Investigations into moulding window of gas assisted injection moulded polystyrene plates with various gas channel designs

Abstract

The design of the gas channel plays an important role in the successful application of gas assisted injection moulding. Although empirical guidelines for gas channel design have been proposed by the various equipment suppliers, quantitative rules based on well designed experiments have not been reported previously. To investigate the effects of geometry on gas penetration for two plate thicknesses, transparent polystyrene (PS) plates designed with semicircular gas channels of differing radii and with rectangular gas channels of differing width to height ratios have been produced using gas assisted injection moulding. Moulding windows and criteria for gas penetration were also chosen so that design rules could be defined quantitatively. The mouldability index was also classified into five levels (excellent, good, fair, poor, and bad) based on the relative areas of the moulding windows. From a plot of mouldability index against R eq, the ratio of equivalent gas channel radius to plate thickness, it was found that to obtain an appropriate moulding window (i.e at least a fair mouldability index) R _eq should be greater than 2. Gas channels with a semicircular cross-section provide better mouldability than those with rectangular cross-sections of the same cross-sectional area. For the same equivalent radius, the ratio of width to height in rectangular gas channels also affects the mouldability index. The present investigation provides part designers with preliminary quantitative design/moulding guidelines for choosing the effective gas channel design that allows the parts to be moulded within an appropriate moulding window, so that the uncertainty in both simulation and process control can be overcome. A methodology for the establishment of guidelines for quantitative design of gas channels is also proposed.