WO2016139359A1

WO2016139359A1 - Wooden mould with a heating frame

Info

Publication number: WO2016139359A1
Application number: PCT/EP2016/054707
Authority: WO
Inventors: Henrick Merle; Jérémie HASSIN
Original assignee: Centre National D'etudes Spatiales; Afc Stab
Priority date: 2015-03-05
Filing date: 2016-03-04
Publication date: 2016-09-09
Also published as: FR3033278B1; FR3033278A1

Abstract

The invention relates to a wooden mould (1) comprising at least one wooden stratum made of a plywood panel which is thermally modified by special molecular transformation (10), and at least one functional layer made of a composite material (11), characterised in that the mould (1) comprises a heating frame (20) in the form of at least one electrical network (20a) arranged between said at least one wooden stratum (10) and said at least one functional layer made of a composite material (11).

Description

Wooden mold with heating screen

GENERAL TECHNICAL FIELD The invention relates to machinable wooden blocks, in particular for the constitution of tools such as wooden molds. Such molds make it possible to manufacture prototypes as well as products in series. A particular application of the invention relates to the manufacture of products made of composite materials and more precisely satellite antenna reflectors.

STATE OF THE ART

The use of a machinable wooden block is known for the manufacture of molds. In particular, a plywood strand and / or panel which has been thermally modified to obtain a molecular conversion, is used, which makes it possible to obtain a material of good quality, good strength and good dimensional stability between 80 and 210 ° C. (ie having a coefficient of expansion less than 5.10 ^"6 mm / ° C), cheaper, easy to repair and reusable, EP 1 982 828 discloses such a block made of wood.

To improve the qualities of such a mold, in particular in terms of performance, mechanical strength and dimensional stability, to approach a one-piece carbon mold the document FR 2 940 926 describes adding to the mold a composite layer with binder, such as resin, and fibers among glass fibers, carbon, polyamide, aramids, bamboo.

This mold has a competitive manufacturing cost and has the advantageous characteristics mentioned above.

The aerospace industry is constantly seeking innovative solutions in order to reduce the costs and duration of manufacturing while guaranteeing performance that meets the needs of more and more demanding. The molds presented above, in wood covered with a layer of composite, are now widely used to manufacture composite parts. They have many advantages compared to the existing technologies of Invar or graphite molds, which are long and expensive to produce.

A particular use of these molds is the manufacture of composite antennas carried out by deposition of the layers of material in contact with the mold, followed by a heating step advantageously carried out under pressure so that the binder, of resin type, polymerizes during the manufacturing process.

The heating stage of the wood and carbon mold (Auto-STAB ™ type), and the molded part can be implemented in several ways.

A known solution is the autoclave (or oven), which transmits heat to the mold and to the convection molded part. The autoclave is used to raise the temperature and pressure, while the oven only allows the temperature rise. This solution has many defects such as high cost, high energy consumption and a relatively long manufacturing cycle time. Another known solution is the use of a heating blanket, with which the mold and the molded part are covered.

The disadvantage of these solutions lies in a lack of homogeneity of the heating of the mold, in particular between the top of the composite part and the bottom which is in direct contact with the mold. Such a lack of homogeneity causes deformations of the mold or especially of the part to be manufactured which are difficult to anticipate and compensate. In addition for the manufacture of antennas, consisting of several layers of composite material stacked, the current solutions are not satisfactory.

In addition, the heating of the molds at high temperatures prevents use "chain", that is to say, with a time between two reduced manufacturing, the mold to be cooled between two manufacture of composite parts.

Therefore, there is a need to improve existing devices and methods.

PRESENTATION OF THE INVENTION

The invention therefore proposes to overcome the disadvantages presented above. For this purpose, the invention provides a wood mold comprising at least one plywood panel plywood panel thermally modified by molecular transformation and at least one functional layer of composite material, characterized in that the mold comprises a heating screen in the form of at least one electrical network disposed between said at least one wooden stratum and said at least one functional layer of composite material.

The advantages of the invention are manifold.

