Plastics extrusion processing

In this section, we discuss the design of a melt extruder and derive the appropriate mathematical model. This will also introduce the reader to the plasticating extrusion process discussed in the following section. 

The lumped-parameter model approach becomes particularly useful when dealing with the plasticating extrusion process discussed in the next subsection, where, in addition to melt flow, we are faced with the elementary steps of solids handling and melting. 

R. Y. Chang, C. W. Hsu, W. H. Yang, W. L. Yang, and D. C. Hsu, Three Dimensional Numerical Analysis of the Single Screw Plasticating Extrusion Process, SPE ANTEC Tech. 

Air can be entrapped in the melt during processing. This can happen when plastic (pellets, flakes, etc.) is melted in a normal air environment (as in a plasticating extrusion process or in an injection barrel, compression mold, casting form, spray system, etc.), and the air cannot escape. Generally the melt is subject to a compression load, or even a vacuum, which causes release of air but in some cases the air is trapped. If air entrapment is acceptable, no further action is required. However, it is usually unacceptable, for reasons of both performance and aesthetics. 

Schott, N. R., Plastics Extrusion Processing Technology Seminar, University of Lowell, 1988. 

By the nature of the plasticating extrusion process, thermal properties are very important. In the early portion of the extruder, solid polymer particles are heated to the melting point. In the midportion of the extruder, the molten polymer is raised in temperature to a level considerably above the melting point while the remaining solid particles continue to heat up and melt. In the last portion of the extruder, the molten polymer has to reach a thermally homogeneous state. When the extrudate leaves the extruder die, it has to be cooled down, usually to room temperature. Through this whole process, the polymer experiences a complicated thermal history. The thermal properties of the polymer are crucial to being able to describe and analyze the entire extrusion process. 

Other sources of parhculates arise from tiie combustion of solid fuels in power stations and generated as dust particles in a number of industrial operations such as slitting and cutting, coating operations, plastic extrusion processes, etc. 

A number of researchers have begun to prepare poly-mer/superconductor composites with the hope of improving the processibility and properties of the hybrid materials (see Table 37.1) [1-8], For example, polymeric matrices loaded with ceramic superconductor components have been used in a plastic extrusion process to prepare superconducting wires and filaments [5,6]. Moreover, hydrophobic polymers have been used as environmentally protective layers [7,8] to slow the parasitic corrosion reactions that occur when cuprate compounds are exposed to water, acids, carbon dioxide, and carbon monoxide. 

Two very important plastics extrusion processes in which molten polymer is used to coat another material are extrusion coating and cable covering. 

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