Tool for moulding self-stiffened panels made from a composite material

For moulding self-stiffened panels (A) from a composite material with a thermosetting matrix, use is made of a tool having a block (10), a sealing bag (12), whose peripheral edge is connected to the block by a sealing bead (14), and solid, non-deformable calibration parts (22). Panel (A) is placed in a tight volume (16) defined between the block and the bag, while the parts (22) are placed outside said volume, above the bag and between the panel stiffeners (C). The calibration parts (22) are connected by a rigid structure (24). Thus, when the volume (16) is placed under a vacuum and when the tool is placed in an oven or autoclave, a uniform pressure is applied to the panel by the bag (12) and the parts (22) ensuring the maintenance of the geometry of the stiffeners.

BACKGROUND OF THE INVENTION 
The invention relates to a tool making it possible to manufacture by 
moulding self-stiffened panels made from a composite material with a 
thermosetting matrix. 
Composite material parts are generally produced by stretch forming several 
superimposed layers, each constituted by impregnated resin fibres. 
According to the envisaged application, the fibres can be of carbon, 
glass, Kevlar, etc. The resin constituting the composite material matrix 
is generally a thermosetting resin, which is thermally polymerized. 
The invention is applicable to all industrial fields using composite 
material parts, i.e. particularly the aeronautical, space, car, maritime 
and railway industries. 
In these different fields, the use of composite material parts makes it 
possible to make structures considerably lighter. This weight gain is due 
not only to the specific strength of these materials, which exceeds that 
of standard metal alloys, but also the possibility offered by these 
materials with regards to obtaining complex shapes by moulding. Therefore 
it is possible to assemble as a single composite material part a 
mechanical subassembly which, conventionally, would be constituted by 
several basic metal parts interconnected e.g. by welding or mechanical 
fixtures (rivets, screws, etc.). 
In the particular case of self-stiffened panels formed from a base plate 
provided with stiffeners, it is possible to produce these panels as a 
single composite material part, whereas according to the prior art the 
panels are made from a base plate to which are connected, e.g. by 
riveting, angle iron-shaped stiffeners. 
The manufacture of such self-stiffened panels made from composite material 
with a thermosetting matrix takes place by moulding using an appropriate 
tool, which is placed in an oven or an autoclave, whereof the temperature 
rise ensures the polymerization of the resin. 
In practice, this moulding operation causes production problems linked on 
the one hand with the functions to be satisfied during the moulding of a 
thermosetting composite material and on the other with the special 
structure of self-stiffened panels. 
The functions to be satisfied during the moulding of a thermosetting 
composite material part are: 
the temperature rise necessary for activating the polymerization reaction; 
the application of a pressure perpendicular to the surface of the layers 
throughout the duration of the baking cycle of the part necessary for the 
good compacting of the layers and the final quality of the part; 
the application of a uniform pressure to the entire surface of the part, so 
as to avoid the local expulsion of the resin by a differential pressure 
effect between two zones, resulting from the low viscosity of said resin 
at the start of the polymerization cycle; and 
the use of a moulding tool, whose pressure application system is able to 
follow the thickness decreae of the parts in all directions occurring 
during polymerization, in order that said system can fulfil its function 
throughout the polymerization cycle. 
Moreover, the manufacture of a self-stiffened panel requires the use of a 
moulding tool which is also able to fulfil the geometrical function of 
maintaining the spacing between the stiffeners and maintaining the 
planeity of said stiffeners, so as to prevent any undulation thereof, 
which might lead to a local buckling of the compressively stressed panels. 
The presently used moulding tools for moulding such structures suffer from 
major disadvantages. A first known tool is constituted by a closed or 
sealed mould, in which are fixed inflatable bags positioned between the 
stiffeners of the panel to be polymerized. In this case, the mould 
containing the part and the bags must be solid, so as to take up the 
internal compressive stresses applied thereto. It is therefore expensive 
and difficult to heat. 
Moreover, as inflatable bags are by their very nature flexible, floating 
elements, the maintaining of the spacing between the stiffeners and the 
maintenance of the planeity thereof are not ensured in a satisfactory 
manner. In other words, the parts obtained suffer from serious geometrical 
defects. 
