Joint Components

The present invention relates to a joint between two components, each of which may be a pipe or a pipe fitting, and a method for forming such a joint. Particularly, but not exclusively, the invention relates to such a joint and method of forming a joint suitable for use in conjunction with a PVC pipe and/or pipe fitting. The invention may also be used with other fusible plastics, such as polyethylene, if desired

In recent years, polyethelene has been widely used for manufacturing pressure pipes for water and gas. One of the advantages of polyethelene, which has contributed to its popularity, is that successive lengths of pipe and associated fittings can easily be welded together. This allows reduction or elimination of comparatively expensive mechanical joints.

In recent years, PVC has been a less popular alternative to polyethelene. This is because PVC is less tough and cannot easily be welded. The difficulties in welding have caused PVC piping systems to require an unsatisfactory number of mechanical joints. The lack of toughness means that PVC pipes have not afforded satisfactory crack resisting properties. It is unfortunate that PVC suffers these drawbacks because for a given pressure of the fluid within the pipe, a PVC

pipe can have considerably thinner walls than a polyethelene pipe. For example, for a given operative pressure, a polyethelene pipe might require walls of 30 to 40 millimetres thickness, whereas a PVC pipe might require walls which are only 10 millimetres thick.

Recent developments have provided an impact modified PVC, known as "PVC-T".

PVC-T is considerably more tough then PVC-U and has considerably better crack resisting properties, even against a so-called "fast-crack" in which a fault propagates rapidly along a considerable length of pipe.

This material therefore solves one of the above discussed problems associated with PVC.

In the light of this development, it is highly desirable to develop a method of welding PVC pressure pipes. Unfortunately, this problem has resisted solution because of temperature constraints enforced by physical properties of PVC. If the welding temperature is too high, the PVC will simply burn, with subsequent continuous degradation. If the welding temperature is too low, the heated component will simply become soft and deform without fusion. These temperature constraints mean that conventional welding techniques

such as hotplate welding or electrofusion welding have failed.

The present invention sets out to provide a method of welding which can be used with, inter alia, PVC pressure pipes and/or pipe fittings. The invention also sets out to provide pipes and fittings for use in conjunction with such a method, and a process for manufacturing these pipes and fittings.

According to a first aspect of the invention there is provided a method of securing a connection between a socket and a spigot, comprising: inserting the spigot in the socket with an induction member and a volume' of thermoplastic material in the region of the junction between the spigot and socket; and inducing a current in the induction member, so as to cause its temperature to rise to a Curie temperature of a portion of the induction member, at which point the rise in temperature is restricted; wherein the rise in temperature of the induction member causes softening of the thermoplastic material so as to effect a weld between the socket and spigot.

The induction member is suitably in the form of one or more coils or rings which are provided in a collar which constitutes the volume of thermoplastic material. The collar is preferably inserted between the inner surface of the socket and the outer surface of the spigot.

Alternatively, the volume of thermoplastic material may be part of the spigot or socket and the induction member may, therefore, be embedded in that part of the spigot or socket defining the volume of thermoplastic material, as appropriate.

The coils or rings can be formed from a material of low resistance coated with a magnetic material. The coating material may be several skin-depths thick. The spigot and socket can each be formed at the end of a respective length of pipe, or on an appropriate fitting such as a union connector, or a T-pieσe or a saddle, for example.

A suitable material for forming the spigot, socket and collar is PVC; however an alternative weldable material such as polyethylene, polybutylene or some other polyolefin, for example, could be used if desired.

The current can be induced by induction coils which are provided in a welding appliance. Usefully, the welding appliance can be a clamp, which can clamp the socket at the same time as inducing a current. This clamping action will serve to squash the socket against the volume of thermoplastic material situated between the socket and spigot, helping to produce a good weld.

Additionally or alternatively, the plastic memory of the socket can be used to effect a good weld. The plastic memory causes the socket to shrink onto the volume of thermoplastic material when the socket is heated. The socket will have been plastically expanded prior to the welding process. The plastic expansion may be effected as part of the manufacturing process for the socket.

