Morphology of Microinjection Molded iPP Parts in Controlled Conditions

A part made of just one material (mono-material) is a new frontier in reducing environmental impact of plastic, being 100% recyclable. The focus of this work is on the possibility of controlling the properties of parts obtained by microinjection molding (mu IM). mu IM tests in variothermal conditions are conducted on an isotactic polypropylene. Two variothermal steps are adopted: the filling and packing stages are conducted at one selected mold temperature, and subsequent annealing steps are conducted while the sample is still inside the mold. A comparison with the parts produced under constant mold temperature is also performed. Electronic microscopy is adopted for the characterization of morphology. Furthermore, atomic force microscopy coupled with HarmoniX (HMX) tool allows detecting simultaneously local morphology and mechanical properties. The part morphology is characterized by a fibrillar layer close to the sample walls and a spherulitic core; a transition zone is also detectable in between. Morphology significantly influences mechanical properties: the transition zone shows the minimum value of elastic modulus, additionally, the elastic modulus increases with spherulite dimensions. Eventually, the annealing step results to be crucial in determining the part properties and can be adopted as a robust method to tailor properties through the process.