Multi-piece molding technology is an important tool in producing complex-shaped parts that cannot be made by traditional two-piece molds. However, designing multi-piece molds is also a time-consuming task. This paper proposes an approach for automatic recognition of mold-piece regions and parting curves for free-form CAD models. Based on the geometric properties of objects and mathematical conditions of moldability, a collection of feasible parting directions is formed from which the sets of visible-moldable surfaces are identified for each parting direction. Moldable surfaces that are partially visible to a particular parting direction are recognized and pided into fragments by silhouette detection and edge extrusion. A set of criteria is proposed to arrange tentative fragments, which can be simultaneously visible to several parting directions, into appropriate regions for mold pieces. Finally, the parting curves for mold pieces are extracted from the corresponding mold-piece regions. The proposed algorithm overcomes the problems found in previous multi-piece molds and at the same time achieves high accuracy and high performance. Examples of industrially complex models are used to demonstrate the performance and robustness of the proposed algorithm. The approach is generic in nature, allowing its application to be extended to any complex geometry in 3-D mold design 69807

1. Introduction

Injection molding is one of the most common manufacturing processes extensively used today; it is able to produce parts with good quality and accuracy. From a geometric perspective, molds can be either two-piece or multi-piece. Conventional two-piece molds have only one primary parting direction, which constrains the two mold pieces to one axis of motion. However, undercuts can often be found in complex parts and, as a result, a number of side-cores is required to form the shapes of these undercuts. The higher the number of side-cores required, the higher the tooling costs and time consumed in the machine. Some complex parts with multiple undercuts may even be impossible to produce with two-piece molds. On the contrary, the use of multi-piece molds can overcome the aforementioned restrictions of traditional two-piece molds. With many different parting directions allowing free movement of mold pieces, undercuts can be eliminated so that no actual side-cores are required, thus significantly reducing

total manufacturing costs. Consequently, multi-piece molding has become an important technology in handling complex geometries that cannot be made solely by using two-piece molds. Fig. 1 shows examples of typical molded parts, two-piece molds, and multi- piece molds provided by Protoform  GmbH.

The main challenge in multi-piece mold design lies in iden- tifying feasible parting directions, mold-piece regions, and the corresponding parting curves and parting surfaces. In this paper, a systematic approach to automating the determination of these requisite data for constructing mold pieces is proposed. The ba- sic problem of multi-piece molds can be described thus: given a free-form CAD model, how do we determine the feasible parting directions D, mold-piece regions Si for each element di of D, and the  corresponding  parting curves?

The remainder of the paper has been organized in the following manner: in Section 2, related works are reviewed and an overview of the proposed algorithm is presented. Background information of surface moldability and ray testing for surface visibility are de- scribed in Section 3. Section 4 discusses the algorithm for au- tomatic recognition of mold-piece regions and parting curves. Several proposed criteria are also included. The algorithm imple- mentation and examples are discussed in Section 5. Conclusions are given in Section 6.

Fig. 1.    Examples of molded parts, two-piece molds, and multi-piece molds.

2. Related works and overview of the algorithm