DIGITAL COATING AND PRINTING

Abstract

Building panels, especially floor panels and a method to produce such building panels that include a decorative surface and a transparent protective layer, which is applied by a digital coating. Also, a vision control system that may be used to adapt a digital print or a digital embossing to a specific panel surface.

Claims

1. (canceled)

2. A method of coating a building panel, comprising the steps of: applying at least one transparent layer on a decorative surface of wood of a building panel, each transparent layer being a UV curable coating layer; and curing the at least one transparent layer with UV light, thereby forming a transparent protective surface layer, wherein said at least one transparent layer comprises a digital print and a transparent digital coating.

3. The method according to claim 2, comprising applying the digital print on a transparent layer of the at least one transparent layer.

4. The method according to claim 2, comprising injecting the digital print into a transparent layer of the at least one transparent layer prior to the step of curing.

5. The method according to claim 2, wherein an upper transparent layer is applied by digital coating above the digital print.

6. The method according to claim 2, wherein a transparent layer of the at least one transparent layer is located under or above the digital print.

7. The method according to claim 2, wherein the transparent digital coating is applied by a digital print head.

8. The method according to claim 7, wherein the digital print head is configured to apply overlapping drops.

9. The method as claimed in claim 7, wherein the digital print head is configured to apply drops having a size of 60-200 picolitres.

10. The method according to claim 2, wherein the at least one transparent layer UV comprises a liquid polyurethane substance or water based UV curable polyurethane.

11. The method according to claim 2, wherein the decorative surface comprises a print.

12. The method according to claim 2, wherein the at least one transparent layer comprises color pigments.

13. The method according to claim 2, wherein an edge of the building panel comprises a bevel.

14. The method according to claim 2, wherein said decorative surface is visible through said transparent protective surface layer.

15. The method according to claim 2, wherein the at least one transparent layer comprises wear resistant particles and/or scratch resistant particles.

16. The method according to claim 2, the at least one transparent layer comprising a plurality of digital prints placed on top of each other with transparent layers between the digital prints, thereby providing a three-dimensional image.

17. The method according to claim 2, wherein the at least one transparent layer comprises a structured surface with cavities and protrusions.

18. The method as claimed in claim 17, wherein the structured surface is in register with the decorative surface.

19. The method as claimed in claim 2, wherein the building panel is a floor panel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0115] The invention will in the following be described in connection to embodiments and in greater detail with reference to the appended exemplary drawings, wherein,

[0116] FIGS. 1a-c illustrate known roll coating and digital printing method;

[0117] FIGS. 2a-b illustrate a digital coating method;

[0118] FIGS. 3a-c illustrate digital injection printing in transparent layers; and

[0119] FIGS. 4a-c illustrate digital printing and embossing.

DETAILED DESCRIPTION OF EMBODIMENTS

[0120] FIG. 2a shows a floor panel 1 comprising a core 3 of solid wood and a decorative surface 5 that is the upper part of the solid wood core. The floor panel 1 may comprise bevels 15a, 15b at edges of the floor panel 1. Several transparent layers of adhesion coat L1, base coat L2 and a top coat L3 are applied on the wood surface. The adhesion coat and the base coat are applied by rollers and UV cured. The topcoat is in this embodiment applied digitally.

[0121] The main principles of a digital coating method and equipment are shown in FIG. 2b. A digital coating unit 36 comprising a digital print head 30 used as a digital coating head. The digital print head 30 is preferably fixed. The digital print head 30 applies a transparent topcoat L3, preferably on a base coat L2. The digital application may be made without any contact between the panel and the digital print head 30, and the coating is applied as drops, which are fired from the digital print head 30 towards the decorative surface that may comprise an adhesive coat L1 under the base coat.

[0122] A UV curing oven 23 with ultra violet light is located preferably after the digital coating unit 36 in the feeding direction and may provide a practically instant curing within a few seconds of the coating, especially if a UV cured polyurethane coating with an appropriate photo polymerization initiator is used.

