Injection Molded Plastic

Injection molded plastic is a variety of parts from very small components to parts the size of a body panel of a car. A mold cavity is made from steel or aluminum in precisely the shape of the desired part. The material for the plastic is mixed in a barrel like container and heated. When it is hot, it is injected into the mold cavity and left to cool. It will harden into the exact shape of the cavity.

Designing the mold is a very complex process. The wall of the part should not be too thick or too thin. Thick walls are expensive because they use more plastic, and thin walls have no strength. A projection such as reinforcement ribs stick out of the wall. These must not be thicker than the wall, or there will be sink marks in the final product. Holes in the part are also important. They should be reduced to a minimum or avoided because they will weaken the part too. Rounded corners are the best design because the hot plastic will pull away from sharp corners when it cools creating a flawed product.

Injection molded plastic parts can be made for almost any two or three dimensional shape. Both simple and complex plastic parts will have a smooth finish. Some of the parts that are made by plastic injection molding are brackets, robot parts, washers, motorcycle parts, auto parts, toy parts, knobs, front panels, enclosures and gears. It is also used for packaging, appliance parts, power tools, electronic enclosures, display windows and valves. Injection molded plastic can make highly technical, intricate parts for fire safety systems, security systems and the defense industry.

This is all done in big injection molding machines. PVC, nylon, ABS, Acetal, Teflon, Acrylic, polycarbonate and polystyrene are some of the plastics that can be used for plastic molding. Each one has a calculated shrinkage amount, and each mold is designed for the specific type of resin to be used.

There are several complications associated with injection molded plastic manufacturing. If the melting temperatures are too high there will be scorching or burning. Warping can happen if the wall thicknesses are not uniform. Bubbles may form if the resin is too hot, and there is too much moisture in it. The cavity of the mold will not be filled properly if there is not enough resin injected, or the injection speed is too slow.