Note: Descriptions are shown in the official language in which they were submitted.
CA 02185701 2005-O1-05
Method and device for manufacturing biaxially oriented
tubing from thermoplastic material.
The present invention relates to a method for
manufacturing biaxially oriented tubing from thermoplastic
material, comprising forcing the tube over a mandrel at an
orientation'temperature of the plastic material, which
mandrel comprises an expansion part which produces
expansion in the circumferential direction of the tube,
while downstream of the mandrel an axial pulling force is
exerted on the tube, and the biaxially oriented tube is
cooled down, and downstream of the mandrel the tube is
subjected to calibration of its external dimensions. In the
context of the present invention the term tube also relates
to hose-type products.
The object of biaxially orienting the plastic
material of a tube, also known as biaxia~ly drawing a tube,
is to improve the properties of the tube through
orientation of the molecules of the plastic material of the
tube in two mutually perpendicular directions. In the case
of a certain known method the biaxial orientation is
produced by forcing the tube over a mandrel, with the
result that the tube is deformed. Upstream of the mandrel
the tube is brought to a suitable orientation temperature.
In practice, the temperature distribution inside the tube
wall is within a suitable range for biaxial orientation of
the molecules of the plastic material of the tube.
In the case of the known method the tube is forced
over the mandrel through an axial pulling force being
exerted on the tube downstream of the mandrel. The mandrel
comprises an expansion part which produces the increase in
the dimensions in the circumferential direction of the
tube. The fact that the tube passes over this expansion
part essentially determines the orientation of the plastic
material in the circumferential direction of the tube. The
axial pulling force essentially determines the orientation
in the axial direction. The biaxial orientation obtained is
fixed (frozen) by cooling~down the tube.
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A method of the type mentioned in the preamble is
known from, for example, Wo 93/19924. This publication
describes how a tube with a cylindrical tube wall is
manufactured by means of an extruder. Viewed in the
direction of movement of the tube, a so~.id, undeformable
mandrel with a conical expansion part i~> situated down-
stream of said extruder. The tube is forced over the
mandrel at an orientation temperature which is suitable for
the plastic material concerned, through an axial pulling
force being exerted on the tube downstream of the mandrel.
At the level of a run-off part of the mandrel, connected to
the expansion part of the mandrel, the tube is cooled
externally in order to fix the orientation obtained.
In order to guarantee th<~t the ultimately manu-
factured biaxially oriented tube has the desired external
dimensions - in this case external diamsater - with suffi-
cient accuracy, this publication proposes that a vacuum
calibration sleeve should be placed downstream of the
mandrel, between the mandrel and the pu7_ling device, The
tube is pulled through this calibration sleeve, in the
course of which the tube is sucked against the inside wall
of the calibration sleeve by means of a vacuum created by a
vacuum source, The result of this is that the tube is
stretched here in the circumferential direction until it
reaches an external diameter determined by the inside wall
of the calibration sleeve. In order to make stretching of
the tube possible, the tube has to be heated in the case of
this known method. The heating is carried out by heating
the calibration sleeve.
The method of calibration of the external dimen-
sions of the biaxially oriented tube proposed in the case
of this known method has proved disadvantageous. In
particular, it has been found that the biaxial orientation
of the plastic material of the tube produced during the
passage over the mandrel is impaired in an undesirable way,
both through increasing the external dimensions of the tube
during calibration and through heating applied to the tube
during the calibration.
The object of the present invention is to eliminate
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the abovementioned disadvantage and also to provide a
method which comprises accurate regulation for the
external dimensions of the tube.
This object is achieved by a method for
manufacturing biaxially oriented tubing from
thermoplastic material, comprising forcing the tube over
a mandrel at an orientation temperature of the plastic
material, which mandrel comprises an expansion part which
produces expansion in the circumferential direction of
the tube, while downstream of the mandrel an axial
pulling force is exerted on the tube, and the biaxially
oriented tube is cooled down, and downstream of the
mandrel the tube is subjected to calibration of the
external dimensions of the tube, characterized in that at
a difference downstream of the mandrel the tube is drawn
through a calibration opening bounded by calibration
means, the calibration opening being such that it
produces a reduction in the external dimensions of the
tube. The calibration means can be in the form of, for
example, a solid draw plate with a calibration opening
formed therein or a number of rollers which together
bound the calibration opening.
