U.S. patent application number 14/055293 was filed with the patent office on 2015-08-20 for apparatus for extruding a polymeric material and extrusion head therefor.
This patent application is currently assigned to Pirelli Tyre S.p.A.. The applicant listed for this patent is Pirelli Tyre S.p.A.. Invention is credited to Francesco D'ORIA, Thomas PONTA, Stefano TESTI.
Application Number | 20150231843 14/055293 |
Document ID | / |
Family ID | 35694348 |
Filed Date | 2015-08-20 |
United States Patent
Application |
20150231843 |
Kind Code |
A9 |
PONTA; Thomas ; et
al. |
August 20, 2015 |
APPARATUS FOR EXTRUDING A POLYMERIC MATERIAL AND EXTRUSION HEAD
THEREFOR
Abstract
An apparatus for extruding a polymeric material, having an
extrusion head which includes a male die, a female die coaxially
arranged with respect to the male die, a conveying channel, and at
least one portion of which is defined between the male die and the
female die. The apparatus further includes a device for adjusting a
cross-sectional area of the at least one portion of the conveying
channel by reciprocally displacing the female die with respect to
the male die in response to an extrusion speed variation of the
polymeric material.
Inventors: |
PONTA; Thomas; (MILANO,
IT) ; TESTI; Stefano; (MILANO, IT) ; D'ORIA;
Francesco; (MILANO, IT) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Pirelli Tyre S.p.A. |
Milano |
|
IT |
|
|
Assignee: |
Pirelli Tyre S.p.A.
Milano
IT
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20140042663 A1 |
February 13, 2014 |
|
|
Family ID: |
35694348 |
Appl. No.: |
14/055293 |
Filed: |
October 16, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11919136 |
Feb 6, 2009 |
8585949 |
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PCT/EP2005/004500 |
Apr 27, 2005 |
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14055293 |
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Current U.S.
Class: |
264/210.1 ;
425/28.1; 425/29 |
Current CPC
Class: |
B29L 2030/003 20130101;
B29C 2948/92571 20190201; B29C 2948/92704 20190201; B29C 2948/92904
20190201; B29C 48/92 20190201; B29C 2948/9258 20190201; B29C 48/302
20190201; B29C 2948/92019 20190201; B29C 48/07 20190201; B29C
48/325 20190201; B29C 2948/92409 20190201; B29C 2948/92514
20190201; B29C 48/266 20190201; B29C 2948/92933 20190201; B29C
2948/92952 20190201; B29C 2948/926 20190201; B29C 48/34 20190201;
B29C 48/09 20190201; B29K 2105/10 20130101; B29C 48/255 20190201;
B29C 2948/92209 20190201; B29C 2948/92466 20190201; B29D 30/08
20130101 |
International
Class: |
B29D 30/08 20060101
B29D030/08 |
Claims
1-36. (canceled)
37. An apparatus for extruding a polymeric material, comprising an
extrusion head which comprises: a male die; a female die coaxially
arranged with respect to said male die; a conveying channel, at
least one portion of which is defined between said male die and
said female die; and a device for adjusting a cross-sectional area
of said at least one portion of said conveying channel by
reciprocally displacing said female die with respect to said male
die in response to an extrusion speed variation of said polymeric
material.
38. The apparatus according to claim 37, wherein the male die is
provided with an inner cavity coaxially extending with respect to a
longitudinal axis of the extrusion head.
39. The apparatus according to claim 37, wherein the device for
adjusting the cross-sectional area of the at least one portion of
the conveying channel comprises a resilient element acting on at
least one die.
40. The apparatus according to claim 39, wherein the resilient
element is associated with the female die.
41. The apparatus according to claim 39, wherein the resilient
element is a spring whose elastic constant K is selected in
response to the range of variation of the extrusion speed.
42. The apparatus according to claim 37, wherein the device for
adjusting the cross-sectional area of the at least one portion of
the conveying channel is a servo-device that detects the variation
of the at least one parameter indicative of the polymeric material
flow conditions and adjusts said cross-sectional area in response
to the detected variation of said at least one parameter.
43. The apparatus according to claim 42, wherein said servo-device
comprises a sensor acting on said polymeric material and generating
a signal representative of the variation of said at least one
parameter.
44. The apparatus according to claim 43, wherein said sensor
detects a pressure variation of the polymeric material flowing
through the extrusion head.
45. The apparatus according to claim 42, wherein said servo-device
comprises a processing device for calculating a new position of at
least one die.
46. The apparatus according to claim 42, wherein said servo-device
comprises a device for moving said at least one die to a new
position.
47. The apparatus according to claim 46, wherein the device for
moving said at least one die to said new position comprises an
actuator device.
48. The apparatus according to claim 47, wherein said actuator
device is associated with said at least one die and a position
sensor.
49. The apparatus according to claim 47, wherein said actuator
device comprises a hydraulic device.
50. The apparatus according to claim 47, wherein said actuator
device comprises a pneumatic device.
51. The apparatus according to claim 47, wherein said actuator
device comprises a gear electro-mechanical device.
52. The apparatus according to claim 47, wherein said actuator
device comprises a linear actuator.
53. A process for manufacturing a tyre, said process comprising the
steps of: forming a crude tyre on a supporting device; moulding
said crude tyre; and curing said crude tyre, wherein the step of
forming the crude tyre comprises the step of extruding at least one
elastomeric material, the step of extruding comprising the steps
of: feeding said elastomeric material to an extrusion apparatus
comprising an extrusion head, said extrusion head comprising: a
male die; a female die coaxially arranged with respect to said male
die; and a conveying channel, at least one portion of which is
defined between said male die and said female die; and adjusting a
cross-sectional area of said at least one portion of the conveying
channel by reciprocally displacing said female die with respect to
said male die in response to an extrusion speed variation of said
elastomeric material.
Description
[0001] The present invention relates to a method for extruding a
polymeric material.
[0002] In particular, the present invention relates to a method for
extruding an elastomeric material to be used in tyre manufacturing
processes.
[0003] The invention further relates to an extrusion head for
extruding a polymeric material and to an extrusion apparatus which
comprises said extrusion head.
[0004] A preferred field of application of the present invention is
a process for manufacturing a tyre wherein elastomeric sheets or
strip-like elements, possibly reinforced with metallic or synthetic
thread-like elements, are used for producing tyre constitutive
elements such as, for example, a carcass ply, a belt layer, a
sidewall, a bead core.
