A more efficient way is to mix extrusion materials with twin screw extruder
When mixing two different additives, a more efficient way is to mix extrusion materials with twin screw extruder. In this method two screw rotate with or against each other and have special mixing features, such as kneading blocks and forward and reverse capabilities. Some single screw extruder systems can, however, be used for mixing.
Though general-purpose screws are available, different plastics require differently calibrated extrusion screws. The length-to-diameter ratio, pitch, length of each zone and helix angles of a screw must all be matched to the plastic type being used. The screw must also be designed to suit a specific type of die.
Extrusion screw design has been improving over the years, with new innovations and ideas that help in perpetually adjusting the process so as to meet the needs of specific applications.
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.