The mixing action within the extruder is controlled by screw geometry, the speed of the screw, and the back pressure provided by the constriction of the melt as it passes out the end of the barrel. The geometry of the screw significantly affect plastic melting and mixing. The angle of the screw flights can be changed to reduce the shearing force on the plastic. The length of each section of the screw changes for different materials. When the length of the compression section is increased, this reduces the shear intensity on the plastic.
Many plastic materials are shear and heat sensitive.
Shear sensitive materials have complex molecules that will decompose if they are compressed too quickly. Polycarbonate is one of these plastics and is called a shear sensitive material. The metering section of the screw can be lengthened to improve the homogeneity of temperature and color additives. There are general purpose screws designed for extruding low melt general purpose plastic materials, and there are specialty screws for compounding shear sensitive materials and for the mixing of abrasive additives. The plastic material and their additives are introduced into the mixing section at one end of the screw.