Effect of Melt-Memory on the Crystal Polymorphism in Molded Isotactic Polypropylene

The influence of γ-quinacridone as a β-crystal nucleating agent in injection molded isotactic polypropylene (iPP) is discussed. Samples are injection molded and characterized via polarized-light optical microscopy and X-ray diffraction. Mold-filling simulation is used to understand the shear and cooling processes during sample preparation. The cooling rate associated with the quench near the mold wall is estimated to be greater than 600 K s⁻¹ using simulation, confirming previous studies that β-crystal growth is not supported at that cooling rate. The non-nucleated samples form β-crystals at a distance of 100–300 µm from the skin and in the core of the sample, which is not expected based on quiescent cooling data. Since the mold-filling simulation does not predict shear in the core, the formation of the β-crystals formed in this region is attributed to shear-induced crystallization effects in the injection unit of the molding machine that are not modeled in flow simulation, as they are typically excluded from any molding simulation analysis. This “melt-memory” effect has shown to be significant, and it is suggested that the prediction of final properties of injection moldings requires understanding and knowledge of the entire shear history of the material including that of the injection unit.