Extrusion and Thermoplastics
In a thermoplastic extrusion, the material is first softened by heating so that it can be shaped. This process is performed by the extruder, or extrusion machine. This heat softening is referred to by different names, such as ‘thermal softening’, ‘plasticization’, or ‘plastication’.
Most extruders are single screw machines. The screw is what forces the material towards, and then through, the die. Shape is imparted by the die, and/or by post-extrusion forming, and the product is then set to shape by cooling while maintaining its shape. The equipment used to perform this process is known as the post extrusion equipment, while the entire system is known as an extrusion line. In addition, there are food extruder machine and feed extruder machine.
Extruder Classification
Extruders may be categorized by three figures, for instance, 1-60-24. The first number specifies the amount of screws the machine has, the second number indicates the screw diameter in millimeters (mm), and the third number indicates the effective screw length as a multiple of the screw diameter. Therefore, in the given example, a single screw machine is being described that has a screw diameter of 60 mm and a length of 24 (i.e., a L/D ratio of 24/1).
Machine Construction
Figure 1 shows a cut-away diagram of a simple single screw machine. This diagram shows the arrangement the different parts of the machine. The two units - screw and barrel - interact together to convey the plastic material, melt the material, and then push it through the die.
The screw is rotated at a predetermined speed with the electric motor drive unit and gearbox. Temperature controllers are connected to heating/cooling elements on the barrel to maintain the temperature at the set-point temperatures.
The capability of the screw and barrel assembly to extrude a given material is based on the characteristics or construction of the barrel and screw, the characteristics of the plastics material, and the circumstances under which the system is operated.