Many kinds of plastic parts and pickup bobbins are made by injection molding more than any other way. A screw plasticating principle widely used in plastic extruders was adapted to injection molding. The tooling is quite elaborate with special plates and gates to control the flow of plastics, which is forced into the cavity of the mold. Molds may be opened hydraulically or mechanically by the use of side-draws, multi-plate molds and automatic screwing devices in the mold. Some molds have one or two cavities for large-volume production runs.

Black or Cream bobbins can be injected depending what gates are open to allow the various colors of plastic materials. There are a wide variety of plastic materials used for injection molding. Some examples are: Nylon, Cellulose, Phenolic, Polybutylene, and Polystyrenes. There are so many combinations and grades of material to choose from. It would be best to go to a library and look up books on plastics and read the many interesting things about plastics. It’s good to learn when you want to experiment and develop new products.