FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a method for manufacturing a board-shaped sandwich material. The invention also relates to a board-shaped sandwich material with a corrugated fiberboard core and hard particle layers as surface layers. Board-shaped sandwich material according to the present invention and board-shaped sandwich material manufactured by means of the method according to the present invention are primarily intended for use in for sample furniture, furnishings and as building material. For some of these applications particle boards are commonly used, which consist of compressed chips and adhesive. The particle board is often constructed such that its surface layers are hard and compact, whereas the core is more porous in order for the particle board to become lighter. In spite of this, particle boards are relatively heavy and thereby entail, among other things, high transport costs.

A board-shaped sandwich material comprising a core made from corrugated fiberboard and cover layers made from particle board is previously known from ZA9706742. The tubular flutes of the corrugated fiberboard extend essentially perpendicularly to the top side and the bottom side of the core and the cover layers are in the form of hardened particle boards which are adhered onto the core. Such a sandwich material can combine the hardness of the particle boards, which are used as cover layers, with the porosity and the lightweightness of the corrugated fiberboard core.

SUMMARY OF THE INVENTION The object of the present invention is to provide a new and advantageous way of manufacturing a sandwich material with a corrugated fiberboard core and covering particle layers. The object is achieved by a method according to claim 1 .

The inventive method of manufacturing a board-shaped sandwich material comprises the steps of:

- providing a board-shaped core made of corrugated fiber- board, which core has a top side and a bottom side and in which core the tubular flutes of the corrugated fiberboard extend essentially perpendicularly to the top side and the bottom side of the core;

- application of chips and adhesive onto the top side and the bottom side of the core so as to form a mat of chips on each side;

- compression of the mats of chips by exerting press force orthogonally to the top side and the bottom side of the core, the adhesive in the mats of chips being brought to harden so as to form hard particle layers on the top side and the bottom side of the core. By forming particle layers by compression and hardening directly on the top side and the bottom side of the corrugated fiberboard core, the bond between the core and the particle layers becomes strong, since the method allows that, in the interface between the particle layers and core, a thin layer is formed in which chips and adhesive have penetrated into the tubular flutes of the corrugated fiberboard. Another advantage of the method according to the invention is that the particle layers can be made very thin, since these never have to be handled separately from the core, but are compressed and hardened in place. The dimension of the finished sandwich material can thereby be adjusted such that it coincides with the standard thickness of a particle board. The sandwich material manufactured by means of the method can thereby be made lightweight but still rigid and strong. According to one embodiment, the mats of chips consist of a dry mixture of chips and adhesive, the adhesive being activated in connection with the compression. According to an additional embodiment, the application of chips and adhesive onto the bottom side of the core is performed by transferring the core to a surface on which a mat of chips is arranged. These embodiments provide for an efficient manufacturing process for sandwich material, since the manufacturing can take place according to the "conveyor belt "-principle.

According to one embodiment, a thin barrier layer is applied onto the top side and the bottom side of the core, respectively, before the application of the chips and adhesive intended for the formation of mats of chips. According to this embodiment, the barrier layer prevents the mixture of chips and adhesive from penetrating too far into the tubular flutes of the corrugated fiber- board, which ensures that the final sandwich material becomes lightweight thanks to that the flutes are kept essentially air filled. According to an additional embodiment, the barrier layers are applied by applying a wet mixture of chips and adhesive onto the top side and the bottom side of the core and hardening the mixture so as to form a thin barrier layer on each side. By applying a wet mixture of chips and adhesive as a barrier layer, it is easy to control the degree of penetration of the mixture of chips and adhesive into the flutes in order to optimize the weight of the sandwich material while at the same time securing a strong bond between core and surface layers. According to one embodiment, the core is manufactured by joining flat cylindrical corrugated fiberboard elements made from rolled-up single face corrugated fiberboard side by side with the envelope surfaces against each other to a board-shaped core. According to this embodiment, the density and the size of the core can easily be varied, for example by means of the number of corrugated fiberboard elements. According to an additional embodiment, the core is compressed before the application of chips and adhesive by exerting force essentially orthogonally to at least one of the side edges of the core. The density of the core can then be varied by the degree of compression when the corrugated fiberboard elements are compressed to a core.

