FR2658116A1 - Method of stiffening the rims of a honeycombed filling structure of a panel made of composite material, method of manufacturing such a panel and panel obtained according to this method - Google Patents

Abstract

In order to manufacture a panel made of composite material, the edge of a honeycombed filling structure (10) is cut at a bevel with an angle between 25 DEG and 40 DEG , and then a film of hot-melt adhesive (14) is deposited onto this edge. This film is heated, so as to impregnate the walls (12) of the cells to a certain depth, and then it is hardened by cooling. The edge of the structure (10) is thus stiffened sufficiently for the latter to be able to withstand, without being deformed, the pressure applied during the polymerisation cycle of the resin which impregnates the fibres constituting the skins (16, 18) with which the honeycombed structure is subsequently coated.

Description

METHOD FOR STRENGTHENING THE EDGES OF A STRUCTURE
HONEYCOMB FILLING OF A MATERIAL PANEL
COMPOSITE, PROCESS FOR PRODUCING SUCH A PANEL, AND
PANEL OBTAINED ACCORDING TO THIS PROCESS.

DESCRIPTION
The invention relates to a method for stiffening the edges of a honeycomb filling structure constituting the core of a panel made of composite material. The invention also relates to a method of manufacturing a panel of composite material incorporating such a method of stiffening the edges.

Finally, the invention also relates to a panel made of composite material obtained according to this latter process.

 Panels made of composite material, the core of which is made up of a honeycomb filling structure, are used more and more frequently to constitute structural elements in various industries and in particular on aircraft.

 In panels made of composite material, the core of which is formed by a honeycomb filling structure, this structure is placed between two thin skins each formed from one or more superimposed layers made up of fibers impregnated with thermosetting resin. The fibers constituting these layers can be woven or non-woven and they are oriented in directions chosen according to the resistance characteristics which it is desired to give to the panels. These fibers can be carbon, glass, kevlar, etc. depending on the intended application.

 To manufacture such panels, the filling structure covered by the skins is placed inside an autoclave in which the panel blank is subjected to a hot and pressurized polymerization cycle. The pressure is for example obtained by placing the blank of the panel in a sealed envelope in which a partial vacuum is created and by applying a certain pressure to the outside of this sealed envelope.

 On the edges of the panel, the skins of the two faces extend beyond the peripheral edge of the honeycomb filling structure and are joined to one another during the polymerization. The area formed by the peripheral edge of the skins enables the panel to be fixed to a support frame or to another panel by any fixing means such as bolts, rivets or glue.

 When the panel blank is pressurized during polymerization, the periphery of the honeycomb filling structure constitutes a critical zone for the correct adhesion of the skins to this structure. Indeed, the force due to the pressure exerted on the blank comprises in this zone a lateral component, oriented inward parallel to the faces of the panel, to which the honeycomb filling structure does not oppose a sufficient back pressure, given its low mechanical resistance in this direction. Consequently, the honeycomb filling structure tends to deform locally at its periphery, so that a satisfactory connection between the skins and this structure cannot be obtained.

 Various solutions have already been proposed in an attempt to remedy this drawback.

 A first known solution consists in cutting the honeycomb filling structure perpendicular to the plane of the panel, then in placing on the cells near the edges a film of an intumescent product. During hot polymerization, this product increases in volume and fills the cells.

In order to prevent a deformation from occurring before this product becomes sufficiently rigid, a system of rules is arranged laterally over the entire periphery of the panel and is used to transmit the pressure to the edges of the panel during polymerization.

 This known method of stiffening the edges leads to a significant increase in the weight of the composite material panels since the weight of the intumescent product is approximately 80 grams per linear meter.

In addition, the need to use a system of rules leads to having complicated tools and a relatively long manufacturing time, and consequently, at an additional cost which is further accentuated by the difficulties in automating this process for the make it industrial.

 Furthermore, document EP-AO 311 931 describes another solution to this problem, consisting in placing a honeycomb frame at the periphery of the honeycomb filling structure, the cells of which are arranged in an inclined direction. relative to the plane of the panel and to cut this bevel frame, so as to form a peripheral edge substantially normal to this inclined direction.

 If this latter solution makes it possible to obtain a panel whose edge is reinforced without any increase in weight, the manufacturing time remains relatively high, as does the cost of the panel, and automated manufacturing seems very difficult to implement.