[0005] A tyre generally comprises: a carcass structure comprising
at least one carcass ply, the ends of which are folded back or
secured to two annular reinforcing elements, i.e. the so-called
"bead cores"; a tread band; a belt structure placed between the
carcass structure and the tread band; and a pair of sidewalls
applied to said carcass structure in axially opposite
positions.
[0006] The tyre portion which comprises the bead core is known as
"bead" and performs the function of fixing the tyre on a respective
mounting rim.
[0007] Generally, in a position radially external to said bead
core, the bead comprises a rubber strip, conventionally called
"bead filling" or "bead apex", which has a substantially triangular
cross-section and extends radially outwardly from the respective
bead core.
[0008] In conventional manufacturing processes, the tyre
constitutive elements are made by using semi-finished products,
i.e. continuous sheets of elastomeric material--possibly in
combination with reinforcing elements such as steel or textile
cords--that are prepared separately and in large quantities
previously to the tyre assembling operations.
[0009] According to said conventional processes, for each tyre
constitutive element, the manufacturing process comprises the steps
of winding a predetermined elastomeric sheet onto a building drum,
cutting (or in some cases pre-cutting) said sheet into a length
approximately equal to the circumference of the drum, and joining
the circumferentially opposite ends of said sheet length directly
on the building drum.
[0010] In more recent times particular attention has been given to
production methods that eliminate or at least remarkably reduce the
preliminary production of said semi-finished products. For example,
European patent N. 928,680 in the name of the same Applicant
discloses the manufacturing of a green tyre by consecutively
producing and assembling together on a toroidal support the tyre
constitutive elements. In detail, the tyre is manufactured by
axially overlapping and/or radially superimposing turns (coils) of
a strip-like element on the toroidal support, said strip-like
element being a strip of an elastomeric material only, or a strip
of elastomeric material embedding reinforcing elements thereinto,
typically textile or metal cords, or a rubberized metal wire or
cord.
[0011] According to said further process, the toroidal support is
moved, preferably by a robotized system, between a plurality of
work stations in each of which, through automated sequences, a
particular building step of the tyre is carried out.
[0012] The manufacturing process further comprises the successive
step of moulding the green tyre, so as to confer to the latter a
desired tread pattern, and the step of curing the green tyre, so as
to confer to the latter a desired geometrical conformation which is
obtained by curing the elastomeric material forming the tyre.
[0013] The moulding and curing steps of the green tyre are carried
out by introducing the green tyre into a moulding cavity defined
within a vulcanization mould, whose shape matches the shape of the
outer surface of the tyre to be obtained, and by introducing a
fluid under pressure into a diffusion interspace (or diffusion gap)
provided between the inner circumferential surface of the green
tyre and the toroidal support. Such a tyre manufacturing process is
described, for instance, in European Patent EP-976,533 in the name
of the same Applicant.
[0014] In the following description, the term "extruded element" is
used to indicate either a strip-like element or a sheet. In
particular, the term "extruded element" is used to indicate a
strip-like element or a sheet which is made of a polymeric material
or which is reinforced with at least one thread-like element. The
term "thread-like element" is used to indicate an element whose
longitudinal dimension is substantially greater than its
transversal dimension, the thread-like element comprising one or
more reinforcing elements, each reinforcing element consisting of a
textile or metallic filament or cord. In case the strip-like
element or the sheet is made of a polymeric material, the
strip-like element or the sheet is preferably obtained by
extrusion. In case the strip-like element or the sheet is
reinforced with at least one thread-like element, the strip-like
element or the sheet is preferably obtained by extruding a
polymeric material onto at least one thread-like element advancing
through an extrusion apparatus.
[0015] Generally, an extrusion apparatus comprises an extrusion
head which includes: a male die; a female die, coaxially arranged
with respect to the male die; and a distributor element for
uniformly distributing the extruded material into a conveying
channel which is provided between the male die and the female
die.
[0016] In case the extruded element is reinforced with at least one
thread-like element, the male die is usually provided with an inner
cavity coaxially extending with respect to a longitudinal axis of
the extrusion head, said cavity being suitable for receiving said
at least one thread-like element advancing along a direction
substantially parallel to said longitudinal axis. The polymeric
material flowing into the conveying channel is thus deposited onto
said at least one thread-like element advancing through the
extrusion apparatus.
[0017] Document U.S. Pat. No. 3,752,614 discloses an extrusion head
for forming insulated wire which includes a fixed threaded hollow
mandrel and a threaded hollow pin disposed internally of, and in
mating engagement with, the mandrel for supporting a male die
member in axial alignment with a female die member mounted within
the head. The threaded portions of the mandrel and the pin are so
engaged that rotation of the pin within the mandrel advances or
retracts the male die member with respect to the female die member
while maintaining the alignment therebetween. This device allows to
compensate for changes in the extruded plastic material, insulation
thickness, or in the pressure or temperature of the system while
the extrusion head is in operation. The relative movement between
the male die member and the female die member is effected by an
operator by manual control.
[0018] Document U.S. Pat. No. 3,583,033 discloses a die for in-line
extrusion of viscoelastic and viscous thermoplastic materials,
comprising a conical male valve member which is advanced or
retracted with respect to a conical seat to vary the degree of
shear and back pressure to which the material is exposed in passing
through the annular conical passegeway. The movement of the conical
male valve member is achieved by rotating a ring nut and is
manually effected and controlled by an operator.
[0019] Document GB-2,060,473 discloses a head for extruding tubes
for blow moulding, including a mandrel supported by one part, the
other part comprising at least one conical wall portion which, with
a corresponding mandrel wall portion, forms a conical flow space
section whose throughflow cross-section can be varied by the
relative displacement of the two telescopically engaging parts. The
relative displacement is manually effected by the operator by means
of an adjusting screw associated with one part of the mandrel and
engaged with a suitable screwthread formed on the other part of the
mandrel. It is described that various remote controlled servo
devices could conceivably be used in place of the adjusting
screw.
[0020] Document U.S. Pat. No. 3,402,427 discloses a crosshead die
body apparatus including a shaping die for extruding and shaping
thermoplastic material comprising polyvinylidene fluoride resin,
wherein the crosshead die body has at least two externally
adjustable internally axially positioned frusto-conically shaped
valving means and at least one annular orifice portion of fixed
uniform annular width and of substantially fixed but adjustably
variable length located axially between said valving means, whereby
the pressure drop and shearing stress between the extruder outlet
and the shaping die may be progressively and precisely controlled.
