Thermoplastics can be formed by injection molding, where heated plastic is forced into a closed mold and allowed to cool.
Injection molding is probably the most common process for forming plastic parts, but it is mainly used when large numbers of a part have to be produced.
Since the steel mold for injection molding may be costly, it usually makes economic sense to invest in it if the cost is going to be spread out over a large number of parts.
Many products are made in large enough quantities to justify injection molding. Examples of plastic products made by injection molding can range from novelty items to power tools.