GB1591305A

GB1591305A - Pipe of oriented thermoplastic polymeric material and a method and apparatus for its manufacture

Info

Publication number: GB1591305A
Application number: GB39506/76A
Authority: GB
Original assignee: IMI Yorkshire Imperial Plastics Ltd; Yorkshire Imperial Plastics Ltd
Current assignee: IMI Yorkshire Imperial Plastics Ltd; Yorkshire Imperial Plastics Ltd
Priority date: 1976-09-23
Filing date: 1976-09-23
Publication date: 1981-06-17
Also published as: NZ185249A; MX149737A; SE7710636L; BE859023A; ZA775690B; DE2742969C2; FI772814A; AT365975B; GR63671B; AU516688B2; ATA684577A; AU2908877A; JPS5342259A; DE2742969A1

Description

(54) PIPE OF ORIENTED THERMOPLA STIC POLYMERIC MATERIAL AND A METHOD AND APPARATUS FOR ITS MANUFACTURE (71) We, IMI YORKSHIRE IMPERIAL PLASTICS LIMITED (formerly Yorkshire Imperial Plastics Limited), a British Company, of PO Box 166, Leeds LSl 1RD, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a pipe of oriented thermoplastic polymeric - material and also to a method of, and apparatus for, manu- facturing such a pipe.

In our UK patent No. 1,432,539 we have taught a method of, and apparatus for, forming a pipe from orientable thermoplastic polymeric material by radially expanding a tubular blank into a mould at a temperature at which expansion of the pipe will cause orientation of the polymer molecules.

In this manner the finished pipe has an oriented structure capable of withstanding a greater hoop stress for a given wall thickness than a pipe made of the same material which has not been oriented. The pipe is also formed with an enlarged oriented socket for carrying a sealing ring. It is accordingly possible to produce a pipe with an integral socket at one end which provides a specified bursting strength with a smaller wall thickness than was previously possible, whereby the volume of plastics used for a given pipe diameter and strength is reduced to minimise the cost of the materials required. Such pipes are typically manufactured from PVC, chlorinated PVC, high or low density polyethylene, polypropylene or ABS, although other suitable orientable polymers may be used. In normal practice we have found that the expansion ratio between the diameter of the tubular blank and the finished oriented pipe is limited to a maximum of about 2:1 primarily because higher expansion ratios require the tubular blank to be of small diameter with extremely high wall thickness and such tubular blanks are extremely difficult to manufacture. As the enlarged socket is of greater diameter than the remainder of the pipe, the wall of the socket is thinner than the pipe wall and consequently more flexible. In practice we have found that, whilst the reduced rigidity of the socket is not critical, large diameter pipes subject to high pressures can incur leaks past the socket seal due to the increased flexibility of the thinner socket wall.

According to one aspect of the invention a method of forming a laminated pipe or a pipe having a laminated socket, wherein at least one lamina is of orientable thermoplastic polymeric material, includes placing two or more coaxial tubular blanks into a female mould having internal dimensions corresponding to the external dimensions of the finished pipe, heating the tubular lamina blanks to a temperature at which their deformation will induce orientation of the polymer molecules, applying internal pressure by means of a fluid medium to the tubular lamina blanks to force them radially outwards against the female mould whilst orienting the polymer molecules, cooling the moulded pipe to a rigid condition and then removing the applied pressure.

Preferably, the internal pressure is- applied to the inner tubular blank. However, the internal pressure may be first applied to the outer tubular blank and subsequently to the or each, inner tubular blank. The internal pressure may alternati,vely be applied simultaneously to the outer tubular blank and to an inner tubular blank, and the internal pressure to the outer tubular blank is released after the outer tubular blank has been expanded thereby causing the internal pressure in the inner blank to expand it towards to outer tubular blank. The tubular blanks are preferably heated by means of a fluid, such as water, at a suitable temperature. This heating fluid may be circu lated around the outer tubular blank so that the inner tubular blank or blanks is heated by heat transfer through the wall of the outer tubular blank. The heating fluid may alternatively or additionally be circulated through the inner tubular blank so that the outer tubular blank is heated by heat transfer through the wall of the inner tubular blank.

In either case a heat transfer fluid may be arranged between the tubular blanks to enhance heat transfer between them. The tubular blanks may have a radial gap between them through which the heating fluid is alternatively or additionally circulated.

