Verification of Appearance Failure in Application of Injection Foam Molding to Automobile Exterior Parts by Short Shot Method ～The Process of Dimple Generation through Surface Observation～

Thinning is one of the measures to reduce the weight of plastic exterior parts of automobiles. Reinforcing fillers such as rubber and talc are contained, but if the plate thickness is reduced, rigidity cannot be maintained. Therefore, reinforcement through use of ribs is increasing, however, this detracts from the exterior appearance due to sink marks on the back side of ribs. One method is to generate internal pressure by a foaming agent to suppress the rib sink, however, foaming technology is not applied to exterior parts because appearance quality is compromised by specific defects unique to foam molding. Swirl marks and dimples occur as appearance defects. The mechanism of swirl marks and measures to address them are known, but the mechanism of dimples is not clear, and measures to counter dimples have not been established. Accordingly, in this paper, we conducted experiments on polypropylene for automotive exteriors containing rubber and talc using the short shot method with the use of a chemical foaming agent, and observed the generation process of dimples, a specific foaming defect, through static and dynamic visualization experiments. As a conclusion, we discovered that there are two modes of dimple formation. We found that dimples are caused by the phenomenon of breaking foam due to swirl marks and because of shrinkage during cooling and solidification.