperature of molten plastic is lowered which going through the sprue and runner,the viscosity will rise;however,the viscosity is lowered by shear heat generated when going through the gate to fill the cavity.It is desirable to keep the distance that the material has to travel down to a minimum to reduce pressure and heat losses.It is for this reason that careful consideration must be given to the impression layout gates design.4.1.SprueA sprue is a channel through which to transfer molten plastic injected from the nozzle of the injector into the mold.It is a part of sprue bush,which is a separate part from the mold.4.2.RunnerA runner is a channel that guides molten plastic into the cavity of a mold.4.3.GateA gate is an entrance through which molten plastic enters the cavity.The gate has the following function：restricts the flow and the direction of molten plastic；simplifies cutting of a runner and moldings to simplify finishing of parts；quickly cools and solidifies to avoid backflow after molten plastic has filled up in the cavity.4.4.Cold slug wellThe purpose of the cold slug well,shown opposite the sprue,is theoretically to receive the material that has chilled at the front of nozzle during the cooling and ejection phase.Perhaps of greater importance is the fact that it provides position means whereby the sprue bush for ejection purposes.The sprue,the runner and the gate will be discarded after a part is complete.However,the runner and the gate are important items that affect the quality or the cost of parts.5、Ejection A molding is formed in mould by injecting a plastic melt,under pressure,into an impression via a feed system.It must therefore be removed manually.Furthermore,all thermoplastic materials contract as they solidify,which means that the molding will shrink on to the core which forms it.This shrinkage makes the molding difficult to remove.Facilities are provided on the injection machine for automatic actuation of an ejector system,and this is situated behind the moving platen.Because of this,the moulds ejector system will be most effectively operated if placed in the moving half of the mould,i.e. the half attached to the moving platen.We have stated previously that we need to eject the molding from the core and it therefore follows that the core,too,will most satisfactorily be located in the moving half.The ejector system in a mould will be discussed under three headings,namely：（）the ejector grid;（）the ejector plate assembly; and（）the method of ejection.5.1、Ejector gridThe ejector grid（Figure5-1） is that part of the mould which supports the mould plate and provides a space into which the ejector plate assembly can be fitted and operated.The grid normally consists of a back plate on to which is mounted a number of conveniently shaped “support blocks”. The ejector plate assembly is that part of the mould to which the ejector element is attached.The assembly is contained in a pocket,formed by the ejector grid,directly behind the mould plate.The assembly（Figure5-2）consists of an ejector plate,a retaining plate and an ejector rod.One end of this latter member is threaded and it is screwed into the ejector plate.In this particular design the ejector rod function not only as an actuating member but also as a method of guiding the assembly.Note that the parallel portion of the ejector rod passes through an ejector rod bush fitted in the back plate of the mould.5.2、Ejection techniquesWhen a molding cools,it contracts by an amount depending on the material being processed.For a molding which has no internal form,for example,a solid rectangular block,the molding will shrink away from the cavity walls,thereby permitting a simple ejection technique to be adopted.However,when the molding has internal form,the molding,as it cools,will shrink onto the core and some positive type of ejection is necessary.The designer has several ejection techniques from which to choose，but in general,the choice will be restricted depending upon the shape of the molding.The basic ejection techniques are as follows：（）pin ejection（）sleeve ejection（）stripper plate ejection and（）air ejection.Figure 2-1aFigure 2-1bFigure 3-1Figure 3-2Figure 4-1aFigure 4-1bFigure 5-1Figure 5-2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：（）较高的成型速度使大批量生产成为可能；（）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；（）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357410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：（）推出支架；（）推板装配机构；（）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：（）推杆脱模；（）推管脱模；（）推件板脱模；（）气动脱模。17
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