Plastic injection is a manufacturing process that involves using pressure to inject molten plastic into a mold. This is then cooled, solidified, and ejected as a finished product. There are several types of plastic injection molding, including overmolding, transfer molding, and compression molding.
Before the molten plastic is injected into the mold, it must be heated and warmed to a liquid state. This is usually accomplished by using a heated barrel of the injection machine and a screw that gradually warms and melts the thermoplastic resin pellets. The screw is controlled by a hydraulic system, which helps control the temperature of the plastic as it is being injected.
The melted plastic is then transferred into the mold via channels that are machined into the injection mould, or “mold”. These are called the sprue bushing, runners and gates. The sprue bushing seals tightly against the nozzle of the injection machine, and it directs molten plastic to the cavity images in the injection mould through the runner channels. When the sprue bushing is sealed, the injection machine shuts off and the valves close to isolate the injection system and chemical precursors. The sprue bushing can also be used to create the main parting line, or “gate”, in an injection-molded component, which is seen as a rough patch or dimple-like feature on the edge of the finished part.
Injection pressure is the force that the screw or plunger exerts to push molten plastic into the cavities of the injection mold until they are 95% filled. The injection pressure is influenced by the flow characteristics of the plastic, such as its viscosity and shear rate. It should be maximized, but not so high that it causes a build-up of pressure inside the mold cavities and early solidification of the sprue and runners.
A coolant system is pumped through the sprue and runners to remove heat and allow the molded part to cool. The cooling is accelerated by the injection pressure, which may be supplemented by holding pressure if needed to improve the compactness of the molded parts and prevent shrinkage. Holding pressure is about half of the injection pressure, and is also required to help control dross formation (residue) and flashing on the edges of the molded parts.
After the molded components have cooled, they are separated and the sprue, runners and gates are removed by cutting or grinding. The final molded product is then trimmed, packaged, and shipped. Typical applications for plastic injection include automotive parts, household items, and office and factory equipment. Common plastics for injection molding include polycarbonate, nylon, and polypropylene, but specialty materials such as PEEK require specialized injection molds. The injection molding process is a very flexible and versatile method of manufacture that can produce a wide range of products with relatively low cost. This flexibility is largely due to the fact that the same tooling can be used for both single-cavity and multi-cavity tools, as well as simple and complex products.