General Extruder Design

2 Extruder Drives The higher specific heat requirements of SPS and lack of preheating through drying necessitates extruder drive with greater capability. On a theoretical basis, extrusion of SPS requires 2.5-2.75 times the drive horsepower typically needed to process a preheated polyethylene terephthalate (PET). For preheated PET extrusion, extruder drives are designed for approximately 12.01b/h/hp room temperature SPS would require approximately lOlb/h/hp [13]. 

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Die channel design, like extruder design, is often a compromise because of the wide array of materials that must be run through the system. Channels designed for tie layers and barrier polymers are generally small because of the small amount of these materials that Is needed. Problems may be encountered if a polymer used In small flows Is switched into a large extruder or die channel, since the residence time may exceed the polymer s heat tolerance. 

As an example, blown film extruders designed to process LLDPE can process mLLDPE generally without difficulty torque, head pressure, and motor load limitations generally do not limit film productivity. However, it is important to understand the differences arising from the different... 

The simple extruder design analyzed here would not be implemented in practice because of obvious mechanical problems, but, as we shall see subsequently, it is sufficiently close to the description of a true single-screw extruder that the calculations done here are all relevant. There are three weaknesses in the analysis. First, we have considered only a Newtonian fluid, while most real polymers have highly shear-dependent viscosities. Second, our heat transfer analysis is inadequate, both because we have considered temperature- and pressure-independent physical properties and because we have been able to obtain explicit solutions only for certain limiting cases. Finally, we have not dealt with the flow in the neighborhood of the transition from the extruder channel to the die. All of these restrictions can be relaxed, as we shall see, but to do so for the latter two generally requires the use of numerical algorithms to solve the full equation set. We shall address this topic in Chapter 8. 

The designer should be aware of the fact that this is to be considered in designing with extruded products. The designer can exercise little control over this pull back condition except to be guided by the experience of the extrusion processor to indicate which materials are particularly susceptible to this problem and what the recommended wall thicknesses are to minimize the effect. In general, one of the best ways to improve the condition is to slow down the rate of extrusion. As a result, products have a tendency to pull back. They also will be more costly to produce. 

Plasticator A very important component in a melting process is the plasticator with its usual specialty designed screw and barrel used that is used in different machines (extruders, injection molding, blow molding, etc.). If the proper screw design is not used products may not meet or maximize their performance and meet their cost requirements. The hard steel shaft screws have helical flights, which rotates within a barrel to mechanically process and advance (pump) the plastic. There are general purposes and dedicated screws used. The type of screw used is dependent on the plastic material to be processed. 

In 1976, Meluch and Campbell at General Motors obtained a patent10 for a steam hydrolysis depolymerization of PUR. High-pressure steam hydrolyzes flexible PUR foams rapidly at temperatures of 232-316°C to form diamines and polyols. The diamines are distilled and extracted from the steam and the polyols are obtained from the hydrolysis residue. In 1977, Bayer AG obtained a patent for a continuous PUR hydrolysis process using a specially designed extruder.11... 

The Potente-Schoppner model provides evidence for why extruders over 150 mm in diameter are not designed with grooved barrel feed sections. In general, resin... 

High-pressure processes require suitable liquid pumps, gas compressors, agitators, extruders and other machinery components. High-pressure production methods are generally expensive to develop, to design, to build, to operate, and to maintain. Their application will only be successful if clear advantages can be achieved with respect to the product quality and the costs in general. 

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