Jointing, Sealing and Welding # 1985 York
A relatively recent innovation in the field of plastic pipes and pipeline systems is the development of electro-fusion as a means of joining pipes and fittings. For this technique special fittings are manufactured which incorporate a heating element close to the surface to be welded. Welding takes place by locating the pipe in the fitting and connecting the terminals of the heater to the appropriate power supply for a suitable time. Low voltage supplies are used.
Various problems can arise if fittings are incorrectly designed (a) With small fittings the heat supplied by the heater can be sufficient to raise the temperature of the entire thickness of the pipe or fitting to above the polymer's melting point, causing distortion.
(b) if the temperature of the polymer adjacent to the heater becomes too high, degradation can occur with a resulting reduction in performance.
(c) If the temperature of the interface between the fitting and the pipe is too low, the weld strength may be inadequate.
An additional restriction is that the maximum power input that can be used is specified in most standards (1).
One method of arriving at a suitable design is to perform comprehensive experimental studies. In some instances this can be an expensive exercise. An alternative is to use a computer simulation of the electro-fusion process to suggest suitable designs that may then be tried experimentally.
Such a computer simulation need not necessarily include all the complexities of the electro-fusion process providing it can be used to point the designer in the correct direction. Such a model is described here.