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filling process. In addition, during filling the melt will force the air out of the cavity
when it reaches the ends of the part. As a result, the cavity will be able to fill up com-
pletely with no air traps.
Different kinds of gates are available for injection moulding, including sprue gates
for temperature-sensitive and high-viscous materials, submarine gates for smaller
parts in multi-cavity moulds, and ring gates for sleeve-like parts with the core
mounted on both sides. A side or edge gating is chosen to mould the 87.58 elbow
as it is the most common type of gating and can be used for all types of products.
This type of gates is also easy to machine. However, a secondary process is needed
to remove the gate from the moulded part.A spherical runner is selected in the design owing to its popularity in practice.
Although the runner may cause a mismatching problem, it is preferred to other
shapes such as a square, a semicircle and a rectangle. The cutters for spherical
runner are readily available in the market and easy to machine as well. The layout
of runners is curved instead of straight so as to prevent sharp corners and sharp
changes in direction. It can be noticed that the flow path between each cavity and
the sprue is of the same length so that all cavities can be filled at the same time
under equal pressure.
In this design, the mould flow analysis software Pro/PLASTIC Advisor is used to
determine the size of runners and gates. After a few trials with different locations,
diameters of runners and gates are determined as Ø12 mm for runners and Ø4 mm
for the gate. Figure 4 shows a picture of the filled up model and the confidence
level of fill. The confidence of fill indicates the probability of a region within the
cavity when being filled with plastic. In figure 4, the result of confidence of fill is
displayed in green colour, indicating that those sections will definitely be filled up.
The yellow pointer indicates the position where plastic is injected.
2.2.3. Design of cooling system. While rapid cooling of the mould improves
the cycle time, uniform cooling improves product quality and dimensional stability.
Inefficient cooling of the mould results in such moulding faults as wrapage, hot
spots and residual stresses in the mould. In order to maintain temperature difference
between the mould and the plastic melt, cooling fluid is used to circulate through
holes or channels within the mould. These holes and channels thus form a complete
system of water circuit. Commonly used cooling media include water, glycol and
water mixture, pure ethylene glycol oil, and so on. The type of fluid flow, either a
turbulent or laminar flow, influences the rate of cooling. The temperature also
depends on the rate of heat transfer through the steel to the cooling media.
The layout of cooling circuit is often complicated by the fact that flow-ways cannot
be drilled too close to any hole in the same mould plate. The mould plate has a large
number of holes and recesses in order to accommodate ejector pins, guide pillars,
guide bushes, sprue bush, inserts, and so on. The laying out of coolant circuit has to avoid possible clashes with these holes. Therefore, it is a good practice to plan the
circuit layout before other mould items such as ejector pins, guide bushes, screws,
and so on are positioned.
The simplest form of circuit is designed for the cavity of the 87.58 elbow (figure 5).
As shown in the figure, holes are drilled longitudinally through the cavity plate along
part geometry. These holes are interconnected to form a U circuit. Each U circuit cir-
culates around one cavity. The holes must not be positioned too close to the impression
as it will cause temperature variation across the impression. In practice, the holes