Extruder Screw Design

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, 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.      

Unfortunately, people involved in screw design are not always up-to-date on extru-sion theory. As a result, many extruder screws in use today perform considerably below maximum possible performance, solely because of improper screw design.An example is the still-common use of the square pitch extruder screw. This is ascrew with constant pitch with the pitch being equal to the diameter of the screw;this pitch corresponds to a helix angle of 17.66. It can be demonstrated quite easily that the square pitch is far from optimum with respect to melting and melt convey-ing for a number of polymers. This fact has been known since the early 1950s, yet most extruder screws in use today still use the constant square pitch design.

      A factor that may have contributed to the state of affairs in screw design is that there has not been a comprehensive text dealing with screw design. The objective of this chapter is to demonstrate how extrusion theory can be used to properly design extruder screws. Hopefully, this will provide a solid foundation, based on engineer-ing principles, from which better and more effective screw designs can be developed in the future. The principles of screw design are not only important in designing new extruder screws, but also in the analysis of processing problems of an existing extrusion line. It is important to be able to recognize whether a problem is related to poor screw design or to another part of the process. Thus, knowledge of the basic principles of screw design is important to essentially every person involved with extruders.