A methodology to deal with the automatic design of internal pins in injection mold CAD via the automatic recognition of undercut features is developed. The approach to automatically identifying the undercut features is first proposed. For the given parting directions, all the inner and outer undercut features are identified based on the topological relationship of geometrical entities. The outer edge loop, which represents the largest cross-section boundary along the given parting directions, is extracted and patched up. The surfaces of molding are then identified based on the classifications of their geometrical entities. To identify the deep inner undercuts in the molding, the projection of the main core, internal pins and their bounding boxes along the parting direction and the pin withdrawal direction are generated. Upon determination of whether the bounding boxes of any two internal pins and the main core projection have intersection area, the deep inner undercuts are located. The complete methodology is finally implemented and verified through case studies and the efficiency of the methodology in handling complex molded parts is thus illustrated.71770

Keywords: Undercut feature Parting direction Parting line Internal pin design Injection mold CAD

1. Introduction

Injection molding is an important manufacturing process. The plastic molded components manufactured by this process, the so- called moldings in this paper, are suffused in our daily life and widely used in industries. To shorten design and development lead-times, automatic and rapid design is critical. New CAD- enabled and tailor-made technology is thus needed to realize the automatic and rapid design in injection mold CAD.

In mold CAD, undercut feature recognition is an important step, as it is related to the determination of parting directions and parting lines, generation of core and cavity, and the creation of secondary molding tools which include side-cores and internal pins. To recognize undercut features, the topological relationships of geometrical entities in a molding need to be extracted. In tandem with this, many researchers conducted research on undercut feature recognition. Fu et al. developed a methodology to categorize and recognize undercut features [1,2]. The undercut features are classified into different categories based on the edge number and edge characteristics of the target surface. The undercut direction is then defined as the direction whereby the local tool can be withdrawn easily and most effectively to avoid the warpage of the molding. It is thus also the withdrawal direction of the local tool by which the undercut is molded. To identify

undercut features, Ye et al. proposed a method for identification of undercut features based on the extended attributed face-edge graph (EAFEG) [3]. The depressions and protrusions in a molding are extracted and defined as the potential undercuts. Ismail et al. presented a methodology for recognition of cylindrical-based features [4]. Based on the edge boundary classification, the types of all the cylindrical-based features are determined according to the three test points. The potential undercut existing in these cylindrical-based features are marked for further recognition. Khardekar et al. provided an algorithm to efficiently identify and display undercuts [5]. With the implementation of Gauss Map, the candidate directions for testing a two-piece mold are chosen.

To determine the parting direction, much research has been conducted. The definitions of surface visibility  in  both global and local interferences are proposed by Chen and Woo [6–8]. The visibility map (V-map), which is a spherically convex region consisting of the visible directions of the entire locally visible surfaces, is implemented to determine the optimal parting direction. The 3D model of the molding is mapped onto a unit sphere and the determination of its visibility is made. Gu et al. [9] presented an approach to recognizing the features automatically. The optimal parting direction is determined according to the candidate parting directions and feature type. Yin et al. conducted an approach to recognizing undercut features for near  net- shape manufacture parts [10]. By using the local freedom of sub-assemblies in the directed blocking graphs, the interacting undercut features and their convex hull are isolated and identified. Dhaliwal et al. also described algorithms for computing the global