Note: Descriptions are shown in the official language in which they were submitted.
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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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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
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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
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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.
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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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l~9S~3B
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
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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
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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
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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
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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
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'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,
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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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