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1. It should be possible to have high-level objects ready to
use while mould designers think on the level of realworld
objects.
2. The representation of assemblies should encapsulate operational
functions to automate routine processes such as
pocketing and interference checks.
To meet these requirements, a feature-based and object-oriented
hierarchical model is proposed to represent injection moulds.
An assembly may be pided into subassemblies, which in turn
consists of subassemblies and/or inpidual components. Thus,
a hierarchical model is most appropriate for representing the
structural relations between components. A hierarchy implies
a definite assembly sequence. In addition, a hierarchical model
can provide an explicit representation of the dependency of
the position of one part on another.
Feature-based design [10] allows designers to work at a
somewhat higher level of abstraction than that possible with
the direct use of solid modellers. Geometric features are
instanced, sized, and located quickly by the user by specifying
a minimum set of parameters, while the feature modeller works
out the details. Also, it is easy to make design changes because
of the associativities between geometric entities maintained in
the data structure of feature modellers. Without features,
designers have to be concerned with all the details of geometric
construction procedures required by solid modellers, and design
changes have to be strictly specified for every entity affected
by the change. Moreover, the feature-based representation will
provide high-level assembly objects for designers to use. For
example, while mould designers think on the level of a realworld
object, e.g. a counterbore hole, a feature object of a
counterbore hole will be ready in the computer for use.
Object-oriented modelling [11,12] is a new way of thinking
about problems using models organised around real-world concepts.
The fundamental entity is the object, which combines
both data structures and behaviour in a single entity. Objectoriented
models are useful for understanding problems and
designing programs and databases. In addition, the objectoriented
representation of assemblies makes it easy for a
“child” object to inherit information from its “parent”.
Figure 2 shows the feature-based and object-oriented hierarchical
representation of an injection mould. The representation
is a hierarchical structure at multiple levels of abstraction,
from low-level geometric entities (form feature) to high-level
subassemblies. The items enclosed in the boxes represent
“assembly objects” (SUBFAs, PARTs and FFs); the solid lines
represent “part-of” relation; and the dashed lines represent
other relationships. Subassembly (SUBFA) consists of parts
(PARTs). A part can be thought of as an “assembly” of form
features (FFs). The representation combines the strengths of a
feature-based geometric model with those of object-oriented
models. It not only contains the “part-of” relations between
the parent object and the child object, but also includes a
richer set of structural relations and a group of operational
functions for assembly objects. In Section 3.1, there is further
discussion on the definition of an assembly object, and detailed
relations between assembly objects are presented in Section 3.2.
3.1 Definition of Assembly Objects
In our work, an assembly object, O, is defined as a unique,
identifiable entity in the following form:
O = (Oid, A, M, R) (1)
Where:
Oid is a unique identifier of an assembly object (O).
A is a set of three-tuples, (t, a, v). Each a is called an