In this respect it is important to recognize that
the tube will shrink downstream of the mandrel through
cooling, in particular produced by cooling means set up
at that point. This shrinkage occurs both in the case of
the method according to the present invention and in the
case of the methods already known. In order to produce
the effective reduction in the external dimensions of the
tube envisaged according to the present invention, the
reduction must therefore lead to smaller external
dimensions than the external dimensions which the tube
would acquire if allowance is made for the shrinkage
which occurs.
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Unlike the above-described known method, the
calibration means produce a reduction in the external
dimensions of the tube. Since a reduction in the
external dimensions now occurs, it is not necessary for
the tube and/or the calibration means to be heated, which
is advantageous for achieving the envisaged biaxial
orientation.
In the case of the method according to the invention
the cross-section of the tube undergoes a progressive
14 reduction between the mandrel and the calibration
opening. The way in which said reduction occurs here, in
particular the speed at which it occurs, can
advantageously be used for controlling the biaxial
drawing
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process, as will emerge from the sub-claims and the
corresponding description.
The tube also encounters a resistance at the moment
when it is passing through the calibration means. This
resistance can also advantageously be used for controlling
the biaxial drawing process.
It is pointed out that European Patent Application
EP 0157601 discloses a method of the type mentioned in the
preamble in which, downstream of the expansion part of the
mandrel, the tube to be oriented is drawn through a draw
plate lying around a cylindrical run-off part of the
mandrel. It is clear that this draw plate does not serve as
a calibration means for the external dimensions of the tube
after the tube has left the mandrel, as in the case of the
present invention. On the contrary, EP 0157601 discloses an
interaction between the draw plate and the part of the
mandrel lying inside it for producing the biaxial
orientation of the plastic material of the tube.
In the case of the method according to the present
invention it is a great advantage if the distance between
the mandrel and the calibration opening is regulated. For
this purpose, it is, of course, necessary for the
calibration means to be movable relative to the mandrel,
which is simple to carry out.
The distance between the mandrel and the calibra-
tion opening is preferably regulated depending on charac-
teristics measured downstream of the calibration opening,
in particular the external dimensions of the biaxially
oriented tube.
The distance between the mandrel and the calibra-
tion opening is advantageously increased if the measured
external dimensions of the biaxially oriented tube are
smaller than the desired external dimensions, and the
distance between the mandrel and the calibration opening is
reduced if the measured external dimensions of the
biaxially oriented tube are greater than the desired
external dimensions.
In an advantageous embodiment of the method
according to the invention the calibration means are
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cooled. The tube is also preferably cooled further when it
is downstream of the calibration means. The influence of
the shrinkage of the tube caused by this cooling on the
external dimensions of the tube can be determined (for
example, experimentally) and used in determining or adjust-
ing the dimensions of the calibration opening, which are
necessary to obtain the desired external dimensions of the
tube.
In a variant of the method according to the present
invention, downstream of the mandrel the tube is drawn
through several calibration openings placed one after the
other and bounded by calibration means, each calibration
opening being such that it produces a further reduction in
the external dimensions of the tube. This means that the
reduction per passage through a calibration opening is
limited, which in a number of cases is advantageous as
regards the influence of said reduction on the biaxial
orientation achieved in the case of the mandrel.
The invention will be explained in greater detail
below with reference to Figure 1,
in which in a top view the part of the exemplary embodiment
of a device according to the invention for manufacturing
biaxially oriented tubing which is relevant for explaining
the invention is shown.
Figure 1 is based on an application in which a
tube with a smooth cylindrical wall is manufactured from
thermoplastic material. It will be clear that the inventive
idea and solutions described here can also be used for the
manufacture of tubular sections of a different cross-
section, if necessary by adapting the embodiment of the
solutions described herein.
Figure 1 shows a part of a tube 1 (in longi-
tudinal section) made of thermoplastic material, which tube
is manufactured in a continuous process by means of an
extruder (not shown). Downstream of the extruder the
plastic material of the tube 1 is brought by means of tem-
perature-regulating means .(not shown) to a temperature
suitable for biaxial orientation, for example cooling by
air or water.
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The plastic material of the tempered tube 1 is
biaxially oriented (drawn) by subsequently forcing the tube
1 over a metal mandrel 3 fixed by a tension member 2 to the
extruder. The mandrel 3 has a cylindrical run-on part 4, a
conical expansion part 5 and a slightly tapered run-off
part 6.
