Note: Descriptions are shown in the official language in which they were submitted.
1~95S51
- The invention relates to a pipe of oriented thermoplastic
polymeric material and also to a method of manufacturing such a pipe.
In our United Kingdom Patent No. 1 1~32 539 we have taught a method
of, and apparatus for, forming a pipe from orientable thermoplastic poly-
meric material by radially expanding a tubular blark into a mould at a tem-
perature at which expansion of the pipe will causeorientationofthe polymer
molecules. In this manner the finished pipe has an oriented structure cap-
able of withstanding a greater hoop stress for a given wall thickness than
- a pipe made of the same material which has not been oriented, and has an
enlarged oriented socket for carrying a sealing ring. In this manner a pipe
was produced with an integral socket having a specified bursting strength
with a smaller wall thickness than was previously possible, thereby reduc-
ing the volume of plastics used for a given pipe diameter and strength and
,:~
minimising 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. 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 conse-
' quently more flexible.
In practice we have found that, whilst -the reduced rigidity of
` the socket is not critical, large diameter pipes subjected 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
pipe of orientable thermoplastic polymeric material having an integral
socket of enlarged internal diameter includes a tubular blank having a por-
tion Of Breater wall thickness in a female mould with the por-tion of greater
-; wall thickness positioned within a portion of the female mould defining the
; - 2 -
'-' '`
:
` ` , ; ' -
: . ~ :
` l`~)9~S~
integral socket, heating tbe tubular blank to a temperature at which defor-
mation will induceorientationofthe polymer molecules, applying internal
pressure to the tubular blank to expand it radially outwards against the
female mould whilst orienting the polymer molecules, the portion of greater
wall thickness being expanded into the portion of thefemale mould defining
the socket, cooling the moulded pipe to a rigid condition, and then remov-
ing the applied internal pressure. The tubular blank is preferably beated
by means of a fluid, such as water, at a suitable temperature. The internal
pressure is preferably applied by compressed gas or liquid under pressure.
The method may also include forming the tubular blank with the portion of
- greater wall thickness by supporting a tube of constant -thickness within a
mould defining the portion of greater wall thickness, heating the tube to a
moulding temperature, compressing the tube axially within the mould to form
the portion of greater thickness, cooling the moulded blank and removing it
from the mould.
; According to another aspect of -the invention a pipe of oriented
thermoplastic polymeric material has an integral socket of wbich the cross-
section of area is greater than the cross-sectional area taken through an-
other portion of the pipe. Tbe thickness of the socket is preferably equal
to, or greater than, the wall thickness of tbe 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 con-taining one form of tub-
ular blank;
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 in accordance with our United Kingdom Patent No. 1 432 539 and
Figures 4 and 5 are scrap sections similar to
3 -
.
.-; - : . . :
., . , : :...... : :
~9sss ~
Figure 3 but illustrating sockets produced at the end of a pipe by
the mould of Figure 1 or Figure 2.
With reference to Figure 1, a tubular blank 10 of an unplasticisea
PVC having a vicat softening point of 82 C is located inside a split female
mould having an upper half 11 and a lower half 12. The blank 10 is sealed
into the mould by means of external seals 13 and 14 so than an annular cham-
:. ber 15 is defined between the outer surface of the blank 10 and the inwardly
facing surfaces of the two mould halves 11 and 12. The annular chamber 15
can be filled with hot water through an inlet pipe 16 and drained by an out-
let pipe ]7. The inwardly facing surfaces of the two mould halves 11 and 12
, are generally cylindrical as shown but define at one end an annular recess
18 for forming a socket a-t the end of the pipe. The two mould halves 11 and
12 would be clarnped sealingly togeth.er in a convenient manner and are
arranged and operated generally as taught in our United Kingdom Patent No.
.i 1 432 539.
However, it will be noted that the external seal 14 has a greater
internal diameter than the external seal 13 and engages a portion 19 of the
blank 10 which is generally of greater wall thickness. After the mould
halves 11 and 12 have been clamped together so that the seals 13 and 14 en-
20 gage the blank 10, hot water at a temperature of 92 C is passed through the
inlet pipe 16 into the annular 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 the blank 10 to a temperature between 82C and 92 C
at which its expansion will result in orientation ofthe polymer molecules.
Hot water may also be recirculated through the bore of the blank 10. Once
~ the blank 10 has achieved the required temperature, the water in the annular
chamber 15 is completely drained away, and hot water is fed under pressure
- into the bore of the blank 10 thereby expanding the thicker portion 19
.
:
.
,
:: '
)9~55~L
radially outwards in-to the anmllar recess 18, and expanding the portion of
normal thickness into the cylindrical portion of the mould. In this manner
the material of the tubular blank 10 is oriented with a corresponding in-
crease in the hoop strength of -the finished pipe. The water pressure ap-
plied to the bore of the blank 10 may conveniently be applied to either, or
both, ends of the blank through appropriate internal or external seals.
