Effect of mold temperature on the structures and mechanical properties of micro-injection molded polypropylene

The effects of the mold temperature of micro-injection molding on the microstructures and mechanical properties of isotactic polypropylene (PP) gears were studied. The micro-injection molded PP gears present a skin–core type of morphology. The core layer is much thicker than the core and shear layers. The generation of β polymorph in the test samples is easily promoted during the micro-molding process with high shear rate at low temperature, or low shear rate at high temperature. Nanoindentation tests show that the modulus decreases along the flow direction and increases with increasing mold temperature. The highly oriented shear layer has the highest nanoindentation modulus compared with the skin and core layers. [Display omitted] •The thickness of different MIM gear layers is affected by combined processing effects.•β polymorph in micro gear can be induced by low mold temperatures and high shear rates.•The phenomenon of high indentation modulus is attributed to micro crystal structure changes.