The Research Of Sequential Co-injection Molding Based On Numerical Simulation

The sequential co-injection molding is a technology to produce plastics part in that the different melts are sequentially injected into the cavity and a multi-layer product that contains complex character is obtained after the melt is freezing. Usually, the melt firstly injected is the skin layer of the part and the melt secondly injected is the core layer. As a new injection molding technology, avoiding the core melt breakthrough and obtaining more core melt in the product are quite significant for theoretic study and practical application of sequential co-injection molding. However, the investigations dealing with co-injection molding which were published in the past three decades could not provide comprehensive engineering-analysis results and system theories how to optimize the product configuration and how to calculate the minimum volume of skin melt prefilled.By the MPI software, the effect of pseudoplasticity on the tendency of core-melt front breakthrough was discussed, and based on this research, the effect of the length-width ratio of rectangular-plate part on the tendency of core-melt front breakthrough was also discussed. The mathematic formulae to reflect factors that affected the tendency of core-melt front and calculate minimum-volume fraction of skin melt prefilled were created. Simultaneously, sequential co-injection molding process was researched by MPI software. The formulae to be set up in the research were proofed and the means that reduce computing error was given.The research result shows that: (1)Under different injection rater, the change of the viscosity ratio of core/skin melt is the important factor which affects the tendency of core-melt front breakthrough. The pseudoplasticity and shear state of the melt affect the viscosity ratio of core/skin melt. If the pseudoplasticity of core melt is less than (equal to, or greater than) that of skin melt, increasing the injection rate of core melt will decrease (not change, or increase) the tendency of core-melt front breakthrough. Whatever the pseudoplasticity of core melt is less than (equal to, or greater than) that of skin melt, increasing the injection rate of skin melt will decrease the tendency of core-melt front breakthrough.(2)The calculation result by the formula created this paper which calculates the tendency of core-melt front breakthrough accords with the result of numerical simulation experiment. If fixing material pair and molding conditions, with the change of length-width ratio of the rectangular plate, block from skin melt and spatial steric hindrance will change average flow-speed of core melt. That was the important reason why the length-width ratio would affect the tendency of core-melt front breakthrough.(3)The formula created in this paper which calculate minimum-volume fraction of skin melt prefilled is rational. When viscosity ratio of core/skin melt is less, the calculate results are greater than the true value, vice versa. Due to the vary of the material pair, the pattern of core melt front changes gradually to protrusive or complicate with the increase of core-melt flowing distance, which will result in increasing or decreasing the average flowing velocity of core melt, then the calculation error from the formula will be less or more than the true value.(4)For the shell of hand-lawnmower base, the absolute error value from the formula in calculating minimum-volume fraction of skin melt prefilled is greater, but the error could be astringed step by step. For the shell, longer penetration length and evener core-melt distribution could be obtained by increasing the breakthrough tendency of core-melt front.