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Figure 3. Cavity layout and runner system for the 87.58 elbow.
UPVC pipe fitting injection mould
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.
Figure 4. Filled-up model and confidence level of fill.
The simplest form of circuit is designed for the cavity of the 87.5° 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 circulates 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 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 interlocks 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.