EP1870246B1 - Method for printing a print substrate - Google Patents

Description

The invention relates to a method for printing a printing material according to the preamble of claim 1. Such a method is known from the document DE 197 45 136 A1 known.

In printing form-based, preferably operating on the offset printing principle printing machines, such. B. in web-fed rotary printing presses and sheetfed presses, find increasingly pressure-free inkjet printing devices use, in particular the customization of printed products produced by offset printing with z. As barcodes, numbering or other markings serve. Such inkjet printing devices have at least one inkjet print head, the so-called continuous inkjet principle, the drop-on-demand inkjet principle, the thermal inkjet principle, the bubble inkjet principle or any other inkjet Principle can be formed. The inkjet printheads usually have a nozzle row of several juxtaposed nozzles over which ink can be directed to a substrate to be printed.

Since the maximum printing speed of inkjet printing devices is significantly lower than the maximum printing speed of offset printing devices, it is difficult to print inline on a substrate after offset printing and after inkjet printing. To increase the achievable printing speed of inkjet printing devices, it is already known from practice to use inkjet printing devices with a variety of inkjet printheads, namely on the one hand with multiple inkjet printheads transversely to the transport direction of the substrate or to the printing direction and on the other with a plurality of inkjet printheads in the transport direction of the printing material or in the printing direction, wherein the plurality of inkjet printheads are arranged in an array-like or matrix-like manner next to one another.

The required number of inkjet printheads transversely to the printing direction is defined primarily by the desired printing resolution in relation to the given printing resolution of the inkjet printhead used and by the desired total printing width based on the given printing width of an inkjet printhead. The required number of inkjet printheads in the printing direction is determined primarily by two points, namely firstly that the desired printing speed is greater than the given printing speed of an inkjet printhead, and secondly that a plurality of printing inks via the inkjet printing device to be applied to a substrate.

Regardless of whether an inkjet printing device with multiple arrayed inkjet printheads or a single inkjet printhead is used for printing a substrate, the achievable printing speed can also be increased by the or each inkjet printhead of an inkjet printing device to the transport direction the printing material and thus obliquely aligned or inclined to the printing direction. The inclination has the consequence that the effective distance of the nozzles transversely to the printing direction or transport direction of the printing material is reduced and thus the printing resolution can be increased transversely to the printing direction. If the printing speed remains unchanged, it can then be printed with a higher areal coverage or optical density. Equally, however, the areal coverage or optical density can be kept constant while increasing the printing speed.

If an increase in the printing resolution and / or increase in the printing speed of inkjet printing devices with the printing direction or to the transport direction of the substrate inclined inkjet printheads worked, so provided in a prepress output data for an image to be printed with the inkjet printing device after be implemented the given geometric conditions.

In the printing process known from practice, this implementation takes place in the hardware of the inkjet printheads, but this has the disadvantage that this conversion is valid only for a defined inclination, only for a defined drop frequency and only for a defined printing speed. Changes z. As the printing speed, so it can not be reacted to, which ultimately results in the printing quality impairing distortions for the print image to be printed with the inkjet printing device.

On this basis, the present invention is based on the problem to provide a novel method for printing a substrate. This problem is solved by a method according to claim 1. According to the invention, output data, in particular an output data matrix, of a print image to be printed by the inkjet printing device depends on a current print speed, depending on a current drop frequency of the or each inkjet print head of the inkjet printer and depending on a current skew angle of the or each nozzle row of or each inkjet printhead with respect to the transport direction of the printing material before transmission of the data to the inkjet printing device in target data, in particular a target data matrix, converted to control the inkjet printing device in real time.

For the purposes of the method according to the invention, it is proposed that the conversion of the output data into the target data for controlling an inkjet printing device inclined in the printing direction be carried out independently of the hardware of the inkjet print heads of the inkjet printing device. The inventive conversion of the output data in the target data is thus carried out before the transfer of image information from the prepress to the inkjet printing device and thus between the prepress and the inkjet printing device. According to the invention, the conversion of the output data into the target data takes place in real time, wherein the current print speed, the current drop frequency and the current skew angle are variable variables in the conversion of the output data into the target data.

As a result, the conversion of the output data in the target data z. B. be adapted to a changing printing speed, so that even with changing printing speeds with the inkjet printing device, a high print quality can be guaranteed.