 The subject of the invention is precisely a new method of stiffening the edges of a honeycomb filling structure making it possible to reinforce the edge of this structure sufficiently so that the latter can withstand the pressure applied during polymerization, without increasing the weight too penalizingly and for a manufacturing time significantly shorter than that of the existing methods of stiffening existing curbs, which implies a significant reduction in the manufacturing cost and in scrap rates, this process being able to be further automated as part of an industrial manufacturing of panels.

To this end and in accordance with the invention, a method is proposed for stiffening the edges of a honeycomb filling structure of a panel made of composite material, characterized in that it consists of - cutting into bevel the edge of said structure of
filling - place a film of hot-melt adhesive on the edge
thus cut out; - heat this film of glue so as to liquefy this
last and cause its deposition by capillarity
on the walls of said structure adjacent to the edge
cut; then - cool the hot-melt adhesive so as to stiffen
the latter and to stiffen the walls of the structure
fill adjacent to the cut edge.

 Advantageously, the edge of the filling structure is cut at a bevel at an angle between approximately 250 and approximately 400 and, more precisely, between approximately 350 and approximately 400 relative to the plane of the panel.

The invention also relates to a method of manufacturing a panel made of composite material incorporating the aforementioned method of stiffening the edges of the honeycomb filling structure of the panel. According to this manufacturing process, the stiffening of the edges of the filling structure is followed by the steps consisting of: - placing the skins on both sides of the structure
honeycomb, to form a panel blank; - subject this blank to a polymerization cycle
including a first phase during which
we mark a plateau at a first lower temperature
higher than a polymerization temperature of the resin
permeating the skins, maintaining the blank in
depression and then a second phase during the
what is the polymerization temperature
resin by applying pressure to the blank
of panel.

 Advantageously, the plateau reached during the first phase of the polymerization cycle corresponds to a temperature of approximately 1250C and lasts approximately 1 hour. This bearing makes it possible to partially polymerize the adhesive in order to ensure the mechanical blocking of the honeycomb filling structure on the two skins.

It occurs before the start of the resin gel which permeates the fibers contained in the two skins.

 Finally, the invention also relates to a panel made of composite material obtained according to the aforementioned manufacturing method, this panel comprising a honeycomb filling structure placed between two skins formed of fibers impregnated with polymerized resin, the edge of the filling structure being cut at a bevel, characterized in that the walls of the filling structure adjacent to the edge cut at a bevel are stiffened by a film of stiffened hot-melt adhesive.

A preferred embodiment of the invention will now be described, by way of nonlimiting example, with reference to the accompanying drawings, in which
- Figure 1 is a sectional view showing the edge of a panel of composite material produced according to the invention;
- Figure 2 is a perspective view, partially cut away, showing the honeycomb filling structure of the panel of Figure 1 after stiffening of the edges of this structure according to the invention; and
- Figure 3 shows, in solid lines, the variation curve of temperature T (in C) as a function of time (in hours) and, in broken lines, the variation of pressure P (in Bars) as a function of time (in hours) during the polymerization cycle of the resin impregnating the fibers contained in the two skins of the composite material panel, during the manufacture of this panel.

 The section of Figure 1 shows the edge of a composite material panel made in accordance with the invention. In this figure, the reference 10 generally designates a honeycomb filling structure formed of juxtaposed cells delimited by walls 12 and whose axes are oriented parallel to a direction perpendicular to the two faces of this structure. The nature of the material constituting the walls 12 of the honeycomb filling structure 10 is determined according to the envisaged application, among the existing products. Because of its particular shape, such a structure has very good resistance to the compressive forces exerted perpendicular to its faces. On the other hand, its resistance to compression forces oriented parallel to its faces is poor.

 According to the invention, a first step in manufacturing the panel illustrated in FIG. 1 consists in cutting the peripheral edge of the filling structure 10 at a bevel. This cutting can be carried out by any suitable means and in particular by means of a machine. cutting by fluid jet under high pressure. The angle formed by the inclined edge thus produced with the two faces of the filling structure 10 is preferably between approximately 250 and approximately 400. A value close to 400 and, more precisely, between approximately 350 and approximately 400 is satisfying health.

 According to an essential characteristic of the invention, the step of cutting the peripheral edge of the filling structure 10 is followed by a step consisting in depositing on the previously cut edge a film of unsupported hot-melt adhesive. This adhesive film is designated by the reference 14 in FIGS. 1 and 2. By way of example, the adhesive sold by the company 3M under the name 3109-2 can be used, this adhesive is based on epoxy resin which can be polymerized to 1250C and able to withstand a temperature of 175 / 1800C.

