In-line rheology and physical properties of thermoplastic foam

High density microcellular foam can reduce production costs without significant deterioration in physical and mechanical properties of polymer parts. However, lack of knowledge of rheological properties for typical thermoplastics/blowing agent mixtures at all stages throughout the injection molding process hampers development of the manufacturing technology.; An in-line capillary rheometer nozzle was constructed to study the viscosity of single-phase low density polyethylene (LDPE), thermoplastic polyolefin (TPO) and glass-fiber reinforced polypropylene (GFPP) solutions with chemical and physical blowing agents at shear rates up to 105 s -1. A model based on a simplified Cross-Carreau model and the free volume concept was proposed to estimate the viscosity reduction resulting from the addition of a blowing agent into a polymer solution. The model employs a scaling method based on concentration-dependent, temperature-dependent and pressure-dependent shift factors to collapse the viscosity measurement to a master curve.; The viscosity reduction observed with chemical blowing agents and physical blowing agents in TPO melt were subsequently compared with the in-line rheometer under shear conditions relevant to industrial processes. It was seen that the exothermic masterbatch CBA (generating nitrogen) had a greater reduction effect on the viscosity of TPO melt than the endothermic masterbatch CBA (generating carbon dioxide). As to the physical blowing agents, carbon dioxide had a greater viscosity reduction effect on the TPO melt than nitrogen, owing to the difference in physical properties between these two gases and their respective solubility in the polymer melt. Comparing the plasticizing influence of the CBA species versus the PBA species, the extent of viscosity reduction was greater for the former additive type based on equivalent gas content in the melt. The differences in plasticization between the CBA and PBA species, and between the CBA species alone, have been attributed to the hetero-phasic nature of the polymer and the manner by which the blowing agents incorporate their gases into the matrix.; In addition, two different forms of an endothermic blowing agent, namely powder versus masterbatch, were compared for the rheology properties in GFPP solutions, showing a greater reduction in shear viscosity for the masterbatch additive. The lower shear viscosity was found to significantly affect the resulting fiber length distribution measured in the molded samples. Longer fibers survived to the mold cavity as the concentration of the masterbatch CBA was increased while in the case of the powder CBA the distribution of lengths more closely resembled those processed in the absence of a foaming agent.; An instrumented slot mold was designed and applied as an in-line slit rheometer to measure the viscosity of an unsteady two-phase polymer with chemical blowing agent suspensions during injection into the mold cavity. In the experiments, blowing agent type and concentration were varied along with processing conditions, to generate controlled cell structures in two different polymers, low density polyethylene (LDPE) and thermoplastic polyolefin (TPO). The viscosity of all gas-polymer suspensions revealed a reduction compared with neat polymer melt in the presence of gas bubbles, due to the reduced volume fraction of polymer matrix. A model has been proposed that yields good agreement with our experimental results for estimating the shear viscosity of two-phase flow in the mold cavity of the injection molding machine.; The effects of process conditions on different aspects of producing thermoplastic polyolefin (TPO) foam were explored, including the developed microstructure, crystallinity, surface quality, and the resulting mechanical properties of foam injection molded parts. Using a fractional factorial design of experiments (DOE) matrix, samples were injection molded by a short-shot technique with two chemical blowing agents (CBA)...