[0123] The digital print head 30, that preferably is a Piezo head, has preferably a capacity to fire drops with a drop size of about 50-200 picolitres or more. The drops may have a diameter of 30-100 microns or more and may create a drop spot on the surface that exceeds 100 microns. The drops are preferably positioned such that they overlap each other. The overlapping may be obtained by a combination of drop size and drop position. Several print heads located after each other in the feeding direction may also be used to create a continuous layer even when individual drops are applied in a raster pattern.

[0124] The UV curing coating is preferably a water based UV curable polyurethane substance with a viscosity that is adapted to the digital print head 30. Water-based polyurethane dispersions are preferred as coating used in the digital print head. They may be environmental friendly and technically superior to similar solvent-based coatings. They may be, for example, free of isocyanate and may have a zero or a very low volatile organic content. They have good properties related to hardness, stain and abrasion resistance, impact strength and temperature flexibility.

[0125] Polyurethane dispersions are fully reacted polyurethane/polyureas of small and discrete polymer particles and such particles may be produced with a size of about 0.01-5.0 microns and may therefore be handled in a digital Piezo print head or other similar heads. They may have 20-70% solid content and a wide range of layers with different hardness may be produced with a digital coating method. Polyurethane dispersions may be blended with for example acrylic emulsions in order to reduce costs in some applications. They may be also comprise small wear and scratch resistant particles, for example, aluminium oxide, that may be handled by the Piezo head. Such particles should be rather small since the nozzle opening in most digital print heads is about 10 microns. It is preferred that the wear resistant particles are smaller than 5 microns and it is even more preferable that the wear resistant particles are about 1 micron or smaller.

[0126] Wear resistant particles may preferably be applied by scattering the particles in dry powder form on, for example, the base coat that preferably is in a liquid state and not yet cured. A second base coat or a top coat may be applied on the wear resistant particle layer. The advantage is that large particles that may clog the nozzles are not applied by the digital print head. Separate scattering allows that particles with a size of 10-100 microns may be applied.

[0127] The coating is stored in liquid form in a coating container 31, which is connected to the digital print head 30 with a coat-feeding pipe 33. A digital control unit 34 connected to the print head and the conveyor with data cables 33 or wireless, controls the drop size and the speed of a conveyor 21 that displaces the panel 1 in relation to the digital print head 30.

[0128] Such a digital coating unit is much more cost efficient than a conventional digital printer since much larger drops may be fired and this gives an increased capacity and less problems with the channels in the head that may be sealed by larger particle in the ink when the printer works with high resolution and small drops. Each digital coating head formed as a digital print head may be designed to apply one layer only and there is no need to coordinate the drop application of such different print heads as in conventional multi-colour digital printing where drops aligned in a raster pattern side by side creates an digital image.

[0129] Special digital print heads may be design that allows applications of very large drops in the range of 200-400 picolitres and more and the nozzle opening may exceed 20 microns. Such digital print heads are generally not suitable to be used as conventional print heads aiming to create a high-resolution image. They may be designed to apply large overlapping drops in considerable quantities and in a cost efficient way. Each print head, that mainly is used as a digital coating head, may be designed to apply 10-20 gr/m2 or more in a single pass coating step.

[0130] The coating line may be very compact and the UV curing oven may be located close to the digital coating unit. The coating may be very precise and the non-impact method provides much better possibilities than roll coating to apply the UV curing coating on the edges, on bevels 15a, 15b formed at the edges, and on surfaces that are not completely flat such as for instance brushed or hand scraped wood surfaces or embossed laminated paper based or powder based floor. UV cured protective layers may be applied by a digital coating method on practically all type of floors in order to improve the surface properties or designs. Some parts of the surface may therefore, according to an embodiment of the invention, be formed with known methods to obtain the basic strengths, designs or structures and the final layers may be applied by digital coating. Floor with surfaces comprising paper, powder, cork, vinyl and even stone and tiles and similar may be digitally coated in order to improve the surface properties.

