Pipe joints or couplings – With assembly means or feature – Molded joint facilitator
Patent
1979-12-21
1982-09-14
Arola, David
Pipe joints or couplings
With assembly means or feature
Molded joint facilitator
285369, 285423, F16L 4702
Patent
active
043492193
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a welding muff with a shell-like sleeve member made from thermoplastic material, the welding heat necessary for melting the thermoplastic material being produced by an electrical current in the turns of the winding of an electrical resistance heating wire arranged in the sleeve member joining zone.
Various constructions of welding sleeves for joining line elements and tubular members, such as pipes, fittings and valves in line systems made from thermoplastic material are known. They are either part of the pipe portions, fitting and valves, i.e. integrally connected thereto, or are constructed as independent components in the form of a sleeve-like body. For joining together the individual tubular parts, their ends are fitted into one another with the aid of the welding sleeve and are then welded together, the heat necessary for melting the thermoplastic material in the joining zone of the thermoplastic parts being produced by supplying electrical current to a resistor.
In a known and frequently used construction of the welding sleeve a winding of an electrical resistance heating wire with axially spaced turns is provided as a resistor in the area of the sleeve member joining zone. The individual turns of the winding are located on the inner wall of the sleeve member. The arrangement on the inner wall of the sleeve member means that the turns of the resistance heating wire are as close as possible to the inner wall and the thin thermoplastic material layer only has a limited thickness of e.g. 0.5 mm and mainly serves as protection against electric shock hazard. The closer the electrical resistance heating wire comes to the inner wall the more reliable the welding of the parts to be joined.
However, problems occur in practice in the manufacture of such welding sleeves which are prejudicial to the above-described ideal solution of a sleeve with filament winding located directly on the inner wall of the sleeve member. There is therefore no need to investigate in greater detail the manufacture of such a welding sleeve. For the manufacture of a known electrical welding sleeve with electrical heating coil a thermoplastic-coated filament is wound in turns onto a mandrel, which is subsequently injection moulded in a mould. In this manufacturing method, the mandrel is always larger than the subsequently moulded part, which on cooling is subject to a certain shrinkage. Due to the shrinkage of the sleeve member, it is not possible to prevent the moulded filament from being under a certain compressive stress. On connecting an electrical heating system to such a welding sleeve the resistance heating wire expands and, because it directly melts the thermoplastic material surrouding it, assumes a position corresponding to a larger diameter than the diameter of the winding turns wound onto the mandrel.
In another manufacturing procedure, the resistance heating wire is placed in a helical slot previously worked into the inside of the sleeve member. Here again, on heating the sleeve member, there is a longitudinal expansion of the wire and consequently the winding is removed from the joining zone.
In the above-mentioned, known constructions of the welding sleeve when making junctions a phenomenon occurs which impairs the joining quality and leads to a movement of the turns in the filament winding. If the distance of the turns from the joining zone is so large that there is not a complete melting of the inner wall, a typical molecular material connection for a weld no longer takes place in the joining zone of the thermoplastic parts to be connected and instead there is only a more or less strong shrink joint.
The problem of the present invention is therefore to so further develop a welding sleeve of the above-defined type, whilst avoiding the disadvantages of the known constructions, that reliable and faultless welding is guaranteed at the connection zone of the thermoplastic parts to be connected.
According to the invention, this problem is solved in that the turns of the resistance heating wire windin
REFERENCES:
patent: 2223434 (1940-12-01), Trilkey
patent: 3094452 (1963-06-01), Von Riegen et al.
patent: 3422179 (1969-01-01), Bauer et al.
patent: 3943334 (1976-03-01), Sturm
patent: 3954541 (1976-05-01), Landgraf
Arola David
Von Roll A.G.
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