Core and cavity generation method in injection mould design M. W. FU, J. Y. H. FUH and A. Y. C. NEE

Core and cavity generation method in injection mould design M. W. FU, J. Y. H. FUH and A. Y. C. NEE

In a computer-aided injection mould design system, the generation of parting surfaces and the creation of core and cavity blocks is usually a bottleneck. The parting surfaces and core/cavity blocks are created based on the parting direction and parting lines. Here, the architecture of an injection mould design system is proposed on the basis of the practical information flow and processing steps in mould production lifecycle. In this architecture, the methodology to generate the parting surfaces and the core/cavity blocks is proposed. To generate the parting surfaces, the parting line edges are classified and the extruded directions specified to the different groups of parting line edges. Extruding the parting line edges to the boundary of the core/cavity-bounding box generates the parting surfaces. To create the core/cavity blocks, the Boolean regularized difference operation (BRDO) is used and the related algorithms are presented. The criterion to identify whether the undercut features need local tools for moulding is proposed. The case studies illustrate and validate the methodology to generate the parting surfaces and core/cavity blocks.

In injection mould design, the main design activities include the determination of parting direction, parting lines and surfaces, selection of mould types, cavity layout, gating, ejection, venting, heating/cooling types, mould materials, and the temperature control system. After the parting direction and lines are determined and the design scheme is decided, the rest of the detailed design activities mentioned above can proceed. Based on the known parting direction and parting lines, the methodologies related to the generation of parting surfaces and core/cavity blocks are presented.

To generate the core/cavity blocks automatically, two methods known as the Boolean-based approach (BBA) and the Euler-based approach (EBA. In BBA, the core/cavity blocks are generated using the Boolean regularized difference operation (BRDO) between the core/cavity bounding box and the moulding. In EBA, the Euler operation is the key process to generate the related core/ cavity block surfaces.

Generation of core/cavity blocks

Core and cavity blocks can be generated based on the following steps.
Step 1. Identify all the through-hole undercut features and `patch’ up all these features.
Step 2. Generation of a bounding box. The bounding box is defined as the material stock that fully contains the moulding with enough space for assembly of other components .Figure shows the bounding box of a moulding.
Step 3. Generation of parting surfaces and `sewing’ all the parting surfaces together.
Step 4. Subtracting the bounding box with the patched part solid by the BRDO and splitting the bounding box using the generated parting surfaces.
The procedures to generate the core and cavity blocks are shown in the figure . After patching all the through-hole features in Step 1, it is necessary to determine the maximum dimensions Lx, Ly and Lz of the moulding in X, Y, Z directions and the bounding box thicknesses a, b, c in three dimensions. The bounding box thicknesses can be determined based on the required mould strength and the moulding parameters. The final bounding box dimensions are (Lxa, Lyb, Lzc). In Step 3, the parting surfaces, generated based on the parting line edges, are sewn together. In the last step, the BRDO is performed between the bounding box and the patched moulding. After the operation, the bounding box has an empty space inside. The sewn parting surface is then used as the splitting surfaces to split the box into two mould halves. One is the core block; the other is the cavity block. The procedures are illustrated in figure.
In an injection mould, if the core and cavity blocks and their inserts cannot mould the undercut features, the incorporation of local tools in the mould structure will be needed. It is hence necessary to identify the undercut features that cannot be moulded by the core and cavity blocks based on the following equation:

Where is the undercut direction and is the parting direction. Above equation means that if the undercut direction is not in the parting direction, the undercut features will become the `real’ undercut features and local tools are needed. The method to generate the local tools is not covered here.


Conclusions
Here, the architecture of an injection mould design system is proposed based on the practical mould design procedures. An efficient methodology for the creation of parting surfaces and the creation of core/cavity blocks is presented according to the generated parting direction and parting lines. Three types of parting, namely flat, stepped or complex partings are proposed. To generate the core/cavity blocks, the Boolean regularized difference operation is adopted. Case studies are used to illustrate the procedures of the methodology and the related algorithms. The results have shown that the methodology is effective in providing a solution for the computer aided injection mould design system automatically to generate the parting surfaces and create the core and cavity blocks.