BACKGROUND TO THE INVENTION In the prior art method to produce curved sandwiched structures, used as a structural part of a vehicle, each of several layers are preformed to get a curved shape and then the layers are fixed together. This method includes many production steps since the pre-forming of each of the individual layers involves at least one operation.

Furthermore, bending of thin sheet metal in a bending apparatus demands greater bending than the desired curvature so as to obtain the desired curvature as a permanent deformation; therefor exact forming requires great skill of the manufacturer. There is also a risk of great variations in quality of the resulting product when fixing such preformed layers together. Inaccurate fit of the individually preformed layers result in differences in thickness of the glue which later on negatively affect the quality of the product in terms of strength and durability.

Moreover, the sheets are easily damaged when handled in the bending apparatus due to the low thickness.

Furthermore, usually not all production steps in the prior art technique can be performed at the same place so that some transportation and handling is necessary.

Curved sheet material is bulky and easily damaged when handled.

From EP 0 247 359 B1 a manufacturing method is known for an interior material for vehicles. The interior material has a light weight and can be produced easily and without high cost. In detail this manufacturing method comprises the following steps; vacuum forming of a plastic surface layer by sucking a thermoplastic resin sheet in a thermo-softened state into a mould with a certain shape; placing a reinforcing fiber mat onto the surface layer via an adhesive in the mould; injecting a polymer composition containing unsaturated mono-alcohol, poly-isocyanate and catalysts into the mould so as to permeate the polymer composition throughout the reinforcing fiber mat; and demoulding an integrally-moulded object from the mould after the completion of curing of the polymer composition.

However, this prior art manufacturing method and material is only directed to an interior material. The lightweight material made of plastic, glass-fiber or carbon fiber could not be used as a structural element bearing heavy loads.

From document EP 0 445 992 B1 a vacuum-mould is known, for vacuum moulding sheet material for producing a trim-covered component. The mould has a device for the application of differential vacuum to the mould cavity. The device comprises a plurality of vacuum sources in communication with the mould cavity by means of vacuum holes. Thus, by means of this vacuum-mould it is possible to get surfaces as desired also when several layers of sheet material are used.

A problem with using sandwich structures made of plastics, glass fibers and/or carbon fibers for vehicles, is that in case of an accident these sandwich structures break and therefore constitute a risk for the passengers.

Therefore, it is an object of the present invention to provide a method for producing curved sandwich structures that reduces the number of production steps, improves the quality of the finished product and reduces the production cost. It is also an object to provide a curved sandwich element related to said method.

SUMMARY OF THE INVENTION According to the present invention these objects are accomplished by a method for producing curved sandwich structures comprising the features as claimed in claim 1 and 10 as well as a curved sandwich structure according to claim 19. Additional advantageous features of the present invention can be found in the subclaims.

With the method of producing curved sandwich structure according to the present invention the several production steps formerly necessary for pre-forming can be eliminated ; now the layers are elastically deformed to their final shape without the necessity of performing greater bending to get the desired curvature. Furthermore, the handling of the material used for the layers is simpler since flat sheets and finished sandwich elements are easier to handle than thin curved layers. Due to fewer production steps and less need of transportation and handling the risk of damages is lower.

These and other objects, features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment of the invention.

In the following there is disclosed a detailed description of a preferred embodiments of the invention in conjunction with the drawings, of which: Fig. 1 a three-dimensional view of a mould used for the method for producing curved sandwich structures according to the present invention, Fig. 2 a cross-sectional view of the mould in a first step of the method according to the present invention, and Fig. 3 a three-dimensional view of the assembly of the layers of the curved sandwich in a further step of the method according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In order to reduce time for production, weight and cost it has been tried to use plastic materials in rail vehicles e. g. as parts of walls, roof etc. Often a sandwich structure comprising plastic material reinforced by glass fiber or carbon fibers is used.

However, apart from these advantages it has been found that the materials used for such sandwich structures have insufficient strength, when exposed to fire or shock.

Thus, in order to avoid the disadvantages of the prior walls for railway vehicles, according to the present invention there is provided method for producing a curved sandwich structure as well as a sandwich structure which can be used as a structural part of a vehicle and that will perform well even in extreme conditions such as shock or fire. For this reason there is provided an outer sheet of the curved sandwich structure being made of thin metal, in particular of steel, stainless steel or aluminum.

In the following first the method for producing a curved sandwich structure is described in greater detail. This method comprises the following steps: Step 1 : A sheet with limited flexural strength, preferably a thin first metal sheet is mainly elastically deformed in a male or female mould by creating a vacuum under the first metal sheet. A pump is connected to recesses in a mould. By pumping air with the pneumatic pump the air velocity is higher in the mould than in surrounding air. The difference in air velocity results in a difference in pressure. The difference in pressure (Q) and the weight of the sheet (P) results in a force on the sheet, making it adapt to the shape of the mould.

Sealing the edges of the first sheet and the mould, so as to form a closed compartment, can enhance the effect, but it is also possible to create a sufficient pressure difference even though the mould is partly open to surrounding air.

An initiating force can also be applied to initiate the deformation.

