GB2246318A

GB2246318A - Abutment welding of plastic pipes and fittings and apparatus therefor

Info

Publication number: GB2246318A
Application number: GB9016510A
Authority: GB
Inventors: David John Hill; Keith Andrew Wilson
Original assignee: Victaulic Co PLC
Current assignee: Victaulic Co PLC
Priority date: 1990-07-27
Filing date: 1990-07-27
Publication date: 1992-01-29
Anticipated expiration: 2010-07-27
Also published as: GB2246318B; GB9016510D0

Abstract

A method of abutment fusion welding of two pipes comprises disposing between the two abutment surfaces of the pipes a heating plate at least one face of which is provided with grooves or contours; heating the plate and forcing onto the faces of the plate the two abutment surfaces, whereby the ends of the pipes are heated by the hot surfaces of the plate, and the at least one face configured with grooves or contours causing the abutment surface of the adjacent pipe to be conformed to that configuration; withdrawing the heating abutment surfaces from the plate and removing the plate and applying the heated abutment surfaces of the pipes together with sufficient force to weld the surfaces together; and allowing the welded ends of the pipes to cool. The invention includes apparatus for carrying out this method.

Description

Abutment Welding of Plastic Pipes and Fittings and Apparatus therefor This invention relates to the abutment welding of plastics pipes and fittings, and to apparatus therefor.

It is to be understood that by the expression "pipe" as used herein is meant any pipe-like member such as a pipe as such, and in addition pipe fittings such as "T" junctions, elbows, bends, reducers and valve inlet members, for example.

The invention is particularly related to the abutment welding of plastics pipes and fittings formed of thermoplastic materials, or materials which have, or can be made to have, thermo-plastic characteristics, such that fusion welding of the relevant surfaces of juxtaposed pipes and fittings can take place to form a joint or junction between those pipes and fittings. Thus, typically the invention is applicable to pipes and fittings of medium and high density polyethylene materials. Such materials may be provided with appropriate fillers or fibre reinforcement.

Abutment welding of such pipes and pipe fittings of the kind to which the invention relates involves heating the abutment surfaces to be joined, for example by means of a heated platten contacting each of the abutment surfaces until the surfaces are softened, and then pressing together the abutment surfaces and permitting the surfaces to cool and harden. Whilst abutment welding of this kind can be used with any appropriate pipes or pipe fittings it is most commonly used within the butt welding of juxtaposed pipes.

Fusion butt welded pipes have advantages over pipes jointed in other ways, in that, for example, it permits the use of a fully end load resistant system made from a single thermoplastics material, there being no compromise on chemical resistance or physical parameters.

With butt welding of pipes however, although techniques have been developed to ensure adequate strength of the weld, the time taken to heat the juxtaposed abutting ends of pipes adequately to ensure strong and safe welding, and then the time taken subsequently to allow cooling, are a major constraint in the use of this technique, since it slows the on-site use most significantly, particularly with large diameter pipes (for example greater than 180 mm) which cannot be coiled onto a circular forming carrier to provide long continuous lengths of pipe, but have to be provided in the form of convenient length separate pipe members each of which have to be butt welded to the next adjacent pipes on either side. Such pipe lengths can. typically, be of 6, 9, 12, 15 or 18 metres in length, and in each instance they need to be joined before laying on-site.

Added to this is a problem in welding semicrystalline materials such as polyethylene in that they have low thermal conductivity and high thermal capacity, which means that heating bulk material, such as pipe ends, to the required melt temperature, is slow. Furthermore, once heating and fusion has been completed, cooling is similarly slow.

It is an object of the present invention to overcome or at least substantially reduce the above mentioned problems.

In accordance with one aspect of the present invention there is provided apparatus for the abutment fusion welding of two pipes or pipe fittings including a heating plate having on at least one side a grooved or contoured face for use in contacting the abutment surface of at least one of the two pipes or pipe fittings to be juxtaposed.

In accordance with another aspect of the present invention there is provided a method of abutment fusion welding of two pipes or pipe fittings comprising disposing between the two abutment surfaces of the pipes or pipe fittings a heating plate at least one face of which is provided with grooves or contours; heating the plate and forcing onto the faces of the plate the two abutment surfaces, whereby the ends of the pipes are heated by the hot surfaces of the plate, and the at least one face configured with grooves or contours causing the abutment surface of the adjacent pipe or pipe fitting to be conformed to that configuration; withdrawing the heated abutment surfaces from the plate and removing the plate and applying the heated abutment surfaces of the pipes or pipe fittings together with sufficient force to weld the surfaces together; and allowing the welded ends of pipes or pipe fittings to cool.

In a preferred embodiment of the invention, both surfaces of the heating plate are provided with grooves or contours.

The present invention improves heat transfer rate to the pipe ends in operation of the invention in that it provides an increase in the area of the pipe end exposed to the heating plate. Thus, the usual planar surfaces (PTFE coated) of the plate are grooved or contoured, preferably by machining, to one of a variety of configurations.