The internal pressure is preferably applied by the heating fluid but may be applied by means of compressed gas or other liquid under pressure. The method may also include forming the outer blank shorter than the inner blank, locating the shorter blank radially outside the longer blank, and locating the shorter blank axially within the mould.

The method may also include expanding a portion of the tubular blanks into an annular recess in the female mould to define a socket at the end of the finished pipe. In this case the method may include forming one of the blanks to a length similar to that of the socket, arranging this socket blank in axial alignment with the annular recess in the female mould whereby at least part of the socket 'blank will be forced into the annular recess to form part of the socket.

The method may include arranging the socket blank around the other blank or blanks and applying the internal pressure first to the socket blank. The method may also include locating the socket blank axially within a portion of the annular recess prior to its expansion.

In another aspect the invention comprises a laminated pipe or a pipe having a laminated socket, wherein at least one lamina is of oriented thermoplastic polymeric material as made by any of the methods previously defined. Some laminae may not be oriented.

One of the laminae may be formed from oriented material to provide enhanced hoop strength for the pipe, the other lamina or ]aminae being provided to give specified properties such as thermal insulation to miniraise heat transfer through the pipe wall, resistance to chemical attack, or to provide a reflective outer surface to minimise absorption of solar energy as taught in our corresponding UK Patent Application No.

39504/76 (Serial No. 1,591,304), to which application attention is hereby directed. The pipe may define an annular socket of greater diameter than the remainder of the pipe.

In this case one of the laminae may be used to increase the wall thickness of the socket and/or to increase the wall thickness of a flange connecting the socket to the remainder of the pipe.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross-section of a mould containing two coaxial tubular blanks; Figure 2 illustrates a modification of Figure 1; Figure 3 is an enlarged scrap section of a socket at the end of a pipe produced by the mould of Figure 1 or Figure 2; Figure 4 is a scrap section through a modified pipe also produced by the mould of Figure 1 or Figure 2; Figure 5 illustrates a modification of the arrangement shown in Figure 1; Figure 6 is an enlarged scrap section of a socket at the end of a pipe produced by the mould of Figure 5, and Figure 7 illustrates a modification of the arrangement shown in Figure 5.

With reference to Figure 1, a tubular blank 10 of an unplasticised PVC having a vicat softening point of 82"C is located inside a split female mould having an upper half 11 and lower half 12. The blank 10 is sealed into the mould by means of external seals 13 and 14 whereby the annular interior space 15 of the mould can be filled with hot water through an inlet pipe 16 and drained by an outlet pipe 17. The two mould halves 11 and 12 are such that the annular interior space 15 is generally cylindrical as shown but defines at one end an annular recess 18 for defining a socket at the end of the pipe.

The two mould halves 11 and 12 would be clamped sealingly together in a convenient manner. A second tubular blank 19 is positioned coaxially within the first tubular blank 10 and is also formed of unplasticised PVC having a similar softening point.

The apparatus shown in Figure 1 is operated by fitting the tubular blank 19 inside the larder tubular blank 10 which is then positioned between the split mould halves 11 and 12. The mould is then damped together so that the seals 13 and 14 engage the outer tubular blank 10, and hot water at a temperature of 92"C is passed through the inlet pipe 16 into the chamber 15 and is recirculated, through the outlet pipe 17 and an unshown water heater back to the inlet pipe 16, for a sufficient time to heat both of the tubular blanks 10 and 19 to a temperature between 82 ' and 92"C at which their expansion will result in oriention of the polymer molecules. Heat transfer from the outer tubular blank 10 to the inner tubular blank 19 may be enhanced either by minimising the radial gap between them or by arranging a heat transfer fluid between them. Alternatively, hot water may also be circulated through the radial gap between the tu'bular blanks 10 and 19. Alter natively, the heating of the tubular blanks 10 and 19 may be effected solely by the circulation of hot water through the radial gap between them or the bore of the inner blank 19. Once the tubular blanks 10 and 19 have achieved the required temperature, the hot water supply is fed under pressure to the bore of the inner tubular blank 19 thereby expanding both of the tubular blanks radially outwards to conform to the internal shape of the mould 11, 12. In this manner the material of both tubular blanks 10 and 19 is oriented with a corresponding increase in the hoop strength of the finished pipe. The water pressure applied to the tubular blank 19 may conveniently be applied to either, or both, ends of the tubular blank through appropriate internal or external seals. Alternatively, water pressure may simultaneously be applied to the radial gap between the tubular blanks 10 and 19 and to the bore of the inner tubular blank 19 so that the outer tubular blank 10 will first be expanded to engage the mould cavity, there being no resultant force applied to the inner tubular blank 10. The water pressure applied to the radial space between the tubular blanks 10 and 19 would then be reduced so that the water pressure acting within the bore of the tubular blank 19 will expand it outwards into contact with the inner surface of the moulded tubular blank 10 whilst simultaneously displacing the water between them.

