Title: Method and machine for producing multiaxial fibrous webs
Abstract: A plurality of unidirectional sheets (30a, 30b, 30c) are superposed in different directions and they are bonded together. At least one of the unidirectional sheets is made by spreading a tow so as to obtain uniform thickness, width not less than 5 cm, and a weight of no more than 300 g/m2, cohesion being imparted to the sheet so as to enable it to be handled prior to being superposed with other sheets. Advantageously, the unidirectional sheets are made of carbon fibers and are obtained by spreading out large tows.
Patent Number: 6,919,118 Issued on 07/19/2005 to Bompard, et al.
| Inventors:
|
Bompard; Bruno (Lyons, FR);
Olry; Pierre (Bordeaux, FR);
Duval; Renaud (Couzon au Mont d'Or, FR);
Bruyere; Alain (Villefontaine, FR);
Coupe; Dominique (Le Haillan, FR);
Aucagne; Jean (La Tour Du Pin, FR)
|
| Assignee:
|
Societe Nationale d'Etude et de Construction de Moteurs d'Aviation-SNECMA (Paris, FR);
Hexcel Reinforcements (Villeurhanne, FR)
|
| Appl. No.:
|
330960 |
| Filed:
|
December 27, 2002 |
Foreign Application Priority Data
| Current U.S. Class: |
428/105; 428/219; 428/102; 428/114; 428/137; 428/220; 428/364; 428/401 |
| Intern'l Class: |
B32B 005/12 |
| Field of Search: |
428/102,105,109,107,114,123,137,212,219,220,364,392,401,408,375,396
156/305
423/447.1,447.2
|
References Cited [Referenced By]
U.S. Patent Documents
| 3250655 | May., 1966 | Adler.
| |
| 3566733 | Mar., 1971 | McClean.
| |
| 3953641 | Apr., 1976 | Marquis.
| |
| 4256522 | Mar., 1981 | Britton.
| |
| 4325999 | Apr., 1982 | Campman et al.
| |
| 4823564 | Apr., 1989 | Wunner.
| |
| 5334419 | Aug., 1994 | Minami et al.
| |
| 5688577 | Nov., 1997 | Smith et al.
| |
| 5945356 | Aug., 1999 | Pott.
| |
| 6319348 | Nov., 2001 | Olry et al.
| |
| Foreign Patent Documents |
| 0 272 088 | Jun., 1988 | EP.
| |
| 0 330 980 | Sep., 1989 | EP.
| |
| 2 185 497 | Jan., 1974 | FR.
| |
| 1190214 | Apr., 1970 | GB.
| |
| 1 447 030 | Aug., 1976 | GB.
| |
Other References
"Faserverbundbauweisen", Chapter "Fasern und Matrices" by Manfred Flemming et
al. (Springer-Verlag 1995) pp. 15, 104 and 105.
"Faserverbundbauweisen", Chapter "Halbzeuge und Bauweisen" by Manfred Flemming
et al. (Spring-Verlag 1996), pp. 51, 52, 54-57, 70-71, 107, 108 and 115.
"Einfuhrung in die technologie der Faserverbundwerkstoffe" by W. Michaeli et
al. (Carl Hanser Verlag, 1989), pp. 59-65.
|
Primary Examiner: Dixon; Merrick
Attorney, Agent or Firm: Weingarten, Schurgin, Gagnebin & Lebovici LLP
Parent Case Text
This application is a divisional application under 35 U.S.C. §120 of U.S.
application Ser. No. 09/381,941, filed Sep. 27, 1999, now U.S. Pat. No. 6,585,842,
with is a US National Phase entry of PCT Application No. PCT/FR98/00597, filed
Mar. 25, 1998.
Claims
1. A fibrous unidirectional sheet showing transverse cohesion and constituted
by juxtaposed unidirectional strips obtained from spreaded tows having at least
12 K filaments per tow, the sheet weighing not more than 300 g/m
2 and
being of width of not less than 5 cm.
2. A sheet according to claim 1, characterized in that it is made of fibers of
a material selected from carbon, ceramics, carbon or ceramic precursors, glasses,
and aramids.
3. A sheet according to claim 1, characterized in that it is made of continuous filaments.
4. A sheet according to claim 1, characterized in that it is made of discontinuous filaments.
5. A sheet according to claim 3, characterized in that cohesion is imparted thereto
by the presence of a bonding agent.
6. A sheet according to claim 5, characterized in that the bonding agent is suitable
for being eliminated.
7. A sheet according to claim 6, characterized in that the bonding agent is a
water-soluble polymer.
8. A sheet according to claim 4, characterized in that cohesion is imparted thereto
by lightly matting discontinuous filaments.
9. A sheet according to claim 4, characterized in that cohesion is imparted thereto
by needling.
10. A composite material part comprising fiber reinforcement densified by a matrix,
the part being characterized in that the fiber reinforcement comprises at least
one unidirectional sheet according to claim 1.
11. A multiaxial fiber sheet in the form of a continuous strip having a longitudinal
direction, comprising a plurality of superposed unidirectional sheets of difference
directions that are bonded together, the sheet being characterized in that it includes
at least one unidirectional sheet according to claim 2.
12. A multiaxial sheet according to claim 11, characterized in that it is constituted
by two unidirectional sheets at angles of +45° and -;45° to the longitudinal
direction of the multiaxial sheet.
13. A multiaxial sheet according to claim 11, characterized in that it comprises
a main unidirectional sheet oriented in the longitudinal direction of the multiaxial
sheet and at least two transverse longitudinal sheets each disposed on a respective
face of the main sheet and extending in directions that make opposite angles with
the direction of the main sheet.
