WO2000013892A1

WO2000013892A1 - Sandwich construction

Info

Publication number: WO2000013892A1
Application number: PCT/SE1999/001523
Authority: WO
Inventors: Göran Langstedt
Original assignee: Linlan Induction AB
Current assignee: Linlan Induction AB
Priority date: 1998-09-03
Filing date: 1999-09-03
Publication date: 2000-03-16
Anticipated expiration: 2001-03-03
Also published as: AU6015099A

Abstract

A sandwich construction comprising a fibre- and thermoplastic-containing composite layer (1), a first metal layer (2), and optionally a second metal layer (3), the composite layer (1) comprising one or more fabric layers (4) containing arranged reinforcing fibres and a thermoplastic, is described, wherein said fibres have a twist value of about 0.1 to 100 m-1 and having a free non-bound fibre length of 0.1-20mm, and the thermoplastic is partially distributed in the composite layer (1) to a penetration depth of about 30-80 % of the total thickness of the composite layer (1), starting from the interface between the first metal layer (2) and the composite layer (1) and optionally from the interface between the second metal layer (3) and the composite layer (1).

Description

SANDWICH CONSTRUCTION

Field of the Invention

The present invention relates to a sandwich construction as well as manufacture and use thereof. Background Art It is generally known that body metal sheets for vehicles, such as automobiles, and casings and chassis for machinery, instruments and apparatus of different kinds are relatively heavy owing to dimensioning in respect of rigidity, strength in complex loads, safety in connection with, for instance, collisions and damping of oscillations and impacts. There is an increasing demand for a reduced weight of such constructions. A problem in screening of electromagnetic radiation from electronic equipment is that electric sheets that are otherwise used in e.g. transformers must usually be applied to the casing or chassis or between sheets therein. There is thus a demand for simpler constructions, where the screening metal layer is integrated in such chassis or casings. However, no technique is presently available that permits the manufacture of such constructions in a mould without reducing their strength and, thus, their safety.

US-A-5 164 141 (Bayer Aktiengesellschaft) refers to a process for the continuous production of laminated sheets, particularly flat fibre reinforced thermoplastic matrix composite laminates with a metal surface for electric shielding purposes.

EP-A2-0 743 174 (YKK Corporation) describes laminated plate material and loom harness frame manufactured therefrom. The loom produced has been designed for weight reduction and minimisation of inertia for the specific purpose of loom shafts. Summary of the Invention

An object of the present invention is to solve the above problems and satisfy the above defined demands. According to the present invention this object is achiev- ed by providing a sandwich construction which has the features defined in the accompanying claim 1, is light and strong and can replace body metal sheets for vehicles as well as casings and chassis for different machinery, instruments and apparatus, for instance those emitting electromagnetic radiation. Preferred embodiments are recited in the dependent claims. The invention also concerns methods for manufacturing this sandwich construction and use of the same in the above-mentioned fields. Brief Description of the Drawings Fig. 1 shows an embodiment of a sandwich construction according to the invention which is provided with only one metal layer.

Fig. 2 shows another embodiment of a sandwich construction according to the invention which contains a steel wool layer.

Fig. 3 shows the ratio of force to deflection for a composite panel based on the sandwich construction according to the present invention. Description of the Invention More specifically, the invention relates to a sandwich construction which is characterised in that it comprises a fibre- and thermoplastic-containing composite layer, a first metal layer, and optionally a second metal layer, the composite layer comprising one or more fabric layers with reinforcing fibres arranged therein, and a thermoplastic distributed in the composite layer, and the composite layer being connected to the first and optionally second metal layers by melting and solidifying of the thermoplastic when heating and simultaneously com- pressing the layers.