Indeed, the heating weft provides homogeneous endogenous heating, which improves the properties of the mold. In addition, a particularly interesting synergistic effect occurs between the wooden mold and the heating screen: while the wood was previously used especially for its qualities related to fast machining at lower cost, It has been found that the wooden mold has a heat reflective effect, which contributes to further improving the homogeneity of heating (less than or equal to a degree of difference between the different parts of the mold) and to reducing the losses of heat. energy. Such a mold is a technical advance over the usually used molds (such as INVAR or graphite) which diffuse loss of some of the heat. Such a wooden mold with a heating screen can now increase in high temperature endogenously to allow a good polymerization of the product to be manufactured.

The invention is advantageously completed by the following characteristics, taken alone or in any of their technically possible combination: the heating screen is composed of a plurality of electrical networks, each being configured to heat a different zone of the functional layer of material composite, so as to regulate the temperature of each zone,

The mold comprises thermocouples arranged near the electrical network or networks, so as to control the effective temperature of each electrical network,

The mold comprises at least one thermocouple for measuring the temperature at the functional layer of composite material,

The mold comprises the following successive stacking: said at least one wooden stratum, a waterproof layer of composite fabric, the heating weft, the at least one functional layer of composite material, - The mold comprises a cooling network disposed between the wood and a layer of composite material, the cooling network being in the form of a heat transfer fluid network, - the cooling network is disposed between the wood and the sealed layer of composite material,

the functional layer of composite material has a thickness relative to its diameter, said layer being composed of a plurality of sub-layers,

the functional layer of composite material has a thickness of between 5 mm and 10 mm for a mold 1.2 m in diameter, knowing that the thickness varies according to the surface of the mold, preferably equal to 8 mm, said layer being composed of a plurality of sub-layers.

The invention also proposes an assembly comprising a mold described above, in which the heating screen is configured to heat a product to be molded endogenously, the assembly comprising an autoclave or drying oven or a heating blanket, adapted to heat said product in such a way exogenous.

The assembly may further comprise a control unit configured to adjust the heating temperature by means of the heating screen, and the autoclave or the oven or the heating blanket.

In particular, the presence of thermocouples allows a control of the different networks of the electrical frame but especially a coordination with other means of exogenous heating, such as the autoclave or the heating blanket. This gives, on both sides of the room, a gap less than or equal to 1 ° C, that is to say that the difference in temperature between the functional surface of the mold and the atmosphere in contact with the workpiece is less than or equal to 1 ° C.

Thus, coupling the heating screen and an autoclave (or a heating blanket), we obtain an endo- and exogenous heating controlled to be optimized.

The invention also proposes a method of manufacturing a product made of composite material by means of a mold or a set described above, comprising the steps of:

- Placing layers of composite material of the product in contact with the mold,

Vacuuming,

Heating the heating screen to polymerize the resin. The process is advantageously completed by the following characteristics, taken alone or in any of their technically possible combination:

the resin is pre-impregnated in the layers of composite material,

the evacuation step comprises a resin infusion step, the heating screen preheats the mold, said preheating then facilitating the step of placing the preimpregnated composite material layers,

The method comprises a subsequent step of cooling the mold by means of the cooling network, the temperature of the heating screen is controlled continuously, by means of the control unit, according to the temperature of the autoclave or the oven or the heating blanket, the product is an antenna reflector intended to be mounted on a satellite.

PRESENTATION OF FIGURES

Other characteristics, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and nonlimiting, and which should be read with reference to the appended drawings, in which:

FIG. 1 represents a view of the mold,

FIG. 2 represents a sectional diagram of a mold according to an embodiment,

FIG. 3 represents electrical circuit networks for a heating screen of a mold according to one embodiment,

FIG. 4 represents a view of the electrical circuit networks as well as the thermocouples of a mold according to one embodiment,

FIG. 5 represents an assembly comprising a mold and thermocouples connected to a control unit,

FIG. 6 represents an assembly comprising a mold and an autoclave according to one embodiment,

FIG. 7 represents different stages of the manufacture of a mold according to an embodiment,

- Figure 8 shows different stages of the manufacture of a product.

