Note: Descriptions are shown in the official language in which they were submitted.
' 13~5933
MANUFACTURE OF PLASTIC JACKETED STEEL PIPE
This invention relates to the manufacture of
plastic jacketed steel pipe.
In certain industries, plastic jacketed steel pipe
is used for various reasons. Such pipe is used in the
telephone industry for introducing pressurized air into
telephone cables so as to pressurize th~ cables for the
purpose of preventing ingress of moisture. Such pipes are
located between an inlet facility to a cable and an air
pressure source. Plastic jacketed steel pipe as used in the
telecommunications industry will be referred to as "air pipe'
throughout the specification.
In the manufacture of ai pipe, a metal tube is
formed from metal strip, e.g. aluminum, which is coated on
both sides with polyolefin. The metal strip is formed into a
metal tube by turning longitudinal edges of the strip
inwardly towards each other to bring the edges into
overlapping relationship and the jacket is extruded around
it. During extrusion, the heat involved in the extrusion
process is sufficient to soften the outer plastic coating on
the metal tube so as to fuse this coating with the jackat
thereby securing the jacket to the metal tube. The heat
obtained during extrusion is also sufficient to soften and
fuse together the opposing plastic coatings located between
the overlapped ends of the metal tube. However, the
accompanying extrusion pressure is disadvantageous in that
without the use of some internal support within the metal
tube, the metal tube will collapse under the extrusion
pressure. In order to prevent this from happening, internal
support is provided by a mandrel which is disposed within the
extruder head in the region at which the extrusion pressure
is applied. ~hile the use of the mandrel operates success-
fully in the manufacture of air pipe~ it has been found that
the constant rubbing of the mandrel against the inner plastic
coating on the tube during movement of the tube along the
~ passline, causes transfer of the plastics material onto the
- mandrel. As a result, the plastics material continually
builds up in thickness upon the mandrel. The mandrel thus
,
i 13~33
requires cleaning regularly to make it operational and this
results in a high percentage of "down time" of the apparatus.
A further disadvantage which is found with the use
of the mandrel relates to the bond strength between the
jacket and the outer plastic coating on the metal tube. A
peel test is applied between the jacket and the coating upon
samples taken from air pipe manu~actured by this conventional
process. It has been found that the minimum required load
for peeling the jacket from the plastic coating i8 consis-
tently below the minimum requirements set for the test.
The present invention seeks to provide a method ofproducing a plastic jacketed steel pipe in which the above
disadvantages are avoided.
Acccrding to one aspect of the present invention,
there is provided a method of making pipe having a metal
inner layer surrounded by an extruded outer layer comprising:
providing a metal strip having on one side a plastic coating
compatible with the material of the outer layer to be formed;
forming the metal strip into a tube by turning longitudinal
edges of the strip inwardly towards each other to bring said
edges into overlapping relationship, one with the other and
so that the plastic coatings oppose one another at the
overlapped edges; forming an assembly of the outer layer and
tube by extruding the outer layer onto the metal tube with
insufficient extrusion pressure to cause collapse of the tube
and with the inside of the tube unsupported; and subsequently
subjecting the outer layer to radial inward pressure which is
also insufficient to cause collapse of the tube, so as to
reduce the outside diameter of the outer layer and press it
against the outer plastic coating on the tube with the
material at the inner sur*ace of the outar layer having
sufficient residual extrusion heat stored therein to causa
softening o~ the outer plastic coating to fuse it to the
outer layer and to cause fusion together of the two opposing
coatings at the overlapped edges of the tube.
