FR2555501A1 - METHOD FOR MANUFACTURING HOLLOW RIGID STRUCTURES IN RESIN-IMPREGNATED FIBERS, DEVICE FOR IMPLEMENTING SAME AND STRUCTURE OBTAINED - Google Patents

Abstract

Process for fabricating hollow box structures (Fig. 1 B) obtained in a single operation of polymerization and reinforced with slubs of fibers (27). The process uses rigid mandrels covered with inflatable sheaths which are draped with sheets of fibers and which are arranged at the same time as the fiber slubs into a mould (Fig. 4) with the outer profile of the finished part and inside which takes place the curing after the elastic sheaths have been inflated. The fabrication in series of such structures by the process herein disclosed does not require any drying stove or autoclave.

Description

 Method for manufacturing hollow rigid structures made of resin-impregnated fibers, device for its implementation and structure obtained.

 The present invention relates to a method for manufacturing hollow rigid structures produced from sheet material of fibers impregnated with resin, as well as to a device for carrying out the method. It also relates, as an industrial product, to the structures obtained according to the process.

 Although the invention can be the subject of various applications in various fields of industry, it has very particular advantages in aeronautics in particular for the constitution of structures of complex shapes which must combine perfect mechanical resistance with lightness as well. great as possible. This is the reason why the invention will be exposed by reasoning from structures used in aeronautics without this being a desire to limit the invention.

 In general, hollow rigid structures, such as aircraft wings for example, currently consist of an outer skin defining two walls spaced from one another and stiffened by a series of ribs and internal beams. This organization is characteristic of metal construction where the skins, ribs and beams are linked most of the time by riveting. When the coverings, the ribs and the side members are made of fibers impregnated with resin, the structure remains the same and the riveting is replaced by bonding of the elements.

 This known process for producing rigid hollow structures made of resin-impregnated fibers is long to implement, requires numerous and delicate operations to be carried out as much as to control, sophisticated equipment (autoclave) and requires a very long manufacturing cycle which is not compatible with mass production.

However, this method has drawbacks with regard to the final product obtained. Indeed, the forces applied during the use of the parts often cause detachments at the points of connection of the elements. Gluing can not be carried out safely whatever the shape of the part, so that it is often necessary to add metal parts to reinforce the connections.

 The present invention provides a new process for manufacturing rigid hollow structures which makes it possible to obtain a one-piece structure, produced in a single polymerization operation and entirely made of fiber materials impregnated with resin.

 To this end, the invention, in its most general aspect, relates to a new process for the manufacture of hollow rigid structures produced from material in plies of fibers impregnated with polymerizable resins, characterized in that - a rigid mandrel equipped on its periphery of an elastically deformable flexible envelope - the plies of fibers are placed on the flexible envelope in such a way that said plies can slide over one another.

- It encloses the assembly thus formed in 'a mold whose rigid walls are spaced from the sheets of fiber previously impregnated with resin.

- After heating, the flexible envelope is caused to expand until the layers of fibers come into contact with the walls of the mold and the resin is polymerized.

- Then what causes the retraction of the flexible envelope and extracts from the mold the mandrel-polymerized structure that is separated from one another, the mandrel can be recovered for other operations.

 For the production of large hollow rigid structures of complex shape which must have a continuous external wall and multiple internal cavities separated by partitions stiffening the external wall as is the case in particular for aircraft wings, the method according to the The invention is characterized in that - several mandrels of section adapted respectively to each part of the desired final shape are prepared, mandrels equipped with a flexible envelope and each coated with sheets of fibers capable of moving relative to each other.

- These coated mandrels are placed side by side in a mold, taking care that the plies of fibers impregnated with resins coating each mandrel are separated from the other plies and from the wall of the mold.

- we recovert.all of the mandrels coated with a second series of layers of fibers.

- The mold is closed and, after heating, the flexible envelopes are caused to expand in order to press the sheets of fibers previously impregnated with resin against one another and against the walls of the mold, after which polymerization of the resin is ensured.

- Then the flexible envelopes are retracted to extract the final structure from the mold, which is separated from the mandrels.

 According to a particular aspect of the invention, it is possible to avoid the use of large-sized heating chambers necessary in conventional processes by ensuring polymerization by the addition of heat in the zone situated on the internal side of the deformable envelope. That is to say either in the chamber formed between the mandrel and the deformable envelope or by circulating inside the mandrel of a heat fluid This technique has the following advantages: the amount of energy required for polymerization is lower than that used in the heating chambers, the temperature distribution is more homogeneous since the thermal inertia of the external mold is neutralized, finally, the time required for the rise in temperature and cooling is considerably reduced. external mold during the rise in temperature are reduced, thus allowing better precision.

