subject: Single Screw Extruder Method And Apparatus [print this page] A single screw extruder is commonly used instrument for the plastication of polymeric material. Polymeric material is useful for the fabrication of a variety of molded or shaped articles. Plastication refers to the softening of polymeric material to such an extent that it flows freely and will assume any shape. In the case of polymeric material which is crystalline, plastication is synonymous with melting. In the case of polymeric material which is amorphous, plastication occurs at about the glass transition temperature.
Knowing the respective bulk densities of different polymeric materials to be plasticated together makes it possible to provide separate plastication conditions in the single screw extruder, each suitable for a different material. This is done by constructing an extruder screw with two or more different plastication sections. The plastication section of an extruder screw typically has a frusto conical shape with the circumference of the cone increasing in the downstream direction. As polymeric material passes through a plastication section of a single screw extruder, the space between the root of the screw and the interior wall of the barrel (the "channel depth") decreases, and the polymeric material is compressed.
Progressively decreasing the volume of the solids bed has the result of keeping it in contact with the interior wall of the barrel as a progressively larger portion of the one of the materials becomes plasticated. In a second section, all of the remaining unplasticated material is plasticated. A good indication that all material has been plasticated is that the single screw extruder maintains an essentially constant output of extruded material and that the amperage of the motor which powers the extruder remains essentially constant.