 As illustrated in FIGS. 1 and 2, this film of adhesive is deposited not only on the beveled face previously cut out from the filling structure 10, but also on the parts of the two opposite faces of this structure adjoining this beveled face. The area for depositing the adhesive film corresponds to the area in which it is desired to obtain mechanical attachment between the honeycomb filling structure 10 and the skins which must subsequently cover it. In practice, the adhesive film is deposited after the layers of protective products which usually cover it for handling and transport have been removed.

The deposition of the adhesive film 14 on the peripheral edge of the honeycomb filling structure 10 corresponds to an increase in weight of approximately 18 grams per linear meter of edge, which is much less than the weight of the intumescent product used. according to
the prior art.

When the adhesive film has been deposited, it is heated, for example by means of the jet of hot air from a blower, to instantly soften it until
make it practically liquid. The glue thus liquefied comes by capillarity to be deposited on the walls 12 of the cells of the honeycomb structure 10, 1 or 2 mm deep. This deposit has been deliberately exaggerated in Figure 1, to facilitate reading.

 The adhesive 14, which then permeates the walls 12 of the cells near the external faces of the peripheral edge of the structure 10 is then cooled.

This cooling can be carried out naturally until the adhesive returns to room temperature or obtained using an appropriate cooling means accelerating the process.

 The adhesive then becomes rigid and has the effect of locally stiffening the walls 12 of the cells, near the faces of the filling structure 10 delimiting the peripheral edge of the latter. This local stiffening of the walls 12 of the cells gives the honeycomb filling structure 10 thus prepared sufficient rigidity to allow its peripheral edge to collect a pressure of up to 3 bars without undergoing any significant deformation.

 This rigidity therefore allows the honeycomb filling structure to withstand without deformation the pressure applied subsequently during the polymerization of the resin contained in the skins between which the filling structure 10 is then placed. It thus guarantees a healthy adhesion of these skins on the filling structure.

 The various operations which have just been described make it possible to produce the edges of the honeycomb filling structure 10.

 When these edges are finished, the manufacture of the composite material panel is continued by placing the filling structure 10 between two skins 16 and 18 shown very schematically in FIG. 1. Each of these skins 16 and 18 consists of a or several layers, woven or non-woven, formed of fibers impregnated with a thermosetting resin. The nature of these fibers, as well as their orientation, are chosen according to the envisaged application.

 The positioning of the skins 16 and 18 is carried out by a draping operation, carried out so that each of the skins extends beyond each of the peripheral edges of the honeycomb filling structure 10, so to be able to be secured to each other as seen on the left of Figure 1. This peripheral zone of the composite material panel in which the skins 16 and 18 are in direct contact constitutes a connection zone of this panel to a supporting frame or to another panel by an appropriate fixing means such as bolting, riveting, gluing, etc.

 When the draping of the skins 16 and 18 on the honeycomb filling structure 10 is finished, the panel blank thus formed is placed inside a sealed envelope comprising for example a rigid and flat wall in contact with the lower face of the skin 18 and by a flexible and waterproof membrane in contact with the upper face of the skin 16. This sealed envelope allows a partial vacuum to be applied to the blank of the panel, for example of approximately -0 , 7 Bar.

 The sealed envelope containing the panel blank is then placed in an autoclave or in an oven, to undergo a pressure polymerization cycle.

 To take account of the presence of the stiffened adhesive film 14 on the peripheral edge of the honeycomb filling structure 10, this polymerization cycle is carried out in two stages as illustrated by the curves in FIG. 3. In this figure, the curve in solid lines represents the change in temperature T (in OC) as a function of time t (in hours) and the curve in broken lines represents the change in pressure P (in Bars) as a function of time t.

 During the first phase (I) of this polymerization cycle, the panel blank is maintained under a vacuum, for example of about 0.7 Bar.

Simultaneously, a temperature rise for example of around 20C / min is carried out, in a conventional manner, but this temperature rise is stopped at the instant tl or a temperature of around 1250C is reached (point
A in Figure 3). A plateau is then marked at this temperature for approximately 1 hour. The purpose of this bearing is to partially polymerize the adhesive in order to ensure the mechanical blocking of the honeycomb filling structure 10 on the skins 16 and 18 pre-impregnated with resin, before the gel begins to gel.