After the blank 10 has been fully expanded into themould 11 and 12~ the re-
sultant pipe is cooled in any convenient manner, for instance by cooling the
mould halves 11 and 12 with cold water, or by passing cold water through the
bore of 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 are separated and
the formed pipe removed. The formed pipe is then trimmed by removing the
surplus material at each end.
~s shown in Figure 3, two finished pipes 20 and 21 are sealed to-
.
; gether by a sealing ring 22 engaged within a recess 23 defined by an inte-
gral socket 24 which is of greater diameter than the main cylindrical por-
tions of the pipes 20 and 21. The integral socket 24 shown in Figure 3 has
been formed in accordance with our United Kingdom Patent No. 1 432 539 from
':,
a cylindrical blank of constant wall thickness and, as a result, it will be
noted that the wall of the integral socket 24 is consequently thinner than
; the pipe wall. This is because the cross-sectional areas of the integral
socket and the pipe wall are equal, and the socket 24 has a greater diameter ~.
than the pipe. Althoughorientationincreases the burst strength of the pipe
roughly pro rata with -the stretch ratio, its effect on stiffness is relat-
ively small. Thus, although -the socket 24 will probably have a higher hoop
tensile strength than the cylindrical section of the pipe by virtue of its
higher stretch ratio, its flexibility will be relatively greater and could
' . .
: , . . ~:
.:
. ; ' I .~!
~ '
1~3g~5~
lead to displacement of the seaLing ring 22 and a consequent failure of the
joint under pressure. However, by using the thicker wall portion 19 as de-
scribed with reference to Figure 1, -the wall thickness of the integral
socket 24 can be increased as shown in Figures 4 and 5 in which the sarne
reference numerals have been used to iden-tify equivaLent components. In
Figure 4 it will be noted tha-t the wall thickness of the socket 24 is equal
to the wall thickness of the pipe, and in Figure 5 it will be noted that the
wall thickness of the socket 24 is greater than -the wall thickness of -the
pipe. In fact, the wall thickness of the finished socket 24 can be closely
controlled by appropriately selecting the wall thickness of the thicker por-
tion 19 of the tubular blank 10. The axial length and position of the
thicker portion 19 can be altered from that shown in Figure 1 to strengthen
any specific portion of the finished pipe which it is desired to strengthen.
It will be noted from Figure 1 that the thicker portion 19 has
been formed by locally increasing the outer diameter of the tubular blank
10. Figure 2 is generally similar to Figure 1 and the same reference num-
erals have been used to identify equivalent components. However, it will be
noted that the seals 13 and 14 have the same external diameter as the outer
diameter of the tubular blank 10 is constant, and the thicker portion 19 is
provided by locally decreasing the internal diameter. Whi:Lst the blank 10
shown in Figure 2 could be rnanufactured as taught in United Kingdom Patent
No. 997 551, it couLd alternatively be manufactured by heating a tubular
blank of constant wall thickness in a mould which defines the internal and
external diameters of the thickened end portion 19, and by compressing the
tubular blank 10 axially within that mouLd to define the thickened end por-
tion 19 and by cooling and removing the blank from that mould. ~he tubuLar
blank 10 illustrated in Figure 1 could conveniently be made in the same
manner.
- 6 -
.
~(;1 9SS51
United Kingdom Patent No. 997 552 teaches how a tubular blank with
a thickened end portion can be formed into a non-oriented tube by expanding
a core within the thickened portion to define an enlarged coupling socket.
Although it has consequently been known for some time that a streng-thened
coupling socket can be formed at the end of a tube by expanding a tubular
blank having a portion of greater wall thickness, it has not previously been
proposed to use such a blank in the production of a pipe of oriented thermo-
.~ plastic polymeric material manufactured, for instance, as taught in our
United Kingdom Patent No. 1 432 539. The reason why we have not considered
this possibility previously is that it was felt that the application of in-
ternal pressure to a tubular blank of non-constant wall thickness would cause
the thinner portion of the tube to expand in preference to the thicker por-
tion and that the operation would either be comp~etely uncontrollable or it
would not be possible to obtain a product which reflected thedimensional
differences present in the blank. To the contrary, we have found that the
process in our United Kingdom Paten-t No. 1 432 539 will expand, in a satis-
factory controllable fashion, a tubular blank having a portion of greater
! wall thickness, and that the result is an orien-ted pipe having an integral
socket of greater rigidity than has hitherto been possible.
The mould used to produce the blank 10 of Figure ] does not have
to be split in the rnanner illustrated in the drawing. An alternative mould
may be in the form of a tube with an end cap, giving a split at the socket
- groove.
.
: . .
., .
,~. ' : :
~' .
