The extruder completes all of the above processes through a barrel with a screw and a helical path. The plastic pellets enter the barrel through the hopper at one end of the barrel and then through the screw to the other end of the barrel. In order to have sufficient pressure, the depth of the thread on the screw decreases as the distance to the hopper increases. External heating and internal heat generated by friction between plastics and screws make the plastic soft and melted. Different polymers and different applications, the design requirements for the extruder are often different. Many options relate to the discharge port, multiple feed ports, special mixing devices along the screw, melt cooling and heating, or no external heat source (adiabatic extruder), the gap between the screw and the barrel is relatively large , And the number of screws. For example, a twin-screw extruder can make the melt more fully mixed than a single-screw extruder. Tandem extrusion is the melt extruded from the first extruder and supplied as a raw material to the second extruder, usually used to produce extruded polyethylene foam.