Finally, such a tool becomes very voluminous when used for moulding parts 
several meters long. 
Another known tool for moulding self-stiffened panels has, like the 
previous one, a solid sealed mould. However, the inflatable bags are 
replaced by elastomer shims. This tool has the same disadvantages as the 
previous tool with regards to the dimensioning of the mould. 
In addition, the pressure applied to the part results from the thermal 
expansion of elastomer cores, so that the pressure and temperature cannot 
be separately controlled. This is prejudicial when using certain resins 
for forming the composite material matrix. 
Moreover, although the geometrical maintenance of the stiffeners is better 
than when using inflatable bags, the creation of local overstresses can 
lead to flow or creep of the material constituting the expandable cores, 
so that the latter then move with them the adjacent stiffeners. The 
maintenance of the spacing between the stiffeners, as well as their 
planeity are consequently not totally assured. 
In addition, the solid elastomer cores do not make it possible to uniformly 
redistribute the stresses in all directions, so that the pressure applied 
to the part being moulded is not uniform. Finally, the thickness reduction 
of the part during polymerization is accompanied by a drop in the pressure 
applied by the elastomer cores, which has an effect on the final quality 
of the part. 
In another known tool, elastomer shims are also placed between the 
stiffeners of the panel to be polymerized, but the thus formed assembly is 
placed between a block on which the base plate of the panel rests and a 
bag, whose peripheral edge is connected in sealed manner to the block, 
said bag covering the elastomer shims. The pressure is applied to the part 
by forming a vacuum in the space defined between the block and the bag and 
containing both the part and the shims. In this case, the elastomer shims 
are used as geometrical shapers and redistribute on the panel the internal 
pressure of the autoclave, which is applied to the sealing bag. 
In this procedure, pressurizing is separate from the temperature and the 
same flexibility for the control of the baking cycles is obtained as when 
using inflatable bags. Furthermore, the tool does not have to take up high 
compressive stresses, because on both faces it is exposed to the pressure 
prevailing in the autoclave. Therefore the block can be formed in a very 
light wall. 
However, as in the preceding arrangement, the pressure applied to the part 
is not perfectly uniform and the maintaining of the spacing between the 
stiffeners and the planeity of the latter are dependent on possible creep 
of the material constituting the shims. 
In a fourth known method for producing self-stiffened composite material 
panels, use is made of a tool comparable to that of the third method 
described hereinbefore, i.e. a block associated with a sealing bag, whilst 
replacing the elastomer material shims by solid, non-deformable shims. The 
bag then applies the pressure prevailing in the autoclave, through the 
non-deformable shims, to the base plate parts located between the 
stiffeners. Moreover, the differential expansion between the shims and the 
block applies a pressure to the faces of the stiffeners. Thus, this method 
makes it possible to guarantee good positioning and planeity 
characteristics of the stiffeners. 
However, the pressure applied to the stiffeners is dependent on the 
temperature and is consequently not identical to that applied to the base 
plate. Consequently there is a differential pressure between the base 
plate and the stiffeners, which tends to expel the resin towards the base 
plate at the end of the polymerization cycle. 
Moreover, this solution is disadvantageous when the base plate has local 
overthicknesses. Thus, thickness variations of the plate during the 
polymerization cycle differ as a function of whether the base plate zones 
are thicker or thinner, so that the non-deformable shims would only bear 
at the end of the cycle on the thinner zones of the base plate. 
Consequently no pressure would then be applied to the thicker zones of the 
latter. 
Thus, at present, there is no moulding method which is completely 
satisfactory for making it possible to produce self-stiffened panels made 
from a composite material with a thermosetting matrix. 
SUMMARY OF THE INVENTION 
The present invention relates to a novel tool making it possible to mould 
self-stiffened panels of composite material, whilst guaranteeing a perfect 
geometry of the stiffeners, as well as the obtaining of temperature and 
pressure conditions which it is desirable to respect during the moulding 
of a composite material with a thermosetting matrix. 
