Replication of Stochastic and Geometric Micro Structures - Aspects of Visual Appearance

The surface quality of injection molded parts depends on the processing conditions, the cavity surface structure, the runner system, the used polymer and the part geometry. Different replication of the cavity surface structure to the molded part influences its surface,function and visual appearance. A descriptive model of the replication process has been developed, taking into account the thickness of the frozen layer during injection. The expected dependencies were proven by systematic injection molding tests with geometric surface micro structures, having depths of 45 or 100 mu m and widths from 400 to 2 000 mu m, that were replicated into high-crystalline polypropylene (HCPP) and polycarbonate (PC). The volumetric structure replication ratio increased with rising mold temperature, melt temperature and cavity pressure. As expected, the mold temperature was dominant for very small micro features. The RMS roughness, determined by confocal microscopy and atomic-force microscopy, was found to be a suitable replication parameter for draw-polished to mirror-finished stochastic cavity surface structures. An abrupt change in wall thickness decelerated the flow front velocity, thus decreased the replicated polymer surface roughness and increased the surface gloss. Moreover, a several micrometer high "surface step" remained, due to the different thickness-dependent shrinkage. This step always ascended from the thicker to the thinner part area. The replication of a mirror-finished mold surface into HCPP was dominated by morphological effects. Local micro shrinkage differences led to micro sink marks, which affected the surface gloss much more than the mold surface structure.