Other preferred embodiments of the method according to the present invention will appear from the dependent claims and the following description.

The invention also relates to a sandwich material manufactured according to the inventive method. The sandwich material com- prises a board-shaped core made from corrugated fiberboard, which core has a top side and a bottom side and in which core the tubular flutes of the corrugated fiberboard extend essentially perpendicularly to the top side and the bottom side of the core, the sandwich material further comprising particle layers on the top side and the bottom side of the core, which particle layers consist of hardened and compressed chips and adhesive and wherein, in the interface between the core and the particle layers on each side, there is a layer in which chips and adhesive have penetrated into the tubular flutes of the corrugated fiber- board and thereby integrated the core with the particle layers. The sandwich material is, at the same time as it is rigid and strong, also lightweight. Furthermore, it exhibits a strong bond between core and particle layers. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be described with reference to the appended drawings, wherein: Fig 1 a-c is a principle sketch which shows the steps in a method according the invention; Fig 2 shows application of a wet mixture of chips and adhesive onto a core made from corrugated fiberboard material so as to form barrier layers;

Fig 3 shows a core made from corrugated fiberboard material with barrier layers and with layers of mixture of chips and adhesive applied before compression and hardening;

Fig 4 shows a cross-section of a sandwich material according to the invention;

Fig 5 is a principle sketch which shows a method according to the invention; and

Fig 6a-b shows manufacturing of a core made from corrugated fiberboard material by joining cylindrical corrugated fiberboard elements.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The term mat of chips in the following refers to a not yet hardened and compressed layer of adhesive-coated chips, manufactured by coating chips with adhesive in liquid form or spray form, or a layer of chips mixed with adhesive, where the adhesive can preferably be in powder form. The term particle layer is used for a hardened and compressed layer of chips and adhesive.

In Fig 1 a-c, a method according to the present invention for manufacturing of a board-shaped sandwich material 1 is schematically illustrated. In the method according to the invention, a board-shaped core 2 made from corrugated fiberboard as shown in Fig 1 a is provided. The core has a top side 3a, a bottom side 3b and side edges 4. The corrugated fiberboard that forms the core has tubular flutes 10, see Figure 2, which extend essentially perpendicularly to the top side 3a and the bottom side 3b of the core. On the top side 3a and the bottom side 3b of the core, chips and adhesive are applied, whereby a mat of chips 5a, 5b consisting of chips and adhesive is formed on each side. These mats of chips 5a, 5b are thereafter compressed as shown in Figure 1 b by exerting press force F F ₂ orthogonally to the top side 3a and the bottom side 3b of the core. The compressed mats of chips 5a, 5b are brought to harden under pressure and thereby form hard particle layers 6 on the top side 3a and the bottom side 3b of the core, see Figure 1 c. A sandwich material 1 comprising a core 2 made from corrugated fiberboard and particle layers 6 on the top side and the bottom side is thereby obtained.

The lower mat of chips 5b, which is intended to bear on the bottom side 3b of a core 2 made from corrugated fiberboard, may according to the invention be formed by arranging adhesive-coated chips in the form of a layer on a belt conveyor. On top of the lower mat of chips 5b the core 2 is placed with the bottom side 3b directed towards the lower mat of chips 5b. An upper mat of chips 5a is thereafter formed on the top side 3a of the core, for example by arranging adhesive-coated chips in the form of a layer on the top side 3a of the core. The core 2 with the two mats of chips 5a, 5b is then subjected to press force F ₇ , F ₂ orthogonally to the top side 3a and the bottom side 3b by means of press rolls or press plates 7, whereby the mats of chips are compressed. In connection with the pressing , the adhesive in the mats of chips 5a, 5b is activated so that the chips adhere to each other, and the particle layers 6 are hardened. Depending on which adhesive is used in the particle mixture, the hardening preferably takes place at an elevated temperature, such as up to 100°C, or in some cases up to about 250°C. A pressure force of approximately 100-300 bar can be used during the hardening.

