FR3141377A1 - METHOD FOR MANUFACTURING A PART MADE OF DISCONTINUOUS LONG FIBER COMPOSITE MATERIAL WITH THERMOSETTING MATRIX - Google Patents

Abstract

The invention relates to a method of manufacturing a part made of composite material, comprising the use of a mold in two parts (32,34) defining between them a cavity (35) intended to contain a thermosetting resin and long fibers discontinuous, the cavity and the two mold parts being arranged along a main axis (Z), the method comprising the following steps: - heating the two parts of the mold (32,34) from two heating elements (40,42) in contact and arranged therewith along the main axis, so as to soften and make the resin sufficiently viscous in an area of the cavity (35) arranged in an axial position, taken along the main axis, which is closer of a heating element than another zone of the cavity, - application of pressure to the material formed by the resin and the fibers in the cavity zone, along a secondary axis (X) oriented differently from the main axis . Figure for abstract: Fig. 3

Description

METHOD FOR MANUFACTURING A PART MADE OF A COMPOSITE MATERIAL WITH LONG DISCONTINUOUS FIBERS WITH A THERMOSETTING MATRIX

This disclosure relates to a method of manufacturing a part made of a composite material with discontinuous long fibers with a thermosetting matrix.

The manufacture by thermocompression of parts in composite material with discontinuous long fibers or DLF (acronym known in English terminology as "Discontinuous Long Fibers") is known and generally requires heating the material and applying pressure to it in order to consolidate the material. Long fibers are generally fibers with a length ranging from 5 to 60 mm. The term DLF is used to designate composite assemblies formed of a matrix and discontinuous long fibers.

Conventionally, the material, particularly with a thermosetting matrix, is introduced into a tool which has been previously heated, in order to minimize manufacturing cycles.

A pre-curing can also be carried out on the material before inserting it into the tool in order to minimize the viscosity of the resin during the pressurization phase in the tool and, thus, to avoid expelling the resin from the tool during this phase.

An installation 10 for manufacturing a part made of composite material generally comprises, as schematically illustrated in the , an upper tool 12 and a lower tool 14 jointly defining between them, when the tools are arranged against each other (closed mold), one or more cavities 16 to be filled with the material which, once solidified, will constitute the part of the desired shape. The two tools 12 and 14 are framed by heating plates, an upper one 18 arranged above the upper tool 12 and the other lower one 20 arranged below the lower tool 14. On the , the cavity 16 has a fairly simple general shape without any notable difference in thickness along the vertical axis Z, nor along the horizontal axis X. The shape of the cavity which extends along the axis Y perpendicular to the two axes X and Z of the is substantially uniform and its representation is not of any real interest here.

In the case of a composite material part with a simple shape as in the , all points of the cavity are substantially located at the same distance from the upper 18 and lower 20 heating plates so that the mass of the upper 12 and lower 14 tools which must be heated before reaching the cavity is identical at all points of the cavity.

On the contrary, when the composite material parts to be manufactured have complex shapes and/or numerous folds and/or differences in thickness, this results in heterogeneity of thermal behavior in the different areas of the tooling due to the differences in distance from the heating plates from one area to another, which can influence the state of the resin during the heating phase (in particular the progress of the resin in the tooling and its viscosity).

There illustrates an installation 10' for manufacturing a composite material part of complex shape in which the upper 12' and lower 14' tools jointly define between them a cavity 16' of more complex shape than that of the . As shown in this figure, particular points P1 to P3 of the cavity 16' have been identified as an example in order to show that, on the one hand, the points P1 and P2 which are closer to the respective heating plates 18 and 20 than the point P3 are heated more quickly than the latter and therefore constitute hot spots and, on the other hand, that the point P3 constitutes a cold spot in the tooling. As an example, heating differences of up to several degrees per minute and occasional temperature differences of up to several tens of degrees can be observed between different zones of the cavity and in particular between the points P1, P2, on the one hand, and P3, on the other hand.

In such a case, it is known to carry out thermal mapping in order to minimize these differences. For example, the temperature of the heating plates is controlled by zone and temperature instructions varying from one zone to another are thus applied during the heating phase.

However, in the case of parts with complex shapes which may, for example, include two portions located in perpendicular planes and exhibiting curvatures, as well as variations in thickness, significant deviations could still be noted, even after optimization of the heating which was made possible by thermal mapping.