 Likewise, after polymerization, the cooling of the part can be improved by ensuring the forced circulation of a cooling fluid either between the deformable envelope and the mandrel or preferably inside the mandrel itself.

 The device which is the subject of the invention therefore makes it possible inter alia to very significantly reduce the times necessary for stabilizing the temperature of the mold and cooling the assembly in order to shorten the manufacturing cycles.

The device for implementing the method is characterized in that it comprises at least one hollow rigid mandrel on which is threaded an elastically deformable envelope, aes means for expanding at least locally the fnveloFixe
deformable to move it away from the hollow mandrels, means for mooting and immobilizing the envelope mandrel assembly in a mold in such a way that the periphery of the envelope is
away from the mold walls before the expansion phase
Finally, the invention relates, as a new industrial product, to a hollow rigid structure comprising a continuous peripheral surface forming the upper and lower faces of the cells s + extending over the entire length of the structure and of the partitions connecting the upper and lower faces over the entire length of the structure and separating the cells from each other, said structure being produced in a single block from materials in layers of fibers impregnated with resin.

 The advantages of the present invention are manifold but the following will be cited above all: 1 - simultaneous polymerization (or baking) of the outer skins of the structure and of the internal stiffeners, which results in the elimination of any sticking and a reduction in the overall mass of the structure with equal mechanical characteristics.

- 2 - cortrôlable evacuation of air and resin contained in the sheets of fibers during the polymerization are on the inflation pressure of the deformable envelope which allows to obtain a product of the required quality.

- 3 - manufacture of rigid structures of great lengths with local resistances perfectly mastered and adapted to needs thanks to an internal distribution of stiffenings according to the profile cut and thanks to the possibility of modulating the thickness of the layers of fibers according to the locations.

- 4 - removal of large autoclaves required in known processes.

- 5 - possibility of introducing wicks of fibers under the outer skin to improve the mechanical characteristics of the structure in tension or compression.

 An embodiment of the invention will now be described, by way of nonlimiting example, with reference to the appended drawings in which - the figure represents it diagrammatically, with partial cutaway, an airplane wing structure produced according to known techniques. - Figure lb shows schematically a wing obtained by the method according to the invention; - Figure 2 is a longitudinal section of a mandrel used to implement the method of the invention - Figure 3 is a cross section of the mandrel of Figure 2; - Figure 4 shows a half mold being fitted with mandrels - Figure 5 is an elevational view of the closed mold - Figure 6 is a section of the mold along line 6-6 of Figure 5 containing the structure obtained according to the invention - Figure 7 shows a drain and Figures 8 to 8d show different configurations of structures obtainable according to the method of the invention.

 As can be seen in the figure the current conventional process for manufacturing a hollow structure such as an airplane wing consists in placing a series of open ribs N in the transverse direction of the wing, these ribs having the profile of the section of the wing and being spaced from each other and rigidly held together by the longitudinal P-beams. The whole is covered with a skin E which completely envelops ribs and beams.

FIG. 1b shows in comparison the structure of a similar wing obtained according to the method of the invention and in which all the elements are made of fibers impregnated with resin forming a single block consisting of a surface skin S stiffened by internal walls I constituting a series of non-deformable related hollow boxes.

 The basic method of the invention will now be described with reference to FIGS. 2 and 3. To obtain a hollow structure with a single box, a hollow mandrel 1 is used, the section of which may be any, but preferably has a polygonal shape which approaches the final shape of the structure as shown in FIG. 3 comprising two lateral faces 2, two upper and lower faces 3 and four inclined faces 4.

The mandrel must be very rigid to withstand radial crushing forces during pressurization and therefore, preferably, must be made of metal with internal reinforcing ribs 5 stiffening each face. The inner chamber 6 of the mandrel is obstructed at its end by cheeks 7. Inside the chamber 6 are provided ducts 8 of pressurized air inlet which are connected to orifices 9 formed in the wall of the mandrel .

The ducts 8 pass through the cheeks 7 at the end of the mandrel and are connected on the one hand to a source of pressurized air which is not shown and on the other hand to a suction system which is not shown. The mandrel 1 is covered on its periphery with an envelope 10 or sheath made of elastically deformable flexible material, the ends of which are forcefully clamped (in a leaktight manner) between flanges II of the mandrel and sealing flanges 12 fixed on the mandrel.

A sealed chamber 13 is thus provided between the mandrel and the gane, chamber into which open the air inlet pipes 8 under pressure, thus ensuring the expansion of the envelope or its retraction at will.