In the figures, the dashed lines represent information exchanges. In all the figures, similar elements bear identical references.

DETAILED DESCRIPTION

In relation to FIGS. 1 and 2, a mold 1 as presented in document FR 2 940 926 is considered for manufacturing a product.

This mold 1 comprises at least one plywood panel plywood panel 10 thermally modified by specific molecular transformation, hence the name "wooden mold". This wood, which will be called "thermally modified", has a hygrometry rate lower than a standard wood because of its transformation, which improves its dimensional stability at temperatures above 100 ° C ( decreased evaporation of water). As previously stated, the properties of this wood are described in document EP 1 982 828. To design the mold, it is possible to assemble several layers of wood.

A so-called functional layer of composite material 11 is added. Functional means a layer which will be in contact with the product to be manufactured 70 (see FIG. 6). The combination of wood and composite material offers performance in terms of dimensional stability and mechanical strength close to a carbon block or INVAR, overcoming its disadvantages: weight, cost, difficult machining and toxic for the operator .

A layer 12 of composite material, called "anti-bimetallic" material, may also be added opposite the functional layer 11, on the other side of the wood 10, to limit the "bimetallic" effect which may notably be related to the temperature rise of the mold.

A heating screen 20 is added in the mold 1, between the wood 10 and the functional layer of composite material 11. It provides endogenous heating, that is to say coming from inside the mold 1. This heating frame 20 is in the form of at least one electrical network 20a, in which an electric current Joule heated. In a preferred embodiment, the heating screen 20 has a plurality of electrical networks 20a, 20e, as shown in Figure 3 (five networks). Each of these networks 20a, 20e covers a zone Za,..., Ze which is different from it.

Each of these networks is independent, that is, each network is self-sufficient and can function without regard to the operation of others.

The combined effect of the endogenous heating provided by the weft 20 and the reflection of the heat by the wood 10 provides a mold 1 whose temperature homogeneity is excellent (less than or equal to 1 ° C difference on the surface of the mold 1).

In addition, the presence of the heating screen 20 allows in one embodiment to overcome the heating by autoclave or oven. This avoids wasting energy by heating a large volume, since the heating is targeted at the functional layer 11 which is in contact with the product to be manufactured 70.

For example, in Figures 1 and 3, the mold 1 has a circular and curved shape. The products to be manufactured are placed in contact with the mold 1 ("on" the mold 1).

In the case of a hollow mold 1, the products to be manufactured are placed "in" the mold 1.

In both cases, the product or the materials that constitute it is (are) more precisely placed on the functional layer of composite material 11 of the mold 1.

The first network 20a covers a circular surface of smaller diameter Za, and the other four networks 20b, 20c, 20d, 20e each cover a quarter of the mold, minus the zone Za of the first network 20a. This geographical independence of the zones Za, Ze allows a quasi-independent control thermally, heat transfer by the mold 1. Each network 20a, 20e comprises a power outlet (not shown in the figures).

In order to ensure good control of these networks and to control the product manufacturing process, thermocouples 30a, 30f have been placed close to the electrical networks 20a, 20e, in each of the areas Za, Ze which they respectively cover. In this way, each thermocouple 30a, 30f is able to account for the heat produced by each network 20a, 20e. FIG. 4 illustrates a placement of these thermocouples 30.

These thermocouples 30 make it possible in particular to detect a malfunction of the heating screen 20.

In addition, a surface thermocouple 31 is provided for measuring the effective temperature of the functional layer of composite material 11. This surface thermocouple 31 may be one of the preceding thermocouples 30a, 30b, ...

The information generated by these thermocouples 30, 31 is transmitted to a control unit 80 by means of a wired or wireless link (not detailed here) (see FIG. 5).

The control unit 80 typically comprises a processor 81 and a memory space 82.

The control unit 80 can advantageously drive the heating frame 20.