If desired, further tubular blanks may be arranged coaxially within the tubular blank 19. Because the tubular blanks can have a relatively low wall thickness, they are much easier to extrude than a corresponding one-piece tubular blank, and expansion ratios in excess of 2:1 are readily obtained; it is thought that expansion ratios in excess of' 3:1 are probably attainable. In the event that the tubular blanks 10 and 19 are expanded one after the other as described, the internal pressure required to achieve this expansion is of course less than that required to expand both tubular blanks 10 and 19 at the same time, or to expand a comparable one-piece tubular blank. Irrespective of the radial gap between the tubular blanks 10 and 19, the resultant laminae in the finished pipe are securely locked together.

After the tubular blanks 10 and 19 have been fully expanded into the mould 11 and 12, the resultant pipe is cooled in any convenient manner, for instance by cooling the mould parts 11 and 12 with oold water, or by passing cold water through the pipe at an appropriate pressure to retain the pipe shape until its temperature has dropped below the point at which reversion could occur. After the pipe has been cooled, the mould halves -11 and 12 can be separated and the formed pipe removed.

The portions of the tubular blanks 10 and 19 expanded into the cylindrical portion of the annular interior space 15 form a cylindrical wall 20 of a finished pipe 21 illustrated in Figure 3, and the portions of the tubular blanks 10 and 19 which are expanded into the annular recess 18 form an integral socket 22 defining an annular groove 23 which receives a sealing ring 24 lof resilient material such as synthetic rubber.

After the moulded pipe has been cooled and removed from the mould halves 11 and 12, it would have an inwardly turned flange defined by the portion of the mould between the annular recess 18 and the seal 14, and a similar flange between the end of the annular interior space 15 and the seal 13.

These flanges would be trimmed off the finished pipe which would then be of the form shown in 'Figure 3. As illustrated in that figure, a second laminated pipe 25, similar to the laminated pipe 21, has its cylindrical end 26 sealingly engaged by the sealing ring 24 within the socket 22 of the first pipe 21.

Figure 2 illustrates a modification of the apparatus described with reference to 'Figure 1 and the same reference numerals have been used to signify components which serve the same function unless stated to the contrary. It will be noted that the inner tubular blank 19 is supported by the seals 13 and 14 and that the outer tubular blank '10 is contained entirely within the split mould halves 11 and 12, being located radially by the inner tubular blank 19 and axially by the portions of the moulds lying immediately adjacent the seals 13 and 14. In this manner heated water is free to flow between the inner surface of the outer tubular blank 10 and the outer surface of the inner tubular blank 19 thereby providing a heat transfer medium between the tubular blanks 10 and 19 and also directly heating the inner tubular blank 19. Pressure for expanding the tubular blanks, after they have achieved the correct temperature, is applied solely to the inner tubular blank 19 which serves to force the outer tubular blank 10 into the mould cavities. This embodiment has the advantage that the length of the outer tubular blank 10 is reduced and the waste material which has to be cut off the ends of the finished pipe is of reduced thickness being formed primarily from the inner tubular blank 19. If desired further tubular blanks may ;be arranged either radially outside the tubular blank 10 or radially inside the tubular blank 19. A problem was initially experienced with the process illustrated in Figure 2 due to difficulty in maintain,ing end sealing. This was caused by the internal axial stress present in the extruded tubular blanks and the stress relief which occurred during heating in the mould 1;1, 12. The resultant axial contraction of the outer blank 10 exposed end portions of the inner blank 19 giving rise to preferential expansion of the exposed portions and consequential difficulties in obtaining a uniform product. However, this problem was solved by using stress free blanks obtained by annealing the extruded blanks in hot water.