During welding, the current can be induced for a sufficient period to allow a quantity of the melted thermoplastic material to escape from between the spigot and socket, so as to be visible to the user as an indication that the heating process is complete.

According to a second aspect of the invention there is provided a socket for forming a connection with a spigot, the said socket being formed at the end of a pipe or pipe fitting and comprising a region of enlarged diameter and a thermoplastic collar fitted within the said region of enlarged diameter, the said collar comprising at least one induction member within its body.

Alternatively, the socket may comprise a region of enlarged wall thickness formed from a thermoplastic material, the region of enlarged wall thickness comprising at least one induction member adjacent or within its body.

According to a third aspect of the invention there is provided a collar assembly for insertion between a spigot and socket during a welding process for welding the spigot into the socket, the said assembly comprising a thermoplastic collar and at least one induction member provided in thermal communication with the said thermoplastic collar.

Preferably the collar assembly is defined by an annular ring or sleeve, which can be made of PVC, with a plurality of coils situated within its body. The annular ring or sleeve can also be made from some other weldable material, such as polyethylene, polybutylene or some other polyolefin, for example, providing that

material is capable of welding with the material forming the spigot and socket

In the case of both the second and third aspects of the - invention, the collar is preferably made from brightly coloured material, and the socket comprises a route for allowing part of the melted collar to escape to an external position, where it can be viewed by a user. The induction member is preferably defined by one or more coils or rings. Both the collar and socket can be formed from PVC or, if desired, some other weldable material, such as polyethylene.

According to a fourth aspect of the invention, there is provided a welding appliance for welding a spigot inside a socket; the said welding appliance comprising two opposed portions, which are separable to allow a spigot and socket assembly to be located therebetween, and which can be brought together to receive the spigot and socket assembly therebetween, at least one of the said portions comprising an induction coil for inducing a current in an induction member provided in the said spigot and socket assembly.

This socket could equally well be formed by an injection moulding process.

Preferably, the portions each comprise a channel for accommodating the spigot and socket assembly in a close-fit manner, such that the spigot and socket

assembly is closely surrounded by surfaces of the portions when the portions are brought together. The welding appliance can be in the form of a clamp and the two portions can be clamping portions for clamping the spigot and socket assembly therebetween during welding, so as to close the socket about the spigot, for the purpose of ensuring a good weld.

According to a fifth aspect of the invention there is provided a method of manufacturing a socket comprising the steps of:

(i) forming a tube with an annular region of increased wall thickness;

(ii) cutting the tube perpendicular to its longitudinal axis in the region of increased wall thickness so as to form two halves, each of which has an annular end region of increased wall thickness;

(iii) fitting a collar assembly comprising a volume of thermoplastic material and an induction member to a former;

(iv) warming and plastically expanding the region of increased thickness of one of the tube halves using the former and collar assembly;

(v) allowing the tube half to cool thereby to retain the collar assembly within an end region defined by the deformed region thereof.

Preferred embodiments of the invention will be described by way of example and with reference to the accompanying drawings in which:

Figure la is a cut-away perspective view of a pipe length;

Figure lb is a cut-away perspective view of a pipe of Figure la after a cutting operation;

Figure 2 is a cut-away perspective view of a collar assembly;

Figure 3 is a partially cut-away perspective view showing the collar assembly of Figure 2 mounted on a former which is addressing the pipe section of Figure lb;

Figure 4 shows the collar assembly and pipe section of Figure 3 following an operation in which the collar is installed within the pipe section to form a first embodiment of socket;

Figure 4a shows a second embodiment of a socket;

Figure 5 is a partially cut-away perspective view of the socket of Figure 4 being addressed by the spigot end of a length of pipe;

Figure 6 is a perspective view of the socket of Figure 5 in combination with a welding appliance;

Figure 7 is a perspective view showing the socket and welding appliance of Figure 6 together with the spigot end of the pipe length of Figure 5 received within the socket;

Figure 8 is a perspective view similar to Figure 7, but with the welding appliance closed around the socket; and

Figure 9 is a side elevation, partially in section, showing an assembled joint formed from the socket and spigot of Figure 5.