For reasons of sealing, a waterproof layer of composite fabric 13 is provided between the wood 10 and the heating screen 20. Sealing is important for vacuum drawing processes, in which the mold is evacuated to expel the air, for which it is not desired to observe porosity or pollution by wood or composite materials.

We then obtain a mold 1 comprising the elements in the following order:

The anti-bimetallic layer of composite material 12,

Wood 10, more specifically the layer of wood of heat-treated plywood panel 10,

- The waterproof layer of composite fabric 13,

- The electric frame 20,

- The at least one functional layer of composite fabric 11.

The functional layer 11 is composed of a plurality of sub-layers 11a, 1f.... Typically between ten and twenty layers is sufficient, more precisely, fifteen sub-layers may be suitable, for example. The term sublayer means a layer forming part of a set of sub-layers forming itself a layer.

The functional layer has a thickness typically between 5 and 10 mm, preferably around 8 mm. This is particularly the case for a mold 1.2 meters in diameter,

More generally, the thickness varies according to the surface of the mold.

The composite materials used are, for example, fibers among glass, carbon, polyamide, aramid, bamboo and ramie fibers.

In order to be able to cool the mold 1 more quickly, a cooling network 40 may be provided between the wood 10 and a layer of composite material 11, 12, the cooling network 40 being in the form of a heat transfer fluid network.

In particular, the network 40 is disposed between the wood 10 and the tight layer of composite fabric 13. A fluid circulation system is provided (pump, exchanger, and / or tank, etc.).

It is possible to control the temperature of the fluid as well as its flow, which offers several cooling speeds.

In connection with FIGS. 6 and 7, a method of manufacturing a mold is described.

First, a step El of preparing a block of wood 10 is implemented.

By preparation is meant wood processing and machining.

For the heat treatment, a molecular transformation for example at 240 ° C under a neutral gas atmosphere (such as nitrogen) is known from the prior art.

An assembly of different wooden panels is made using a resin, for example epoxy 200 ° C.

Finally, the machining of the mold 1 is performed.

This preparation step El makes it possible to obtain the desired shape for the mold 1. Thanks to the wood, the machining is fast and inexpensive, which makes it possible to obtain prototyping molds 1 in a short time frame for reduced budgets.

Then, a step E2 of laying the tight layer of composite material 12 is performed.

Then, a step E3 of placing the electrical frame or frames 20, possibly with the thermocouples, is performed. This step requires not to leave a zone without an electrical network nearby, to limit the risks of inhomogeneity of heating. Then, a step E4 for laying the sublayers of composite materials l ia, l lf ... forming the functional layer 11 is performed.

For this, we have one by one the underlays on each other.

Then, a step E5 of emptying is performed. A vacuum bag can be used depending on the composite material used.

According to an embodiment, the layers of composite material are already impregnated with a resin matrix and consequently the evacuation step E5 is intended to drive out the air and to promote the veneering between them of the various elements which form the mold 1.

According to another embodiment, the layers of composite material are not impregnated in resin matrix. During the evacuation, resin is injected and / or infused, for example into the vacuum bag; this will then diffuse into the tissues of the different layers thanks to the depression.

The resin is typically epoxy, the same type as that used for wood to ensure good compatibility.

Then, a step E6 of polymerization of the resin is carried out. A first heater at 60 ° and a second at 180 ° C can be made, or a single heater at 180 ° C.

The temperature generally depends on the temperatures of use of the customer.

For this purpose, an autoclave 50 (or oven) or a heating blanket (not shown in the figures) may be used. They respectively comprise a thermometer 51 or a thermocouple. They are also advantageously controlled by the control unit 80 (FIG. 6). The heating blanket typically includes a heating blank in a silicone rack.

Alternatively, we take advantage of the heating screen 20 of the mold 1 to heat the resin present in the mold 1. A combination of the heating screen 20 and the autoclave 50 or the heating blanket is also possible.

The autoclave stages are generally carried out under pressure, for example three bars (3 bars).

