Study on the processes and warpage of double-injection-molded parts containing PP recycled materials

Double-injection molding involves the use of two injection units, A and B, which can combine two different plastics into one part in a single injection molding process. Double-injection molding can promote the recycling of plastic waste as well as improve the performance and dull appearance of plastic products made from recycled materials. However, the warpage of parts is a significant challenge in double-injection molding. Warpage is significantly affected by the properties of the recycled material and injection molding process. In this study, the relationship between the warpage of double-injection-molded parts using raw and recycled materials was studied through uniform design experiments. To investigate the processes and warpage, the material was recycled several times, and its properties were tested. A regression model was established to describe the quantitative relationship between the important parameters and warpage. The number-average molecular weight and weight-average molecular weight exhibited the largest decrease after the first recycling cycle with a 21.11% and 41.11% decrease, respectively. The melting temperature decreased from 164.61 to 159.35(o)C, and the crystallinity did not change significantly. The processing parameters varied with the change in the properties of PP recycled materials, with the melt temperature decreased from 225 to 210(o)C. In addition, the number of recycling cycles and holding time for B were the most important factors influencing the warpage of double-injection-molded parts, followed by holding time for A and melt temperature for A. Warpage can be reduced by increasing holding times B and A. The experimental results provide important data for reducing the warpage of double-injection-molded parts with recycled materials.