The contours on either side of the heater plate may be complementary and may form a male/female pairing in the abutment surfaces when subsequently brought together. The contours may, alternatively, be of a matching configuration such as to provide a male to male or female to female match on the abutment surfaces after removal of the heating plate.

In some embodiments, the contouring may be such as to significantly reduce the possibility of air entrapment between the abutment surfaces and also may be such as to reduce manufacturing costs, or such as to simplify manufacture. Desirably, the contours are also such as to enable easy release from the molten polymer from the abutment surfaces, and to enable easy cleaning and maintenance.

In a typical conventional butt weld fusion process two plain prepared ends of the pipes to be joined are clamped in axial alignment. A heated plate (preferably provided with a non-stick coating on both sides) is placed between the aligned pipe ends, which are simultaneously brought into contact with either side of the plate for a predetermined time (soak time). When heated sufficiently, the pipe ends are withdrawn slightly from the plate, and the plate is removed. The two pipe ends are then brought together under controlled applied pressure and are held together while fusion takes place.

The appearance of a characteristic weld bead on the outside (and inside) of the pipe joint confirms that a satisfactory weld is being developed. The pressure is released and the joint is allowed to cool before being unclamped from alignment. On further cooling the joint achieves similar strength to the pipe. Completion of a satisfactory weld may be confirmed by examination of the weld bead, which may be removed if required.

The purpose of the "soak time" is to allow sufficient mass of the polymer in the weld zone to reach or approach a sufficient temperature so as to minimise the temperature drop that occurs when the heater plate is removed and before the joint is made, and so that a weld bead may form. The formation of the weld bead is concurrent with formation of new contact surface in the weld and to some extent has a purging effect on the previously heated surface.

All stages of the welding procedure are controlled, and typically the following paramaters are specified: (a) Size of pipe, (b) Material, (c) Fusion temperature, (d) Heating/Fusion pressure, (e) Soak time, (f) Cooling time, A butt weld machine would typically be used having: (i) Pipe clamps, (ii) Hydraulic ram mechanism, (iii) Separate or integral hydraulic unit, (iv) Heating plate with controller and temperature indicator (v) Planing tool (for removing the bead), (vi) Power supply or generator, By these means and utilising the invention very good quality fusion welded joints can be made in PE pipe for example.

In order that the invention may be more readily understood, one embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is an isometric view of apparatus according to the invention for fusion butt welding of two polyethylene pipes; Figures 2 and 3 are respectively a schematic front view and a schematic section of a heating plate in accordance with the invention; Figures 4 and 5 are a schematic front view and a schematic section of another form of heating plate; and Figures 6 and 7 are a schematic front view and a schematic section (on line A-A of Figure 6) of yet a further form of heating plate.

Referring to the drawings (particularly Figure 1) it will be seen that the apparatus for butt fusion pipes comprises clamps 3,4 adapted to fit around the ends of two adjacent polyethylene pipes 1,2, the clamps being mounted on longitudinal rods 5,6 on which they may be moved when subjected to hydraulic means (not shown) for pressing the two pipe ends together. Prior to connecting the two ends of the pipes, these are subjected to planing to ensure that their ends are parallel with each other and satisfactorily planar in configuration.

Disposed between the two pipe ends in Figure 1 is a heating plate 7 having a lifting handle 8 and connecting means 9 to electrical power supplies.

Figures 2 and 3 show a heater plate 10 having a circumferential groove 11,12 on each face having a male/female alignment between opposite faces. This is easy to manufacture, but does involve some possibility of alignment problems and air entrapment unless carefully utilised.

Figures 4 and 5 show a heater plate 13 having on each face all over pyramidal grooving having a male/female alignment between opposite faces. This is easy to manufacture, allows easy alignment and is unlikely to incur air entrapment problems.

Figures 6 and 7 show a heater plate 16 with radial grooves 17,18 on each face having a male/female alignment between opposite faces. Although of greater difficulty to manufacture, easy alignment is possible, and easy escape of air to avoid entrapment thereof is achievable.

The method of use of the apparatus illustrated is similar to conventional butt welding described hereinabove and as usual the ends of pipes 1 and 2 are prepared to a clean planar finish. On contact with the heater plate 7 and under the controlled application of pressure, heating of the surfaces of the ends of the pipes 1 and 2 causes them to deform rapidly so that there is contact across the entire contoured area. The heat transfer rate from heater plate 7 to pipe end is then increased, and if required the soak time may be reduced. On withdrawal of the pipes 1 and 2 and removal of the heater plate 7, a contoured surface to the melt on each pipe end is observed. The two complementary pipe ends are then brought together under controlled pressure and a weld bead may form.Owing to the contoured surface finish of the melt at the ends of the pipes prior to jointing, natural air flow across the surfaces and hence convective cooling of the pipe ends is reduced. In addition, owing to the formation of the contoured surface, a clean surface from within the pipe end has been formed without recourse to formation of a large weld bead and consequently the mass of polymer heated may be lower than in conventional planar butt welding ( a weld bead, albeit in different form to conventional butt welding, may still be formed).

