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rotational degrees of freedom.
The plan fragment table is an exhaustive enumeration of all
the states in the search space for the problem of moving an
object to satisfy a series of constraints between markers on
the object and markers fixed in the global coordinate frame.
To enumerate the combination of different values of the above
three parameters, 82 entries will be generated [7]. If the search
space for the problem can be reduced the number of entries
in a plan fragment table will decrease. To achieve this, the
number of enumerate values for entry parameters must be
decreased. For example, for a specified constraint type, if the
enumeration values of TDOF change from {0,1,2,3} to {0,3},
then the search space is reduced.
Because of these restrictions on the constraint sequences,
the number of entries in our plan fragment table is substantially
reduced. To solve for one, two or three constraints allowed in
our system, only nine entries are required. For interactive
addition of components to the assembly, more constraint types
and free sequences will increase the flexibility for users. However,
in automatic assembly modelling for an injection mould,
as the spatial relationships are predefined in assembly objects,
some of the sequence restrictions do not matter. With the
above-defined synthesised constraints, the structural relationships
of a component part can be specified in the database of
the components. When adding a component part to the mould
assembly, the system will first decompose the synthesised
constraints into primitive constraints, then generate a group
of fragment plans to orient and position the component in
the assembly.
5. Automated Assembly Modelling of
Injection Moulds
Any assembly of injection moulds consists of productindependent
parts and product-dependent parts. The design of
inpidual product-dependent parts is based on the geometry
of the plastic part [1,2]. Usually the product-dependent parts
have the same orientation as that of the top-level assembly,
and their positions are specified directly by the designer. As
for the design of product-independent parts, conventionally,
mould designers select the structures from the catalogues,
build the geometric models for selected structures of productindependent
parts, and then add the product-independent parts
to the assembly of the injection mould. This design process is
time-consuming and error-prone. In our system, a database is
built for all product-independent parts according to the
assembly representation and object definition described in Section
3. This database not only contains the geometric shapes
and sizes of the product-independent parts, but also includes
the spatial constraints between them. Moreover, some routine
functions such as interference check and pocketing are encapsulated
in the database. Therefore, the mould designer must select
the structure types of product-independent parts from the user
interfaces, and then the software will automatically calculate
the orientation and position matrices for these parts, and add
them to the assembly.
5.1 Mould Base Subassembly
As can be seen from Fig. 1, the product-independent parts can
be further classified as the mould base and standard parts. A
mould base is the assembly of a group of plates, pins, guide
bushes, etc. Besides shaping the product, a mould has to fulfil
a number of functions such as clamping the mould, leading
and aligning the mould halves, cooling, ejecting the product,
etc. Most moulds have to incorporate the same functionality,
which results in a similarity of the structural build-up. Some
form of standardisation in mould construction has been adopted.