The Injection Molding Process

Injection Molding is the process of forcing melted plastic in to a mold cavity. Once the plastic has cooled, the part can be ejected. Injection molding is often used in mass-production and prototyping. Injection molding is a relatively new way to manufacture parts. The first injection molding machines were built in the 1930’s.

There are six major steps in the injection molding process:

1. Clamping

An injection molding machine constists of three basic parts; the mold plus the clamping and injection units. The clamping unit is what holds the mold under pressure during the injection and cooling. Basically, it holds the two halves of the injection mold together.

2. Injection

During the injection phase, plastic material, usually in the form of pellets, are loaded into a hopper on top of the injection unit. The pellets feed into the cylinder where they are heated until they reach molten form (think of how a hot glue gun works here). Within the heating cylinder there is a motorized screw 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 mold through a sprue, while the pressure and speed are controled by the screw. Note: some injection molding machines use a ram instead of a screw.

3. Dwelling

The dwelling phase consits of a pause in the injection process. The molten plastic has been injected into the mold and the pressure is applied to make sure all of the mold cavities are filled.

4. Cooling

The plastic is allowed to cool to its solid form within the mold.

5. Mold Opening

The clamping unit is opened, which seperates the two halves of the mold.

6. Ejection

An ejecting rod and plate eject the finished piece from the mold. The un-used sprues and runners can be recycled for use again in future molds.

Advantages of Injection Molding
High production rates
High tolerances are repeatable
Wide range of materials can be used
Low labor costs
Minimal scrap losses
Little need to finish parts after molding

Disadvantages of Injection Molding
Expensive equipment investment
Running costs may be high
Parts must be designed with molding consideration