EP2313281B1 - Method for producing a printed surface on a flat workpiece - Google Patents

Abstract

The invention relates to a method for producing a decoratively printed surface on a flat workpiece, comprising the following steps: pretreating a flat workpiece substantially made of wood or wood components by applying at least one liquid base coating (12, 22); partially or completely drying the at least one applied liquid base coating (12, 22); digitally inkjet-printing in passing on the pretreated flat workpiece, wherein inkjet print heads dispense droplets having a variable droplet size during the inkjet printing, wherein the smallest droplet size is < 20 pl and the small droplets differ from the larger droplets by a factor of at least two.

Description

For some time now, there have been various methods for decorative coating of flat workpieces. In this case, a decorative coating, e.g. be a print of a replicated wood structure or a stone tile motif, the flat workpieces are e.g. Wood-based panels, e.g. MDF (medium density fiberboard), HDF (high density fiberboard) or chipboard panels, which must be decorative coated in their surface for use as furniture parts, kitchen fronts or flooring (e.g., laminate flooring).

Also, various attempts have been made in the past to find a cheap wood with little visible structure, e.g. Pine wood, to change its appearance by color printing so that it costs a more expensive and e.g. stronger grained wood looks similar. This applies both to printing on solid wood panels and to printing on veneers or veneered panels, e.g. veneered wood-based panels.

As a flat workpiece in the context of the present invention, both a plate-shaped material, e.g. a chipboard, other fibreboard or cement fiber board or the like understood as well as a present in the form of roll or web workpiece, e.g. a veneer roll. A flat workpiece is thus understood as a workpiece to be printed essentially two-dimensionally.

The prior art discloses two problem areas: The decorative printing is carried out either by so-called indirect gravure printing machines, in which the ink is transferred from an engraved gravure roll over a rubber roller interposed to compensate for tolerances in the workpiece and then remains on the workpiece surface. This has the disadvantage of lack of flexibility, since a change of the decor (decorative image) inevitably leads to a change of the gravure roll and / or the ink.

Alternatively, the digital inkjet technology is already used, but in the direct printing on the workpiece surface with relatively large Drops (usually> 40 pl droplet volume) mean that fine structures can not be reproduced. In addition, the color of large-volume droplets in the case of open-pored wood is absorbed by them and runs in the direction of the wood grain. Such a digital printing device is in the US 2002/0061389 disclosed.

As a result, the selection of woodgrains to be printed or Holzanmutungen on the veneer or wood surface is severely limited.

Even with digital printing on pretreated white-primed surfaces, the large droplet volume in ink-jet printing presents itself as a problem in the present state of the art. In order to obtain a good resolution and thus a natural-looking printed image, the surface must be offset several times by a half droplet size or less be printed. This is done according to the prior art with a moving printhead, which is repeatedly reciprocated on the standing medium to be printed and thereby prints images together. Alternatively, with a simple pressure over the area with large droplet volume, the poor quality is sometimes deliberately accepted.

task

Against this background, the method according to the invention is intended to solve the problem of enabling an industrial production method of decorative printed, flat workpieces, in particular HolzwerkstofFplatten, solid wood panels or veneer, in which a very lifelike appearance is made possible by a high resolution of the printed image at the same time industrial production capacities. At the same time a capacity of> 500 m ² / h, more preferably> 1000 m ² / h should be achieved at the same time instead of the usual today with ink jet technology with larger droplet volume low capacity of 40-80 m ² / h.

In addition, in the case of printing a veneer, which is often supplied industrially as roll goods, industrial processing without the risk of cracking in the veneer should be possible.

This problem is solved with the method according to the invention according to claim 1. In the method according to the invention for producing a decorative printed surface on a flat workpiece, a flat workpiece consisting essentially of wood or wood components is first coated by application of at least one liquid base layer, which is then partially or completely dried. Next, digital ink jet printing is performed on the pretreated sheet workpiece, with inkjet printheads emitting droplets of variable droplet size in inkjet printing, and the smallest droplet size is <20 pl and the small droplets differ from the larger droplets by at least a factor of two.

This results in an effective coating of the workpiece with high capacity and high quality. The printing of the workpiece on the base layer with different sized droplets allows a very high resolution of an image, so that visually appealing products are obtained. In addition, if required, a full-surface coating can be provided by large droplets. This increases the potential use of the printing device. Furthermore, the droplet size can also be selected depending on the selected base layer, so that larger droplets are used in an absorbent substrate than in a non-absorbent substrate.

