Method for manufacturing biaxially oriented tubing from thermoplastic material

A tube is extruded from a thermoplastic material and is then forced over a mandrel at an orientation temperature of the material. The mandrel has an expanding section that expands the tube in its circumferential direction. A tube speed controller upstream of the expanding section of the mandrel exerts an axial force on the tube, and a puller downstream of the mandrel exerts an axial pulling force on the tube. Upstream of the tube speed controller, an outer layer of a wall of the tube is brought to a temperature below the orientation temperature. That outer layer is thick enough to withstand the force exerted by the tube speed controller.

FIELD OF THE INVENTION 
The present invention relates to a method for manufacturing biaxially 
oriented tubing from thermoplastic material according the the preamble of 
claim 1. 
DESCRIPTION OF RELATED ART 
In WO 93/19924 a method of making tubing is disclosed. The object of 
biaxial orientation of the plastic material of a tube, also known as 
biaxially stretching a tube, is to improve the properties of the tube 
through orientation of the molecules of the thermoplastic material in two 
mutually perpendicular directions; the axial direction and the hoop or 
circumferential direction. 
In order to effect the biaxial orientation it is desired that the tube is, 
uniformly over the wall thickness of the tube, at the orientation 
temperature suitable for the thermoplastic material concerned. This has 
been disclosed in DE 23 57 210 and EP 0 441 142 (Petzetakis). In practice, 
this orientation temperature is the temperature at which the plastic 
material becomes form-retaining when it cools down. For PVC 
(polyvinylchloride) the orientation temperature lies in a range just above 
the glass transition temperature of PVC. PE (polyethylene) and other 
polyolefins exhibit no transition temperature, but an "alpha phase", which 
indicates the transition from a crystalline through a partially 
crystalline to an amorphous structure. The orientation temperature of such 
a plastic material lies just above the temperature range appertaining to 
the "alpha phase". The biaxial orientation is fixed (frozen) by cooling 
the tube. 
To obtain the orientation temperature of the plastic material the tube 
which exits from the extruder at a high temperature is cooled. In 
practice, this cooling is achieved by passing the extruded tube through a 
cooling device placed downstream of the extruder, which device cools the 
tube externally and/or internally. 
According to WO 93/19924 the extruded tube is passed through a constant 
temperature area before reaching the expanding section of the mandrel to 
obtain the desired homogeneous temperature of the tube before it is 
biaxially oriented. WO 93/19924 also proposes to place a pushing device in 
this area which has belts which grip the outside of the tube and push the 
tube towards the mandrel. This pushing device therefore acts as a tube 
speed controlling means. It is noted that most extruders allow the 
extrusion speed to be set at a desired value, but this possibility does 
not allow a control of the biaxial orientation process. 
SUMMARY OF THE INVENTION 
In the context of the present invention, the tube speed controlling means 
are separate means placed between the extruder and the expanding section 
of the mandrel. 
The transmission of an axial force to a tube by the tube speed controlling 
means, without damaging the surface thereof upon which the tube speed 
controlling means engages (the inner and/or the outer surface of the 
hollow tube), will be based upon frictional forces between the tube speed 
controlling means and the tube. 
It has been found that the axial force which can be transmitted by the tube 
speed controlling means to the tube with the known method is limited since 
the thermoplastic material of the tube has its orientation temperature as 
the tube speed controlling means acts upon the tube, and the tube is 
rather soft at that temperature. In order to exert an axial force on the 
tube the tube speed controlling means must apply a large radial force on 
the tube and due to the softness of the tube these forces would damage the 
tube, in particular at the locations where the tube speed controlling 
means grips the tube. 
The object of the present invention is therefore to provide a method which 
allows a substantial axial force to be exerted by the tube speed 
controlling means on the extruded tube without the tube being damaged and 
without other detrimental consequences for the following biaxial 
orientation. It is a further object of the invention to provide a energy 
efficient method for the manufacturing of biaxially oriented tubing. 
Another object of the invention is to make it possible to manufacture 
biaxially oriented tubing in a continuous process in a precisely 
controllable manner. 
The present invention provides a method, which is characterized in that 
upstream of the tube speed controlling means the plastic material in an 
outer layer of the wall of the tube, upon which layer the tube speed 
controlling means acts, is brought to a temperature which is below the 
orientation temperature, this layer being so thick that it can withstand 
the force exerted by the tube speed controlling means. 
The present invention therefore proposes that the area of the tube on which 
the tube speed controlling means acts should have a cold, and consequently 
strong and hard outer layer, which is also referred to herein as a "skin". 