According to a first advantageous development of the invention, the conversion of the output data into the target data takes place via a transformation such that an output data matrix is scaled and sheared in the printing direction as well as transversely to the printing direction. A scaling factor for scaling the output data matrix transversely printing direction is determined from the current skew angle, namely from the ratio of the expansion of the print image transversely to the printing direction with inclined inkjet printing device for extending the same transversely to the printing direction in non-inclined inkjet printing device. A scaling factor for scaling the output data matrix in the printing direction is determined from the current print speed and the current drop frequency. A shear angle to shear the output data matrix is determined from the current skew angle.

According to a second, alternative advantageous embodiment of the invention, the conversion of the output data into the target data takes place in such a way that an output data matrix is scanned step by step depending on the current skew angle, the current print speed and the current drop frequency, wherein if one or more nozzle positions of the inkjet Printer to hit a pixel in an output data matrix, a corresponding pixel is set in a target data matrix.

Fig. 1:

eine schematisierte Darstellung zur Verdeutlichung der Druckverhältnisse bei einer nicht schräggestellten Inkjet-Druckeinrichtung;

Fig. 2:

eine schematisierte Darstellung zur Verdeutlichung der Druckverhältnisse bei einer schräggestellten Inkjet-Druckeinrichtung;

Fig. 3

eine erste Darstellung zur Verdeutlichung einer ersten Variante des erfindungsgemäßen Verfahrens;

Fig. 4:

eine zweite Darstellung zur weiteren Verdeutlichung der ersten Variante des erfindungsgemäßen Verfahrens;

Fig. 5:

eine dritte Darstellung zur weiteren Verdeutlichung der ersten Variante des erfindungsgemäßen Verfahrens;

Fig. 6:

eine erste Darstellung zur Verdeutlichung einer zweiten Variante des erfindungsgemäßen Verfahrens;

Fig. 7:

eine zweite Darstellung zur weiteren Verdeutlichung der zweiten Variante des erfindungsgemäßen Verfahrens; und

Fig. 8:

eine dritte Darstellung zur weiteren Verdeutlichung der zweiten Variante des erfindungsgemäßen Verfahrens.

Preferred embodiments of the invention will become apparent from the dependent claims and the description below. Embodiments of the invention will be explained in more detail with reference to the drawing. Showing:

Fig. 1:

a schematic representation to illustrate the pressure conditions in a non-tilted inkjet printing device;

Fig. 2:

a schematic representation to illustrate the pressure conditions in a tilted inkjet printing device;

Fig. 3

a first representation to illustrate a first variant of the method according to the invention;

4:

a second representation for further clarification of the first variant of the method according to the invention;

Fig. 5:

a third representation for further clarification of the first variant of the method according to the invention;

Fig. 6:

a first representation to illustrate a second variant of the method according to the invention;

Fig. 7:

a second illustration for further clarification of the second variant of the method according to the invention; and

Fig. 8:

a third illustration for further clarification of the second variant of the method according to the invention.

Before the process according to the invention for printing on a printing material is described in detail below, reference should be made in advance to FIG Fig. 1 and 2 Pressure conditions are described on inkjet printing devices. So shows Fig. 1 schematically an inkjet printhead comprising a nozzle row 10 of a plurality of juxtaposed nozzles 11, which are positioned along a row or line 12 equidistant from each other. The spacing of the nozzles 11 of such a nozzle row 10 is predetermined by the technology used in the inkjet printing head.

For printing a printing substrate, the printing material to be printed is preferably moved in the direction of the arrow 13 relative to the preferably stationary ink-jet printing head, wherein in the case of a constant drop frequency of ink and a constant printing speed, the in Fig. 1 illustrated Raster of possible positions 14 for drops of ink results. In Fig. 1 In this case, the droplet frequency, printing speed and the distance of the nozzles are chosen such that four adjacent positions 14 of ink droplets describe a square 15 with a defined surface area. A distance X of the positions 14 in the printing direction determines the resolution in the printing direction, a distance Y of the positions 14 transverse to the printing directions determines the resolution transverse to the printing direction, wherein Fig. 1 these two resolutions are the same size. It should be noted that, of course, the resolution in the printing direction can be different in size than the resolution transverse to the printing direction. From the distance Y of the nozzles 11 transversely to the printing direction multiplied by the number of nozzles 11 results in the printable width or the extent of a printable with the ink-jet print head print image transversely to the printing direction, according to Fig. 1 the nozzle row 10 of nozzles 11 with the pressure direction 13 forms an angle α of approximately 90 °. The area of the square 15 of the Fig. 1 is a measure of the area coverage or optical density that can be used to print.

