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1、1The 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 ther

2、mosetting 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 condit

3、ions 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 melti

4、ng 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：（ⅰ）a high molding spe

5、ed adapter for mass production is possible；（ⅱ）there is a wide choice of thermoplastic materials providing a variety of useful properties；（ⅲ）it is possible to mold threads,undercuts,side holes,and large thin section.2、The

6、 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 sc

7、rew 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 gra

8、dually 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 a

9、ctivates 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 f

10、orce 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 3is opened.To achieve this sequence,springs may be incorpor

11、ated 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

12、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 op

13、eration,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

14、 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

15、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 pl

16、ate 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 bolt

17、s.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 t

18、o 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 unt

19、il 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 t

20、o 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