As a material for the first metal sheet e. g. steel, stainless steel or aluminum is used.

The material can also be preformed to a first shape and receive a final shape after The method can also include sealing the first metal sheet to the mould or to certain parts of the mould, surrounding the outlets. If a tight seal is obtained, the force holding the first metal sheet to the mould is retained when the pump is turned off.

Step 2:

The next step is to build a double skin sandwich with a core material and a second sheet. The core material can be chosen from a vide variety of lightweight materials.

Most preferably the core material comprise an insulating material such as foamed plastic, or lightweight wood. Other materials to be considered are metal honeycomb, foam metal. Very stiff or brittle core materials have to be preformed. Furthermore, said core material can comprise one or several layers that are particularly suitable for special purposes such as insulation, damping of sound or damping of vibrations.

The core material could also include stiffening members (not shown), to take up forces in structures that will be heavily loaded, and terminations, that will protect the core and form joint to other parts of the vehicle.

Materials suitable for the second sheet are metals such as steel, stainless steel, aluminum or non-metals such as plastic, fiber reinforced plastic, even decorative panel materials as wood laminate can be considered. Very stiff or brittle second sheet materials have to be preformed or partly preformed. If metal is chosen the inner layer will significantly contribute to the strength of the sandwich.

Step 3: In one embodiment layers of adhesive are applied so as to bind the sheets and the core, either on the part which is placed in the mould or on the part which will be placed in the mould next. The adhesive does not have to be applied as an even layer but could be applied in strands or dots, because the adhesive will be evenly distributed in a subsequent step of the method.

In another embodiment spacers are placed between the layers of the sandwich to make an interspace which will be filled with adhesive in a subsequent step of the

method. The spacer could be a flexible, perforated sheet member or for example a net or mesh. The adhesive will fill the perforations or holes in the spacer.

Step 4: When all compounds of the sandwich are placed in the mould, a membrane is used to close the mould. The membrane is sealed to the mould, thus forming a closed compartment. Air is evacuated from the compartment. Since the pressure on the outside is greater than inside the compartment the core material and the second sheet is forced to adapt to the shape of the mould.

The curved sandwich structure produced by the method according to the invention has the following major advantage, which e. g. is relevant in case of accidents: The rigidity of a double skin sandwich is much greater than the rigidity of the metal skins per se, as stiffness is proportional to the thickness raised to the third power.

The forces introduced in the sandwich by the elastic deformation of the metal skin are taken up by the surface of the sandwich as membrane tensions.

Some of the production steps of the method for producing curved sandwich structures according to the present invention will in the following be discussed in greater detail with reference to the drawings.

Fig. 1 shows a mould with characteristic details as it is before a first metal sheet is placed thereon.

In particular, in fig. 1 there is shown a mould 1, which defines the final shape of the detail. The mould 1 has one or more guides or, as shown in fig. 1, a guiding means 2 to facilitate the placing of the first metal sheet. The surface of the mould 1 has recesses 3, which can be holes or grooves. The recesses 3 are connected to conduits leading to a pneumatic pump via a valve 5. The recesses are small so that there will be no marks on the finished product.

Now the first production step is described with reference to fig. 2. Fig. 2 shows the placing of a first sheet 6 on the mould 1. By placing the first metal sheet 6 in the mould 1 to the guiding pin 2, a difference in ve) ocity Vi-Vo arises because of the airflow under the sheet. The difference in velocity causes a difference in pressure Q, which affects the first metal sheet. The weight of the sheet also affects the sheet. The resulting force makes the first metal sheet adapt to the mould. The pressure Q can be maintained even when the pneumatic pump is turned off if the first metal sheet 6 is sealed to the mould 1.

In the further production steps are shown in fig. 3. When the first metal sheet 6 is formed and fixed in the mould 1 as described with reference to fig. 2, a layer of glue is applied in the next step. Thereafter, a preformed insulation material 8 is placed on the top of the first metal sheet 6. A second layer of glue is applied on the insulation material 8 before placing a second sheet 7 on the insulation material 8. In the next step an airtight membrane 9 covers the mould 1 with the first and second sheet 6,7 and the intermediate core material 8 and the air is evacuated from the mould 1. Due to the evacuation the difference in pressure forces the second sheet 7 to take the shape of the underlying structure, thus forming a double skin sandwich. In a last step,

when the adhesive is cured, the airtight membrane 9 is removed and the mould 1 is emptied. The finished sandwich element will retain its curved shape.

In conclusion, the present invention discloses a method for producing a curved sandwich structure by means of which with lower costs, less production steps and better quality a curved sandwich structure can be produced. In particular, in the method according to the present invention a sandwich structure comprising at least two outer layers and an intermediate layer is produced. Merely the intermediate layer may be preformed in a curved shape, at least the outer layers are formed into the curved shape without separate process steps when being mounted together with the intermediate layer by means of layers of glue.

Although the invention has been described with reference to a specific embodiment thereof, it will be apparent to those skilled in the art that numerous changes and modifications may be made therein without departing from the scope of the invention.

It is, therefore, to be understood that it is not intended to limit the invention to the embodiment shown but only by the scope of the claims, which follow.