Twin-screw corotating mixing extruders have been used since the 1950s to produce filled, force-fed and other plastic compounds. The location and method of feeding materials, the design of the optimum screw geometry, the solutions for removing air and volatile components are all strongly dependent on the type of materials to be mixed, but over the years a number of generally applicable laws have been discovered and many technical innovations have been made to significantly improve productivity and specific energy consumption.