CA1095438A - Pipe of oriented thermoplastic polymeric material and a method and apparatus for its manufacture - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A laminated pipe of oriented thermoplastic polymeric material is manufactured by a method in which at least two coaxial tubular blanks are expanded in a female mould after heating to a temperature that will induce orientation.
The moulding apparatus is arranged to support the blanks which may be of different materials chosen for their individual properties. One of the blanks may be of short length and be used to reinforce the pipe at a chosen area such as a coupling sheet.

Description

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The invention relates to a pipe of oriented thermoplastic polymeric material and also to a method of, and apparatus for, manufacturing 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 withstand-ing a greater hoop stress for a given wall thickness than a pipe made of the same mateTial 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 thatJ 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

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lO~S438 flexibility of the thinner socket wall.
~ ccording to one aspect of the invention there is provided a method of forming a pipe of orientable thermoplastic polymeric material including placing an outer tubular blank and at least one inner tubular blank, said blanks being spaced apart and said blanks being coaxial and of circular transverse cross-section, into a female mould having internal dimensions corresponding to the external dimensions of the finished pipe, heating the blanks to a temperature at which their deformation will induce orientation of the polymer lecules, applying internal pressure to the blanks to expand said blanks radially outwards against the mould whilst orienting the polymer lecules in what had been both of the blanks, subsequently cooling the moulded pipe within the mould to a rigid condition and then reving 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 alternatively 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 the outer tubular blank. The tubular blanks are preferably heated by means of a fluid, such as water, at a suitable temperatuTe. This heating fluid may be circulated 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 10~5438 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.
According to another aspect of the invention there is provided a pipe of oriented thermoplastic polymeric material formed from at least two 2Q concentric laminations including an annular socket of greater diameter than the remainder of the pipe, and in which at least a part of the socket is reinforced by one of the laminations. Some laminations may be of different materials. Some of the laminations may not be oriented. One of the laminations may be formed from oriented material to provide enhanced hoop strength for the pipe, the other lamination or laminations being provided to give specified properties such as thermal insulation to minimise heat transfer through the pipe wall, resistance to chemical attack, or to provide .

' , 10~5438 a reflective outer surace to minimise absorption of solar energy. The pipe may define an annular socket of greater diameter than the remainder of the pipe. In this case one of the laminations may be used to increase the wall thickness of the socket andtor to increase the wall thickness of a flange connecting the socket to the remainder of the pipe. An outer lamination may be in the form of a preformed ring comprising at least part of the outer wall of the annular socket.
According to a further aspect of the invention, apparatus for moulding a pipe of oriented thermoplastic polymeric material includes a female mould having internal dimensions corresponding to the external dimensions of the finished pipe, means for supporting two or more coaxial tubular blanks within the female mould, means for heating the tubular blanks to a temperature at which their deformation will induce orientation of the polymer molecules, and means for applying internal pressure to the heated tubular blanks to expand them radially outwards against the female mould.
The apparatus preferably also includes means for cooling the oriented pipe before the internal pressure is released. The means for supporting the blanks may include sealing means for sealingly engaging opposite ends of the outer tubular blank~ Alternatively the means for supporting the blanks may include sealing means for sealingly engaging opposite ends of the inner tubular blank, the outer tubular blank or blanks being located radially by the inner tubular blank and axially by the mould. The mould preferably defines an annular recess for forming a socket on the oriented pipe. The annular recess may serve to locate an outer tubular blank, or a preformed ring axially within the mould whereby this outer tubular blank or preformed ring will form part of the socket.

109543~3 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 tub-ular blanks;
Figure 2 illustrates a modification of Figure l;
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 l;
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 82C is located inside a split female mould ha*ing 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 tbrough 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 de-fines 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 with-in 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 ,' :' .' "

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blank 19 inside the larger tubular blank 10 which is then positioned between the split mould halves 11 and 12. The mould is then clamped 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 82C and 92C at which their ex-pansion will result in orientation 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 tubular blanks 10 and 19. Alternatively, 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 re-quired 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 ex-panded 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 ~0~543~

pressure acting within the bore of the tubular blank 19 will expand it out-wards into contact with the inner surface o~ 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. Be-cause the tubular blanks can have a relatively low wall thickness, they are much easier to extrude than a corresponding one-piece tubular blank, and ex-pansion ratios in excess of 2:1 are readily obtained; it is thought that ex-pansion 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 ra-dial 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 cold 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 rever-sion could occur. After the pipe has been cooled, the mould halves 11 and12 can be separated and the formed pipe removed.
~ The portions of the tubular blanks 10 and 19 expanded into the cy--~ lindrical 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 2~ of resilient material such as synthetic rubber. After the moulded pipe has~been cooled and removed from the mould halves 11 and 12, it would -- 8 _ 109S43~

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 Fig-ure 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 con-trary. 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 ad~acent 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 sur-face 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 il-lustrated in Fi~ure 2 due to difficulty in maintaining end sealing, This _ g _ `

