Experimental and numerical investigation of shrinkage and sink marks on injection molded polymer gears: a case study

Injection molding (IM) is the most efficient and cost-effective method for mass-production of plastic gears. This method aids in reducing the time and cost required to create appropriate shape. Injection molded gears experience shrinkage on addendum diameter and sink marks on rim during cooling and after ejection. To achieve accurate dimensional precision of the gear, these defects should be carefully minimised to improve gear life. The influence of packing pressure, packing time, and melt temperature on diametric shrinkage, mass, and sink marks of polypropylene (PP) gears was studied numerically and experimentally. The findings demonstrate that increasing packing pressure and packing time reduces diametric shrinkage and increases mass. On the other hand, contrasting effect of melt temperature was observed i.e. increase in shrinkage and decrease in mass. Experimental and numerical study revealed minimum diametric shrinkage of 1.606% and 1.853%, respectively, with the identical injection molding process parameters. Sink marks were seen on the rim surface between the hub and the dedendum circle. As the packing pressure increases, the depth of sink marks had reduced. It does, however, rise as packing time and melt temperature increases. The minimal depth of sink marks was found to be 0.07 mm and 0.115 mm, respectively, in experimental and numerical study.