14. A composite material part comprising fiber reinforcement densified by a matrix,
the part being characterized in that the fiber reinforcement comprises at least
one multiaxial sheet according to claim 11.
15. A sheet according to claim 2, characterized in that:
it is made of continuous or discontinuous filaments;
cohesion is imparted thereto by the presence of a bonding agent;
the bonding agent is suitable for being eliminated and is a water-soluble polymer.
16. A composite material part comprising fiber reinforcement densified by a matrix,
the part being characterized in that the fiber reinforcement comprises at least
one unidirectional sheet according to claim 8.
17. A composite material part comprising fiber reinforcement densified by a matrix,
the part being characterized in that the fiber reinforcement comprises at least
one unidirectional sheet according to claim 9.
18. A multiaxial fiber sheet in the form of a continuous strip having a longitudinal
direction, comprising a plurality of superposed unidirectional sheets of different
directions that are bonded together, the sheet being characterized in that it includes
at least one unidirectional sheet according to claim 8.
19. A multiaxial fiber sheet in the form of a continuous strip having a longitudinal
direction, comprising a plurality of superposed unidirectional sheets of different
directions that are bonded together, the sheet being characterized in that it includes
at least one unidirectional sheet according to claim 9.
20. A multiaxial sheet according to claim 18, characterized in that it is constituted
by two unidirectional sheets at angles of +45° and -;45° to the longitudinal
direction of the multiaxial sheet.
21. A multiaxial sheet according to claim 19, characterized in that it is constituted
by two unidirectional sheets at angles of +45° and -;45° to the longitudinal
direction of the multiaxial sheet.
22. A multiaxial sheet according to claim 18, characterized in that it comprises
a main unidirectional sheet oriented in the longitudinal direction of the multiaxial
sheet and at least two transverse longitudinal sheets each disposed on a respective
face of the main sheet and extending in directions that make opposite angles with
the direction of the main sheet.
23. A multiaxial sheet according to claim 19, characterized in that it comprises
a main unidirectional sheet oriented in the longitudinal direction of the multiaxial
sheet and at least two transverse longitudinal sheets each disposed on a respective
face of the main sheet and extending in directions that make opposite angles with
the direction of the main sheet.
Description
FIELD OF THE INVENTION
The invention relates to making fiber sheets, and more particularly multiaxial
sheets formed by superposing and linking together a plurality of unidirectional
fiber sheets disposed in different directions.
A field of application of the invention lies in making multiaxial fiber sheets
for forming reinforcing plies for preparing composite material parts. The intended
materials are particularly those constituted by fiber reinforcement which can be
organic or inorganic, or precursors therefor such as fibers of polymer, glass,
carbon, ceramic, para-aramid, . . . , which reinforcement is densified by an organic
matrix, e.g. a resin, or an inorganic matrix, e.g. glass, carbon, or ceramic.
STATE OF THE ART
It has been known for a long time to make multiaxial fiber sheets by superposing
unidirectional sheets, i.e. made up of threads or fibers that are oriented essentially
in a single direction, the unidirectional sheets being superposed in different directions.
A common technique consists initially in making the unidirectional fiber sheets,
and in giving them sufficient cohesion to enable them to be handled without dispersing
the elements making them up.
A commonly proposed solution consists in bonding together the elements forming
the warp of the unidirectional sheets by threads extending in the weft direction.
This inevitably results in undulations being formed which, when a plurality of
sheets are superposed and pressed against one another, can cause fibers to be crushed
and broken, thereby creating discontinuities. That degrades the multiaxial sheets
made in that way and consequently degrades the mechanical properties of the composite
material parts prepared from such multiaxial sheets.
To remedy that drawback, a well-known solution consists in using bonding threads
of number and weight that are as small as possible. Document GB-A-1 190 214 (Rolls
Royce Limited) concerning sheets of carbon precursor fibers, and document FR-A-1
469 065 (Les Fils d'Auguste Chomarat & Cie), concerning sheets of glass fibers,
illustrate that approach. Nevertheless, it is clear that the above-mentioned drawback
is diminished but not eliminated.
It is also proposed in document EP-A-0 193 478 (Etablissements Les Fils d'Auguste
Chomarat & Cie) to use bonding fibers but made of a heat-fusible material. During
the preparation of the composite material, the temperatures used can cause the
bonding threads to melt at least in part, thereby reducing the extra thickness
where they cross the warp elements. However it is necessary for the material of
the bonding fibers to be compatible with the nature of the matrix of the composite
material, which greatly limits the use of that method.
Another solution mentioned in document FR-A-1 394 271 (Les Fils d'Auguste
Chomarat & Cie) consists in placing glass fiber threads parallel to one another
and in bonding them together chemically, the binder used being soluble in the matrix.
In that case also, the need for compatibility between the binder and the matrix
limits applications of the method. Furthermore, no means is described to enable
the threads to be placed parallel to one another, and it will readily be understood
that making wide sheets on an industrial scale gives rise to real practical difficulties.
Finally, the resulting sheet is not free from undulations resulting from the threads
being placed side by side.
Yet another solution consists in spreading out a plurality of tow, bringing together
the resulting unidirectional fiber strips in a side by side configuration to form
a sheet, and in imparting transverse cohesion to the sheet by needling. Such a
method is described in particular in document U.S. Pat. No. 5,184,387 (assigned
to Aerospace Preforms Limited) where the tows used are made of carbon precursor
fibers capable of being needled without being broken. Nevertheless, multiaxial
sheets are not made by superposing those unidirectional sheets. According to that
document, annular sectors are cut out from the unidirectional sheet to form annular
plies which are superposed and needled.