[0131] UV cured liquid coating offers the advantage that the coating is liquid until it is exposed to the UV light. This increases the productivity of the digital coating unit and many problems related to for example solvent inks may be avoided.

[0132] The digital coating equipment may of course have several print heads. The digital coating may be applied on individual panels or on a large sheet that after the coating is divided into several panels.

[0133] The above described principles may preferably be used to apply a base coat L2 and and/or the adhesion coat L1. It is possible to apply all transparent layers that are used to protect a wood surface or a printed image. Digital coating may be used to apply transparent layers with a film thickness that corresponds to 5-10 g/m2. Even thicker layers may be produced for example 10-20 g/m2 and a total thickness of about 100-120 g/m2 may be reached with 5-10 coating stations. The panel may also pass a coating station several times.

[0134] FIG. 3a shows that a digital print P1 may be applied on a transparent layer and such print is characterized in that a transparent layer L2 is located under the print and another transparent layer L3 may be located above the print P1. The panel surface, that preferably is the upper part of a wood material 5 may be visible and may together, with the print P1, form a part of the final surface dcor and the decorative surface 2. The floor panel 1 may be provided with bevels 15a, 15b at its edges. At least the upper transparent layer is preferably applied by digital coating. A print P2 may also be injected into the wet transparent coating prior to the UV curing.

[0135] Such methods where a print P1 is applied on a transparent layer or when a print P2 is injected into a transparent layer may be used to obtain improved design properties since several images may be placed on top of each other with transparent layers between the prints and this may provide three dimensional images similar to a stone surface with transparent or semi-transparent crystals. Such three dimensional images may be partly or completely formed by conventional roller application methods that may be combined with digital printing or coating.

[0136] FIG. 3b shows a first production step of a combined digital printing and digital coating line. A digital print P1, P2 is applied on a cured layer or on a wet transparent layer. The print may be cured by UV 23, if the print, for example, is injected into a wet polyurethane layer.

[0137] FIG. 3c shows a digital coating unit 36 that may apply a transparent base coat or top coat on the digital print. The coat is UV cured in a UV oven 23. A transparent or semi-transparent adhesion coat may be applied digitally under the print.

[0138] It is an advantage if the digital coating unit 36 is connected, wireless or with a data cable, to a digital printing unit 35 as shown in FIG. 3c such that a digital print and a digital coating may be controlled and adapted to specific designs that are based on a combination of print and coating. A vision system 39 may also be digitally connected and digital coating and/or printing may be adapted to specific surface structures and designs. A vision system may for example be used in line with the printing and coating steps to create a digital image of a surface of a panel that preferably comprises a wood based surface 2. The panel may be an individual floor panel that may be a solid wood panel or a panel comprising a core covered by a wood layer with at thickness of for example 0.3-3 mm. Defects of the surface such as extreme deviations in colour, cracks, knots, repetitive effects etc. may be detected and analysed by the vision system 39, preferably by analysing the digital image of the surface of the panel and comparing the image with images already stored in the memory of the vision system. The system may be intelligent and new images and adjustment instructions may improve the efficiency. Specific software may be made for different wood species The visible surface of the panel may be at least partly digitally printed based on an analysis of the digital image such that defects are covered or adjusted with a digital print or that some surface portions are printed in order to create a more attractive wood design, such as creating an improved wood grain structure. As an alternative or complement to digitally printing, a transparent coating may be applied based on an analysis of the digital image. For example, the gloss grade of the surface of the panel may be changed and an increased amount of a liquid substance may be applied to cover cracks or cavities detected in the wood surface. The surface may be exposed to a various types of lights when a digital camera takes a picture and this may improve the possibilities to detect specific defects.

