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1. Introduction Plastic materials have been more and more used in the piping systems for public and industrial water supplies and sewage transport. The unplasticized polyvinyl chloride (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 character-istics, 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. 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 24570
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cores (Pye 1983).
A groove or 3608 internal undercut is a feature commonly found in UPVC pipe fit-
tings. A gasket such as an ‘O’ ring is supposed to be fitted to the groove. Figure 1 shows
an 87.58 pipe fitting elbow with a groove undercut. Groove features require a special
type of core known as a collapsible core, which can form a 3608 internal undercut
and collapse inward to facilitate demoulding of circular parts during the ejection
stage. There are more than a dozen patents approved on the design of collapsible
cores. 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 mould-
ing design (Chung et al. 1998, Ye et al. 2000) with focus on the recognition of under-
cut 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 model-
ling of undercut features (Huang 1995, Chung 1999), analysis of geometrical features
for a collapsible core (Tor et al. 2000a), or algorithms of generating computer-aided
design (CAD) models for undercut mould design (Zhang 1996). A number of compu-
tational and analysis programs are now available at different levels of sophistication to
aid mould design; for example, computer flow simulation and knowledge-based
systems (Ong et al. 1995). These tools facilitate different tasks associated with injec-
tion 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 (Ye et al. 2000), 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
presents the design of a UPVC pipe fitting injection mould with consideration of a col-