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Design of an unplasticized polyvinyl chloride pipe fitting injection mould with a three-stage collapsible core.1. Introduction Plastic materials have been more and more used in the piping systems forpublic and industrial water supplies and sewage transport. The unplasticized polyvinylchloride (UPVC) material suggests itself as an ideal substitute for copper and cast iron pipes owing to such merits as high strength to weight ratio, excellent flow characteristics, and high impact strength with exceptional resistance to corrosion or chemical attack (Miller 1981).UPVC piping systems make use of pipe fittings such as elbows, tees, reducers and valves to connect pipes of different sizes and to change the direction or angle of fluid flow.
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These pipes and fittings are available in sizes ranging from 14 mm to 600 mm in diameter (Janson 1989).These pipe fittings are mainly produced by an injection moulding process. The design of UPVC pipe fitting injection moulds is usually a difficult assignment because of the unique shapes of fittings and the presence of undercuts in them. An undercut may be any feature that obstructs the extraction of a moulded part from the mould along the direction in which it opens (Ye et al. 2001). In the past, undercut features for pipe fittings were often created through a post process after the part was moulded. This results in an increase in the production cost. Therefore, designers have brainstormed some mechanisms to form the undercut features during the moulding process, thus eliminating the post-processing stage.There are a range of such tools for moulding parts with undercuts as form pins, split cores, side cores and cavities, rotating cores for unscrewing, and collapsible cores (Pye 1983).10179
A groove or 360o internal undercut is a feature commonly found in UPVC pipe fittings. A gasket such as an ‘O’ ring is supposed to be fitted to the groove. Figure 1 shows an 87.5° pipe fitting elbow with a groove undercut. Nevertheless, many of them will never see the market due to the complexity of the parts or high tooling cost. There are a few standard collapsible cores available in the market (for example, DME 2002), which however are only suitable for parts smaller than 3.5 inches in diameter (Tor et al. 2000a). As a result, special collapsible cores must be custom-designed for UPVC pipe fittings that often involve grooves with large diameters (Tor et al. 2000b).
Research into undercuts has been geared towards computer-aided injection moulding design (Chung et al. 1998, Ye et al. 2000) with focus on the recognition of undercut features from moulded parts (Fu et al. 1999, Ye et al. 2001) or the determination of an ejection direction (Chen et al. 1993). Other research emphasizes the solid modeling of undercut features, analysis of geometrical features for a collapsible core, or algorithms of generating computer-aided design (CAD) models for undercut mould design. A number of computational and analysis programs are now available at different levels of sophistication to aid mould design; for example, computer flow simulation and knowledge-based systems. These tools facilitate different tasks associated with injection mould design including cavity layout, gating location, parting-line determination, ejection and venting layout, runner system design, and mould base selection. Since all these tasks are interrelated in mould design, trade-offs are always needed. Therefore, the mould design for UPVC pipe fittings cannot be isolated from the design of collapsible cores for the groove feature. In this regard, this paper from the design of collapsible cores for the groove feature. In this regard,
Figure 1. Example product: an 87.5 8 elbow with a groove undercut.
UPVC pipe fitting injection mould
This paper presents the design of a UPVC pipe fitting injection mould with consideration of a collapsible core unit for the 360°internal undercut. The product shown in figure 1 is used as an example to illustrate the trade-offs involved in the interactive design process, as well as systematic procedures for UPVC pipe fitting injection mould design.