Powder based balancing layer

POWDER BASED BALANCING LAYER

Technical field

The disclosure generally relates to the field of panels with a decorative surface layer and a balancing and/or protective layer, preferably floor and wall panels. The disclosure relates to production methods to produce such panels.

Field of Application

Embodiments of the present invention are particularly suitable for use in production of floating floors, which are formed of floor panels comprising a core and a decorative wear resistant solid surface layer comprising fibres, binders and wear resistant particles that have been applied on the core as a powder as described in WO 2009/065769. The field of application comprises products with other types of decorative layers such as paper based, laminated panels (DPL), where a balancing and/or a protecting layer is used. The following description of technique, problems of known systems and objects and features of the embodiments of the invention will therefore, as a non-restrictive example, be aimed above all at this field of application and in particular at floorings which are similar to traditional floating wood fibre based laminate floorings. The invention does not exclude floors that are glued down to a sub floor. Embodiments of the invention can also be used in building panels such as for example wall panels, ceilings, and furniture components and similar.

Background

Wood fibre based direct pressed laminated flooring (DPL) usually comprises a core of a 6-12 mm fibre board, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing and/or protective layer of laminate, plastic, paper or like material.

The surface layer of a laminate floor is characterized in that the decorative and wear properties are generally obtained with two separate layers one over the other. The decorative layer is generally a printed paper and the wear layer is a transparent overlay paper, which comprises small aluminium oxide particles.

The printed decorative paper and the overlay are impregnated with melamine formaldehyde resin and laminated to a wood fibre based core under heat and

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WO 2012/141647 ₂ PCT/SE2012/050386 pressure. The balancing layer is used to keep the panel flat after production and when the panel is exposed to variations in moisture conditions that cause the surface layer to swell and shrink.

Recently new “paper free” Wood Fibre Floor (WFF) types have been developed with solid surfaces comprising a substantially homogenous mix of fibres, binders and wear resistant particles.

The wear resistant particles are preferably aluminium oxide particles, the binders are preferably thermosetting resins such as amino resins and the fibres are preferably wood based. Other suitable wear resistant materials are for example silica or silicon carbide. In most applications decorative particles such as for example colour pigments are included in the homogenous mix. In general all these materials are preferably applied in dry form as a mixed powder on a HDF core and cured under heat and pressure to a 0.1 -1.0 mm solid layer.

Several advantages over known technology and especially over conventional laminate floorings can be obtained:

• The wear resistant surface layer, which is a homogenous mix. can be made much thicker and a wear resistance is achieved, which is considerably higher.

• New and very advanced decorative effects can be obtained with deep embossing and by separate decorative materials, which can be incorporated into the homogenous surface layer and coordinated with the embossing.

• An increased impact resistance can be reached with a homogenous surface layer, which is thicker and has a higher density.

• The homogenous surface layer can comprise particles that have a positive impact on sound and moisture properties.

• Production costs can be reduced since low cost and even recycled materials can be used and several production steps can be eliminated.

Powder technology is very suitable to produce solid decorative surface layer, which are much thicker than conventional laminate surface layers. Such solid powder based layers create a much higher tension on the panel when they

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WO 2012/141647 3 PCT/SE2012/050386 shrink (or swell) during curing and balancing is a problem It is therefore difficult to produce a panel with a thick and a high quality surface, which is also well balanced after pressing as well as stable and flat in variable moisture conditions.

Definition of Some Terms

By surface layer is meant all types of surface layers, which give the panel its decorative properties and its wear resistance. By a “WFF mix” is meant a mix of materials comprising fibres, binders, wear resistant particles and, optionally, a colour substance, which is preferably applied as powder on a carrier.

By “WFF floor” is meant a floor panel comprising a solid surface, which is obtained by a WFF mix that is preferably applied as dry powder on a core, such as for example HDF, and cured under heat and pressure.

