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Abstract In the design of plastic injection molds, the depressions and protrusions (DP) preventing the removal of molded part from mold are referred to as undercuts. This paper describes an approach to recognize DP feature within the curvature region representation based on the curvature properties of entities in B-rep model. The recognized depression features and the concave transitions between protrusion features are potential undercuts which affect the choice of parting directions of the molded part. The possible withdrawal directions of the potential undercuts are derived by geometric reasoning and visibility map computation. This method can recognize isolated and intersecting features with planar, quadric, and freeform surfaces. The identified potential undercuts with their possible withdrawal directions can provide decision support information for mold design such as selecting parting direction, parting lines and surfaces, etc. The case study is presented to verify the feasibility of the developed method.Keywords Injection molding . Freeformfeature 19777
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recognition .Undercut .Withdrawal direction
1 Introduction
The concept of form features originated in the process
planning of machined parts. From the machining viewpoint,
features are usually defined as concave portions of a part
that can be generated with metal removal processes. While in injection mold design, both depression and protrusion
(DP) can become undercuts which prevent the removal of
molded part from mold. Such undercuts are main factor
affecting the moldability of a part and determine the
structure and cost of the mold. So the extraction of
undercuts should be the first issue to be addressed before
other mold design activities such as determination of the
parting direction, parting lines, and parting surfaces can be
carried out.
Although much of the research has been done for the
recognition of machined feature, it is still necessary to
develop a methodology to recognize feature used in
molding processes such as injection molding and die
casting, because machined features may differ from those
required for molding processes. One representative work
has been presented by Chen et al. [1], which uses boolean
regularized difference operation (BRDO) technique. It
involved taking the regularized difference between the
convex hull of the part with the part itself (CH[P]−P). And
the resultant pockets are referred to as undercuts. The
possible withdrawal directions of the undercut are the
intersection of all visibility map (V-Map) of the surface in
the undercut. However the requirement to form the entire
pocket by a single mold piece is too rigorous. It may fail to
derive the possible withdrawal directions of some models
such as the one in Fig. 1a. Based on their method, the V-
Maps of the recognized undercut is null. But obviously the
undercut can be demolded by a pair of opposite withdrawal
direction as shown in Fig. 1a. To make up this deficiency
Hui [2] and Yin et al. [3, 4] decompose the volume of the
undercuts into convex cells by intersecting it with half
spaces of its faces having concave edges (Fig. 1b). Then
these convex cells are amenable to processing for mold-
ability analysis based on their V-Maps or freedom cones.
However, these algorithms are limited to the parts com- posed of polyhedra. The methodology may be general in
nature and can be applied to curved object (Fig. 1c)by
approximating the surfaces with a polygonal mesh, but the
running time of the computation of convex hull and volume
decomposition would increase significantly the more accu-
rately the approximation fits the original geometric surface.
Fu et al. [5, 6, 17, 18] attempted to provide a complete
solution for computer-aided injection mold design including