Method for the online quality control of decorative prints on substrate materials

Abstract

The invention relates to a method for online quality control of decorative prints on substrate materials, including similarity comparisons of actual and target images and adjusting decorative prints if deviations of color values are detected. The method may include the steps of: a) producing a hyperspectral digital image of a print decoration; b) calibrating the print decoration via a hyperspectral digital image; c) producing and storing a digital target image of the print decoration; d) creating a first print decoration on a first substrate material; e) producing and storing a digital actual image of the printed decoration on the first substrate material; f) determining color deviations between the digital target image and the digital actual image via a computer program; and g) printing on at least one side of substrate materials so as to form a decorative layer. The invention also relates to a device for carrying out the method.

Claims

1. A method for online quality control of decorative prints on substrates, comprising a similarity comparison of an actual image and a target image of print decors and an adjustment of the decorative print when deviations of color values of the actual image from color values of the target image are detected during production of a batch of substrates with a decorative layer, wherein the method comprises the steps of: a) generating a plurality of hyperspectral digital images of a print décor formed on a like plurality of substrates; b) calibrating the print decor by means of the plurality of hyperspectral digital images by generating from the plurality of generated hyperspectral digital images an average hyperspectral digital image of the print decor; c) generating and storing a digital target image, in the form of a digital photograph, of the print decor by converting the average hyperspectral digital image of the print decor having a one resolution into an image having another, reduced resolution in the range of 4 to 36 megapixels; d) printing at least one first print decor on at least one first substrate; e) generating and storing at least one digital actual image, in the form of a digital photograph, of the printed decor on the at least one first substrate with a resolution in the range from 4 to 36 megapixels, by means of a color scanner or a digital camera; f) determining of color deviations for one or more color values of the L*a*b* color space and/or the L*C*h° color space between the digital target image and the at least one digital actual image by a computer program; and g) printing at least one side of further substrates while forming a decorative layer in such a way that color deviations between the digital target image and the at least one digital actual image of the printed decorations on the further substrates occur only below a predetermined nominal value or only within a predetermined tolerance range.

2. The method according to claim 1, wherein the generation of each hyperspectral digital image of the print decor is carried out by means of a hyperspectral scanner.

3. The method according to claim 1, wherein the digital target image is generated in method step c) by converting a calibrated hyperspectral image into the image with the reduced resolution in the range from 4 to 36 megapixels by means of a computer program or by printing the print decor on a substrate and subsequently scanning it by means of a color scanner or photographing the printed decor by means of a digital camera.

4. The method according to claim 1, wherein the generation of each digital image according to method steps c) and e) is carried out under equivalent conditions.

5. The method according to claim 1, wherein during the production of a batch of substrates with the formation of a decorative layer, the generation of the at least one digital actual image is carried out continuously and that the method steps f) and g) are repeated continuously.

6. The method according to claim 1, wherein the printing of the substrates according to method steps d) and/or g) is carried out by means of gravure printing or digital direct printing.

7. The method according to claim 1, wherein, if color deviations are detected in method step f), a warning signal is issued to operators of a printing line for substrates, or an automatic adjustment of one or more color values of the L*a*b* color space and/or the L*C*h° color space is carried out during the production of the print decor on the substrate in such a way that color deviations between the digital target image and the at least one digital actual image of the printed decor on further substrates occur only below a specified nominal value or only within a specified tolerance range.

8. The method according to claim 1, wherein the substrates are selected from a group comprising paper, glass, metal, foils, wood materials, in particular MDF or HDF boards, WPC boards, veneers, paint layers, plastic boards and inorganic carrier boards.

9. The method according to claim 1, wherein a print decor produced by means of digital printing or a print decor produced by means of gravure printing using printing rollers is printed on uniformly basecoated substrates.

10. The method according to claim 1, wherein a protective layer is applied to the print decor or the print decors.

11. The method according to claim 1, wherein at least one of the printed substrates, which may be provided with a protective layer, is fed to a short-cycle (SC) press for further processing.

12. A device, configured to carry out a process for online quality control of print decors on substrates according to claim 1, the device comprising: at least one means for generating the plurality of hyperspectral digital images of a print decor; at least one means for calibrating the print decor based on the average hyperspectral digital image generated from the plurality of hyperspectral digital images of the print decor; at least one means for generating and storing the digital target image of the print decor with a reduced resolution in the range of 4 to 36 megapixels; at least one means for printing the at least one first print decor on the at least one first substrate; at least one color scanner or a digital camera for generating and storing the at least one digital actual image of the printed decor on the at least one first substrate with a resolution in the range of 4 to 36 megapixels; at least one means for determining color deviations between the digital target image and the digital actual image in the L*a*b or L*C*h° color space; and at least one means for adapting/controlling the decor printing when printing the at least one side of further substrates while forming the decorative layer in such a way that the color deviations in the L*a*b or L*C*h° color space between the digital target image and the digital actual image of the printed decors on the further substrates occur only below the predetermined nominal value.

13. The device according to claim 12, wherein the means for generating the plurality of hyperspectral digital images of the print decor is a hyperspectral scanner.