Known Technique and Problems thereof

The new “paper free” WFF floors with a solid surface comprising a WFF mix of fibres, preferably wood fibres, small hard wear resistant particles and a binder are produced according to a production method where the WFF mix is applied in powder form on a core. The wood fibres are generally refined, mechanically worked, and of the same type as used in HDF and particleboard, i.e. treated in a way that the lignin content is essentially unchanged. They comprise natural resins such as lignin. The wear resistant particles are preferably aluminium oxide particles. The surface layer comprises preferably also colour pigments or other decorative materials or chemicals. Processed fibres e.g. bleached wood fibres may also be used. Processed fibres may be semi-transparent, and are preferably transparent in a cured binder.

A preferred binder is melamine or urea formaldehyde resin. Any other binder, preferably synthetic thermosetting or thermoplastic resins, may be used. The WFF layer is generally scattered in dry powder form on a wood based core, such as for example HDF.

The backside of the core is covered by a balancing layer, which comprises one or several papers impregnated with melamine resin in order to get a balanced product after pressing. The panel is generally produced with a surface that is slightly convex in order to be able to stay flat even in very dry conditions.

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The total build up with decorative layer, core and balancing or protective layer is transferred into a press, where the decorative layer and the balancing or protective layer are cured, under the influence of heat and pressure.

By a protective layer is meant a layer adapted to protect the backside of the core.

The curing of a melamine formaldehyde (MF) resin involves shrinkage of the MF resin matrix and the balancing layer is needed to balance the shrinkage forces from the decorative layer.

Paper based backer materials can create problems as the paper always has a dominating fibre direction either in the production direction or perpendicular to the production direction. The shrinkage during curing is always higher in the transverse direction of the fibres as the fibres can be packed together closer in this direction. The implication of this behaviour is that the balancing layer has different shrinkage in the production direction compared with the transverse direction.

As the backer is used to balance the decorative layer, which may as an example comprise a powder mix with random fibre direction, the consequence is that an optimised balancing only can be obtained in one direction, either along or perpendicular to the production direction. It is also complicated to adapt the thickness of the paper to different thicknesses of the surface layer.

However, the paper based balancing layer offer the advantages that the paper is rather easy to transport together with the core under the scattering units and into a press.

It is known from WO 2009/065769 that the balancing layer can be created with a powder layer that is applied on the backside of the core. This known disclosure does not show how a powder layer on a back side of a core should be handled in order to allow a cost efficient production, especially when discontinuous presses are used which require that the core with its powder based layers can be fed in high speed with clamping devices into a press.

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The above description of various known aspects is the applicants' characterization of such, and is not an admission that any of the description is prior art.

Objects and Summary

An objective of certain embodiments of the invention is to provide a building panel, preferably a floor panel with a decorative surface layer and a balancing and/or protective layer which can be produced in a cost efficient way.

A first aspect of the invention is a method of manufacturing a building panel with a decorative surface layer, a core and a balancing and/or protective layer. The method comprises the steps below and preferably performed in the listed sequence:

• applying a first layer of a first powder based mix, comprising wood fibres and a thermosetting binder on a core;

• applying a liquid substance on the first powder based mix;

• drying the first powder based mix;

• turning the core with the dried first powder based mix such that the first powder based mix points downwards;

• applying a second layer on the upper part of the core; and • curing the first and second layers simultaneously by providing heat and pressure, wherein the first layer forms the balancing and/or protective layer and the second layer forms the decorative surface layer in the building panel.

The method offers the advantages that a powder based balancing or protective layer can be applied in a cost efficient way by scattering on a core. The powder may be fixed to the core with a liquid substance that swells, partly dissolves or dissolves the resin, thereby making the powder tacky and stick together. A drying step removes part of the water thus leaving the powder glued not only to itself but also to the core. The attachment makes the first layer stable such that the core with the first layer can be turned and fed under scattering units and into a press. A paper based balancing or

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WO 2012/141647 θ PCT/SE2012/050386 protective layer can be replaced in a cost efficient way by a wood fibre based layer with random fibre direction that has similar shrinkage properties as the decorative surface layer.

The building panel may be a floor panel.

The core may be a HDF or MDF board.

The second layer may comprise a veneer layer. Thereby, the veneer layer forms the decorative surface layer. The second layer may comprise a sheet-shaped layer of wood. A curable lacquer may be applied on top of the veneer layer.

The second layer may comprise a first paper sheet. In this embodiment, the second layer forming the decorative surface layer is a laminate such as a DPL (direct pressed laminate) or HPL (high pressure laminate).

