Printing method using compensated printed images for equalizing inkjet printing defects

Abstract

The invention relates to a printing method for producing printed products, in which optical defects caused during inkjet printing, in particular by inhomogeneous wetting of the printed substrate with the inkjet ink and / or by defective inkjet printing nozzles, are compensated by means of a compensating print image printed before inkjet printing.

Description

Technical Field

[0001] The invention relates to a method for producing a printed product and to a printed product obtained by the method according to the invention. Background Art

[0002] When printing on substrates (e.g. sheet- or web-shaped substrates such as paper, cardboard or plastic), defective printing results sometimes occur during inkjet printing. This means that the printed image in the prepress phase and the finished printed image on the substrate deviate from each other in an undesirable manner. In this case, the substrate has, for example, too much or too little ink in certain areas. The cause of the defective printing result can be traced to the substrate, the inkjet system (in particular the inkjet print head) or both.

[0003] A known problem in the production of printed products is that the substrates used do not have uniform, ideal surface properties, but rather have inhomogeneous surfaces. Such surfaces can, for example, have an inhomogeneous wetting behavior with inkjet inks. Sometimes, the substrates used for printing all have the same inhomogeneous surface. When such substrates are printed by means of inkjet printing, unless special measures are taken, printed products are obtained that all exhibit the same altered printed image, which has the same defective optical appearance.

[0004] Another known problem in the production of printed products is traced back to the inkjet print heads of inkjet printers, which in some cases have defective inkjet print nozzles. These defective inkjet print nozzles then do not print onto the substrate in a parallel manner to the adjacent inkjet print nozzles, but rather print skewed. Thus, an inkjet print head with inkjet print nozzles that print skewedly can also lead to defective printed images on the substrate.

[0005] In order to deal with defective printed images, various methods have been proposed. For example, it is known in principle to optically evaluate the substrate by means of a camera system before printing and to modify the printed image in the prepress phase based on this evaluation in such a way that the desired error-free printed image is obtained on the substrate as a result.

[0006] DE 43 34 712 A1 describes a reproduction system comprising a printing press, in which distortions of the printed image are detected by means of a camera. Based on this optical detection, correction data for compensating the printed image are generated.

[0007] WO 2005 / 092613 A2 describes a printing press having an optical detection device which assesses the quality of the print on the basis of the detected print image data and, if necessary, changes parameters of the printing press based thereon.

[0008] The methods known from the prior art are relatively complex and usually require analytical evaluation of each individual substrate prior to the actual printing process. Summary of the invention

[0009] The object of the present invention is to provide a method by means of which a printed image which is as free of defects as possible can be obtained during inkjet printing. The method should be simple to implement with conventional printing presses and should not require complex image analysis of each individual substrate. In particular, the method should also be suitable for continuously compensating for recurring surface inhomogeneities on successively printed sheets or webs.

[0010] It has been found that defect-free printed images can be obtained in a simple manner by means of inkjet printing if suitable compensating printed images are applied to the substrate before inkjet printing. These compensating printed images are preferably colorless or have essentially the same color as the substrate, so that an observer of the finished printed product cannot detect these compensating printed images with the naked eye in the ideal case, but an optically defect-free printed image is obtained after inkjet printing. These compensating printed images or intermediate images can be applied to the substrate in a simple manner by means of printing methods known in principle, such as offset printing, flexographic printing or screen printing.

[0011] Accordingly, the present invention relates to a method for producing a printed product, wherein a substrate is printed with the aid of at least one inkjet print head, wherein the substrate and / or the at least one inkjet print head have at least one defect that results in a defective printed image on the substrate, characterized in that before printing with the aid of the at least one inkjet print head, at least one defect of the substrate and / or the at least one inkjet print head is equalized by printing (Aufdrucken) an intermediate image on the substrate, thereby obtaining a substantially defect-free printed image on the substrate.

