[0001] The invention relates to a method for continuous lateral stretching of films to a
constant width. The invention relates in particular to a method for lateral stretching
of reticulate film material to a constant width. By 'reticulate sheet material' is
understood film material with a large number of openings, offering very little resistance
to lateral stretching.
[0002] Various methods for stretching a web of film or fabric in a direction normal to the
length are known in the plastics and textile industries.
[0003] Such stretching may be done, for instance, as described in the French patent specification
No.1.020.595 and in the Swiss patent specification No.461.419, by means of a bent
stretching bar along which the film is drawn. In this method the degree of stretching
depends on the bending radius of the stretching bar, the tensile force in the film,
and the coefficient of friction between te film and the stretching bar.
[0004] Another method makes use of a stretching roller, as represented in the Dutch patent
application No.7015574, fig. 3. The circumference of this roller is fitted with single-threaded
or multiple-headed helical ridges that extend towards the two extremities of the roller
and are arranged so that their pitches are opposite. This stretching roller acts as
a kind of screwing conveyor that forces the fibres towards the ends of the roller.
[0005] Still another method is that employing the so-called banana roller. This roller consists
of a bent shaft to which rollers have been fitted independently of each other. A sleeve
of flexible material is slipped around these rollers. If a sheet under tension is
passed over the flexible outer sleeve in a direction normal to the length of the roller,
the outer sleeve will rotate owing to friction. The sleeve then makes an outward movement
between the point with the smallest and the point with the greatest bending radius.
The film material follows this movement owing to friction.
[0006] A device that is widely used in the textile and plastics industries for lateral stretching
of fabrics and films and with which very great stretching forces can be applied is
the so-called stretching frame. A device like this has been described in, inter alia,
the Netherlands patent application.No.6802553. This device consists of two chain tracks
running in guides according to a desired pattern. Both chains are provided with clamps
or needles that grip the film and carry it along in the direction of transport of
the chains. The width of the stretched film depends on the distance between the chain
tracks at the delivery end. A problem encountered in this method of stretching is
to make allowance for the reduction in length that occurs during lateral stretching.
[0007] The proper functioning of the above equipment, which is used for lateral stretching
of film and fabrics, mostly depends on the dimensional stability of the material to
be stretched and/or the resistance of the film to stretching. The dimensional stability
of, for instance, fibrillated film is very little and the resistance to lateral stretching
is virtually negligible, so that the equipment in question is unsuitable for processing
of this type of material.
[0008] The US patent No.3.838.481 relates to a device for stretching of fabrics or similar
material, the side edges of the web being passed over tensioning rollers fitted alongside
the web. These tensioning rollers are motor-driven and the degree of stretching is
adjusted by changing the speed of rotation of the rollers and, secondly, by changing
the angular position of the rollers. This results in translation of the web together
with the rollers, so that the web is tensioned and also assumes a different position.
By changing the angular position of the rollers, the force exerted by the web on the
rollers is enlarged or diminished, so that the transversal transport of the web proportionally
increases or decreases, respectively. The points of rotation of the rollers are situated
relatively far from the side edges of the web, so that changing of the angular position
of the rollers causes undesired deformations and displacements of the side edges.
This method is therefore unsuitable for accurate width control of reticulate sheet.
[0009] It is difficult, if not impossible, with all the above-described devices to stretch
films to a constant width, while the achieved widening is furthermore limited.
[0010] In further processing of the stretched material, which may be used for various purposes,
i.a. for reinforcement of clothing, building material, etc., it is of importance that
the film has a constant width over its entire length. If it has not, the film will
have to be cut to the desired length, which results in much waste, while deviations
in width may furthermore cause differences in strength owing to irregular stretching.
[0011] The object of the invention is to eliminate said drawbacks and to provide a method
of stretching film material to a desired constant width.
[0012] According to the invention, this is achieved by passing the unstretched web of reticulate
film - possibly via guideand/or tensioning rollers - to pairs of clamping rollers
mounted on either side of the web which clamp the web at the desired width and which
are so operated by a controlling mechanism that, when the desired web width is deviated
from, the angle included by the clamping roller shafts and the sides of the web in
the plane of the web is readjusted.
[0013] By 'pairs of clamping rollers' is understood sets of rollers of which the treads
are in contact with each other so as to clamp a web of film material which is fed
through. These pairs of clamping rollers have appeared to be very suitable for lateral
stretching of reticulate film material to a constant width.