~0~5438 was caused by the internal axial stress present in the extruded tubular blanks and the stress relief which occurred during heating in the mould ll, 12. The resultant axial contraction of the outer blank lO exposed end portions of the inner blank 19 giving rise to preferential expansion of the exposed portions and consequential difficulties in obtaining a uniform prod-uct. 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 trap-ping 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 an-nular interior space 15 before the blanks are expanded.
As shown in Figure 4, a pipe 27 can be formed from say three tub-ular 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 i 27, and the outer lamina 28 serving to protect the foam lamina 29 and incor-porating say a white pigment to minimise the absorption of solar energy.
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 con-trary. It will be noted that the inner tubular blank l9 is supported by the seals 13 and 14 and that an outer tubular blank 31 is considerably shorter than the equivalent tubular blank lO illustrated in Figures 1 or 2. The out-er 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 blank l9 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 l9 109543t3 so that its axial position can be accurately determined before the mould halves 11 and 12 are shut. 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 casir.g 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 es-sentially thinner than the contiguous cylindrical wall 20 as it bas been ex-panded 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 tbe socket and, more importantly, increasing its rigidity. In practice the difference between the diameters 20 of the tubular portion 20 and the socket 22 is such that the reduced thick-ness of the lamina 33 does not substantially decrease tbe 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 tbe sealing ring 2~ is positively located to prevent leakage between the pipe 32 and a second laminated pipe 35, which is similar to the laminated pipe 32, and has its cylindrical end 36 se~lingly engaged by the sealing ring 24.

~095438 The axial length and axial position of the lamina 34 can be con-trolled 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 de-sired postion. 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 con-trary. It will be noted that the outer tubular blank 31 shown in Figure 5 has been replaced by an outer tubu~ar blank 38 which is of greater diameter and isaxially located within the annular recess 18. In this manner, the mould halves 11 and 12 positively locate theouter 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 circulation 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 per-; mit 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 ei~ther by applying pressure to the interior of the inner tubular .

'1095438 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 blank 19 is expanded. Alternatively, the outer tubular blank 38 may be re-placed by a ring of convenient shape which is positioned in the annular re-cess 18 and into whichthetubular blank 19 is expanded. Such a ring may be made of any convenient material, for instance a plastics material 19, or other material which will be keyed mechanically to the expanded tubular blank 19. In the latter event the ring may be formed of metal of a shape similar to the lamina 3~ of Figure 6 so that the expansion of the tubular blank 19 will key the pipe 32 axially and radially to the ring.
It will be appreciated that the present invention teaches the for-mation 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 appro-priate temperature and expanded in a convenient sequence.
It will be appreciated that sockets of a different configurationto 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 an-other or are interconnected by a short length of pipe. Although the descrip-tion has related to the production of a pipe having a straight central axis, ~095438 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 ar-rangement, the mould is formed as a tube with an end cap, so that a split is produced at the socket groove.

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Claims (21)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of forming a pipe of orientable thermoplastic polymeric material including placing an outer tubular blank and at least one inner tubular blank, said blanks being spaced apart and said blanks being coaxial and of circular transverse cross-section, into a female mould having internal dimensions corresponding to the external dimensions of the finished pipe, heating the blanks to a temperature at which their deformation will induce orientation of the polymer molecules, applying internal pressure to the blanks to expand said blanks radially outwards against the mould whilst orienting the polymer molecules in what had been both of the blanks, subsequently cooling the moulded pipe within the mould to a rigid condition and then removing the applied pressure.

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

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

4. A method, as in Claim 1, including heating the blanks to a temperature at which deformation will induce orientation of the polymer molecules, applying equal pressures simultaneously to the inner blank and to the space between the inner and outer blanks so as to maintain the position of the inner blank while shaping the outer blank against the mould, then releasing pressure between the blanks thereby causing the pressure applied to the inner blank to expand it and shape it against the shaped outer blank.

5. A method, as in Claim 1, including heating the blanks by means of a fluid at a suitable temperature and circulating the heating fluid around the outer blank whereby the inner blank is heated by heat transfer through the wall of the outer blank.

6. A method, as in Claim 5, including arranging a heat transfer fluid between the blanks to enhance the heat transfer between the blanks.

7. A method, as in Claim 1, including heating the blanks by means of a fluid at a suitable temperature, and circulating the heating fluid through the inner blank whereby the blank is heated by heat transfer through the wall of the inner blank.

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

9. A method, as in Claim 5 or 7, including circulating the heating fluid between the blanks.

10. A method, as in Claim 1, including 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.

11. A method, according to Claim 1, including expanding a portion of the blanks into an annular recess in the female mould to define a socket at the end of the finished pipe.

12. A method, as in Claim 11, including forming one of the blanks to a length similar to that of the socket, and 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.

13. A method, as in Claim 12, including arranging the socket blank around the other blank or blanks, and applying the internal pressure first to the socket blank.

14. A method, as in Claim 12, including locating the socket blank axially within a portion of the annular recess prior to its expansion.

15. A method, as in Claim 14, including heating the socket blank by circulating the heating fluid between the socket blank and the annular recess.

16. A method, as in Claim 1, including locating a preformed ring in an annular recess in the female mould, and expanding a portion of at least one of the blanks into the preformed ring.

17. A pipe of oriented thermoplastic polymeric material formed from at least two concentric laminations including an annular socket of greater diameter than the remainder of the pipe, and in which at least a part of the socket is reinforced by one of the laminations.

18. A pipe as in Claim 17 in which some of the laminations are formed from different materials.

19. A pipe as in Claim 17 in which one lamination is formed from oriented material to provide enhanced hoop strength for the pipe, and another lamination is formed from a material selected to give at least one property from the group of properties comprising thermal insulation to minimise heat transfer through the pipe wall, resistance to chemical attack, and reflective outer surface to minimise absorption of solar energy.

20. A pipe as in Claim 17 in which a flange connects the socket to the remainder of the pipe, and at least a portion of the flanged is reinforced by one of the laminations.

21. A pipe as in Claim 17 in which a preformed ring comprising at least part of an outer wall of the annular comprises the outer lamina.