A finishing step E7, in which the composite material on the surface of the mold 1 is machined and then prepared to improve its appearance. The manufacturing process of the mold may comprise a step EO of setting up a layer 12 "anti-bimetal" on the rear face of the wood

10, that is to say on the face of the mold 1 opposite to the functional layer

11. The thickness of this layer 12 is substantially equal to that of the functional layer 11, in order to compensate the bimetal effect as accurately as possible.

In addition, according to the mold 1 that is desired, the method may comprise a step El 'of setting up the cooling network 40, before the establishment of the sealed layer of composite material 12. For this, pipes allowing circulating a coolant are disposed on the surface of the mold.

The requirement of homogeneity does not have to be verified here, since it is a question of cooling the mold 1 more quickly, and not of manufacturing a product 70.

Tests can be performed to ensure the quality of the mold. We can cite :

- Heating of the frame 20 only at 100 ° C and / or 180 ° under vacuum. The objective, achieved, is to have a difference in temperature of less than or equal to 1 ° C on the whole of the mold 1,

a vacuum tightness test of 7 bars and 180 ° C.

- Continuous dimensional control at 110 ° C, for a maximum tolerance of one millimeter. Now, with reference to FIG. 8, a method of manufacturing a product 70 and in particular an antenna reflector 70 is described. For this purpose, the mold 1 is preferably circular and of dimension greater than one meter (for example 1.2m or 2.4m).

In a first step F1, there are layers of composite materials 71 in contact with the mold, and more particularly on the functional layer 11. Advantageously, the mold 1 can be preheated to facilitate the draping of said layers. Indeed, they soften and become more manageable than at room temperature. The heating screen 20 allows such preheating. Such preheating is particularly advantageous in the case of composite materials pre-impregnated with resin.

In a second step F2, the mold is evacuated with the layers of composite material 71. Indeed, it is necessary to apply a pressure on the composite material 71 during the polymerization. The evacuation can be done by means of a vacuum bag to impregnate the resin layers. Alternatively, step F2 may be included in step F3 (see below), in the case of an autoclave 50 under a neutral gas atmosphere which can both heat and pressurize. Otherwise, the vacuum bag is typically used even for an autoclave because the bag protects against combustion when the atmosphere comprises oxygen.

Two embodiments for the impregnation of the resin exist in particular: the resin is pre-impregnated in said layers 71, or else the step F2 comprises a sub-step of infusion of resin in the vacuum bag.

In a step F3, the assembly is heated to polymerize the resin, with temperatures up to 180 ° C, by means of the heating screen 20 in particular. can set several heating modes:

the first is simply to use the heating screen 20, to limit energy losses by heating only in contact with the product 70, and not heating a large volume of air. A vacuum bag is then used to keep under vacuum,

the second is to use in parallel the heating screen 20 and the autoclave (or oven) 50. Using the surface thermocouple 31 and the thermometer 51, the temperature of the heating screen 20 is controlled so that the temperature between atmosphere of the autoclave 50 and the functional layer 11 of the mold 10 is identical. In this way, the product 70 is heated in a perfectly homogeneous manner on both sides (less than 1 ° C apart on either side of the product 70). In the case of the oven, a vacuum bag is used; in the case of the autoclave 50, the pressurization of the step F2 can be carried out thanks to the autoclave 50 if the atmosphere is in neutral gas. Otherwise, a vacuum bag is used, the third is to use in parallel the heating frame 20 and the heating blanket that covers the product 70. A vacuum bag is used to vacuum the mold and the blanket. Using the thermocouple of the heating blanket and the surface thermocouple 31, co-coilote simultaneously in the vacuum bag at atmospheric pressure (between 0.6 and 1.1 bar). In the same way, this co-piloting makes it possible to have an identical temperature on both sides of the product 70 (less than 1 ° C apart). In addition, the heating is targeted in contact with the product, which minimizes the waste of energy,

a fourth mode consists in using the heating screen 20, the heating blanket and the autoclave 50, except that in this case the autoclave 50 does not rise in temperature and is used mainly for pressurizing. In all these alternatives, the data of thermocouples 30, 31, thermometer 51, etc. are recovered by the control unit 80. As explained above, depending on the program chosen, it can adjust the heating screen 20 and / or the autoclave, oven 50, heating blanket to optimize the polymerization of the resins.