According to the invention, this result is obtained by means of a tool for 
the moulding of self-stiffened panels of composite material with a 
thermosetting matrix, formed from a base plate provided with stiffeners, 
incorporating a block, a sealing bag, whereof one peripheral edge is 
tightly connected to the block, a tight volume being defined between the 
block and the bag, as well as solid calibration parts which can be placed 
between the stiffeners of a panel to be moulded placed in said tight 
volume, characterized in that the solid calibration parts are located 
outside said tight volume. 
As a result of this arrangement, the pressure is applied directly by the 
sealing bag both to the base plate and to the stiffeners. Therefore the 
pressure is distributed in a perfectly uniform manner and follows the 
thickness variations of the part resulting from the polymerization of the 
resin. The tool is also perfectly adapted to the manufacture of panels, 
whose base plate has a variable thickness, e.g. due to the presence of 
local reinforcements. Moreover, the positioning and straightness of the 
stiffeners are ensured by the solid calibration parts positioned above the 
bag. 
Finally, like the other known tools using a sealing bag tightly connected 
to a block, the tool does not have to take up the pressure, so that it can 
be formed by relatively thin parts and the panel is easy to heat, no 
matter what its dimensions. 
Preferably, the solid calibration parts are interconnected by a rigid 
connecting structure, which has a thermal expansion coefficient 
approximately equal to that of the panel to be produced. In this case, a 
detachable bracket can be joined to the connecting structure in order to 
position the latter on the block. 
Advantageously, in order to ensure a satisfactory surface state of the 
panel after baking, elastomeric material covers are interposed between the 
sealing bag and the panel to be moulded, so as to protect the latter from 
the creases or folds of the bag. 
Elastomeric material shims can also be placed within the tight volume, in 
the extension of each stiffener. These shims make it possible to protect 
the ends of the stiffeners and prevent pinching or sliding of the fibres 
of the composite material at this location. 
In order that the calibration parts can easily be fitted whilst still 
fulfilling their function with regards to the position and planeity of the 
stiffeners, between each of these parts and each of the stiffeners there 
is a clearance between a minimum clearance and a maximum clearance of 
predetermined nature at the polymerization temperature of the panel, the 
calibration parts having an expansion coefficient such that, at ambient 
temperature, said clearance is at least equal to 0 and preferably at the 
most equal to said maximum clearance. The minimum and maximum clearances 
can be respectively approximately 0.1 and 0.2 mm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the tool used according to the invention for carrying out the 
moulding under pressure of a self-stiffened panel A of a composite 
material, whose matrix is formed from a thermosetting resin. Panel A 
comprises a base plate B, which can be planar or slightly curved and whose 
thickness can be constant or variable, as well as stiffeners C all 
projecting over the same face of base plate B, perpendicular thereto, said 
stiffeners being parallel to one another and generally having a constant 
spacing. 
The tool of FIG. 1, in which has been previously formed the panel A in a 
manner to be described hereinafter, is intended to be placed in an oven or 
autoclave making it possible to subject said panel to a predetermined 
temperature cycle ensuring the polymerization of the resin forming the 
panel matrix. 
The tool according to the invention firstly comprises a block 10 in the 
form of an e.g. metallic plate of limited thickness and which is intended 
to be positioned horizontally. The upper face of said plate has a shape 
complimentary to that of the outer surface of panel A to be moulded. For 
simplification purposes said upper face is shown in planar form in FIG. 1, 
but it is readily apparent that it can also be curved, as a function of 
the shape of the panel to be produced. 
The tool of FIG. 1 also comprises a sealing bag 12, which is positioned 
according to the invention immediately above the panel A to be moulded, so 
as to follow the contours of the latter. The dimensions of the bag exceed 
those of the panel, so that the peripheral edges of bag 12 can be tightly 
connected to block 10 all around the panel, e.g. by a sealing mastic bead 
14 or by any other means fulfilling the same function. 
The assembly formed by block 10 and by the sealing bag 12 defines a tight 
volume or pocket 16 in which is placed the panel A to be moulded. This 
tight volume 16 communicates with a vacuum pump 18 by a duct 20, e.g. 
connected to the sealing bag 12 by a cap or ferrule provided on the 
latter. 
