Customized 2 shot plate injection molding Factory:Plastics can also be divided into general plastics and engineering Plastics. Its main purpose is to define a wide range of applications. Polyethylene and polypropylene, for example, are cheap and can be produced on a variety of different types of machines. Engineering Plastics are expensive, but the stability and physical properties of raw materials are much better.In general, they are both rigid and resilient.