This the temperature and thickness of the skin should be such that it can 
withstand mechanical influences of the tube speed controlling means 
without being damaged. It has been found that the cold outer layer can be 
thin compared with the total thickness of the tube wall. The temperature 
of the outer layer required for achieving the necessary strength and 
hardness depends on the plastic material, but will in any case be clearly 
lower than the orientation temperature of the plastic material. For a 
plastic material like PVC, the glass transition temperature of which lies 
in the range between approximately 80 and 85.degree. C., it is found that 
cooling of the extruded tube to a temperature of approximately 70.degree. 
C. on the area of the tube upon which the tube speed controlling means 
acts is adequate for obtaining a sufficiently thick and strong outer 
layer. Other plastic materials exhibit no clear transition temperature for 
the strength properties of the material. In the case of PE and other 
polyolefins there is the abovementioned "alpha phase". In that case the 
outer layer needs to be cooled to just below the temperature range 
appertaining to the "alpha phase". 
The skin is preferably formed by cooling this outer layer of the extruded 
tube to a lower temperature than the part of the wall of the tube not 
comprised in the outer layer. A suitable value for the temperature of the 
skin (or temperature range) is in any case below the orientation 
temperature of the plastic material concerned. The part of the wall of the 
tube not comprised in the outer layer or skin is at a higher temperature 
than the outer layer, preferably approximately the desired orientation 
temperature. 
The hard skin will distribute a force applied locally on the tube over a 
larger surface area of the tube. Therefore it is now possible to exert 
forces on the tube which would otherwise damage the tube locally, e.g. 
apply a radial force which would otherwise lead to a depression or hole 
being formed in the tube. 
A highly energy efficient method for producing biaxially oriented tubing is 
obtained when the heat content of the material not comprised in the cold 
layer is maintained such that the entire wall of the tube can reach the 
orientation temperature before it reaches the mandrel. 
Preferably the tube speed controlling means acts upon the tube under 
deformation of the initial cross-section of the tube, which it has 
upstream from the tube speed controlling means. This measure according to 
the invention is based on the realization that it is allowed to deform the 
tube at this point since there is a certain period of time to restore the 
tube to its underformed shape before the tube reaches the mandrel. 
The radial surface pressure created by the deformation of the tube by the 
tube speed controlling means makes it possible to exert a great axial 
force on the tube by the tube speed controlling means. The strong and hard 
outer layer increases the resistance to deformation of the tube, with the 
result that when the tube is deformed by the tube speed controlling means 
the surface pressure thereby produced is greater than it would be without 
the cold outer layer. The outer layer also prevents undesirable damage to 
the tube. 
When a biaxially oriented cylindrical tube is being manufactured with the 
method of the present invention, which is the type of tube for which there 
will be the greatest demand in practice, the tube to be biaxially oriented 
comes out of the extruder in the form of a tube with a smooth cylindrical 
tube wall, which is then deformed through the tube speed controlling means 
acting upon the tube, for example to an oval shape by compresssing it 
radially. If the tube subsequently moves over a mandrel with an 
essentially round cross-section, the ultimately desired shape of tube is 
obtained. After it has passed over the mandrel, the diameter of the tube 
will decrease to the ultimately desired dimensions as the result of 
cooling (shrinkage) and the pulling force (constriction) exerted on the 
tube. For this reason the diameter of the mandrel will preferably be 
somewhat greater than the inner diameter of the tube to be manufactured. 
The speed of the extruded tube is preferably controlled by tube speed 
controlling means which act upon the tube over a length thereof, the 
engagement of the tube speed controlling means with the tube being 
achieved by several active elements of said tube speed controlling means, 
which clamp the tube between them. A lower limit for the surface area with 
which the tube speed controlling means act upon the tube is formed by the 
maximum admissible surface pressure between the tube and the tube speed 
controlling means. Said surface pressure may not be so great that it can 
lead to damage to the tube. 
The tube speed controlling means can comprise a plurality of driven endless 
tracks disposed around the circumference of the tube, each track engaging 
on a longitudinal strip of the tube. The parts of the tracks acting upon 
the tube need not fully enclose the tube in the circumferential direction, 
because the strong and hard outer layer distributes the radial and axial 
forces exerted by the tracks over a large surface area of the tube. This 
makes it possible to control the speed of the tube with devices which 
until now have been used in particular as pulling devices for tubular 
sections in the plastics industry. 
The present invention also proposes cooling the tube internally after it 
has passed over the mandrel, in particular if the wall of the biaxially 
oriented tube is so thick that external cooling alone of the tube would 
lead to an undesirably long cooling section and undesirably slow cooling. 
