Parts are produced by injection of liquid styrene plastic under very high pressure into complex steel molds. These molds are generally composed of two-halves that sandwich the parts; however, 'slide molds' may consist of many steel components to allow greater levels of detail to be incorporated into a single sprue. Once the plastic cools, it is removed from the mold. In the 1960s and 1970s, typical vehicle kits might contain 50 to 200 individual parts. Today it is common for a single vehicle kit to contain from 300 to 1200 parts. Each part must be carefully cut from the 'sprue' (the plastic channels that allow the plastic to flow into the mold and which hold the parts in place), cleaned of any flaws or mold marks, and then assembled.