The depth of machining of the contoured surface of the heater plate 7 should provide sufficient groove depth significantly to increase contact area, but be shallow enough to enable rapid contour deformation of the pipe end. Pipe wall thickness is a factor in determining the length across the plate between repeats of the grooving or contouring, it being desirable that any pattern repeat length is significantly less than the wall thickness.

Suitable groove depths may typically be in the range 1 - lOmm, and may extend across substantially the entire heater plate surface, so that a wide range of pipe sizes may be welded using the same heater plate, or may extend across only sufficient of the plate for welding of a particular group of pipe sizes.

The contoured surface may extend across the entire heater plate surface but have groove depths and pattern repeat lengths appropriate to the diameter and wall thickness of pipe to be welded. Small diameter pipes generally have lesser wall thickness than larger diameter pipes (for similar pressure resistance). Such grooves, therefore, may be shallower and more closely spaced near the heater plate centre, than near to the periphery. The dimensions of such contours may increase gradually from centre to periphery or may be so arranged as to be in bands having contours of fixed dimensions, which increase step-wise from centre to periphery. Thereby a preferred predetermined number of pattern repeats may be utilised across the wall thickness of pipes of any size.

The contoured heater plate may be prepared by machining from solid, casting, etching or by any other suitable forming technique so as to produce the required surface detail.

The heater plate may be formed of PTFE coated machined aluminium with high energy input electric heating to ensure rapid heat transfer and easy release from the polymer melt. This may be aided by appropriate radii to the edges of the contouring or grooving. Contoured or grooved sub-plates may be attached by bolts or other means to one or both sides of a conventional planar heating plate. Such sub-plates may then be removed or attached to the conventional heating plate as required. This would enable, for example, the provision of spare sub-plates with the PTFE coating in good condition, to be exchanged with used sub-plates requiring cleaning or renovation.

We have appreciated, when considering the effectiveness of the present invention, that in order to achieve the desired effect of reducing heating and cooling times in the butt joint welding of plastics pipes it is desirable that at least one of the following be achieved while not affecting the strength performance of the joint:- 1. Reduce heat content of the weld by (a) Lower melt temperature (b) Less heated material 2. Improve heat transfer rate by (a) Greater exposed heated area (b) Materials used for heater plate By means of the invention conditions l(b) and 2(a) are met and both soak time and cooling time can be less than conventional butt fusion welding. Furthermore, the effect on l(b) and 2(a) can be such that the temperature required by the heater plate is reduced, and hence l(a) can also be met. Normally, the increase in melt viscosity that results from the use of a lower temperature would lead to a poorer weld, but this effect is offset by the improved mixing resulting from the contoured weld interface. Another advantage of the lower temperature and reduced welding cycle, is the suitability of the process to be used with less thermally stable materials.

It is to be understood that the foregoing is merely exemplary of abutment fusion pipe jointing methods and apparatus therefor in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention.

Claims

1. Apparatus for the abutment fusion welding of two pipes are pipe fittings including a heating plate having on at least one side a grooved or contoured face for use in contacting the abutment surface of at least one of the two pipes or pipe fittings to be juxtaposed.

2. Apparatus as claimed in Claim 1 wherein both sides of the heated plate are provided with grooved or contoured faces.

3. Apparatus as claimed in Claim 2 wherein the contours on either side of the heater plate are complementary and are arranged to form a melelfernale pairing in the abutment surfaces of the two pipes or pipe fittings to be fusion welded when brought together.

4. Apparatus as claimed in Claim 1 or 2 wherein the contours on either side of the heater plate are of a matching configuration such as to produce in the abutment surfaces of the two pipes or pipe fittings to be fusion welded a male to male and/or female to female match after removal of the heating plate.

5. A method of abutment fusion welding of two pipes comprising disposing between the two abutment surfaces of the pipes a heating plate at least one face of which is provided with grooves or contours; heating the plate and forcing onto the faces of the plate the two abutment surfaces, whereby the ends of the pipes are heated by the hot surfaces of the plate, and the at least one face configured with grooves or contours causing the abutment surface of the adjacent pipe to be conformed to that configuration; withdrawing the heating abutment surfaces from the plate and removing the plate and applying the heated abutment surfaces of the pipes together with sufficient force to weld the surfaces together; and allowing the welded ends of the pipes to cool.

6. A method as claimed in Claim 5 wherein both surfaces of the heating plate are provided with grooves or contours.

7. A method as claimed in Claim e in which the abutment faces of the pipes are shaped prior to contacting the heater plate to a configuration at least partly complementary to that of the heater plate whereby to increase initial contact the area between the face of the heater plate and the relevant abutment face of the pipe.

8. Apparatus for the abutment fusion gelding of two pipes substantially as shown in and as hereinbefore described with reference to the accompanying drawings.

9. A method of abutment fusion welding of two pipes substantially as hereinbefore described with reference to the accompanying drawings.