Further advantageous embodiments of the method according to the present 
invention are disclosed in the appended claims and the following 
description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1, 2, 3 and 4 are based on an application of the method according to 
the invention in which a tube made from thermoplastic material (such as 
PVC or PE) and having a smooth cylindrical wall is being manufactured. It 
will be clear that the inventive idea and solutions described here can 
also be used for manufacturing tubular sections with a different 
cross-section, if necessary by adapting the embodiment of the parts 
described herein. 
FIG. 1 shows an extruder 1 by means of which a hollow tube 2 made of 
thermoplastic material is manufactured in a continuous process. On leaving 
the extruder 1, the tube 2 has a round annular initial cross-section. 
The tube 2 leaving the extruder 1, is passed through an external 
calibration sleeve 3 and subsequently through a cooling device 4, in this 
example a water cooling device. 
The tube 2 is biaxially oriented by forcing the tube 2 at a suitable 
orientation temperature of the plastic material of the tube 2 over a 
mandrel 6 which is held in place by a tension member 5 passing through the 
hollow tube 2 and connected to the extruder 1. 
In practice, this orientation temperature is the temperature at which the 
plastic material becomes form-retaining when it cools down. For PVC the 
orientation temperature lies in a range just above the glass transition 
temperature of PVC. PE and other polyolefins exhibit no transition 
temperature, but an "alpha phase", which indicates the transition from a 
crystalline through a partially crystalline to an amorphous structure. The 
orientation temperature of such a plastic material lies just above the 
temperature range appertaining to the "alpha phase". 
The mandrel 6 has a cylindrical run-on section 7, an expanding section 8 
having the form of a truncated cone, and an essentially cylindrical 
run-off section 9 which is tapered slightly towards the downstream end 
thereof. 
For controlling the speed with which the tube 2 moves towards the mandrel 
6, a tube speed controlling device 12 is present at a distance upstream of 
the upstream end of the mandrel 6, viewed in the direction of movement of 
the tube 2, which device 12 acts upon the outside of the tube 2. 
The diagrammatically shown device 12 will be explained further below. 
A pulling device 20 is present downstream of the mandrel 6, for exerting an 
axial pulling force on the tube 2. Said pulling device 20 can be of a 
design which is generally known in the prior art. 
The forcing of the tube 2 over the mandrel 6 is not only effected by the 
pulling force exerted by the device 20 on the tube 2, but also by means of 
the device 12 which is in this case set to exert an axial pushing force 
towards the mandrel 6 on the tube 2. The object of the additional pushing 
force is to permit a reduction in the pulling force to be exerted by the 
device 20 on the tube 2. This is advantageous because the tearing strength 
of the tube 2--which is at the orientation temperature while passing over 
the mandrel 6, and is therefore viscous--constitutes a limitation for the 
pulling force to be exerted on the tube 2 by the device 20. A great degree 
of orientation in hoop direction, and consequently advantageous tube 
properties, can be achieved by this method. 
According to the present invention the outside of the tube 2 upstream of 
the device 12 is cooled by the cooling device 4 in such a way that the 
plastic material in an outer layer adjoining the outside of the tube 2 is 
brought to a temperature which is clearly lower than the orientation 
temperature of the plastic material. This ensures that the wall of the 
tube 2 acquires a cold and therefore relatively strong and hard outer 
layer of a suitable thickness, so that this outer layer can withstand 
mechanical influences, caused in particular by the device 12 acting upon 
the tube 2. In other words, the present invention proposes providing the 
tube 2 with a hard "skin", by cooling the outer layer to a temperature 
which is lower than the orientation temperature desired for biaxial 
orientation when the tube is passing over the mandrel 6. 
For a plastic material like PVC, the glass transition temperature of which 
lies in the range between approximately 80 and 85.degree. C., it is found 
that cooling to approximately 70.degree. C. on the outside of the tube is 
adequate for obtaining a sufficiently thick and strong outer layer. The 
temperature in the outer layer defined according to the present invention 
in the case of PVC therefore lies between 80.degree. C. on the inside of 
the outer layer and 70.degree. C. on the outside of the outer layer. In 
FIG. 2 the outer layer is indicated by the 80.degree. C. isotherm shown by 
a dashed line "l" in the wall of the tube 2. Other plastic materials show 
no clear transition temperature for the strength properties of the 
material. In the case of PE there is an "alpha phase", which indicates the 
transition from a crystalline through a partially crystalline to an 
amorphous structure. The outer layer in this case should be cooled to just 
below the temperature range appertaining to the "alpha phase". 
