Question

Briefly describe, probably with the aid of an illustration, the injection molding process. For full credit, state what general category of polymer it is used for.

Answer 1

The plastic injection moulding process produces large numbers of parts of high quality with great accuracy, very quickly. Injection moulding is wonderful for this kind of high volume production because the high initial costs pay the manufacturer back over time with low per unit costs. For this same reason injection moulding can be a poor choice for low volume production.

Thermoset or thermoplastic material in granular form is fed through a hopper into a heating barrel. The plastic is heated to a predetermined temperature and driven by a large screw through the gate(s) and into the mould. Once the mould is filled, the screw will remain in place to apply appropriate pressure for the duration of a predetermined cooling time. Upon reaching this point, the screw is withdrawn, the mould opened, and the part ejected. Gates will either shear off automatically or be manually removed. This cycle will repeat over and over, and can be used to create hundreds of thousands of parts in a relatively short amount of time.

Injection moulding is wonderful for this kind of high volume production because the high initial costs pay the manufacturer back over time with low per unit costs. For this same reason injection moulding can be a poor choice for low volume production.

The six main steps in injection moulding are as follows:

• Clamping – the clamp unit consists of metal plates (or platen). The process begins with the mould being clamped together under pressure to accommodate the injection and cooling processes.
• Injection – the molten thermoplastic material, which has been melted by pellet form in the barrel of the machine, is injected under pressure into the mould through either a screw or ramming device.
• Dwelling – once the molten plastic is injected into the mould, more pressure is exerted to make sure all the mould’s cavities are filled, using hydraulic or mechanical pressure.
• Cooling – the plastic is left to cool and solidify within the mould.
• Opening – the movable platen is separated from the fixed platen to separate the mould.
• Ejection – ejection is completed by the use of rods, a plate or an air blast to remove the plastic component completely from the mould.

POLYMERS USED TO BE FORMED BY INJECTION MOULDING PROCESS ARE:-

According to recent reports 80% - 90% of all polymers used in the plastic moulding industry are polyethylene compounds (HDPE, LPDE and LLPDE). Although ABS, PVC, Polypropylene, Polyether Imide (PEI), Styrene, Acrylic (PMMA), Nylon 32% Glass Fiber, Nylon, Polyoxmethylene (POM), Polycarbonate are also used.