GB2207882A

GB2207882A - Joining plastics workpieces by fusion

Info

Publication number: GB2207882A
Application number: GB08719020A
Authority: GB
Inventors: Alan John Dickinson
Original assignee: British Gas PLC
Current assignee: British Gas PLC
Priority date: 1987-08-11
Filing date: 1987-08-11
Publication date: 1989-02-15
Anticipated expiration: 2007-08-11
Also published as: GB2207882B; GB8719020D0

Abstract

Workpieces for example pipes 10, 12 of polyethylene or other plastics material are joined by fusion by heating end-faces 40,42 by hot air from a blower 24 and heater 26 then forcing the faces together between a sleeve 30 and an outer annular wall 50. The pipes are held in clamps 16,18 relatively movable by a hydraulic cylinder 20. The end faces 40,42 are forced together for a period during which melt zones at either side of the interface are confined by the wall 50 and the sleeve 30 as the melt zones cool. The structure 22 can slide along the pipes so that the wall 50 can be positioned opposite the sleeve 30 once the end faces have been heated to fusion temperature. Alternatively, the end faces 40,42 are inclined to form a scarf joint. <IMAGE>

Description

METHOD & APPARATUS FOR JOINING PLASTICS WORKPIECES BY FUSION The invention relates to methods and apparatus for joining plastics workpieces by fusion, particularly though not exclusively tubular workpieces of material such as polyethylene, for example.

A method of joining plastics workpieces by fusion according to the invention comprises heating faces of the workpieces to fusion temperature by passing hot fluid over said faces through a gap therebetween, bringing the workpieces together to bring said faces into mutual engagement within an assembly of walls which confine melt zones at either side of the engaged faces, forcing the workpieces together for a period during which said melt zones are pressurised within the confinement of said walls and while said melt zones cool, discontinuing the force upon the workpieces and relatively separating the walls and the joined workpieces.

Preferably said confinement afforded by said walls is completed when said faces have attained fusion temperature by relative sliding movement between a wall and first one then subsequently both of the workpieces in a direction across said gap.

Where the workpieces are tubular said walls preferably comprise an inner wall initially extending across said gap and engaging the inner surfaces of end regions of the workpieces and an outer wall which initially engages the outer surface of one workpiece and which by said relative sliding movement after said fusion temperature has been reached is positioned so as to confine said melt zones between itself and said inner wall.

Apparatus for use in joining plastics workpieces by fusion, according to the invention, comprises means for holding the workpieces with their end faces mutually opposed across a gap between them, means for passing hot fluid across said end faces to heat them to fusion temperature, and a wall assembly comprising two paced walls between which the workpieces can be received, said means being operable to move the workpieces relatively to bring said end faces into mutual engagement between said two walls after fusion temperature is attained and to maintain melt zones at either side of the engaged faces pressurised for a period while the melt zones cool.

Where the workpieces are tubular, said means for passing hot fluid across said end faces preferably comprises structure which can be releasably held in encircling relationship around the workpieces, said structure defining an annular duct communicating with said gap, said structure comprising annular surfaces conforming to the outside surfaces of the workpieces and slidable relatively thereto said surfaces including an annular surface of one of said walls, the other of said walls being an annular wall insertable inside the workpieces to conform closely to the inside surfaces thereof.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which Figure 1 is a diagrammatic longitudinal section through two tubular workpieces with square end faces representing a typical butt joint configuration and apparatus arranged in position to heat the end faces of the workpieces to fusion temperature; and Figure 2 shows a later stage in the method of joining the workpieces shown in Figure 1 after the fusion temperature has been reached and the apparatus confines the melt zones as the workpieces are forced together.

Figures 1 & 2 show two tubular workpieces 10, 12 which are shown as pipes, by way of example. Other examples of tubular workpiece pairs are a pipe and a fitting such as a valve, elbow or tee. Typically such tubular workpieces are components of a pipe system such as a natural gas distribution system or a water supply or sewage disposal system or some other industrial or process system for example. The workpieces 10, 12 will be typically of polyethylene but the invention is applicable to joining workpieces of other plastics materials, whether both workpieces are of the same material or not.