Figures 1 to 4 relate to the steps involved in the manufacture of an embodiment of a socket according to a first embodiment of the invention for use in the welding process of the present invention. Figures 5 to 8 relate to the welding process of the invention.

Manufacture of the socket will be discussed first.

A length of PVC pipe 2 is extruded using a variable opening die technique, so that a local section 4 of the pipe has an increased wall thickness.

The pipe 2 is then cut in the region of the area of increased thickness 4 and perpendicular to its longitudinal axis, so as to form a pipe section 6, which has an area 4a of increased thickness at one end 8.

A collar 10, as shown in Figure 2, is moulded around a number of coils 12. The coils are formed from coated copper wire. The wire coating is made from a magnetic material and has a thickness of several skin depths at the operating current frequency. In the present context a "skin depth" is defined in relation to the so-called

"skin effect" and is the depth from the surface within which 63% of an applied high frequency current is contained by a conductor. Skin depth decreases with increased current frequency and is lowest for materials which are the most magnetic.

The body of the collar is made from highly stabilised, easy- flow PVC material and is brightly coloured for good visibility. It has been found that red is a particularly good colour for the collar.

To form the socket 20 which is shown in Figure 4, the collar 10 is first fitted around a former 14 as shown in Figure 3. The former 14 is then introduced to the thicker end 4a of the pipe section 6 as shown in Figure lb. The thick section of the PVC pipe is heated, and is thermoformed into a socket of increased diameter by application of the former 14 whilst using the collar 10 as the former tip. When the former 14 is removed, the collar assembly 10 is left behind in intimate contact with the inside of the pipe section 6. The fitting between the collar assembly and the inside of the pipe section 6 should be tight enough to prevent accidental displacement. The resulting socket 20 is shown in Figure 4.

This socket could equally well be formed by an injection moulding process.

Use of the socket in forming a welded joint will now be described.

A pipe length 22 having a spigot end 24 which has an outer diameter smaller than the inner diameter of collar 10 to enable it to conjoin with the socket 20 is cleaned about the spigot end. The inside of the collar 20 is also cleaned. The socket 20 is then positioned within a welding appliance in the form of a welding clamp 30 and the spigot is inserted into the socket 20.

The welding clamp is in two halves 32, 34 and comprises one or more induction coils within its body. The induction coils cannot be seen in the figures.

In order to weld the pipe lengths 22 and 6 together, the induction coils are energised. The induction coils induce a high frequency current in the coils 12 situated within the collar assembly 10 of the socket 20. The current flows in the coating of magnetic material which is situated on the copper coils 12. This causes the magnetic layer to heat up very quickly. Eventually, the coating reaches its Curie temperature, and the coating material loses its magnetic properties. When this happens, the relative magnetic permeability of the coating decreases to unity. The skin depth of the material is inversely proportional to its magnetic permeability, and, therefore, increases significantly as the magnetic permeability decreases. This causes the

current to travel through the copper coils. Because the copper is a better conductor, the overall resistance of the coil system drops significantly.

Due to the above described drop in resistance of the coil system, the maximum temperature which can be induced in the coils 12 is the Curie temperature of the coating. By carefully selecting the ratio of the composite materials (nickel and iron, for example) of the coating, the coil system 12 can be designed to self-regulate at a suitable temperature. For example, if a coating material which has a Curie temperature which is slightly above the melting point of the collar 10 is used, the coil system can be heated to its maximum temperature, thus ensuring melting of the collar, whilst safely avoiding burning of the collar material.

The rise in temperature will also cause adjacent regions of the spigot and socket to melt, thereby allowing for fusion with the collar material upon cooling.