[0014] Readjustment of the angle included by the clamping roller shafts and the sides of
the web of reticulate material is effected by turning the clamping rollers around
an vertical axis which is situated within the width of the clamping rollers and preferably
coincides with or is near to the desired position of the side edges of the stretched
material.
[0015] When the desired web width is deviated from, it is also possible to readjust the
distance between the pairs of clamping rollers by means of a controlling mechanism.
[0016] The controlling mechanism for readjustment of the angle included by the clamping
roller shafts and the sides
Qf the web in the plane of the web may consist of photocells that are mounted after
the pairs of clamping rollers, as seen in the direction of transport of the web, and
which, when the desired web width is deviated from, send a signal to a transmitter
coupled to a servo mechanism that operates a hydraulic cylinder whose piston is pivoted
to the pairs of clamping rollers, so that the angle included by the clamping roller
shafts and the sides of the web in the plane of the web can be readjusted.
[0017] Another possibility of controlling said readjustment consists in that photocells
have been mounted before the pairs of clamping rollers, as seen in the direction of
transport of the web, which ensure that the side edges of the web, run, e.g. midway
between the pairs of clamping rollers fitted on either side of the web. When deviations
occur, readjustment is effected in the same way as indicated in the first-mentioned
possibility of readjustment.
[0018] It is also possible to have the distance between the pairs of clamping rollers readjusted
by photocells that, when the desired web width is deviated from, send a signal to
a transmitter coupled to a hydraulic cylinder whose piston rod is so coupled, by means
of a guide, to the pairs of clamping rollers that they can be moved laterally to the
direction of transport of the web. This possibility of readjustment can be used in
addition to the before mentioned readjustment.
[0019] After the pairs of clamping rollers, as seen in the direction of transport, there
are preferably pairs of rollers at either side of the web, which pairs of rollers
are driven independently of each other and between which the side edges of the web
are clamped. The driving mechanisms of the pairs of rollers are controlled by photocells
positioned at either side of the web, before the first-mentioned pairs of clamping
rollers. When during the stretching operation one of the sides of the web sags, for
instance because an excess of material is fed to one of the sides, the relevant photocell
will send a signal to the driving mechanism which causes the relevant pair of rollers
to run faster until the side edge has been readjusted. It is evident that when both
side edges sag, the photocells at both sides will respond, speeding up the driving
mechanisms of both pairs of rollers until readjustment has been effected.
[0020] After the driven pairs of rollers, as seen in the direction of transport, there can
be at either side of the web so-called side stretchers for the side edges of the web,
which have not been stretched yet. These stretchers consist of two small plates, one
placed at a small distance above the other, between which the side of the sheet are
passed. The upper or the lower plate is provided with little holes, oriented laterally
as well as in the direction of transport, through which a medium, for instance air
or water, can be blown in order to stretch the side edges of the sheet. By means of
chain guides, the stretched reticulate film is subsequently passed through a stabilization
unit. The distance between the chains at either side of the stabilization unit can
be varied, if necessary.
[0021] It has been found that in the above-described way reticulate film material can be
continuously stretched laterally to a constant width without complicated and, hence,
expensive equipment being required. The method according to the invention is particularly
suitable for lateral stretching of a material that offers little resistance in this
respect, e.g. a maximum force of 1 N on the pairs of clamping rollers at a lateral
stretch of 1000%.
[0022] The invention also relates to a device for effecting the method according to the
invention.
[0023] The device according to the invention consists of:
- guide and/or tensioning rollers for the unstretched web of reticulate material;
- pairs of clamping rollers fitted on either side of the web at the desired web width,
and also photocells which monitor the side edges of the web and which are coupled
to controlling mechanisms for the pairs of clamping rollers;
- photocells before the pairs of clamping rollers, coupled to the driving units for
pairs of rollers after the pairs of clamping rollers;
- side stretchers for the unstretched side edges of the web.
[0024] Preferably, the device further comprises a device for stabilization of the stretched
web at the desired width, chain guides for the web in the stabilization device, guide
rollers for the stabilized web, and, finally, a winding reel and pressure roller for
the stabilized web of reticulate material.
[0025] The invention will be further elucidated with reference to a non-restrictive, schematic
drawing, in which:
Fig. 1 is a bottom-view of the stretching device.
Fig. 2 is a side view of the stretching device.
