Study On Processing Conditions And Products Properties Of Water Assisted Injection Molding

Water assisted-injection molding technology is a new technology developed on the basis of the conventional injection molding techniques and gas-assisted injection molding technology. Because the thermal conductivity of water is 40 times the nitrogen and heat capacity is 4 times the nitrogen, the cycle time of the water assisted injection molding is significantly shortened than that of gas-assisted injection molding. At the same time, water-assisted injection molding has advantages over its better known competitor, gas-assisted injection molding. Because it incorporates a shorter cycle time to successfully mold a part, due to the higher cooling capacity of the water during the molding process. So the water assisted injection molding has a great advantage and application of space. But systematic research of the WAIM is very less.The objective of this dissertation is to study water penetration length and residual wall thicknesses of the molded parts by water assisted injection molding, mainly through the establishment of theoretical models and experiments and the study of water assisted molding products morphology, to search the law of crystallization.The main purpose of the study mainly include following aspects:The governing equations for viscous compressible non-Newtonian polymer melt in the second penetration of Water-Assisted Injection Molding (WAIM) were constructed using dimensionless method. To get the numerical solution, the varational equation for pressure was derived with integration by parts. The formula for velocity expressed by pressure gradient was also derived by the same method. In order to simulate water advancing distance along the radial direction during the second penetration, a formula for interface velocity depending on water pressure, melt density, viscosity, penetrated length and penetrated radius was generalized using integration by parts and mechanical principle. Finally, a program was developed to get the numerical solution for pressure, temperature, velocity and residual thickness during WAIM.Through the experiments, analysis of injection process parameters such as melt temperature, melt injection pressure, water injection pressure, injection delay time, packing time, volume of melt influence on the residual wall thickness and length of water penetration in water assisted molding has been made. The results show that:the processing conditions determine the residual wall thickness and the water penetration length of the products. Rising of polymer melt temperature will reduce the residual the wall thickness and the length of water penetration. Increasing of the injection pressure will reduce the length of water penetration. Increasing of the water injection pressure of water will increase the residual of the wall thickness of products and the length of water penetration. Increasing of the water injection delay time will increase the length of water penetration. Increasing of the holding time of water will increase the length of water penetration.A theoretical study of the internal morphology of polypropylene (PP) products manufactured by the water assisted injection molding has been carried out, using polarizing microscope (PLM). The formation mechanism of crystalline morphology has been analysed, comparing with the products of CIM. The results show that:the wall of products manufactured by the water assisted molding can be divided into inner surface, the core layer and the outer surface, in the thickness direction with the laws of crystalline morphology. The crystal structure of inner surface within the surface next to the product surface area showed a dense crystalline structure of the spherulites crystals, other regions showed slightly larger in diameter and irregularly shaped spherulites crystals structure. Crystal structure in the core layer showed the disordered arrangement of the main spherulites crystals, and the spherulite growth of the fully, large diameter, crystal and amorphous regions are clearly different. The crystal structure of the outer surface of products is close to the outer surface of the wall region (0-50μm) and close to the wall surface area similar products, also showed a compact spherulites crystals.