Due to these thermal heterogeneities, the resin introduced into the cavity polymerizes heterogeneously between the different zones of the cavity where temperature differences are observed.

However, during the pressurization phase in the tool, the pressure must be applied, on the one hand, to a material that is sufficiently molten so as not to damage it by shearing it and, on the other hand, to a material that is sufficiently viscous so that it is not driven out under the effect of the pressure, which would risk creating areas in the cavity (and subsequently in the part) rich in fibers and lacking in resin.

When the pressure is applied simultaneously and homogeneously to all areas of the cavity as described above, if the pressure is applied appropriately for a hot area, i.e. for the area where the material will be sufficiently softened and viscous, it will however not be appropriate for a cold area in which the material will still be too rigid. Conversely, if the pressure is applied appropriately for a cold area, it will however not be appropriate for a hot area in which the material will be too fluid.

In view of the above, it would therefore be interesting to design a process to at least partially remedy the drawbacks set out above when manufacturing a part made from thermosetting discontinuous fibre composite material.

The invention thus relates to a method for manufacturing a part made of composite material with discontinuous long fibers with thermosetting resin, comprising the use of a mold in at least two parts which jointly define between them at least one cavity intended to contain a thermosetting resin and discontinuous long fibers, said at least one cavity and said at least two mold parts being arranged along a main axis, the method comprising the following steps:
- heating the two parts of the mold from at least two heating elements which are in contact respectively with said at least two mold parts and arranged therewith along the main axis, so as to soften and render sufficiently viscous the resin in at least one zone of said at least one cavity, said at least one cavity zone being arranged in an axial position, taken along the main axis, which is closer to one of the heating elements than another zone of said at least one cavity,
- applying pressure to the material formed by the softened and sufficiently viscous resin and the long discontinuous fibers in said at least one cavity zone, along a secondary axis oriented differently from the main axis and at a different time from the application of pressure along the main axis.

The above method thus makes it possible to take advantage of the heterogeneity of heating of the mold parts and the areas of the cavity by compressing as a priority the area(s) of the cavity that are closest to the heating element(s) and which, therefore, have been heated more quickly than other areas of the cavity, or even than the rest of the cavity. This makes it possible to avoid compressing areas of the cavity in which the material has not reached the desired temperature and therefore the appropriate viscosity. The application of pressure is thus carried out at a different time for one or more areas of the cavity compared to other areas that will be compressed later. The secondary axis may or may not be perpendicular to the main axis.

According to other possible characteristics, taken individually or in combination:
- the method comprises applying pressure along the main axis to the material occupying the remainder of the cavity after applying pressure along the secondary axis;
- the application of pressure to the material depends on the prior heating step;
- the application of pressure along the secondary axis is carried out when the viscosity of the resin is between 5 and 1000 Pa.s, preferably between 5 and 100 Pa.s;
- the application of pressure along the secondary axis is carried out at the end of a first predetermined period of time from the start of heating;
- the application of pressure along the main axis is carried out at the end of a second predetermined period of time greater than the first predetermined period of time;
- the application of pressure along an axis perpendicular to the main axis and to the secondary axis is carried out at the end of a third predetermined time period greater than the second predetermined time period.

The invention also relates to an installation for manufacturing a part made of composite material with discontinuous long fibers with thermosetting resin, comprising:
- a mold in at least two parts which jointly define between them at least one cavity intended to contain a thermosetting resin and long discontinuous fibers, said at least one cavity and said at least two mold parts being arranged along a main axis,
-at least two heating elements which are in contact respectively with said at least two mold parts and arranged therewith along the main axis,
-at least one pressure application device which is configured to apply pressure, along a secondary axis oriented differently from the main axis (for example perpendicular), to a material formed by softened and sufficiently viscous resin and by long discontinuous fibers and which is located in at least one zone of said at least one cavity, said at least one cavity zone being arranged in an axial position, taken along the main axis, which is closer to one of the heating elements than another zone of said at least one cavity.

Such an installation is used for the implementation of the method briefly described above. In one embodiment, the heating elements of this installation can be relatively simple since it is sufficient that they ensure uniform heating of the two mold parts.

According to other possible characteristics, taken alone or in combination:
- said at least one pressure application device is chosen from a press, a jack;
- said at least one device for applying pressure along the secondary axis is controlled by the heating elements;
- said at least one device for applying pressure along the secondary axis is programmed so as to apply pressure as a function of the thermal response of the two mold parts or in relation to a prior thermal study (a possible thermal study provides for example for installing thermocouples in the mold cavity, performing heating/cooling cycles, analyzing the responses of the different thermocouples and adjusting the cycles/zones accordingly);
-the installation comprises at least one pressure application device which is configured to apply pressure, along the main axis, to the material located in the cavity.