The second layer may further comprise a second paper sheet.

The first and/or the second paper sheet may be impregnated with a thermosetting resin such as melamine formaldehyde

The first paper sheet may be arranged such that the fibre direction of the first paper sheet is extending in a first direction, and the second paper sheet may be arranged such that the fibre direction of the second paper sheet is extending in a second direction, said second direction being transverse to the first direction.

The second layer may comprise a second powder based mix comprising wood fibres, binders, preferably a thermosetting binder, and wear resistant particles. Thereby, the second layer can be applied in the same way as the powder based balance or protective layer by scattering on the core.

The method may comprise the further step of applying a print or a colour substance into the second mix.

A second aspect of the invention is a method of manufacturing a building panel with a decorative surface layer, a core and a balancing and/or protective layer. The method comprises the steps below and is preferably performed in the listed sequence:

• applying a first layer of a first powder based mix, comprising wood fibres and a thermosetting binder on a carrier, preferably of a thin material with

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WO 2012/141647 γ PCT/SE2012/050386 a thickness that does not exceed the thickness of the decorative surface layer;

• placing the core material on the first powder based mix;

• applying a second layer on the upper part of the core; and • curing the first and second layers by providing heat and pressure wherein the first layer forms the balancing and/or protective layer and the second layer forms the decorative surface layer of the building panel.

The method offers the advantages that a powder based balancing or protective layer can be applied in a cost efficient way by scattering on a carrier that keeps the powder based balancing or protective layer connected to the core during transport to the device that applies the surface layer and finally when the core is fed into the press.

The building panel may be a floor panel.

The core may be a HDF or MDF board.

The carrier may be a fibre based material.

The second layer may comprise a veneer layer. Thereby, the veneer layer forms the decorative surface layer. A curable lacquer or powder mix layer may be applied on top of the veneer layer.

The second layer may comprise a first paper sheet. In this embodiment, the second layer forming the decorative layer is a laminate such as a DPL (direct pressed laminate) or HPL (high pressure laminate).

The second layer may further comprise a second paper sheet.

The first and/or the second paper sheet may be impregnated with a thermosetting resin such as melamine formaldehyde.

The first paper sheet may be arranged such that the fibre direction of the first paper sheet is extending in a first direction, and the second paper sheet may be arranged such that the fibre direction of the second paper sheet is extending in a second direction, said second direction being transverse to the first direction.

The second layer may comprise a second powder based mix comprising wood fibres, binders, preferably a thermosetting binder, and wear resistant particles.

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Thereby, the second layer can be applied in the same way as the powder based balance or protective layer by scattering on a core.

The method may comprise the further step of applying a print or a colour substance into the second mix.

A third aspect of the invention is a method of manufacturing a separate and continuous powder based balancing and/or protective layer, which is essentially uncured. The method comprises the steps below and is preferably performed in the listed sequence:

• applying a powder mix comprising fibres and a thermosetting binder on a carrier;

• applying moisture on the powder mix such that the powder mix is connected together to an essential uncured balancing and/or protective layer; and • releasing the uncured balancing and/or protective layer from the carrier.

The separate and continuous powder based balancing or protective layer may be used in a method for producing a building panel which method comprises the steps below and is preferably performed in the listed sequence:

• placing the uncured balancing and/or protective layer under a core material;

• applying a surface layer on the core material; and • curing the surface layer and the uncured balancing and/or protective layer under heat and pressure.

The method offers the advantages that a powder based balancing or protective layer can be produced as a separate uncured layer that can be handled as a conventional paper based balancing or protective layer. The moisture, that preferably comprises water, may be sprayed on the powder such that the binder, preferably a melamine resin, moisture swells, partly dissolves or dissolve the resin, thereby making the powder tacky and stick together. The fibres in the mix will be connected to each other when the water dries. The fibres may also be connected by applying heat, e g., IR heating, thereby removing the moisture and connecting the fibres in the mix. A powder based uncured balancing or protective

81773865 layer may be formed as a flexible thin sheet and a core material may be placed over the balancing or protective layer.

The surface layer may comprise a powder based surface layer, at least one paper sheet or a veneer layer.

