MELTING PLASTIC TO MAKE A STRONG JOINT
A widely used method of joining Polyethylene water and gas pipes is to incorporate coils of wire that form heating elements on the inside joint fittings. When an electric current is passed through the wire element the plastics melt and expand to make a strong joint between the fitting and pipe. This has become commonly known as electrofusion.
The applied voltage can be either 40 volts for fittings to join pipes up to and including 315mm diameter or 80 volts for fittings above that. In these fittings, the wire coil is usually a continuous heating element with closely wound zones at each end and a single coil joining them across the middle.
There is, however, an option to have 40-volt fittings above 315mm but the power limitations would mean that each end of the fitting would need to be welded to the pipe separately. The element, therefore, would need to consist of separate windings, making a connection to both ends of the element impossible as the inside wire connection is a direct leak path out of the pressure side of the joint.
PATENTED BURIED LEAD TECHNIQUE
In the past it has been common to use two windings in parallel with the wire forming the return path running adjacent to the wire forming the ingoing path. However, should the wires become displaced, and there is always some wire movement during the welding process, there is the risk of a full voltage potential short circuit which could lead to a severe overheat situation and result in weld failure.
Rutland Plastics has developed and patented the buried lead technique which does not exhibit this problem. In this method, the lead in is buried beneath the subsequent coil winding so the risk of full voltage short circuits is considerably less.
The buried lead process can also be used in saddle-shaped tee joints instead of the usual two-shot moulding route where the element is wound on to a pad or carrier which is then moulded into the saddle face. Some of these fittings are ingeniously designed to permit side pipes or branch-offs to be attached to either live gas or water mains without turning off the supply.
The 3 axis digitally controlled machinery used for all these wire laying processes has also been developed by Rutland Plastics.
By using these machines and various machining techniques, grooves can be cut and wires inserted in any desired manner, to different depths, in any arbitrary shape.
Rutland Plastics uses the technique to manufacture fittings for joining pipes from 20mm up to 630mm. The range consists of straight couplers, reducers, elbows, tees and saddles. Also manufactured are a range of transition fittings where one end is an electrofusion socket and the other a steel or brass threaded connector.
New uses for products utilising this method are constantly being pioneered. Latest developments include fittings to permit pipes to be joined to the sides of plastic vessels, either flat sided or curved.