To avoid the need to give even temporary cohesion to unidirectional sheets for
making multiaxial sheets, it is known to make the multiaxial sheets directly by
forming a plurality of unidirectional sheets and by superposing them in different
directions without any intermediate handling. The superposed sheets can be connected
to one another by bonding, by sewing, or by knitting.
Documents illustrating that technique are, for example, documents: U.S.
Pat. Nos. 4,518,640, 4,484,459, and 4,677,831.
In document U.S. Pat. No. 4,518,640 (assigned to Karl Mayer) reinforcing threads
are introduced into the sheet while it is being formed, thereby making it possible
to provide bonding without piercing through the fibers. Nevertheless, that gives
rise to openings being present in the multiaxial sheet, which openings produce
surface discontinuities.
In document U.S. Pat. No. 4,484,459 (assigned to Kyntex Preform), each unidirectional
sheet is formed by causing a thread to pass around spikes carried by two parallel
endless chains, such that the portions of the threads that extend freely between
the spikes are mutually parallel. Unidirectional sheets are formed by guiding the
respective threads in different directions, and they are bonded to one another
by sewing. With that technique it is not possible to have reinforcing threads in
the longitudinal direction of the multiaxial sheet; unfortunately, it is often
necessary to place reinforcing elements in that main direction. In addition, if
a large amount of tension is exerted on the threads to guarantee parallelism in
each sheet, then the portions of the threads extending between the spiked chains
can tend to become rounded by the fibers tightening, thereby giving rise to openings
in the multiaxial sheet. Finally, it will be observed that that technique does
not make a very high production speed possible given the time required for forming
each unidirectional sheet.
In document U.S. Pat. No. 4,677,831 (assigned to Liba Maschinenfabrik GmbH),
the
technique described consists in displacing a main unidirectional sheet longitudinally
parallel to the direction of the elements which make it up, and in laying transverse
unidirectional sheets thereon in directions that make predetermined angles with
the direction of the main sheet (0°), for example +45° and -;45°
and/or +60° and -;60°. The transverse sheets are laid by a laying process
between two spiked chains situated on either side of the main sheet. That technique
which does not necessarily require a main sheet to be present, also suffers from
several drawbacks.
Thus, it is necessary to eliminate the marginal zones where the transverse
sheets turn around the spikes. Unfortunately, the wider the transverse sheets,
the larger the marginal zones, and the larger the losses of material due to their
being eliminated, and it is also more difficult to turn the sheets on the spikes.
This greatly limits the width that can be used for the transverse sheets. In addition,
the above-mentioned drawback of possible irregularity in the multiaxial sheet is
also to be found, in particular due to the formation of holes because of the tensions
that it is necessary to apply to the elements of the transverse sheets in order
to hold them parallel during laying.
In addition, relatively high stitch density is necessary immediately after laying
in order to confer sufficient strength to the resulting multiaxial sheet. In addition
to making it impossible to preserve a smooth surface state, this high stitch density
affects the flexibility of the multiaxial sheet and limits its deformability in
use, e.g. by draping.
Furthermore, when a main sheet (0°) is provided, it is necessary
to support it while the transverse sheets are being laid, such that all of them
are to be found on the same side of the main sheet. Reinforcing elements are indeed
provided that extend in the main direction (0°), but the resulting multiaxial
sheet is not symmetrical between its faces. Unfortunately, such symmetry is advantageous
to facilitate the construction of regular reinforcement and it is therefore desirable
to place the main direction at 0° in the middle of the multiaxial sheet, between
its faces.
It should also be observed that a drawback common to those techniques using threads
for forming unidirectional sheets lies in obtaining multiaxial sheets which firstly
present surface roughness due to the threads, and secondly cannot be as thin as
it is sometimes desired.
Finally, a method of making a multiaxial sheet from unidirectional sheets
is also described in document GB-A-1 447 030 (Hyfil Limited). A first unidirectional
sheet of warp-forming carbon fibers is pre-needled and another, weft-forming unidirectional
sheet is bonded to the first, likewise by needling. The pre-needling of the first
sheet seeks to displace fibers from the side where the second sheet is to be placed,
in order to contribute to bonding therewith. It will be observed that the unidirectional
sheets used are made coherent by a bonding thread, as described in above-mentioned
document GB-A-1 190 214, with the drawbacks that result therefrom.
It should also be observed that the above-mentioned known techniques all suffer
from a drawback which lies in the relatively high cost of multiaxial fiber sheets
when they are made using carbon fibers. There exists a need to reduce the cost
of such sheets, in particular so as to extend their field of application.
OBJECTS OF THE INVENTION
An object of the invention is to propose a novel method of making multiaxial
fiber
sheets, in particular to enable the cost of making such sheets to be reduced, so
as to cause multiaxial sheets made with fibers that have the reputation of being
expensive, such as carbon fibers, to be more attractive.
Another object of the invention is to propose a method enabling "mirror"
multiaxial sheets to be made, i.e. multiaxial sheets presenting symmetry relative
to a midplane, in particular relative to a main unidirectional sheet (0°),
which sheet is therefore situated between transverse unidirectional sheets making
opposite angles relative to the main direction.
Another object of the invention is to propose a method enabling multiaxial
fiber sheets to be made that present a surface of smooth appearance without irregularities
such as holes or roughnesses.