A fourth aspect of the invention is a separate and continuous powder based balancing and/or protective layer, which is essentially uncured. The separate layer comprises powder mix comprising connected fibres and an essentially uncured thermosetting binder.

The powder based balancing or protective layer is preferably produced according to the method of the third aspect.

A fifth aspect of the invention is a building panel, comprising a core, preferably a wood-based core such as MDF or HDF board, a decorative surface layer arranged on a first surface of said core, the decorative surface layer comprising at least one paper layer, and a balancing layer arranged on a second surface of said core, said second surface being opposite to said first surface, wherein the balancing layer is formed of a powder based mix comprising wood fibres and a thermosetting binder.

The building panel is preferably produced according to the first or second aspect of the invention.

In some embodiments of the invention, there is provided a method of manufacturing a building panel with a decorative surface layer, a core and a balancing layer, wherein the method comprises the steps: applying a first layer of a first powder based mix, comprising wood fibres and a thermosetting binder on the core; applying a liquid substance on the first powder based mix wherein the core is a HDF or MDF board; drying the first powder based mix; turning the core with the dried first powder based mix such that the first powder based mix points downwards; applying a second layer on an upper part of the core; and curing the first and second layers by providing heat

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9a and pressure, wherein the first layer forms the balancing layer and the second layer forms the decorative surface layer in the building panel.

Brief Description of the Drawings

The invention will in the following be described in connection to preferred 5 embodiments and in greater detail with reference to the appended exemplary drawings, wherein,

Figs 1 a-e illustrate a method to form a balancing and/or protective layer.

Figs 2a-e Illustrate a method to form a balancing and/or protective layer.

Fig 3 illustrates a building panel wherein the decorative layer is laminate.

Fig 4 illustrates a building panel wherein the decorative layer is a veneer layer.

Detailed Description of Embodiments

The powder intended to be used as a balancing and/or protective layer 3 is applied as a first powder mix on a core 2, preferably a MDF or HDF material, preferably by one or several scattering units 11 as shown in figure 1a.

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Figure 1b shows that the powder is thereafter pre-stabilized by a fluid 7, preferable based on water, which is applied by a stabilizing device 9. The first powder layer is thereafter dried. Such drying can be performed off-line in a controlled environment or in-line by applying heat 8 with a heating device 10 on the powder mix 3. The heating device 10 may comprise infrared light.

The water based fluid 7 and the following drying 8, attaches the powder layer 3 to the core 2, to such an extent that the core 2 with the powder can be turned 180° to bring the powder based balancing or protective layer 3 downwards, as shown in figure 1c, such that it may be transported further along the production line or be stacked on a pallet for intermediate storage before further production takes place.

Oversized or undersized fibres from the hammer mill that produces the fibres for the surface layer may be used in the balancing or protective layer mix The fibre size is not as critical as in a powder based surface layer and may vary from a fibre length of several mm to less than 0.1 mm. The most preferred average fibre length is 0,1-1,0 mm.

A second powder based surface layer mix 1 is thereafter applied on the upper side of the core 2 as showed in fig. 1d. A digital print 4 may be printed into the surface layer 1. The surface layer 1 preferably also comprises wood fibres, a colour substance, thermosetting binders and aluminium oxide particles.

The wood fibres in both the first and second powder mixes in all embodiments of the invention may be virgin, unrefined, refined and/or processed, comprising lignin and without lignin, e.g. α-cellulose fibres or holocellulose. A mixture of refined and unrefined fibres may also be used. It is also contemplated that vegetable fibres as jute, linen, flax, cotton, hemp, bamboo, bagasse and sisal may be used. Also mineral fibres and carbon fibres may be used.

As an alternative to the second powder based surface layer mix, the second layer forming the decorative surface layer may comprise at least one paper sheet 21 adapted to form a laminate, which is shown in fig. 3. Said at least one paper sheet 21 may be arranged on the upper side of the core. Preferably, the second layer comprises a first paper sheet 21 and a second paper sheet 22. The first and second paper sheets 21, 22 may be arranged such that the fibre direction of the first paper sheet 21 is extending in a first direction, and the fibre

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WO 2012/141647 -j ₁ PCT/SE2012/050386 direction of the second paper sheet 22 is extending in a second direction being opposite to the first direction. By arranging the fibre direction of the sheets transversely, the second layer has fibre directions in more than one direction. Thereby, during shrinkage, the second layer obtains properties more similar to the balancing or protective layer having a random fibre direction compared to a surface layer having only one dominating fibre direction.

