Injection molding is a popular manufacturing process that facilitates producing a large volume of high-quality, uniform and cost-effective plastic parts. During the injection molding process, a plastic or resin material is heated to its melting point and then injected into a custom mold. The part forms inside the mold and is released once it has properly cooled. Injection molding is automated and can be repeated thousands and sometimes millions of times, creating exact copies of a custom part. 

The first step of getting a plastic part injection molded is to have a computer-aided design (CAD) model of the part produced by a design engineer. The three-dimensional (3D) CAD model then goes to an injection molding company where a plastic mold maker (or toolmaker) will make the mold (tool) that will be fitted into an injection molding machine to make the parts. 

Molds are precision-machined usually from steel or aluminum and can become quite complex depending on the design of the part. Plastic materials shrink at different rates when they cool, so the mold has to be constructed with consideration for the shrinkage rate of the material being used for the parts. In other words, a formula is applied in the construction of the mold to slightly increase the size so that when the plastic shrinkage occurs, the part will be to the dimensional specifications of the CAD model.