The hard skin produced according to the invention upstream of the device 12 
around the warmer and softer wall material of the tube 2 prevents any risk 
of damage to the tube 2 by the device 12. Other advantages of the skin 
will be described further below. 
Because the device 12 is spaced apart from the mandrel 6 a period of time 
is available to effect the reheating of the outer layer to the desired 
orientation temperature. During the time that the tube 2 moves from the 
device 12 to the mandrel 6 the plastic material enclosed by the outer 
layer, which material is at a higher temperature than the outer layer, 
gradually releases part of its heat to the colder outer layer. The result 
of this is that the outer layer defined according to the invention will 
gradually become thinner if the outside of the tube 2 is no longer cooled. 
This heating can ultimately lead to the disappearance of the outer layer 
defined according to the present invention. The temperature of the inner 
part of the tube wall is then preferably regulated in such a way, for 
example by internal cooling/heating of the tube 2, that at the moment when 
the tube leaves the device 12 said temperature is higher than the 
orientation temperature. As the heat of this inner part of the tube is 
partly transfered to the outer layer the inner part is cooled to the 
desired orientation temperature. This heat transfer from inside to outside 
means that the tube, including the outer layer, is at the orientation 
temperature desired for biaxial orientation when it is passed over the 
mandrel 6. 
It can be seen in FIG. 2 that the thinning of the outer layer (line "l") 
begins immediately after leaving the cooling device 4. In order to ensure 
that a sufficiently strong outer layer remains present at least while the 
device 12 is being passed, the device 12 can be provided with a cooling 
system. 
It can also be seen from the illustration in FIG. 2 that the cold outer 
layer of the tube 2 decreases further in thickness after leaving the 
device 12, through heating from the inside of the tube 2. In order to 
ensure that on reaching the expanding section 8 of the mandrel 6 the tube 
2 is at the desired orientation temperature as uniformly as possibly, 
provision is made for a heating device 40 placed near the mandrel 6, which 
heating device will be explained below. 
After passing over the expanding section 8 of the mandrel 6, the tube 2 is 
cooled on the outside by a diagrammatically indicated cooling device 25. 
In FIG. 2 this is shown by dashed line "k", which shows the 80.degree. C. 
isotherm in the wall of the tube 2. The tube 2 is also cooled internally 
downstream of the mandrel 6 by supplying cooling liquid through a supply 
channel 26, extending partly through the tension member 5, to a space 27 
which is bounded by the tube 2, the mandrel 6 and a sealing device 28. The 
sealing device 28 comprises a flexible disc which rests in a sealing 
manner against the inside of the tube 2. The influence of the internal 
cooling on the temperature of the tube 2 is indicated by line "n", which 
shows the 80.degree. C. isotherm in the wall of the tube 2. 
The cooling liquid supplied, which is heated in the space 27, leaves the 
space 27 through passage openings 29 (see FIG. 2) disposed in the mandrel 
6, and subsequently passes into a space 30 upstream of the mandrel 6. The 
space 30 is bounded by the tube 2, the mandrel 6 and a sealing device 31. 
The sealing device 31 can also lie closer to the extruder 1, depending on 
the situation. The sealing device 31 in this example, like the sealing 
device 28, comprises one or more flexible discs resting in a sealing 
manner against the inside of the tube. The liquid leaves the space 30 
through a discharge channel (not shown) in the tension member 5. The fact 
that the liquid flows through the inside of the mandrel 6 produces cooling 
of the mandrel 6. It is clear that individually adjustable liquid flows 
can also be supplied to the spaces 27 and 30. It is also possible to cool 
the mandrel 6 by means of a separate cooling liquid flow. 
The device 12 used in this example comprises a frame, bearing two chains 
14, 15 of rubber blocks 16, 17 respectively, which can be moved along a 
corresponding closed track. For the sake of clarity, only a number of 
pairs of the rubber blocks 16, 17 are shown. Each closed track has an 
active part in which the blocks 16, 17 belonging to the two chains 14, 15 
act upon sectors of the outer circumference of the tube 2 situated on 
either side of the tube 2. The device 12 is designed in such a way that 
the distance between the blocks, and thus the passage for the tube 2 to be 
oriented, can be altered. 
The way in which the device 12 acts upon the tube 2 will now be explained 
with reference to FIGS. 1 and 3. 
In the section of FIG. 3 a pair of blocks 16, 17 can be seen, belonging 
respectively to the chains 14, 15 of the pushing device 12 shown in FIG. 