The first paper sheet 21 may be a printed paper. The second paper sheet 22 may be a transparent overlay paper. The second paper sheet 22 may form a wear layer and may comprise small aluminium oxide particles. The first and second paper sheets 21, 22 may be impregnated with melamine resin such that they can be laminated to the core under heat and pressure simultaneously as curing the balancing and/or protective layer. The second paper sheet 22 may be arranged on top of the first paper sheet 21.

Alternatively, the second layer may comprise a veneer layer 23 or sheet-shaped layer of wood arranged on the upper side of the core, which is shown in fig. 4.

The second layer may further comprise a curable lacquer layer applied on top of the veneer layer or sheet-shaped layer of wood. The curable lacquer layer may be cured simultaneously as curing the balancing and/or protective layer.

Other décor layers may also be used as a second layer, such as cork, rubber, plastics, especially a thermoplastic décor layer.

The cor© 2 with th© surface layer 1 and th© balancing or protective layer 3 is thereafter fed into a press where the layers are cured under heat and pressure. The balancing layer may be optimized in thickness and material composition in order to create a perfect balancing of the surface layer. The fibre orientation in the layers may be essentially the same.

Recycled fibres from the machining of the panels, for example when a mechanical locking system is formed, may be used in the balancing or protective layer mix.

The balancing or protective layer 3 may preferably comprise a homogenous powder mixture comprising about 50 weight % recycled MDF fibres with a moisture content of preferably 3-8 % and about 50 weight % Melamine Formaldehyde (MF) resin (Prefere 4865, Dynea). The MF resin amount can vary between 30-70 w-%, preferably 40-65 w-% and most preferably 45-60 w-%. The

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WO 2012/141647 ₁₂ PCT/SE2012/050386 fibre content may vary from 30 - 70%, most preferably between 40-55%.

If needed the powder formulation can be modified such that thermosetting particles, pigments, hard particles, release agents, wetting agents and similar materials are included into the mix. The thermosetting plastic particles may be mixed at random into the powder or applied as a separate thin layer and may be used to provide a sealing against moisture penetration into the core.

The core of an HDF board preferably has a moisture content of 0-8 %

The scattering unit 11 may have capacity of scattering of 100-1000 g/m² with a tolerance of +/- 5% as measured using a calibration cup with an area of 100x100 mm cross- and length wise the board. The amount of stabilization fluid may vary between 0 up to 200 g/m².

The scattering unit 11 may comprise a needle belt and a scattering roller 12. The scattering roller is provided with needles in the range of about 30-120, preferably about 50-100, and most preferably about 70-90 needles per cm². The needle length is about 0.5-2.5 mm, preferably about 1-2 mm and most preferably about 1.5 mm. Several scattering units 11 may be used to even out differences in the applied powder mixture.

Furthermore, the scattering unit may be provided with a needle belt with needles with a preferred length of about 15-20 mm, a preferred frequency in the range of about 500-1000 rpm preferably about 1000 rpm, and a stroke length of about +/3 mm.

The stabilizing device 9 and the heating device 8 may be used to stabilize the surface layer 1 and/or the balancing or protective layer 3.

The stabilization device 9 may stabilize the powder using steam, nozzle spray coating or ultra sonic spray coating.

Stabilization fluid may comprise solvents such as non-polar solvents, polar aprotic solvents and polar protic solvents or mixtures thereof. Preferred solvents are polar protic solvents such as isopropanol, ethanol and water. Most preferred is water.

The stabilization fluid can further comprise additives such as wetting agents, defoamers, release agents, anti-slip agents and catalysts.