The mats of chips 5a, 5b are normally composed of dry sub- stance. Preferably, chips which have been adhesive-coated and dried are used, but also a dry mixture of chips and particulate adhesive, or chips and adhesive which are applied simultaneously and are first being mixed when within the mat of chips, can be used. The mats of chips 5a, 5b can for example have a thickness of approximately 6-9 mm, in which case the finished particle layers after pressing and hardening become approximately 3 mm thick. The thickness of the particle mats 5a, 5b should be slightly more than double the size of the desired thickness of the finished particle layers 6. For a finished thick- ness of the particle layers 6 of about 3 mm, the chips should not be more than approximately 3-5 mm long, preferably less than 3 mm.

In Figure 2, a method according to the invention, where thin bar- rier layers 8 are applied onto the top side 3a and the bottom side 3b of the core before the application of chips and adhesive intended to form mats of chips 5a, 5b, is shown. The barrier layers 8 have as one of their objects to plug the transverse flutes 10 of the corrugated fiberboard, in order to prevent chips and adhesive from the mats of chips 5a, 5b from penetrating deep into the flutes 10 and thereby increase the density of the sandwich material 1 . The barrier layers 8 can preferably be formed from a wet mixture of chips and adhesive 9, which is spread onto the top side 3a and the bottom side 3b of the core by means of applicators 1 1 . The mixture 9 of chips and adhesive can then be hardened before mats of chips 5a, 5b are arranged on each side 3a, 3b, as is shown in Figure 3. It is also possible to directly arrange mats of chips 5a, 5b on each side, without intermediate hardening steps, in which case the barrier layers 8 and the particle layers 6 are being hardened in the same hardening step. It is desirable for a certain penetration to take place of the mixture 9 of chips and adhesive of the barrier layers 8 into the tubular flutes 10 of the corrugated fiberboard, as is shown in the enlargement in Figure 2, since this increases the bond between barrier layers 8 and core 2. The mixture 9 shall however not penetrate too far, since the density of the core 2 thereby increases. A penetration of about 1 mm is sufficient to achieve a strong bond between the core 2 and the particle layers 6. The barrier layers 8 make it possible to keep a lightweight core 2, while at the same time the mats of chips 5a, 5b can be compressed and hardened to particle layers 6 under large pressure. The sandwich material 1 thereby obtains a light core 2 and a hard and durable surface. A sandwich material 1 with barrier layers 8 manufactured using a method according to the invention is shown in cross-section in Figure 4.

The barrier layers can also be composed of some other kind of filler than a mixture of chips and adhesive, for example, solely adhesive can be used as a barrier layer, in which case the adhesive should be viscous in order not to penetrate too far into the tubular flutes of the corrugated fiberboard. According to one embodiment, the barrier layers can be composed of a thin plastic film which is applied onto the core. The plastic film can embrace the entire core or cover only its top side and bottom side. The plastic film seals the flutes so that chips and adhesive are prevented from penetrating thereinto when applying chips and adhesive so as to form the mats of chips. The plastic film should be chosen such that it melts upon pressing and hardening of the mats of chips at an elevated temperature, such that chips and adhesive are allowed to penetrate a short way into the flutes. Strong adhesion of the particle layers to the core is thus achieved.

The chips which are used in surface layers and possible barrier layers are preferably wooden chips, preferably saw dust, since these have a suitable size distribution for formation of particle layers with a thickness of a few millimetres. It is also possible to use other chips than wooden chips, for example chips from flax straw or hemp.