Other characteristics and advantages of the subject of the present disclosure will emerge from the following description of embodiments, given as non-limiting examples, with reference to the appended figures.

There is a schematic general view of an installation for manufacturing a part made of composite material of simple shape according to the prior art;

There is a schematic general view of an installation for manufacturing a composite material part of complex shape according to the prior art;

There is a schematic general view of an installation for manufacturing a composite material part of complex shape according to one embodiment of the invention.

Detailed description

As shown in the and designated by the general reference noted 30, an installation for manufacturing composite material parts of complex shapes according to an embodiment of the invention will now be described.

The composite material part that is manufactured in this example is made of long discontinuous fibers and can be applied in many industrial sectors such as aeronautics, automobiles, railways, etc. More particularly, the parts that can be manufactured by the installation 30 can be applied to aircraft engines, landing gear or nacelles and can, for example, be flow straighteners, cowlings, thrust reverser grilles, acoustic panels, structural panels, etc.

The installation 30 comprises a mold comprising at least two parts formed here by an upper tool 32 and a lower tool 34. Each of the tools comprises an internal surface which is shaped in a suitable manner so that, when the two tools are brought against each other as in the and that the mold is thus closed the two internal surfaces facing the two tool parts which each have a relief form, jointly, the outline or the envelope of a closed cavity 35 internal to the mold and which will house the part to be manufactured. The outline of the cavity defines the external outline of the future part. In the example illustrated on the , the part has a section following a view in a plane X, Z which has the general appearance of a lying T whose bar 36 is arranged substantially vertically (along the Z axis) on the and the leg 38 is arranged substantially horizontally. It will be noted that the leg 38 is however not straight and is slightly curved with its free end 38a raised upwards and a central portion 38b directed downwards. The part also extends in a third direction Y perpendicular to the plane of the , for example, by retaining the general shape of the section illustrated on the .

The two upper and lower mold parts or tools are arranged here along a vertical Z axis, one above the other. However, other spatial arrangements can be envisaged and, for example, an arrangement in which the two tools are arranged along a horizontal axis.

The installation 30 also comprises two heating elements 40 and 42 which are in the form of heating plates, one upper 40 arranged above the upper tool 32 and in contact with the latter in order to optimize the heat transfer between these two elements, and the other lower 42, arranged below the lower tool 34 and in contact with the latter for the same reasons. The dimensions along the X axis of the heating plates are for example greater than the dimensions of the tools because the heating plates are generally those of the press used for compression.

It will be noted that, in this embodiment, the installation makes it possible to have relatively simple heating elements since it is not necessary to provide, as in the prior art, heating by heating zone of the heating elements.

In this embodiment, the Z axis is considered the main compression axis while the X axis is considered the secondary compression axis. However, other arrangements are possible and the secondary axis is not necessarily perpendicular to the main axis, it is in any case oriented differently from the main axis.

A compression device along the main axis which is conventionally used in the type of installation of the prior art as illustrated in FIGS. 1 and 2 is part of the installation 30 and the heating plates 40 and 42 are part of this compression device.

In the example illustrated on the , the internal cavity 35 is defined by the internal surfaces of the two tools 32 and 34 and also by a device 50 for applying pressure along the secondary axis X. This device 50 is positioned laterally relative to the two tools arranged one above the other and thus closes laterally, by a so-called application end of the device, the cavity 35. It will be noted that the two tools are configured laterally so as to partially accommodate the application end of the device 50. The device 50 may be a jack which, under the action of a command, applies pressure to the material arranged in the area of the cavity opposite the device. This is the area where the future branch 36 of the T-shape of the part to be manufactured is located. The application end of the device is shaped in a manner adapted to the shape of the portion of the part located in the cavity area opposite.

In general, the part to be manufactured can take different shapes (the cavity is therefore designed according to such shapes) and in particular shapes where a portion of the part extends in a first direction and another portion of the part extends in a second perpendicular direction. The portion of the part which extends in a direction arranged perpendicular to the heating elements of the installation is arranged on one of the sides of the part to allow priority to be given to receiving lateral pressure in a direction perpendicular to the direction of extension of the portion of the part. In the example of the , the portion of the part in question is portion 36, here vertical.