Another problem experienced with the Figure 2 process is the trapping of globules of water between the two blanks during their inflation; this can be avoided by ensuring that all the water has been drained from the annular interior space 15 before the blanks are expanded.

As shown in Figure 4, a pipe 27 can be formed from say three tubular blanks to form laminae 28, 29 and 30 as shown, the inner lamina 30 being of say oriented thermoplastic polymeric material to provide enhanced hoop strength for the pipe, the middle lamina 29 being formed say from a foamed material which serves to minimise heat transfer through the walls of the pipe 27, and the outer lamina 28 serving to protect the foam lamina 29 and incorporating say a white pigment to minimise the absorption of solar energy as taught in our corresponding UK Patent Application 39504/76 (Serial No. 1,591,304).

Figure 5 illustrates a modification of the apparatus described with reference to Figure 1 and the same reference numerals have been used to signify components which serve the same function unless stated to the contrary. It will be noted that the inner tubular blank 19 is supported by the seals 13 and 14 and that an outer tubular blank 31 is considerably shorter than the equivalent tubular blank 10 illustrated in Figures 1 or 2. The outer tubular blank 31 has an axial length similar to the axial length of the annular recess 18, and is located axially of the inner tubular Iblank 19 in the position shown. This location may be achieved by arranging the outer tubular blank 31 to be a close sliding fit over the inner tubular blank 19 so that its axial position can be accurately determined before the mould halves 11 and 12 are shut. If it is desired that there should be a radial gap between the inner tubular blank 19 and the outer tubular blank 31, it will probably be necessary to locate the outer tubular blank 31 axially within the casing to prevent it from being moved by the flow of heating water through the annular interior space 15.

'Figure 6 illustrates part of a pipe 32 formed from the tubular blanks 19 and 31 by the apparatus of Figure 5. It will be noted that the cylindrical wall 20 of the finished pipe 32 is formed entirely from the inner tubular blank 19, and that the integral socket 22 and annular groove 23 are formed from separate laminae 33 and 34, and lamina 33 ;being formed from the inner tubular blank 19, and the outer lamina 34 being formed from the outer tubular blank 31. As the inner tubular blank 19 is of constant cross-section, the lamina 33 defining the inner wall of the socket 32 is essentially thinner than the contiguous cylindrical wall 20 as it has been expanded to a greater diameter as shown. The outer lamina 34 therefore serves to increase the wall thickness of the socket 22 and the annular groove 23 thereby increasing the hoop strength of the socket and, more importantly, increasing its rigidity. In practice the difference between the diameters of the tubular portion 20 and the socket 22 is such that the reduced thickness of the lamina 33 does not substantially decrease the hoop strength of the lamina as the greater expansion of this portion of the inner tubular blank 19 causes a correspondingly greater orientation of the material in the lamina 33. The prime function of the lamina 34 is accordingly to stiffen the socket 22 and annular groove 23 so that the sealing ring 24 is positively located to prevent leakage between the plipe 32 and a second laminated pipe 35, which is similar to the laminated pipe 32, and has its cylindrical end 36 sealingly engaged by the sealing ring 24.

The axial length and axial position of the lamina 34 can be controlled by the initial position and axial length of the outer tubular blank 31. In this manner, the lamina 34 may be extended to strengthen a radial flange 37 interconnecting the cylindrical wall 20 and the socket 22, or may be shortened to strengthen the socket 22 and/or annular groove 23 at any desired position. If desired, the socket 22 and/or annular groove 23 may be strengthened with additional laminae, similar to lamina 34, by providing a plurality of coaxial tubular blanks similar to the outer tubular blank 31 shown in Figure 5.

Also the cylindrical wall 20 and the inner lamina 33 may be reinforced by one or more additional laminae in the manner taught with reference to Figures 1 to 4. The tubular blank 31 shown in Figure 5 may alternatively be positioned coaxially within the tubular blank 19.