The sandwich construction shown in Fig. 1 comprises a fibre- and thermoplastic-containing composite layer 1, which in turn comprises one or more fabric layers 4. A plurality of fabric layers 4 can be oriented in different directions with a view to increasing the strength of the sandwich construction. The fabric layers 4 can also be arranged in a varying manner in one and the same plane, for instance when different degrees of strength are desirable in different parts or directions of the sandwich construction. The fabric layer 4 can originally be a pure fibrous tissue or originate from a material where a fibrous tissue or mat is preimpregnated with polymer material or where the polymer material is in the form of fibres or film or is mixed, interlaced, woven, braided, non-woven or knitted in the fibrous tissue, such as a prepreg. Moreover, ground-down fibrous fragments can be present in the composite layer 1. Use is preferably made of fabrics of the Splitfilm Warpnits type, whereby in varying forming of the sandwich construction the fibres can, during compression, be draped over complex curved surfaces and stretched without losing their strength. The fibre material can thus consist of any type of fibres which has satisfactory strength properties. Use is preferably made of glass fibres, armid fibres or carbon fibres, but also hemp and other textile fibres, such as flax and sisal, are usable. In the preferred embodiments of the present invention, the fabric layer (s) 4 contain (s) arranged reinforcing fibres twisted into yarn structures. The fibre diameter is about 5-25 μm for glass, armid and carbon fibres, and is normally about 1-50 μm for textile fibres. Yarns made from such fibres normally have a diameter of about 0.1-1.0 mm. Further, each fibre has a low twist value of about 0.1-200 m"¹, preferably 5-100 m^"1. The yarns may be present in the composite layer 1 in a wave shaped configuration having a wave length of 2-100 mm. Yarns of different wave configurations, both as to amplitude and as to wave length, may be present in the composite layer 1. In one embodiment, the amplitude of the yarn wave length is close to the interface between the metal layer 2, and optionally 3, and the composite layer 1.

The term "twist" value "as used herein is defined as the amount of turns of a fibre string around its own axis per length unit, expressed in m^-1.

The thermoplastic component in the composite layer 1 can be any suitable thermoplastic polymer, e.g. high- temperature plastics, such as polyimides, but preferably use is made of polypropylene, polyamides or polyesters. Examples of usable polyesters are PET and PBT. It may have been added as raw material to the composition in the form of polymer filaments extending adjacent to the fibre filaments in the layer of fabric layer 4. The thermo- plastic component can also have been added in the form of recycled fibre polymer composite material or in the form of surface-treated material from recycled PET bottles. A preferred material is the above-mentioned prepreg, i.e. a material where a glass fibre fabric is impregnated with polypropylene or polyamide, or polypropylene or polyamide is interlaced, woven, braided, non-woven or knitted in the fabric. The polymer can initially also be added to the layer of fibrous tissue 4 in the form of a granulate. When manufacturing the inventive sandwich construction, the composite layer 1 is subjected to a heating and compression procedure that will be described below and in which the thermoplastic present in the composite layer 1 is being molten and distributed in parts of or the entire composite layer 1. According to a preferred embodiment of the present invention, the thermoplastic is molten in a controlled way such that it is only partially distributed in the composite layer 1. The degree of distribution is defined as the penetration depth of the total thickness of the composite layer 1, starting from the interface between the metal layer 2 and the composite layer 1. In the embodiment with a second metal layer 3, the penetration depth amounts to the total penetration depths, i.e. that seen from the interface between the composite layer 1 and the metal layer 2 and that seen from the interface between the composite layer 1 and the metal layer 3. Preferably, the penetration depth is about 30-80%, de- pending on the final use of the sandwich construction. Further, the penetration depth is not exactly the same throughout the composite layer. Instead, slight variations in penetration depth may occur depending on the fluctuations in the heat treatment conditions and in homogenity in the starting materials used. Therefore, the amounts of penetration depth indicated throughout the application are intended to be mean values. Thus, in the composite layer 1 there is a part of 20-70% that is not infiltrated by the thermoplastic, and this part essen- tially contains fabric material . Such a composite layer 1 partially containing a thermoplastic has been found to be particularly strong, tough and energy absorbing. Further, with the non-penetrated part essentially containing fabric in the composite layer 1, undesired stiffness of the sandwich constructions is avoided. Also, deep drawing of such a construction is facilitated.

