The Single Screw Extrusion of Pastes

Screw extrusion is widely used but little is known about the design and operation of this type of equipment save in the polymer industry. Particularly with the advent of high-performance, high-value ceramic materials, it has become important to extend knowledge. Here a mathematical model for a flooded screw brings together recent paste rheology and a model of screw extrusion developed previously for frictional solids flow. The model suggests that helix angles in current use are significantly below the optimum. Furthermore it appears that the deepest channel screws will produce the highest pressure gradients, a fact already established in practice which is in contrast with screws for polymers. Experiments both with and without flow on a 20 mm diameter laboratory extruder used screws of varying angle and channel depth with pastes of various rheologies. Pressure measurements from experiments without flow could show a marked offset from predictions with screws of low helix angles, this apparently being the result of an increased surface shear stress at the barrel-paste interface. The effects of changing the screw speed were reliably predicted. Experiments with flow carried out on a 76 mm diameter industrial machine showed general consistency with predictions.