Prediction of morphology development within micro–injection molding samples

The understanding of the mechanisms that determines mechanical performances during micro-injection molding is crucial to meet the increasing demand for advanced plastic products with improved durability and performance. These depend on the morphology developed during the process, which, on its turn, is determined by the complex thermomechanical history experienced by polymer chains. Thus, the ability to correctly predict morphology opens the possibility to forecast final part properties. Semi-crystalline polymers parts show a morphology mainly composed of spherulites in the core and fibrils close to the sample walls. In this work, a commercial software for injection molding simulation was adopted for describing the thermomechanical history experienced by polymer chains during micro–injection molding tests, whereas a crystallization model was introduced for the description of the two morphologies mentioned above. The molecular stretch parameter was adopted as the main variable for describing the effect of flow on the molecular arrangement. A threshold mechanism based on the molecular stretch was also introduced in the crystallization model for controlling the growth of fibrillar morphologies. Different micro-injection molding tests were conducted in an end-less cavity with the aim of testing the capability of the model in predicting the morphology developed along the sample thickness. The proposed model was able to predict the formation of fibrils in the regions close to the sample wall, furthermore, the reduction of fibrillar layer thickness with the mold temperature increase was consistently described.