Another object of the invention is to propose a method enabling multiaxial
fiber sheets to be made requiring only a very low density of bonding transversely
to the unidirectional sheets making them up in order to ensure coherence, thereby
enabling good deformability of the multiaxial sheets to be preserved.
Another object of the invention is to provide multiaxial fiber sheets having
the above properties while also being of great length, and of small thickness and
weight (per unit area).
Another object of the invention is to propose a laying method and machine
enabling multiaxial fiber sheets to be made from unidirectional sheets that can
be relatively wide, while conserving good surface regularity and limiting losses
of material.
BRIEF SUMMARY OF THE INVENTION
In one aspect, the invention provides a method of making a multiaxial fiber sheet,
the method comprising the steps consisting in superposing a plurality of unidirectional
sheets in different directions, and in bonding the superposed sheets together,
in which method, to make at least one unidirectional sheet, at least one tow
is
spread so as to obtain a sheet of substantially uniform thickness, having a width
of not less than 5 cm and a weight of not more than 300 grams per square meter
(g/m
2), and cohesion is imparted to the unidirectional sheet enabling
it to be handled prior to being superposed with at least one other unidirectional sheet.
In a feature of the method, to make at least one of the unidirectional sheets,
a plurality of tows are used, the tows are spread so as to form unidirectional
strips, and the strips are placed side by side so as to obtain a unidirectional
sheet having a width of not less than 5 cm and weighing not more than 300 g/m
2.
To further improve an advantage of the method, in particular when using carbon,
at least one of the unidirectional sheets is preferably obtained by spreading at
least one tow having a number of filaments equal to or greater than 12 K (12,000
filaments) and possibly as many as 480 K (480,000 filaments) or more.
A similar technique can be used with all technical fibers.
An advantage of the method is thus to use large tows, in particular the largest
tows available for various kinds of fiber.
For given weight, particularly with carbon, the cost of a fat tow is much less
than that of a thin tow or thread of the kind which, so far as the Applicants are
aware, are those used in the state of the art for making multiaxial sheets.
By way of illustration, the following table applies to commercially available
carbon threads or tows formed using different numbers of filaments, and gives the
weights that can be obtained for a unidirectional sheet, depending on whether it
is formed by mutually parallel threads as in the prior art, or by spreading tows
as in the present invention. The threads or tows are made of high strength or high
modulus carbon with a polyacrylonitrile or an anisotropic pitch precursor.
| |
|
Unidirectional |
| Thread or tow |
Unidirectional |
sheet made by |
| Number of |
sheet made up of |
spreading and |
| filaments |
parallel threads |
fixing |
| 3K |
150 to 200 g/m2 |
|
| 6K |
200 to 250 g/m2 |
| 12K |
250 to 300 g/m2 |
100 to 150 g/m2 |
| 50K |
|
100 to 250 g/m2 |
| 320K |
|
100 to 300 g/m2 |
| 480K |
|
200 to 300 g/m2 |
A tow is spread or a plurality of tows are spread and juxtaposed, so as to form
at least one unidirectional sheet having weight per unit area no greater than 300
grams per square meter (g/m
2), thus making it possible from a limited
number of heavy tows to provide a sheet of relatively broad width, i.e. at least
5 cm, and preferably at least 10 cm.
The use of unidirectional sheets of relatively light weight makes it possible
to conserve this property in multiaxial sheets made up of such unidirectional sheets.
In addition, contrary to the above-mentioned prior art techniques using sheets
of parallel threads, spreading tows until lightweight sheets are obtained causes
multiaxial sheets to be made that do not have surface defects such as holes or
undulations, and that have smooth surface appearance. It is also possible with
the method of the invention to use fibers that are fragile.
When the unidirectional sheet is built up from discontinuous filaments, cohesion
can be imparted thereto by matting the filaments to a small extent. To this end,
the sheet can be subjected to needling or it can be exposed to a jet of water under
pressure, the sheet being disposed over a plate. The sheet can then be widened
without losing its cohesion.
In all cases, regardless of whether the unidirectional sheet is made of filaments
that are continuous or discontinuous, cohesion can be imparted thereto by providing
a chemical bonding agent which may optionally be suitable for being eliminated
(or sacrificed). The agent is advantageously applied by spraying a liquid compound
onto the sheet or by passing it through a bath. Cohesion can also be provided by
dusting a heat-fusible or thermo-adhesive polymer in powder form onto the sheet.
It is also possible to envisage imparting transverse cohesion to at least one
of the unidirectional sheets used by fixing by means of at least one heat-fusible
or thermo-adhesive film or thread, or indeed by forming a line of adhesive, e.g.
an adhesive in solution in an evaporatable solvent.
The method of the invention seeks more particularly to make a continuous multiaxial
sheet having a longitudinal direction, by fetching at least one transverse unidirectional
sheet onto a moving support that moves in a direction of advance parallel to the
longitudinal direction of the multiaxial sheet, the or each transverse unidirectional
sheet being fetched as successive segments that are adjacent or that overlap in
part and that form the same selected angle relative to the direction of advance.
The cohesion of the superposed unidirectional sheets makes it possible to make
multiaxial sheets without constraints on laying the unidirectional sheets relative
to one another, thus providing great flexibility concerning the order in which
the unidirectional sheets are superposed. It is thus possible to make multiaxial
sheets that present symmetry relative to a midplane ("mirror" symmetry), in particular
relative to a longitudinal middle unidirectional sheet whose direction is parallel
to the direction of advance, together with at least two transverse unidirectional
sheets disposed on either side of the longitudinal sheet and forming opposite angles
relative thereto.
