Although plastics have been around for centuries, the processing of man-made varieties is a relatively modern phenomenon. The first injection molding machine was patented in the 1870’s and together with profile extrusion, came into common commercial usage during the 1930’s. Compression molding had developed about a decade earlier. Blow molding, as we know it today, did not develop until the 1940’s.
Other processing methods, such as rotational molding, did not come into usage until the 1960’s, and the processing of recycled plastics, like the technology used in the production of plastic lumber, developed only in the 1980’s.
Plastic raw materials are roughly divided into 2 types: commodity grade and engineering grade. Commodity grade resins are more widely used and include polyethylene, polypropylene, ABS (acrylonitrile butadiene styrene), polystyrene and PVC (polyvinyl chloride).
Engineering grade resins are generally more difficult to process but have characteristics which make them desirable for specialized use. Some of the more widely used engineering grade thermoplastics are acetal, polyester, nylon and polycarbonate. Some of the considerations for raw material selection include impact strength, high and low temperature characteristics, warpage, resistance to ultraviolet light and the friendliness of the material to printing or adhesives.
Plastic resins are also broadly classified as either thermoplastic or thermoset. Thermoplastic resins can be remelted after processing. Flash and rejected parts can be reground and added to virgin resin for reprocessing. Thermoset plastics, typically used in compression molding and SMC molding, cannot be remelted. Once processed, if they are reheated, they simply burn up and disintegrate. Thus, rejected parts and flash cannot be reused.