In other words, the method according to the
invention is based on the requirement that the adhesion
between the outer layer and the plastic coating on the tube
131~933
takes place after and downstream of the extrusion process
itself. Thus, if the extrusion process takes place at a
small or insignificant pressure upon the tube, then this
pressure will be insufficient to cause mutual overall
adhering contact between the outer layer and the outer
coating on thP tube so that the required degree of bonding
between the two materials cannot be obtained. However, with
the further steps of the process according to the invention,
the assembly of the outer layer and the tube is then moved
downstream from the extruder and is subjected to a radial
inward pressure which is not related to the pressure at the
extrusion step. However, the radial pressure may be applied
in such a way that the degree of pressure can be controllably
varied whereby collapse of the tube does not result while at
the same time a substantial bond is obtained between the
jacket and the outer plastic coating on the tube.
It is possible that the radial inward pressure on
the outer layer may be provided by fluid pressure, e.g. by
passing the assembly through a fluid pressurized chamber.
However, in a more practical application, the assembly is
drawn throuyh a polishing die. A conventional wire polishing
die may be used.
More particularly, the invention provides a method
of making a pipe having a metal inner layer surrounded by an
extruded outer layer comprising providing a metal strip
having on each side a plastic coating compatible with the
material of the outer layer to be formed; forming the metal
strip into a tube by turning longitudinal edges of the strip
inwardly towards each other to bring said edges into over-
lapping relationship, one with the other and so that theplastic coatings oppose one another at the overlapped edges,
forming an assembly of outer layer and tube by extruding the
outer layer onto the metal tube with insufficient extrusion
pressure to cause collapse of the tube and with the in~ide of
the tube unsupported; and then passing the assembly through a
polishing die without causing collapse of the tube while:-
l) ensuring there is no adherence between the outer surface
of the outer layer and the die; and 2) reducing the diameter
~31~93~
of the outer layer and pressing it against the outer plastic
coating on the tube with the material at the inner sùrfac~ of
the outer layer having suffi~ient residual extrusion heat
stored therein to cause so~tening of the outer plastic
coating and fuse it to the jacket and to cause fusion of the
two opposing coatings at the overlapped edges of the tube.
The amount of radial pressure applied by the
polishing die to the outer layer will depend upon the
reduction in diameter of the outer layer as it passes through
the die. This reduction may easily be controlled by adjust-
ing extrusion pressure to control the outside diameter of the
outer layer during extrusion. Thus, the invention provides a
process which is easily controllable by extrusion pressure to
pro~ide the required degree of inward pressure at the
polishing die. In contrast, in ths conventional method o~
forming pipe, if a mandrel is not used, the extrusion
pressure cannot be controlled to prevent tube collapse while
also effecting an intimate and sufficiently strong fusion
bond between outer layer and metal tube. In the case of the
present invention it is believed that a substantially small
reduction in diameter of the jacket when passing through the
polishing die is suf~icient to produce the required radial
inward pressure to effect adequate fusion bonding of the
jacket to the outer plastic coating on the tube. By way of
example, in an air pipe having an overall diameter of ~.745
inches as it issues from the polishing die, and with a satis-
factory fusion bond the diameter of the outer layer upstream
of the polishing die was of the order of 0.748 inches.
Hence, a reduction in diameter of 3 mil has been found
sufficient to produce the desirPd bonding results. Control
o the extrusion pressure may change the diameter of the
outer layer upstream of the polishing die by less than 1 mil
if required thereby effecting change in radial pressure to
obtain the desired bonding results.
In addition, it has been found that the method
according to the invention using a polishing die increases
the strength of the bond between the outer layer and the
13~33
outer coating of plastic upon the tube beyond that which i5
achievable with the conventional process.
