注塑成型塑料部件设计英文文献和翻译(4)
including both positional modification variables and sizing mod-
ification variables, is introduced into the CAD-CAE features.
The shape modification variable of the part feature is the overall
thickness (a sizing modification variable). By changing this mod-
ification variable, the part shape is modified such that the thick-
ness of all the component features is changed to the same thick-
ness. The thickness may be varied until the part shape achieves
the optimal mouldability and other quality measures. Obviously,
part feature does not have any positional modification variable.
Component features have both positional and sizing modi-
fication variables. Before proceeding further, it is necessary to
emphasize that most of the component features have a corres-
ponding profile defined in the underlying CAD system. A profile
is a number of 2D curves either connected or not connected with
each other. By extruding or rotating a profile, a 3D CAD feature
can be created, which is then used to construct more complex
CAD geometry by means of Boolean operations. Figure 2 shows
the profile of a wall feature. Examples of some commonly used
profiles of a hole, boss, and rib feature include a circle, two con-
centric circles, and a 2D triangle respectively.
With the help of the feature profile, modification variables
can easily be defined. For example, the positional modification
variables of a hole feature can be defined by the coordinates of
the centre of the circle that is used as the profile to create the
hole feature. The sizing modification variable is the hole radius.
Figure 3 illustrates these modification variables.
Similarly, the wall feature, rib feature, and boss feature also
have their respective positional and sizing modification vari-
ables. Table 1 lists the modification variables of all the CAD-
CAE features.
It is important to note that by specifying shape modification
variables, different types of shape modification problems, either
positional or sizing, or modification to different features, can be
handled in a unified manner. It is not necessary to develop indi-
vidual methods for the different problems on different features.
Hence it benefits both model consistency and software develop-
ment efforts.
Fig. 3. Positional and sizing modification variablesAssociated with each modification variable are four parame-
ters: current value, starting value, ending value, and step value. For
sizingmodification variables such as thickness, radius, and height,
these parameters are sufficient in defining how the correspond-
ing variable should be varied during the part-shape-modification
process. For the positional modification variables, which are the
coordinates of the relevant key point on the profile, e.g. the cen-
tre of a circle, it may be necessary to specify how they are varied.
For example, the hole position may be varied within a rectangular
range or a circular range on the surface of the hole profile. This is
problem specific, hence it is up to the designer to specify.
3.2 Criteria construction variables and the criteria
for shape modification
The designer should also specify the mouldability and other
quality measuring criteria as constraints or targets of the intended
shape modification operation. Some of these criteria are [16]:
• Shear stress should not exceed the maximum recommended
for the material type.
• Shear rate should not exceed the maximum recommended for
the material type.
• Flow front temperature should not be more than 20 ◦Cbelow
the melt temperature.
• Cooling time should be uniform and minimised.
• All extremities should fill at the same time.
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