In a preferred implementation of the method, each of the successive segments
forming
a transverse sheet is fetched by moving the sheet over a length substantially equal
to the dimension of the multiaxial sheet as measured parallel to the direction
of the transverse sheet, by cutting off the segment fetched in this way, and by
depositing the cutoff segment on the moving support or the multiaxial sheet that
is being made. Advantageously, the transverse sheet is reinforced in the zones
where it is cut, e.g. by fixing a film on at least one of its faces.
It will be observed that laying transverse sheets in successive cutout segments
makes it possible to limit losses of material compared with the known technique
of laying by turning the sheet around spikes. In addition, working in this way
avoids damaging the fibers, and therefore makes it possible to lay fibers that
are fragile, such as high modulus carbon fibers or carbon fibers based on anisotropic
pitch, or ceramic fibers. In addition, restarting the laying process after a break
in transverse sheet feed is made much easier compared with the case where the transverse
sheets are formed by a set of parallel fibers that are not bonded together.
In another aspect, the invention provides a unidirectional or multiaxial fiber
sheet as obtained by the above method.
In yet another aspect, the invention provides making composite material parts
that comprise fiber reinforcement densified by a matrix, in which parts the fiber
reinforcement is made from at least one such unidirectional or multiaxial sheet.
In a further aspect, the invention provides a laying machine enabling the preferred
implementation of the method to be performed.
To this end, the invention provides a laying machine for making a multiaxial
fiber
sheet by superposing unidirectional fiber sheets in different directions, the machine comprising:
apparatus for advancing the multiaxial sheet, the apparatus comprising
support means for supporting the multiaxial sheet that is being made and drive
means for driving the support means in a direction of advance;
feed means for feeding longitudinal unidirectional sheet in a direction parallel
to the direction of advance;
a plurality of cross-laying devices each including feed means for feeding the
cross-laying
device with continuous unidirectional sheet, a moving grasping head for taking
hold of the free end of a sheet, and means for laying successive segments of sheet
parallel to a transverse direction at a selected angle relative to the direction
of advance, said laying means comprising means for driving the grasping head; and
bonding means for bonding the superposed unidirectional sheets together,
the bonding means being located downstream from the support means in the direction
of advance,
in which machine:
each cross-laying device includes cutter means; and means are provided for performing
successive cycles comprising, for each cross-laying device, grasping the free end
of a unidirectional sheet by means of the grasping head, moving the grasping head
to fetch a segment of unidirectional sheet, cutting off the fetched segment of
unidirectional sheet, and laying the cutoff segment of unidirectional sheet on
the support means.
An important advantage of such a machine lies in the possibility of laying unidirectional
sheets of relatively broad width, including in the transverse directions.
Superposed unidirectional sheets can be bonded together in various ways,
e.g. by sewing, by knitting, by needling, or by adhesive, e.g. by spraying an adhesive
agent or by inserting a heat-fusible or thermo-adhesive film or thread between
the sheets. A bonding agent that may possibly have been used for providing cohesion
within unidirectional sheets can be reactivated to bond the sheets to one another.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood on reading the following description
given by way of non-limiting indication with reference to the accompanying drawings,
in which:
FIG. 1 is a fragmentary overall view of an installation enabling coherent unidirectional
sheets to be made;
FIG. 2 is a fragmentary diagrammatic plan view of the FIG. 1 installation;
FIG. 3 is a fragmentary view of a first variant embodiment of the cohesion means
of the FIG. 1 installation;
FIG. 4 is a fragmentary view of a second variant embodiment of the cohesion
means of the FIG. 1 installation;
FIG. 5 is a diagrammatic view showing part of the making and widening of a coherent
unidirectional sheet that is made up of discontinuous fibers;
FIGS. 6A and 6B are a highly diagrammatic overall plan view of a laying machine
for making multiaxial fiber sheets in an implementation of the invention;
FIG. 7 is a diagrammatic elevation view showing a detail of the device for putting
local reinforcing films into place in the machine of FIGS. 6A-6B;
FIGS. 8A to 8C show the successive steps of putting the reinforcing
film into place using the FIG. 7 device;
FIG. 9 is a diagrammatic view in lateral elevation showing a detail of the device
in the machine of FIGS. 6A-6B for cutting the transverse unidirectional sheet into
segments and for fixing a cutoff segment;
FIG. 10 is a diagrammatic end elevation view of the cutting and fixing device
of FIG. 9;
FIGS. 11A to 11C show the successive steps of fetching, cutting, and
fixing a segment of transverse unidirectional sheet in the machine of FIGS. 6A-6B;
FIG. 12 is highly diagrammatic and shows part of a variant embodiment of the
laying machine of FIGS. 6A-6B;
FIGS. 13A to 13D show the successive steps of fetching, cutting, and
fixing a segment of a transverse unidirectional sheet in another variant embodiment
of the laying machine of FIGS. 6A to 6B;
FIG. 14 is highly diagrammatic and shows a variant implementation of the fixing
of segments of transverse unidirectional sheet in a laying machine such as that
of FIGS. 6A-6B;
FIG. 15 is highly diagrammatic and shows a variant implementation of laying
transverse unidirectional sheets;
FIG. 16 is highly diagrammatic and shows a variant implementation of laying
in which the transverse unidirectional sheets overlap partially; and
FIGS. 17, 18, and 19 are highly diagrammatic and show first,
second, and third variant embodiments of the means for bonding together the superposed
unidirectional sheets in a laying machine.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Making a Unidirectional Sheet (FIGS. 1 to
5)
Tows are spread individually and the resulting unidirectional strips are optionally
juxtaposed to form a unidirectional sheet whose cohesion is provided by supplying
a bonding or attaching agent between the filaments making up the sheet, prior to
storing the sheet on a reel.