A third pressure application device along the third Y axis can also be used to compress the part being produced axially along this axis.

The manufacture of the part begins, in a known manner, by arranging inside the cavity of the mold long discontinuous pre-impregnated fibers, for example carbon (or for example glass or aramid), which fit the walls (internal surfaces of the tools) of the cavity forming the contour thereof. In practice, one of the tools, in particular the upper tool 32, is moved away along the Z axis from the lower tool 34 to be able to fill the bottom of the cavity which is arranged in the lower tool with fibers. When this step is carried out, the upper tool 32 can come into contact with the lower tool 34 along a joint plane and the device 50 comes into contact with lateral zones of the upper and lower tools in order to close the mold laterally. As an example, we start with a pre-impregnated ply with continuous, unidirectional or woven fibers. The ply is cut into coupons, for example 50 mm x 8 mm or 12.5 x 12.5 mm. It is also possible to start from continuous, cut pre-impregnated wicks.

This material is either introduced in bulk into the tooling or is supplied in sheets (e.g. Hexcel’s HexMC® (registered trademark) material with 8552 resin) from which patterns are cut and then assembled into a preform for introduction into the tooling. The assembled/formed preform may undergo a pre-cure cycle to increase the minimum viscosity of the resin.

In the next step, heating is applied via the heating plates 40 and 42 in order to heat the upper 32 and lower 34 tools and the material in the cavity 35.

The heating is applied in a controlled manner, for example by following a temperature rise ramp with a predetermined number of degrees per minute which makes it possible to reach, after a predetermined time, the desired temperature at the hot points of the cavity, in particular P'1 and P'2 on the . These points are those which are located in a zone of the cavity which is located closest respectively to the two heating elements 40 and 42 and which will therefore heat more quickly than the other points of the cavity such as for example the point P'3 which is in a distinct zone of the cavity, further away from the heating elements. In other words, the point P'3 is positioned axially in a position further away from the heating elements than the points P'1 and P'2. It will be noted that the same principle can be applied to cavities of different shapes of which one or more zones are located closer to the heating elements than other zones of the cavity. This is particularly the case with cavity zones perpendicular to each other as explained previously with the portions of parts to be manufactured of complex shapes. This positioning difference can also come from differences in thickness at the level of different zones of the part to be manufactured and/or from different folds in different zones of the part to be manufactured.

The temperature profile and the applied temperature values are predetermined (The thermal profile that is desired at any point of the tooling is determined by resin advancement and rheology models that are developed from physicochemical tests on the materials. Adaptation to the part is carried out iteratively during thermal mapping.) and the heating profile can be stopped or modified (for example, the heating temperature can be maintained at a plateau value) when the desired temperature for the hot spots P’1 and P’2 is reached. At this temperature, the resin in the area considered with the hot spots is softened but remains sufficiently viscous to be subjected to external pressure and flow in a controlled manner inside the cavity without risking being forced out of the cavity under the effect of this pressure.

For example, the viscosity of the resin is between 5 and 1000 Pa.s, preferably between 5 and 100 Pa.s for the 8552 resin mentioned in the example above, so that it can be subjected in a controlled manner to external pressure.

In this example, the pressure application device 50 is controlled by the heating plates 40 and 42 so that the application of pressure by the device is controlled when the temperature rise profile of the heating plates reaches a predetermined value. The pressurization of the area of the cavity 35 where the hot spots are located is for example controlled after a predetermined period of time, after reaching the temperature value predetermined by the temperature rise profile. This localized pressurization of the material along the secondary compression axis is thus particularly suited to the viscosity state of the latter in the cavity area considered. It would indeed have been detrimental to wait until all the points of the cavity were at a higher temperature to compress the material as a priority along the main compression axis.

When pressure is to be applied along the secondary axis X (lateral pressure), the temperature of the area where the cold point P’3 is located is too low for the resin present there to be sufficiently softened, but heating continues when pressure is applied so that the softening of the resin occurs in all areas of the part.

When the pressure has been applied satisfactorily to the material most suitable for compression (located in an area closer to the heating elements than the rest of the cavity) and the other points of the cavity are at the desired temperature for the resin to be softened and sufficiently viscous, the material present in the cavity can be compressed along the main compression axis Z (the other areas of the cavity not yet compressed during the first compression are therefore compressed here; it should be noted that the first areas of the cavity that have already been compressed are compressed again but along another axis and in a time-shifted manner). This second compression phase occurs after a predetermined period of time after the first compression phase along the secondary axis.

