Plastic Pipes Conference Association # 1995 Edinburgh
Crawford, Xu, Spence
Currently plastic pipes and fittings are manufactured by extrusion or injection moulding. However, in certain circumstances there are considerable advantages to be gained by using rotational moulding for these types of products. In particular, rotationally moulded products are stress-free, can have greater than normal wall thickness (up to 40 mm) and they are produced at a cost which is considerably less than injection moulded or extruded products. These advantages make rotational moulding of interest to the pipe industry, particularly for the manufacture of fittings such as bends, T-pieces, etc. However, before this moulding method can become widely accepted in the pipe industry it has to be recognised that rotational moulding has a number of disadvantages which have limited its widespread use. These include long cycle times, void content within the mouldings and low ESCR due to the higher MFI grades of material which have to be used in rotomoulding. This paper describes the results of recent research which attempts to address these problems. A computer simulation has been developed which allows cycle times to be predicted and the effects of material and process variables to be examined. The work also shows that void-free articles can be manufactured using new developments of the moulding process. These recent advances result in lower cycle times and better quality products. The paper also describes some results on the rotomoulding of cross-linked materials which are likely to be of interest to the pipe industry.
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