The veneer to be printed is available as a roll product with typical lengths of 50 to 100 m. The plate product is usually available with dimensions of 80-3,000 mm wide and 100-6,000 mm long, more preferably 100-1,400 mm wide and 300-3,000 mm long. The usually coated plate thicknesses are 1.5 to 50 mm, more preferably 3-20 mm thickness.

The following are the appended FIGS. 1-7 described. Thereafter, the inventive method is shown in various embodiments.

DESCRIPTION OF THE FIGURES 1-7

FIG. 1 shows a cross section through a decorative coated flat workpiece. It shows a wood-based panel, such as chipboard or MDF board 10, which is coated with a veneer 11, ie the veneer is glued to the wood-based panel. Then comes one or more liquid applied basecoats, which act as dye-receiving layers 12, which are then printed by the method according to the invention with a layer of ink 13. Subsequently, a wear protection layer 14 can be applied. In an alternative embodiment, this wear protection layer can also be omitted.

In an alternative embodiment, the wood material board or other type of board, e.g. a cement fiber board or other fiberboard glued no veneer layer 11, but the one or more color receiving layers 12 are applied directly to the plate 10.

FIG. 2 : Here is a cross section through a veneer, which is unrolled from the roll and coated shown. It shows the veneer of the roll 21, the one or more color receiving layers 22, on which the digital printing ink 23 applied and a possible anti-wear layer 24 applied in an alternative method according to the invention.

FIG. 3 : In this FIG. 3 the method according to the invention with the method steps is shown. It shows a development for a roll-shaped or web-shaped workpiece 31. Thereafter follows a first coating station for the application of a liquid coating 32 as a base layer, and a drying station 33, for example, for a UV drying of UV-curing acrylates. The stations 32 and 33 can also be used several times in series for a layer structure with several layers. The station 34 shows a digital printer for the passage of the web-shaped or sheet-shaped workpiece. The station 34 comprises four printing bars 35 each printing one color, for example cyan, magenta, yellow and black. Alternatively, a smaller or a larger number of pressure bars 35 can be used. Behind each pressure bar 35, a predrying station 36 is alternatively arranged, which may also be omitted in an alternative embodiment.

Finally, a complete drying / curing in a station 37 and the subsequent rewinding of the web-shaped material 38 takes place.

FIG. 4 : In an alternative embodiment, for printing on roll or sheet material, the method is divided into two steps. In a first step, the material is unwound from the roll in a station 41. Thereafter, it is coated in the station 42 and dried in the station 43. Again, the stations 42 and 43 may be connected several times in a row. The then rewound material is unwound again in a second process step and printed in the station 44. Here again four pressure bars 45 are provided. The possible intermediate drying behind the individual pressure bars as in FIG. 3 is not indicated here, but may also be present. 47 denotes a drying station and 48 a subsequent winding station of the web-shaped or roll-shaped material.

FIG. 5 illustrates an example printed flat workpiece 51, which has been printed in this case with a wood effect. The workpiece 51 comprises some long wood pores or wood grains 52. An indicated cutout 54 contains a knothole 53.

This cut-out 54 is also shown enlarged at 55. Here it can be seen how the knothole printed from different droplets 56 and 57 from the ink-jet printer is shown. By way of example, two droplets 56 for a large droplet volume of e.g. more than 40 pl as well as a small droplet 57 for e.g. less than 20 pl shown. The droplet size of the droplets 56 and 57 may be different by a factor of 2 to 10 for the design of the printed image. It can be seen that with large and small droplets a correspondingly good visual resolution and thus lifelike reproduction of the knothole can be achieved without the need to repeatedly print across the image. In addition, print images can be produced with high resolution and print quality.

FIG. 6 shows an additional coating station for a so-called screened coating, which in the method according to the invention in the sequence which in FIG. 3 is shown, can be inserted. For example, after passing through the web material through the stations 32 and 33 of the FIG. 3 another station, as in FIG. 6 and then a new drying, such as 33.

