FIELD OF THE INVENTION AND PRIOR ART

This invention relates to a sheet structure having comparatively rigid surface layers and an intermediate layer, e.g. of cellular material, located therebetween. The invention also relates to methods for production of such sheet structures and sheet elements obtainable from such sheet structures.

Conventional art within this field is to manufacture sandwich structures in question with surface layers and an intermediate layer having substan¬ tially the same thickness over the entire width of the sheet structures. When special loads have made it necessary to have thicker and thereby more stable surface layers, the thickness of the surface layers was in¬ creased over the entire sheet structure. However, such methods mean a considerable price raise and weight increase, since the surface layers being stronger are more expensive and heavier than the material in the intermediate layer, which primarily functions as spacing layer between the surface layers.

Furthermore, it is known to apply various reinforcing arrangements on the outer surfaces of sheet structures of this type. However, this is rather time consuming and leads to a deterioration from the estetic point of view too.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a sheet structure and production methods connected thereto which make it possible to elimi- nate or reduce problems discussed in the introduction.

With respect to the sheet structure this is obtained by the fact that the surface layers as well as the intermediate layer have projections and re¬ cesses facing each other and are designed so that the projections of the surface layers are received by the recesses of the intermediate layer and vice versa. Thus, the surface layers may already in the production thereof be provided with projections forming thickenings in correspondence with the locations where particularly high loads in the intended user environ¬ ment motivate this, e.g. such thickenings may be provided at such loca- tions where other components are intended to be secured through the sheet structure by means of arbitrary securing means, such as screws, nails, rivets or the like. Accordingly, a stable sheet structure having the strength required in particularly exposed positions is obtained as a con¬ sequence of the thickenings without any need of any external indications thereof. It may be mentioned that it is of course possible to apply any type of marking, suitably discrete, which for the sake of facilitating the mounting inform the operators about within which regions of the sheet structure an increased strength is present as a result of a greater thick¬ ness of the surface layers.

It should also be mentioned that the thickenings of the surface layers dis¬ cussed may generally have to function as reinforcements, namely if the case that the thickenings have the character of reinforcing patterns dis¬ tributed over the extension of the sheet structure. It is then intended that the thickenings may form mesh work or cross-sectioned patterns, so that the thickenings accordingly will get the character of reinforcing beams.

The characteristics of the methods according to the invention are more closely disclosed in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a specific de- scription of embodiments of the invention cited as examples.

In the drawings:

Fig 1 is a schematic view illustrating layers of a sheet structure in an ex- ploded state,

Fig 2 is a perspective view illustrating the finished sheet structure,

Fig 3 is a schematic view illustrating a possible method for producing the sheet structure,

Fig 4 is a view illustrating a further method for producing the sheet structure,

Fig 5 is a view illustrating another possible production method,

Fig 6 is a cross-section through a sheet structure, and

Figs 7 and 8 are cross-section views illustrating sheet elements having means for interconnection thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A sheet structure having comparatively rigid surface layers 1 , 2 and an intermediate layer 3, e.g. of cellular material, located therebetween is il¬ lustrated in Fig 2. The intermediate layer is intended to normally have to act spacingly between the surface layers, which are the essential ones from the strength points of view. The intermediate layer 3 is suitably made of foamed plastic with a comparatively low density, while the sur-

face layers are made of material having a higher density and thereby a better strength. Although the material in the surface layers 1 , 2 is not re¬ stricted hereto, it may be suitable to make the surface layers of any polymeric material.

The surface layers 1 , 2 as well as the intermediate layer 3 have projec¬ tions and recesses facing each other and designed so that the projec¬ tions 4 of the surface layers 1 , 2 are received by the recesses 7 of the intermediate layer 3 and vice versa.

It is more exactly illustrated in Fig 1 how the intermediate layer 3 has a pattern of projections 6 and recesses 7.

It appears from Fig 2 that the outer sides of the surface layers 1 , 2 are intended to be substantially plane. Accordingly, the projections 4 of the surface layers 1 , 2 facing the intermediate layer 3 will form thickenings thereof. These thickenings mean local reinforcements of the surface layer and thereby of the entire sheet structure and the sheet structure is as already described in the introduction so constructed that these thicken- ings are in reality located where they are required as a consequence of loads existing, need of securing points etc. These thickenings may ex¬ tend mesh like or cross-section like over the sheet structure. It would of course also be possible to let the thickenings in certain cases extend substantially parallelly to each other over the sheet structure without in- tersecting each other should the load conditions require this. On the whole it is a basic object of the invention to create prerequisites for thickenings and thereby an increased strength only where this is really required, so that an unnecessary weight increase of the sheet structure is avoided.

