Pipes made from plastics in particular polypropylene and polyethylene are widely used for gas, water and effluent transmission and in industrial pipework, and are commonly joined by butt fusion welding. This is a slow process, particularly for diameters greater than 250mm, but developments arising from the Core Research Programme at TWI have resulted in significant productivity improvements and therefore cost savings.
The major part of the weld cycle time in butt fusion welding is cooling, which takes place slowly because plastics have low thermal conductivity. The welding conditions currently used have been chosen to provide tolerance to equipment characteristics and to allow the operator latitude in timing the sequence (the hot plate is removed from the equipment by hand at the end of the heating phase). Relatively long heat soak times are used to provide a large volume of softened material so that the pipe ends do not cool too much during the hot plate removal time. Unfortunately this results in slow cooling after the pipe ends have been brought together to make the weld. Cycle times range from 12 minutes for 125mm diameter pipe to over an hour for 400mm diameter pipe.
Automatic butt fusion equipment is available which reliably achieves a hot plate removal time of under three seconds. Trials at TWI using this equipment have shown that it is possible to use much shorter heat soak times without the temperature of the pipe ends falling during plate removal to a temperature at which a poor joint is formed.
Implanted thermocouples showed that, when welding 125mm diameter pipe with total heating times as short as 15 seconds, the temperature fell to about 140oC; poor welds were formed only if the temperature fell to around 120oC the polymer crystallisation temperature.
Weld quality was assessed by tensile testing and stress crack resistance testing at 80oC. As much less heat was used, the welds cooled much more rapidly, and were sufficiently rigid to be removed from the welding machine within a minute, an overall improvement of a factor of ten.
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