Plastics extrusion refers to the process whereby a polymer —usually in the form of pellets or powder—is mixed, heated, melted, compressed and pumped through a shaped die to form a continuous stream of product of the desired cross-sectional shape. A typical extrusion system is shown in Figure 1. Typical extrusions include pipe, sheet, film, coated wire and cable, monofilaments, parison for blow-molding, foams, fibres and various profiles [Hensen (1988)]. Twin-screw extruders are common in applications requiring superior mixing or improved heat and mass transfer [White (1990)].
Figure 1. Illustration of a Single Screw Plasticating Extruder (from “Computer Modeling for Extrusion”, K. T. O’Brien (Ed.), Carl Hanser Verlag, Munich. (1992).
In performing its task, the extruder functions as a solids pump, a heat exchanger and melting device [Davis (1993)], a melt pump, a chemical reaction or mass transfer vessel [Davis (1993); Deason (1983)] and an intensive mixer or homogenizer. Heat transfer is important in all extruder applications, but becomes critical in cases such as reactive extrusion [Davis (1993)] and foam extrusion. In practice, there is little difficulty in melting polymer granules since, besides heating through the barrel, significant amounts of mechanical energy are added to the system through the rotation of the screw. However, removing heat is not so straightforward. Polymer viscosity increases sharply with decreasing temperature and for this reason, some of the energy removed through cooling from the barrel is unavoidably added back to the system in the form of viscous heating.
Of all components of an extruder, the single most important is the screw. Its effect on performance is so great that, in many respects, extruder design is equivalent to screw design. Screw configuration varies depending on the particular application. Figure 2 shows various typical plasticating screw designs.
Figure 2. Plasticating Screw Designs (with permission from “Computer Modeling for Extrusion”, K. T. O’Brien (Ed.), Hanser, Munich, 1992).
It is convenient for analysis to divide an extruder screw in three “zones”: The Feed or Solids Conveying Zone is designed to preheat the polymer granules and convey them to subsequent zones. In the Transition or Melting or Plasticating Zone, the polymer granules are melted and the resulting melt compressed. It has been shown in most extrusion applications (with the exception of twin-screw and pin-barrel extruders or certain PVC extrusions [Rauwendaal (1986)]) that during melting the polymer particles are pressed together to form a contiguous solid bed which is pushed against the trailing flight flank [Rauwendaal (1986); Tadmor and Klein (1970)]. In the Metering or Pumping Zone, the screw channel is full of molten polymer which is pumped towards the die. The motion of the screw relative to the barrel induces a drag flow which, because of the geometry of the screw, has the net effect of pushing the polymer forward. This is resisted by the pressure gradient operating in the opposite direction, and the net melt flow rate through the metering zone is the sum of these two terms [Tadmor and Klein (1970); Pearson and Richardson (1983); Rao (1986)].
The basic theoretical understanding of plasticating extrusion was developed in the 1960s. Models for solids conveying, melting of polymer granules and melt flow in the metering section have been systematically presented in the classical textbook of Tadmor and Klein (1970). Software for extruder screw design became available in the early seventies and have continued to evolve ever since [Pearson and Richardson (1983); Rao (1986)]. A variety of general purpose and specialized (CAD) codes can be used for the rigorous simulation of the extrusion process [Vlachopoulos (1992) and other references therein]. Such simulations help designers gain a better understanding of flow and heat transfer in an extruder, identify reasons for possible product defects and develop better physical models for the processes involved.
Davis, W. M. (1993) Heat transfer in extruder reactors. Reactive Extrusion. M. Xanthos, (Ed.). Hanser, Munich.
Denson, C. D. (1983) Stripping operations in polymer processing. Advances in Chemical Engineering. 12: 61.
Hensen, F. (Ed.). (1988) Plastics Extrusion Technology. Hanser, Munich.
Pearson, J. R. A. and Richardson, S. M. (Eds.). (1983) Computational analysis of polymer processing. Elsevier.
Rao, N. S. (1986) CAD of Plasticating Screws. Hanser, Munich.
Rauwendaal, C. (1986) Polymer Extrusion. Hanser, Munich.
Tadmor, Z. and Klein, I. (1970) Engineering Principles of Plasticating Extrusion. Van Nostrand Reinhold. DOI: 10.1016/0032-3861(72)90093-6
Vlachopoulos, J., Silvi, N., and Vlcek, 1. (1992) PolyCAD: a finite element package for polymer flow. Computer Modelling for Extrusion and Other Continuous Polymer Processes. O’Brien, K. T. (Ed.). Hanser, Munich.