The expressions "projections" and "recesses" used here should be inter¬ preted in a broad sense. Primarily, a targeted relief structure with a con¬ siderable constructive importance is intended, i.e. substantially more than only porosities or other surface irregularities in micro scale. The

thickenings obtained by the projections 4 of the surface layer are in the practice intended to have a thickness exceeding the thickness of the surface layers 1 , 2 in the region of the recesses 5 by at least 25%. More preferred percentage in this respect is 50% and in particular 100%. The recesses 7 of the intermediate layer are here intended to have a depth being at least 2%, preferably at least 5% and in particular at least 10%, of the greatest thickness of the intermediate layer 3.

Furthermore, it is pointed out that the definitions "projections" and "recesses" used herein include that the relationship between the surfaces occupied by projections and recesses, respectively, may vary within very broad limits. It may in certain load cases be sufficient to have the projec¬ tions and recesses, respectively, so arranged that the surface layers get thickenings only over a very small part of the surface of the sheet struc- ture, e.g. less than 25, less than 10 or even less than 1% of the surface. The thickenings may in other cases extend over more than 75% of the surface of the sheet structure. It should in this context be observed that it should be an aim to produce a part of the surface of the sheet structure over which the thickenings extend, since this results in a reduction of the relationship weight/strength.

In a possible method for producing the sheet structure described, the in¬ termediate layer 3 is firstly produced with the projections 6 and recesses 7 in question on the lateral surfaces thereof in a way indicated in Fig 1. Surface material is after that applied on these lateral surfaces, so that this fills the recesses 7 of the intermediate layer 3 while forming the sur¬ face layers 1 , 2 illustrated in Fig 2 and having a thickness varying in cor¬ respondence with the projections and the recesses of the intermediate layer. It would then for instance be possible to cast surface layer material on the intermediate layer 3 illustrated in Fig 1 , so that this flows out over the intermediate layer and forms a smooth outer surface. The casting of a surface layer on the lower side of the intermediate layer 3 in Fig 1 could take place by placing the intermediate layer in a mould in which the surface layer material already have been applied or to which such

material is later supplied by injection. The intermediate layer 3 could alternatively after the application of the surface layer on the top side thereof be turned and the surface layer casting be repeated on the other side of the intermediate layer then turned upwardly.

Another alternative is schematically illustrated in Fig 1 , namely to form the surface layers 1 , 2 partially by the outer coating layers 8, 9 illus¬ trated, which here are intended to have the character of thin sheets of substantially constant thickness, said sheets would be applied against the intermediate layer 3 after a flowing cast of a medium having on one hand the function of an adhesive and on the other the ability to function as a casting agent on the lateral surfaces thereof so as to fill up the recesses in the intermediate layer 3. With respect to the function as an adhesive it is of course the question of establishing an intimate and powerful mutual connection of the intermediate layer 3 and the coating layers 8, 9.

Another production method is illustrated in Fig 3. The surface layers 1 , 2 are according to this method produced with projections and recesses 4 and 5, respectively, at one flat side thereof, i.e. those which are intended to be located inwardly towards the intermediate layer 3. The outer sides of the surface layers 1 , 2 are intended to be substantially plane. The material, especially for plastic, forming an intermediate layer 3 is then injected through one or more suitable injecting devices or nozzles 10 between two surface layers 1 , 2 positioned with respect to each other in an appropriate manner, so that this material fills up the volume between the surface layers and gets a thickness varying in correspondence with the projections 4 and recesses 5 of the surface layers.

It is of course understood that in the embodiment shown in Fig 3 the sur¬ face layers 1 , 2 should suitably be located in a mold or the like limiting the possibilities of intermediate layer material injected to depart laterally out of the volume between the surface layers.

A method for continuous production of the sheet structure according to the invention is schematically illustrated in Fig 4. The intermediate layer 3 is as previously described produced with projections 6 and recesses 7 on the lateral surfaces thereof and a surface layer material is applied thereon so that this fills the recesses 7 of the intermediate layer 3 while forming surface layers 1 , 2 having a thickness varying in correspondence with the projections and recesses of the intermediate layer 3.

The intermediate layer 3 is in the embodiment according to Fig 4 in- tended to move (the arrow 11) while the surface layer material is applied on the lateral surfaces thereof. This movement takes suitably place from the top and downwardly, and it is understood that means suitable for this task, such as guide pulleys 12, are arranged so as to define the path of movement of the intermediate layer. Suitable guide means 13, possibly also guide pulleys, are of course also present so as to define the path of movement of the sheet structure obtained after the surface layers 1 , 2 are formed.

It is preferred that liquid surface layer material, preferably any suitable polymer, is supplied in excess to the intermediate layer 3, so that the bathes 14 of material is created at the surfaces of the intermediate layer 3. Accordingly, the recesses 7 in the outer surfaces of the intermediate layer will be filled up as the intermediate layer passes these bathes 14.

There are means 15 for bringing the surface layer material in the bathes 14 to obtain a substantially plain outer surface, which are here illustrated as being any form of fixed quiding element, but they could also be consti¬ tuted by rotating cylinders or the like. The task of these means is to de¬ termine the position of the bathes 14 of surface layer material discussed and define the thickness of the surface layers 1 , 2 as the sheet structure under production moves along these means 15.

