Published on Jan 19, 2016
Injection molding is the most common method used to produce plastic molded items. Highly complex parts can be made in one operation, ranging from tiny gears, to videocassette cases, and even bathtubs.
The Plastic pellets are fed from a hopper into the heated screw barrel of the injection molding machine and are conveyed by the screw action into molds. Electric heater bands around the barrel plus the shear heat generated in the screw itself melt the plastic. The screw pushes itself back against the melted plastic accumulating at the front until sufficient melt has been formed to fill the mold. The screw then acts as a hydraulic ram to force the melted plastic into the closed mold where it is cooled. Injection molding machine ratings are expressed in terms of tons of clamping pressure. After the part is sufficiently cooled, the mold is opened and the finished part ejected.
The operation is primarily hydraulic: pumps work in conjunction with hydraulic cylinders, accumulators and tanks, and are all coordinated by valves and controls.
Since many thermoplastic resins are hygroscopic, continuous hopper dryers are used to dry pellets prior to molding. While most machines use electric dryers, gas dryers have become more popular to reduce costs. Radio frequency drying may be superior to both in some cases.
Resin pellets are poured into the Feed hopper, a large open bottomed container, which feeds the granules down to the screw. The screw is rotated by a motor, feeding pellets up the screw's grooves. The depths of the screw flights decreases towards the end of the screw nearest the mold, compressing the heated plastic. As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets. Heaters on either side of the screw assist in the heating and temperature control during the melting process.
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