1. The blocks 16, 17 are shown in the position in which they are situated 
in the active part of the closed track along which they move. The tube 2, 
which has left the extruder 1, the calibration sleeve 3 and the cooling 
device 4 with a round initial cross-section, is compressed to a tube 2 
with an oval cross-section through the blocks 16, 17 acting thereon. For a 
better understanding of the invention, the external circumference with 
round initial cross-section of tube 2 is shown in FIG. 3 by a dashed line. 
In FIG. 3 the inner boundary line of the cold outer layer of the tube 2 is 
also indicated by a dashed line. In this example the tube is made of PVC 
and the inner boundary line corresponds to the 80.degree. C. isotherm 
(dashed line "l"). 
As a reaction to the deformation of the tube 2 brought along by the device 
12, a surface pressure (normal force) is created between the tube 2 and 
the blocks 16, 17 of the device 12. Said surface pressure is the result of 
the resistance of the tube 2 to the imposed deformation; it is clear that 
the strong outer layer makes an important contribution to the overall 
deformation resistance of the tube 2. With the same deformation, the 
presence of the outer layer thus leads to a greater surface pressure than 
if there were no outer layer. The greater surface pressure makes it 
possible to exert a greater axial force on the smooth tube 2. 
The surface pressure between the blocks 16, 17 and the tube 2 can therefore 
be regulated by regulating the passage between the chains 14, 15. 
Moreover, tubes of mutually differing diameters can be handled without 
major adjustments being made to the device 12. The device 12 can be 
provided with temperature-regulating means for regulating the temperature 
of the blocks 16, 17. For example, it may be desirable to cool the blocks 
16, 17, in order in this way to prevent premature heating of the cold 
outer layer of the tube 2. 
The tube 2 is then moved over the mandrel 6, which has a round 
cross-section corresponding to the tube to be manufactured. The 
deformation of the tube 2 caused by the device 12 is allowable because the 
biaxial orientation of the molecules of the thermoplastic material 
occurring at the mandrel 6 is essentially the determining factor for the 
properties of the ultimately manufactured tube 2. 
It can be seen in FIG. 1 that the blocks 16, 17 of the device 12 act upon 
the tube 2 at a distance upstream of the upstream end the mandrel 6. The 
tension member 5 is also made so thin that the tube 2 cannot come into 
contact internally with the tension member 5 at the place where these 
blocks 16, 17 act upon the tube 2 and compress the tube 2. The risk of the 
tube 2 becoming jammed between the blocks 16, 17 and the tension member 5 
is thus avoided. 
The distance between the point where the blocks 16, 17 act upon the tube 2 
and the expanding section 8 of the mandrel 6, preferably 5-10 times the 
tube diameter at this point, is advantageous for the abovementioned 
heating of the outer layer from the inside. Furthermore, a relatively 
large distance between the device 12 and the expanding section of the 
mandrel 6 leads to a damping of any pulsations which may occur in the 
axial force exerted by the device 12. In conjunction with the hard outer 
layer, the state of stress of the wall material of the tube 2 at the 
position of the mandrel 6 remains very constant. This is not only 
advantageous for controlling the biaxial orientation process, but in 
particular prevents undesirable wrinkling in the wall thickness from 
occurring in the axial direction of the manufactured tube 2. 
In the care of the method according to the present invention the suitable 
distance between the device 12 and the expanding section of the mandrel 6 
will have to be determined for each individual situation. Various 
parameters, for example the dimensions of the tube, the degree of 
deformation in the circumferential direction of the tube while it is 
passing over the expanding section of the mandrel, the envisaged axial 
force exerted by the tube speed controlling means, and the properties of 
the plastic material of the tube, will be found to be important. 
The distance between the device 12 and the mandrel 6 also has the advantage 
that the tube 2 undergoes a gradual transition from the deformed oval 
cross-section at the device 12 to the cross-section at the mandrel 6. 
Between the device 12 and the expanding section 8 of the mandrel 6, the 
tube 2 is subjected to an axial pressure load by the device 12 when the 
device 12 exerts a pushing force. In combination with the envisaged 
distance between the device 12 and the expanding section of the mandrel 6 
the tube 2 will have the tendency to buckle. The risk of buckling is 
limited in the case of the method according to the invention by the strong 
outer layer (skin) of the tube 2, which does, however, become increasingly 
thin further away from the device 12, due to the heating of the outer 
layer. Therefore the tube 2 is preferably supported in the lateral 
direction in the region between the device 12 and the expanding section of 
the mandrel 6. The tube 2 need not be supported over the entire distance 
here, and it can be supported either on the inside or on the outside. The 
tube 2 is advantageously supported by a run-on section of the mandrel 
placed upstream of the expanding section of the mandrel. This run-on 
section then forms an internal support for the tube near the expanding 
section of the mandrel. If a suitable length is selected for the run-on 
section, buckling can be prevented over the entire distance between the 
device 12 and the expanding section of the mandrel. 
