The present invention relates to a method of fusing an electrofusion joint having an electric resistance heating wire embedded therein, a method of confirming the state of fusion and an electrofusion joint suitable for use in such methods.
Electrofusion joints have been developed recently and are finding spreading use in various fields such as jointing of plastic pipes of water works, sewage pipes and gas pipes and in various forms such as a saddle joint, an elbow joint, a T joint and a cap joint. In general, an electrofusion joint is a molded article with an electrical resistance heating wire embedded therein. Fusion of this type of joint to a pipe is conducted by applying an electrical current at a constant voltage and for a predetermined time while fitting the joint on a pipe end or pressing the same against the pipe end so as to cause the interface to be molten thereby to melt and fuse the interface. In some cases, however, electrofusion is conducted without setting any definite time, i.e., by terminating the supply of electrical current upon detection of melting. For instance, the melting of the interface is detected by providing a detection hole b extending to the region near the electrical resistance heating wire a as shown in FIG. 11 and detecting the pressure of molten resin flowing into the hole by means of a pressure switch. According to the method of this type, bonding by fusion can simply be accomplished by fitting or pressing a joint to a pipe and then supplying an electrical current to the joint. Thus, electrofusion method of the type described enables the fusion bonding to be completed easily and in a short period of time as compared with the conventional method which requires the joint to be fitted or pressed to the pipe after thermally fusing the abutting surfaces of these joint and pipe. In addition, this type of method offers a comparatively high reliability of bonding by fusion.
One of problems encountered with this type of method is that completion of fusion cannot easily be confirmed through an appearance check, particularly when the length of time of the electrical power supply is fixed.
Conventionally, operators are obliged to lift, swing or pull the joint in order to confirm the pipe joint after the fusion, in order to confirm whether the fusion has been conducted safely.
Various other methods also have been proposed and used for the purpose of confirming the state of fusion. For instance, in one of these known methods, a detection hole 60 leading to a region in the vicinity of the electrical resistance heating wire is provided as shown in FIG. 4 and the state of fusion can be detected by observing the state of expansion of the molten resin through the hole 60. Alternately, a hole is formed in the joint so as to reach a region in the vicinity of the electrical resistance heating wire and a radially movable indicator is received in the hole so that the completion of fusion is confirmed by recognizing a radial movement of the indicator caused by spreading of the molten resin. In another known method, a temperature indicator tape capable of changing its color according to the temperature is adhered to the surface of the pipe joint so that the completion of fusion is detected through observation of the color of the temperature indicator tape.
The arrangement in which molten resin spreads into a hole leading to a region near the electrical resistance heating wire, as is the case of the aforementioned arrangement in which the fusion is detected by a pressure switch responsive to flow of molten resin into a hole, suffers from a problem in that a large quantity of molten resin is required for the purpose of confirmation of the fusion particularly when the joint has a large wall thickness because in such a case the volume of the hole is considerably large. In order to enable a visual check of the molten resin flowing out of the hole, it is necessary that the molten resin protrudes to a height of 5 mm or so above the pipe surface. This requires that a significantly large quantity of molten resin flows into the hole and, therefore, that the fluidity of the resin around the interface between the joint and the pipe increases to such a high level as to cause various troubles such as overheating due to short-circuiting of electrical resistance heating wire, reduction in the pressure of the molten resin around the interface, deformation of the joint or the pipe in amount corresponding to the quantity of the resin which has been spread into the hole resulting in a large residual internal stress to reduce the quality of the bonding by fusion. In the method which relies upon the spreading of the molten resin out of the hole, it has been difficult to adequately control the quantity of the resin flowing out of the hole regardless of the conditions such as the size of the gap between the environmental temperature, the size of the gap between the pipe and the joint, and so forth.
The method which relies upon a radially movable indicator also is disadvantageous in that the indicator tends to move by vibration or force of gravity applied to the joint with the result indication indicative of completion of fusion bonding is made even when the bonding actually has not been done.
The method which makes use of a temperature indication tape does not cause any adverse effect on the fusion performance because it does not require spreading or flowing of the molten resin out of the joint. With this method, however, it is difficult to exactly predict the molten resin temperature which are important factor of the fusion from the exterior of the joint. Thus, the method relying upon the temperature indicator tape tends to suffer a large error. In general, where the ambient air temperature is around 20.degree. C., the temperature of the joint surface does not rise beyond 60.degree. C. when the inner surface of the joint has been molten. To enable the fused state to be detected correctly, therefore, the tape has to be selected such that its color is changed at, for example, 50.degree. C. Such a tape will easily change its color when irradiated directly with sunlight in summer. Thus, the method which makes use of the temperature indicator tape is also unsatisfactory from a practical point of view.