Topworks Weekly Digest

Injection Moulding Process

by Steven Cheng on September 04

In the injection moulding process a relatively cold mould, shaped as the negative of the component to be

produced，is filled under pressure by the injection of a pre-determined quantity of heated and plasticized

material. The injected material solidifies, and after a period of further cooling the mould is opened and the

product removed.

The following illustrate the main phases of the process as carried out on a horizontal plunger type

injection moulding machine The course of the material from granulate to the finished product as

illustrated in these diagrams is as follows:

The granulate is loaded into the hopper of the measuring or dosing device above the injection unit. From

the hopper, a pre¬determined quantity of material falls in front of the plunger. This charge of material is

pushed forward by the plunger so that it is deposited，first on to the injection plunger and then, after

withdrawal of the latter, in front of it.

As the injection plunger moves forward, it forces the material through the plunger guide into the heating

cylinder In this cylinder which is heated on its outer surface by electric resistance- type heater bands, the

material is brought to a state of thermoplastic melt. Even heating of the material as it passes through the

cylinder is aided by a spreader inside the cylinder. This spreader, or torpedo as it is also known, forces the

material against the hot inner wall of the cylinder.

As successive strokes of the injection plunger force the material through the heating cylinder it's

consistency changes until, at the front part of the cylinder, it is in the state of a homogeneous melt ready

for injection. The melt is forced into the mould through the injection nozzle. This operation is also

referred to as shoot¬ing \ while the speed of injection, which is regulated to suit the material being used

and the component, is known as the c shot rate

The mould, which is clamped or bolted in position in the injection machine consists basically of halves;

one member mounted on the stationary platen and one on the moving platen.

From the injection nozzle the material passes through the sprue bush in the nozzle or fixed side of the

mould. It is then distributed to the cavities through interconnecting channels or 'runners.When the cooling

period has elapsed the mould is opened and the components (T) are ejected from the moving mould-half

by means of ejector pins which, as the mould approaches the fully open position，are operated by the

ejector tail striking the stroke limiting screw.

Thus, the injection moulding cycle may be divided into five stages, as follows :

The measuring out, in front of the injection plunger, of a quantity of granulate equivalent to the

capacity of the mould cavity or cavities.

The melting of this material in the heating cylinder to a consistency suitable for injection.

Injection of the melted material into the closed and relatively cold mould.

Cooling the injected material to the point of solidification.

Opening the mould and removing the product.

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Mold Center/ West District

Huangyan, Zhejiang,China 318020