The core 2 made from corrugated fiberboard can be produced in different ways within the scope of the invention. In Figure 5, an example is schematically shown, where a corrugated fiberboard web 12 made from single-face corrugated fiberboard, that is, corrugated fiberboard with one flat side and one corrugated side, is wound up on a roll 13, from which it is being unwound and rolled into a cylinder 14. Pre-coating with adhesive is carried out by applying adhesive onto the corrugated fiberboard web 12 by means of a spray nozzle 15. The adhesive can be applied either solely onto the flat side of the corrugated fiber- board web 12 or onto both sides. The adhesive is dried before the corrugated fiberboard web 12 is being rolled into the cylinder 14. The longitudinal axis of the cylinder 14 extends in the same direction as the tubular flutes 10 of the corrugated fiber- board. When the cylinder 14 has reached a suitable diameter, the corrugated fiberboard web 12 is cut, whereafter the cylinder 14 is sealed and crosscut by a set of knives 16, whereby flat cylindrical corrugated fiberboard elements 17 are formed. These are guided into one or several folds 18 and are carried further to a press frame 19, where a number of corrugated fiberboard elements 17 are placed side by side with the envelope surfaces 17a against each other as shown in Figure 6a. The corrugated fiberboard elements 17 are thereafter compressed to a rectangular core 2 by means of the press frame 19 pressing against at least one of the sides which are to constitute the side edges 4 of the core, see Figure 6b, with a pressure adjusted for desired density and rigidity of the core 2. The degree of compression in this way controls the density of the core 2. The adhesive which was previously applied to the corrugated fiberboard web 12 may now be activated and hardened in order to join the core 2, or alternatively, the core can be held under pressure against the side edges 4 when the barrier layers 8 and/or the particle layers 6 are applied and hardened. It is of course not necessary that the roll 13 and the cylinder 14 are horizontal during the manufacturing of the flat cylindrical corrugated fiberboard elements 17 as shown in Figure 5. On the contrary, the longitudinal axes of the roll and cylinder may be at an angle of up to 90° with respect to the underlying surface.

An alternative method of manufacturing of the core 2 is to pre- coat a corrugated fiberboard web with adhesive and then cut it into strips, which strips are rolled into flat cylindrical corrugated fiberboard elements 17. The cylindrical corrugated fiberboard elements 17 are joined as described above side by side with the envelope surfaces 17a against each other into a board-shaped core 2.

The core 2 can also be constructed from corrugated fiberboard sheets which have been joined together without first having been rolled into cylindrical corrugated fiberboard elements. In this case, other kinds of corrugated fiberboard than single-face corrugated fiberboard may be used, such as single-wall corrugated fiberboard. The core can for example be manufactured by joining corrugated fiberboard sheets or strips of corrugated fiberboard into cuboids of suitable size, which if required are crosscut with a cut across the tubular flutes of the corrugated fiberboard such that a core with a top side and a bottom side essentially perpendicular to the tubular flutes of the corrugated fiberboard is formed.

A board-shaped sandwich material 1 manufactured by means of the inventive method has a core 2 made from corrugated fiber- board with a top side 3a and a bottom side 3b and side edges 4. The tubular flutes 10 of the corrugated fiberboard extend essentially perpendicularly to the top side and the bottom side of the core. The sandwich material 1 further comprises particle layers 6 on the top side 3a and the bottom side 3b of the core, which particle layers consist of hardened and compressed chips and adhesive. In the interface between the core 2 and the particle layers 6 on each side, there is a layer in which chips and adhesive, alternatively another material which forms a barrier layer 8, have penetrated into the tubular flutes 10 of the corru- gated fiberboard and thereby has integrated the core 2 with the particle layers 6 alternatively the barrier layers 8. The bond between the particle layers 6 and the core 2 is therefore strong.

The inventive method is of course not restricted to the embodi- ments described above. On the contrary, several modifications of these should be obvious to a person skilled in the art without departing from the scope of the invention as defined in the appended claims.