Flow Patterns And Gas Penetration In Gas-assisted Injection Molding Process

Gas-assisted injection molding, as a newly developed polymer processing technique, is widely used in manufacture. It makes the production of parts with complex geometry, precise dimension, high surface quality and high strength to weight ratio. And it also makes reduction of weight and cycle times, smaller holding pressure and less clamp forces and provides higher flexibility in part design.Skin melt ratio was deduced as function of non-Newtonian index and pressure gradient rate of gas and melt front and its variation trend and range was analyzed. A mathematic model of the velocity and displacement of gas and melt front was founded, and a simple analysis was given.Three flow patterns were found in gas-assisted injection molding process: complete by-pass flow, recirculation flow and stagnation point. The distribution of streamline and velocity were observed and studied, and it was found that the influence of gas on the flow field was only local.Because the value of fractional coverage for temperature sensitive Newtonian fluid is constant under a high capillary number in isothermal capillary tube, any deviation from the constant value of 0.6 would be caused by temperature gradient. The capillary tubes were put into two water baths with different temperature early or late, and then temperature gradient formed in the tubes. The frozen layer model was founded to calculate the fractional coverage. Then, the influence of temperature and delay time on gas penetrating process was studied, and the delay time of the fractional coverage reached the top value and the influence of temperature gradients between the two water baths on fractional coverage was obtained in the study.