During start-up of the coating process, the surface and body
characteristics of the extrudate are observed by the operator of
the machine and are modified by manipulation of the valving means
until the optimum extrudate characteristic are obtained. Then, by
continued observation and manipulation of the valving means, either
by manual or automatic control, the optimum characteristics can be
maintained by the operator throughout the extruding and shaping
operation without reaching or exceeding the yield point of the
resin.
[0021] The Applicant has noted that in case the step of extruding a
polymeric material to obtain an extruded element is part of a
complex process, such as a tyre manufacturing process, generally
the extrusion speed of said material needs to be regulated on the
basis of the process step subsequent to the extrusion step, while
the extrusion apparatus is continuously fed with the polymeric
material to be extruded.
[0022] In particular, in the more recent tyre manufacturing
processes--disclosed, for instance, in European patent N. 928,680
mentioned above--wherein the tyre constitutive elements are
produced and consecutively assembled on a toroidal support while
the tyre is being manufactured so that the storage of semi-finished
products is substantially avoided, the Applicant has noted that the
extrusion speed needs to be increased when the extruded
elements--which are used for forming a specific tyre
component--have to be obtained and applied onto the tyre being
manufactured, and, on the contrary, the extrusion speed needs to be
decreased when the extruded elements do not have to be deposited so
that waste of raw materials is avoided or at least remarkably
reduced.
[0023] The Applicant has noted that the regulation of the extrusion
speed has to be carried out while ensuring that the extruded
elements maintain the desired quality.
[0024] In fact, the Applicant has noted that the quality of the
extruded elements remarkably influences the quality of the tyre
reinforcing structures including said extruded elements. Therefore,
great attention and care have to be paid to the production of the
extruded elements in case the latter are reinforced with
thread-like elements.
[0025] In particular, uniformity and homogeneity of the extruded
material in the production of the extruded elements have to be
carefully controlled and a substantially constant thickness thereof
has to be ensured so that uniformity of the tyre is guaranteed.
Moreover, in case the extruded elements are reinforced with
thread-like elements, great attention and care have to be paid in
order to obtain extruded elements wherein the polymeric coating
layer uniformly adheres to the thread-like elements so that the
thickness of the coating layer is substantially constant along the
longitudinal extension of the extruded elements.
[0026] The Applicant has noted that the quality of the extruded
elements depends on the geometry of the extrusion head and, at a
predetermined geometry thereof, on the physico-chemical
characteristics of the extruded material as well as on the process
parameters of the extrusion process.
[0027] For example, particular care is required when polymeric
materials that are sensitive to temperature are used. In fact,
scorching at high temperatures and/or clots at low temperatures can
arise, principally if long periods of permanency thereof are caused
to occur in the extrusion head. Scorching and clots need to be
avoided since they negatively influence uniformity and homogeneity
of the extruded elements, and thus of the tyre reinforcing
structures including said extruded elements. Moreover, said defects
can cause the extrusion process to be stopped in order to allow the
extrusion head to be cleaned from the clots and/or the scorched
material.
[0028] Furthermore, the Applicant has noted that stagnation zones
of the polymeric material can originate in the extrusion head,
particularly when small flow rate values of the polymeric material
occur, for instance when the output of the extrusion process is
decreased. As a consequence, the period of permanency of the
polymeric material in the extrusion head increases and, as
mentioned above, scorching or overheatings of said material can
occur.
[0029] Moreover, the Applicant has noted that, at a predetermined
and constant extrusion speed of the extruded elements, a remarkable
decrease of the flow rate of the polymeric material flowing through
the extrusion head can also cause the formation of very thin
extruded elements and, sometimes, can even cause the breakage
thereof. In case the extruded elements are reinforced with
thread-like elements, areas of the outer surface thereof can be
even devoid of the extruded material. On the contrary, at a
predetermined and constant extrusion speed of the extruded
elements, a remarkable increase of the flow rate of the polymeric
material flowing through the extrusion head can cause the formation
of too thick extruded elements.
[0030] The Applicant has noted that a variation of the extrusion
speed, which is chosen in response to the process step subsequent
to the extrusion step, causes a variation of the flow rate of the
extruded polymeric material and, consequently, a variation of the
pressure inside the extrusion head.
[0031] In the present description and in the following claims the
term "extrusion speed" is used to indicate the linear velocity of
the polymeric extruded element exiting from the extrusion head. In
case the extruded element consists only of the polymeric material
(i.e. the extruded element does not embed any reinforcing element),
the extrusion speed can be varied by modifying the rotational speed
of the extruder screw. In case the extruded element comprises at
least one thread-like element embedded in the polymeric material,
the extrusion speed can be generally varied by modifying the linear
velocity of the at least one thread-like element.
[0032] In particular, the Applicant has noted that an increase of
the extrusion speed requires an increase of the flow rate of the
extruded polymeric material and thus causes an increase of the
pressure inside the extrusion head. Therefore, in order not to
mechanically stress the extrusion head and not to scorch the
polymeric material, the flow rate of the polymeric material needs
to be kept below a predetermined maximum value, fact which
inevitably limits the maximum extrusion speed value. On the other
side, a decrease of the extrusion speed requires a decrease of the
flow rate of the extruded polymeric material and thus causes a
decrease of the pressure inside the extrusion head. As a
consequence, the period of permanency of the polymeric material in
the extrusion head increases and, as mentioned above, scorching of
the polymeric material as well as formation of stagnation zones can
occur. Therefore, the flow rate of the polymeric material needs to
be kept over a predetermined minimum value, fact which inevitably
limits the minimum extrusion speed value.
[0033] The Applicant has perceived the need of increasing the
operative range of the flow rate of the polymeric material to be
extruded so that the range of variation of the extrusion speed can
be remarkably increased and can be suitably fitted to the working
conditions of the process step subsequent to the extrusion
step.
[0034] In particular, the Applicant has perceived that the above
goal can be achieved by modifying the geometry of the extrusion
head during the extrusion process, the variation of geometry of the
extrusion head being carried out in response to the working
conditions of the process step subsequent to the extrusion
step.
[0035] In detail, the Applicant has found that, once the desired
range of variation of the extrusion speed has been chosen on the
basis of the working conditions of the process step subsequent to
the extrusion step, the extrusion head can operate at the
corresponding operative range of the flow rate of the polymeric
material to be extruded by adjusting the cross-sectional area of
the conveying channel in response to the actual flow rate flowing
through the extrusion head, and thus in response to the actual
extrusion speed which is required in the specific phase of the
extrusion process.
