Injection Molding

Injection molding is one of the most popular processing operations in the plastics industry. In recent years, more than half the processing machinery manufactured were injection-molding machines. The equipment is basically designed to achieve the melting of the resin, injecting the melt into a cavity mold, packing the material into the mold under high pressure, cooling to obtain solid product, and ejecting the product for subsequent finishing. It is different from extruders in that a mold is used instead of a die, requiring a large force to pack the melt into the mold. A machine is typically classified by the clamping force (which can vary from 1 to 10,000 tons!) and the shot size determined by the size of the article to be manufactured. Other parameters include injection rate, injection pressure, screw design, and the distance between tie bars.

The machine is generally made of (a) a hydraulic system, (b) plasticating and injection system, (c) mold system, and (d) a clamping system. The hydraulic system delivers the power for the operation of the equipment, particularly to open and clamp down the heavy mold halves. The injection system consists of a reciprocating screw in a heated barrel assembly and an injection nozzle. The system is designed to get resin from the hopper, melt and heat to correct temperature, and deliver it into the mold through the nozzle. Electrical heater bands placed at various points about the barrel of the equipment allow close control of the melt temperature. The mold system consists of platens and molding (cavity) plates typically made of tool-grade steel. The mold shapes the plastic melt injected into the cavity (or several cavities). Of the platens, the one attached to the barrel side of the machine is connected to the other platen by the tie bars. A hydraulic knock-out system using ejector pins is built into one of the platens to conveniently remove the molded piece.

The machine operates in an injection-molding cycle. The typical cycle sequence is, first, the empty mold closes, and then the screw movement delivers an amount of melt through the nozzle into it. Once the mold is full, the pressure is held to "pack" the melt well into the mold. The mold is then cooled rapidly by a cooling medium (typically water, steam, or oil) flowing through its walls, and finally the mold opens to eject the product. It is common for this cycle to be closely monitored and to be mostly automated by the use of sophisticated control systems. Figure 2.12 shows a diagram of a simple injection-molding machine indicating the hydraulic, injection, and mold systems. The mold filling (a), compaction (b), cooling (c), and ejection (d) steps are also illustrated in Figure 2.12. Figure 2.13 shows a modern injection-molding machine.

When a multicavity mold designed for several "parts" is used, the ejected product is complex, consisting of runners, a spruce, and flashing that needs to

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