GB2137297A

GB2137297A - Branch pipe fitting for thermoplastic pipe

Info

Publication number: GB2137297A
Application number: GB08308799A
Authority: GB
Inventors: Eric Bridgstock; David Michael Anthon Kenworthy
Original assignee: Fusion Plastics Ltd
Current assignee: Fusion Plastics Ltd
Priority date: 1983-03-30
Filing date: 1983-03-30
Publication date: 1984-10-03
Also published as: GB8308799D0; GB2137297B

Abstract

An electro-fusion saddle fitting 2 for connecting a branch pipe to a main pipe of thermoplastic material comprises a body (1) of thermoplastic material and a heating element at least partially embedded in the body at the surface which contacts the main pipe, the element being formed from uncoated resistance wire. Several methods of making the fitting are described, one of which comprises forming a mat preform by pressing the heated wire into a thin sheet of thermoplastic material, and then injection moulding over the preform. <IMAGE>

Description

SPECIFICATION Branch pipe fitting for thermoplastic pipe This invention relates to branch or saddle fittings for connecting a branch pipe to a main pipe of thermoplastic material, such as polyethylene. A saddle fitting generally consists of a part-cylindrical body having a concave surface corresponding to the external shape of the main pipe to which it is to be attached, and an integral tubular boss projecting outwardly from the body for connection of a branch pipe, the fitting being moulded from a material which is compatible with, and preferably the same as that of the main pipe so that the fitting can be welded directly to the main pipe.The invention is particularly concerned with saddle fittings of the electro-fusion type which include an electrical resistance heating element to which an electric current can be supplied to heat and fuse the material of the fitting to that of the main pipe.

In order to ensure a satisfactory weld between the saddle fitting and pipe it is desirable that the heating element be located as close as possible to the surface of the pipe so that the material of the pipe is sufficiently heated and softened. The invention resides in a novel method of manufacturing a saddle fitting and more especially of mounting the heating element at the inner surface of the fitting. The invention also includes a fitting made by the method.

The features of the invention will become clear from the following detailed description which is given with reference to the accompanying drawings in which: Figure 1 is a plan view of a saddle fitting; Figure 2 is an end view of the fitting; Figure 3 is a top view showing a preform used in the manufacture of the fitting in preparation; and Figure 4 is a section taken along the line IV-IV in Fig. 3.

The saddle fitting illustrated in Figs. 1 and 2 has a generally semi-cylindrical body 1, an upwardly projecting tubular boss 2 for connection of a branch pipe, lateral flanges 3 for clamping the fitting to a pipe during welding thereto, and electrical connector sockets 5, 6 having pin contacts 7, 8 connected to the opposite ends of an electrical resistance heat- ing winding located at the inner surface of the body 1. The fitting is at least for the most part formed by injection moulding and the heating element is fixed at the inner surface of the fitting as will become clear from the description of the manufacturing process which follows.

Figs. 3 and 4 illustrate the preparation of a preform used in the production of the saddle fitting. A platen 10 of electrically insulating material has a flat upper surface in which is formed a spiral groove into which is laid a length of uncoated electrical resistance wire 11 to form a spiral winding, the maximum depth of the groove being less than half the wire diameter so that a major part of the wire protrudes above the surface of the platen 10.

The ends of the wire are passed down through holes in the platen and connected to an electrical power supply. A thin sheet 13 of thermoplastic material, preferably the same material as the main body of the fitting, is placed over the wound wire and is pressed firmly down onto the winding as electric current is supplied to it. The thermoplastic material is softened by the heat generated and the wire becomes embedded in the sheet. The electric current is then terminated and the assembly allowed to cool before the sheet 13 is lifted away from the platen 10 taking with it the embedded heating coil 11. To ensure positive retention of the wire in the sheet it is preferably pressed into the sheet to a depth greater than half the wire diameter.

To assist in winding the wire into the spiral groove the thermoplastic sheet 13 can be applied against the platen 10 first to act as a light pressure plate.

