Mechanical performance of double gated injected metallic looking polypropylene parts

Abstract

The metallic effect obtained by incorporation of metal particles in polymers by injection molding has the advan- tage of eliminating post-processing techniques reducing production cost and time. Nevertheless, undesired defects in the final appearance of parts are common. In this work PP/aluminum pigments were obtained by direct injection molding and the influence of metallic particles on the aesthetic, morphological and mechanical properties of the parts was assessed. Aes- thetic aspects could be improved by manipulating processing conditions: high melt temperatures diminished differential shrinkage and made weld lines less noticeable. Also at high melt temperatures Al particles increased thermal conductivity of PP generating a thicker skin, which combined with an inherent gradient temperature and typical shear stresses developed during injection molding, induced the formation of !-PP phase. Mechanical performance of parts showed to be dependent on PP morphology. Distinct deformation behaviors were seen according to the presence of PP polymorph, !-PP counteract- ing the detrimental effect of not bonded Al flakes, and making PP-Al moldings to have similar toughness as PP moldings with the added value of metallic looking.