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Figures 2a-2e show how a powder based balancing or protective layer can be applied by applying the powder based balancing or protective layer as a first mix 3 on a carrier 5. The core is applied on the mix (fig 2c) and a powder based surface layer 1 is applied on the core 2. Alternatively, a surface layer of at least one paper sheet 21 or veneer layer 23 is applied to the core for forming a decorative surface layer, as described above with reference to figs. 3 and 4. Stabilizing and/or heating of the balancing and/or surface layers may be made as described above. The core and layers are fed into a press and cured under heat and pressure to a building panel as shown in figure 2e.

The carrier may be a paper with a weight of for example 100 - 200 gr. It may also be a non woven fibre based material or a foil.

Powder may also be applied on a carrier that preferably is a conveyor belt 6 and stabilized with fluid and dried such that it can be released from the conveyor 6 and handled as a separate sheet without the carrier 5 as shown in figure 2b. Such a separate and continuous sheet or layer is essentially uncured. The sheet or layer may be flexible such that it may be bent. The powder may comprise fibres, preferably wood fibres, and a thermosetting binder.

EXAMPLE

In example 1 below the powder formulation for the balancing layer used comprising 50 weight-% recycled MDF fibre (Vâlinge Innovation Sweden), 50 weight-% Melamine Formaldehyde resin (Prefere 4865, Dynea).

Example 1: Powder based product obtained through heat compression of product produced with powder based balancing layer and a treated powder as decorative layer.

A balancing layer was formed by scattering 500 g/m² of powder on a 9.7 mm HDF core.

g/m² of stabilisation fluid with 5 w-% wetting agent, 6 w-% release agent and 3 w-% catalyst was applied by spraying on the powder based balancing layer mix

The powder and the stabilisation fluid were applied at a line speed of 2.7 m/min. The powder based balancing layer was dried at the same line speed with IR with an effect of 19 kW.

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The resulting intermediate product with a stabilized powder based balancing layer was turned 180° and stacked on a pallet before use in the next operation where a decorative surface layer of 500 gr/m² was applied in powder form on the core.

The core with the surface and balancing layers was pressed in a press with a pressure of 40 kg/cm², during 25 seconds. The upper press table applied a heat of 170 degrees C on the surface layer and the lower press table applied a heat of 175 degrees C on balancing layer.

A panel with a small pre tension backwards and a slightly convex surface was obtained.

In example 2 below the powder formulation for the balancing layer used comprising 42 weight-% recycled MDF fibre (Vâlinge Innovation Sweden), 58 weight-% Melamine Formaldehyde resin (Prefere 4865, Dynea).

Example 2: Powder based product obtained through heat compression of product produced with powder based balancing layer and a treated powder as decorative layer.

A balancing layer was formed by scattering 320 g/m² of powder on a 9.7 mm HDF core.

g/m² of stabilisation fluid with 1 wt-% wetting agent, 6 wt-% rerelease agent and 1 wt-% catalyst was applied by spraying on the powder based balancing layer mix

The powder and the stabilisation fluid were applied at a line speed of 2.0 m/min. The powder based balancing layer was dried at the same line speed with IR with an effect of 19 kW.

The resulting intermediate product with a stabilized powder based balancing layer was turned 180° and feed into the next operation where a decorative surface layer of 550 gr/m² was applied in powder form on the core.

The core with the surface and balancing layers was pressed in a press with a pressure of 40 kg/cm², during 37 seconds. The upper press table applied a heat of 184°C on the surface layer and the lower press table applied a heat of 175°C on balancing layer.

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A panel with a small pre tension backwards and a slightly convex surface was obtained.

It is contemplated that there are numerous modifications of the embodiments described herein, which are still within the scope of the invention as defined by 5 the appended claims.

For example, it is contemplated that the balancing layer may be provided with an orientation in some embodiments. The wood fibres based the balancing layer generally has a random fibre direction. However, in order to form a balancing layer having similar properties as the decorative surface layer, an orientation may be formed in the balancing layer. Such orientation may be provided with by means of scattering the powder in a specific pattern, by a pattern in the press plate, or by means of a rake forming a pattern.

The steps of the method claims do not necessarily have to be performed in the above described order. It is for example contemplated that the second layer may 15 be applied before the first layer, and that the core is turned after the second layer has been applied. Thereafter the first layer is applied, liquid is applied, the first powder based mix is dried and the core is turned with the dried first powder based mix such that the first powder based mix points downwards.