[0012] In a preferred embodiment, a substrate with an inhomogeneous surface is used and the properties of the intermediate image are selected such that the intermediate image substantially compensates for the inhomogeneous surface of the substrate. It is particularly preferred to use a substrate that has an inhomogeneous wetting behavior with respect to the inkjet ink of the at least one inkjet print head. This is to be understood in particular as substrates that do not have a uniform wetting behavior with respect to the inkjet ink, in particular aqueous inkjet ink, over the entire surface of the substrate. Accordingly, the surface of the substrate can be easily wetted by aqueous inkjet inks in one area, for example, and difficult to wet with aqueous inkjet inks in another area.

[0013] In another preferred embodiment, the at least one inkjet print head has a non-ideal image generation behavior. This non-ideal image generation behavior of the at least one inkjet print head can be traced only to the at least one inkjet print head or other components of the entire inkjet system (in particular the electronic control device or the ink supply system). An example of a non-ideal image generation behavior of the at least one inkjet print head is the presence of such inkjet printing nozzles: these inkjet printing nozzles show a behavior that deviates from the majority of inkjet printing nozzles and these inkjet printing nozzles apply the inkjet ink to the substrate in particular skewed. Thus, the inkjet print head in this preferred embodiment particularly includes one or more inkjet printing nozzles with non-ideal printing behavior.

[0014] In another preferred embodiment, the intermediate image is a dot / grid. A periodic grid is particularly preferred. As a periodic grid, a grid that can compensate for the anisotropic properties of the substrate is particularly preferred. The result is that the best possible isotropy is obtained on the substrate for the printed image that is finally printed. For example, the substrate may have a preferred ink flow direction as an anisotropic property, and this preferred ink flow direction is at an angle of about 90° relative to the reference line. Then, such a periodic grid is usually selected as the intermediate image: the grid compensates the preferred ink flow direction in the substrate so that the printed image finally obtained no longer has a preferred direction, that is, it is finally isotropic. Specifically, the substrate (such as a paper sheet) may have a preferred ink flow direction, which results in: if there is no intermediate image, a stripe pattern will be printed parallel to the edge of the sheet. But it is desired that no stripe pattern appears. In this case, such an intermediate image is usually selected: the intermediate image has a stripe grid oriented perpendicular to the edge of the sheet. In this way, the anisotropy of the intermediate image compensates for the anisotropy of the substrate and an error-free (in this case isotropic) printed image is obtained as an overall result.

[0015] In the method according to the invention, an intermediate image is printed onto the substrate. Preferred printing methods for printing the intermediate image are selected from form-dependent / form-bound printing methods, in particular: offset printing methods, flexographic printing methods, screen printing methods and combinations thereof.

[0016] As printing media for applying the intermediate image to the substrate in the printing method, in principle all printing media that are usually usable in the respective printing method used (in particular printing pigments and printing varnishes) are suitable. In a preferred embodiment, the at least one printing medium is colorless. In another preferred embodiment, the at least one printing medium has essentially the same color as the substrate. Printing media with different colors can also be used, in particular when the substrate has different colors.

[0017] Preferred printing media that can be used to produce the intermediate image are water-repellent. Oil-based offset printing inks are particularly preferred. With the aid of such or similar printing media, the substrate is partially sealed by printing on the intermediate image. This has the additional advantage when using, in particular, water-based inkjet inks, that the water-based inkjet ink printed onto such partially sealed areas remains on the substrate surface for a longer time and does not penetrate into the substrate. As a result, thermal drying of the inkjet ink in the printing press can be achieved more easily. Another effect is that when using paper or cardboard as a substrate, deformation of the substrate due to penetrating water is less likely.

[0018] The substrate can be used in the method according to the invention in different forms, preferably in the form of sheets or in the form of strips. If the substrate is in the form of strips, one or more cutting devices can be provided before or after the printing method for cutting the printed strips into individual sheets.

[0019] In principle, all types of printing materials that can be processed in a printing press are suitable as substrates for the method according to the invention. For example, these may be plastics (especially plastic films), metals (especially metal films), paper or cardboard. In a preferred embodiment, the substrate is selected from paper, cardboard and plastic.

[0020] The printed product obtained by the process according to the invention is also a subject of the present invention.