In FIG. 1 it can be seen that the mandrel 6 is provided with a cylindrical 
run-on section 7, which section is placed upstream of the expanding 
section 8 of the mandrel 6 and is integral therewith. Said run-on section 
7 then forms an internal support for the tube 2 and has for example a 
length of at least three times the tube diameter. Buckling of the tube 2 
is prevented near the device 12 by the strong outer layer still present 
there, and is prevented near the mandrel 6 by the run-on section 7 of the 
mandrel 6. 
The axial pressure to which the tube 2 is subjected in this case also leads 
to upsetting of the tube 2. The result of this is that the cross-section 
of the tube 2 upstream of the mandrel 6 will be slightly larger, generally 
a few per cent (1-5%) than upstream of the device 12. For accurate 
guidance of the tube 2 relative to the mandrel 6, it is desirable for the 
tube 2 to be centered before the expanding section 8 of the mandrel 6. 
This is achieved through the fact that, when the diameter of the run-on 
section 7 is being determined, the increase in the internal diameter of 
the tube 2 as a result of the upsetting effect is taken into account. An 
advantageous effect of the upsetting of the tube 2 is that it also causes 
a greater surface pressure between the blocks 16, 17 of the device 12 and 
the tube. 
Although a heating of the cold outer skin is effected by heat transfer from 
the inside of the tube 2, a controlled heating of the tube 2 between the 
device 12 and the expanding section of the mandrel is preferred to be able 
to insure that the plastic material of the tube wall is at the orientation 
temperature when passing over the mandrel 6. On the basis of the 
abovementioned automatic heating of the outer layer from the inside, 
reaching the orientation temperature uniformly could not always be 
guaranteed with certainty. 
It is preferred that heating of the tube comprises influencing the 
temperature of the plastic material of the tube by a system which is 
adjustable sector-wise in the circumferential direction of the tube. The 
sector-wise adjustment of the heating is preferably carried out depending 
on the measured cross-section profile of the biaxially oriented tube. This 
measure is based on the following idea: 
While the tube is passing over the mandrel the plastic material of the tube 
encounters a resistance which counteracts the movement of the tube over 
said mandrel. This resistance depends on several parameters, such as the 
temperature of the plastic material, the wall thickness of the tube 
upstream of the mandrel, the friction between the tube and the mandrel, 
and the shape of the mandrel. Since the plastic material is in a readily 
deformable state while it is passing over the mandrel, the distribution of 
the plastic material around the mandrel will therefore be influenced by 
differences in resistance to the movement of the tube over the mandrel 
seen in the circumferential direction of the tube. This can lead to 
differences in the wall thickness of the tube, viewed in a cross-section 
at right angles to the axis of the mandrel, when the tube is leaving the 
mandrel. In the sector of the tube where there is a variation in the wall 
thickness the biaxial orientation obtained will also not correspond to 
that in the other circumferential sectors of the tube. Any influence which 
the tube speed controlling means may have on the homogeneity of the tube 
can also be compensated for by this measure according to the invention. 
This method of influencing the resistance by means of the temperature of 
the tube wall can be achieved in a simple way in practice, and can be 
carried out from the outside of the tube or also, possibly in combination, 
from the inside of the tube. Through a local rise in the temperature, the 
plastic material of the tube will flow more easily at that point under the 
load which occurs. So this in fact influences the resistance encountered 
by the tube when it is passing over the mandrel. Likewise, through a local 
change in the temperature of the plastic material on the inside of the 
tube, an influence can be exerted on the friction resistance between that 
part of the tube and the mandrel. In this case the mandrel can be provided 
with individually adjustable heating units disposed around the 
circumference of the mandrel. 
The heating device 40 in this example comprises eight infrared heating 
units 41, which are placed near the mandrel 6 at regular intervals around 
the path where the tube 2 passes through the device 40. Each unit 41 can 
supply an adjustable quantity of heat to the tube 2. The infrared heating 
units 41 are set up in such a way that each of them can exert influence on 
the temperature of the plastic material of the tube 2 in a sector of the 
circumference of the tube 2. The heating device 40 designed in this way 
can be used to bring the tube 2 accurately to the temperature desired for 
the biaxial orientation. 
With the heating device 40, sector-wise influencing of the resistance 
encountered by the tube 2 when it is passing over the mandrel 6 is also 
possible, as explained earlier. 