14. The device according to claim 12, further comprising at least one means for applying a protective layer to each substrate provided with the respective print decor.

15. The method of claim 1, wherein the image of method step c) having the reduced resolution in the range of 4 to 36 megapixels is an image in an L*a*b or L*C*h° color space.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in greater detail below on the basis of an exemplary embodiment with reference to the figure, in which:

(2) FIG. 1: shows the prior art of the quality control of printed decorations on the basis of hyperspectral scans; and

(3) FIG. 2: shows the online online quality control of decorative prints on substrate materials according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) In the prior art, as illustrated in FIG. 1, the calibration of a print decoration (D1), in which images of a plurality of substrate materials which have been printed and thus produced (P1, P2, P3) with the same print decoration (D1), comprises the generation of hyperspectral digital images by means of ACMS® (Advanced Colour Measurement System) (ACMS P1 D1; ACMS P2 D1, ACMS P3 D1). The generation of hyperspectral digital images by means of ACMS (ACMS P1 D1; ACMS P2 D1, ACMS P3 D1) can be effected for example by a hyperspectral true-color scanner having the following properties: Xenon light source Conforming to ASTM/CIE/ISO color measurement standards Geometry: 45°/0° Measurement area: 80×220 mm Spatial resolution: 125 μm Spectral range: 380-780 nm Spectral resolution: 5 nm

(5) On the basis of the hyperspectral digital image obtained it is possible to carry out a contrast assessment and a quality evaluation of multicolored surfaces. On the basis of a similarity comparison of the hyperspectral images (ACMS P1 D1; ACMS P2 D1, ACMS P3 D1), similarity factors are calculated for one or more color values of the decoration (D1). Said similarity factors can be exchanged as a standardized protocol between a customer (producer of the substrate material) and a supplier (producer of the decoration). However, creating a calibrated decoration by means of hyperspectral digital images according to ACMS (D1/ACMS D1) is expensive and time-consuming and unsuitable for the online quality control of production batches (Pn) of substrate materials of the decoration (Pn D1). According to this method, moreover, hyperspectral digital images of printed substrate materials in ongoing production (Pn D1/ACMS) would have to be continuously generated and processed by means of ACMS (Pn D1/ACMS Pn D1), in order to enable a comparison with the calibrated decoration on the basis of hyperspectral digital images (D1/ACMS D1).

(6) The method according to the invention, as illustrated in FIG. 2, is a development of the method according to FIG. 1. A plurality of decorative prints and the respective generation of a hyperspectral image by means of ACMS are effected in the calibration. Provided that the values obtained here correspond to the similarity stipulations, this decoration is released for further processing. The creation of the calibrated decoration on the basis of hyperspectral digital images (D1/ACMS D1) is followed by the generation of a digital target image (F D1) with lower resolution in the range of 4 to 36 megapixels. For the online quality control method according to the invention, this digital target image is stored in a database and used for a rapid comparison in any further production of this decoration. In the present case, the digital target image (F-D1) was generated as a digital photograph. Digital actual images (F Pn D1) are generated from the production batches (Pn D1) at continuous time intervals. The digital actual images are recorded in each case under approximately identical conditions, in a manner largely shielded from variable external light influences, such as, for example, by means of a tube which is open only toward the object and in which the recording device and an illumination system that may be needed are positioned. The digital actual images (F Pn D1) are continuously subjected to a similarity comparison vis-à-vis the digital target image (F-D1). In this case, for example, one or more color values of the L*a*b* color space and/or of the L*C*h° color space are determined and compared with one another. In the event of one or more color values of the digital actual images exceeding the predefined target value, which is 20%, for example, for the color values of the L*a*b* color space, it is possible to intervene in production and to influence the production process. This can be done manually. For this purpose, if corresponding deviations of color values occur, this is indicated to the operator of a production installation by means of a signal. This signal can be, for example, an acoustic signal or a visual signal in the form of signal luminaires or colored displays on the monitor of a process computer. A visual signal in the color green indicates for example that no color deviations have been measured between the digital target image and the digital actual image. A visual signal in the color yellow indicates color deviations. However, the latter vary within a predefined tolerance range or below a predefined target value and there is no need for any intervention in the position process. A visual signal in the color red indicates color deviations which vary outside a predefined tolerance range or above a predefined target value. Here it is necessary for the operator of the production installation to intervene in the production process. By way of example, said operator can stop ongoing production or correct the printing for individual or a plurality of color channels such that color deviations occur only within the predefined tolerance range or below a predefined target value. Alternatively, if color deviations have been ascertained, an automatic adaptation of the decorative print can be effected by means of a process computer and corresponding control software, such that color deviations no longer occur or they are kept small enough that the color deviations occur only within a permissible tolerance range or below a predefined target value.