Preferably, according to the invention, there is
provided a method of making pipe having a metal inner layer
surrounded by an extruded outer layer comprising: providing
a metal strip having on each side a plastic coating com-
patible with the material of the outer layer to be formed;
forming the metal strip into a tube by turning longitudinal
edges of the strip inwardly towards each other to bring said
edges into overlapping relationship, one with the other and
so that the plastic coatings oppose one another at the
overlapped edges; forming an assembly of tube and outer layer
by extruding the outer layer onto the tube with the inside
extruded diameter of the outer layer being larger than the
outside diameter of the coated metal tube; collapsing the
extruded outer layer so that it contacts the outer plastic
coating on the tube; passing the assembly of tube and outer
layer through a rooling fluid to cool and increase the
hardness of the outer sur~ace of the outer layer; and then
passing the assembly through a polishing die without causing
collapse of the tube while: 1) passing the cooling fluid
between the outer layer and the die to act as a lubricant;
and 2) reducing the diameter of khe outer layer and pressing
it against the outer plastic coating on the tube with the
material at the inner surface of the outer layer having
sufficient residual heat from extrusion stored therein to
cause softening of the outer plastic coating to fuse it to
the outer layer and to cause fusion of the two opposing
coatings at the overlapped edges of the tube.
In th~ preferred method, no extrusion pressure is
applied onto the tube so that collapse of the tube at the
extrusion stage o~ the process i6 impossible. After ex-
trusion of the outer layer and before entry into the polish-
ing die, the outer layer is collapsed onto the coated tube.
This is conveniently achieved by applying tension to the
outer layer as by the speed of the downstream r~eling
apparatus.
131 ~933
Also, the invention includes apparatus for making
pipe having a metal inner layer surrounded by an extruded
outer layer, the apparatus comprising in a direction
downstream along a passline for the pipe; means for forming
metal strip into a tube with overlapped edges extending
longitudinally of the tube; an extruder having an extruder
head having a core tube surrounding a passline for the metal
tube, the extxuder head having a die orifice for the outer
layer surrounding the core tube with a downstream end of the
core tube disposed in relation to the die orifice such that
the outer layer is extruded onto the metal tube with a larger
inside diameter than the outside diameter of the metal tube;
tension applying means ~or causing collapse of the outer
layer onto the outPr coating on the tube; and means for
subjecting the outer layer to radial inward pres~ure in-
: sufficient to cause collapse of the tube.
One embodiment of the invention will now be
described by way of example, with reference to the accompany-
ing drawings, in which:~
Figure 1 is an isometric view of a sectioned air
pipe made by the method of the embodiment;
Figure 2 is a side elevational diagrammatic view of
apparatus according to the embodiment;
Figure 3 is a cross-sectional view through a metal
tube taken along line III-III in Figure 2;
Figure 4 is a cross-sectional view through an
extruder head which is an enlargement of part of the ap-
paratus shown in Figure 2;
Figure 5 is an isometric view showing part of a
cooling trough on larger scale than and forming part of the
apparatus of Figure 2, and
Figure 6 is a cross-sectional view through a
polishing die located within the cooling trough in Figure 5.
As shown in Figure 1, in the e~bodiment, an air
: 35 pipe 10 comprises an aluminum tube 12 formed from strip and
having overlapped edges 14. Surrounding the tube 12 is an
extruded outer layer or jacket 16 of polyethylene.
'
1 3 ~ 3
To manufacture the air pipe 10, apparatus as shor"n
in Figure 2 is used. As shown by Figure 2, this apparatus
comprises, in a downstream direction along a passline, a
give-up 18 for aluminum tape, a tape ~orming device 20 an
extruder head 22 attached to an extruder not shown, a cooling
trough 24, a capstan 26 and an air pipe take-up 28. The
give-up 18, the tape forming device 20, capstan 26 and take-
up 28 arP all of conventional construction and will be
described no further.
The extruder head 22 is designed such that when the
jacket is exkruded onto the tube 12, the jacket has a larger
inside diameter than the outside diameter of the tube. To
provide for this, the extruder head 22 as shown in Figure 4,
has a core tube 30 extending along the passline ~or the tube
30, the core tube having a downstream end 32 which lies
substantially in alignment with the extrusion orifice 34 of
the extruder. This type of extruder head is of conventional
construction and is designed for extruding jackets onto cable
cores by the well known "tubing" technique. The extruder
head will be described no further.