Fig. 2 illustrates the pressure conditions, which then set when the nozzle row 10 is tilted from the nozzles 11 by an angle β to the printing direction 13, wherein in Fig. 2 the angle β is for example 30 °. It follows immediately that the distance Y of the ink droplet positions 14 decreases transversely to the printing direction, whereby the resolution can be increased transversely to the printing direction. Should be compared to Fig. 1 are printed with unchanged area coverage or unchanged optical density, the distance X between the positions for ink droplets in the printing direction can be increased by increasing the printing speed.

In Fig. 2 the achievable area coverage or achievable optical density is visualized by a parallelogram 16 spanned by four adjacent positions 14, the area of the parallelogram 16 of FIG Fig. 2 the area of the square 15 of the Fig. 1 equivalent.

From the referring to Fig. 1 and 2 Accordingly, it follows that by an inclination of a row of nozzles of an inkjet printhead transversely to the printing direction 13 while maintaining the achievable area coverage or optical density and reducing or reducing the printing width and thus extension of the printable image across the printing direction 13, the printing speed of inkjet Printing facilities can be increased. This effect is utilized in the process according to the invention for printing on a printing material in order to increase the printing speed of inkjet printing devices and thus inkjet printing processes and thus to print a substrate inline to form a printing plate-based printing process with a pressure-free inkjet printing process.

Due to the fact that the inkjet printheads or nozzle rows thereof are inclined relative to the transport direction of the printing material and thus the printing direction, there is a need to provide output data of a print image to be printed with the inkjet printing device in target data for controlling the inkjet printing device in a prepress stage to change. This is done in the sense of the present invention prior to the transmission of data to the inkjet printing device immediately after the provision of the output data in the prepress, said conversion depending on a current printing speed of the printing form based printing process and thus the inkjet printing process, depending on a current drop frequency of the or each inkjet printhead of the inkjet printing device and depending on a current skew angle of the or each row of nozzles of the or each inkjet printhead with respect to the transport direction and thus the printing direction.

The conversion of the output data in the target data is done in real time, so that during printing z. B. changing printing speed, the target data for controlling the inkjet printing device can be changed so as to always provide an optimal print image with the inkjet printing device under changing printing conditions. The current print speed, the current drop frequency and the current skew angle are therefore variable Sizes in the conversion of the output data of the prepress stage in the target data for controlling the inkjet printing device.

As noted above, the skew angle of the or each nozzle row of the or each inkjet printhead of the inkjet printing device is a variable size of the method of the invention, but the skew angle is ideally chosen so that at a given maximum printing speed and a given maximum printing speed maximum drop frequency just a geometric area coverage or optical density of 100% is given. The maximum printing speed is then only by physical parameters such. B. limits the drop speed itself and the associated placement accuracy of the ink droplets on the substrate. However, the skew angle can also be selected to provide areal coverage of less than 100%.

The actual drop frequency is either the same or variable for all the nozzles of an inkjet printhead, and when a continuous inkjet printer is used, the drop frequency is the same for all nozzles, and then, if a drop-on-demand inkjet Pressure device is used, the drop frequency is variable.

The current printing speed is detected by measurement via a sensor and represents a variable input variable for the conversion of the output data of the prepress for the inkjet printing device to be printed image in the target data for controlling the inkjet printing device.

In order to convert the output data into the target data, according to a first variant of the method according to the invention, the output data is transformed into the target data via a transformation, namely such that output data in the printing direction and transversely to the output data are present in the form of an output data matrix, in particular an output bitmap print direction are scaled and further sheared to provide a target data matrix, in particular a target bitmap, for driving the inkjet printing device. The detailed procedure in this transformation will be described below with reference to FIG Fig. 3 to 5 explained.

In Fig. 3 the reference numeral 17 indicates an output data matrix provided in a prepress stage for a print image to be printed with an inkjet printer device, this output data matrix 17 being an orthogonal output bitmap which has been screened using known methods. In the simplest case, there are purely binary data per pixel, which means that one pixel of the output data matrix 17 is either set and thus black or unset and thus white. In the exemplary embodiment shown, the printed image to be printed is an A, wherein in Fig. 3 an unoccupied and thus white pixel with the reference numeral 18 and a set and thus black pixel is designated by the reference numeral 19.

In Fig. 3 In order to convert the output data matrix 17 into a target data matrix, first a scaling of the output data matrix is performed transversely to the printing direction 13, whereby a scaling factor for the scaling transversely to the printing direction results from the current skew angle. Thus, the scaling factor for the scaling transversely to the printing direction results from a ratio of the printing width of the inclined inkjet printhead in relation to the printing width of the non-inclined inkjet printhead.