[0026] On the reel 1 is a web, with a width varying from a few centimetres to several metres,
of polypropylene film fibrillated by, e.g., rollers with pins, which is unwound at
a rate of up to about 150 m/min., say 50 m/min., and is passed to the pairs of clamping
rollers 5 and 6, possibly by way of a guide roller 2, a tensioning roller 3 and another
guide roller 4. It is also possible to feed more than one web of the film material,
one above the other in which case a corresponding number of pairs of tensioning rollers,
one above the other, may be applied. In a preferred mode of realization of the invention,
a package consisting of several webs of the film, one on top of the other and connected
to each other at the side edges, is stretched by the method according to the invention.
The pairs of clamping rollers 5 and 6 are mounted on either side of the web at a relative
distance corresponding to the desired web width after stretching, e.g. from one to
several metres. The stretched web leaving the pairs of rollers 5 and 6 is passed over
the driven rollers 7a, 7a' and 7b, 7b' and the side stretchers 28 and 29 to the stabilization
device 8, in which the reticulate film is clamped in between chains at the sides and
passed through the device 8 in which the web is stabilized at the desired width thermally
or by spraying on a stabilizing agent.
[0027] In the case of several webs above each other, the side edges of the separate webs
can also be welded together after the stretching operation and the webs thus united
are passed through the stabilizer as one single web. The stabilized web leaves the
device 8 and is wound onto reel 11 by way of rollers 9, 9' and 10. Roller 12 acts
as a pressure roller and can slide in guide 13.
[0028] The tension in the web is controlled by a tensioning roller 3, which is known in
itself, coupled to the electromagnetic brake 15 via potentiometer 14.
[0029] The desired width of the stretched web is constantly checked by the photocells 16
and 17. When the web width is deviated from, these photocells 16 and 17 send a signal
to the transmitter 18 and 19 and then to the servo mechanisms 20 and 21, each of which
operates a hydraulic cylinder 22 or 23, whose piston rod 24 or 25 is pivoted to the
pairs of clamping rollers 5 and 6, so that the angle included by these rollers and
the sides of the web in the plane of the web can be readjusted, which enables the
web to be stretched to the correct width. The axes of rotation 5' and 6' of the clamping
rollers coincide with or are near to the desired position of the side edges of the
material to be stretched. Another possibility of obtaining the correct web width consists
in the installation of photocells 26 and 27 before the pairs of clamping rollers 5
and 6, as seen in the direction of transport of the web. These photocells ensure that
the side edges of the web run, e.g., midway between the clamping rollers. When deviations
occur, the angular position of the pairs of clamping rollers 5 and 6 will be readjusted
in the'same way as in the above possibility of readjustment.
[0030] When one of the side edges la or lb sags, for instance as indicated by the dotted
line 1b', the photocell 30 will send a signal to the driving mechanism of the rollers
7b and 7b' which speeds up the rollers until readjustment has been effected. In that
case, the photocell 31 will send a signal to the driving mechanisms of the rollers
7a and 7a' so that these are speeded up until readjustment has been effected.
[0031] An additional possibility of readjustment, particularly intended to offset major
irregularities in the web fed in, enables the distance between the pairs of clamping
rollers in the direction normal to the web to be changed when the web width is deviated
from. To this end, photocells 16' and 17' can send such a signal to a transmitter,
servo mechanisme and a hydraulic cylinder whose piston is coupled to the pairs of
clamping rollers by way of a guide that the relative distance between the pairs of
clamping rollers 5 and 6 is readjusted when the web width is deviated from. This possibility
of readjustment may be combined with one of the above possibilities of readjustment.
[0032] As mentioned before, there can be side stretchers 28 and 29 after the pairs of rollers
7a, 7a' and 7b and 7b' as seen in the direction of transport. These stretchers consist
of two small plates, one placed at a small distance above the other, between which
the side edges of the web are passed. The upper or the lower plates are provided with
little holes, oriented laterally as well as forward, through which compressed air
or another medium is supplied in order to stretch the side edges of the web.
[0033] The devive for effecting the method according to the invention consists of, for instance,
a feed reel 1 on which unstretched sheeting has been wound, possibly guide rollers
2 and 4, a tensioning roller 3 which is coupled to the potentiometer 14 that operates
the brake 15 on the reel 1, pairs of clamping rollers 5 and 6 with axes of rotation
5' and 6', mounted on either side of the web, controlling mechanism 16 through 27
for the pairs of clamping rollers 5 and 6 and the pairs rollers 7a, 7a' and 7b, 7b'
with photocells 30 and 31, side stretchers 28 and 29, a device 3 with guide chains
for the stretched web for stabilization of this web at the desired width, guide rollers
9,9' for the stabilized web, and finally a winding reel 11 and a pressure roller 12
for the stabilized web of reticulate material, and a guide 13 for the pressure roller
12.