One important parameter in this context is the "non- bound free fibre length" . When the thermoplastic is molten to a controlled penetration depth in the composite layer 1 during the production process, some parts of the yarns in the fabric are infiltrated by the thermoplastic, while other parts thereof remain non-infiltrated. The yarn parts infiltrated are, of course, located closest to the interface between the composite layer 1 and the metal layer 2, and vice versa concerning the optional metal layer 3. The parts of the wave-formed fibre yarns not infiltrated are defined as the "non-bound free fibre length" , and the remaining infiltrated parts are defined "bonded fibre lengths" . The "non-bound free fibre length" is 0.1-20 mm, preferably 1-9 mm.

Also, sandwich constructions in which the entire composite layer 1 is infiltrated with thermoplastic, have reduced strength and energy absorbing capacity, but are nevertheless useful in certain applications.

The design and operation of the press tool to be used mean that the composite layer 1 can be made compact with no or just a few inclusions of air.

The thickness of the composite layer 1 is not critical, but depends on the intended use of the sandwich construction. However, the thickness should essentially exceed the thickness of the first metal layer 2. As shown in Fig. 2, the composite layer 1 can optionally also comprise one or more metal wool layers 5, e.g. steel wool or aluminium wool, and a second metal layer 3. If steel wool is used, it must be clean, i.e. it must not be of the lathery type intended for domestic use. The steel wool, which should preferably also be degreased and etched, or the other metal wool, can be supplied in a conventional manner in the form of a roll. The purpose of the presence of metal wool is on the one hand that it has great fractional toughness giving protection against penetration by external objects, e.g. fork trucks and, on the other hand, that it improves the thermal conduction in the manufacturing process described below, which results in increased toughness of the final product and a shorter process time owing to the thermoplastic melting more rapidly. Satisfactory sandwich constructions, however, can also be prepared without the presence of metal wool. If more than one metal wool layer 5 is present in the composite layer 1, at least one fabric layer 4 should be arranged between two metal wool layers 5. Moreover, various metal wool layers 5 can be arranged in different directions .

The first metal layer 2 on one side, preferably the upper side, of the composite layer 1 may consist of conventional vehicle sheet metal with satisfactory deep- drawing qualities. Preferably the metal layer 2 is made of aluminium which besides can be pretreated to improve adhesion to the composite layer and to prevent oxidation and corrosion. Its thickness is about 0.05-0.5 mm. The metal layer 2 gives the sandwich construction a metal surface and an impression of a compact metal construction. If e.g. car bodies are involved, it is for instance not possible to distinguish that the body material is not made of metal throughout . Further the metal layer 2 of the prepared sandwich construction can be painted just like a conventional car body, e.g. car sheet metal, or the metal layer 2 can even before the manufacturing process be coated with a heat-resisting layer of primer, e.g. paint, which can be accomplished, for instance, by using suitable solvent-based systems. The surface of the first metal layer 2 that is to be bound to the composite layer 1 can optionally be pretreated to facilitate the binding between the two layers. The second metal layer 3 which optionally is bound to the other side of the composite layer 1 is not necessary in all applications, but may be desirable in other applications of the sandwich construction, e.g. in car bodies. Depending on the final application of the sandwich construction, a composite layer 1 may, as mentioned above, consist of one or more fabric layers 4 with fibrous tissues oriented in the same or different direction in the plane of the sandwich construction. More- over, the thickness of the layer, the fibre diameter and extent of the selected fibre material are not critical, but may also vary according to the application of the sandwich construction. As mentioned above, a plurality of metal wool layers 5, e.g. steel wool layers, may also be included in the composite layer 1, and the thickness and extent of these layers are not critical either, but may vary according to the application of the sandwich construction.

