Relative influence of injection molding processing conditions on HDPE acetabular cups dimensional stability

The effects of processing conditions on the characteristics of injection-molded high density polyethylene (HDPE) acetabular cups for the total hip replacement implants have been studied. The study has been focused on the effects of filling rate (injection speed), packing pressure, packing time and hot nozzle temperature of the mold on the dimensional stability, weight variation and visual appearance of the acetabular cups. A considerable influence of these processing conditions on the weight and dimension variation was found. Information regarding cycle time duration was obtained through numerical simulation of the entire molding process using a commercial dedicated code Moldflow Plastics Insight 6.1 (Moldflow Corporation, 2007). Within the influencing process parameters studied, the packing pressure was found to be the most significant one affecting the part dimensions and weight. An increase in the injection speed (decrease in the injection time) had a negative effect on the weight and on the most of the part’s analyzed dimensions. The influence of the hot nozzle temperature and packing time was considered irrelevant, for the range of process parameters under analysis. For all the set-point variation tests, two singularities on the linear dimension variations were identified: anisotropic shrinkage in flow and normal to flow direction and a slight plastic expansion at the location around the hall furthest away from the injection point.