For forcing the tube 1 over the mandrel 3 a pulling
device (not shown) is present downstream of the mandrel 3,
for exerting an axial pulling force on the tube 1. A
pushing device may also be placed upstream of the mandrel
3, for exerting a pushing force on the tube 1.
In order to fix the biaxial orientation of the
molecules of the plastic material of the tube 1 during the
passage over the mandrel 3, the tube is subjected to
external cooling downstream of the expansion part 5 of the
mandrel 3. For this purpose, a diagrammatically indicated
cooling device 8 is set up at this point, for example with
spray nozzles for cooling water.
At a distance downstream from the mandrel 3, there
is a calibration and cooling device 10 according to the
invention, which will be explained below. The calibration
and cooling device 10 comprises a draw plate 11, in the
form of a metal disc with a central calibration opening 12.
The draw plate 11 is slidably mounted on guide bars 13 of
the fixed frame of the calibration device 10, in such a way
that the distance between the draw plate 11 and the mandrel
3 can be set within a suitable range. The calibration
device 10 comprises a diagrammatically indicated
displacement unit 14 for moving the draw plate 11.
Arms 15 with cooling medium spray nozzles 16 are
fixed to the draw plate 11, for cooling the biaxially
oriented tube 1 during and after its passage through the
draw plate 11. The cooling medium, for example water, is
supplied through a pipe 17 to the spray nozzles 16. The
cooling medium is collected in a tank 18 placed around the
calibration and cooling device 10.
The diameter of the calibration opening 12 of the
draw plate 11 is selected in such a way that the external
diameter of the tube 1 is reduced when it is passing
~,
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WO 95I256Z8 PCT/NL95100099
through the draw plate 11. The reduction which the draw
plate 11 produces in the external diameter relative to the
external diameter of the tube 1 at the moment of leaving
the mandrel 3 is greater than the reduction in the external
diameter of the tube 1 resulting from the shrinkage through
cooling of the tube in the path between the mandrel 3 and
the calibration opening 12. In other words, the draw plate
il exerts an effective force on the tube 1, thereby
reducing the external diameter of the tube 1. By making an
allowance for further shrinkage of the tube 1 after it
leaves the calibration opening, the diameter of said
calibration opening 12 can be selected in such a way that
the ultimately manufactured tube 1 acquires the envisaged
external diameter With great accuracy.
The present invention provides a solution to
effectively controlling the external diameter of the
biaxial oriented tube 1. To this end, a diagrammatically
shown measuring device 20 is placed downstream of the
calibration device 10. This measuring device 20 measures
the external dimensions, in this case the external
diameter, of the tube 1. The measuring device 20 transmits
a signal representing the external dimensions to a control
unit 21, which compares this signal with a signal
representing the desired external dimensions of the tube 1.
A control signal based on the difference between these two
signals is supplied by the control unit 21 to the
displacement unit 14 of the calibration device 10. This
produces the movement of the draw plate 11 relative to the
mandrel 3. If it is found by the control unit 21 that the
external diameter of the tube is smaller than the desired
external diameter, the control unit 21 transmits such a
signal to the displacement unit 14 that the distance
between the mandrel 3 and the draw plate 11 increases.
However, if the external diameter of the tube 1 is greater
than the desired external diameter, the draw plate 11 is
moved towards the mandrel 3.
The basic principle of this method can be explained
by the speed at which the cross-section of the tube is
reduced. This speed depends on the distance between mandrel
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3 and draw plate 11. If the speed of reduction of the
cross-section is relatively great, the ultimate diameter
reduction is found to be greater than that at a lower speed
(a great distance between the mandrel and the draw plate).
The resistance formed by the draw plate 11 to the
tube 1 passing through it can also be used advantageously
for the biaxial orientation to be achieved. Although this
orientation occurs essentially when the tube 1 is passing
over the mandrel 3, it is found that the axial tension in
the tube 1 in the path after it has passed through the draw
plate 11 has an influence on the ultimately manufactured
tube 1. In particular, the cooling of the tube can be regu-
lated suitably by means of the cooling device 8 so that the
biaxially oriented tube 1 is cooled in the path between the
mandrel 3 and the draw plate 11. This then leads to an
increase in the resistance formed by the draw plate 11 to
the tube 1 passing through it. This change in the resis-
tance, combined with the pulling force exerted on the tube
1, leads to a change in the axial tension in the tube 1.
This method of changing the axial tension in the tube 1 can
advantageously be used for obtaining the envisaged biaxial
orientation.