In FIG. 1, a single tow spreading device is shown so as to clarify the drawing.
A tow
10a is taken directly from a box in which it was stowed. In
a variant, the tow can be taken from a reel carried by a creel.
Tows of various kinds can be used depending on the use intended for the sheet.
For example, the tows may be of carbon fibers or ceramic fibers, or of fibers that
are precursors of carbon or ceramic, glass fibers, aramid fibers, or a mixture
of different kinds of fiber. Suitable ceramics are in particular silicon carbide
and refractory oxides, e.g. alumina and zirconia. The tows can be made of continuous
filaments or of discontinuous filaments, and if they are discontinuous they can
be obtained, for example, by bursting tows of continuous filaments. With tows made
of discontinuous filaments, it is possible to use hybrid tows comprising filaments
of different materials that are intimately mixed together. That can be achieved
by fetching burst tows or ribbons made of different materials and mixing the fibers
thereof by passing them through a gill box.
When possible, heavy tows are used, specifically for the purpose of reducing
the cost price of the resulting sheets. The term "heavy" tow is used herein for
a tow made up of at least 12 K filaments (i.e. a tow made up of 12,000 filaments),
preferably a tow having a number of filaments not less than 50 K, and possibly
as many as 480 K or even more.
The tow
10a passes over a picker and disentangler device
12
formed by a plurality of bars
12a extending between two end plates
12b, the entire assembly being rotated about an axis parallel to
the bars under drive from a motor
13. The bars
12a, e.g. four
bars, are disposed regularly around the axis of rotation.
After passing over two deflector rolls
14 and
16 mounted to rotate
freely, the tow
10a reaches a tension-adjustment device
18
made up of four rolls
18a,
18b,
18c,
and
18d that are likewise mounted to rotate freely. These rolls constitute,
in well-known manner, a parallelogram that is deformable under drive from an actuator
19 which makes it possible by acting on arms carrying the rolls to lengthen
or shorten the path of the tow
10a through the tension-adjusting
device so as to keep the tension constant.
Thereafter, the tow
10a passes successively over a plurality
of fixed curved rolls
22a,
22b,
22c that
are "banana" shaped. These rolls, of which there may be three for example, operate
in known manner to spread out the ribbon so as to form a thin unidirectional strip
20a.
The tension in the strip
20a is measured in conventional manner
by passing over rolls
24a,
24b, and
24c,
in which the roll
24b is movable vertically while being biased by
an elastic force. Information about variation in the tension of the strip as supplied
by measuring the displacement of the axis of the roll
24b is used
to control the actuator
19 so as to keep the measured tension constant.
The strip
20a is placed adjacent to other strips
20b,
20c,
20d, and
20e that are identical
or similar on a roll
25 that is free to rotate, thereby forming a unidirectional
sheet
30. The strips can thus come from tows that are identical or different,
e.g. if different, from tows of different weights, or made of fibers of different
kinds, thereby making it possible to obtain a hybrid sheet.
The strips
20b to
20e are obtained by means of tow-spreading
devices identical to the device described above.
As shown in FIG. 2, the various spreader devices are mounted on respective frames
26a,
26b,
26c,
26d, and
26e represented by chain-dotted rectangles. The frames are located
alternately above and below a common horizontal plane so as to avoid interfering
with one another.
The strips
20a to
20e coming from different spreader
devices meet on the roll
25. In order to adjust the positions of the strips
so that they are exactly adjacent, the transverse positions of the frames relative
to the advance direction of the tows, can be adjusted. Thus, each frame, e.g.
26e,
can be moved along transverse guiding slideways
28e under drive from
a motor
29e.
In a variant, the unidirectional strips can be placed one beside another in a
manner that is not adjacent, but that includes partial overlap. Smaller tolerance
is required compared with placing the strips exactly edge to edge, however the
portions situated along each edge in the resulting sheet will need to be sacrificed.
Transverse cohesion can be imparted to the sheet
30 by projecting
a liquid compound thereon downstream from the roll
25, said compound containing
a chemical bonding agent, e.g. a compound comprising a polymer in solution.
Various polymers can be used. Advantageously, the polymers used can be suitable
for being sacrificed, i.e. they should be easy to eliminate, e.g. by being dissolved
or by applying heat treatment. Amongst such polymers, mention can be made of polyvinyl
alcohol (PVA) or polyvinylpyrrolydone type polymers that are soluble in water,
and of soluble polyester. It is also possible to envisage using polymers that are
compatible with a matrix that is deposited at a later stage when making a composite
material using a reinforcing fabric made from a multiaxial sheet including the
unidirectional sheet. The term "polymer compatible with the matrix" is used herein
to designate a polymer, e.g. a resin, having the same kind as or suitable for dissolving
in the matrix, or indeed a polymer that is of a different kind but whose presence
in contact with the matrix does not affect the properties of the composite material.
The liquid compound is delivered to nozzles
32 via a feed pipe
34.
After the compound has been sprayed, the sheet passes between two rolls
36
which are pressed against each other at adjusted pressure so as to distribute the
desired quantity of liquid compound uniformly over the entire surface of the sheet
30. Thereafter, the sheet
30 passes beneath a strip dryer
38
for eliminating the solvent contained in the liquid compound. The coherent sheet
30 can then be stored on a reel
40 that is rotated by a motor
39.