The end of the level corresponds to point B in Figure 3.

 The second phase of the polymerization cycle is designated by (II) in FIG. 3. This second phase begins at the end of the plateau for approximately 1 hour during which the temperature is maintained at approximately 1250C. It is characterized by the application to the panel blank of a pressure of approximately 2 bars and by the resumption of the conventional polymerization cycle.

 Thus, the temperature rise continues, for example at around 20C / min, until the polymerization temperature is obtained, close for example to 1800C at time t2 (point C in Figure 3). A plateau is then marked for approximately 2 hours at this latter temperature (point D), then the panel blank is cooled in the usual way. When the temperature becomes sufficiently low, the pressure applied to the workpiece blank is reduced to 0.

 A comparison of the process according to the invention with the earlier process of coating the peripheral edge of the honeycomb filling structure with an intumescent product shows that the process according to the invention makes it possible to appreciably reduce the manufacturing time of a panel as well as the increase in mass resulting from the stiffening of its edges. Taking these two factors into account leads to a significant reduction in the cost of the panel.

 Furthermore, the mechanical rigidity obtained by filling the peripheral edge of the honeycomb filling structure with adhesive in accordance with the invention makes it possible to obtain panels practically free from local deformation and in which the quality of the connection between the skins and the honeycomb filling structure is very satisfactory. The scrap rate due to an insufficient quality of this link is therefore significantly reduced.

 Of course, the invention is not limited to the embodiment which has just been described by way of example, but covers all its variants. In particular, different types of glue can be used and it is easy to see that the temperatures which must be reached, in particular when the glue is heated and during the first phase of the polymerization cycle, can vary, as can the duration of the marked step. during the first phase of this cycle, depending on the nature of the adhesive. The angle a of the bevel cut of the peripheral edge of the honeycomb filling structure can also vary in particular to take account of the thickness of the panel.

Claims (11)

 1. method for stiffening the edges of a honeycomb filling structure (10) of a panel made of composite material, characterized in that it consists of - cutting the edge of said structure at a bevel  filling; - place a film of hot-melt adhesive (14) on the  edge thus cut; - heat this film of glue so as to liquefy this  last and cause its deposition by capillarity  on the walls of said structure adjacent to the edge  cut; then - cool the hot-melt adhesive so as to stiffen  the latter and to stiffen the walls of the structure  fill adjacent to the cut edge.  2. Method according to claim 1, characterized in that the edge of the filling structure is cut at a bevel at an angle (a) between about 250 and about 400 relative to the plane of the panel.  3. Method according to claim 2, characterized in that said angle (a) is between approximately 350 and approximately 400.  4. Method for manufacturing a composite material panel comprising a honeycomb filling structure (10) placed between two skins (16,18) formed of fibers impregnated with polymerized resin, characterized in that it consists to - bevel the edge of said structure of  filling; - place a film of hot-melt adhesive (14) on the  edge thus cut; - heat this film of glue so as to liquefy this  last; - cool the glue so as to stiffen it  last; - place said skins on both sides of the struc  honeycomb structure, to form a blank of  panel - subject this blank to a polymerization cycle  comprising a first phase (I) during which  we mark a plateau at a first lower temperature  higher than a polymerization temperature of the resi  do, keeping the blank under vacuum, then  a second phase (II) during which we reach  the polymerization temperature of the resin in application  as a pressure on the workpiece blank.  5. Method according to claim 4, characterized in that the first temperature is around 1250C.  6. Method according to any one of claims 4 and 5, characterized in that the duration of said level is approximately one hour.  7. Method according to any one of claims 4 to 6, characterized in that the edge of the filling structure is cut at a bevel at an angle (a) between about 250 and about 400 relative to the plane of the sign.  8. Method according to any one of claims 4 to 7, characterized in that the said angle (a) is between approximately 350 and approximately 400.  9. Composite material panel comprising a honeycomb filling structure (10) placed between two skins (16,18) formed of fibers impregnated with polymerized resin, the edge of the filling structure being cut in a bevel, characterized by the fact that the walls of the filling structure adjacent to the bevel cut edge are stiffened by a film of rigidified hot-melt adhesive (14).  10. Panel according to claim 9, characterized in that the edge cut at a bevel makes an angle between about 250 and about 400 relative to the plane of the panel.  11. Panel according to claim 10, characterized in that said angle is between approximately 350 and approximately 400.