Twin screw extruder
The screw is usually contained inside a tight fitting barrel driven by a variable speed motor and gearbox. It is a highly efficient device capable of processing several tons of plastic per hour, even in modestly sized extruders. The screw is divided into three division the feed section, the compressor and the metering section.
The feed section of the screw mechanism stores and delivers resin pellets from a gravity fed hopper into the jacketed screw to begin the plastic longitudinal movement. Twin screw extruder threads move the plastic using axial rotation. Within the barrel, heaters help the plastic develop a tack to increase its friction against the barrel wall. Without this friction the plastic could not be conveyed forward and would merely rotate inside the screw.
In the transition or melt stage, the root diameter of the screw increases while the height of the flight decreases. The resin is melted here because of compression, shearing and heating produced in the barrel. Now the melt moves to the metering section, where the screw diameter remains constant. The melted plastic which is under high pressure now is pumped into the extruder die.
The resin is sometimes not completely melted in the basic extrusion screw. The barrier screw in designed to counter this problem. Additional flights are attached to the transition section so as to separate molten and solid plastic to different channels. As the solid pellet moves forward it melts due to shear against the wall and thus melts and flows into the liquid channel. Thus the solid channel narrow gradually and the liquid channel widens.