The Single Screw Plasticating Extrusion Process

Turning to Eq. 9.2-44 and substituting X, yields g0 = 0.654 gi = 0.277 g2 = 0.059 g3 = 0.008, which implies that in this particular case that 65.4% does not pass over the flight, 27.7 passes once, 5.9% twice, and 0.8% three times, and so on. Good dispersion, as mentioned in Chapter 7, requires 20-30 passages over high-shear zones. 

It is easy to visualize the mechanisms that take place in the solids conveying zone of a screw extmder, and to develop appropriate mathematical models describing these mechanisms. This was done by Darnell and Mol (28) in 1956, and their model is discussed later in this section. 

However, the melting or plasticating process that follows the solids conveying zone is a relatively complex one, and unlike the melt extrusion and solids conveying processes, the detailed physical melting mechanisms cannot be easily visualized, predicated, and modeled from basic principles without experimental investigation. 

Analyzing the experimental results, we note that throughout most of the extruder, the solid and melt phases coexist side by side, and are clearly segregated from each other, with 

It should be noted that melting takes place along most of the extruder. Indeed, the production capacity of plasticating extruders is frequently determined by their plasticating capacity. Further visual analysis of the experimental results reveals a tendency of the melt pool to penetrate under the solid bed and, occasionally, to completely surround it the continuity of the solid bed is frequently broken and a melt filled gap appears (e.g., turn 15.5, Fig. 9.23). This tendency for solid bed breakup seems to originate in the tapered sections of the extruder, and it appears to be a source of surging (i.e., fluctuation in time of temperature, pressure, and flow rate) of the extmdate at the die, as well as a source of entrapping some air bubbles into the melt stream. 

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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. 

The process of film extrusion by the film-blowing method was conducted on a Une specially designed for film manufacture in the Department of Food Process Engineering, Lubhn University of life Science (PL), based on a single-screw plastic extruder of L/D = 35 (Figure 9.1). The Une was produced by SAVO Ltd Co., Poland. 

In the method of producing a hollow foamed plastic body from polyethylene or polypropylene by means of an extrusion blow-moulding process using a single-screw... 

Screw extruders usually are built with a single screw as shown, but may have as many as four screws and the die may have multiple holes of various cross sections. An 8 in. dia screw can have a capacity of 2000 Ib/hr of molten plastics. Tubing can be extruded at 150-300 ft/min. To make pellets, the extrudate goes to cutting machines in which parts as small as washers can be made at rates as high as 8000/min. The extrusion of plastics is described at length by Schwartz and Goodman (Plastics Materials and Processes, Van... 

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