FIG. 4 shows diagrammatically in sectional view a part of the production 
line of FIG. 1 wherein the biaxial orientation of the tube 2 is effected. 
An important difference with the production line shown in FIG. 1 is the 
alternative embodiment of the mandrel, which is indicated with reference 
numeral 50 in FIG. 4. 
As in FIG. 1 the mandrel 50 is connected to the extruder (not shown) with a 
tension member 51. The mandrel 50 consists essentially of two sections; a 
heated section 52 which comprises an essentially cylindrical run-on 
section 50a and a conical expanding section 50b and a cooled section 53 
which comprises an essentially cylindrical run-off section 50c. 
A ring-shaped disc 54 of a thermal insulating material, such as a plastic, 
is placed between the heated section 52 and the cooled section 53 of the 
mandrel 50. 
A warm fluid, e.g. warm water, is fed through a conduit 55 in the tension 
member 51 to one of more channels 56 provided in the essentially solid 
metal mandrel section 52. Each channel 56 ends in a recessed 
circumferential groove 57 provided in the outer conical surface of mandrel 
section 52. The fluid supplied through conduit 55 forms a layer between 
the tube 2 and the heated section 52 of the mandrel 50 and will flow from 
this groove 57 against the direction of movement of the tube 2. The warm 
fluid then flows into an annular chamber 58 which is defined by a sealing 
device 59, the tube 2 and mandrel section 52. Finally the fluid leaves the 
chamber 58 via a further conduit 60 provided in the tension member 51. The 
warm fluid will flow not in the same direction as the moving tube 2 since 
an effective fluid seal is established by the contact pressure between the 
tube 2 and the mandrel 50 downstream of the groove 57 in the area of the 
transition between the conical section 52 and the run-off section 53 of 
the mandrel 50. 
In case of the biaxial orientation of a tube made of PVC the preferred 
temperature of the warm fluid is about 95.degree. C., the pressure of the 
fluid is preferably no more than is necessary to form and maintain the 
fluid layer between the tube 2 and the heated mandrel section 52. 
A cold fluid, e.g. cold water, is fed through a conduit 61 in the tension 
member 51 to one or more channels 62 provided in the essentially solid 
metal mandrel section 53. Each channel 62 opens in a recessed 
circumferential groove 63 provided in the outer surface of section 53. The 
fluid will flow from this groove 63, against the direction of movement of 
the tube 2, towards a second circumferential groove 64 provided in the 
outer surface of the mandrel section 53, and flows from there via one or 
more channels 65 to a chamber 66 downstream of the mandrel 50. A layer of 
fluid is hereby established between the cooled section 53 of the mandrel 
50 and the tube 2. The chamber 66 is defined by a sealing device 67, the 
part of the tension member 51 extending downstream from the mandrel 50, 
and the mandrel section 53. The fluid entering the chamber 66 will leave 
this chamber 66 via a conduit 69 provided in the tension member 51. 
The groove 63 is spaced such a distance from the downstream end of mandrel 
section 53 that an effective fluid sealing is established by the contact 
pressure between the tube 2 and mandrel section 53. This pressure is 
mainly a result of the tendency of the tube 2 to shrink as the tube is 
cooled down. The flow of cold fluid between the run-off section 53 of the 
mandrel 50 and the tube 2 cools the tube 2 from the inside immediately 
after the radial expansion of the tube 2 has been effected. In case of the 
biaxial orientation of PVC the temperature of the cold fluid is preferably 
about 20.degree. C. when fed into the conduit 61. 
It is noted that the thickness of the fluid layers between the tube 2 and 
the sections 52 and 53 of the mandrel 50 is exagerated in FIG. 4. 
As is clear form the above and from FIG. 4 the tube 2 is only in contact 
with the mandrel 50 in the area between the groove 57 on the conical 
section and the groove 64 on the run-off section and in the area between 
groove 63 and the downstream end of the run-off section. The total area of 
contact is therefore considerably smaller than with the mandrel of FIG. 1. 
and the friction between the mandrel and the tube is greatly reduced. Due 
to this reduced friction the phenomenon can be observed that the pulling 
force exerted by the pulling device 20 (FIG. 1) on the tube 2 downstream 
of the mandrel 50 is not completely dissipated by the expansion of the 
tube 2 and frictional forces occurring at the mandrel 50, but there still 
is a residual pulling force on the tube 2 upstream of the mandrel 50. This 
would result in the tube 2 being pulled from the extruder 1 at a greater 
speed than intended and eventually the tube 2 could rupture. To eliminate 
this undesirable effect the tube speed controlling means 12 (FIG. 1) 
placed between the extruder 1 and the mandrel 50 are in this case set to 
exert an axial braking force on the tube 2, i.e. an axial force directed 
away from the mandrel 50. This braking force can be obtained by having the 
tracks 14, 15 of the tube speed controlling means 12 moving in the 
direction of movement of the tube 2 at a predetermined constant speed. 
