Investigation of effect of recycled material concentration with process parameters on residual stresses in injection molding

Residual stresses are inevitable in injection molding. Selection of proper process conditions for the reduction of the negative effects of these is an important case. In this study, two polymeric materials as polystyrene (PS) and high density polyethylene (HDPE) with their recycles were used. The effects of injection conditions such as melt temperature, mold temperature and cooling time on residual stresses were investigated by changing them successively. The main aim of this thesis is to investigate the effects of recycled polymeric materials on residual stresses in plastic injection and there has not been seen any study on this subject in the literature review. In this study, after the recycled of polystyrene and high density polyethylene were obtained, blends were prepared by adding recycled materials to their virgins by weight of 30 %, 50 %, 70% and 100%. Melt flow index and differential temperature scanning tests were applied and morphologies of the blends were investigated with electron scanning microscope. The effects of the recycled materials on the elastic modulus, yield strength and notched impact strength of the injected standard specimens were investigated under various process conditions. For determining the residual stresses, layer removal method was applied. According to these thermal, rheological, morphological and mechanical tests, while the increase of recycled concentration reduced residual stresses for polystyrene blends, just the opposite was seen for the high density polyethylene blends. Of the polystyrene blends, the blend with 30 % recycled material gave higher elastic modulus and yield strength than that of virgin polystyrene but at the same time residual stresses of this blend was the highest. Lowest residual stresses were seen especially in the blend of 50 % concentration and it gave close values of mechanical properties to the virgin polystyrene?s. Among the blends of the high density polyethylene, the blend of 30% concentration gave the lowest residual stresses. Although a little reduction of elastic modulus was seen in this blend, because the residual stresses did not rise so much, it showed an ideal blend characteristics with its higher notched impact strength. For the HDPE blends over the recycled concentration of 30 %, while elastic modulus and yield strength increased, notched impact strength decreased and residual stresses increased. Keywords: Residual stresses, plastic injection, recycled material