Intelligent CAD/CAM tools for mold insert and EDM electrode design

Abstract

Plastic parts, especially those designed for consumer electronics products, are becoming more compact and freeform in shape. This requires the construction of compact and delicate geometric features in the injection moulds that produce these parts. It is now common practice to employ electric discharge machining (EDM) whenever the conventional machining process fails to machine such delicate features. Before the EDM process can machine such a feature in a mould insert, however, an EDM electrode has to be designed and manufactured. Although CAD/CAM systems are widely used in mould design and manufacturing, and specific commercial CAD/CAM systems that are customized for injection mould applications are also available, there are still no intelligent CAD tools that address the specific requirements of EDM electrode design. As a result, the quality of electrode design depends largely on the experience of design engineers, and the electrode design process takes up a substantial amount of the time allotted for the design of the injection mould. The aim of this research is to develop an intelligent CAD tool that supports EDM electrode design. There are two major challenges. First, the design of an electrode depends very much on the geometric shape and machinability of the feature and its relationship with the mould insert. However, there are no existing rules or formulae that explicitly govern the design of an electrode. In this research, the designs of a large number of electrodes are studied to identify general design practices and major design considerations. The second challenge is the investigation of a new technique that can automatically construct CAD models of electrodes. The basic approach of this automatic design technique involves the extraction of the region that requires EDM from the CAD model of the mould insert, the recursive partitioning of the extracted region into sub-regions according to the design guidelines, the construction of a 3D model of the electrode for each sub-region, and the possible grouping of individual electrodes into a larger electrode. The new technique developed for recursive region partitioning is inspired by the hint-based method for the automatic recognition of machining features. The major contribution of this research is the development of a new technique that automates the design of EDM electrodes. This new technique has been implemented as a CAD tool and integrated into a commercial CAD/CAM system through its application program interface. To demonstrate the capability of the CAD tool to deal with practical design problems, it has been tested with a group of industrial parts that have complex and delicate features. The improvement in the performance of the design process was measured in terms of the reduction in both design time and the amount of data input required from the user.