The embodiment also includes a means for subjecting
the extruded jacket to a radial inward pressure which is
insufficient to collapse the tube. As shown in Figures 5 and
6, this radial inward pressure subjecting means comprises a
polishing die 36 which is disposed within the cooling trough
24 so as to be immersed within the cooling fluid 38 con~ained
within the trough. The cooling fluid 38 is normally water.
The polishing die 36 may be a conventional wire
polishing die. For the avoidance of confusion, a polishing
die is not the same structure as a drawing die~ A polishing
die has a smooth, slightly tapered inner surface and does not
have a diamond which is part of the structure o~ a drawing
die. The polishing die is movable laterally and is also
pivoted about two axes to enable the polishing die to follow
the change in position of the passline of the assembly of
tube and jacket moving along the trough. The reason for this
movement is to minimize any lateral forces applied by the
polishing die upon the jacket material which may not have
1315933
solidified when it passes through the polishing die and could
be sub~ected to deformation. In this regard, the polishing
die i9 mounted upon a free cantilever arm 40 of a bra~ket 42
which is secured to the base of the cooling trough 24. The
arm 40 is raised above the bottom of the trough so as to be
flexed vertically and permit vertical movement o~ the
polishing die dependent upon the vertical displacem~nt of the
jacket tube. In addition, the polishing die 36 is mounted
within a U-shaped holder 44 about a horizontal pivotal axis
46 and the holder 44 is also mounted to the arm 40 about a
vertical axis 48. Thus the polishing die 36 has pivotal
movement in two directions to enable the die to ~ollow slight
angular changes in the longitudinal axis of the jacketed tube
12 which, it is expected, will not have a true rectilinear
axial direction.
In use of the apparatus as shown in Figure 1, a
metal tape 50 is drawn from the give-up 18 by the take-up 28.
The metal tape has on each side a thin plastic coating (not
shown) which is compatible with the jacket to be formed. The
plastic coating is preferably ~ormed of a polyolefin material
which may be polyethylene. The coated metal strip is passed
through the tape forming device 20 in which it is formed into
the tube 12 by the side edges 52 of the tape being turned
inwards towards each other so that the edgas 14 are in
overlapping relationship as shown in Figure 3O This opera-
tion is conventional for forming tube in the manufacture of
telecommunications cables. The tube 12 is then drawn through
the core tube 30, as shown in Figure 4, and as it emerges
~rom the core tube 30, the molten plastics material for
forming the jacket is extruded around it through the die
orifice 34 as shown. As can be seen ~rom Figure 4, because
- of the presence of the core tube 30 at the extruder orifice
34, the jacket 16 is extruded onto the tube 12 with an inside
diameter which is greater than the outside diameter of the
tube. Thus, the tube 12 i5 not subjected to the extrusion
pressures present in the extruder head 22 and no forces are
involved which could cause collapse of the tube. It should
be noted that as the tube 12 is fed through the core tube 30
~31~33
and as the jacket is being extruded onto it, the inside of
the tube 12 is complet~ly unsupported, i.e. no internal
mandrel is present. Nevertheless, as already stated, no
collapse of the tube can result.
Although the extruded material upon leaving the
extruder head, has a normal extrusion temperatur~, e.g.
between 450 and 500F~ nevertheless, the jacket does not
form intimately to the outside of the plastic coated tube 12
because the jacket is not applied to the tube under extrusion
pressure. Hence, there is little or no fusion between the
jacket material and the plastics coating on the tube 12 after
the jacket is formed. This is indicated diagrammatically in
Figure 6, by a nominal gap 53 between sleeve 12 and jacket 16
as they pass together into the polishing die 36. The
necessary pressure is however provided by the polishing die
36.