[0036] In a first aspect the present invention relates to a method
for extruding a polymeric material, said method comprising the
steps of: [0037] feeding said polymeric material to an extrusion
apparatus including an extrusion head, said extrusion head
comprising: [0038] a male die; [0039] a female die, coaxially
arranged with respect to said male die, and [0040] a conveying
channel, at least one portion of which being defined between said
male die and said female die; [0041] adjusting a cross-sectional
area of said at least one portion of the conveying channel by
reciprocally displacing said female die with respect to said male
die in response to an extrusion speed variation of said polymeric
material.
[0042] In accordance with the present invention, since the
cross-sectional area of the conveying channel is adjusted in
response to the variation of the extrusion speed, critical flow
conditions can be avoided and thus scorching or overheating of the
polymeric material as well as the formation of stagnation zones in
the extrusion head or mechanical damages thereof can be avoided or
at least substantially reduced.
[0043] In other words, according to the present invention the
geometry (i.e. the cross-sectional area) of the conveying channel
can be automatically adapted to the different flow conditions of
the polymeric material by means of a reciprocal displacement of the
female die with respect to the male die.
[0044] In fact, according to the present invention, an increase of
the extrusion speed requires a corresponding increase of the flow
rate of the polymeric material to be extruded and thus causes a
corresponding increase of the pressure in the extrusion head. The
resulting pressure increase has the effect of axially displacing
the female die from the male die so that the cross-sectional area
of the conveying channel increases and the pressure losses in the
conveying channel decrease. As a result of the conveying channel
cross-section variation, the pressure increase in the extrusion
head can be limited and the desired flow rate value of the
polymeric material can be guaranteed. On the contrary, a decrease
of the extrusion speed requires a corresponding decrease of the
flow rate of the polymeric material to be extruded and thus causes
a corresponding pressure decrease and an increase of the period of
permanency of the polymeric material in the extrusion head. The
resulting pressure decrease has the effect of axially moving the
female die towards the male die so that the cross-sectional area of
the conveying channel decreases and the pressure losses in the
conveying channel increase. As a result of the conveying channel
cross-section variation, the period of permanency of the polymeric
material in the extrusion head can be suitably controlled and the
desired flow rate value of the polymeric material can be
guaranteed.
[0045] According to the method of the present invention, the step
of adjusting the cross-sectional area of at least one portion of
the conveying channel by reciprocally regulating the position of
the female die with respect to the male die comprises the step of
partially counteracting the force exerted on at least one die by
the polymeric material flowing in the conveying channel.
[0046] Preferably, the step of adjusting comprises the step of
partially counteracting the force exerted on the female die by the
polymeric material flowing in the conveying channel.
[0047] Preferably, the counteracting force is substantially
parallel to the extrusion head longitudinal axis.
[0048] According to an embodiment of the present invention, the
step of partially counteracting the force exerted on at least one
die is carried out by means of a resilient element acting on said
at least one die along said longitudinal axis.
[0049] Preferably, said resilient element is associated with the at
least one die which is allowed to be axially displaced.
[0050] Preferably, said resilient element is a spring whose elastic
constant K is selected in response to a predetermined range of
variation of the extrusion speed.
[0051] The method of the present invention further comprises the
step of extruding the polymeric material.
[0052] The method of the present invention further comprises the
step of choosing the extrusion speed variation of the polymeric
material in response to working conditions of a process step
subsequent to the extrusion step.
[0053] According to a further embodiment, the method of the present
invention further comprises the steps of: [0054] detecting a
variation of at least one parameter indicative of the polymeric
material flow conditions, the variation of said at least one
parameter being associated to the extrusion speed variation of said
polymeric material, and [0055] adjusting the cross-sectional area
of at least one portion of the conveying channel in response to the
detected variation of said at least one parameter.
[0056] Preferably, said at least one parameter indicative of the
polymeric material flow conditions is the pressure. Preferably, the
pressure is detected in the extrusion head.
[0057] Preferably, said at least one parameter indicative of the
polymeric material flow conditions is detected with a predetermined
frequency value. Alternatively, said at least one parameter is
continuously detected.
[0058] Preferably, the step of detecting the variation of at least
one parameter indicative of the polymeric material flow conditions
comprises the step of generating a signal representative of said
variation by means of a sensor acting on said polymeric material
flowing through the extrusion head.
[0059] Preferably, the step of adjusting the cross-sectional area
of the conveying channel comprises the steps of: [0060] calculating
a second position of at least one die in response to a variation of
said extrusion speed occurring at a first position, and [0061]
moving said at least one die to said second position.
[0062] Preferably, the step of calculating comprises the step of
calculating the second position of the female die and the step of
moving comprises the step of moving the female die to said second
position.
[0063] Preferably, the step of moving said at least one die to said
new position is carried out by means of an actuator device.
Preferably, the actuator device is associated with said at least
one die and said sensor.
[0064] Preferably, the method of the present invention is suitable
for extruding a cross-linkable material, the latter being
particularly sensitive to temperature variations.
[0065] In a second aspect thereof, the present invention relates to
an apparatus for extruding a polymeric material, said apparatus
including an extrusion head which comprises: [0066] a male die;
[0067] a female die, coaxially arranged with respect to said male
die; [0068] a conveying channel, at least one portion of which
being defined between said male die and said female die, and [0069]
a device for adjusting a cross-sectional area of said at least one
portion of said conveying channel by reciprocally displacing said
female die with respect to said male die in response to an
extrusion speed variation of said polymeric material.
[0070] Preferably, the male die is provided with an inner cavity
coaxially extending with respect to a longitudinal axis of the
extrusion head, said cavity being suitable for receiving at least
one thread-like element which is used for reinforcing an extruded
element.
[0071] In a first embodiment of the extrusion apparatus of the
present invention, the device for adjusting the cross-sectional
area of said conveying channel comprises a resilient element which
acts on at least one die and partially counteracts the force
exerted on said at least one die by the polymeric material flowing
in the conveying channel.
[0072] Preferably, said resilient element is associated with the
female die.
[0073] Preferably, said resilient element is a spring whose elastic
constant K is selected in response to the desired range of
variation of the extrusion speed.
[0074] According to a further embodiment of the extrusion apparatus
of the present invention, the device for adjusting the
cross-sectional area of said conveying channel is a servo-device
for detecting the variation of at least one parameter indicative of
the polymeric material flow conditions (the variation of said at
least one parameter being associated to the extrusion speed
variation of said polymeric material) and for adjusting said
cross-sectional area on the basis of the detected variation of said
at least one parameter.