The invention is particularly applicable where it is required to join one workpiece, for example a length of pipe, of material such as polyethylene, say, having a low melt flow index (MFI) less than 0.2 grammes/10 minutes to another workpiece, for example a fitting, of a material such as polyethylene, say, having a high MFI greater than 1 gum/10 minutes. British Standard No. 3412 may be referred to for explanation of the term Melt Flow Index.

The apparatus shown in the drawings is designed to confine the melt zones which are frmed in the method described below.

The apparatus comprises the fllowing principal components: a force-frame 14; clamps 16, 18; a hydraulic cylinder 20; a hollow air-feed structure 22; an air blower 24; an air-heater 26; air re-circulation ducting 28; and a sleeve 30.

The force-frame 14, clamps 16, 18 and cylinder 20 need not be described in detail since they form a well-known combination very similar to conventional equipment used for effecting the joining of pipes of plastics materials by conventional butt-fusion techniques using a heater plate to heat the ends of the pipes, before they are forced together.

However in the apparatus shown in Figure 1, although the functions of the frame, clamps and cylinder are essentially similar to the corresponding functions in the known equipment, no heater plate is present.

Accordingly, it will be understood that the clamps 16, 18 can be operated to grip the pipes 10, 12, respectively, and hold them with their respective end faces 40, 42 in accurate alignment. Conventional means (not shown) such as a planer device is preferably provided on the frame 14 operable to dress the faces 40, 42 to ensure fresh faces free of contamination and, in the case of the butt joint shown, in planes accurately orthogonal to the line of relative approach of the clamps 16, 18 along the frame 14.

Alternatively, the end faces can be cut e.g. by planing so as to be inclined at matching angles so that a scarf joint can be formed.

At least one clamp 16 is movable relatively to the frame 14. Relative movement of the clamps 16, 18 is effected by operation of the cylinder 20, here shown supported by an abutment 44 on the frame 14 while the piston rod 46 extends out of the cylinder 20 and acts on the clamp 18 assumed to be the sole clamp movable relatively to the frame 14 in this example of construction.

Figure 1 shows the apparatus in the heating stage of the method. The clamps 16, 18 hold the pipes 10, 12 with their end faces 40, 42 separated by a gap equal in width to the wall thickness of the pipes 10, 12. Hot air from the blower 24 is forced into the structure 22 which encircles the ends of the pipes 10, 12. The structure defines an annular duct 46 which communicates at its radially inner open margin with the gap between the end faces 40, 42. The inner margin of the gap is closed by the sleeve 30, which had been positioned within the pipes 10, 12 previously and which conforms closely to the inner surfaces of the pipes. Thus, hot air is forced to flow over the end faces 40, 42 through the gap between them. The air leaves the duct 46 through a port 48 and is conducted through the return ducting 28 to the inlet of the blower 24. Typically, the sleeve 30 preferably has an insulated and non-stick outer surface which can engage the pipes 10, 12 without risk of sticking particularly at the softened end faces 40, 42. The structure 22 is typically of insulating material to conserve heat but the inner surfaces engaging the pipes 10, 12 are also insulated and non-stick.

Once the end faces 40, 42 reach fusion temperature the hot air flow is discontinued. The structure 22 is moved along the pipes 10, 12 leftwards (as seen in the drawings) by sliding the structure manually or by powered mechanism (not shown), so that an annular wall 50 (Figure 2) is positioned opposite the sleeve 30, closely conforming to the outside of the pipes 10, 12.

The pipes 10, 12 are moved relatively endwise to bring the end faces 40, 42 into mutual engagement by relative movement of the clamps 16, 18 by energisation of the cylinder 20. The cylinder is maintained pressurised so that the faces 40, 42 are forced together under a predetermined load for a predetermined period. The melt zones indicated at 52, which are the annular end regions of the pipes at each side of the common interface 54 where the end faces 40, 42 engage, are completely confined between the wall 50 and the wall provided by the sleeve 30 and between the remainder of each pipe.

The pressurisation of the melt zones at 52 as they cool enables a sound butt fusion joint to form between the two pipes 10, 12, even where one pipe has a low MFI for example and the other has a much higher MFI.

