SIMULATION | PROCESSING The challenges of multi-cavity
co-injection moulding Moldex3D explains in this article how its CAE software can help overcome the moulding issues of a difficult process
Multi-cavity co-injection moulding is one of the most commonly used processes to manufacture automotive components and structural reinforce- ment products, and it has been widely applied in many other industries. The benefits of multi-cavity co-injection moulding include the ability to reduce material waste and cost, and further enhance the productivity of co-injection moulded parts. However, the same general guidelines for developing a single-cavity co-injection mould cannot be fully applied in the development of a multi-cavity co-injection mould. The key to success- ful multi-cavity co-injection moulding is proper core/skin distribution. Co-injection moulding is already a complex process itself. By combining the multi-cavity moulding process, which often results in flow imbalance, it would be very difficult to achieve the desired distribution of materials. The following case study illustrates how Moldex3D CAE software is used to evaluate the effects of injection flow rate and cavity design for designing a better multi-cavity co-injection mould. The runner geometries and the cavity used in this multi-cavity co-injection simulation experiment are shown in Figure 1. The material of the core and skin is Polyrex PG-22 [polystyrene from Chi Mei]. In the moulding process, a certain percentage of skin is injected first, and then the core material is injected to finish the filling process. The skin to core ratio is 72:28.
The comparison of the simulation and experi-
mental results of the core layer melt front is shown in Figure 2 according to the study. As shown, at a low injection flow rate (10.2 cm³/s), Branch 1 has the longest core penetration distance, while at a higher injection flow rate (51 cm³/s), Branch 2 is the longest. Both simulation and experimental results show similar trends. The following experiment was designed to
further investigate the effects of different injection flow rates on the low-viscosity core material penetration. As shown in the analysis results, when the injection flow rate is at 10 cm³/s, the core material in Branch 1 reaches the cavity first. When the injection flow rate increases to 16 cm³/s, the core material in Branch 2 reaches the cavity first.
Above: Co-injection moulded part
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Fig. 2 The blue lines show the melt front result measured in the experiments, and the coloured red area is the melt front simulation result
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Fig. 1 The cavity and the runner geometries used in the multi-cavity co-injection moulding experiment
November/December 2017 | INJECTION WORLD 33
PHOTO: STEPHEN WOOLVERTON
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