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(such as PE). Less screw and barrel wear are also found. The output is, how- ever, dependent on die-head pressure and at high pressures the material resi- dence time distribution becomes broader (the clearances between the flanks of the screws are usually greater for this type of machine). At the high shear rates possible in this type of machine, this non-uniform residence time can result in decomposition of heat sensitive materials. However, the greater inter- channel flow results in better mixing or compounding. Tapered Machines Because of the very positive feeding characteristics of counter-rotating twin screw machines, very high forces may be generated. These forces can be so high that thrust-bearing failure is a real danger. Such machines therefore often have to be starve fed and/or run at low screw speeds. This is because it is dif- ficult to incorporate adequate thrust bearings in the small space available. Tapering the screw diameter from the feed section to the die can increase the space available. Thus the screws have a larger diameter at the feed end and larger thrust bearings can then be incorporated, permitting higher outputs. The shear seen at the screw tips is also reduced. Compounding Extruder The mixing efficiency of a twin screw extruder can be increased by incorporat- ing mixing elements along the screws. These and other elements may be slot- ted onto a central shaft to build up the screw section required. On some machines the length, number, and form of the elements used may be easily changed. The elements may take various forms such as reversed screw flights, kneading discs, pins, etc. Thus, in the process, the material may be heat-softened in an extrusion section, passed into the mixing section, and then conveyed into another section. This process may be repeated several times, for example in devolatilizing extruders or machines used to carry out chemical reactions. Cascade Extruder The simplest type of cascade extruder consists of two single screw extruders (two screw and barrel assemblies, one mounted above the other) where the output (melt) from the first machine feeds the second. A passageway that may be vented for devolatization connects the two screw assemblies. This type of machine may also be classified as a vented extruder. It offers the advantage that each screw may be driven at a separate speed, so their output can be matched. Valves may be also used to optimize the output from each stage. Gear Pump A gear pump is a very simple twin-screw extruder that moves a fluid material through the action of two intermeshing gears (Figure 3.). A gear pump extruder may be used to produce fibers from polymers that have low viscosi- ties at their processing temperatures. Examples of this type of polymer are polyamides, polypropylenes, or PET. A gear pump fitted between the end of the barrel and the die of a single-screw extruder will produce a steady high pressure and a more consis- tent and higher output from the die. Thus, a gear pump can make the output of an extruder virtually independent of back-pressure and, within reason, of screw wear. As the accuracy of output is improved, significant down gauging of the extrudate is possible (approximately 10%). Since the gear pump is used to build up pressure in the die (>70 MPa/10,000 psi), the screw is not required to develop high pressures. This usually results in a reduction in melt 5 section 1: introduction to extrusionPDF Image | Dynisco Extrusion Processors Handbook
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