In a variant, cohesion can be imparted to the sheet by spraying a compound containing
a liquid resin, and then curing the resin. Advantageously, a resin is used which
can be cured by being exposed to ultraviolet radiation, with the strip dryer
38
being replaced by a UV source. By way of example, such a resin can be a UV-curable acrylate.
Further techniques could also be used, e.g. dusting a powder of heat-fusible
or thermo-adhesive polymer onto the sheet, or depositing a heat-fusible or thermo-adhesive
film or thread on the sheet, and then exposing it to a heater device. It is also
possible to envisage forming "lines of adhesive" on the sheet constituted by an
adhesive in solution, with the solvent subsequently being evaporated.
Depending on the weight and the number of tows used, it is possible to
obtain a sheet
30 of greater or lesser width. Starting from tows having
a relatively large number of filaments, as already mentioned, the method has the
advantage of enabling wide sheets to be obtained, i.e. sheets that are at least
5 cm wide, that are preferably at least 10 cm or more wide, while using a limited
number of tows, and thus of spreader devices. Another characteristic of the method
is to enable thin sheets to be obtained, weighing no more than 300 g/m
2 and
of uniform thickness.
A bonding agent can be applied to the sheet for fixing purposes equally well
when
the sheet is made of continuous filaments and when it is made of discontinuous filaments.
When the sheet is designed to be used to form fiber reinforcement of a composite
material part obtained by densifying the fiber reinforcement with a matrix, it
is preferable to select the bonding agent as a function of that use. For example,
a bonding agent suitable for being sacrificed can be used, which is capable of
disappearing by being dissolved or by the application of heat prior to densification
by means of the matrix of the composite material. It is also possible to use a
bonding agent that is compatible with the matrix, i.e. that is capable of dissolving
in the matrix or of remaining without reacting chemically therewith, so that the
properties of the matrix are not degraded.
Other methods of fixing that impart sufficient transverse cohesion to the sheet
to enable it to be handled can also be envisaged when the sheet is made up of discontinuous
filaments. These relate in particular to methods of fixing that serve to attach
parallel discontinuous filaments to one another.
FIG. 3 shows the sheet
30 formed by the adjacent unidirectional strips
20a to
20e passing through a device
33 for spraying
jets of water under pressure onto the sheet while the sheet is passing over a metal
plate
33a. By rebounding on the plate
33a, the jets
of water perform a moderate amount of matting of the discontinuous filaments. Thereafter,
the sheet
30 passes in front of a drying strip
38 prior to being
stored on the reel
40.
In another variant shown in FIG. 4, the strip
30 passes through a needling
device
35. This device comprises a needle board
35a driven
with vertical reciprocating motion, and a support
35b over which
the strip
30 passes. The support
35b has perforations in register
with the needles of the board
35a. As a result, the needles penetrate
through the entire thickness of the sheet
30 while displacing the discontinuous
filaments, thereby giving rise to a limited amount of transverse matting which
provides the desired transverse cohesion. The needled sheet is stored on the reel
40.
Although the spreader device shown in FIG. 1 can be used with tows made
up of filaments or fibers that are continuous or discontinuous, it is most particularly
suitable for tows of continuous filaments.
Advantageously, the operation of forming a unidirectional sheet or
strip made up of discontinuous filaments includes spreading a tow of continuous
filaments as shown in FIG. 1, so as to obtain a sheet
20a of continuous
filaments. This is taken to a stretching and bursting device
21 (FIG.
5).
The stretching and bursting technique is well known per se. It consists in causing
the sheet to pass between several successive pairs of drive rolls, e.g.
21a,
21b, and
21c, which are driven at respective speeds
v
a, v
b, and v
c such that v
c>v
b>v
a.
By drawing the sheet at increasing speeds, the continuous filaments are broken.
The distance between the pairs of rolls, and in particular between
21a
and
21b determines the bursting pattern, i.e. it determines the
mean length of the burst filaments.
After stretching and bursting, the sheet
20′
a is stretched,
however its weight (per unit area) is significantly reduced compared with that
of the sheet
20a. The stretched sheet
20′
a made
up of discontinuous filaments is optionally juxtaposed side by side with or partially
overlapping other similar sheets
20′
b to
20′
e,
and is then made coherent by the above-described moderate matting means, e.g. by
being subjected to a jet of water under pressure as in the implementation of FIG.
3, or to needling by a needling device
35, as in the embodiment of FIG.
4.
The resulting sheet
30 can be widened so as to further reduce its weight
(per unit area), without the sheet losing its cohesion. This ability of being widened
is given by the cohesion technique used (water jet or needling).
Widening can be performed, for example, by causing the coherent sheet
30
to pass over one or more pairs of curved rolls
37 prior to being stored
on the reel
40.
It will be observed that the sheet can be widened after it has been stored on
the reel
40, e.g. when it is taken from the storage reel in order to form
a multiaxial sheet.
Other known techniques for obtaining unidirectional sheets by spreading tows
can also be used, for example the techniques described in Rhône Poulenc Fibres
documents FR-A-2 581 085 and FR-A-2 581 086. In those documents, a tow for spreading
is taken to rolls which include resilient elongate elements at their peripheries
that are disposed along generator lines and that are provided with spikes. For
the portion of its path where it is in contact with a roll, the tow is engaged
on the spikes and it is spread by the elastic elements extending parallel to the
axis of the roll.
Making a Multiaxial Sheet
Reference is now made to FIGS. 6A-6B which show a laying machine constituting
an embodiment of the invention suitable for making a continuous multiaxial sheet
from a plurality of unidirectional sheets, at least one of which can be obtained
by a method as described above.