[0075] Preferably, said servo-device comprises a sensor acting on
said polymeric material and generating a signal representative of
the variation of said at least one parameter.
[0076] Preferably, said sensor detects a pressure variation of the
polymeric material flowing through the extrusion head.
[0077] Preferably, said servo-device further comprises a device for
calculating a new position of at least one die and a device for
moving said at least one die to said new position.
[0078] Preferably, the device for moving said at least one die to
said new position is an actuator device. Preferably, said actuator
device is associated with said at least one die and a position
sensor. The position sensor has the function of detecting the
position of said at least one die.
[0079] According to a preferred embodiment, said actuator device
comprises a hydraulic device.
[0080] According to a further embodiment, said actuator device
comprises a pneumatic device.
[0081] According to a further embodiment, said actuator device
comprises a gear electro-mechanical device.
[0082] According to a further embodiment, said actuator device
comprises a linear actuator.
[0083] In a third aspect thereof, the present invention relates to
a process for manufacturing a tyre, said process comprising the
steps of: [0084] forming a crude tyre on a supporting device;
[0085] moulding said crude tyre, and [0086] curing said crude tyre,
wherein the step of forming the crude tyre comprises the step of
extruding at least one elastomeric material, the step of extruding
comprising the steps of: [0087] feeding said elastomeric material
to an extrusion apparatus including an extrusion head, said
extrusion head comprising: [0088] a male die; [0089] a female die,
coaxially arranged with respect to said male die, and [0090] a
conveying channel, at least one portion of which being defined
between said male die and said female die; [0091] adjusting a
cross-sectional area of said at least one portion of the conveying
channel by reciprocally displacing said female die with respect to
said male die in response to an extrusion speed variation of said
elastomeric material.
[0092] Preferably, the step of forming the crude tyre comprises the
step of extruding at least one elastomeric material in the form of
a strip-like element to be deposited onto the crude tyre being
manufactured. In this case, preferably the supporting device is a
toroidal support.
[0093] Alternatively, the step of forming the crude tyre comprises
the step of extruding at least one elastomeric material in the form
of a sheet to be deposited onto the crude tyre being manufactured.
In this case, preferably the supporting device is a building
drum.
[0094] Throughout the present description and the following claims,
the term "elastomeric material" is used to indicate a composition
comprising at least one elastomeric polymer and at least one
reinforcing filler. Preferably, the composition further comprises
additives, such as a cross-linking agent and/or a plasticizer.
[0095] The method of the present invention can be advantageously
used either for extruding a semi-finished product (in the form of a
sheet) to be used in a conventional tyre manufacturing processes,
or for extruding strip-like elements which are employed in more
recent tyre manufacturing processes as described above.
[0096] However, the present invention can be also applied to
technical fields different from a tyre manufacturing process. In
particular, the present invention can be applied to any technical
field wherein the extrusion of a polymeric material--to obtain an
extruded element--is required.
[0097] Further characteristics and advantages of the present
invention will become clearer from the description made hereafter
with reference to the attached drawings in which, for illustrative
and non limiting purposes, four embodiments of an extrusion head
for carrying out the method of the present invention are shown. In
the drawings:
[0098] FIG. 1 is a schematic cross-sectional view of a first
embodiment of an extrusion head in accordance with the present
invention;
[0099] FIG. 2 is a schematic cross-sectional view of a second
embodiment of an extrusion head in accordance with the present
invention;
[0100] FIG. 3 is a schematic cross-sectional view of a third
embodiment of an extrusion head in accordance with the present
invention;
[0101] FIG. 4 is a schematic cross-sectional view of a fourth
embodiment of an extrusion head in accordance with the present
invention;
[0102] FIG. 5 is a graph showing the pressure variation of the
polymeric material as a function of the flow rate thereof through
an extrusion head in accordance with the present invention in
comparison with a conventional extrusion head provided with a fixed
geometry of the conveying channel;
[0103] FIG. 6 is a graph showing the temperature variation of the
polymeric material as a function of the flow rate thereof through
an extrusion head in accordance with the present invention in
comparison with a conventional extrusion head provided with a fixed
geometry of the conveying channel;
[0104] FIG. 7 is a graph showing the period of permanency of the
polymeric material as a function of the flow rate thereof through
an extrusion head in accordance with the present invention in
comparison with a conventional extrusion head provided with a fixed
geometry of the conveying channel.
[0105] FIG. 1 schematically shows an extrusion head, indicated with
reference number 1, for extruding a coating layer 100--made of a
polymeric material 2--at a radially outer surface of an elongated
element 3 advancing through the extrusion head 1 along a direction
indicated by arrow A.
[0106] The extrusion head 1 has a longitudinal axis X-X and is part
of an extrusion apparatus which is not illustrated in detail as
being conventional per se.
[0107] As mentioned above, the extrusion head 1 of the present
invention can be used in a process for manufacturing a tyre. In
such a case, the elongated element 3 can be a metallic or synthetic
thread-like element which is covered by extrusion with an
elastomeric material to form an extruded element to be used in the
manufacturing of tyre reinforcing structures, such as, for example,
the carcass structure, the belt structure, the "bead cores".
[0108] According to the embodiment shown in FIG. 1, the extrusion
head 1 comprises: a distributor element 11, a male die 12, a female
die 13 and an annular body 10. The annular body 10 is coaxially
arranged with respect to the distributor element 11, the male die
12 and the female die 13 and positioned radially external
thereto.
[0109] In particular, the annular body 10 is provided with an inner
cavity 14 coaxially extending with respect to the longitudinal axis
X-X and suitable for housing the distributor element 11, the male
die 12 and the female die 13.
[0110] The extrusion head 1 is further provided with an inlet duct
15 for feeding the polymeric material 2. The inlet duct 15 is
associated to the annular body 10 at a feeding duct 16 which is
formed in the annular body 10 and which extends, in the illustrated
embodiment, in a direction substantially perpendicular to the axis
X-X.
[0111] In a way known per se, for example through pipes not
illustrated, the inlet duct 15 and the feeding duct 16 are in fluid
communication with an extruder barrel provided with at least one
extruder screw (not illustrated since conventional per se).
[0112] The distributor element 11 comprises a tubular body 17 on
the outer surface of which is provided at least one pair of
distribution channels 18, only one of which being shown by a dashed
line in FIG. 1.