After a predetermined period of time, the arrangement is allowed to cool. The welding clamp 30 can then be removed and the connection can be inspected. If the weld has been successful, PVC from the collar assembly 10 will have been forced out from between the socket 20

and the spigot end 24 of the pipe 22. This will have been caused by a combination of the clamping action of the clamp and the contraction of the part of the socket 20 which is formed by the pipe section 6. The contraction occurs because this part of the pipe section has been expanded during manufacture and will have a plastic memory which causes shrinkage of the pipe back to its original shape as the pipe section is heated. This shrinkage reduces the volume between the pipe section 6 and the pipe section 22, thereby forcing the melted PVC from the collar assembly 10 from between the two; providing that the welding clamp 30 has successfully melted the PVC of the collar assembly 10. The PVC of the collar assembly is brightly coloured so as to enable it to be clearly seen when it has been forced out.

It is not necessary for both the clamping action of the welding clamp and the plastic memory of the socket to be used in order to contract the socket. Either of these principles could be used independently. Obviously, if contraction of the socket is to be provided purely by shrinkage due to plastic memory, then the welding apparatus need not be in the form of a clamp and may simply comprise induction coils suitable for surrounding the spigot and socket assembly to a degree that induces sufficient current in the coils 12.

In the foregoing embodiment, the welded PVC acts for both end load retention and for sealing. However, if desired, an elastomer seal could be incorporated inside the socket, along with the collar, so as to ensure a good seal. The heat provided by the welding clamp would still cause the plastic memory of the socket to cause the socket to shrink, thereby securely clamping the elastomer seal between the pipe section 6 and the pipe section 22. In this case, the welding need only be sufficient for end load retention purposes.

The assembled joint is shown in Figure 9. In this figure, a return 50 is shown on the socket. This return could be omitted if desired.

Figure 4a shows an alternative embodiment of the socket 120. In this embodiment, the region of enlarged diameter is replaced with a region of greater wall thickness 110. The region of greater wall thickness 110 has an induction coil 112 embedded in it and is intended to melt when the induction coil 112 is heated in the same manner as the coil 12 of the previous embodiment.

The region of greater wall thickness 110 can, therefore, be seen as equivalent to the collar 10 but an integral part of the socket 120 rather than a separate component.

Obviously, the socket 120 is formed using a different process to the one described for socket 20 above. The above described extrusion process would, in this case, be inappropriate, because it would not allow the coil to be included in the region of greater wall thickness. Instead, the socket is formed using an injection moulding technique, with the body of the socket being formed around the coil - which is inserted into the mould prior to the injection step. Of course, the socket 20 according to the first embodiment of the invention could also be formed by injection moulding. In fact, injection moulding is preferred over extrusion where the socket forms part of a fitting such as a connector. Additionally, socket 120 could be formed by a modified extrusion process.

Although the foregoing description relates to a spigot and socket coupling of two PVC pipes, the invention is not restricted to such an arrangement.

For example, in an alternative arrangement the spigot pipe is formed from a ceramics material or from iron. In this case, the outer surface of the spigot end is provided with a series of annular radially projecting ribs. Upon melting, the collar material flows between the ribs and is urged into position by the clamping action of the welding appliance. Upon cooling, the collar material sets in position and forms a seal and

mechanical link by its engagement with the ribs. The ribs could be replaced by some other suitable formation such as an annular channel provided in the outer spigot surface.

In another embodiment, two pipes or fittings which are to be connected do not comprise a respective spigot and socket. In this case the two are positioned end-to-end and a sleeve is positioned inside the two ends, so as to overlap the connection. The sleeve is generally similar in construction to the above described collar, but has different proportions; i. e. it is shaped rather more like a tube.

Upon application of the welding appliance, the sleeve melts and fuses with melted adjacent portions of the pipes and/or fittings.

Many further modifications and adaptations will suggest themselves to those versed in the art upon making reference to the foregoing, which is given by way of example and is not intended to limit the scope of the invention in any way.