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4. Design of collapsible core
As shown in figure 1(c), there is a groove undercut associated with the 87.5° elbow at side B. The undercut has a 16° 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
Figure 8. Side core inside the 87.58 elbow.
UPVC pipe fitting injection mould
Figure 9. Mechanism of the sliding side core.
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.
5. Design evaluation
The main criteria for the evaluation of injection mould design include the functionality of design, the manufacturing process and the cost of manufacturing. The functionality of the injection mould can be evaluated based on the injection moulding process, which involves mould filling, cooling and demoulding.
The purpose of mould-filling analysis is to ensure that, with the designed runner system, the mould will definitely be filled up during the injection process. Based on the result obtained from the analysis, modifications can be suggested to ensure the quality of the moulded part meets specifications. Plastic Advisor is one of the mould-filling analysis programs available in Pro/Engineer design solutions. The software is capable of suggesting the location of gates, the number of gates, the kind of material, thicker wall section, size of runners and gates, and so on. It can also simulate mould filling for injection-moulded plastic parts, thus providing manufacturability insights for reducing design changes and mould costs. In Plastic Advisor, designers start from a solid model (for example, figure 4) and subsequently conduct such analyses as determining the fill time of the cavities, confidence of fill, injection pressure, flow front temperature, weld lines and air traps.
A sufficient cooling system has been designed to run on the cavities and the core. It helps make the molten plastic into shape as soon as possible. These designs help prevent the mould from overheating and ensure the moulded part is cooled as soon as possible so as to reduce the cycle time. In addition to cooling, the moulded part has to be ejected with no damage. This stage depends a lot on how the mechanism is designed and incorporated to the mould. Using the side core and the collapsible core unit helps solve problems in demoulding the undercut. With the development of CAD models, the mechanism can be verified to function virtually through the computer. Interference between the core and the moulded part during demoulding has been checked using the commands available in the software.
To minimize the cost of the mould, most of the parts are designed with minimum complexity. Only such simple manufacturing processes as milling, turning, electrode deposition machining and wire cutting are needed to produce the mould to the required tolerances. In addition, proper materials are selected based on the functionality of the moulded part. Inexpensive materials such as medium carbon steel are used for parts with a high wear rate, and chromium steel is used for the cavities and the core. In addition, overall dimensions have been set small in order to reduce the material cost.
6. Conclusions
Moulding design involves quite a few inpidual parts. Direct gating is used in the design. Based on the mould-filling analysis using Plastic Advisor, the location and size of the gate and runners are determined. The software is also used to identify possible places of air traps and weld lines to be present in the moulded part. Cavity design is carried out after the gating system is established and cooling channels are arranged to cool the cavity. Accordingly, a number of ejector pins, guide pins, tapered interlocks and screws are positioned. A simple basic ejector system is also developed. In the design, springs are used as the return system.三通管可折叠硬聚氯乙烯管件注射模设计
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