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2. Design of injection mould for an 87.5° elbow
An injection mould is an assembly of parts that provides an impression within which plastic material is injected and cooled. The impression is formed by the cavity and the core. The cavity is the female portion of the mould and is used to generate the external form of the moulded part; the core is the male portion of the mould and is used to form the internal shape of the moulded part. Besides shaping the part, an injection mould needs to fulfil a number of tasks such as distribution of melt, cooling of molten material, ejection of the moulded part, transmitting motion, guiding and aligning the mould halves (Ye et al. 2000). The specification of the UPVC pipe fitting elbow is given as ‘DIN 1953 Ø50 mm _ 87.5°’, where DIN 1953 is a German standard for UPVC rubber joint fittings.
2.1. Selection of parting line
A parting line is a mark on the moulded part, where the two mould halves meet. The parting surface can be flat or non-flat depending on the shape of the part. The flat parting surface or plain flash line is the best parting line as it is the simplest to manufacture and maintain. The other parting surfaces, such as a stepped parting surface, profiled parting surface, angled parting surface and complex edge forms, are also available. They are used for parts with complex geometry. The 87.5° elbow is cylindrical and highly curved in shape. Hence the parting line should be the symmetrical line between the two halves of the component for easy ejection. Due to the presence of the groove undercut, another parting line should be added to allow the ejection of the part during demoulding. These parting lines are shown in figure 2.
Figure 2. Parting lines for the 87.5° elbow.
2.2. Design of cavity
Cavity design involves the layout of cavities, the type of runners, cooling system layout, as well as guiding units. These inpidual tasks directly affect the quality and dimensions of the moulded part.
2.2.1. Layout of cavities.
For most injection moulding machines, the barrel is positioned in the central axis of the stationary platen. This determines the position of the sprue, which is a part of the plastic part that leads the molten plastic to the mould. The cavities should be arranged relative to the centre sprue so that: (1) all cavities can be filled at the same time with melt of the same temperature; (2) the flow length is short to keep scrap to a minimum; (3) the distance from one cavity to another is sufficiently large to provide space for cooling lines and ejector pins, and leaves an adequate cross-section to withstand forces from injection pressure; and (4) the sum of all reactive forces is in the centre of gravity of the platen.
As shown in figure 1(b), side A of the part should make use of a sliding side core to produce the undercut. For easy incorporation of the side core mechanism, a two-cavity layout is determined as shown in figure 3. With such a layout, the pressure forces will be balanced with respect to the sprue location. In addition, cavities will be filled at the same time since they are placed at an equal distance from the starting injection position. The distance between two cavities will provide the space for cooling lines and ejector pins.
2.2.2 Design of runner system.
Figure 3 also shows the runner system designed for the 87.5°elbow. The gate is chosen at the location that is of equal distance from both ends of the elbow. In this way, the cavity will be filled up more uniformly during the 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 completely 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 87.5° 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 neede to remove the gate from the moulded part.
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