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4 Gear Box Main Feed Port Twin Screws with Changeable Elements Down Stream Feed Removable Removable Barrel Sections Drive Motor Figure 2. Twin Screw Extruder (courtesy of Krupp Werner & Pfleiderer) sive. Thus, for economic reasons, the amount of these heat stabilizers must be kept as low as possible. One way of doing this is to compound and extrude in one step. This saves a further heating stage (if the material is first com- pounded, cooled and then re-extruded). Twin screw machines are widely used to make UPVC pipe and profiles. Today they are often used to compound other plastics, or resins, with additives to make compounds for use in other extrusion or injection molding operations. Operation and Characteristics With a single screw extruder the hopper is filled and the screw takes in the material at the rate it wants. Such a scheme, called flood feeding, is often not possible with twin screw machines. This is because the very positive feeding characteristics of twin screw machines may result in very high forces being generated. These forces may be so high that thrust-bearing failure is a real danger. Twin-screw extruders are therefore often starve fed, resulting in a through-put independent of screw speed. The average residence time is inversely proportional to the screw speed and feed rate and, therefore, decreases with increasing output. The Specific Energy Consumption, or the energy input per unit mass of material, also decreases with increasing throughput. The output is often virtually independent of the size of die used. Because of the mixing action, twin screw machines can achieve more melting, mixing and conveying in a shorter machine length than a single screw machine. They are, however, much more expensive. Co-Rotating or Counter-Rotating Machines Both types of twin screw extruders have distinct advantages that lead to their use in specific applications. Counter-rotating machines are used for the extru- sion of UPVC and co-rotating machines are used for compounding applica- tions. The counter-rotating machine has very positive material feed and con- veying characteristics. The residence time and material temperature control in the machine are also uniform. However, air entrapment, generation of high pressure, low maximum screw speed and output are disadvantages usually encountered. The advantages of the co-rotating machine are that the screws wipe each other clean (self-wiping), high screw speeds and high outputs are possible, particularly for materials which are not very shear, or heat, sensitivePDF Image | Dynisco Extrusion Processors Handbook
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