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should not be closer than 16 mm. The mould plate is plugged at one end to ensure
that the flow can form the U circuit and to prevent leakage of water.
2.2.4. Position of ejector pins, tapered interlocks and screws. All thermoplastic
materials contract as they solidify and thus cause the mould to shrink onto the core
used to form the mould. Such shrinkage makes the mould difficult to remove and
hence ejector pins are used to eject the moulded part out of the mould. Tapered inter-
locks are also used to ensure precise alignment between the core and the cavity.
Ejector pins should be placed at positions where ejection of the moulded part
results in little distortion. To ensure distortion free ejection, there should be enough
number of ejector pins for each cavity. The number of ejector pins depends on the
part’s shape and size. Ejector pins are always arranged at a location that does not
affect the aesthetic appearance of the mould. Figure 6 shows the location of ejector
pins for the 87.58 elbow moulding. Two ejector pins are introduced to eject each
cavity during ejection. They are placed at an approximately equal distance from
location of the gate, thus firmly supporting the moulded part during ejection. An
additional ejector pin is arranged at the sprue location to provide enough strength in
order to eject the runner system.
There are mainly two kinds of guiding components. The first kind is guide pins and
guide bushes, which are used to guide the moving half into the fixed half. A standard
injection mould usually requires four guide pins with two pins placed in each cavity.
The second kind is tapered interlocks used to ensure precise alignment between the
core and the cavity. To ensure that the two halves of cavities for the 87.58 elbow
moulding are closely matched, four interlocks are arranged as shown in
figure 6. Precise alignment of the two halves can also reduce the visibility of
parting lines to improve the aesthetic appearance of the part. Moreover, screws are
used to tie the cavity to the support plate. Their locations are also shown in figure 6.2.2.5. Cavity for the fixed half. After completing the cavity for the moving half, the
cavity for the fixed half is almost a direct copy of that of the moving half. The only
difference is that there are no ejector pins for the fixed half cavity as the ejector
system is located at the moving half. A sprue is assembled to the fixed half to guide
the molten plastic into the cavity. The moving half cavity is attached to the support
plate while the fixed half cavity is attached to the front clamping plate.
2.2.6. Design of ejector system and support plate. The main function of the ejector
system is to hold ejector pins and to eject the moulded part out of the mould. The
ejector system consists of an ejector plate assembly and an ejector plate return
system. The ejector plate assembly is located directly behind the support plate, and
rests on the back clamping plate. Support pillars and support bars are placed
between the support plate and the back clamping plate to provide spacing for the
ejector assembly to move (figure 7). Guide pins are attached at the back clamping
plate and pass through the ejector plate assembly and the support plate. A spring is
placed around each guide pin to allow the ejector plate to return to its original position
after ejection.
During ejection, an ejector rod of the machine will hit the ejector plate assembly,
causing it to move forward and eject the moulded part. The springs are compressed
during the ejection process, and automatically decompress and return the plate to its original position when the ejector rod retracts. The ejector stroke is the distance
between the ejector assembly and the bottom of the supporting plate. As a rule of