The Influences Of Material Combination And Process Parameters On The Quality Of Co-injection Molded Parts

Co-injection molding is an innovative molding technology developed from the traditional injection molding, which involves the flows of the core and skin melt under the complex thermodynamic history in the cavity. The interaction between the core and skin melts makes the selection of materials and the interacting of molding conditions more complicated than the traditional injection molding technology. Therefore, it is of great significance to investigate the effect of material combination and process parameters on the molding process and the quality of products for co-injection molding.The paper mainly focuses on the effect of materials selection and process parameters on the core melt breakthrough phenomena and the distribution and uniformity of core material. The distance between the core melt front and the skin melt front is defined as the chase distance in the paper. The research of core front breakthrough process was switched to the problem of the chase distance of the flow. The works and achievements included:1. The co-injection process was analyzed using Moldflow Co-injection analysis module with LDPE, HDPE, PS and ABS assembled as core and skin materials respectively. The effects of material selection on chase distance and the distribution and uniformity of core material was investigated. The influence laws of the viscosity ratio of core/skin melt on the quality of products were obtained: with the increase of the melt viscosity ratio of core and skin layers, the decrease tendency of the chase distance with filling time was weakened, and the chance of core melt breakthrough was less, but the results of the distribution and uniformity of core material get worse.2. Based on the Taguchi method, the effects of the process parameters of co-injection molding on chase distance and the distribution uniformity of core material were investigated using certain material combination. The effect degrees of all correlation factors on chase distance and uniformity of the distribution of core material were obtained. The results showed: the melt injecting velocity has notable effect, the core melt temperature and mold temperature have significant effect, and the skin melt temperature has the feebler effect on the chase distance and the distribution uniformity of core material. An approximately optimum combination of process conditions was obtained as the baseline of process setups for the subsequent single factor experiments.3. Based on the results of orthogonal experiments and the optimum combination of the process conditions, the three significant factors (melt injecting velocity, core melt temperature, mold temperature) were selected for the single-factor experiments to investigate the effects of the factors on the chase distance and the distribution uniformity of core material, and the influence laws of process conditions on the part quality were obtained. The results indicted that the decrease tendency of the chase distance with filling time weakens and the core melt breakthrough phenomena occurs less with the increase of melt injecting velocity and mold temperature, but the distribution and uniformity of core material get worse.