Plastic Injection Moulding Explained
Injection Moulding is the process of forcing melted plastic in to a mould cavity. Once the plastic has cooled, the part can be ejected. Injection moulding is often used in mass-production and prototyping and is a relatively new way to manufacture parts, the first machines appearing in the 1930’s.There are six major steps in the injection moulding process:
An injection moulding machine consists of three basic parts: the mould, plus the clamping and injection units. The clamping unit holds the two halves of the injection mould together during the injection and cooling.
During the injection phase plastic material, usually in the form of pellets, is loaded into a hopper on top of the injection unit. The pellets feed into a cylinder where they are heated until they reach molten form.Within the heating cylinder there is a motorised screw or ram that mixes the molten pellets and forces them to end of the cylinder. Once enough material has accumulated in front of the screw, the injection process begins. The molten plastic is inserted into the mould through a sprue (channel), while the pressure and speed are controlled by the screw.
The dwelling phase consists of a pause in the injection process. Once the molten plastic has been injected into the mould, the pressure is applied to make sure all of the mould cavities are filled.
The plastic is allowed to cool to its solid form within the mould.
The clamping unit is opened, which separates the two halves of the mould.
An ejecting rod and plate eject the finished piece from the mould. The unused sprues and runners can be recycled for use again in future moulds.