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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
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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,