[0113] The tubular body 17 of the distributor element 11 is
provided, similarly to the annular body 10, with an inner cavity 19
coaxially extending with the longitudinal axis X-X and intended for
receiving the elongated element 3 advancing along the direction
A.
[0114] In operation, preferably the advancing direction A of the
elongated element 3 is substantially parallel to the longitudinal
axis X-X of the extrusion head 1.
[0115] The extrusion head 1 further comprises an annular conveying
channel 20. A first portion 20' of said conveying channel (i.e. the
conveying channel portion positioned in proximity of the feeding
duct 16) is coaxially defined between a radially inner surface of
the annular body 10 and a radially outer surface of the tubular
body 17 of the distributor element 11. A second portion 20'' of
said conveying channel (i.e. the conveying channel portion
positioned in proximity of the exit of the extrusion head) is
defined between the male die 11 and the female die 12.
[0116] The conveying channel 20 is used for conveying the polymeric
material 2 to be deposited onto the outer surface of the elongated
element 3. To this purpose, the conveying channel 20 defines a
substantially annular and continuous passageway which is coaxial
with the longitudinal axis X-X.
[0117] The distribution channels 18 are formed on the outer surface
of the tubular body 17 and each channel is in fluid communication
with the feeding duct 16.
[0118] In the embodiment illustrated in FIG. 1, the distribution
channels 18 have a development of a curvilinear type, preferably of
helical type, and extend on radially opposite sides with respect to
the longitudinal axis X-X. Each distribution channel 18 carries out
the function of distributing the polymeric material entering the
inlet duct 15 as much homogeneously as possible in the conveying
channel 20 so as to allow a uniform production of the desired
coating layer 100.
[0119] Similarly to the annular body 10 and the tubular body 17 of
the distributor element 11, the male die 12 and the female die 13
are provided with an inner cavity 21 for allowing the elongated
element 3 to pass through, while the polymeric material 2--which
flows in the conveying channel 20--is deposited on the outer
surface of the elongated element 3.
[0120] In accordance with the present invention, the female die 13
is slidably associated with the annular body 10 so as to be axially
movable with respect to the male die 12.
[0121] According to the present invention, the provision of a
female die 13 which can be displaced with respect to the male die
12 allows the cross-sectional area of the second portion 20'' of
the conveying channel 20 to be modified during operation on the
basis of the extrusion speed variation of the polymeric material
2.
[0122] To this purpose, the extrusion head 1 comprises a device for
adjusting the cross-sectional area of the second portion 20'' of
the conveying channel 20 on the basis of said extrusion speed
variation. In particular, said device acts on the female die 13 to
adjust the position thereof with respect to the male die 12 along
the longitudinal axis X-X in response to the extrusion speed
variation.
[0123] According to an alternative embodiment (not shown), a
similar result can be achieved by providing a device for adjusting
the cross-sectional area of the second portion 20'' of the
conveying channel 20 which acts on a male die which is movable
along the longitudinal axis X-X with respect to a stationary female
die, i.e. to a female die that is in a fixed position.
[0124] In the embodiment illustrated in FIG. 1, the device for
adjusting the position of the female die 13 with respect to the
male die 12 along the longitudinal axis X-X comprises a resilient
element 22 which is interposed between the female die 13 and a
supporting element 23 at least partially fitted onto the annular
body 10 by any conventional fastening means (not shown). The
resilient element 22 illustrated in FIG. 1 is a spring. The
supporting element 23 houses the spring 22 and is provided, at a
free end thereof, with a passage 24 for allowing the elongated
element 3, coated with the polymeric material 2, to advance along
the direction A and to come out from the extrusion head 1.
[0125] The value of the elastic constant K of the resilient element
22 is calculated in such a way that the rigidity thereof can at
least partially counteract the force exerted on the female die 13
by the polymeric material 2 flowing through the conveying channel
20 whatever is the extrusion speed (and thus the flow rate) of said
material, said extrusion speed being comprised in the desired
extrusion speed variation.
[0126] With reference to the embodiment of the extrusion head
described above and illustrated in FIG. 1, the method according to
the present invention for depositing by extrusion a polymeric
material 2 on an elongated element 3 advancing in the extrusion
head 1 along a direction A to obtain the coating layer 100
comprises the following steps.
[0127] In a first step, after having conveyed the elongated element
3 within the longitudinal cavity 19 of the extrusion head 1, the
polymeric material 2 is fed into the feeding duct 16 of the
extrusion head through the inlet duct 15. The polymeric material 2
is caused to flow into the conveying channel 20 through the
distribution channels 18.
[0128] In a second step, the force exerted by the polymeric
material 2 on the female die 13 is at least partially counteracted
by the elastic force exerted by the spring 22 which allows an
adjustment of the cross-sectional area of the second portion 20''
of the conveying channel 20. Therefore, according to the present
invention, the cross-sectional area of the second portion 20'' of
the conveying channel 20 is automatically regulated by adjusting
the position of the female die 13 with respect to the male die 12
on the basis of the actual value of the desired extrusion speed
(and thus of the resulting flow rate of the polymeric material 2
flowing through the conveying channel 20).
[0129] Therefore, in operation, it is advantageously possible to
increase the flow rate of the polymeric material 2 flowing in the
extrusion head 1, for example in order to increase the extruded
element production yield, while ensuring at the same time that the
values of pressure, temperature and period of permanency do not
cause mechanical damages to the extrusion head 1 as well as
scorching or overheating of the polymeric material.
[0130] This aspect is shown in detail in the graphs reported in
FIGS. 5 and 6, wherein the variation of pressure and temperature,
respectively, of the polymeric material as a function of the flow
rate thereof through an extrusion head in accordance with the
present invention in comparison with a conventional extrusion head
provided with a fixed geometry of the conveying channel are
shown.
[0131] With reference to the graph shown in FIG. 5, the variation
of pressure as a function of the flow rate at three different
positions (X.sub.1, X.sub.2, X.sub.3) of the female die 13 along
the longitudinal axis X-X is shown. In detail, the value of
position X.sub.1 is smaller than the value of position X.sub.2 and
the value of position X.sub.2 is smaller than the value of position
X.sub.3 (i.e. X.sub.1<X.sub.2<X.sub.3) while considering the
female die axially moving according to the direction A, i.e.
axially departing from the male die, so that the cross-sectional
area of the second portion 20'' of the conveying channel is caused
to increase.