Without the tube speed controlling means 12 actually braking the tube 2 it 
can be observed that, using a mandrel 50 of the type shown in FIG. 4, the 
tube 2 does not become stretched in the axial direction thereof, or at 
least not in a sufficient manner. 
Therefore a balance has to be established between the pulling force exerted 
on the tube by the pulling device 20 downstream of the mandrel 50 and the 
axial force exerted by the tube speed controlling means upstream of the 
mandrel. This balance is obtained by regulating the speed of both devices. 
Also the area of contact between the tube 2 and the mandrel 50 according to 
the present invention can be made adjustable to allow control the 
frictional forces between the mandrel 50 and the tube 2. This can be done 
either by having a plurality of mandrels with differing locations of the 
grooves on the outer surface of the mandrel or by providing a mandrel with 
valve means that allow the fluid to exit from one or more selected grooves 
on the outer surface of the mandrel. 
During start-up of the production line shown in FIG. 1 but provided with 
the mandrel of the type shown in FIG. 4 it is obvious that no fluid layer 
can be formed between the tube 2 and the mandrel 50 and that the pulling 
device 20 downstream of the mandrel 50 cannot aid the forcing of the tube 
2 over the mandrel 50. During the start-up procedure the tube speed 
controlling means 12 are then advantageously set to exert a pushing force, 
towards the mandrel 50, on the tube 2. To be able to exert a significant 
pushing force on the tube 2 the cooling device 4 is already in operation 
to form the cold outer skin on the tube 2 as disclosed hereinbefore. 
The presence of a fluid film between the mandrel 50 and the tube 2 in a 
process for the manufacture of biaxially oriented tubing has already been 
disclosed in DE 23 57 210. In this document the fluid is introduced 
between the hollow plastic tube and the run-on section of the mandrel 
upstream from the expanding section of the mandrel. This fluid is to be 
dragged along by the moving tube over the mandrel. In DE 23 57 210 it is 
noted that the angle of the conical expanding section is limited to 
prevent the fluid film from being disrupted. 
According to a preferred embodiment of the present invention the fluid is 
supplied between the tube and the mandrel through channels formed in the 
mandrel and opening on the outer surface of the mandrel, in particular on 
the expanding section and the run-off section thereof. This allows a far 
greater angle of the expanding section than with the method and mandrel of 
DE 23 57 210. The inventive design of the mandrel and manner of providing 
the fluid layer can therefore also be used to improve this known method 
without the forming of the cold outer skin on the extruded tube. 
Another phenomenon that can be observed is that, due to the axial tensile 
forces present in the tube 2 between the device 12 and the mandrel 50, the 
tube 2 tends to contract in radial direction. This effect is counteracted 
by the cold outer skin of the tube 2, but also the device 12 could be 
provided with biassing means which bias the blocks 16 and 17 towards the 
tube 2 to maintain a sufficient radial contact pressure between the blocks 
16, 17 and the tube 2. 
To obtain a stable thickness of the fluid layer between the mandrel section 
53 and the tube 2 a volumetric pump, i.e. a pump having a constant output 
independent from the fluid pressure, is preferably used to circulate the 
fluid. A similar type of pump is preferably used to circulate the warm 
fluid which forms a fluid layer between the tube 2 and mandrel section 52. 
As can be seen in FIG. 4 the tube 2 is also cooled externally after the 
orientation in circumferential direction has been effected. A cooling 
device 70 is provided to achieve this external cooling. 
A plate 75 having a calibrating opening where the tube 2 passes through is 
provided downstream of the mandrel 50. The plate 75 is movable with 
respect to the mandrel 50 as is indicated by arrow C in FIG. 4. Downstream 
of the plate 75 a measuring device 80 is located, which device 80 can 
determine the wall thickness and shape of the cross-section of the tube 2 
passing through the device 80. The signal representing the measurements of 
the device 80 are fed into a control device 81, which compares this signal 
with a signal representing the desired tube dimensions. On the basis of 
this comparison the position of the plate 75 with respect to the mandrel 
50 can be controlled. The same comparison is also used to control the 
working of the heating device 40 which has been described above referring 
to FIG. 1.