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cular undercut, at least six segments are needed. A four-segment collapsible core is
also possible for groove undercuts. However, it can only be used for shallow undercuts
since the movement of segments is restricted.
A literature review of various collapsible core designs for continuous internal
undercuts suggests that all existing designs use two or more different sets of segments.
Theoretically, this is possible but in practice only the two-set scheme is commonly
used. A two-stage collapsible core consists of a set of small segments together with
a set of large segments. The set of small segments retracts inward before the set of
large segments collapses on them. This is due to the smaller inertia of the small
segment and the way the segments are cut. The small segment is tapered towards
the outer edge, which allows the segment to collapse first and to regain its location
in the expanded position. Usually, the number of segments is equal for the two sets
while one type of segments is placed alternately after another. As a result, the total
number of segments that form the core is always even.
Figure 10(c) illustrates a design for the 87.58 elbow, where two patterns of seg-
ments are placed one after another. Four segments of type A collapse inwards followed
by four segments of type B. The amount of tapered joining of each segment is deter-
mined by interference analysis using Pro/Engineer. This is discussed in section 4.5.
4.2. Actuating mechanism for collapsible core
To cause an inward retraction of segments, an actuating mechanism is needed.
A few techniques such as a finger cam, springs, taper inner core, lever and spring
steel actuations are available. The lever and spring actuation is used in the 87.58
elbow moulding with the following considerations.
During the solidification process, the plastic contracts and shrinks onto the core.
Therefore, the spring is used in addition to the lever actuation in order to remove
the segment positively from the moulded part. The spring will provide a small force
to provide the initial actuation of the lever. A type of loaded compression springs is
placed as shown in figure 11(a) to force the segments to swivel towards the centre
when the inner ring moves down. In the lever actuation design, each segment is
pivoted at one end through a pin. An inner ring is located inside the sleeve of segments to keep the segments in the expanded position and to withstand injection forces during
demoulding. A protrusion is added to the inner diameter of the segment. As the inner
ring is moved along the direction indicated in figure 11(b), it touches the protrusion
and drags the segments along the direction. The extent of collapse depends on the
tapered protrusion on the segment.
4.3. Interference factors
To check the interference between the segments themselves and that between the
segment and the moulded part during collapsing, one set of matching segments and the
inner ring are used for study. Figure 12(a) shows the cross-section of a segment and
the groove undercut. It is obvious that in order to clear the undercut, the segment
must retract inwards sufficiently. This depends on the rotation angle of the segment
and the tapered edge of the undercut. The length of the segment starting from the
pivot also affects the condition to clear the undercut. Furthermore, there is a potential
interference between two segments during collapsing. As shown in figure 12(b), the
rotation angle of the segment must be controlled. Another factor that can also cause
the collision of the two segments is the width of the segment. The smaller the
width, the less the possibility that they hit each other.
4.4. Mechanism of collapsible core
To mould groove undercuts found in UPVC pipe fittings, a two-stage, lever-