GB2189190A - Branch pipe fitting for thermoplastic pipe - Google Patents

Abstract

A preform for use in the manufacture of an electro-fusion saddle fitting has a spiral winding of resistance wire 3 carried on a thermoplastic mat 1 injection moulded over the winding and including integral projections 15 spaced apart along the wire and embracing the wire. The preform is made by winding the wire into a spiral groove 6 in a die plate 5, there being recesses 7 of enlarged cross- section spaced along the groove, and injection moulding over the die plate. <IMAGE>

Description

SPECIFICATION Branch pipe fitting for thermoplastic pipe This invention relates to a branch or saddle fitting of the electro-fusion type, for connecting a branch pipe to a main pipe of thermoplastic material, such as polyethylene. Such fittings generally comprise a part cylindrical body of thermoplastic material compatible with and preferably the same as the material of the main pipe to enable the fitting to be welded directly to the main pipe. The body has a concave surface shaped to conform to the external surface of the main pipe, and an electrical resistance heating element is incorporated in the fitting near to the concave surface, and electric current being supplied to the heating element for heating and fusing the material of the fitting and the main pipe when they are to be welded together.

In one known method of manufacturing an electrofusion saddle fitting a flat mat of thermoplastic material having a spiral groove on one face is first prepared by injection moulding. Plain, uncoated resistance wire is laid into the spiral groove to form the heating winding, and the preform thus produced is loaded into an injection mould for forming the main part of the body over the preform. In the final fitting the material of the initial mat defines the concave welding surface and the heating wire is embedded in the fitting adjacent this surface.

When the preform is inserted in the mould it is bent from its flat form into an arcuate shape and measures must be taken to ensure that the wire is not dislodged from the groove during this bending, which is a disadvantage.

For example, the spiral groove may be shaped to hold the wire securely, or a hot tool may be applied to the mat over the wire to melt the thermoplastic over section of the wire to hold it in place. Another disadvantage arises because of the thickness of the mat which cannot easily be reduced owing to practical limitations set by the moulding process.

In making an electro-fusion welded joint the time and energy needed to complete a satisfactory weld depends on the values necessary to produce the required temperature at the welding interface. Therefore, the deeper the winding is embedded in the fitting the greater the energy and time required. It has been found that even a reduction of 0.1 mm in the thickness of material covering the heating wire can make a significant difference to the electrical energy needed for fusion.

The present invention seeks to at least alleviate the disadvantages of the prior art preform described above, and the invention resides in a preform for use in the manufacture of an electro-fusion saddle fitting, comprising a winding of electrical resistance wire and a thin layer of thermoplastic material moulded in situ over the winding and forming a carrier for the winding the carrier including integral projections embracing the wire to retain the wire securely on the carrier.

By forming the carrier or mat directly over the winding the thickness of the mat between the wire and the opposite face which defines the welding surface in the completed fitting, may be reduced to about 0.3mm. The integral projections ensure that the wire will not become detached when the preform is bent into the curved shape necessary for moulding the body of the fitting over the preform.

The invention also resides in a method of making the preform comprising the steps of winding resistance wire into a groove in a die plate, the groove including sections of enlarged cross-section, moulding thermoplastic material over the wire and die plate to form a thin layer over the wire, the thermoplastic material flowing into said enlarged groove sections to form projections embracing the wire, and removing the wire and thermoplastic layer from the die plate.

Also provided by the invention is an apparatus for use in making the preform, comprising a die plate having a flat face, a spiral groove and recesses spaced apart along the groove and having greater width and depth than the groove.

A better understanding of the invention will be had from the following detailed description in which reference is made to the accompanying drawings, in which Figure 1 is a schematic perspective view of an apparatus for making a preform according to the invention; Figure 2 is an enlarged scale cross-section showing the wire wound into the groove of a support plate; Figure 3 is a detailed perspective view of part of the support plate; Figure 4 is a section showing the thin carrier being moulded over the wire, and Figure 5 is a scrap section through part of a finished preform.

The apparatus illustrated in making a preform consisting of a thin rectangular mat 1 having a central hole 2 and carrying on one side a spiral winding of uncoated resistance wire 3 of circular cross section. The preform is made on a die plate 5 which progresses through a series of stations. On its upper face the plate 5 has a spiral groove 6 of uniform, substantially semi-circular cross-section.

Spaced apart along the groove are recesses 7 of greater width and depth than the groove.

