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    Department of Mechanical and Production Engineering, National University of Singapore, Singapore An injection mould is a mechanical assembly that consists of
    product-dependent parts and product-independent parts. This paper addresses the two key issues of assembly modelling for injection moulds, namely, representing an injection mould assembly in a computer and determining the position and orientation of a product-independent part in an assembly. A feature-based and object-oriented representation is proposed to represent the hierarchical assembly of injection moulds.This representation requires and permits a designer to think beyond the mere shape of a part and state explicitly what portions of a part are important and why. Thus, it provides an opportunity for designers to design for assembly (DFA). A simplified symbolic geometric approach is also presented to infer the configurations of assembly objects in an assembly according to the mating conditions. Based on the proposed representation and the simplified symbolic geometric approach,automatic assembly modelling is further discussed.23036
    Keywords: Assembly modelling; Feature-based; Injection
    moulds; Object-oriented
         1. Introduction
    Injection moulding is the most important process for manufacturing
    plastic moulded products. The necessary equipment consists
    of two main elements, the injection moulding machine
    and the injection mould. The injection moulding machines used
    today are so-called universal machines, onto which various
    moulds for plastic parts with different geometries can be
    mounted, within certain dimension limits, but the injection
    mould design has to change with plastic products. For different
    moulding geometries, different mould configurations are usually
    necessary. The primary task of an injection mould is to shape
    the molten material into the final shape of the plastic product.
    This task is fulfilled by the cavity system that consists of core,
    cavity, inserts, and slider/lifter heads. The geometrical shapes
    and sizes of a cavity system are determined directly by the
    plastic moulded product, so all components of a cavity system
    are called product-dependent parts. (Hereinafter, product refers
    to a plastic moulded product, part refers to the component of
    an injection mould.) Besides the primary task of shaping the
    product, an injection mould has also to fulfil a number of
    tasks such as the distribution of melt, cooling the molten
    material, ejection of the moulded product, transmitting motion,
    guiding, and aligning the mould halves. The functional parts
    to fulfil these tasks are usually similar in structure and geometrical
    shape for different injection moulds. Their structures
    and geometrical shapes are independent of the plastic moulded
    products, but their sizes can be changed according to the
    plastic products. Therefore, it can be concluded that an injection
    mould is actually a mechanical assembly that consists of
    product-dependent parts and product-independent parts. Figure
    1 shows the assembly structure of an injection mould.
    The design of a product-dependent part is based on extracting
    the geometry from the plastic product. In recent years,
    CAD/CAM technology has been successfully used to help
    mould designers to design the product-dependent parts. The
     
    automatic generation of the geometrical shape for a productdependent
    part from the plastic product has also attracted a
    lot of research interest [1,2]. However, little work has been
    carried out on the assembly modelling of injection moulds,
    although it is as important as the design of product-dependent
    parts. The mould industry is facing the following two difficulties
    when use a CAD system to design product-independent
    parts and the whole assembly of an injection mould. First,
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