Why Extrude?

Why use an extruder to pump and melt plastic? Why not simply put it in a container and melt it with heat? The reason is that polymers are very poor conductors of heat, so that attempts to melt them with heat produce large differences in temperature. High temperatures develop which easily degrade (burn) the polymer, and the process is very slow. Consider the extruder machine with the screw turning within the barrel. The space between the screw and the barrel is fundamentally simple shear-strain. When the polymer is contained within the extruder and is subject to this shear strain, it is vigorously plastically deformed so that heat develops. The shear strain is everywhere in the polymer, so the heat generation is everywhere in the polymer. No heat conduction is necessary to warm the interior of the polymer, and a uniform melt temperature is efficiently developed. Also, the extruder screw is configured to have a helical passage so as simultaneously pump the polymer as it is being sheared (heated.) It then serves to simultaneously melt, heat, and transport polymers very efficiently and robustly. Useage of such an extruder machine has persisted for at least six decades, during which time it has been modified and challenged by other devices. However, the same shear-strain principle is still the predominant method in modern extruders.

The Kinematic Model

Therefore, the fundamental starting point of understanding the single screw extruder is one of simple kinematics. The myriad of extruder screw designs can be easily and robustly characterized by the calculation of the shear-strain field that exists as the screw turns in the barrel. This calculation solely involves only the screw and barrel dimensions and the speed at which the screw is turning. All of these factors can be accurately measured, which makes this most basic kinematic analysis very precise and robust. This is a statement of one understanding of the physics. The next step is to represent this in equations and express them with a computer program and is under development.

Solids Conveying

The solids conveying section of an extruder screw is often a primary source of problems. The Extruder Tech model for solids conveying is available which includes: *Cylindrical coordinates, *Heat transfer with barrel and screw, and *Developing temperature in the solids. Pressure and temperature development in the solids are predicted along with the onset of melting. Data needed are screw dimensions. Friction factors can be supplied by the user, but consultation on defining the friction factors is available. The model can be setup to include the friction factors as a function of temperature, stress, and velocity. The application of this model is particularly useful for deep feed sections where the need for cylindrical coordinates is critical. Most solids models neglect the curvature of system or artificially adjust for it. The basis for this work is "Control Volume Analysis for Feed Flow in Extruders" by S. J. Derezinski, Society of Plastic Engineers, ANTEC '98, pp 142-147.