The flowing surface layer material may be supplied for forming the bathes 14 by means of nozzles 16 or the like, and these nozzles or

further such could be designed to project a coating of surface layer material on the intermediate layer 3 passing.

As far as the method illustrated in Fig 4 has been described so far it could be characterized as continuous casting of the surface layers 1 , 2 onto the intermediate layer 3. Although the surfaces layers 1 , 2 could be formed entirely of liquid surface material, it is preferred that one or more web-like materials are included in the surface layers. These are sche¬ matically indicated at 17 and are intended to be supplied from a suitable source, for instance rolls, so that the webs 17 are applied on the inter¬ mediate layer 3 by means of the means 15 already described and acting as guiding elements. The webs 17 may be made of a fibrous material, for instance glass fibre. The webs could then be permeable to the liquid surface layer material forming the bathes 14, so that the webs accord- ingly will be more or less imbedded in the liquid material. The webs 17 could in accordance with an alternative also consist of continuous whole coating layers or other webs impermeable to the liquid surface layer ma¬ terial used, so that these would be located outermost in the sheet struc¬ ture obtained, i.e. that the liquid surface layer material from the bathes 14 would harden between these outer mostly located webs and the intermediate layer 3.

The variant illustrated in Fig 5 is based on the fact that the surface layers 1 and 2 are produced with projections 4 and recesses 5 on one flat side thereof and that the material forming the intermediate layer 3 is injected between the surface layers 1 , 2, so that this fills up the volume between the surface layers 1 , 2 and get a thickness varying in correspondence with the projections and recesses of the surface layers.

In the embodiment according to Fig 5 it is preferred that the surface lay¬ ers 1 , 2 are moved in the direction of the arrow 18, preferably from the top and downwardly, while the intermediate layer material is injected between the surface layers, e.g. by means of one or more nozzles or other injection devices 19.

The surface layers 1 , 2 are feed from suitable sources, for example sup¬ ply rolls or manufacturing units for the surface layers 1 , 2, and are led through sets of guide means 20, i.e. rolls, so that they are located at the predetermined distance from each other, which may be defined by further guide means or rolls 21 , two of which are illustrated for the sake of ex¬ emplifying. The requirement with respect thereto is that the surface lay¬ ers 1 , 2 are supported from the outer sides thereof in the region where the intermediate layer material is injected between the surface layers and hardened, so that a substantially constant thickness of the sheet structure obtained is established.

A cross section of the sheet structure according to the invention is illus¬ trated in Fig 6. The thickness of the surfaces 1 , 2 is indicated by a where the recesses 5 are present and the thickness by b where the projections 4 are present. The distance e indicates the distance between two thickenings of the surface layers 1 , 2 formed by projections 4.

A sheet structure as that shown in Fig 6 may advantageously be utilised for forming such sheet elements which appear schematically from Fig 7 and 8, namely sheet elements 22, 23, which at edge portions 24, 25 thereof have female and male formations, respectively. More exactly, the female formation is obtained by means of tongues 26 intended to externally grip around an extension of the element 23 forming the male formation 25. The tongues 26 are formed by surface layers 1 , 2 of the element 22, while the outer surfaces of the projection 25 are formed by the surface layers 1 , 2 of the element 23.

The tongues 26 have hook means 27 intended to function engagingly with corresponding hook means 28 on the projection 25 when the sheet ele¬ ments are brought together into the position according to Fig 8.

In a method for production of sheet elements 22, 23 having said means 27, 28 for interconnection at the edge portions 24, 25 a sheet structure

similar to that in Fig 6 having comparatively rigid surface layers 1 , 2 and an intermediate layer 3, e.g. of cellular material, located therebetween is first of all produced, wherein the surface layers are made to have thick¬ ened portions 4, which are received in recesses 7 in the intermediate layer 3, whereupon the sheet structure is divided into the sheet elements 22, 23 by means of cuts applied in the thickened portions 4 of the surface layers 1 , 2, so that parts of these thickened portions 4 will be present on the edge portions 24, 25 of the sheet elements 22, 23. Finally, the sheet elements are processed in their remaining parts of the thickened portions so as to obtain the connecting means 27, 28 described.

Should there be a desire to produce sheet elements 22, 23 having con¬ necting means 27, 28 along for instance four circumferential sides should the sheet structure according to Fig 6 be produced with a cross section pattern of thickened portions 4 designed in correspondence therewith, so that the following division of the sheet structure takes place in these thickenings arranged as checks. A rational manufacture of the sheet ele¬ ments according to Fig 7 and 8 may then be obtained.

It is evident that the invention is not only restricted to those aspects which have been described above and shown in the drawings. Several modifications could instead be made by those with the skill in the art without departing from the very inventional idea. It may be mentioned as an example that it is of course completely within the scope of the inven- tion to provide only of the surface layers 1 , 2 and only one of the flat sides of the intermediate layer with projections/recesses while the other surface layer and the other flat side of the intermediate layer 3 would be completely plane.