Injection Molding

Injection Molding is the process of forcing melted thermo plastic into a mold cavity. Once the plastic has cooled, the part can be ejected. The first injection molding machines were built in the 1930's.

There are six major steps in the injection molding process:

Clamping
An injection molding machine consists of three basic parts; the mold, the clamping unit, and the injection unit or units. The clamping unit holds the halves of the mold under pressure during the injection and cooling.

Injection
During the injection phase, plastic material, usually in the form of pellets, are pulled from a hopper on top of the injection unit. The pellets feed into the cylinder where they are heated until they reach molten form. Within the heating cylinder there is a motorized screw that mixes the molten pellets and forces them to the 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 controlled by the screw or in some cases a ram.

Dwelling
The dwelling phase consists 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 mold cavities are filled.

Cooling
The plastic cools to its solid form within the mold.

Mold Opening
The clamping unit is released, which separates the halves of the mold.

Ejection
An ejection rod and plate eject the finished part from the mold.

Some Advantages of Injection Molding

• High production rates
• Repeatable high tolerances
• Wide range of materials can be used
• Low labor costs
• Minimal scrap losses
• Little need to finish parts after molding

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