Crystallization degree dependent effective thermo-elastic and thermal properties of an injection molded polypropylene component. Part 1: Multiscale simulation scheme and effective lamella properties

In injection molding processes of semi-crystalline polymers, inhomogeneous solidification of the melt occurs resulting in complex warpage of the final part. They present a strongly different cooling behavior at mold walls and in their center. Thus, locally different spherulite microstructures are formed in the component leading to residual stresses formed during the injection molding process. To determine the effect of these inhomogeneities on the local thermo-elastic and thermal properties, the injection molding of an isotactic polypropylene (α-iPP) stepped plate is investigated. The previously developed multiscale simulation scheme has been extended to address thermo-elastic homogenization of semi-crystalline polymers. A new Representative Volume Element (RVE) of the cross-hatched crystalline-amorphous α-iPP lamella is introduced at the nanoscale, leading to a stiffer and less anisotropic effective lamella behavior. Besides, a relationship between the local crystallization degree and the cooling rate is derived, based on DSC and Flash-DSC measurements. Corresponding to the local crystallization degree, a specific RVE either with or without secondary branches is designed. In this way, the effect of locally different crystallization degrees on the effective thermo-elastic and thermal properties of the effective semi-crystalline α-iPP lamella is first determined at the nanoscale. The predicted values for the effective elastic Young’s and shear moduli are smaller at mold walls and stiffer in the core area of the part than the corresponding modules, predicted with a constant, mean crystallization degree ξ over the plate thickness; whereas the mean effective thermal expansion α m decreases continuously with the crystallization degree ξ over the half plate section. Highlights : • Derivation of an exponential relationship between cooling rate and crystallization degree via an enthalpy approach and DSC and Flash-DSC measurements. • Accurate design of the cross-hatched α-iPP lamella RVE improves substantially its effective Hooke matrix, which becomes stiffer and less anisotropic. • Specific lamella RVE’s are generated for each local crystallization degree in a plate section by varying length and spacing of its daughter branches in its centre and by omitting these branches at both surfaces. • Effective local elastic lamella properties vary strongly with the crystallization degree over a plate section: their reduction near both plate surfaces is more pronounced than their increase in the plate centre. • Thermo-elastic homogenization of semi-crystalline polymers, which uses mixed stabilized finite elements to handle accurately the quasi-incompressibility of the amorphous phase.