The number of recesses is not important and three or four per turn of the spiral groove should be satisfactory. It may be preferable for the recesses of adjacent turns to be staggered in the circumferential direction.

At a winding station 1, the end of a supply of plain, uncoated resistance wire fed from a supply reel 8 is engaged in the inner end of the spiral groove 6 and is held in place by a pressure plate 9 applied to the die plate 5.

The two plates 5, 9 are then rotated together, e.g. by motor 10, so that the wire 3 is drawn off of the reel 8 and laid into the groove 6 of plate 5, as seen in Fig. 2. When the winding has been completed the supply of wire is cut through, and the die plate 5 carrying the welding is transferred to a moulding station II at which the plate 9 is removed and a mould cover 12 is clamped against the plate 5. Molten thermoplastic material is injected through the cover to form a layer 14 over the spiral wire winding, as seen in Fig. 4. The gap between the mould cover 12 and the wire may be as small as 0.3mm, such a narrow gate for the molten material being possible due to the circular cross-section of the wire 3.In addition, the thermoplastic material enters the recesses 7 and flows around the wire in these regions to form integral eyes or loops 15 which embrace and extend completely around the wire for holding it securely to the flat mat portions formed over the winding. When the thermoplastic material has set the cover 12 is removed and the die plate carrying the finished preform is conveyed to an ejection station Ill. The die plate 5 is returned to the winding station for producing another preform, and the finished preform is used to manufacture a saddle fitting by loading the preform into an injection mould and moulding over it.

The integral eyes 15 formed on the mat 1 ensure that the winding is retained securely in position when the preform is bent into curved shape in the injection mould.

If required terminal contacts, e.g. pins may be attached to the ends of the spiral wire winding during manufacture of the preform, these contacts being used to supply electric current to the heating wire in the finished fitting. The operation of attaching the contacts could be carried out before or after the moulding operation as found most convenient.

Claims (18)

1. A preform for use in the manufacture of an electro-fusion saddle fitting, comprising a winding of electrical resistance wire, a thin layer of thermoplastic material moulded in situ over the winding and forming a carrier for the winding, and integral projections on the carrier spaced apart along the wire and embracing the wire to retain the wire securely on the carrier.

2. A preform according to claim 1, wherein the winding is a spiral winding and a plurality of projections are provided for each turn of the winding.

3. A preform according to claim 2, wherein the projections number three or four per turn of the winding.

4. A preform according to claim 1, 2 or 3, wherein the projections form with the carrier loops extending completely around the wire.

5. A preform according to any one of claims 1 to 4, wherein the thickness of the carrier between the winding and the surface of the carrier remote from the winding is about 0.3 mm.

6. A method of making a preform as defined in claim 1, comprising the steps of winding resistance wire into a groove in a die plate, the groove haing sections of enlarged cross-section spaced apart therealong, moulding thermoplastic material over the wire and die plate to form a thin layer over the wire, the thermoplastic material flowing into said enlarged groove sections to form projections embracing the wire, and when the thermoplastic material has set removing the thermoplastic layer and wire winding carried thereby from the die plate.

7. A method according to claim 5, wherein the wire is wound into the groove by rotating the die plate and drawing the wire from a supply.

8. A method according to claim 7, wherein a pressure plate is applied to the die plate for clamping the wire thereto, and the die and pressure plates are rotated together.

9. A method according to claim 6, 7 or 8, wherein the thermoplastic material is injection moulded over the wire winding, the molten material being injected through a mould cover applied to the die plate and defining a mould cavity therewith.

10. A method according to claim 9, wherein the wire and mould cover define a gap of about 0.3 mm.

11. An apparatus for use in making a preform by the method of claim 6, comprising a die plate having a substantially flat face, a spiral groove in said face, and recesses spaced apart along the groove and being of greater width and depth than the groove.

12. An apparatus according to claim 11, wherein a plurality of recesses are included along each turn of the spiral groove.

13. An apparatus according to claim 12, wherein the recesses of adjacent turns are staggered in the circumferential direction.

14. An apparatus according to claim 10, 11 or 12, wherein the recesses project either side of the groove for forming moulded projections completely enclosing the wire.

15. An apparatus according to claim 14, wherein the groove is substantially semi-circular in cross-section and the recesses are rectangular in cross-section.

16. A preform for use in manufacturing an electrofusion saddle fitting, substantially as herein described with reference to the accompanying drawings.

17. A method of making a preform substantially as herein described with reference to the accompanying drawings.

18. An apparatus for making a preform substantially as herein described with reference to the accompanying drawings.