[0132] For each position of the female die, the variation of
pressure as a function of flow rate (i.e. the pressure/flow rate
curves indicated with references a, b, c respectively) is obtained
by varying the flow rate value of the polymeric material and
measuring the corresponding pressure value at the inlet duct of the
extrusion head by means of a pressure sensor. In the case a
conventional extrusion head (which is provided with a fixed
geometry of the conveying channel, i.e. the female die and the male
die are not reciprocally displaceable) is considered, the
stationary female die of which being located at the position
X.sub.1, an increase of the flow rate of the polymeric material
from Q.sub.1 to Q.sub.2 causes a corresponding increase of the
pressure from P.sub.1 to P.sub.2. In fact, since the female die is
stationary at the position X.sub.1, the only possible path for
passing from Q.sub.1 to Q.sub.2 is along curve a.
[0133] On the contrary, in the extrusion head of the present
invention, an increase of the flow rate from Q.sub.1 to Q.sub.2
causes a displacement of the female die position from X.sub.1 to
X.sub.3 so that it is possible to obtain the flow rate Q.sub.2 at a
pressure value P'.sub.2 which is smaller than P.sub.2 (the pressure
value P.sub.2 corresponds to the flow rate value Q.sub.2 while
moving along curve a, i.e. in the presence of a conventional
extrusion head) since, according to the present invention, the path
for passing from Q.sub.1 to Q.sub.2 is along curve S. The extremes
of curve S are used to calculate the elastic constant K of the
resilient element 22.
[0134] Similarly, with reference to the graph shown in FIG. 6, the
variation of the temperature as a function of the flow rate at
three different positions (X.sub.1, X.sub.2, X.sub.3) of the female
die 13 along the longitudinal axis X-X is shown. The female die
axially moves according to the direction A, i.e. axially departs
from the male die, so that the cross-sectional area of the second
portion 20'' of the conveying channel is caused to increase.
[0135] For each position of the female die, the variation of
temperature as a function of flow rate (i.e. the temperature/flow
rate curves indicated with references d, e, f respectively) is
obtained by varying the flow rate value of the polymeric material
and measuring the corresponding temperature value at the inlet duct
of the extrusion head by means of a temperature sensor. In the case
a conventional extrusion head (which is provided with a fixed
geometry of the conveying channel, i.e. the female die and the male
die are not reciprocally displaceable) is considered, the
stationary female die of which being located at the position
X.sub.1, an increase of the flow rate of the polymeric material
from Q.sub.1 to Q.sub.2 causes a corresponding increase of the
temperature from T.sub.1 to T.sub.2. In fact, since the female die
is stationary at the position X.sub.1, the only possible path for
passing from Q.sub.1 to Q.sub.2 is along curve d.
[0136] On the contrary, in the extrusion head of the present
invention, an increase of the flow rate from Q.sub.1 to Q.sub.2
causes a displacement of the female die position from X.sub.1 to
X.sub.3 so that it is possible to obtain the flow rate Q.sub.2 at a
temperature value T'.sub.2 which is smaller than T.sub.2 (the
temperature value T.sub.2 corresponds to the flow rate value
Q.sub.2 while moving along the curve d, i.e. in the presence of a
conventional extrusion head) since, according to the present
invention, the path for passing from Q.sub.1 to Q.sub.2 is along
curve S'. The extremes of curve S' are used to calculate the
elastic constant K of the resilient element 22.
[0137] Thus, with reference to the embodiments of the extrusion
head 1 described above, the present invention allows to increase
the flow rate of the polymeric material 2 flowing through the
extrusion head 1 while ensuring at the same time that the values of
pressure and temperature remain within an acceptable range of
values, so as to avoid that critical flow conditions in the
extrusion head can occur.
[0138] On the other hand, the present invention makes also
advantageously possible to decrease the flow rate of the polymeric
material in the extrusion head while ensuring, at the same time,
that the values of pressure and period of permanency of the
polymeric material remain within respective acceptable range of
value, so as to avoid the formation of stagnation zones as well as
scorching or overheating of the material being extruded.
[0139] This aspect is shown in the graph illustrated in FIG. 7,
wherein the period of permanency of the polymeric material as a
function of the flow rate at three different positions (-X.sub.1,
-X.sub.2, -X.sub.3) of the female die 13 along the longitudinal
axis X-X is shown. The three different positions are indicated with
negative values since in this case the movement of the female die
with respect to the male die occurs in a direction opposite to that
of arrow A; in fact, the cross-sectional area of the second portion
20'' of the conveying channel is caused to decrease.
[0140] For each position of the female die, the variation of the
period of permanency as a function of the flow rate (i.e. the
period of permanency/flow rate curves indicated with references g,
h, i respectively) is obtained by varying the flow rate value of
the polymeric material and calculating the corresponding period of
permanency in the extrusion head. In the case a conventional
extrusion head (which is provided with a fixed geometry of the
conveying channel, i.e. the female die and the male die are not
reciprocally displaceable) is considered, the stationary female die
of which being located at the position -X.sub.1, a reduction of the
flow rate of the polymeric material from Q.sub.1 to Q.sub.2 causes
a corresponding increase of the period of permanency from t.sub.1
to t.sub.2. However, the value t.sub.2 is quite close to the
critical value t.sub.scorch,2 which represents the period of
permanency at which scorching of the polymeric material occurs
(indicated by the curve t.sub.scorch). In fact, since the female
die is stationary at the position -X.sub.1, the only possible path
for passing from Q.sub.1 to Q.sub.2 is along curve g.
[0141] On the contrary, in the extrusion head of the present
invention, a reduction of the flow rate from Q.sub.1 to Q.sub.2
causes a displacement of the female die position from -X.sub.1 to
-X.sub.3 so that it is possible to obtain the flow rate Q.sub.2 at
a period of permanency t'.sub.2 which is smaller than t.sub.2 (the
period of permanency t.sub.2 corresponds to the flow rate value
Q.sub.2 while moving along the curve g, i.e. in the presence of a
conventional extrusion head) since, according to the present
invention, the path for passing from Q.sub.1 to Q.sub.2 is along
curve S''. The extremes of curve S'' are used to calculate the
elastic constant K of the resilient element 22. Therefore,
according to the present invention, the period of permanency
increases from t.sub.1 to t'.sub.2, the latter being smaller than
t.sub.2 and far away from the critical value t.sub.scorch,2.
[0142] In FIGS. 2 to 4 further embodiments of the extrusion head
according to the present invention are shown.
[0143] The elements of the extrusion head which are structurally
and/or functionally equivalent to those previously illustrated with
reference to FIG. 1 are indicated with the same reference numbers.
