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thumb, the ejector stroke should be 5–10 mm more than the height of the moulded
part. The ejector assembly and cavities are mounted on different mould plates.
Mould plates normally possess recess holes used for mounting purposes.
3. Design of side core
Considering the shape of the part at side A as shown in figure 1(b), we decide to use
a side core mechanism. Figure 8 shows the 87.58 elbow together with the side core,
which is pided into three parts. Core A will be mounted on a sliding plate that
moves in the direction indicated by the arrow. During the moulding process, core A
will be inside the cavity as shown in the figure. When the moulding process is over,
the sliding plate, where the core is attached, will move out of the mould and thus
allow the moulded part to be ejected. The tapered lock is produced as one component
of core A and used to guide the side core into the cavity precisely so as to ensure that
the matching of the core is accurate.
Figure 9 shows the side core assembly. The core is mounted on the sliding plate.
Besides the core, the sliding plate consists of recess holes for mounting the shaft and
springs. As the core will indirectly contact with the molten plastic, it should be tolerant
to high temperature. Therefore, cooling channels are drilled inside the core as shown
in figure 9.
The working principle of the sliding side core is illustrated in figure 9. The sliding
plate rests on the shaft that is mounted between the moving half cavity and the
stopper. A bush is used to guide the sliding plate along the shaft and to prevent wear
of the recess hole. The cam block is used to lock the sliding plate in position during
the moulding process while the cam plate is used to activate the sliding plate. When
the mould is closed, the cam plate is locked to the sliding plate by the slots. When the
mould opens, the fixed half consisting of the cam block and the plate will cause the
sliding plate to slide along the shaft, thus moving the side core away from the cavity.
4. Design of collapsible core
As shown in figure 1(c), there is a groove undercut associated with the 87.58 elbow
at side B. The undercut has a 168 tapered edge and is 4.75 mm in depth. This groove undercut is of 50 mm in diameter, and thus a custom-designed collapsible core is
required. This section details the design of such a collapsible core. A collapsible
core generally consists of three parts: a centre pin, a collapsible core, and a sleeve.
The collapsible core is basically a hollow cylinder with matching slots parallel to
the cylinder, which changes part of the cylinder into the matching segments. These
vertical segments are the flexing segments that form the undercut. The centre pin
expands the flexing segments of the core and provides cooling of the moulding
length. The collapsible core forms the undercut with the expanded flexing segments
and releases the part for ejection with segments in a collapsed position. The sleeve
functions as a backup unit to collapse the core segments if segments fail to collapse
on their own.
4.1. Pattern of segments
While a number of collapsible core designs have been developed and patented, the
fundamental principle remains the same—the core should be able to collapse inward
to clear the undercut before demoulding. As shown in figure 10(a,b), the inward col-
lapse of the core causes a reduction in the projected cross-section area from the
expanded position to the contracted position. In order to achieve the inward collapse,
the core needs to be pided into several segments. The number of segments is deter-
mined according to the type of undercuts. For example, to mould interrupted internal threads, a minimum of three segments is necessary (McCready 1996). For a full cir-