As the assembly 54 of the jacket and metal tube
emerges from the extruder head 22, the jacket diameter is
reduced by the pulling force exerted by the take-up 28, i.e.
by tension applied to the jacket. This causes the jacket to
conform more closely to the outside shape of the plastic
coated tube 12. The assembly 54 is then passed into the
cooling trough 24 and the cooling fluid 38 commences to cool
the outer regions of the jacket material. The assembly ~4
then pAsses through the polishing die 36. As can be seen
from Figure 6, as the assembly 52 passes through the polish-
ing die, the jacket is subjected to radial inward pressure
which is also insufficient to cause collapse of the tube, but
nevertheless causes a slight reduction in the outside
diameter of the jacket to press it against the outer plastic
coating on the tube and results in fusion bonding to the
outer coating. It has been found that the use of a polishing
die in this respect provides insufficient pressure to cause
tube collapsa even though the inside of the tube remains
unsupported.
Various controls ara required throughout the
process to ensure that at the polishing die, the desired
result, i.e. bonding of the jacket to the plastic coating on
the tube 12, is achieved. One of these controls is concerned
with producing the desired relationship of incoming jacket
diameter to outgoing jacket diameter to provide sufficient
draw down and radial inward pressure to produce the bond.
This control is easily maintained hy a slight change in the
extrusion pressure within the Pxtruder head 22 so as to vary
the outsi~e diameter of the jacke~ as it emerges from the
extruder orifice 34. A slight change in extrusion pressure
may vary the outside diameter by one or more mil~ It has
also been found that a small reduction in diameter of the
jacket at the polishing die is sufficient for the bonding
purpose. For instance, where the outside diameter of the air
pipe 10 is required at 0.745 inches as it emerges from the
die, then an incoming diameter of 0.748 inches will be
sufficient to produce the desired inward pressure. This
incoming outside diameter may of course be varied as just
discu~sed by a slight change in extrusiQn pressure.
Further controls which are required concern the
temperature of the jacket material itself. These controls
are achieved by a combination o~ factors, e.g. the tempera-
; ture of the cooling fluid, the location of the polishing die
36 in the cooling trough 24 from the entrance to the trough,
and the throughput speed of the product itself. These
controls are provided to ensure that as the assembly p~sses
throuyh the polishing die, there is no adherence between theouter surface of the jacket and the die and also that at the
inner surface of the jacket there is still sufficient
residual extrusion heat stored therein to cause softening of
the outer plastic coating and fuse it to the jacket. This
heat should also cause fusion of the two opposing coatings at
the overlapped edges 14 of the tube 12.
Hence, as the assembly 54 proceeds through the
; cooling trough towards the polishing die, the surface of the
jacke~ material becomes slightly hardened under the reduction
in temperature and may ~e formed with a sXin. As the
assembly 54 proceeds through the polishing die, the cooling
fluid lies between the jacket and the die surface as an
extremely thin film so as to act as a lubricant. Thus the
131~933
11
hardening of the outside of the jacket and the presence of
the lubricating cooling fluid is sufficient to prevent
destruction or tearing away of the outer regions of the
jacket as it passes through the die.
While the invention as highlighted by the embodi-
ment produces a process in which the pressure to create
bonding between the jacket and the tube 12 is produced at a
different staye in the process from the extrusion step, a
surprising result is also achieved. This is that the degree
of bond achieved between the jacket and the plastic coating
on the outside of thP tube is much stronger than has pre-
viously been achievable under conventional processes.
Conventional strength tests have been used upon samples of
air pipe 10 made according to the process according to the
embodiment and it has heen found that it has been impossible
to remove the jacket material from the plastic coating on the
tuhe 12 during these tests. Thus, these tests indicate that
the product 16 has a bond which far exceeds the minimum
requirement for this product which renders the product far
superior to the conventional product made by the conventional
process.
It follows, th~refore, that not only do the
apparatus and method of the invention as described in the
embodiment avoid disadvantages in the use of a mandrel, but
they also provide a product which is far superior to that
which is otherwise producibleO
