Understanding of the extrusion process

The concepts of shear rate, shear stress, and viscosity are extremely important in developing a thorough understanding of the extrusion process (and other polymer processing operations). Therefore, a few examples will be given to illustrate how shear rate and shear stress can be determined in simple geometries. 

This appendix contains the detailed development for the equations that are presented in Chapter 3. Full understanding of these developments is not required for detailed analysis and troubleshooting of the extrusion process. They are presented here for those who desire a deep understanding of the mathematics involved with the screw rotation analysis. Some of the equations and figures are duplicated in this appendix for clarity. The nomenclature used here is consistent with that used earlier. The reader is directed to Chapter 3 for nomenclature. 

The lower the body moisture content the higher the plastic body attrition forces it is thus easily understandable that, whatever the other parameters of the extrusion process described above, that the water contained in the ceramic mass must provide the body with enough cohesion to equilibrate the attrition forces with the extrusion walls and the extrusion mouth. Should this not be so, the result will be fissures crossways to the direction of extrusion that are concentrated at the edges of the formed body (herring bone pattern). 

In closing, significant development in the theoretical analysis of the extrusion process has contributed to a better understanding and design of new equipment, an increase of output in existing machinery, and development of concise control via computers. 

Chapter 7 covers the actual analysis of the extrusion process. The process is analyzed in discrete functional zones with particular emphasis on developing a quantitative understanding of the mechanisms operational in each zone. 

It is almost always the case in industrial food production that its processing machinery is borrowed from other industries, and that our understanding of the relevant materials science of processing follows the empirical development of significant new products. Extruders and extrusion is a classic case. 

---