Figure 7 illustrates a modification of the apparatus described with reference to Figure 5 and the same reference numerals have been used to signify components which serve the same function unless stated to the contrary. It will be noted that the outer tubular blank 31 shown in Figure 5 has been replaced by an outer tubular blank 38 which is of greater diameter and is axially located within the annular recess 18. In this manner, the mould halves 11 and 12 positively locate the outer tubular blank 38 in the correct axial position relative to the annular recess 18. If desired, the inlet pipe 16 may be provided with additional connections in the mould halves 11 and 12 to ensure a circula- tion of hot water about both the inner and outer faces of the tubular blank 38. Alternatively the tubular blank 38 may be provided with sufficient axial clearance within the annular recess 18 to permit circulation of the heating water, or may alternatively have its ends scalloped so that it is positively located within the annular recess 18 whilst permitting water circulation. The tubular blanks 19 and 38 may be expanded either by applying pressure to the interior of the inner tubular blank 19 which will then expand into contact with the mould halves 11 and 12 and into contact with the inner surface of the tubular blank 38 which will then be expanded radially into the annular recess 18, or the outer tubular blank 38 may be expanded into the annular recess 18 before the inner tubular iblank 19 is expanded.

It will be appreciated that the present invention teaches the formation of laminated pipe in which the laminae may either be formed from the same material or be formed from different materials which each impart a property to the final pipe. In the event that the pipe is to be formed from blanks of different orientable thermoplastic polymeric materials which must be heated to different temperatures for inducing orientation of the polymer molecules during their deformation, each blank may be heated to the appropriate temperature and expanded in a convenient sequence.

It will be appreciated that sockets of a different configuration to the socket 22 can be produced with apparatus similar to the described but having the annular recess 18 appropriately modified and also that the moulds 11 and 12 may be modified to form sockets at both ends of the pipe or, if short lengths of tubular blank are used, that a double coupling can be formed comprising a pair of sockets which are either connected directly to one another or are interconnected by a short length of pipe. Although the description has related to the production of a pipe having a straight central axis, it will be appreciated that a curved pipe may be produced by expanding or otherwise forcing the tubular blanks into a correspondingly shaped mould.

The mould used to produce the blank 10 of Figure 1 does not have to be split in the manner illustrated in that Figure. In an alternative arrangement, the mould is formed as a tube with an end cap, so that a split is produced at the socket groove.

WHAT WE CLAIM IS:- 1. A method of forming a laminated pipe or a pipe having a laminated socket, wherein at least one lamina is of orientable thermoplastic polymeric material, including placing two or more coaxial tubular blanks into a female mould having internal dimensions corresponding to the external dimensions of the finished pipe, heating the tubular lamina blanks to a temperature at which their deformation will induce orientation of the polymer molecules in the or each lamina of orientable thermoplastic polymeric material, applying internal pressure by means of a fluid medium to the tubular lamina blanks to force them radially outwards against the female mould whilst orienting the polymer molecules, cooling the moulded pipe to a rigid condition and then removing the applied pressure.

2. A method as claimed in Claim 1, including applying the internal pressure to the inner tubular blank.

3. A method as claimed in Claim 1, including applying the internal pressure first to the outer tubular blank and subsequently to the, or each, inner tubular blank.

4. A method as claimed in Claim 1, including applying the internal pressure simultaneously to the outer tubular blank and to an inner tubular blank, and releasing the internal pressure to the outer tubular blank after the outer tubular blank has been expanded thereby causing the internal pressure in the inner tubular blank to expand it towards the outer tubular blank.

5. A method as claimed in any preceding Claim including heating the tubular blanks by means of a fluid, such as water, at a suitable temperature.

6. A method as claimed in Claim 5, including circulating the heating fluid around the outer tubular blank so that the inner tubular blank is heated by heat transfer through the wall of the outer tubular blank.

7. A method as claimed in Claim 5 or Claim 6, including circulating the heating fluid through the inner tubular blank so that the outer tubular blank is heated by heat transfer through the wall of the inner tubular blank.

8. A method as claimed in Claim 6 or Claim 7, including arranging a heat transfer fluid between the tubular blanks to enhance the heat transfer between them.

9. A method as claimed in any of Claims 5 to 7, including circulating the heating fluid between the tubular blanks.

10. A method as claimed in any of Claims 5 to 9, including applying the internal pressure by the heating fluid.

11. A method as claimed in any preceding Claim including forming the outer iblank shorter than the inner blank, locating the shorter blank radially outside the longer blank, and locating the shorter blank axially within the mould.