Since the sandwich construction consists of a plura- lity of different layers, it is understood that all the described layers are parallel or essentially parallel with each other. The sandwich construction can practi- cally have any shape and extent whatever, provided that this can be achieved by means of the press tool or compression moulding die. Furthermore, the cross-section of the construction should be essentially identical in re- spect of the thickness of the various layers even if it has been bent or deformed in some other manner during mould compression, e.g. by deep-drawing.

The sandwich construction according to the present invention is manufactured by first placing the components included therein in the desired order in a cold press tool or compression moulding die, which optionally may first have been coated with a release agent. Any press tool which yields the conditions stated below can be used, but preferably use is made of a press tool as de- scribed in PCT Applications SE97/00600 (priority claimed from Swedish Patent Application No. 9600130-0) , applicant Linlan Induction AB, i.e. under the action of conductive heat and with the aid of vibration assistance from the heating frequency and its harmonics. The press tool of SE97/00600 is said therein to be intended only for the manufacture of products wholly or partly consisting of plastic or a composite material. This press tool, however, has been found to be useful in the manufacture of the sandwich construction according to the present inven- tion. By using this press tool under the conditions that are necessary according to the present invention and with the specifically developed components, the present inventor has now, in a previously quite unknown manner, managed to manufacture a new sandwich construction with properties that in many respects are advantageous.

When all ingredients that are to be included in the sandwich construction have been arranged on top of each other in the desired order in the cold press tool die, this is closed. The material therein is heated to 100-420°C, preferably 200-350°C or 190-280°C and more preferably 220-240°C, depending on what polymer is present in the composite layer 1, and at a pressure from almost 0 up to 50 kg/cm², for instance 10-50 kg/cm². It has been found that particularly good properties of the sandwich construction are achieved at a low pressure of about 0.5-2 kg/cm² for a prepressed metal sheet. This corresponds to a good fit, and can be sufficient to achieve bonding via the surface forces of the polymers. For conventional metal sheet, a pressure of 0.5-50 kg/cm² is usable and is dependent on the surface state of the metal sheet, the type of polymers and the heating tem- perature. At this high temperature, the polymer melts, is bonded to the fibres and distributed to a controlled, desired penetration depth in the composite layer 1. Then the press tool die is cooled under pressure to about 50°C. In this fashion the polymer is cooled and solidi- fies, thereby achieving a strong bond between the thermoplastic of the composite layer 1 and the first and optionally the second metal layer 2, 3. The die is then opened, and the prepared sandwich construction can be removed. The total process time including cooling is below 5 min, preferably 0.5-5 min. The actual heating step lasts from about 10 s to about 3 min, depending on the size of the press tool die and the supplied power.

The satisfactory strength properties, such as excel- lent final strength in large deformations, that are achieved at the low pressure can be attributed to, for example, the composition of the starting textile, i.e. discrete weft yarns in a textile structure consisting of the matrix material, the yarns having a long twist value of 0.1-100 m^_1, preferably about 2 m^-1. This gives a periodic bonding of the fibres, and the non-bonded free length defined above, i.e. 0.1-20 mm of the fibres between the bonded parts of the fibres distributes the load over a volume of material which limits the local mechanical stresses.