[0139] The Vision Controlled Digital Printing method as described above may also be used in other floor than wood floors. Conventionally printed paper or foil surfaces comprise repetition effects from the printing cylinders. Such effects may be partly eliminated by the vision controlled digital printing method. The method is preferably combined with a production step where the printed substrate is connected to a core prior the final digital printing step. A major advantage is that it is not necessary to position the already printed substrate in a precise manner on a panel since the exact position and/or the specific design may be detected by the vision system 39 and the digital printer may apply the complementary design with great accuracy. If paper is used, it is may be advantageous if such paper is a raw paper without any resins. The necessary thermosetting resins may be injected from a base layer under the substrate and/or from a top layer applied on the printed surface. The top layer may be a powder layer comprising wear resistant particles and thermosetting resins or a conventional overlay and the resins may be cured by heat and pressure. Digital coating as described above may also be used as top layer.

[0140] The VCDP method may also be used to create a dcor in a WFF floor with a powder based surface. Powder comprising one or several colours may be scattered on a board and a basic design may be created. A vision system and a computer system may analyse the basic design and give necessary digital input to a digital printer that in a second step may adjust or improve the basic design. The advantage is that the basic design may provide the major part of the pigments and the amount of ink applied by the digital printer may be reduced considerably, preferably to a few g/m2, for example, 1-5 g/m2. In some applications ink content of 3 g/m2 or less may be sufficient. This method may also be used to create a surface design on a ceramic tile.

[0141] The VCDP method is very suitable to combine with a Binder And Powder printing method wherein a pattern or image may be formed digitally by an ink head that only applies a liquid binder on a surface without any pigments. The binder may be a water based substance comprising glycol that provides a suitable viscosity. The pigments are scattered randomly by a second device over the liquid pattern. The binder connects some pigments that form the same pattern as the binder while other non-bonded pigments are removed. This two-step process, where the pigments and a liquid binder are applied separately, may provide an image with the same quality as convectional digital printing technology and is a very cost efficient method to form a basic design that is adjusted or improved in a final digital printing step comprising pigment based ink, preferably water based inks. The powder may be a transparent substance, for example bleached wood fibres, and BAP may be used to create an embossed transparent surface.

[0142] FIG. 4a shows that the digital print P may be applied to, or injected in, the top coat L3 and/or the base coat L2. The decorative surface 5 may be a wood layer, for example solid wood, a wood sheet or a wood veneer that may be glued to a core 3, for example a lamella core, HDF or plywood. The digital print is used to improve the design of the decorative surface.

[0143] FIG. 4b shows that digital coating may be used to provide a Digitally Embossed Surface, hereafter referred to as DES. A first top layer with a discontinuous cross section L3a-L3d may be provided that for example imitates a wood grain structure. The layer is applied in the form of a transparent image having a vertical extension. The wet three-dimensional structure may be UV cured and preferably a second or a third coating with a discontinuous structured cross section may be applied on the first layer. The discontinuous layers may be used to build a vertically extending structured surface with similar or different gloss levels. Embossed structures with for example a depth of about 0.1 mm and more may be formed with any shapes.

[0144] This method may be used to form advanced structures in a very flexible way and the embossing may be precisely coordinated with a printed surface, preferably a digital print as shown in FIG. 4c. The print P is coordinated with the structure L3a-c and individual parts of a digital image may be provided in well-defined surface structures where for example a particular grain is printed in a part of the surface that forms small cavities 37 and other parts of the image may be formed in the surface that comprises small protrusions 38. Such a Digital Embossed In Register Surface, hereafter referred to as DERIS technology makes it possible to create individual panels that are unique and without any repetition effect related to design or structures. Since no embossed cylinders or matrix films are needed, as in conventional technology, a wide variety of practically unlimited structured designs may be produced in a very flexible and cost efficient way.

[0145] DERIS is preferably used together with a digitally printed image. Such image may be printed on a paper, foil, on a board material or injected into powder-based surfaces as described above. The digital printer 35 and the coating unit 36 that forms the top layers L3 are preferably digitally connected to a computer that coordinate the surface design and the surface structure.