Exemplary Embodiment 1—Analog Decoration Direct Printing

(7) An HDF board (8 mm) coated with a basecoat containing titanium dioxide was decorated by means of indirect intaglio printing using a plurality of rollers. The decoration chosen was a wood decoration. A 3-ink system was employed here, with 3 roller application mechanisms, which successively apply different inks. The ink application rollers were correspondingly engraved, such that ink was taken up into the engraving, transferred to a rubber roller and then printed onto the substrate. After the production of the printed decoration, a digital photograph (actual image) was generated with a resolution of 12 megapixels by means of a digital camera. The color values of the L*a*b* color space of this actual image were compared with the color values of the L*a*b* color space of a digital target image. The digital target image had been produced by way of identical decoration calibrated beforehand by means of hyperspectral digital images (ACMS) and the generation of a digital photograph thereof with a resolution of 12 megapixels. This process was repeated for every tenth HDF board in each production batch. The digital actual images were recorded in each case under identical conditions by way of a tube which was open only toward the object and in which the digital camera and an illumination system were positioned. As a target value for allowed color deviations, 20% was defined for the individual values of the L*a*b* color space. Only deviations that were below the predefined target value were ascertained during the similarity comparison of the color values of the digital target image and the digital actual image. Intervention in the production process was therefore not necessary and subsequent substrate boards were printed with the same decoration using the same printing conditions.

Exemplary Embodiment 2—Digital Printing

(8) A multiplicity of HDF boards (8 mm) coated with a basecoat containing titanium dioxide were printed with a wood decoration using a digital printer. The printer, which had four rows of print heads, printed with a modified color set (rY=reddish yellow, gY=greenish yellow, C=red and K=key or black). The inks were water-based. A digital photograph (actual image) of every tenth printed HDF board was generated with a resolution of 8 megapixels by means of a digital camera. The color values of the L*a*b* color space of said actual image were compared with the color values of the L*a*b* color space of a digital target image. The digital target image had been produced beforehand by calibration of hyperspectral digital images of the same decoration by means of ACMS and generation of a digital photograph thereof with a resolution of 8 megapixels. The digital actual images were recorded in each case under identical conditions by way of a tube which was open only toward the object and in which the digital camera and an illumination system were positioned. As a target value for allowed color deviations, 15% was defined for the individual values of the L*a*b* color space. During the similarity comparison of the color values of the digital target image and the digital actual image, deviations for the color values a* and b* of 17% and 20%, respectively, were ascertained in the digital actual images. The color deviations were communicated to the control computer of the printing line. With the aid of RIP software on the control computer, the digital printing was automatically adapted to the color values of the digital target image. After the adaptation of printing, a digital photograph (actual image) of every tenth printed HDF board was furthermore generated with a resolution of 8 megapixels by means of a digital camera. The renewed similarity comparison revealed that color deviations for all color values of the L*a*b* color space now occurred below the predefined target value of 15%.

Exemplary Embodiment 3—Finishing

(9) The printed HDF boards in accordance with exemplary embodiments 1 and 2 are subjected to further processing as follows:

(10) The printed HDF boards were separated upstream of the production line and transported through the following production installation at a rate of 28 m/min.

(11) In a first roller application unit, approximately 70 g of liquid melamine resin (solids content: 55% by weight) comprising the conventional auxiliaries (hardeners, wetting agents, etc.) are applied to the board surface. A melamine resin is likewise applied to the board underside by the first roller application unit (quantity applied: 60 g of liquid resin/m.sup.2, solids content: approximately 55% by weight).

(12) A scattering apparatus is then used to scatter 14 g of corundum/m.sup.2 (F 200) onto the surface. A distance of approximately 5 m before the dryer is reached allows the corundum to sink into the melamine resin. The board then passes through a convection dryer. A quantity of 25 g/m.sup.2 of a melamine resin layer (solids content: 55% by weight) is then applied. Again, this comprises the conventional auxiliaries. A melamine resin is likewise applied to the board underside by a roller application unit (quantity applied: 50 g of liquid resin/m.sup.2, solids content: approximately 55% by weight). Again, the board is dried in a convection dryer.

(13) A melamine resin that additionally also comprises glass beads is then applied to the board surface. Said glass beads have a diameter of 60-80 μm. The applied quantity of the resin is approximately 20 g of liquid melamine resin/m.sup.2 (solids content: 61.5% by weight). The formulation also comprises a release agent, alongside the curing agent and the wetting agent. The applied quantity of glass beads is approximately 3 g/m.sup.2. A melamine resin is likewise applied to the board underside by a roller application unit (quantity applied: 40 g of liquid resin/m.sup.2, solids content: approximately 55% by weight). Again, the board is dried in a convection dryer, and is then again coated with a melamine resin comprising glass beads. Cellulose (Vivapur 302) is included as a further component. Again, approximately 20 g of liquid melamine resin/m.sup.2 (solids content: 61.6% by weight) are applied. Here again, approximately 3 g of glass beads and 0.25 g of cellulose/m.sup.2 are applied. The formulations also comprise a release agent, alongside the curing agent and the wetting agent. A melamine resin is likewise applied to the board underside by a roller application unit (quantity applied: 30 g of liquid resin/m.sup.2, solids content: approximately 55% by weight). Again, the resin is dried in a convection dryer, and then the board is pressed at 200° C. and a pressure of 400 N/cm.sup.2 in a short-cycle press. The press time was 10 seconds. Structure was provided by using a press plate with a wood structure.