Advantageously, the control is carried out continuously.

When the heating is complete, a cooling step F4 is performed.

The residual heat of the mold 1 is thus transferred into the coolant flowing in said network 40, which will accelerate the cooling of the mold 1. Then, the part is demolded in a step F5.

Subsequently, since the mold 1 remains hot by thermal inertia and in particular if the demolding takes place at a still high temperature, it is possible to put layers of composite material to manufacture another composite part 70 only if the temperature is below a threshold.

In order to optimize the time available, once the demoulding F5 has been carried out, the cooling network 40 is activated or maintained activated (even the flow rate is increased and / or the fluid temperature is decreased to further accelerate the cooling) in a step F6 .

In this way, it is possible to start manufacturing another part 70 more quickly.

The manufactured product is for example a composite antenna reflector intended to be embedded on a satellite.

Claims

claims

Wooden mold (1) comprising at least one plywood panel plywood panel thermally modified by molecular transformation (10) and at least one functional layer of composite material (11), characterized in that the mold (1) comprises a weft heater (20) in the form of at least one electrical network (20a) disposed between said at least one wooden layer (10) and said at least one functional layer of composite material (11).

Mold according to the preceding claim, wherein the heating screen (20) is composed of a plurality of electrical networks (20a, 20b, 20c, 20d, 20e), each configured to heat a different zone (Za, Zb, Zc, Zd) of the functional layer of composite material (11), so as to regulate the temperature of each zone.

Mold according to any one of the preceding claims, further comprising thermocouples (30a, 30f) arranged near the electrical network or networks (20a, ... 20e), so as to be able to control the effective temperature of each electrical network ( 20a, ... 20e) or at least one thermocouple (31) for measuring the temperature at the functional layer of composite material (11).

Mold according to any one of the preceding claims, comprising the following successive stacking: said at least one wooden stratum (10), a composite fabric impervious layer (13), the heating weft (20), the at least one functional layer of composite material (11).

5. Mold according to any one of the preceding claims, comprising a cooling network (40) disposed between the wood (10) and a layer of composite material (11, 12), the cooling network (40) being in the form of a heat transfer fluid network.

6. Mold according to claims 4 and 5, wherein the cooling network (40) is disposed between the wood (10) and the sealed layer of composite material (12).

A mold according to any one of the preceding claims, wherein the functional layer of composite material (11) has a thickness relative to its diameter, said layer (11) being composed of a plurality of sub-layers.

An assembly comprising a mold (1) according to any one of the preceding claims, wherein the heating screen (20) is configured to heat a product to be molded (70) endogenously, the assembly comprising an autoclave (50). or oven or heating blanket adapted to heat said product (70) exogenously.

9. Assembly according to the preceding claim, further comprising a control unit (80) configured to adjust the heating temperature by means of the heating screen (20), and the autoclave (50) or the oven or blanket heating.

A method of manufacturing a composite material product (70) by means of a mold (1) or an assembly according to any one of the preceding claims, comprising the steps of:

- (Fl) Placing layers of composite material (71) of the product (70) in contact with the mold (1), - (F2) Vacuum, (F3) Heating of the heating screen (20) polymerizing the resin.

11. Method according to the preceding claim, wherein the resin is pre-impregnated in the layers of composite material (71).

The method of claim 10, wherein the evacuation step (F2) comprises a resin infusion step.

The method of claim 11, wherein the heating screen (20) preheats the mold (20), said preheating then facilitating the step of placing (F1) pre-impregnated composite material layers (71).

14. A method according to any one of claims 10 to 13, comprising a subsequent cooling step (F4, F6) of the mold (1) by means of the cooling network (40).

15. The method according to claim 10, in combination with an assembly according to claim 8, wherein the temperature of the heating screen is controlled by means of the control, depending on the temperature of the autoclave (50) or the oven or the heating blanket.