[0146] DERIS technology may even be used to coordinate an individual structured surface coating with individual wood panels that have different designs or with any other panels where a surface design or structure is already formed. This is an advantage especially in application where digital printing is used to improve the design of a wood surface. Wood species of lower quality may be improved by digital printing as described above. For example a digital picture of a wood surface of a panel may be made by the vision control system comprising a digital camera or scanner. This is preferably made in line prior to the application of the transparent coating that provides the final digital embossed surface. The vision control system may be used together with a computer program that may analyse the digital picture and may adjust the digital coating such that it is coordinated with the wood grain structure and design of each individual panel.

[0147] Such Vision Controlled Digital Embossing methods are not used in flooring applications. VCDP and VCDE may be combined and very cost efficient advanced decors and structures may be formed.

[0148] VCDE is especially suitable to form embossed structures on panels where parts of the dcor are not formed digitally in line in a prior printing step. Even when this is the case, production efficiency and quality may be improved since there is no need to use high quality transportation units that moves the panel with high accuracy between different printing, coating and application steps that all must be coordinate in order to give a high final surface quality. The vision system that preferably also may be combined with a position system between different production steps may be very cost efficient compared to known methods.

[0149] All described embodiments may be combined and all transparent layers may comprise colour pigments.

[0150] The digital coating method may also be used to apply other chemicals on a panel, for example glue, preferably a water based glue.

[0151] The method to apply a UV cured polyurethane digitally in order to obtain 3D structures may also be used for other applications and not only building panels as described above. A digital printer and an UV oven may be used to apply and cure many layers of various types of UV cured polymers vertically in order to create complicated structures and models with a vertical extension of 1-10 mm and even more. A first layer may be applied on a substrate that is moved in an UV oven and back again thereafter a new layer is applied. Two digital Piezo heads may for example also be positioned with a UV oven in between and the substrate may move from a first print head through the UV oven and into the second print head and back again. This sequence may be repeated many times and each layer may be 0.1-0.3 mm thick or even more depending on the viscosity of the liquid polymer that for example may be polyurethane. In general no additional substances are needed to cure the polyurethane and the two printer heads may use the same type of UV cured liquid polyurethane. The two printers may also use different substances, which may be used to decrease the curing time. This production method may be combined with a visions system and partial 3D structures may be formed on specific well-defined portions of various objects that may comprise various materials. 3D structures may have different colours injected digitally into the UV cured polyurethane.

[0152] Such digital 3D methods may be used to provide a matrix that may be used as a sheet that provides an embossed structure when laminate and powder based floors are pressed in a press in order to cure the surface.

[0153] 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 the appended claims.

[0154] It is for example contemplated that curing of the curable coating layer with UV may be replaced by another radiation curing method. It is also contemplated that the present invention may be used for other types of radiation curing coating layers.

[0155] As apparent from the description, the coating layer may be applied with the digital print head directly on the surface of the building panel forming the decorative layer, and may be applied on an intermediate layer arranged on the surface of the building panel.

EXAMPLES

Example 1

[0156] A digital image was applied on a panel comprising a HDF board. The image was created with a single pass printer comprising 5 fixed Piezo print heads. The ink was a water-based ink comprising colour pigments.

[0157] A Piezo print head with a drop size of 50 picolitres was used to apply a transparent layer of water based UV cured polyurethane that corresponds to a film of 10 g/m2.

[0158] The above material was cured in a UV oven and a digital image with a digitally coated transparent layer was obtained.

Example 2

[0159] A digital image with a wood design was applied on a panel comprising a HDF board. The image was created with a single pass printer comprising 5 fixed Piezo print heads. The ink used was a water-based ink comprising colour pigments.

[0160] A Piezo print head with a drop size of 50 picolitres was used to apply a transparent layer of water based UV cured polyurethane that corresponds to a film of 10 g/m2.

[0161] The above material was cured in an UV oven.

[0162] A second similar transparent layer with a weight of 5 g/m2 was applied with a Piezo print head with a drop size of 50 picolitres. The layer was applied as a transparent image coordinated with the digital image.

[0163] The above material was cured in an UV oven and a wood grain design with a wood grain structure in register with the wood grain design was obtained.

