【正文】 s through the sprue,main runner,branch runner and gate before entering the the temperature of molten plastic is lowered which going through the sprue and runner,the viscosity will rise。The Injection MoldingThe 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 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 principle of injection molding is quite similar to that of process consists of feeding a plastic pound in powered or granular form from a hopper through metering and melting stages and then injecting it into a a brief cooling period,the mold is opened and the solidified part machine advantage of injection molding are：（ⅰ）a high molding speed 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.The injectionmolding machineSeveral methods are used to force or inject the melted plastic into the most monly used system in the larger machines is the inline reciprocating screw,as shown in Figure screw acts as a bination injection and plasticizing the plastic is fed to the rotating screw,it passes through three zones as shown：feed,pression,and the feed zone,the screwflight depth is gradually reduced,force the plastic to work is converted to heat by conduction from the barrel the chamber in front of the screw bees 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 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 ruleofthumb can be used to determine the tonnage required for a particular is based on two tons of clamp force per square inch of projected the flow pattern is difficult and the parts are thin,this may have to go to three or four tons.Many reciprocatingscrew machines are capable of handing thermosetting plastic these materials were handled by pression or transfer materials cure or polymerize in the mold and are ejected hot in the range of 375176。C～410176。however,the viscosity is lowered by shear heat generated when going through the gate to fill the is desirable to keep the distance that the material has to travel down to a minimum to reduce pressure and heat is for this reason that careful consideration must be given to