Equipment for melting, pressurising and homogenising plastics by means of a rotating screw. Different configurations are possible; the most simple one being a _ single screw extruder mainly used for conversion processes like cast film extrusion, fibre spinning, film blowing or pipe and profile extrusion. For more demanding applications like compounding of polymer powder before _ pelletisation or the mixing of polymers, twin screw extruders are used where the two normally intermeshing screws can be moved in a corotating (same direction) or counterrotating (opposite direction) fashion. These machines normally contain different mixing elements, most important being kneading blocks for the local creation of extensional flow.

Environmental stress cracking
Under certain conditions of stress and environment like oils, detergents or soaps, ethylene plastics may fail mechanically by cracking. This phenomenon is called environmental stress cracking resistance (ESCR). ESCR properties can be tested in accelerated tests by presence of starter notches, surface active agent, elevated temperature and mechanical loading. Test result is the time when 50 % of the samples have failed.