In a preferred embodiment of the method for manufacturing of the sandwich construction according to the present invention for use as a vehicle body, an about 0.2-mm-thick aluminium sheet is placed in the above described press tool die. Then a 4-mm-thick composite layer 1 is applied, comprising a prepreg as defined above, i.e. a fabric with, for instance, preimpregnated or interlaced polypropylene. The fibre twist value in the prepreg is about 10 m"¹ and the non-bonded free fibre length is 5-10 mm. Then a steel wool layer 5 is applied, followed by a composite layer 1 identical with the one described above, and a second metal layer 3. The material in the press tool die is then heated for a period of 30 s max to about 230°C at a pressure of about 20 kg/cm² during the compression of the aluminium sheets . The total process time, including the steps of placing the in- gredients in the compression moulding die and cooling, amounts to 5 min max and the penetration depth for the thermoplastic is 20% starting from the interface between the composite layer 1 and each of the metal layers 2,3, i.e. totally 40%. In an alternative embodiment of the preferred method above, the first and optionally the second metal layer 2, 3 can be premoulded by compression before the ingredients in the composite layer 1 are applied. In the subsequent heating and compression step, this metal layer or these metal layers then act as a mould with the necessary support and can be considered to replace the metal surface of the compression mould. Thus, the ingredients in the composite layer 1, optionally one or more steel wool layers 5 and the already prepressed metal layer or layers are then compressed under the same conditions as just above, but at a pressure of about 1 kg/cm² to a sandwich construction according to the present invention.

By reverting the manufacturing method described above, i.e. heating the surface parts of metal in a sand- wich construction to a temperature where the metal is separated from the composite part, the metal can also be recovered separately, as can also the composite part with fibres and polymer.

In a further alternative embodiment of the method according to the present invention, the manufacture of the sandwich construction occurs by continuous feeding of one or more metal layers from a roll and feeding of one or more fabric layers with thermoplastic from a roll to the compression moulding die. The same temperature and pressure conditions as stated above are applicable. Optionally, the metal layer or layers fed can be painted in advance. Since the atmosphere inside the press tool die is deoxidised, the paint withstands the high temperature without being degraded. When a sufficient length of metal layer and fibrous tissue layer with thermoplastic has been supplied to the press tool die from the respective rolls, the layers are cut off, whereupon moulding occurs and the process is then repeated. The process cycle time is 0.5-5 min, preferably 1-3 min. Alternatively, the end product may be cut off when exiting the press tool die.

In a preferred embodiment of this method, two metal sheet layers and one glass fibre layer with thermoplastic in the form of a prepreg are supplied to the compression moulding die in such manner that the glass fibre layer is positioned between the two plastic layers.

Alternatively, the metal layer (s) may be premolded by compression and put into the press tool before the other components are fed from the rolls to the tool. It is to be understood that in all of the embodiments above comprising continuous feeding, the manufacture conditions and the features of the components to be included in the sandwich construction are the same as in the preferred non-continuous method described above.

In another method embodiment of the present inven- tion the heating and the compression of the sandwich construction is performed in two separate steps. In this embodiment a first metal layer 2, a fabric layer with thermoplastic, one or more metal wool layers 5 and a second metal layer 3 are stapled in this order to a layer aggregate and is placed in a first induction heated furnace (frequency: 6 kHz) . Optionally, the metal layers 2 and 3 have been premoulded to their final shape . In the induction heated furnace the metal layers 2 and 3 are heated to such a temperature that the thermoplastic in the layer aggregate melts enough for to adhere to the metal layers 2 and 3 , whereby the whole layer aggregate becomes an integral construction. In a first alternative as to the further processing, the layer aggregate heated in the first furnace is allowed to cool, followed by optional cutting of the whole layer aggregate into its final shape, if desired. Thereafter, the layer aggregate is placed into a second induction heated furnace, in which it is heated for melting of the thermoplastic in a controlled way to the penetration depth desired, dependent on the final use. In the molten state, the thermoplastic acts as a glidant between the metal layers and the fibres. Then, when the thermoplastic is still in a molten state, the layer aggregate is quickly transferred to a cold press tool, in which the compression to the final shape takes place. This alternative is particularly advantageous when the aggregate formed during the first heating step is to be transported to another geographic location for the final heating and compression treatment .

In a second alternative of this embodiment, e.g. when the whole process is to be performed at the same place, the layer aggregate formed after the first heating step is transferred directly to the cold press tool, i.e. without any second heating step. In this alternative, the heating conditions in the induction heated furnace are more stringent in a view to obtaining the exact penetration depth desired.