[0164] Digital coating may also be used to seal the edges and/or the locking system 9, 10 against moisture, to eliminate squeaking sound, or to change friction properties of active surfaces in the locking system. The major advantage is that a liquid substance may be applied with high precision and unwanted over spraying on for example the surface may be avoided. A print head is generally used to apply a liquid substance vertical with a distance to the surface of a few mm. Print heads may be used to apply liquid substances from a distance of up to 10 mm and more and the application may be made in various angles against the surface for example 0-10, 10-20, 20-45 degrees or even more then 45 degrees from above or from below. This allows that locking systems with advanced geometries may be coated with combination of several print heads positioned in several angles. The liquid substance may be liquid wax that after cooling to room temperatures becomes a soft layer.

EMBODIMENTS

[0165] 1. A method of coating a building panel having a decorative surface of wood, comprising the steps of: [0166] applying a UV curable coating layer with a digital print head on a decorative surface of wood of a building panel; and [0167] curing the UV curable coating layer with UV light, thereby forming a transparent protective surface layer, wherein said decorative surface is visible through said transparent protective surface layer.

[0168] 2. The method as in Embodiment 1, wherein the building panel is a floor panel.

[0169] 3. The method as in Embodiment 1 or 2, wherein the building panel is an individual panel having a size, which is essentially the same as the final building panel comprising machined edges.

[0170] 4. The method as in any one of the preceding Embodiments, wherein the building panel comprises a mechanical locking system at two opposite edges.

[0171] 5. The method as in any one of the preceding Embodiments, wherein the building panel comprises a bevel at an edge.

[0172] 6. The method as in any one of the preceding Embodiments, wherein the UV curable coating layer is a liquid polyurethane substance.

[0173] 7. The method as in any one of the preceding Embodiments, wherein the UV curable coating layer is water based UV curable polyurethane.

[0174] 8. The method as in Embodiment 1, wherein the decorative surface comprises a print.

[0175] 9. The method as in any one of the preceding Embodiments, wherein the digital print head is a Piezo print head.

[0176] 10. The method as in any one of the preceding Embodiments, wherein the digital print head is designed to apply drops, preferably with a size of about 60-200 picolitres.

[0177] 11. The method as in any one of the preceding Embodiments, wherein the UV curable coating layer comprises wear and/or scratch resistant particles.

[0178] 12. The method as in any one of the preceding Embodiments, wherein the UV curable coating layer comprises a structured surface with cavities and protrusions.

[0179] 13. The method as in Embodiment 12, wherein the structured surface is in register with the decorative surface.

[0180] 14. A floor panel having a core, a surface layer comprising a wood material surface, a print and transparent layers, [0181] wherein a lower transparent layer is located below the print, and an upper transparent layer is located above the print, wherein the lower transparent layer comprises a UV curable polyurethane, and [0182] wherein a part of the wood material surface and the print form a part of a visible surface and the print is at least partly synchronized with the visible design and/or structure of an individual floor panel.

[0183] 15. A floor panel as in Embodiment 14, wherein the upper transparent layer comprises water based polyurethane.

[0184] 16. A floor panel as in Embodiments 14 or 15, wherein the upper transparent layer is embossed.

[0185] 17. A floor panel as in any one of the Embodiments 14-16, wherein the upper transparent layer is embossed in register with the print.

[0186] 18. A method of forming a decor on a building panel with a digital vision control system that provides digital input to a digital print head, comprising the steps of: [0187] creating a digital image of a surface of a building panel by the digital vision control system; [0188] using the digital vision control system to provide digital input to the digital print head based on said digital image; [0189] digitally printing at least a part of said surface of the building panel with the digital print head and with a print that is at least partly adapted to the digital image of said surface of the building panel.

[0190] 19. The method as in Embodiment 18, wherein the building panel is a floor panel.

[0191] 20. The method as in Embodiments 18 or 19, wherein the print comprise colour pigments.

[0192] 21. The method as in any one of the Embodiments 18-20, wherein the surface of the building panel comprises a transparent substance that is UV cured and that after curing forms an embossed structure.