METHOD OF MANUFACTURE OF A HEAT SEALABLE FITTING AND FITTING MADE BY SUCH METHOD

Abstract: A heat sealable fitting primarily intended for joining thermoplastic gas service pipes is manufactured by winding a heated resistance wire on to a preform of thermoplastic material. The winding and preform are then encapsulated in a further layer of thermoplastic material.

Description: Description This invention relates to heat-sealable fittings for making connections to tubing or pipes and to methods of manufacture of such fittings. In the installation of service pipes for the supply of gases it is essential to make a perfect seal at joints in the pipe or between pipes and fittings. One method of making such connections is to use thermoplastic materials which are heated until they fuse to the fitting or pipe to be joined. In one prior art method a tubular jointing fitting was constructed with an integral heater wire. The fitting of thermoplastic material was placed over the ends of two lengths of pipe to be joined and the heater wire coupled to a power supply. The thermoplastic material softened and formed a bond with the material of the service pipes. One method of manufacture of such fittings s described in U.K. Patent No. 1 373 761. A pair of steel bands were placed on a mandrel and a helix of resistance wire was wound between them and its ends connected thereto. A ribbon of thermoplastic tape from an extruder was then wound round the helix of resistance wire and mandrel and a predetermined thickness of thermoplastic material built up. Preferably, the thermoplastic tape was stretched as it was wound so that it was pre-stressed and would shrink on to a pipe when the resistance wire was heated. Finally the complete fitting was removed from the mandrel. In another method of manufacture described in UK' Patent No. 1 40 713 a heater wire was wound in a helical thread on a thermoplastic preform and a thermoplastic jacket subsequently moulded to hold it in position. Stress was then applied by forcing the fitting over a tapered mandrel. The first method suffers from the disadvantage that the method of prestressing the fitting by stretching the thermoplastic tape is extremely sensitive to variations in the mechnical properties of the thermoplastic material whilst the second method demands an additional prestressing step after the fitting has been manufactured. In order to overcome these and other disadvantage, an improved method of manufacture of heat-sealable thermoplastic fittings has been devised. According to the present invention there is provided a method of manufacture of heat sealable fittings comprising the steps of anchoring one end of a length of resistance wire to a tubular preform of thermoplastic material, heating said resistance wire and applying it whilst hot to form a helical winding on said tubular preform, anchoring the other end of said length of resistance wire to said preform and encapsulating said helical winding with a jacket of thermoplastic material. A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figures la to lc show steps in the manufacture of a heat-sealable fitting; and Figures 2a to 2c are illustrations of constructional features of a fitting made by the method of Figure 1. Referring now to Figures la to lc of the drawings. a cylindrical mandrel 1 is loaded into a two-part injection moulding tool 2 (only one part of which is shown). The moulding tool and mandrel cooperate to form a cavity 3 into which a thermoplastic material such as polyethylene, polybutylene or polypropylene is injected through channels 4 to produce a moulded preform 5. The mandrel 1 may comprise two or more components according to the shape and size of the preform 5 to be moulded. For example, at least two components will be-required if an internal annular central stop is required to be moulded on the preform 5. After the injection moulding step, the moulding tool 2 is opened and the mandrel 1 bearing the preform 5 is mounted on a winding lathe 6. One end of a length of resistance heater wire 7 is attached to a terminal post 8 on the preform 5 and the mandrel rotated to form a helical winding on the preform. As the resistance wire is fed from a drum 9 it passes through a heater 11. When the winding is complete the other end of the heater wire 7 is attached to a second terminal post 10. The mandrel 1, bearing the preform 5 is removed from the lathe 6 and placed in a further two-part moulding tool 12 (Figure lc). The heater winding is then encapsulated in a jacket 13 formed by injecting a further layer of thermoplastic material through channels 14. The complete mandrel and mouldings are rempved from the injection tool 12 and allowed to cool. The completed fitting is then removed from the mandrel 1. Features of the heat-sealable fitting are shown in greater detail in Figures 2a to 2c. Figure 2a is a plan view of the wire-wound preform 5 of a heat-sealable fitting. Figure 2b is a section taken along AA' of Figure 2a but including the encapsulating iacket 13, and Figure 2c is a s rap view in the direction X indicated on Figure 2a. The preform 5 has two sections 22,23 to carry the heater winding 24. Winding posts 25 with wire-locking slots 26 are provided to anchor the ends of the heater winding 24. Metal terminal caps 27 fit over the posts 25 and make electrical contact with the heater winding 24. A thicker middle portion 28 on the preform 5, separating the sections 22,23 has a wider spaced convolution of the heater winding so that it will not melt when power is applied. Guide ribs 29 and central locating pegs 30 position and hold the heater wire at a change of winding direction. Variations of the method are within the ambit of the invention. For example, the resistance wire may be heated so that it simply softens the thermoplstic preform or it may be heated to a higher temperature so that it melts the preform forming a fillet which partially covers the wire. The tension created by contraction of the wire after cooling may be sufficient, or further stress may be applied by stretching the wire in addition. One or more stops 31 may be provided on the interior surface of the preform 5 (Figure 2b) so as to enable the ends of the pipes to be correctly located before joining. Also, means such as longitudianl ribs may be provided on the interior wall of the preform so as resiliently to grip the ends of the pipes prior to joining. Furthermore, suitable visible heat-indicator means can be provided on the fitting whereby, in use, to indicate that a correct welding temperature has been attained for completing the welding of the fitting to the pipes.

Claims: Claims 1. A method of manufacture of heat-sealable fittings comprising the steps of anchoring one end of a length of resistance wire to a tubular preform of thermoplastic material, heating said resistance wire and applying it whilst hot to form a helical winding on said tubular preform, anchoring the other end of said length of resistance wire to said preform and encapsulating said helical winding with a jacket of thermoplastic material. 2. A method of manufacture of heat-sealable fittings as claimed in Claim 1, wherein the resistance wire is heated sufficiently to soften the thermoplastic preform when it is applied thereto. 3. A method of manufacture of heat-sealable fittings as claimed in Claim 2, wherein the resistance wire is heated sufficiently to melt the thermoplastic preform to form a fillet partially covering the surface of the resistance wire. 4. A method of manufacture of heat-sealable fittings as claimed in any one of the preceding claims including the further step of stretching the resistance wire prior to winding. 5. A method of manufacture of heat-sealable fittings substantially as herein described with reference to and as shown in the accompanying drawings. 6. A heat-sealable fitting mdde by a method claimed in any one of the preceding claims. 7. A heat-sealable fitting as claimed in Claim 6 wherein the thermoplastic material is polyethylene. 8. A heat-sealable fitting as claimed in Claim 6 wherein the thermoplastic material is polypropylene. 9. A heat-sealable fitting as claimed in Claim 6 wherein the thermoplastic material is polybutylene. 10. A heat-sealable fitting substantially as herein described with reference to and as shown in Figures 2a, 2b and 2c of the accompanying drawings.

Inventors: EWEN, ALEXANDER, MAINE, LESLIE
Applicants: BRITISH GAS CORPORATION, VULCATHENE GLYNWED TUBES AND FITTINGS LTD.
Application year: 1981
Main classification: F16L47
IPC codes: F16L13, F16L47, B29D23, B29C70