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Besides the above methodology, Ganter and Skolgund [6]also reported an approach in recognizing and extracting three classes of concave undercut features, viz. Internal voids, single- and multi-surface holes and boundary perturbations from a B-Rep model for the development of casting core in casting mould design. The features recognized are limited to concave features. The recognition of convex features is not mentioned in their work. The convex portions of the moulding may also be possible undercut features, and hence the method is not suitable for recognizing and extracting all the undercut features in a moulding. Rosen et al. [7]classified undercut features into external undercut features,accessible laterally with respect to the mould closure direction, and internal undercut features, accessible only through the mould core. For a given parting direction, the first step is to ignore all the surfaces with a positive or zero dot product between the surface’s normal vector and the parting direction, and all the other surfaces are then projected onto the plane perpendicular to the parting direction. If the projected surfaces overlap, then the surfaces form an undercut feature.This method can be used to design side-cores and sidecavities. Hui [8] presented a mouldability analysis based on the geometry of moulded parts. The notions of the internal and external undercuts are defined and the mouldability is studied based on these notions. However, the recognition of these undercut features and the undercut feature parameters, which greatly influence mouldability, are not discussed.
In this paper, the definition and classification of undercut features are presented. Based on the undercut feature characteristics, the relationship of mouldabilty and undercut features is described. The definition of undercut feature parameters and their determination are also presented. For the different undercut features, the recognition criteria of these features are also proposed.
2. Definition and classification of undercut features
2.1. Definition of undercut features
In a moulding, concave regions such as local recesses,slots, pockets and holes are potential undercut features;while convex regions such as cylinders, bosses, cones and spheres can also be undercut features. If the main core and cavity and their inserts cannot mould the undercut features,side-cores or side-cavities or other local tools are required to be incorporated in the mould structure. In Fig. 1, there are three undercut features A, B, C in the moulded part. If the parting direction is selected as shown in the figure, then undercut feature A can be moulded in by the main core,but undercut features B and C cannot be moulded in by the main core, cavity and inserts, and would need sidecores and side-cavities for moulding. Consequently, they are the real undercuts. It is noted that whether A, B or C would become the real undercut is dependent on the particular parting direction.
2.2. Classification of undercut features
Based on the mould structure and the local tools (e.g.side-cores, side-cavities, form pins, and split cores) required for a moulding, the undercut features could be classified into two types: External Undercut Features (EU) and Internal Undercut Features (IU). EUs are moulded in by the sidecores or side-cavities; while the IUs are moulded by form pins or split cores inside the core and cavity. Side-cores and side-cavities are withdrawn outwards from the moulded part before injection; while form pins or split cores are withdrawn inwards from the moulded part. EU is the restriction region, which prevents the moulding from being withdrawn from the cavity; while IU prevents the moulding from being ejected from the core. On the contrary, according to the geometric characteristics of the EU and IU features, EUfeatures can be further classified into Inside External Undercut features (IEU) and Outside External Undercut features (OEU). In order to define IEUs as shown in Fig. 2, the target surface, which is defined as the surface in which there are undercut features to be identified, is investigated. It is found that the edges of the target surface form different edge-loops in which the edges are linked together. In a target surface,the largest edge-loop is its outside boundary and is designated as the external edge-loop (an external edge-loop 1 is shown in Fig. 2(b)). The other edge-loops lying inside the largest edge-loop are known as the internal edge-loop (e.g.internal edge-loops 2 and 3 as illustrated in Fig. 2(b)). On the contrary, from the viewpoint of surface features, the adjacent surfaces of a target surface are classified into external and internal adjacent surfaces. The internal adjacent