Method of Printing

The invention relates to a method of printing an image on a substrate in a printing machine comprising at least a first printing unit and an inkjet printing unit.

It has become more and more popular that print products are created in a combination of several different printing process technologies in order to take advantage of special capabilities or properties of the mutually different printing methods. For example, inkjet technology is frequently used for imprinting variable information into static images printed by offset or flexographic printing. Typical print products in such applications might be labels or packaging, hi other examples, special visual effects can only be achieved in combined printing.

When ink-jet imprinting is performed as usual into a blank space of the static background image, e. g. onto the unaffected surface of the printing material, the process is rather uncritical. However, it has been discovered in experiments that problems can arise in the situation that ink is jetted onto a pre-printed printing material. A difference in spread characteristics of ink for inkjet printing on certain papers, notably gloss papers, and preprinted offset ink has been observed. A differential spread of ink on a heterogeneous surface can cause serious, in particular visible image quality defects, when ink is jetted on partially pre-printed (ink-covered) areas of a substrate, for instance a raster image area.

It is accordingly an object of the present invention to avoid quality defects in images partially printed using inkjet ink on a substrate partially already covered by ink. In particular, an object of the invention is to provide a method of printing, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods of this general type.

The technical problem to solve is the avoidance or the correction of differential spread of ink jet ink on preprinted ink, in particular offset or flexographic ink, on a substrate.

This problem is solved by a method of printing according to claim 1. Further improvements and advantageous embodiments and refinements are defined by the limitations set out in the dependent claims.

According to the invention a method of printing an image on a substrate or a printing material in a printing machine comprising or including at least a first printing unit and an inkjet printing unit is provided. In the method at least the following steps are performed: The substrate is moved through the printing machine. A raster image consisting or formed of image dots is printed on the substrate at a first moment using at least the first printing unit. At least one contiguous area of inkjet dots in the raster image is printed at a second moment after the first moment using the inkjet printing unit, whereby substantially all inkjet dots forming the contiguous area are printed at dot locations having similar surface wetting properties. Expressed differently, all inkjet dots in a set of contiguous inkjet dots which are significant for the perception of the shape (or form or appearance) of the contiguous area are printed into the area of the raster image at dot locations having similar surface wetting properties at a second moment after the first moment using the inkjet printing unit. In still other words, a or at least one set of contiguous inkjet dots is printed into the area of the raster image only at dot locations having the same surface wetting property at a second moment after the first moment using the inkjet printing unit.

The printing machine can be an offset printing machine, a flexographic printing machine or a printing machine capable of executing both offset printing and flexographic printing. The raster image can be a halftone image. The set of contiguous inkjet dots can form a line or a full-coverage area of ink. The area of the set of contiguous inkjet dots can have at least one extension of greater length than the intra-dot distance between neighbouring dots of the raster image. Similar surface wetting properties are alike or like one another. Identical surface wetting properties are also similar. The surface wetting properties can be comparable or matching. Similar surface wetting properties can be similar (e. g. different, but not identical) or essentially the same surface wetting properties in an interval of measurement errors or acceptable discrepancies or differences. In particular they can be exactly the same or identical surface wetting properties. Discrepancies or differences can

be acceptable if they do not play a role for the printing process or for the perception of the printed product by the human eye. The inkjet ink can be colorful, in particular can be black. The surface wetting property can be the hydrophilicity or the hydrophobicity, for instance a (quantitative) measure for its degree. The raster image can be a multicolor image, in particular a CMYK-standard color image. The raster can be a frequency modulated raster. The substrate can be in the form of a sheet (preferred) or in the form or a web. The substrate can be selected from the group or can be one out of the group consisting of paper, cardboard, carton, and polymer foil.

In a first advantageous embodiment of the method the inkjet dots are printed at locations not overlapping with the image dots of the raster image. InkJet dots are only printed between raster image dots. More precisely, the positions of the inkjet dots can be chosen in function of the positions of the image dots of the raster image.

In a second advantageous embodiment, the method comprises at least the additional step that raster image data is generated, the raster dot positions of the raster image being chosen in function of the inkjet dot positions for avoiding overlap between raster dots and inkjet dots. For instance, this can be achieved by adjusting the raster screening algorithm of the raster image print to eliminate regular patterns of raster dots which can cause periodic and therefore more visible errors. One specific approach comprises the use of a frequency modulated raster screen instead of a conventionally used amplitude modulated one. In addition, the maximum spot size is controlled to be small enough to minimize interaction with the inkjet-printed ink. As a further measure, the screen angle of the raster in amplitude modulated raster screens can be also changed or varied.

In a third and preferred advantageous embodiment, the method comprises at least the additional step that the raster image is covered by a coating to produce a full coverage area after the first moment, e. g. after raster image printing, and before the second moment, e. g. before ink-jetting. The coating can be a colorless and/or clear varnish. The gloss of the coating can be matched to the gloss of the substrate. The coating can be applied to the substrate using a contact transfer method.

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In a fourth advantageous embodiment of the method the inkjet dots are printed only at sub- areas of the raster image featuring full coverage of ink. For example, this can be achieved by printing full coverage in sub-areas using a lower density ink instead of a raster image with the same color perception, whenever inkjet printing will take place in order to eliminate the variation in spreading. For example, if the desired final image involves black variable inkjet-printed text on light blue background, the light blue is printed as a 100% coverage instead of printing a partial coverage of full density cyan and magenta ink.