This station applies a screened coating to the surface of the flat workpiece 64. This means that the flat workpiece 64 is not uniformly acted upon by a liquid coating, but rather a recurring structured coating takes place via an anilox roller, similar to the gravure printing process. This structure may, for example, take place such that on the surface of the flat workpiece 64 after the coating individual small "hills" and "valleys" are formed, which have a uniform distance from each other and are microscopically fine. In FIG. 6 Such an anilox roller 61 is shown as known from the intaglio printing process, as well as an original 62 with a liquid coating medium, for example lacquer, and a preferably rubberized transfer roller 62, the rastered by the anilox roller 61 in small droplets and placed on the rubberized transfer roller 62 liquid medium on the to be coated flat workpiece 64 transmits.

As a result, a raster-shaped application of individual spaced-apart coating dots, as shown in a cutout 66, results on the surface of the workpiece 64.

FIG. 7 shows a device for coating a plate-shaped flat workpiece. A coating station 71 with a liquid medium is arranged in front of a drying station 72 and a digital printing station 73 with individual pressure bars 74. Furthermore, between the individual pressure bars 74 and intermediate drying device may be provided. Finally, the ink is dried in a station 75. In the then following station 76 a possible in an alternative embodiment wear protection layer can be applied as a liquid medium, which is then dried in a station 77.

In the method according to the invention, the flat workpiece is first coated as a roll product or as a sheet material with a liquid dye-receiving layer in order to ensure the best possible subsequent printing.

In alternative embodiments, a station as in FIG FIG. 6 shown interposed, for example, behind the drying station 72 to produce a corresponding screened surface on the dye receiving layer for a better printing result before printing.

A very important improvement of the prior art by the method according to the invention is achieved by the application of the base layer, which has a surface that thereby significantly smaller droplet sizes (in particular <20 pl) can be used. Furthermore, the bleeding of the individual droplets and the different colors into one another is prevented in the ink printing process. This is a prerequisite for reproducing very fine structures and thus to draw a higher-quality and true-looking image on the veneer or on the white / monochrome surface of the workpiece (eg chipboard) than in the prior art, with high capacity at the same time the one-time printing in the pass.

In the following, an exemplary embodiment of the method of the present invention will once again be explained comprehensively. First, the sheet workpiece, in this example a wood-based panel, e.g. a chipboard, provided with a liquid coating. This liquid coating may already be the surface to be printed (dye-receiving layer). In an alternative embodiment, it is also possible to choose several liquid coatings with complete or partial intermediate drying, e.g. around existing pores or depressions in the workpiece, e.g. the chipboard, close. The uppermost and last applied liquid layer is in this case the dye-receiving layer at the same time.

In an alternative embodiment, the sheet workpiece may also have a colored, e.g. white or otherwise monochrome melamine coated wood-based panel, e.g. Chipboard, be. In this case, the wood-based panel (chipboard, MDF board or the like) on one or both sides with a plain decorative paper, which has been impregnated with a melamine resin, in a hot press (eg short-cycle press or continuous double belt press) has been coated so that the Melamine resin cured and polymerized.

This so prepared wood-based panel is then provided with a liquid coating. In this case, the liquid coating comprises a primer or consists of a primer that mediates adhesion between the digital ink and the underlying cured melamine layer. Alternatively, the cured melamine layer can also be ground to be provided with said liquid coating. Third Possible alternative of this embodiment may also be sufficient only a grinding of the melamine layer and to dispense completely with the liquid base layer in the form of a primer or adhesion promoter.

In an alternative embodiment, as the last layer on the existing dye-receiving layer, a fine grid of a same, alternatively be applied from a different liquid medium as in FIG. 6 shown. This microscopic grid may have a spacing of less than 0.01 mm between each grid dot. In an alternative embodiment, the spacing of the halftone dots is between 0.01 mm and 0.25 mm. The advantage of this screened surface as a dye-receiving layer is that the propagation of both the smaller and the larger inkjet droplets, ie less than 20 pl but also greater than 40 pl, in particular 6pl to 40pl, is controlled. It gives a clearer picture and less bleeding of the droplets.

Subsequently, the sheet-like workpiece as a roll or web or - as in our example - the chipboard as a sheet the printing station 34. The printing station consists of pressure bar for each color, so for example in a four-color printing of four pressure bar. Each individual pressure bar consists of a large number of print heads, which are lined up in such a way that they can deliver the corresponding print image across the entire width of the workpiece. In an alternative embodiment, an intermediate drying 36 can still be arranged between the individual pressure bars 35. For example, if the ink used is a UV curable ink, this intermediate drying 36 may be formed of UV emitters.

