Understanding the Injection Moulding Process

 3.1 INJECTION MOULDING MACHINE

Injection moulding is the most widely used polymeric fabrication process. It evolved from metal die casting, however, unlike molten metals, polymer melts have a high viscosity and cannot simply be poured into a mould. Instead a large force must be used to inject the polymer into the hollow mould cavity. More melt must also be packed into the mould during solidification to avoid shrinkage in the mould. The injection moulding process is primarily a sequential operation that results in the transformation of plastic pellets into a moulded part. Identical parts are produced through a cyclic process involving the melting of a pellet or powder resin followed by the injection of the polymer melt into the hollow

When stage 3 is completed, the mould closes again and the cycle starts over again.

Stage 1
INJECTION OF THE PLASTIC MELTS INTO THE MOULD: In stage 1, the mould is closed and the nozzle of the extruder is pushed against the sprue bushing of the mould. The screw, not rotating at this point, is pushed forward so that the plastic melt in front of the screw is forced into the mould.

Stage 2
HOLDING PRESSURE AND PLASTICATING: When the mould is completely filled, the screw remains stationary for some time to keep the plastic in the mould under pressure, this is called the “hold” time. During the hold time additional melt is injected into the mould to compensate for contraction due to cooling. Later, the gate, which is the narrow entrance into the mould, freezes. At this point the mould is isolated from the injection unit. However, the melt within the mould is still at high pressure. As the melt cools and solidifies, the pressure should be high enough to avoid sink-marks, but low enough to allow easy removal of the

As most modern injection moulding machines are screw machines, heat input is relatively easy. Heat removal from the plastics material contained in the mould is, however, difficult as plastics material contains a lot of heat and has a low thermal conductivity.

Cooling allows the plastic to solidify and become dimensionally stable before ejection. Heat that has been transferred to the mould by the molten plastic is carried away by a coolant that circulates through cored passages in the mould. Coolant temperature and flow rate determines the efficiency of heat removal. Cooling the moulded components uniformly may mean either, cooling the mould with different flow rates of cooling medium in different areas or, using the same flow rate throughout the mould but with different temperatures of cooling medium. The objective is to cool the components as quickly and as uniformly as possible, while ensuring that defects such as poor surface finish and changes in physical properties are not