Once the period during which pressurisation of the melt zones 54 has expired, the cylinder 20 is de-pressurised and the structure 22 is dis-assembled from the pipes 10, 12. Preferably, the structure 22 is in two halves which can be releasably clamped around the pipes. The sleeve 30 is also removed. The joint between the pipes 10, 12 is free from any bead either at the outside or inside, so that there is no obstacle to flow of fluid e.g. natural gas through the pipeline formed by lengths of pipe such as the pipes 10, 12.

The joining of fittings to pipe can be performed in a manner similar to that just described. A typical application of the invention is in a factory in which short tubular lengths are joined to fittings. The short lengths would preferably be of material identical with pipe to be joined in field operations to form a pipeline.

In a modification (not shown) the method can be applied to joining workpieces other than tubular workpieces. For example, in the joining of planar workpieces the melt zones could be straight and could be confined between flat walls extending along the zones. In addition two further walls would be required, one at each end of the melt zones.

Instead of air, some other gas (e.g. nitrogen) or liquid may be used to convey heat to the end faces of the workpieces. Such gas (whether air or another gas) or liquid is generally referred to herein as "fluid".

In another modification (not shown), the liner 30 or a corresponding liner is left in place instead of being removed. The liner in that case need not be of non-stick nature. For example, in that case the liner could be of plastics material and if preferred could partially fuse and become joined to the workpieces by fusion.

Claims

1. A method of joining plastics workpieces by fusion comprising heating faces of the workpieces to fusion temperature by passing hot fluid over said faces through a gap therebetween bringing the workpieces together to bring said faces into mutual engagement within an assembly of walls which confine melt zones at either side of the engaged faces, forcing the workpieces together for a period during which said melt zones are pressurised within the confinement of said walls and while said melt zones cool, discontinuing the force upon the workpieces and relatively separating the walls and the joined workpieces.

2. A method according to claim 1 said confinement afforded by said walls being completed when said faces have attained fusion temperature by relative sliding movement between a wall and first one then subsequently both of the workpieces in a direction across said gap.

3. A method according to claim 2 the hot fluid being fed into said gap by a structure which is displaced by said relative sliding movement.

4. A method according to claim 2 or 3 the workpieces being tubular and said walls comprising an inner wall initially extending across said gap and engaging the inner surfaces of end regions of the workpieces and an outer wall which initially engages the outer surface of one workpiece and which by said relative sliding movement after said fusion temperature has been reached is positioned so as to confine said melt zones between itself and said inner wall.

5. Apparatus for use in joining plastics workpieces by fusion comprising means for holding the workpieces with their end faces mutually opposed across a gap between them, means for passing hot fluid across said end faces to heat them to fusion temperature, and a wall assembly comprising two spaced walls between which the workpieces can be received, said means being operable to move the workpieces relatively to bring said end faces into mutual engagement between said two walls after fusion temperature is attained and to maintain melt zones at either side of the engaged faces pressurised for a period while the melt zones cool.

6. Apparatus according to claim 5 for use in joining tubular workpieces, said means for passing hot fluid across said end faces comprising structure which can be releasably held in encircling relationship around the workpieces, said structure defining an annular duct communicating with said gap, said structure comprising annular surfaces conforming to the outside surfaces of the workpieces and slidable relatively thereto said surfaces including an annular surface of one of said walls, the other of said walls being an annular wall insertable inside the workpieces to conform closely to the inside surfaces thereof.

7. A method of joining tubular workpieces according to claim 1 substantially as herein described with reference to the accompanying drawings.

8. Apparatus according to claim 5 substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A method of joining plastics workpieces by fusion comprising heating faces of the workpieces to fusion temperature by passing hot fluid over the faces through a gap#therebetween bringing the workpieces together in butt or scarf joint relationship, forcing the faces into mutual engagement within an assembly of walls which present wall surfaces closely engaging the workpieces and bridging the workpieces melt zones at either side of the engaged faces being totally confined by the workpieces and the wall surfaces, continuing to force the workpieces together for a period during which the melt zones are pressurised within the confinement provided by the walls and the workpieces and while the melt-zones cool, discontinuing the force upon the workpieces and relatively separating the walls and the joined workpieces.