In the center of the cheeks 7 of the mandrel are fixed two hollow supports 13 intended to ensure the seating of the mandrel in a mold as will be seen with reference to FIG. 4. These supports are connected to pipes 14 - 15. The pipe 14 provides the introduction of a heating fluid or a cooling fluid
dissement inside the chamber of the mandrel while the tube 15 ensures the evacuation of these fluids. For this purpose the pipes 14 and 15 are selectively connected to sources of hot or cold fluids (water or a gas for example) not shown.
The mold intended to receive a series of mandrels like that of FIG. 3 is shown in FIGS. 4 to 6.

The lower part 16 of the mold has an imprint 17 on the underside of the final structure to be obtained and the upper plate 18 has an imprint 19 on the upper face of the structure. The junction plane of the mold includes a number of seats 20 in which are intended to rest the supports 13 of the mandrels which will therefore protrude from the mold. The mold also has passages for the pipe 8 for supplying pressurized fluid. An orifice 21 is also provided to create a slight depression inside the mold, this orifice being connected by a pipe 22 to a suction device not shown.

 The implementation of the process is carried out in the following manner: the sheath 10 not being inflated, a series of layers of fibers 23-24 is draped first on each of the mandrels 1, ensuring that the layers can follow the expansion of the sheath 10. Preferably, as shown in FIG. 3, one or more plies 23 covering a little more than the upper half of the mandrel will be used alternately with one or more plies 24 covering a little more than the lower half of the mandrel so that the edges of the sheets overlap, however allowing their relative free movement.

 The lower part 16 of the mold is prepared by covering the imprint 17 with one (or more) ply 25 and then the mandrels coated with the fiber plies are placed in the mold.

All the mandrels are coated with an (or more iejrs) sheet, upper matching the imprint 19 of the upper plate of the mold and the mold is closed. Naturally, all the layers of fibers are impregnated with resin as is well known.

 It will be noted that in this position, the sheets of fibers draping the mandrels are spaced from the mold cavities and they are spaced from one mandrel to the other in order to allow the expansion of the inflatable sheaths 10.

 Furthermore, it is possible preferably to place in the areas left free between the inclined faces 4 of two successive mandrels of the wicks or drains 27 as shown in FIGS. 7 and 4. These drains which can for example have the shape of a half cylinder as in the drawings comprise a rigid hollow core 28 and porous (for example pierced with a multitude of holes) coated with a sleeve 29 of fibrous material impregnated or not with resin. The purpose of this drain is to trap air bubbles and excess resin discharged from the mold holes where the fiber sheets are subjected to significant pressure during polymerization.

These drains are intended to remain embedded in the final structure by making an accessory reinforcement. The drains are placed in the mold before closing by inserting them into the housings 30 provided in the mold (see FIG. 4). It will be noted that these drains can possibly protrude horadu from the mold in order to connect them, to a suction device for create a slight aspiration and strengthen their drainage role. It will be noted that it is possible to improve (for very precise parts) the relative prior positioning of the draped mandrels and the drains by exerting traction on the protruding ends of the mold of the mandrels and the drains, that the assembly is located in a position horizontal or in a vertical position.

 When the mold is closed, the assembly begins to heat by circulating a hot fluid in the inner chamber of the mandrel through the tubes 14-15. It will be noted that, as a variant, the heating could advantageously be obtained by a series of electrical resistors (heating cords) 32 fixed to the wall of the mandrel in the space reserved between the latter and the deformable sheath 10 as shown in the figures. 6 and 3.

 When the desired viscosity of the resin is reached, the sheaths 10 are caused to inflate by admitting the fluid under pressure via is pipes 8. The sheets of impregnated fibers are then pressed against each other, an inflation control being able to be provided to ensure spontaneous and correct placement of the sheets.

 When the polymerization is complete, the assembly is cooled from the inside of the beams by circulating a cooling fluid either through the pipes 8 or through the pipes 14-15.

 When the cooling is sufficient, the tubes 10 are deflated, the mold is opened and the mandrels are extracted from the final structure.

 It will be noted that the progressive inflation of the sheaths 10 ensures progressive contacting of the layers of excess fibers in the direction of the spaces occupied by the drains.

 The triangular cavities which remain between the plies of the boxes and the outer skins are therefore filled with resin and the drain so that the outer skin is supported even in this place which is not in direct contact with the plies of box fibers.

 Depending on the composition of the rigid core of the drain, this can be used. the latter to improve the mechanical characteristics of the structure in tension and / or in compression.

Finally, it should be noted that the (non-descriptive) examination of the additional resin accumulated in the drain provides a useful indication of the forces undergone by the structure during its use; operational tion; security is thus improved.

