Fitting of weldable thermoplastic material

DETAILED DESCRIPTION OF THE INVENTION The fitting illustrated in FIG.
1 of the drawing is constructed as a welding sleeve generally denoted by reference numeral 1.
Welding sleeve 1 serves to connect pipe elements, such as, the ends of pipes 2 and 2' of thermoplastic material.
A fitting member 3 of thermoplastic material includes a heating element 4 arranged at the inner circumference forming a connecting surface 8.
Heating element 4 includes a spirally wound resistance wire 5 which is provided with a jacket 6 of thermoplastic material.
A compact winding is obtained by welding the individual coils together.
The ends 7 of the winding extend out of the ends of the fitting member 3 for connection to a device with a current source.
The supply of heating energy is controlled in the device in such a way that the heat generated in the welding region at the connecting surface 8 ensures a problem-free welding between the fitting member 3 and the pipe ends 2, 2'.
In order to obtain the welding pressure required for good welding, the fitting member 3 has a shrinkage stress or tension which is released during heating and presses fitting member 3 against pipe ends 2, 2'.
Weld indicators 10 are provided at both ends of the welding region.
As can be seen from FIG.
2 of the drawing, weld indicator 10 is formed by a hollow space 11.
Hollow space 11 is formed as a cylindrical bore which is open at the fitting member 3 towards connecting surface 8 and is covered only by heating element 4 after the winding has been introduced.
Toward the outer periphery 9 of the fitting member 3, hollow space 11 has a portion 12 of reduced width with a retaining and cooling surface 13 and an outlet opening 14.
As illustrated in FIG.
3, when heat is generated by means of electrical current in heating element 4, the material of the jacket of the heating element 4 and of the adjacent inner portions of fitting member 3 and the outer portions of pipe ends 2, 2' are plasticized, wherein a portion of the material is forced by the resulting welding pressure into hollow space 11