Design Guide - Solvay Plastics

Figure 85: Boss Design - General Guidelines
Figure 86: Thermostat Housing Showing Beads
O.D. = 2 X I.D.
O.D.
I.D.
0.3T T
Undercuts
Some design features, depending on orientation,
can place portions of the mold in the way of ejecting
the part. These features are called undercuts and
can require special mold configurations, such as
slides or cams, to move prior to ejection. In some
cases, the material being molded will have enough
flexibility that the part can be pushed off the undercut
without damage.
For example, the typical automotive thermostat housing,
as shown in Figure 86, has beads to provide for leakproof
hose connections. To provide a smooth surface
for the hose connection, the designer has specified that
the parting line cannot be in this area. This results in an
undercut at each bead.
Under some circumstances, it may be possible to eject
a part with an undercut if the core can be pulled first and
the undercut ratio is 8 or less.
Figure 87: Undercut Diagram
B
C
A
Undercut
Ratio (R)
=
100 (Bead diameter (A) - Tube OD (B))
Tube ID (C)
If the part must be molded without side pulls and the
undercut ratio calculated is greater than 8, the design
should be modified. One possible modification is to
taper the tube inside diameter effectively reducing the
wall thickness under the bead. For tubes with inside
diameters less than 25 mm (1 in.), it may be necessary
to modify the bead geometry to get an undercut ratio
of 8 or less.
The success of this approach is based upon removing
the part from the mold while the part is still hot and
therefore more flexible than it will be a room temperature.
80 Amodel ® PPA Design Guide
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