The embodiments shown in FIGS. 2 to 4 differ from that reported in
FIG. 1 in that the device for adjusting the position of the female
die 13 with respect to the male die 12 along the longitudinal axis
X-X comprises, in place of the resilient element 22, a servo-device
32 which detects the extrusion speed variations (and thus the
polymeric material flow rate and pressure variations) and adjusts
the position of the female die 13 with respect to the male die 12
along said longitudinal axis X-X on the basis of the detected
variations.
[0144] In particular, in the embodiments illustrated in FIGS. 2 to
4, the servo-device 32 comprises a pressure sensor 33 associated
with the extrusion head 1 at the inlet duct 15.
[0145] The servo-device 32 further comprises a processing device 34
operatively associated with the pressure sensor 33. The processing
device 34 calculates the new positions of the female die 13 along
the longitudinal axis X-X in response to the variations detected by
the pressure sensor 33.
[0146] The servo-device 32 further comprises a device 35 for moving
the female die 13 to the calculated new positions, said device 35
being operatively associated with the processing device 34.
[0147] In the embodiment illustrated in FIG. 2, the device 35 for
moving the female die 13 to the new positions calculated by the
processing device 34 comprises a hydraulic actuator which includes
a pump 40 and a hydraulic cylinder 45 reciprocally connected by
means of a connecting duct 41.
[0148] The pump 40 is operatively associated with the processing
device 34, while the hydraulic cylinder 45 comprises a stem 46 that
is connected to the female die by the interposition of a crank gear
47.
[0149] According to this embodiment, a position sensor 48 is
associated with the stem 46 of the hydraulic cylinder 45 with the
processing device 34 to detect the position of the stem 46 (this
position corresponding to a respective position of the female die
13) and to send a corresponding electrical signal to the processing
device 34 to allow the latter to calculate a possible new position
for the stem 46.
[0150] According to a further embodiment of the present invention
(not shown), the hydraulic actuator is replaced by a pneumatic
actuator. Specifically, the pump 40 is replaced by a container of
pressurized fluid, while the hydraulic cylinder 45 is replaced by a
pneumatic cylinder.
[0151] FIG. 3 shows a further embodiment of the extrusion head 1 of
the present invention.
[0152] The elements of the extrusion head which are structurally
and/or functionally equivalent to those previously illustrated with
reference to FIGS. 1 and 2 are indicated with the same reference
numbers.
[0153] The embodiment shown in FIG. 3 differs from that of FIG. 2
in that the device 35 for moving the female die 13 to the new
position calculate by the processing device 34 comprises a linear
actuator 55. The linear actuator 55 is provided with a driving
means (not shown) which are operated and regulated by a processor
50.
[0154] According to this embodiment, a position sensor 58 is
associated with the linear actuator 55 and with the processing
device 34 to detect the position of the female die 13 and to send a
corresponding electrical signal to the processing device 34 to
allow the latter to calculate any possible new position for the
female die 13.
[0155] FIG. 4 shows a further embodiment of the extrusion head 1 of
the present invention.
[0156] The elements of the extrusion head which are structurally
and/or functionally equivalent to those previously illustrated with
reference to FIGS. 1, 2 and 3 are indicated with the same reference
numbers.
[0157] The embodiment of FIG. 4 differs from that of FIGS. 2 and 3
in that the device 35 for moving the female die 13 to the new
position calculated by the processing device 34 comprises a gear
electro-mechanical device including a gear mechanism 65 driven by
an electric motor 66 and coupled with the female die 13. The gear
mechanism 65 which is coupled with the female die 13 is housed in a
supporting element 23 of the type illustrated and disclosed with
respect to FIG. 1. The gear electro-mechanical device further
comprises a processor 67 which operates and regulates the electric
motor 66. The processor 67 is associated with the electric motor 66
and with the processing device 34.
[0158] According to this embodiment, a position sensor 68 is
associated with the electric motor 66 and with the processing
device 34 to detect the position of female die 13 and to send a
corresponding electrical signal to the processing device 34 to
allow the latter to calculate a possible new position of the female
die 13.
[0159] With reference to the preferred embodiments of the extrusion
head 1 described above and illustrated in FIGS. 2 to 4, the method
of the present invention for depositing by extrusion a polymeric
material 2 on an elongated element 3 advancing within the extrusion
head 1 along a direction A comprises the following steps.
[0160] In a first step, similarly to the method described above
with respect to the extrusion head 1 illustrated in FIG. 1, after
having conveyed the elongated element 3 within the longitudinal
cavity 19 of the extrusion head 1, the polymeric material 2 is fed
to the feeding duct 16 of the extrusion head through the inlet duct
15 by one or more extruder screws (known per se and not shown in
the figures). The polymeric material 2 is caused to flow into the
conveying channel 20 through the distribution channels 18.
[0161] In a second step, the pressure sensor 33 detects--preferably
at a predetermined frequency value--the pressure at the inlet duct
of the extrusion head (said pressure value being correlated to the
flow rate value and the latter being, in turn, correlated to the
extrusion speed value) and generates a corresponding electrical
signal which is sent to the processing device 34.
[0162] Once a pressure variation is detected, in a third step of
the method of the present invention the processing device 34
calculates a new position of the female die 13 along the
longitudinal axis X-X on the basis of the detected variation and
sends a corresponding signal to the actuator device 35 which moves
the female die 13 to the calculated new position, thus adjusting
the cross-sectional area of the second portion 20'' of the
conveying channel 20. As mentioned above, since this
cross-sectional area is correlated to the pressure (and thus to the
flow rate) of the polymeric material flowing through the extrusion
head, the possibility of adjusting this area allows to extend the
working field of the extrusion head. In other words, according to
the present invention it is possible to increase the range of
variation of the flow rate of the polymeric material flowing
through the extrusion head while ensuring that the other process
parameters (in particular, temperature and period of permanency)
remain within acceptable ranges of values so that critical flow
conditions (and thus scorches, overheating, stagnations of the
polymeric material as well as mechanical damages of the extrusion
head) do not substantially occur.
[0163] The considerations given herein above with respect to the
graphs reported in FIGS. 5, 6 and 7 apply also to the embodiments
shown in FIGS. 2 to 4, with the only exceptions that the extremes
of curves S' and S'', along which the female die 13 moves from
position |X.sub.1| to position |X.sub.3|, are used to calculate the
calibration of the actuator device 35 instead of the elastic
constant K of the resilient element 22 of FIG. 1.
* * * * *