[0021] The method according to the invention is also suitable for continuously compensating for repeated inhomogeneous surfaces on sheet-like or web-like substrates. In this case, the sheet-like or web-like substrate preferably has the same inhomogeneous surface in each printed section. This can be, for example, a stack of paper sheets in which all individual sheets have the same inhomogeneous wetting behavior with inkjet inks. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 : Explain the method according to the present invention by way of example;

[0023] Figure 2 : exemplarily shows an intermediate image which can be used to compensate for the inhomogeneous surface of a substrate. DETAILED DESCRIPTION

[0024] Figure 1The method according to the invention is to be explained in more detail. The left path shows the method according to the invention, the right path shows a method not according to the invention for comparison purposes. The starting point of the invention is a substrate (1) with an inhomogeneous surface, for example a paper sheet, which has an inhomogeneous wetting behavior on its own surface with respect to aqueous inkjet inks. According to the invention, the paper sheet, i.e. the substrate (1) with an inhomogeneous surface, is printed with an intermediate image (2), which is constructed in the form of a periodic grid (anisotropy) (shown only schematically). This printing is carried out by means of offset printing with offset printing colors in an offset printing press (not shown). Subsequently, a circular image is printed as a printed image (3) on the intermediate image (2) with an aqueous inkjet ink by means of an inkjet print head (5). The intermediate image (2) compensates for the inhomogeneous surface (anisotropy) of the substrate (1). As a result, a circular optical image, i.e. a substantially defect-free printed image (3), is obtained on the substrate (1). The anisotropy of the substrate and the anisotropy of the intermediate image are balanced. In contrast, if a circular image is printed directly onto a substrate (1) having an inhomogeneous surface without the intermediate step of printing the intermediate image (2) with aqueous inkjet printing by the inkjet print head (5), a distorted circular image, i.e. a defective printed image (4), is obtained.

[0025] Figure 2 An intermediate image (2) is shown by way of example, which can be used to compensate for the inhomogeneous surface of a substrate (1). Such a printed image can be printed simply by means of offset printing in an offset printing press.

[0026] Reference numerals list

[0027] 1 Base

[0028] 2 Intermediate Image

[0029] 3. Essentially defect-free printed image

[0030] 4 Defective printed images

[0031] 5 Inkjet Print Head

Claims

1. A method for producing a printed product, in, Printing the substrate (1) with the aid of at least one inkjet print head (5), wherein the substrate (1) has at least one defect, the at least one defect causing a defective printed image (4) to appear on the substrate (1), It is characterized in that Prior to printing with the aid of the at least one inkjet print head (5), at least one defect of the substrate (1) is additionally compensated by printing an intermediate image (2) on the substrate (1), thereby obtaining a substantially defect-free printed image (3) on the substrate (1), wherein a substrate (1) having an inhomogeneous surface is used and the properties of the intermediate image (2) are selected so that the intermediate image (2) substantially compensates for the inhomogeneous surface of the substrate (1).

2. The method according to claim 1, in, The substrate (1) having an inhomogeneous surface has an inhomogeneous wetting behavior with respect to the inkjet ink of the at least one inkjet print head (5).

3. The method according to claim 1, in, The at least one inkjet print head (5) has at least one defect, wherein, before printing with the aid of the at least one inkjet print head (5), the defects of the substrate (1) and the at least one inkjet print head (5) are compensated by printing an intermediate image (2) on the substrate (1), thereby obtaining a substantially defect-free printed image (3) on the substrate (1).

4. The method according to claim 3, in, The at least one inkjet print head (5) has non-ideal image generation performance.

5. The method according to claim 4, in, The at least one inkjet print head (5) has one or more inkjet print nozzles having a non-ideal printing performance.

6. The method according to any one of claims 1 to 5, in, The intermediate image (2) is a periodic grid.

7. The method according to any one of claims 1 to 5, in, The intermediate image (2) is applied by a form-dependent printing method.

8. The method according to any one of claims 1 to 5, in, The intermediate image (2) is applied by an offset printing method, a flexographic printing method or a screen printing method.

9. The method according to any one of claims 1 to 5, in, The intermediate image (2) is colorless or has substantially the same color as the substrate (1).

10. The method according to any one of claims 1 to 5, in, The substrate (1) is in sheet or web form and is selected from the group consisting of paper, cardboard and plastic.

11. A printed product obtained by the process according to any one of claims 1 to 10.

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