Correlation of gas penetration and permeation to the structural performance of gas-assisted injection-molded parts

The correlation of insufficient gas penetration and gas permeation to struc tural performance in gas-assisted injection-molded polystyrene parts, desig ned with gas channels having five different types of cross sections but wit h the same section area, was investigated via tensile experiments and bendi ng tests. Test results were also compared with those of standard injection- molded parts. Based on the measured results, it was found that for specimen s with insufficient gas penetration, under tensile test and bending test th e breaking region occurred in the neighborhood of the gas front tip near th e gas entrance. This indicates that the effect of the hollow core developed by the gas, resulting in reduction of the cross-sectional area, is larger than the effect of stress concentration or residual stress at the gas front tip. In tensile tests, it is evident that gas permeation and insufficient gas penetration show significant influence on the tensile properties of par ts, lowering the ultimate tensile strength. Moreover, in bending tests, the stress concentration or residual stress occurring at the gas front tip of insufficient gas penetration and the surrounding area of gas permeation sho ws less significant influence on part bending properties. At the same time, the cross-sectional geometry plays an important role in determining bendin g properties. The present investigation provides part designers design guid elines for understanding the correlation of insufficient gas penetration an d gas permeation to structural performance of gas-assisted injection-molded parts. (C) 1999 John Wiley & Sons, Inc.