The distinctive characteristics of the four mentioned embodiments might be used separately or in combination in actual embodiments in practice.

In the method according to the invention, the raster image can be printed using a contact printing process. In particular, the raster image can be printed using an offset printing process (preferred) or a flexographic printing process. The at least first printing unit can comprise a printing plate or a printing master.

In the method, the inkjet dots can be printed only at ink-covered or at ink-free locations.

In concretely realized embodiments of the method, it is preferred to use an inkjet printing unit which comprises at least one drop-on-demand inkjet printing module. The inkjet printing unit can be suitable for multicolor printing.

In the method according to the invention, all inkjet dots can be printed only at locations having the same surface wetting property.

The scope of the present invention comprises also a substrate featuring or comprising an image obtained by executing the method with limitations or combinations of limitations as disclosed in this specification.

Further improvements, refinements, and advantageous embodiments, features and characteristics are described below and explained in more detail by referring to the attached drawings. It should be understood that the detailed description and specific examples given, while indicating the preferred embodiment, are intended for purpose of illustration only and are not intended to unduly limit the scope of the present invention. Although the invention is illustrated and described herein as embodied in a method of printing, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and rage of equivalents of the claim. The construction and method of operation of the invention together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein:

Figure 1 is a fragmentary, diagrammatic, plan view of a substrate showing an image defect occurring in overprinting an area of offset-printed raster dots by an inkjet line,

Figure 2 is a longitudinal-sectional view of a printing machine suitable for performing a preferred embodiment of the method according to the invention, and

Figure 3 is a block diagram illustrating the steps executed in the preferred embodiment of the method.

Referring now to the figures of the drawings in detail and first, particularly, to Figure 1 thereof, there is seen a diagrammatic representation of a magnified example of a printed area on a substrate 10 without taking advantage of the invention in order to demonstrate the technical problem to overcome. The area features a regular or amplitude modulated offset- printed raster 12 leaving some space of ink-free surface between individual raster image dots. When an inkjet-printed line 14 is printed in the form of individual but contiguous inkjet dots an image defect 16 is observed. Due to the differential spread between ink-

covered and ink-free locations in the raster image area the inkjet-printed line is distorted. In other words, the inkjet ink spreads onto the ink- free surface of the substrate 10 or moves away from the preprinted raster image dots of the offset-printed raster 12. The differential spread in a halftone image can yield a repeating irregularity at a spatial frequency visible to the eye.

Figure 2 is a diagrammatic view of a printing machine 18 suitable for performing a preferred embodiment of the method according to the invention. Along a transport path 20, sheets of substrates 22, notably paper sheets, are moved through the printing machine 18. hi a concrete embodiment, the transport path 20 can be curved or wound around cylinder surfaces. The sheets can be gripped and passed on from transport means to transport means, for instance from cylinder to cylinder. The sheets of substrates 22 are individualized in a feeder from a pile for feeding 24 and stacked into a pile for delivery 26 in the delivery of the printing machine 18. The embodiment of the printing machine 18 shown in Figure 2 has four offset printing units 28, ε. g. is capable for multicolor printing using the four standard colors cyan (C), magenta (M), yellow (Y), and black (B). The sheets of substrates 22 pass through the printing units 28 and receive a four-color raster image on one of its surfaces. Along the transport path 20, the printing units 28 are followed by a coating unit 30 capable to apply a varnish to the four-color raster printed surface of the sheets of substrates 22. Eventually, inkjet printing is performed in inkjet printing unit 32.

A uniform offset-printed background area for the inkjet printing is created in coating unit 30 to give a homogeneous surface. The surface is homogeneous at least in the sense that the local differences in the surface wetting properties are so small that their influence of the inkjet ink-spread can be safely neglected. The homogeneous surface is obtained by a clear varnish coating applied in the coating unit 30 of the printing machine 18, at least in locations where inkjet printing will be done. Although there is complexity added to the offset print job with respect to the situation shown in Figure 1, a single varnish type can be used in this approach for all offset jobs. Different varnishes are needed to match a specific gloss of the underlying substrate to make the coating less visible. Furthermore, the varnish

can be adapted to have good sticking properties for the inkjet ink. In a refinement of this embodiment the background area covered by the varnish is used to provide positive visual effects as a differential gloss.

Figure 3 refers to the steps executed in the preferred embodiment of the method explained in conjunction with Figure 2. The method is executed on substrates being moved through a printing machine 18 along a transport path 22. hi the first step, the raster printing step 34, a multicolor raster image consisting of image dots is printed on the substrate using four offset printing units 28. After that, in the second step, the covering step 36, the raster image is covered by a coating of a colorless and clear varnish to produce a full coverage area using the coating unit 30. After that, in the third step, the inkjet printing step 38, a set of contiguous inkjet dots is printed into the area of the raster image using the inkjet printing unit 32. The area has obtained the same surface wetting property at ink-covered locations and at ink- free locations due to the varnish applied both the ink-covered and ink-free parts of the raster image.

REFERENCE NUMERAL LIST

substrate offset-printed raster inkjet-printed line image defect printing machine transport path sheets of substrates pile for feeding pile in delivery offset printing unit coating unit inkjet printing unit raster printing step covering step inkjet printing step