A SIMPLE APPROACH TO DETERMINING 3-DIMENSIONAL SCREW CHARACTERISTICS IN THE METERING ZONE OF EXTRUSION PROCESSES USING A TOTAL SHAPE FACTOR

It is of great importance in designing extruders to predict the screw characteristics, that is, the throughput in terms ofthe screw speed and the pressure gradient along the down-channel direction. The screw characteristics depend on the extruder geometry, the operating conditions, and material properties of non-Newto nian fluids. This paper suggests a simple approach to determining the screw characteristics for a three-dimensional flow in a channel with a finite aspect ratio (ratio of a width to a depth, W/H) by introducing a Total Shape Factor (F-t) to correct a two-dimensional flow analysis for a channel with an infinite aspect ratio. In the present study, the Total Shape Factor (F-t) was defined as a ratio of a net flow rate obtained by the three-dimensional analysis to that by the two-dimensional analysis. In the proposed approach, the quantity, partial derivative F-t/partial derivative(H/W), which turns out to be almost constant, offers important information for understanding the effects of the flights. Therefore, partial derivative F-t/partial derivative(H/W) is extensively reported in this paper in terms of several dimensionless parameters. This simple approach with such a database will be very useful for extruder designers to predict screw characteristics.