 Furthermore, it will be understood that by using sheaths 10 which do not have a constant thickness, it will be possible to modulate the shape or the section of the final product or to obtain a differentiated evolution of the pressures from one zone to another.

 The forms that can be obtained by the method are numerous and there are shown in Figures 8a, 8b, 8c, 8d some examples of possible embodiments.

 For simplicity, the process descriptions which have been given assume that fibers impregnated with resins which polymerize under heat are treated. it is obvious that the same process, excluding the heating (and possibly cooling) means, applies to the treatment of fibers impregnated with resin which polymerizes when cold.

Claims (11)

EVENDICATIONS  1 - Method for manufacturing hollow rigid structures of complex shapes produced from material in plies of fibers impregnated with polymerizable resins and which must have an outer wall continuous envelope stiffened by internal partitions forming with the outer wall multiple cavities characterized in that that - We prepare several section mandrels respectively adapting to each part of the desired final shape, mandrels equipped with deformable loen envelope. - The plies of fibers are placed on each of the mandrels fitted so that said plies can slide over each other. - - These coated mandrels are placed side by side in a mold. - We cover together mandrels coated with a second series of layers of fibers. - The mold is closed, the controlled envelopes are caused to expand in order to press the sheets of fibers previously impregnated with resin against each other and against the walls of the mold, then  which ensures the polymerization of the resin. - Then the flexible envelopes are retracted, the final structure of the mold is estimated and the mandrels are separated which can be reused for other operations.  9 - Process according to claim 1 characterized in that lton ensures the polymerization of the resin by adding heat to the area located on the internal side of the deformable envelopes. 3 - A method according to any one of claims l to 2 characterized in that after polymerization is provided a forced cooling of the final structure by cooling the area located on the inner side of the deformable envelopes using a cooling fluid. 4 4 - Method according to one of claims 1 to 3 characterized in that one leaves free spaces between the sheets of fibers carried by two successive mandrels and the outer skin so that the air bubbles and the resin contained in excess in the layers of fibers can be concentrated there during the expansion of the envelopes. 5 - Method according to claim 4 characterized in that it promotes the migration of air and excess resin to the free spaces by creating in them a slight depression. 6 6 - Process according to any one of claims 4 to 5 characterized in that the free spaces are filled with a clean drain to absorb the excess resin such as a wick of fibers. 7 - Method according to any of the claims ia - characterized in that, in the free spaces located between the mandrels previously coated with fibers impregnated with resin and the outer skin, there are rigid drains extending over the entire length of the mandrels and consisting of a hollow core whose wall is perforated and a peripheral sheath advantageously made of fibers intended to play the role of wicking to capture the resin, the hollow core being able to be put under vacuum to promote the evacuation of the air and excess resin. 8 - Method according to one any of the preceding claims characterized in that the number of mandrels equipped is reduced to one. 9 9 - Device for implementing the method according to 1 "any one of the preceding claims characterized in that it comprises at least one hollow rigid mandrel on which is threaded an elastically deformable envelope, means for at least locally expanding the deformable envelope in order to move it away from the hollow mandrels, means for mounting and immobilizing the mandrel-envelope assembly in a mold so that the periphery of the envelope is separated from the walls before the phase of expansion of the envelope . 10 - Device according to claim 9 wherein the deformable envelope is fixed in sealed manner to the mandrel outside the zone to deform it - Device according to any one of claims 9 to 10 characterized in that the means for expanding the deformable envelope consist of a series of tubes housed inside the mandrel, these tubes being connected to orifices made in the periphery of the mandrel and opening into the space between the mandrel and the deformable envelope, said tubes being connected to a source of pressurized fluid through their other end. 12 - Device according to any one of claims 9 A it characterized in that the mandrel-envelope assembly is equipped with heating and cooling means acting in the area located inside the deformable envelope. 13 - Device according to any one of claims 9 to 12 characterized in that the mandrels have the form of tubes of polygonal section. 14 - As an industrial product, a new hollow rigid structure comprising a continuous peripheral surface forming the upper and lower faces of the cells extending over the entire length of the structure and of the partitions connecting the upper and lower faces over the entire length of the structure. and separating the cells from each other, the said structure being produced in a single block from materials in layers of fiber impregnated with resin.  15 - Structure according to claim 14 characterized in that each cell is delimited by a first internal series of sheets of fibers and in that "a second external series of sheets of fibers surrounds all of the cells, the partitions being obtained by portions of the first internal series of tablecloths. 16 - Structure according to any one of claims 14 and 15 characterized in that wicks of fibers are inserted under the second external series of plies and in the extension of each partition, these wicks being embedded in the resin and extending in the length of the structure.