In a comparison of the method comprising heating and compression in the same press tool with the two alter- natives described above for separate heating and compression, the total processing time from start to the final product is reduced with about 50% with one heating step and with about 25% with two heating steps. Thus, when using two heating steps, two articles per minute can be produced, which is a considerable improvement compared to known techniques .

In a preferred embodiment of the method with separate heating and compression steps for the production of a sandwich construction for use in a car body, the metal layers 2 and 3 are made of low carbon mild steel or aluminium having a thickness of 0.2 mm, the fibres are non-woven polypropylene and surface-treated PET from bottles, and the metal wool is steel wool. The tempera- ture in the first furnace for inductive heating is about 220°C, the heating temperature is at most 10 s and the pressure is about 1 kg/cm². The layer aggregate is allowed to cool to less than 50°C during a period of less than 20 s. The conditions during the second heating step are essentially the same as during the first heating step.

The sandwich construction according to the present invention can, as mentioned above, be used as vehicle body, preferably for passenger automobiles, but also for lorries, busses and trains. It can also be used in air freight containers and construction components in aircraft, such as panels, and in passenger automobiles as side airbags and frames in car seats and children's safety seats. A further application is wind deflectors for trucks and other vehicles for reducing the air resistance, but also casings or chassis for various machinery, instruments and apparatus, e.g. apparatus for domestic electronics. Further applications of the sandwich construction are in safety doors and cabinets, furniture and kitchen fittings. Moreover, the sandwich construction can replace packings in plate heat exchangers where the composite constitutes the packing. This results in a heat exchanger which need not be pressed by applying a high pressure for the packings not to leak, i.e. a type of welded heat exchanger with no welding seam.

For e.g. screening of magnetic radiation, a sandwich construction in which an electric sheet in the form of a sheet having a thickness of at least 0.1 mm has replaced the first layer of metal sheet 2, can be used. The sandwich construction according to the present invention can also be used in casings or chassis for machinery, instru- ments and apparatus protecting against external electromagnetic radiation.

Claims

1. A sandwich construction comprising a fibre- and thermoplastic-containing composite layer (1) , a first metal layer (2) , and optionally a second metal layer (3) , the composite layer (1) comprising one or more fabric layers (4) containing arranged reinforcing fibres and a thermoplastic, cha rac t e r i s ed in that said fibres have a twist value of about 0.1 to 100 m^_1 and having a free non-bound fibre length of 0.1-20 mm, and the thermoplastic is partially distributed in the composite layer (1) to a penetration depth of about 30- 80% of the total thickness of the composite layer (1) , starting from the interface between the first metal layer (2) and the composite layer (1) and optionally from the interface between the second metal layer (3) and the composite layer (1) .

2. A sandwich construction as claimed in claim 1, c ha rac t er i s ed in that the composite layer (1) also comprises one or more metal wool layers (5) , preferably steel of aluminium wool layer (s) .

3. A sandwich construction as claimed in claim 1 or 2, chara c t e r i s e d in that the first metal layer (2) mainly consists of aluminium.

4. A sandwich construction as claimed in claims 1-3, c hara c t e r i s e d in that the first metal layer (2) is a film with a thickness of 0.05-0.1 mm.

5. A sandwich construction as claimed in claims 1-3, c hara c t e r i s e d in that the first metal layer

(2) is a metal sheet with a thickness exceeding 0.1 mm.

6. A sandwich construction as claimed in any one of the preceding claims, chara c t e r i s e d in that the composite layer (1) has a thickness essentially exceeding that of the first metal layer (2) .

7. A sandwich construction as claimed in any one of the preceding claims, charac t e ri s ed in that a plurality of fabric layers (4) are oriented in different directions.

8. A sandwich construction as claimed in any one of the preceding claims, c hara c t e r i s e d in that a plurality of metal wool layers (5) , preferably steel wool and/or aluminium wool layers, are oriented in different directions .

