Simulation of the development of the microstructure of injection-moulded semi-crystalline thermoplastics by means of a multi-scale approach under consideration of shear-induced crystal forms (alpha and beta)

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Semicrystalline thermoplastics have partially spherical spherulitic microstructure. Despite the geometrical similarity of all spherulites, there are three distinct forms (α-, β- and γ-form) that arise from unique unit cells. This results in different local mechanical, thermal and optical component properties. The α-form represents the predominant crystal modification. In general, the lamellae are formed in the form of folded chains in the crystallization of semi-crystalline homopolymers in a quiescent melt. Isotactic polypropylene (iPP) in the α-form shows the unique phenomenon that lamellae are arranged in a so-called "cross-hatched" structure. The β-form has a lower degree of order compared to the α-form, which results inter alia in a higher degree of crystallization and a lower melting point. The formation of the crystalline regions of the β-form can be promoted by the addition of nucleating agents, by the choice of thermal conditions as well as by shear stress. The formation of crystallites of the γ-form occurs relatively rarely in industrially produced components and only in small proportions. It should be noted that the developed model at IKV for describing the crystallization process can not distinguish between different crystal forms. Which means all spherulites are considered the same and in the α-form, which does not correspond to reality, since the influence of β-spherulites are neglected. In the scientific literature, no work or approach is known which simulates the crystallization process in an injection-moulded component made of semicrystalline thermoplastics taking into account the different crystal forms.The aim of this project is the simulation of microstructure formation taking into account the shear-induced crystallization and consequently two dominating crystal forms (α and β) to calculate realistic microstructures and thus to precisely predict the component properties. To this end, the developed model at the IKV will be adapted so that unique nucleation and growth models for each crystal form will be derived and the calibrated using experimental experiments. These models are then implemented in the software SphäroSim, developed at IKV. The boundary conditions for the microstructure simulation come from filling simulation results based on a self-developed approach that describes the boundary conditions with a high resolution. The simulated microstructure will be then compared with the components, which will be produced under the same injection moulding process, and verified.