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

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.