The mat preform produced as described above and incorporating the spiral heating coil, is after attaching contact pins 7, 8 to the opposite ends of the wire 11, loaded into an injection moulding machine to complete the manufacture of the fitting. The preform is laid over the mould surface which defines an inner surface of the finished saddle fitting and the contact pins 7, 8 are inserted into location holes provided in the mould. The mat preform may be positioned either way up depending upon whether or not it is desired to have the heating wire exposed in the completed fitting.

During the injection process the material of the sheet is softened and fuses to the injected material which forms the body of the fitting.

The completed fitting when removed from the mould has the heater winding located very close to and perhaps even exposed at the inner surface of the saddle.

Instead of winding the wire 11 into a shallow groove in the platen 10, a relatively deep groove may be provided together with an ejector for pushing the coiled wire out of the groove to embed itself into the thermoplastic sheet, The need to have a platen with a spiral groove may be avoided by winding the wire into a spiral directly onto the thermoplastic sheet, the wire being pre-heated and pressed into the sheet as they are brought together.

The preform produced in this way is then used as described above to complete the saddle manufacture.

As an alternative to making a mat preform incorporating the spiral heating element the fitting could be injection moulded directly over the coiled wire. If steel wire is used it may be retained within a shallow spiral groove on the mould surface which defines the inner surface of the saddle by making the mould part which provides this surface magnetic.

The magnetic attraction will retain the wire in place until and during injection but it will be lifted away easily with the finished fitting.

According to another proposal the wire could- be wound into a relatively deep spiral groove in the appropriate mould portion and then be pushed out of this groove by means of an ejector while the injected plastics material is still soft, thereby to embed the wire into the inner surface of the saddle fitting during the moulding process.

In all the methods of manufacturing saddle fittings described above uncoated resistance wire is used, which ensures maximum freedom of choice for the wire to be used in a fitting and can allow a reduction in manufacturing costs.

Claims

1. A electro-fusion saddle fitting for connecting a branch pipe to a main pipe, comprising a body of thermoplastic material having a welded surface for contact with the main pipe, and an electrical resistance heating winding, said winding being formed from uncoated wire and being at least partially embedded in the material of the body at said welding surface.

2. A method of making an electro-fusion fitting as defined in claim 1, wherein the body is formed substantially by injection moulding and the winding is fixed to the body during the injection moulding step.

3. A method according to claim 2, wherein the wound resistance wire is supported on a surface of the injection mould which defines said welding surface of the fitting, and the body is moulded directly over the winding.

4. A method according to claim 3, wherein the uncoated wire winding is made of steel wire, and said mould surface is magnetic and is provided with a shallow groove to support the winding in predetermined position.

5. A method according to claim 3, wherein the uncoated wire winding is located in a relatively deep groove in said mould surface, and is pushed out of said groove by ejector means after the mould has been filled with the thermoplastic material but while said material is still soft.

6. A method according to claim 2, a mat preform is prepared consisting of a thin sheet of thermoplastic material with the wire of the winding partly embedded in the sheet material, and the body is injection moulded over said preform.

7. A method according to claim 5, wherein the wire is embedded in the thermoplastic sheet by heating the wire and pressing it into the sheet.

8. A- method according to claim 7, wherein the wire is pressed into the sheet to a depth greater than half the wire diameter.

9. A method according to claim 7 or 8, wherein the uncoated wire is wound onto the sheet, the wire being pre-heated and pressed into the sheet as they are brought together.

10. A method according to claim 7 or 8, wherein the uncoated wire is wound onto a support member, the wire is then heated and pressed together with the thermoplastic sheet.

11. A method according to claim 10, wherein the wire is heated by passing an electric current through the wire.

12. A method according to claim 10 or 11, wherein said support member has a groove into which the uncoated wire is wound.

13. A method according to claim 12, wherein the groove has a depth less than half the wire diameter.

14. A method according to claim 12, wherein the groove is relatively deep and the wire is pushed out of the groove by ejector means to embed the winding in the sheet.

15. A method of manufacturing an electro-fusion saddle fitting substantially as herein described.

16. An electro-fusion fitting made by the method of any one of claims 2 to 15.