Filling behavior of micro-ribs on injection molded parts depending on micro-structure orientation and processing parameters

In order to better understand the formation of micro-structures on macroscopic parts during the filling stage of injection molding, an experimental study was performed on a simple test part with micro-channels placed parallelly and perpendicularly to flow direction. The single channels had cross-sectional areas of 55 × 45 µm² (height × width) and were placed on a plate shaped part with global dimensions of approximately 75 × 25 × 1 mm³. Short shots with a highly fluent Polypropylene grade were injection molded with the melt front stopping in the structure fields. The barrel and mold temperature, the injection rate as well as the use of a variothermal heating system were varied in a systematic Design of Experiments. The shape of the flow front was then investigated with the optical measurement system Alicona InfiniteFocus. The gained data was analyzed with Matlab scripts and provided the needed distance to completely fill the structures as a reference value (defined as “filling difference”). The influence of the mentioned processing parameters on the filling difference was analyzed using the statistics software Minitab. The investigations showed that the structures orientated in melt flow direction were filled better than the structures perpendicular to it. Regarding the process settings the use of the variotherm heating system always resulted in complete filling of the micro-structures. Therefore, the experiments with active variothermal system were excluded from further evaluation in order to better recognize the influence of the other parameters. As expected, the most important factors were the mold and barrel temperature both resulting in a better filling of the micro-structures when increased.