Crystallization, microstructure, residual stresses and birefringence in injection molding of semicrystalline polymer: Simulation and experiment

This research is an attempt to predict the crystallinity, microstructure, residual stresses and birefringence in injection molded articles of semicrystalline polymers. Isotactic polypropylenes (i-PP) with various molecular weights were chosen for the study.; First of all, a phenomenological approach was employed for the quantitative description of the extent of the molecular orientation induced by the nonisothermal flow. A unified crystallization model, which can describe both quiescent and flow-induced crystallization phenomena, was proposed, and utilized to describe the formation of multilayer microstructure arising in the molded polymers, via the “competing mechanism” for introducing various microstructure layers. The application of transport laws, along with the unified crystallization model and the modified Cross model, to the injection molding led to the process simulation with prediction of crystallinity and microstructure development in the moldings during the molding cycle. Material constants as required in the simulations were determined from various experiments. Measurements of crystallinity and microstructure of injection molded i-PP's samples were conducted, and compared with the simulations.; Based upon the modifications of the Indenbom theory for inorganic glasses and linear viscoelasticity, thermal residual stresses in freely quenched semicrystalline polymer slabs were modeled and compared with the experiments. These stresses were introduced to consider the thermally-induced residual stresses in the molded articles. The modifications were made to include the crystallization and its effect on the shear modulus of the polymer. In the case of the Indenbom theory, a polymer during crystallization due to quenching was assumed to undergo an abrupt transition from an ideal plastic state to an elastic state upon the completion of crystallization. For the theory of linear viscoelasticity, the Morland-Lee constitutive equation was utilized with the effect of crystallization on the time-temperature dependent shear modulus taken into account. The Spencer-Gilmore equation of state was employed to determine the specific volume changes during the quenching and the local thermal loading contribution to the thermal residual stresses. To predict the flow-induced residual stresses and birefringence in the moldings, an idealized viscoelastic simulation of injection molding of semicrystalline polymers were conducted in terms of a filling and a cooling stage. In the simulation, an idealized problem was solved for one-dimensional, unsteady, nonisothermal flow based upon the Leonov viscoelastic constitutive equation. The unified crystallization model was incorporated to take into account the effect of crystallization on the flow-induced residual stresses and birefringence. Based upon both thermally- and flow-induced contributions to the residual stresses and birefringence, it was found that the thermal stresses were greater in magnitude than the flow stresses, and the multilayered microstructure pattern significantly influences the birefringence distribution in the moldings.