Investigations into surface visual quality of gas-assisted injection moulded polypropylene parts

Abstract

Although gas-assisted injection moulding (GAIM) provides many advantages compared with conventional injection moulding (CIM), its applications are limited to surface visual quality studies. In the present study, polypropylene plate parts designed with gas channels having five different types of cross-section but with same cross-sectional area were gas-assisted injection moulded. In addition, various plate thickness parts designed with semicircular gas channels of different radius were also moulded. The surface visual quality of GAIM parts was investigated via gloss and chromatics measurements. The effects of processing parameters and geometrical factors, introduced by part thickness, shape and associated dimensions of gas channels on glossy difference and chromatic aberration of GAIM parts were investigated. The effect on the surface visual quality of gas channel with fillet design and cavity surface with texturing treatment was also examined. It was found that glossy difference is very sensitive to the degree of crystallinity whereas gas channel residual skin melt thickness plays a dominant factor for chromatic aberration. The processing conditions significantly affect surface visual quality. Gas channel design of semicircular cross-section (shape A) provides a better surface visual quality than the other designs. In addition, in order to obtain best surface visual quality, the ratio of equivalent radius to plate thickness should be approximately equal to 2.3. Alternatively, surface visual quality can be improved by texturing treatment on the cavity surface of the core-side. The present study provided part design guidelines for choosing the most effective gas channel design to achieve the best surface visual quality.