In the example shown, a multiaxial sheet
50 is made up of three unidirectional
sheets
30a,
30b, and
30c making the following
angles respectively with the longitudinal direction: 0°, +60°, and -;60°.
The sheet at 0° (sheet
30a), i.e. the "main" sheet, is a coherent
unidirectional sheet as obtained by the above-described method, unreeled from a
reel
40a. The transverse sheets at +60° (sheet
30b)
and at -;60° (sheet
30c) are unidirectional sheets which can
also be coherent sheets obtained by the above-described method and which are unreeled
from respective reels
40b and
40c. The unidirectional
sheets used need not necessarily have the same width. Thus, in the example, the
transverse sheets
30b and
30c both have the same width
which is smaller than that of the longitudinal sheet
30a. In general,
the transverse sheets will normally be of a width that is smaller than that of
the main sheet (0°).
It will be observed that the angles formed by the transverse sheets relative
to
the sheet at 0° can be other than +60° and -;60°, for example they
can be +45° or -;45°, or more generally they can be angles that are preferably
of opposite sign, but that are not necessarily equal. It will also be observed
that more than two transverse sheets can be superposed with the 0° sheet,
e.g. by adding a sheet at 90° and/or by adding at least one other pair of
sheets forming opposite angles relative to the longitudinal direction.
As shown in FIG. 6A, the multiaxial sheet
50 is formed on a support constituted
by a horizontal top segment of an endless belt
42 of a conveyor
44
passing over a drive roll
46 driven by a motor
47, and over a deflection
roll
48 (FIG.
6B). It will be observed that the width of the belt
42 is narrower than that of the sheet
50 so that the sheet projects
slightly from both sides
42a and
42b of the belt
42.
The sheet is made by fetching juxtaposed segments
30b at +60°
onto the belt
42 and then depositing the sheet
30a that is
oriented at 0° thereon, and then bringing over that juxtaposed segments of
the sheet
30c oriented at -;60°. It is an advantageous feature
to be able to make a multiaxial sheet
50 in which the 0° sheet is situated
between the transverse sheets, thereby conferring a symmetrical nature to the sheet
50. This is made possible by the cohesion intrinsic to the sheet
30a.
Also advantageously, the unidirectional sheet at 0°, as obtained by a method
as described above, is of relatively great width, not less than 5 cm, and preferably
at least 10 cm, thus making it possible to make multiaxial sheets of great width.
The devices
60 for fetching, cutting, and laying successive segments of
the sheets
30b and
30c are identical, so only the device
associated with the sheet
30c is described.
The sheet
30c is unreeled from the reel
40c by means
of a grasping head
70 having at least one clamp capable of taking hold of
the free end of the sheet
30c.
The sheet
30c is pulled from an edge
42a of the conveyor
belt
42 over a length that is sufficient to cover the width of the longitudinal
sheet. The segment thus fetched is cut off in the longitudinal direction at the
edge of the sheet
30a which is situated over the edge
42a
of the conveyor belt by means of a cutter device
80. Simultaneously,
the cutoff segment of sheet
30c is fixed by means of its end which
has just been cut so as to conserve its position on the conveyor belt relative
to the previously fetched segment, and thus relative to the sheets
30a
and
30b which have already been laid.
In order to cut the sheet
30c without deformation or fraying, local
reinforcement in the form of a segment of film or tape
92 is fixed on each
face of the sheet
30c at each location where it is to be cut. The
film
92 can be fixed, for example, by adhesive, by thermo-adhesive, by high
frequency welding, by ultrasound welding, . . . by means of a device
90.
For example, a polyethylene film is used that can be fixed by thermo-adhesion.
It will be observed that a reinforcing film could be fixed over one face only of
the sheet
30c.
The grasping head
70 is carried by a block
62 which slides in a
slideway
64 of a beam
66. By way of example, the block
62
is fixed on an endless cable
68 driven in the slideway
64 by a reversible
motor
69. The beam
66 supports the reel
40c, and also
the devices
80 and
90 for cutting off and laying segments of the
sheet, and for putting reinforcing film into place.
A detailed description of how the head
70 and the devices
80 and
90 are implemented is given below. It will be observed that the grasping
head can be swivel mounted relative to the block
62 as can the devices
80
and
90 relative to the beam
66. As a result, the angle made by the
deposited transverse sheet relative to the longitudinal direction (0°) can
easily be modified by appropriately adjusting the orientation of the beam
66
and by adjusting the positions of the head
60 and of the devices
80
and
90 relative to the beam. Operation of the head
70 and of the
devices
80,
90 is controlled by a control unit
100 to which
they are connected by a bundle of cables
102 running along the beam
66.
A segment of each sheet
30b and
30c is fetched, cut
off, laid, and fixed while the conveyor
44 is stationary. Thereafter, the
conveyor is caused to advance over a length equal to the size of the sheets
30b
and
30c as measured in the longitudinal direction (0°),
and the process is repeated. On each advance of the conveyor
44, the same
length of the longitudinal sheet is unreeled.
After being superposed, the sheets
30a,
30b, and
30c are bonded together. In the example shown in FIG. 6B, this bonding
is performed by needling by means of a needle board
52 which extends across
the entire width of the multiaxial sheet
50, as it leaves the conveyor
44.
During needling, the sheet
50 is supported by a plate
52a carrying
a base felt
52b, e.g. made of polypropylene, into which the needles
can penetrate without being damaged. Needling is then performed each time the conveyor
advances. Bonding by needling is particularly suitable for sheets made of discontinuous
filaments or of continuous filaments that are not liable to be excessively damaged
by
