The extruder is indisputably the most important piece of machinery in the polymer processing industry. To extrude means to push or to force out. Material is extruded when it is pushed through an opening. When toothpaste is squeezed out of a tube, it is extruded. The part of the machine containing the opening through which the material is forced is referred to as the extruder die. As material passes through the die, the material acquires the shape of the die opening.
Extruders in the polymer industry come in many different designs. The main distinction between the various extruders is their mode of operation: continuous or discontinuous. The latter type extruder delivers polymer in an intermittent fashion and, therefore, is ideally suited for batch type processes, such as injection molding and blow molding. As mentioned earlier, continuous extruders have a rotating member, whereas batch extruders have a reciprocating member. A classification of the various extruders is shown in Table 2.1.
In this chapter, the hardware components of a typical single screw extruder will be described. Each major component will be discussed with respect to its major function, the possible design alternatives, and how important the component is to the proper functioning of the extruder.
Before going into a detailed process analysis of extrusion, it may be useful to review the basic principles that will be applied in analysis. One can think of the basic principles as the tools used in the process analysis. This chapter is meant to be a review, not an exhaustive dissertation on the subject. For more in-depth and detailed information, the reader is referred to general texts, such as the one by Bird et al. [1] and others [2–4].
To understand the extrusion process, it is not enough just to know the hardware aspects of the machine. To fully understand the entire process, one also has to know and appreciate the properties of the material being extruded. The characteristics of the polymer determine, to a large extent, the proper design of the machine and the behavior of the process.
Chapters 5 and 6 on fundamental principles and important polymer properties are meant to be a preparation for the material covered in this chapter. In this chapter, the central part of this book, the six main functions of an extruder will be described and analyzed. The main functions are solids conveying, plasticating or melting, melt conveying or pumping, devolatilization, mixing, and die forming. The material in this chapter is key to developing a thorough understanding of the extrusion process and indispensable in taking an engineering approach to screw design, die design, and troubleshooting.
The single most important mechanical element of a screw extruder is the screw. The proper design of the geometry of the extruder screw is of crucial importance to the proper functioning of the extruder. If material transport instabilities occur as a result of improper screw geometry, even the most sophisticated computerized control system cannot solve the problem. Screw design is often still considered to be more of an art than a science. As a result, misconceptions about certain aspects of screw design still abound today. Since the theory of single screw extrusion is now well-developed (see Chapter 7), the design of screws for single screw extruders can be based on solid engineering principles. Thus, screw design for single screw extruders should no longer be an art, but a science based firmly on the principles of polymer processing engineering.
Die design is one aspect of extrusion engineering that has remained more of an art than any other aspect. The obvious reason is that it is difficult to determine the optimum flow channel geometry from engineering calculations. Realistic analysis of flow through dies in many cases requires computation of three-dimensional velocity profiles inside the die.
Troubleshooting is often the most critical element of extrusion engineering because of the huge financial impact that extrusion problems can have. As a result, this topic may be the most important in the whole book. For that reason it was decided to devote a separate publication to this subject. The book “Troubleshooting the Extrusion Process” [159] is an expanded text of this chapter, with many detailed case studies.
In most polymer processes, the quality of the final part is greatly dependent on the melting, flow and mixing of the polymer. The optimization of the equipment and manufacturing process, as done today, is time consuming and expensive. It is often necessary to build complex flow visualization equipment, i. e., model extruders with transparent barrels, to qualify the flow during processes.
