Krzysztof Mrozek

Influence of Processing Parameters on Residual Stress in Injection Molded Parts

Residual stresses are the source of shrinkage and warpage of the parts manufactured with injection molding technology and strongly influences its final dimensions. In complicated parts residual stresses are very difficult to predict without numerical tools, along with the warpage, what leads to problems with manufacturing parts that meet the expected tolerances. Residual stresses have also strong influence on mechanical performance of the part, where its high value can results with self-cracking during ejection from the mold. In this work numerical simulations injection molding process were performed to analyze the presence of residual stresses in manufactured plastic parts by this technology. Numerical simulations were used to find the relations between the processing parameters and the distribution and magnitude of residual stresses. Occurrence of residual stresses were analyzed with new 3D residual stress model implemented in Autodesk Moldflow® software. Qualitative strain-optics observations were performed to verify the differences between different sets of processing parameters. From investigated parameters the strongest influence on residual stresses was observed with packing time, while the weakest influence was observed with injection time.

Optimization of Injection Mold Design Through Application of External Selective Induction Heating

The aim of the presented research project is to optimize the injection mold for a better quality of thin-walled moldings without significant increase in cycle time, through application of external selective induction heating. The injection process of thin-walled parts is one of the most problematic procedures in plastics processing. Most commonly occurring defects of said parts are: short shots, diesel effect, welding lines, excessive internal stresses and warping. In order to reduce the number of these deficiencies, electromagnetic induction heating in concern with water cooling has been proposed to achieve rapid temperature cycling during the injection process. High-frequency induction is one of the most efficient methods of injection mold surface heating, by virtue of electromagnetic induction effect. The present study investigates selective induction heating of the cavity surface for three different thin-walled parts. Feasibility of the localized heating method was investigated through simulation of melt flow through the heated areas. The latter were selected on the basis of defects occurrence. To evaluate the applicability and efficiency of the induction heating process (within an area of 32×20 mm), a the plate with cooling channels and a movable heated insert were constructed. During the heating process, an inductor with a concentrator and a 10 kW generator. The measurement was performed by means of the Flir T620 thermographic camera and a Pt100 temperature sensor. Preliminary results allowed continuation of the experiment. For this purpose, an injection mold, consisting of three exchangeable cavity inserts on the stamp and matrix side, was assembled. In order to evaluate the applicability of induction heating, the injection process was carried out and its results were compared to those obtained through utilization of conventional means. The investigation showed that selective heating of the cavity surface to 150 °C afforded elimination of the majority of defects.Copyright

Research on External and Internal Induction Heating Effectiveness of Injection Molds by Means of Thermovision Measurements

The aim of this work is to compare the effectiveness of two induction heating methods of injection molds by means of thermovision measurement. The problem of selecting external or internal induction heating for thin-walled moldings used in electrical and electronic industry is taken into consideration. At first, the boundary conditions were defined. Then a group of three moldings with different defects were selected. The defects that have been taken to remove by means of induction heating are: weld lines, breaking hinges, air traps and diesel effect. In order to compare the methods of heating two models were created. The first one was made as a block of steel with milled grooves with a width of 2mm and a depth varying from 1 to 12mm. The second model consisted of two parts, one being placed in the second.The research stand consisted of prepared models, induction generator with power of 10kW, specially shaped inductor, thermovision camera and temperature sensor of PT100 type as a reference. First, the surface with milled grooves was heated in four different sectors (because of the shape of inductor) in time of 2s. The area of low-depth grooves heated up to 154°C while the surface with 12 mm grooves heated up to 120°C. It comes from eddy currents flowing. This phenomenon shows that effectiveness of external heating of grooved surface decreases with increasing of grooves depth.In the second case the cavity insert was heated as a coil inside the inductor which was located inside the mold. The measured value was the time of heat transfer from heated area to the forming surface in three configurations.Copyright