9. A sandwich construction as claimed in any one of the preceding claims, chara c t e r i s e d in that it comprises a first metal layer (2) , a composite layer (1) and a second metal layer (3) , the composite layer (1) comprising two fabric layers (4) , said fabric being non- woven polypropylene and surface-treated PET from bottles, and having a free non-bonded fibre length of 1-9 mm and fibres with a twist value of 10 m^"1 and a thermoplastic partially distributed in the composite layer (1) to a penetration depth of 40% of the total thickness of the composite layer (1) , starting from the interface between the first metal layer (2) and the composite layer (1) and from the interface between the second metal layer (3) and the composite layer (1) .

10. A sandwich construction as claimed in any one of the preceding claims, wherein the fabric layer comprises a prepreg, the thermoplastic being polypropylene.

11. A sandwich construction as claimed in any one of the preceding claims, charac t e r i s e d in that the fabric layer is a glass-fibre, carbon-fibre, armid, hemp, flax or sisal fabric and is woven, knitted, braided or non-woven fibre yarn construction, and that the thermoplastic is polypropylene, polyamide or polyester.

12. A method for manufacturing a sandwich construction according to claim 1, c h a r a c t e r i s e d by the steps of a) placing a first metal layer (2) , optionally coat- ed with a layer of primer, in a cold press tool, b) placing on the first metal layer one or more layers of fabric with preimpregnated or interlaced thermoplastic and optionally one or more steel wool layers in the desired order, and finally an optional second metal layer (3) , c) closing and optionally evacuating the press tool, d) heating the material in the tool to 100-420°C, preferably 190-280°C, at a pressure of about 0-50 kg/cm² for 0.5-5 min, whereby the thermoplastic present in the composite layer (1) melts and bonds to the fabric, and e) cooling the press tool die under pressure to about 50°C, the thermoplastic in the composite layer (1) being strongly bound to the first and optionally the second metal layer (2, 3) .

13. A method as claimed in claim 12 for manufacturing a car body component , c h a r a c t e r i s e d by placing on top of each other in the following order in a cold press tool: a first film (2) of low carbon mild steel with a thickness of 0.2 mm, a first fabric layer (4) of non-woven polypropylene and PET, one steel wool layer (5) , a second layer (4) of non-woven polypropylene and PET and optionally a second film (3) of low carbon mild steel, closing the press tool, and heating the material in the press tool to a temperature of about 220°C at a pressure of about 1 kg/cm² and for a period of

10 s max.

14. Method according to claim 12, wherein the first metal layer (2), the fabric layer(s) with thermoplastic, the metal wool fabric (s) (5), and the second metal layer (3) are separately and continuously fed from roll(s) to the press tool, and that each layer is cut before enter- ing the press tool or the finished article produced is cut at or after exciting the press tool.

15. Method according to claim 12, wherein after the steps a) and b) the aggregate of layers is heated in a first induction heated furnace, whereafter, instead of steps c) -e) , the aggregate of layers is allowed to cool, followed by heating of the aggregate of layers in a second induction heated furnace to a temperature at which the thermoplastic melts and by transfer of the aggregate of layers comprising molten thermoplastic to a cold press tool, in which the final compression to the sandwich construction takes place, wherein the layer aggregate after heating in the first induction heated furnace optionally is transferred directly to the cold press tool.

16. Use of a sandwich construction as claimed in any one of claims 1-11 as body material for vehicles, preferably automobile, trucks, buses and trains.

17. Use of a sandwich construction as claimed in any one of claims 1-11 as casing or chassis for machinery, instruments and apparatus emitting electromagnetic radiation on the one hand for screening emitted electromagnetic radiation and, on the other hand, for protection against external electromagnetic radiation.

18. Use of a sandwich construction as claimed in any of claims 1-11; in air freight containers, in construction components in aircraft, in safety doors and cabinets, in furniture and kitchen fittings, as side airbags in passenger automobiles, as frames in car seats and children's safety seats as wind deflectors for trucks, and as packing replacements in plate heat exchangers.