12. A method as claimed in any of Claims 1 to 10, including expanding an axial portion of the tubular blanks into an an

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. mould halves 11 and 12 to ensure a circula- tion of hot water about both the inner and outer faces of the tubular blank 38. Alternatively the tubular blank 38 may be provided with sufficient axial clearance within the annular recess 18 to permit circulation of the heating water, or may alternatively have its ends scalloped so that it is positively located within the annular recess 18 whilst permitting water circulation. The tubular blanks 19 and 38 may be expanded either by applying pressure to the interior of the inner tubular blank 19 which will then expand into contact with the mould halves 11 and 12 and into contact with the inner surface of the tubular blank 38 which will then be expanded radially into the annular recess 18, or the outer tubular blank 38 may be expanded into the annular recess 18 before the inner tubular iblank 19 is expanded. It will be appreciated that the present invention teaches the formation of laminated pipe in which the laminae may either be formed from the same material or be formed from different materials which each impart a property to the final pipe. In the event that the pipe is to be formed from blanks of different orientable thermoplastic polymeric materials which must be heated to different temperatures for inducing orientation of the polymer molecules during their deformation, each blank may be heated to the appropriate temperature and expanded in a convenient sequence. It will be appreciated that sockets of a different configuration to the socket 22 can be produced with apparatus similar to the described but having the annular recess 18 appropriately modified and also that the moulds 11 and 12 may be modified to form sockets at both ends of the pipe or, if short lengths of tubular blank are used, that a double coupling can be formed comprising a pair of sockets which are either connected directly to one another or are interconnected by a short length of pipe. Although the description has related to the production of a pipe having a straight central axis, it will be appreciated that a curved pipe may be produced by expanding or otherwise forcing the tubular blanks into a correspondingly shaped mould. The mould used to produce the blank 10 of Figure 1 does not have to be split in the manner illustrated in that Figure. In an alternative arrangement, the mould is formed as a tube with an end cap, so that a split is produced at the socket groove. WHAT WE CLAIM IS:-

1. A method of forming a laminated pipe or a pipe having a laminated socket, wherein at least one lamina is of orientable thermoplastic polymeric material, including placing two or more coaxial tubular blanks into a female mould having internal dimensions corresponding to the external dimensions of the finished pipe, heating the tubular lamina blanks to a temperature at which their deformation will induce orientation of the polymer molecules in the or each lamina of orientable thermoplastic polymeric material, applying internal pressure by means of a fluid medium to the tubular lamina blanks to force them radially outwards against the female mould whilst orienting the polymer molecules, cooling the moulded pipe to a rigid condition and then removing the applied pressure.

nular recess in the female mould to define a socket at the end of the finished pipe.

13. A method as claimed in Claim 12, including forming one of the blanks to a length similar to that of the socket, arranging this socket blank in axial alignment with the annular recess in the female mould whereby at least part of the socket blank will be forced into the annular recess to form part of the socket.

14. A method as claimed in Claim 13, including arranging the socket blank around the other blank or blanks, and applying the internal pressure first to the socket blank.

15. A method as daimed in Claim 13 or Claim 14, including locating the socket blank axially within a portion of the annularrecess prior to its expansion.

16. A method as claimed in Claim 15, including heating the socket blank by circulating the heating fluid between the socket blank and the annular recess.

17. A method of forming a laminated pipe or a pipe having a laminated socket, wherein at least one lamina is of orientable thermoplastic polymeric material, substantially as hereinbefore described with reference to Figures 1, 2, 5 or 7 of the accompanying drawings.

18. A laminated pipe or a pipe having a laminated socket wherein at least one lamina is of oriented thermoplastic polymeric material. m; > nllfactured by the method of any preceding claim.

19. A pipe, as claimed in Claim 18 in which at least one lamina is formed from oriented material to provide enhanced hoop strength for the pipe, and at least another lamina is provided to give another property such as thermal insulation to minimise heat transfer through the pipe wall, resistance to chemical attack, or to provide a reflective outer surface to minimise absorption of solar energy.

20. A pipe as claimed in Claim 19 in which at least one of the lamina is used to increase the wall thickness of the socket.

21. A pipe as claimed in Claim 20 in which at least one of the lamina is used to increase the wall thickness of a flange connecting the socket to the remainder of the pipe.

22. A laminated pipe wherein at least one lamina is of oriented thermoplastic polymeric material substantially as hereinbefore described with reference to and as shown in Figure 3 or 4 of the accompanying drawings.

23. A pipe having a laminated socket wherein at least one lamina is of oriented thermoplastic polymeric material substantially as hereinbefore described with reference to and as shown in Figure 6 of the accompanying drawings.