Similarly, when this second phase of compression along the main axis is completed, a third phase of compression of the material along the third Y axis occurs after a predetermined period of time after the second phase of compression.

For example, pressures ranging from 20 bar to 150 bar can be used for composites with long discontinuous fibers. Possible temperatures can be 100°C, 150°C or 180°C, depending on the complete thermal cycle with the ramps.

In an alternative embodiment, it will be noted that it is possible to carry out thermal mapping in order to characterize the temperature differences between the different points of the different zones of the cavity, and this, with a view to trying to reduce these differences. To do this, the heating plates comprise zones whose temperature setting can be controlled independently of each other.

The parts thus manufactured may exhibit reduced dispersion of the fiber volume ratio and/or reduced porosities. Although the present description refers to specific exemplary embodiments, modifications may be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various embodiments illustrated or mentioned may be combined in additional embodiments. Accordingly, the description and drawings are to be considered in an illustrative rather than restrictive sense.

Claims (12)

Manufacturing process for a part made of composite material with long discontinuous fibers with thermosetting resin, comprising the use of a mold in at least two parts (32,34) which jointly define between them at least one cavity (35) intended to contain a thermosetting resin and long discontinuous fibers, said at least one cavity and said at least two mold parts being arranged along a main axis (Z), the method comprising the following steps:
- heating the two mold parts (32,34) from at least two heating elements (40,42) which are in contact respectively with said at least two mold parts and arranged therewith along the main axis, so as to soften and render sufficiently viscous the resin in at least one zone of said at least one cavity (35), said at least one cavity zone being arranged in an axial position, taken along the main axis, which is closer to one of the heating elements than another zone of said at least one cavity,
- applying pressure to the material formed by the softened and sufficiently viscous resin and the long discontinuous fibers in said at least one cavity zone, along a secondary axis (X) oriented differently from the main axis and at a different time from the application of pressure along the main axis. A method according to claim 1, comprising applying pressure along the major axis (Z) to the material occupying the remainder of the cavity after applying pressure along the minor axis (X). A method according to claim 1 or 2, wherein the application of pressure to the material is dependent on the prior heating step. Method according to one of the preceding claims, according to which the application of a pressure along the secondary axis (X) is carried out when the viscosity of the resin is between 5 and 1000 Pa.s, preferably between 5 and 100 Pa.s. Method according to one of the preceding claims, according to which the application of pressure along the secondary axis (X) is carried out at the end of a first predetermined period of time from the start of heating. Method according to the preceding claim, according to which the application of pressure along the main axis (Z) is carried out at the end of a second predetermined period of time greater than the first predetermined period of time. Method according to the preceding claim, according to which the application of pressure along an axis perpendicular to the main axis (Z) and to the secondary axis (X) is carried out at the end of a third predetermined time period greater than the second predetermined time period. Installation (30) for manufacturing a part in composite material with discontinuous long fibers with thermosetting resin, comprising:
- a mold in at least two parts (32, 34) which jointly define between them at least one cavity intended to contain a thermosetting resin and long discontinuous fibers, said at least one cavity and said at least two mold parts being arranged along a main axis (Z),
-at least two heating elements (40,42) which are in contact respectively with said at least two mold parts (32,34) and arranged therewith along the main axis (Z),
-at least one pressure application device (50) which is configured to apply pressure, along a secondary axis (X) oriented differently from the main axis (Z), to a material formed by softened and sufficiently viscous resin and by long discontinuous fibers and which is located in at least one zone of said at least one cavity, said at least one cavity zone being arranged in an axial position, taken along the main axis (Z), which is closer to one of the heating elements (40,42) than another zone of said at least one cavity. Installation according to the preceding claim, in which said at least one pressure application device (50) is chosen from a press, a jack. Installation according to claim 8 or 9, in which said at least one pressure application device (50) along the secondary axis is controlled by the heating elements (40, 42). Installation according to one of claims 8 to 10, in which said at least one pressure application device (50) along the secondary axis is programmed so as to apply pressure as a function of the thermal response of the two mold parts or in relation to a prior thermal study. Installation according to one of claims 8 to 11, comprising at least one pressure application device (50) which is configured to apply pressure, along the main axis, to the material located in the cavity.