| Inserts containing rough surfaces are more desirable for microscale parts and parts with micro and nanostructured surfaces because they provide controlled flow of the polymer melt inside the cavity. To evaluate the effect of tooling surface roughness on part filling, polypropylene parts with a nominal thickness of 2 mm were injection molded using etched stainless steel tooling inserts with smooth and rough surfaces. First, the rough tooling produced parts with greater mass and narrower mass distributions than the smooth tooling. For the smooth tooling, average part mass increased linearly with fill time at a rate of 0.30 g/s. Although a similar rate of increase was observed for the rough tooling at high switchover pressures, at low pressures, the rough tooling showed significant faster filling. This behavior suggests that surface-roughness-induced flow effects increased the flow of the polymer melt across the rough surfaces, but that high pressures affect the flow effects. Second, all parts molded using the rough tooling showed contact angles of about 85°, indicating complete replication of the tooling surface. In contrast, parts molded using the smooth tooling exhibited contact angles that varied from 94° to 104° and increased with higher melt pressures; this variation suggests that the part surfaces were modified, probably during part ejection. Finally, switchover by machine pressure yielded parts with narrow mass distributions and narrower contact angle distributions than the transfer by position. |
