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

Abstract

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 μm and widths from 400 to 2000 μm, 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.

Vacuum Venting Enhances the Replication of Nano/Microfeatures in Micro-Injection Molding Process

“…The lowest depth replication achieved in the vertical channels was probably due to the feature's higher aspect ratio (see Table 2). This coincides with studies suggested that better replication can be achieved with features having lower aspect ratios [11,12]. However, features aligned at 45° and which had lowest aspect ratios showed lower depth replication compared to the horizontal features.…”

“…A study showed that vertical channels parallel to the melt flow direction in a dog-bone sample gave higher filling compared to horizontal channels which are perpendicular to the melt direction [6]. The better filling in the horizontal channels in this present study indicated that sample shape could affect the filling of features with a particular orientation [11].…”

Review of Factors that Affect Shrinkage of Molded Part in Injection Molding

“…affected by mold temperature both in longitudinal to (the third in ranking order) and transversal to (the second in ranking order) the flow direction. By considering semicrystalline (POM [106,111,112], HDPE and PP [105], HCPP and PC [113,114], PP and PA [111] or PA mixed with 316L [115]), amorphous polymers (PMMA [105,113,116], PS [105,113], COC [117], ABS [118]) or ceramic feedstock (Zr [119]), results identified the mold temperature as a critical parameter affecting shrinkage. Also, authors [11,112,120] determined that by increasing this parameter, led to an increased shrinkage.…”

“…This parameter affected shrinkage in semicrystalline (POM [11], HDPE [105], HCPP [114], PC [113,114], PP [105]) amorphous (PMMA [105,110,113], PS [105,113]) or cyclic olefin polymers [117].…”

Selected methods of injection molds cooling

“…Wykorzystuje się ją w wielu gałęziach przemysłu, począwszy od produkcji zabawek, artykułów gospodarstwa domowego, sprzętu elektronicznego i medycznego, a skończywszy na branży samochodowej, lotniczej i chemicznej. Technologia polega na wtryskiwaniu uplastycznionego tworzywa sztucznego do gniazda formy wtryskowej [6]. W ten sposób powstaje wypraska odwzorowująca kształt gniazda o specyficznych właściwościach fizycznych, zależnych od przebiegu procesu technologicznego [1].…”