The printing on the flat workpiece takes place in such a way that the print heads are arranged vertically and the flat workpiece is transported under the print heads. In this case, the printed image is applied in one pass over the entire width of the flat workpiece. At no point of the workpiece is therefore printed over each other several times. The print heads in the print bars (35) are so pronounced that they can form different droplet sizes, especially small droplet sizes of less than 20 pl per droplet. The number of different droplet sizes ("grayscale") is up to 15 different shades of gray, in a particularly preferred form at 3-8 different shades of gray. That way in a run printed picture, which is in FIG. 5 is shown again is visually appealing and represents by the different droplet sizes a high visual resolution and thus a very realistic representation of the wood decor or stone decor or other decorative surface.

Subsequently, a wear protection layer can be applied to the so decorative printed surface, which is also applied in a liquid state and then dried. In an alternative embodiment, these liquid anti-wear layer can still be added admixtures that make the surface properties positive. These admixtures may be, for example, corundum (AlO ₂ ) to increase the abrasion resistance or chemical nanoparticles that improve the scratch resistance of a liquid applied paint. In an exemplary embodiment, the liquid-applied medium prior to printing (dye-receiving layer) may be a UV-curing acrylate, which is printed using inkjet-based UV printing inks which are also acrylate-based. Of course, other liquid media and correspondingly adhering digital inks can be used for the printing press used.

Claims (15)

1. Method for producing a decoratively printed surface on a flat workpiece, comprising the following steps:

   - pre-treating a flat workpiece, substantially composed of wood or wood components, by applying at least one liquid base layer (12, 22);

   - partly or completely drying the at least one liquid base layer (12, 22) applied;

   - digital inkjet printing onto the pre-treated flat workpiece in a continuous pass, during the inkjet printing inkjet print heads discharging droplets with a variable droplet size and the workpiece being printed with droplets of different size, the smallest droplet size being < 20 pl and the small droplets differing from the larger droplets by at least the factor two.
2. Method according to Claim 1, characterized in that the at least one liquid base layer (11, 12) is applied as a colour receiving layer in the form of a UV-curing acrylate varnish, and the following drying is carried out by UV-radiation curing.
3. Method according to Claim 1 or 2, characterized in that the flat workpiece is a wooden board already previously coated with a cured melamine film, and the base layer (12, 22) applied as a liquid is an adhesion promoter between the melamine and the following printing ink for the inkjet printing.
4. Method according to one of the aforementioned claims, characterized in that the flat workpiece consists of a wooden board coated with an impregnated melamine film and cured in a hot press and which, before the following liquid coating, is sanded in a sanding machine, the last sanding being carried out with a grain size of 180 or finer, particularly preferably with a grain size of 220 or finer.
5. Method according to Claim 4, characterized in that, following the sanding of the melamine layer, the digital inkjet printing is carried out directly on the sanded melamine surface.
6. Method according to one of the aforementioned claims, characterized in that, following the application of one or more liquid base layers (12, 22), a screened application is carried out with a liquid medium with a spacing of the screen dots between 0.001 and 0.25 mm, in particular between 0.01 and 0.1 mm.
7. Method according to one of the aforementioned claims, characterized in that, after the digital printing, a further coating is carried out with one or more wear-resistant layers.
8. Method according to one of the aforementioned claims, characterized in that the workpiece coated is a veneer, and, following the drying of the colour receiving layer, the veneer remains flexible in such a way that it can be used as an edge material that can be laminated on for furniture and floors.
9. Method according to one of the aforementioned claims, characterized in that, during the printing, the volume of the individual digitally printed droplets lies in a range from at least 15 pl to 30 pl, in particular at least 10 pl to 40 pl.
10. Method according to one of the aforementioned claims, characterized in that the volume of the smallest droplets used during the printing is less than 20 pl.
11. Method according to one of the aforementioned claims, characterized in that the printing is carried out with a plurality of colours, preferably with four colours.
12. Method according to one of the aforementioned claims, characterized in that the printing is carried out with UV-curing printing ink.
13. Method according to one of the aforementioned claims, characterized in that the printing ink is cured following the printing of all the colours used.
14. Method according to one of the aforementioned claims, characterized in that, following the printing of at least one of the colours used, intermediate drying is carried out and printing with a further colour is then carried out.
15. Method according to one of the aforementioned claims, characterized in that the printed droplets are printed at a distance from one another and, as a result, do not flow into one another but remain microscopically visible as droplets as a result of rapid drying.

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