1. Method for continuous lateral stretching of filmsto a constant width, characterized
in that an unstretched web of reticulate film material is passed to pairs of clamping
rollers mounted at either side of the web which clamp the web at the desired width
and which are so controlled by a controlling mechanism that, when the desired web
width is deviated from, the angle included by the clamping roller shafts and the sides
of the web in the plane of the web is readjusted.
2. Method according to claim 1, characterized in that the web of reticulate film material
is passed to the pairs of clamping rollers by way of guide and/or tensioning rollers.
3. Method according to claim 1-2 characterized in that readjustment of the angle included
by the clamping roller shafts and the sides of the web in the plane of the web is
effected by turning clamping rollers around an axis which coincides with or is near
to the desired position of the side edges of the stretched material.
4. Method according to the claims 1-3, characterized in that the distance between
the pairs of clamping rollers, as seen in the direction normal to the web, is also
readjusted by a controlling mechanism when the desired web width is deviated from.
5. Method according to claim 1-4, characterized in that the controlling mechanism
for readjustment of the angle included by the clamping rollers shafts and the sides
of the web in the plane of the web consists of photocells that are mounted after the
pairs of clamping rollers, as seen in the direction of transport of the web, and which,
when the desired web width is deviated from, send a signal to a transmitter coupled
to a servo mechanism that operates a hydraulic cylinder whose piston rod is pivoted
to the pairs of clamping rollers.
6. Method according to claim 1-5, characterized in that the controlling mechanism
for readjustment of the angle included by the clamping roller shafts and the sides
of the web in the plane of the web consists of photocells that are mounted before
the pairs of clamping rollers, as seen in the direction of transport of the web, and
which, when the desired web width is deviated from, send a signal to a transmitter
coupled to a servo mechanism that operates a hydraulic cylinder whose piston is connected
to the pairs of clamping rollers.
7. Method according to the claims 1-6, characterized in that pairs of rollers mounted
after the pairs of clamping rollers are driven independently of each other, the driving
mechanisms being controlled by photocells which are mounted at either side of the
web before the pairs of clamping rollers and which, when one of the sides of the web
sags, send a signal to one of the driving mechanisms so that it runs at a higher speed
until the sagging has been eliminated.
8. Method according to the claims 1-7, characterized in that after the driven pairs
of rollers, as seen in the direction of transport of the web, there are side stretchers,
which stretch the unstretched sides of the web by means of compressed air or another
medium.
9. Method according to claims 1-8, characterized in that, when the desired weg width
is deviated form, a signal is furthermore sent by photocells to a transmitter coupled
to a servo mechanism that operates a hydraulic cylinder whose piston rod is coupled,
by means of a guide, to the pairs of clamping rollers, so that the distance between
the pairs of clamping rollers normal to the web is readjusted.
10. Method according to the claims 1-9, characterized in that the web of reticulate
material to be stretched laterally offers very little resistance to lateral stretching.
11. Method according to claims, 1-10 characterized in that a package consisting of
several webs of reticulate material, one on top of the other and connected to each
other at the side edges is laterally stretched to a constant width, after which the
package of reticulate webs is passed through a stabilization divice.
12. Method as described, and represented in the drawing.
13. Device for continuous lateral stretching of a web of reticulate material to a
constant width, characterized in that the device comprises:
- guide and/or tensioning rollers (2,4 and 3, respectively) for the web of unstretched
reticulate material;
- pairs of clamping rollers (5 and 6) mounted at either side of the web at the desired
web width, as well as photocells (16, 17) monitoring the side edge of the web and
coupled to controlling mechanisms for the pairs of clamping rollers;
- axes of rotation for the pairs of clamping rollers (5 and 6), situated within the
width of the clamping rollers.
14. Device according to claim 13, characterized in that the device is also provided
with:
- photocells (30 and 31) before the pairs of clamping rollers (5 and 6), coupled to
the driving mechanisms for the driven pairs of rollers (7a, 7a' and 7b, 7b') after
the pairs of clamping rollers;
- side stretchers (23 and 29) for the unstretched side edges of the web.