Plastic Injection Molding When identical plastic products are to be manufactured in large quantities industry tends to use Injection Molding. This system is of course automated in industry but tends to be operated by person on the school workshop environment. A shot of granulated which has been heated is injected under pressure into a pre-heated mould. The plastic is injected quickly to prevent the plastic hardening before it fills the mould. Once inside the mould the pressure is maintained to prevent the plastic from creeping back into the screw chamber. The pressure also prevents hollows from occurring inside the plastic molded shape.The plastic cools and takes on the shape of the mould cavity. When the plastic component has solidified the mould is opened and the molding ejected.

• Sharp corners and sudden changes in section must be avoided. This may weaken the plastic molding wall.
• Large, flat surfaces should be avoided on a mould as the plastic will not come out completely flat.
• All surfaces on the mould should be smooth and highly polished.

The Sprue (the hole in which the plastic is injected) needs to be carefully designed. It needs to round or half round in section. This allows ease of flow. The sprue also needs to be large enough to allow the mould to be filled very quickly.