Under the effect of the pressure difference existing between the interior 
of the oven or autoclave in which the tool is placed and the tight volume 
16, when the vacuum pump 18 is actuated, a pressure independent of the 
temperature prevailing in the oven or autoclave is uniformly applied by 
the bag 12 to the base plate B on the edges of the stiffeners C and even 
to the rounded connection zones formed at the base of said stiffeners. The 
pressure applied in this way to the panel A is independent of the 
thickness reduction of said panel occurring during the polymerization of 
the resin, as well as thickness variations of the base plate B, e.g. 
resulting from the presence of local reinforcements therein. 
In order to ensure the maintenance of the geometry of stiffeners C during 
moulding, i.e. the maintenance of the spacing of said stiffeners, as well 
as the maintenance of their planeity, the tool according to the invention 
also comprises solid calibration parts 22 which, according to the 
invention, are placed above the sealing bag 12, i.e. outside the tight 
volume 16, between the stiffeners C of the panel A to be moulded. As shown 
by FIG. 1, these calibration parts have an approximately U-shaped 
cross-section and extend between the stiffeners of panel A over the entire 
length of the latter. 
It should also be noted that the calibration parts 22 are made from a solid 
material, i.e. a rigid material which does not creep and whose shape 
remains constant throughout the panel polymerization cycle, except for 
thermal expansions. This material can in particular be a metal or metal 
alloy. 
As shown in FIG. 1, the positioning of the calibration parts 22 with 
respect to one another and the maintenance of their spacing are assured by 
a connecting structure formed by at least two spaced assemblies 24 
distributed over the entire length of the panel to be moulded. 
Each of the assemblies 24 is constituted by several independent elements 
24a, each of which has a column, whereof one end is intended to be fixed 
to one of the calibration parts 22. The rigidity of each element 24 is 
assured by groups of two pins simultaneously traversing each pair of 
adjacent elements 24a. 
In order to ensure that the feet of the stiffeners C do not move during the 
polymerization cycle, the connecting structure formed by assemblies 24 
have a thermal expansion coefficient approximately equal to that of the 
panel A to be produced. 
Advantageously and as is also shown in FIG. 1, the tool according to the 
invention also has detachable brackets 26 making it possible to ensure the 
positioning of each assembly 24 of the connecting structure carrying the 
calibration parts 22 relative to the block 10 prior to polymerization. 
Each bracket 26 is joined to one of the elements 24a of the connecting 
structure 24 by two pins 24b and it rests on the upper face of block 10, 
so that it can e.g. be brought against a detachable abutment 28 projecting 
over the latter. 
Each of the brackets 26 is retracted when the tool is placed in the oven or 
autoclave, so that the assembly floats, except in the special case where 
the block 10 is made from a material identical to that of the connecting 
structure. 
The dimensions of the calibration parts 22 are chosen so as to respect the 
polymerization temperature, whereby the calibration parts must not lock 
the stiffeners, so that only the sealing bag bears on said stiffeners, 
which is necessary for applying a uniform pressure to the entire panel and 
the polymerization temperature of the panel, whereby it is also desirable 
that the calibration parts are sufficiently close to the stiffeners to 
avoid any local deformation of the latter. 
In practice, for respecting these two conditions, the calibration parts are 
dimensioned in such a way that at the polymerization temperature, the 
clearance between each of these parts and each of the adjacent stiffeners 
is min. 0.1 mm and max. 0.2 mm. 
Moreover, the calibration parts 22 must not squeeze or press on the edges 
of the stiffeners C at ambient temperature, so that said parts can be 
mounted on elements 24a. Thus, the expansion coefficient of the material 
from which the calibration parts 22 are made must be chosen in such a way 
that at ambient temperature, the clearance between each calibration part 
and each adjacent stiffener is at least equal to 0 and preferably at the 
most equal to the maximum clearance of 0.2 mm. 
The material from which the calibration parts 22 is made is chosen as a 
function of these objectives, the expansion coefficient of the parts being 
lower the greater the spacing and the thicker the stiffeners. Moreover, 
the thicker the stiffeners, the greater the swelling between the fresh 
composite material and the baked composite material. 
As is more particularly illustrated in FIG. 2, in its part located 
immediately within the sealing mastic bead 14, the sealing bag 12 overlaps 
a profile 30 resting on the upper face of the block 10, whilst surrounding 
panel A. According to conventional procedures, a separating film 34 and a 
draining fabric 32 are placed in this order on the inner face of the 
sealing bag 12, in the part of the latter projecting beyond the peripheral 
edge of panel A. 
