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IMPORTANCE OF PROCESS DESIGN

The intention of this chapter is not merely to present the technology of polymer processing but to initiate the concepts required in the design of polymer processes. A knowledge of the types of polymers available today and the methods by which they are processed is certainly needed, but this is available in several sources such as Modern Plastics Encyclopedia (Green, 1992) and the Plastics Engineering Handbook (Frados, 1976). In this chapter we present primarily an overview of the major processes used in the processing of thermoplastics. In Section 1.1 we begin by classifying the various processes and point out where design is important. In Section 1.2 we present a case study concerned with film blowing to illustrate how the final physical properties are related all the way back to the melt flow of a polymer through the die. Finally, in Section 1.3 we summarize the principles on which polymer process design and analysis are based.

1.1 CLASSIFICATION OF POLYMER PROCESSES

The major processes for thermoplastics can be categorized as follows: extrusion, postdie processing, forming, and injection molding. We describe specific examples of some of the more common of these processes here.

The largest volume of thermoplastics is probably processed by means of extrusion. The extruder is the main device used to melt and pump thermoplastics through the shaping device called a die. There are basically two types of extruders: single and twin screws. The single-screw extruder is shown in Figure 1.1. The single-screw extruder basically consists of a screw (Fig. 1.2) that rotates within a metallic barrel. The length to diameter ratio (L/D) usually falls in the range of 20 to 24 with diameters falling in the range of 1.25 to 50 cm. The primary design factors are the screw pitch (or helix angle, θ) and the channel depth profile. The main function of the plasticating extruder is to melt solid polymer and to deliver a homogeneous melt to the die at the end of the extruder. The extruder can also be used as a mixing device, a reactor, and a devolatilization tool (see Chapter 8).

FIGURE 1.1 Typical single-screw extruder. (Reprinted by permission of the author from Middleman, 1977.)

FIGURE 1.2 Two different extruder screw geometries along with the various geometric factors that describe the characteristics of the screw. (Reprinted by permission of the publisher from Middleman, 1977.)

There are an equal number of twin-screw extruders in use as single-screw extruders today. There are many different configurations available including corotating and counterrotating screws (see Fig. 1.3) and intermeshing and nonintermeshing screws. These extruders are primarily adapted to handling difficult to process materials and are used for compounding and mixing operations. The analysis and design of these devices is quite complicated and somewhat out of the range of the material level in this text. However, some of the basic design elements are discussed in Chapter 8.

FIGURE 1.3 Cross-sectional view of corotating and counterrotating twin-screw extruders.

The extruder feeds a shaping device called a die. The performance of the single-screw and corotating twin-screw extruders is affected by resistance to flow offered by the die. Hence, we cannot separate extruder design from the die design. Problems in die design include distributing the melt flow uniformly over the width of a die, obtaining a uniform thermal history, predicting the die dimensions that lead to the desired final shape, and the production of a smooth extrudate free of surface irregularities. Some of these design problems are accessible at this level of material while others are still research problems (see Chapter 6).

There are many types of extrusion die geometries including those for producing sheet and film, pipe and tubing, rods and fiber, irregular cross sections (profiles), and coating wire. As an example, a wire coating die is shown in Figure 1.4. Here metal wire is pulled through the center of the die with melt being pumped through the opening to encapsulate the wire. The design problems encountered here are concerned with providing melt flowing under laminar flow conditions at the highest extrusion rate possible and to give a coating of polymer of specified thickness and uniformity. At some critical condition polymers undergo a low Reynolds number flow instability, which is called melt fracture and which leads to a nonuniform coating. Furthermore, the melt expands on leaving the die leading to a coating that can be several times thicker than the die gap itself. (This is associated with the phenomenon of die swell.) The problems are quite similar for other types of extrusion processes even though the die geometry is different. The details associated with die design are presented in Chapter 7.

FIGURE 1.4 Cross-head wire coating die. (Reprinted by permission of the publisher from Tadmor and Gogos, 1979.)

We next turn to postdie processing operations. Examples of these processes include fiber spinning (Fig. 1.5), film blowing (Fig. 1.6), and sheet forming (Fig. 1.7). These processes have a number of similarities. In particular, they are free surface processes in which the shape and thickness or diameter of the extrudate are determined by the rheological (flow) properties of the melt, the die dimensions, cooling conditions, and take-up speed relative to the extrusion rate. The physical and, in the case of film blowing and sheet forming, the optical properties are determined by both the conditions of flow in the die as well as cooling rates and stretching conditions of the melt during the cooling process. Furthermore, slight changes in the rheological properties of the melt can have a significant effect on the final film or fiber properties. Design considerations must include predictions of conditions which provide not only the desired dimensions but the optical and physical properties of the film, fiber, or sheet.

FIGURE 1.5 Fiber melt spinning process. (Reprinted by permission of the publisher from Tadmor and Gogos, 1979.)

FIGURE 1.6 Film blowing process. (Reprinted by permission of the publisher from Richardson, 1974.)

FIGURE 1.7 Flat film and sheet process. (Reprinted by permission of the publisher from Tadmor and Gogos, 1979.)

The third category of processing of thermoplastics is forming. Three examples of this type of process are blow molding (Fig. 1.8), thermoforming (Fig. 1.9), and compression molding (Fig. 1.10). Blow molding is primarily employed for making containers used to package a wide variety of fluids. Although polyolefins, such as high density polyethylene (HDPE), or polyethyleneterephthalate (PET), both of which can be considered as commodity resins, are commonly used, interest is growing in using this technique for the processing of higher performance engineering thermoplastics. Essentially a parison, which has been extruded or injection molded, is inflated by air until it fills the mold cavity. The inflated parison is held in contact with the cold mold walls until it is solidified. Considering the process of thermoforming, a sheet of polymer is heated by radiation (and sometimes cooled intermittently by forced convection) to a temperature above its glass transition temperature or in some cases above the crystalline melting temperature and then pressed into the bottom part of the mold (female part) either using mechanical force, pressure, or by pulling a vacuum. The key flow property is the extensional flow behavior of the melt, which controls the uniformity of the part thickness. Sometimes the deformation is applied at a temperature just below the onset of melting, in which case the process is referred to as solid phase forming. At other times the sheet is extruded directly to the forming unit and is formed before it cools down. (This is called scrapless or continuous thermoforming.) Some of the key design considerations are the time required to heat the sheet, the final thickness of the part especially around sharp corners, and the cooling rate which controls the amount and type of crystallinity. In compression molding a slug of polymer is heated and then pressure is applied to squeeze the material into the remaining part of the mold. Some aspects of forming are discussed in Chapter 10.

FIGURE 1.8 Blow molding process. (Reprinted by permission of the publisher from Holmes-Walker, 1975.)

FIGURE 1.9 Plug-assisted vacuum thermoforming. (Reprinted by permission of the publisher from Greene, 1977.)

FIGURE 1.10 Compression molding process. (Reprinted by permission of the publisher from Tadmor and Gogos, 1979.)

The last general category is that of injection molding, which is shown in Figure 1.11. Polymer is melted and pumped forward just as in a screw extruder. The screw is then advanced forward by a hydraulic system that pushes the melt into the mold. Because of the high deformation and cooling rates, a considerable degree of structuring and molecular orientation occurs during mold filling. The physical properties of injection molded parts can be affected significantly by processing conditions. Design considerations include the required injection pressure to fill the mold cavity, the location of weld lines (places where two melt fronts come together), cooling rates, length of hold time in the...