This invention relates to electrofusion couplers

In relation to the connection together of plastics pipes, or the connection of fittings to

pipes, the employment of electrofusion couplers is very well-known What is provided is a sleeve-like member of plastics material having a winding of electrical resistance heating wire in close proximity

to the inner surface of the sleeve-like member, each end ofthe wire being connected to a terminal

accessible externally of the sleeve-like member to enable connection of the winding to a source of electrical supply With, for example, such a sleeve-like member applied to adjacent ends of plastics pipes, the application of electrical power causes the heating wire to heat up to a temperature to melt

the interface between the sleeve-like member and the pipe ends, such that on cooling the sleeve-like member is fused to each pipe end, hence providing a most effective connection between the pipe

ends

With electrofusion couplers of relatively small size, to accept pipe diameters of, for

example, 60 to 125mm, it has been traditionally so that thin copper wire, e g approximately 0 4mm diameter has been used, capable of producing very good joints with the advantage of being of

relatively low cost. However, this wire cannot be used in electrofusion couplers to accept relatively large diameter pipes of over 125mm, because it has low specific resistivity which does not allow larger winding diameters to be employed to extend across a required fusion zone whilst maintaining acceptable spacings or pitch between adjacent turns ofthe wire As a result, much more expensive wire such as copper alloy for example 95%Cu, 5%Sn, or laquered copper alloy, has been employed,

with attendant design problems in ensuring that such larger diameter wire is not wound with too close a spacing or pitch that will cause shorting during use, or too wide a spacing or pitch that will leave

areas between the turns that are inadequately heated, with resultant poor and unacceptable joints

The object ofthe present invention is to provide an electrofusion coupler able to employ

less expensive relatively small diameter copper wire for its winding of electrical resistance heating

element, irrespective as to its diameter

According to the present invention, an electrofusion coupler comprises a sleeve-like member with a winding of electrical resistance heating wire over a length of its bore, and said winding

being composed of a number of wires connected in parallel.

According to a second aspect of the present invention, a method of manufacture of an

electrofusion coupler comprises encasing at least two electrical resistance heating wires in a ribbon of a plastics material, attaching exposed ends ofthe at least two electrical resistance heating wires at each end ofthe ribbon to each other to connect the wires in parallel and to a terminal, winding the ribbon helically about a cyhndrical mandrel strategically locating the terminals, applying a mould to

the mandrel supplying molten plastics material to the mould and whereby to embed the ribbon of

plastics material on the mandrel in the surface of a bore through the coupler formed in the mould, the mould having socket forming sections to form sockets on the coupler within which the terminals are contained

Thus, any number of wires, two, three, or more of a required length can be attached to common terminals at each of their ends, and the grouping of two, three or more wires, helically wound along the bore ofthe coupler and in very close proximity to the surface ofthe bore. With the number of wires embedded in spaced, parallel relationship in an extruded ribbon of a compatible plastics material to that of the coupler, the ribbon wound on the mandrel located in the mould in which the coupler is moulded is readily assimilated into the plastics of the coupler, thereby ensuring that the winding of parallel wires in close proximity to the surface ofthe bore ofthe coupler.

.As a consequence of being connected in parallel, wires of considerably smaller diameter than such as copper alloy can be employed, with much smaller spacings between adjacent wires in

each tum. As a result, there is the total elimination ofthe possibility of there being any inadequately heated areas, whilst at the same time there is avoided any possibility of shorting across adjacent wires

ofthe turns.

By way of explanation, if for a particular coupler, copper alloy wire of a diameter of

1.51mm would ordinarily be used for a given requirement of electrical properties, relatively simple calculation confirms that, for the same electrical properties, copper wire of 0.7mm diameter would be required. However, for the same length of wire and the same fusion zone required, copper wire of 0.7mm diameter single wound at the same pitch as copper alloy wire at 1.51mm diameter has the

automatic result that the distance between the adjacent edges of the turns of the copper wire is greater than the distance between the turns of copper alloy wire, with a consequent increase in the prospect of areas of inadequately heated sectors mid-way between two turns.

In general the resistance of a single wire at a particular temperature can be expressed as

R = DL

A where

R = resistance p = specific resistivity L = wire length

A = wire cross-sectional area

If a single wire is replaced by a number of wires ofthe same material, it is necessary for the aggregate cross-sectional areas ofthe number of wires to be the same as the cross-sectional area ofthe single

wire if the same electrical resistance is to be maintained. Therefore, for a given length L, and wire

diameters d, and d ₂ ,

___ ⁼ ____._ : 4 4

or

d, = v ^/ n

For the example mentioned above, where a single winding of copper alloy of 1 51mm diameter would be used, known to be equivalent to copper wire of 0 7mm diameter, 3 copper wires of 0 4mm

diameter connected in parallel, can be provided to give substantially identical electπcal properties but

with spacings between the three wires themselves and between the turns formed by the three spacings that ensures the absence of voids without attracting the possibility of shorting On a direct cost comparison, a given length of copper alloy wire at, say, 1 51mm is

considerably more expensive than three times that length of copper wire of 0 4mm diameter,

resulting in couplers that are not only more efifective than hitherto, but which have lower cost

factors than hitherto

One embodiment of the invention will now be descnbed with reference to the accompanying drawings in which -

Figure 1 is a schematic sectional side elevation of a coupler in accordance with the invention,

Figure 2 is an enlargement of detail A of Figure 1 , and

Figure 3 is a schematic representation of a mandrel on which a coupler ofthe invention is formed

In Figure 1 a coupler 1 is formed as a tubular member 2 of a appropnate plastics mateπal, the coupler having a bore 3 As is shown more particularly by Figure 2, embedded in the surface of the bore 3 is a πbbon 4 of a compatible plastics mateπal containing two spaced wires 5 of copper, the πbbon being formed such as by known extrusion means At each end of the πbbon 4, it passes

through the wall of the coupler 1, the copper wires 5 being directly connected at each end of the

ribbon to a terminal 6 located within a socket 7

To enable the production of the coupler, and as is indicated by Figure 3, the πbbon 4

containing the copper wires 5 is wrapped on a cylindncal mandrel 8, with the terminals 6 positioned to extend to the same side of the mandrel and to be co-planar there being a suitable means to hold

the terminals in this position within a mould 9 applied to the mandrel and in which the coupler is

moulded During the moulding ofthe coupler the πbbon 4 becomes an integral part ofthe bore of the coupler ensuring that the copper wires connected in parallel e in close proximity to the surface

ofthe bore

By having the copper wires connected in parallel encase in a πbbon, the application ofthe

πbbon to the mandrel can be effected with relative ease, and to so locate the πbbon as to provide an even spacing of the copper wires along its length and, consequently, along the length of the bore

moulded on the mandrel

Thus with the end of adjacent pipe lengths each inserted into opposite ends ofthe coupler,

and with the terminals 6 suitable connected to a source of electncal supply, the embedded copper wires heat the surface ofthe bore and the adjacent extemal surface ofthe inserted pipe ends to cause the fusion ofthe pipe ends to the coupler and hence produce an effective joint