Preferably, elastomeric material covers 36 are placed between the sealing 
bag 12 and the panel A, so as to completely cover the latter. These covers 
are placed edge to edge, so that their adjacent edges are located in the 
extension of the ends of the stiffeners C. The particular function of the 
covers 36 is to prevent any creases or folds formed in the bag from 
damaging the surface state of the panel. The elastomeric material 
constituting these covers must be able to deform so as to integrally 
transmit to the panel the pressure applied by the bag. 
Elastomeric material shims 38 are placed within the tight volume 16 in the 
extension of each of the stiffeners C. The thickness of shims 38 is 
approximately equal to the thickness of the stiffeners, which avoids 
pinching or sliding of the fibres at the end of the latter. 
A two-part, elastomeric material frame 40 surrounds the base plate B of 
panel A, the thickness of said frame being approximately equal to that of 
plate B. 
As illustrated in FIG. 2, the elastomer covers 36 cover both panel A, shims 
38 and frame 40. 
According to a conventional procedure, the upper and lower faces of base 
plate B of the panel are covered by a glass fabric 42, called delamination 
fabric, whose function is to ensure the draining of the gases released 
during polymerization and protects the panel after removal from the mould. 
A separating film 44 is also placed between the elastomer cover 36 and the 
delamination fabric 42 in the peripheral zone of the panel and covering 
the frame 40. Finally, a mould removal film 46 coated with 
polytetraflouroethylene, such as TEFLON, is placed directly on the block 
10 and covers the entire surface of the latter located within the sealing 
mastic bead 14. 
Apart from the advantages referred to hereinbefore regarding the uniformity 
of the pressure applied and the maintenance of a good geometry of the 
stiffeners, the tool according to the invention, when placed in an oven or 
autoclave, makes it possible to ensure a rapid heating of the part. 
Moreover, its overall dimensions and weight are reduced, because the 
dimensions of said tool do not have to take up the compressive forces. 
The use of the tool described initially takes place by a draping operation. 
With the mould open, said operation consists of draping onto the block 10 
a certain number of layers of preimpregnated fibres of thermosetting 
resin, in order to form the lower part of base plate B. 
Separately a certain number of layers of preimpregnated fibres of 
thermosetting resin is draped onto the calibration parts 22. The sealing 
bag 12 and the elastomer covers 36 are placed on the calibration parts 
prior to the start of draping. The U-shaped profiles formed in this way on 
each of the calibration parts are then joined to one another and connected 
by the rigid connecting structure 24 in such a way as to form the 
stiffeners C and the upper part of base plate B. 
After the auxiliary parts of the tool, such as the elastomer frame 40 and 
the profile 30 have been placed on block 10, the assembly formed by the 
rigid connecting structures 24, the calibration parts 22 and the portion 
of the panel to be produced previously draped onto said parts is turned 
over and placed above the block in the position illustrated in FIG. 1. The 
correct positioning of the thus turned over portion of the panel with 
respect to the portion previously draped on the block is brought about 
with the aid of detachable brackets 26. The elastomer shims 36 are placed 
in the extension of the ends of the stiffeners. The pins 24b are removed 
and the calibration parts 22 are withdrawn and then repositioned 
individually so as to permit the putting into place of bag 12. When 
structure 24 is formed again by fitting pins 24b, the bracket 25 is 
removed and the sealing mastic bead 14 put into place. 
The tool can then be placed in an oven or autoclave, where it undergoes the 
temperature cycle necessary for the polymerization of the resin 
constituting the matrix of the panel, after the tight volume 16 has been 
placed under a vacuum by vacuum pump 18. When polymerization is ended, the 
tool is removed from the oven and the panel removed from the mould. 
Obviously, the invention is not limited to the embodiment described in 
exemplified manner hereinbefore and covers all variants. Thus, the 
connecting structure of the calibration parts can be produced so as to 
permit the adjustment of the spacing between these parts, which makes it 
possible to use a single structure for moulding panels with a variation in 
the spacing between the stiffeners.