In other words, the scaling factor for the scaling transversely to the printing direction results from the ratio of the extent of the printed image transversely to the printing direction with an inclined inkjet printing device for extending the printed image transversely to the printing direction with non-inclined inkjet printing device. In Fig. 3 For example, a data matrix scaled by this scaling factor is identified by the reference numeral 20.

Subsequently, in the embodiment of the Fig. 3 a scaling in the printing direction, wherein a scaling factor for scaling in the printing direction from the current print speed and the current drop frequency is determined.
Then, when the drop frequency is unchanged, the scale factor in the print direction corresponds to the ratio of the print speed of the inkjet print head in non-skewed mode to the print speed thereof in the skewed mode. Fig. 3 numbers a data matrix, which is scaled both transversely to the printing direction and in the printing direction, with the reference numeral 21.

In Fig. 3 Then, following the above scaling in the printing direction and transversely to the printing direction, a shear of the output data matrix takes place, wherein a shear angle is determined from the current skew angle. A scaled by both scaling factors and the shear angle transformed data matrix corresponding to the target data matrix for driving the inkjet printing device is in Fig. 3 designated by the reference numeral 22.

It should be noted that the order of the scalings and the shear of the output data matrix is arbitrary. All transformations can also be done in one step.

Fig. 4 Visualizes the effects on the transformation of the output data matrix when the print speed is higher than in the embodiment of Fig. 3 , Otherwise, in the embodiment of Fig. 4 all parameters compared to the embodiment of Fig. 3 unchanged. By increasing the printing speed results in a different scaling in the printing direction, so that the data matrix 21 and thus ultimately the target matrix 22 against Fig. 3 is changed. Because opposite Fig. 3 However, only the printing speed has changed, the scaling factor across the printing direction and the shear angle remains unchanged.

Fig. 5 Visualizes the relationships in the transformation of the output data matrix 17 in a target data matrix 22 for the case in which the inkjet printing device relative to a curved guide element, such. B. a cylinder, is tilted for printing material to be printed. In this case, in addition to the two scales and the shear, a transformation is carried out to compensate for or compensate for differences in the transit time caused by the different distances of the nozzles of the or each inkjet printhead of the inkjet printing device to the printing substrate. In Fig. 5 This transformation is done to compensate for the different distances of the nozzle to the substrate between the scaling in the printing direction and the shear, but here too the order is arbitrary. In Fig. 5 is a scaled in both directions and transformed to compensate for the different nozzle distances data matrix with the reference numeral 23, wherein the target data matrix 22 is additionally sheared by the shear angle.

To compensate for differences in the ink run-time, the procedure is preferably such that the output data of the print image to be printed is adjusted so that such print image information associated with the nozzles at a greater distance from the substrate to be printed, compared with such print image information, the nozzles with a smaller distance are assigned by printing material to be printed, be moved to a position earlier in the printing direction.

A second variant of the method according to the invention for converting the output data of the prepress stage into the target data for controlling the inkjet printing device is described below with reference to FIG Fig. 6 to 8 described, wherein this conversion of the output data in the target data according to the second variant of the present invention takes place in that an output data matrix depending on the current skew angle, the current print speed and the current drop frequency is scanned stepwise, wherein if one or more nozzle positions of the inkjet -Druckeinrichtung hit a pixel in the output data matrix, a corresponding pixel in the target data matrix is set.

Fig. 6 shows an example of an L to be printed by means of an inkjet printing device an output data matrix 24 of 8x12 pixels, wherein set for printing pixels in Fig. 6 are shown as rounded squares 25. In the presentation of the Fig. 6 is assumed to be the resolution for the output data matrix 24 in both directions of the same 200 dpi, so that a screen width 26 of 127 μm results in both directions.

This output data matrix 24 of the Fig. 6 is according to Fig. 7 assuming a skew angle β of a nozzle row 10 of nozzles 11 is virtually sampled relative to the printing direction 13, wherein when one or more nozzle positions 11 strike a set pixel 25 in the output data matrix 24, a corresponding pixel 27 is set in the target data matrix. A step size 28 of this sample, which corresponds to the raster width of the target data matrix, depends on the current print speed and the current drop frequency. The step size of the scan and thus the raster width of the target data matrix is greater, the greater the print speed. In Fig. 7 In order to provide a clearer representation, pixels 27 set in the target data matrix are represented as circles which are shown somewhat smaller than they could cover in terms of area.

In Fig. 8 is compared to Fig. 7 increases the step size of the scan or the raster width 28 of the target data matrix by increasing the printing speed at a constant drop frequency, wherein Fig. 8 the pixels 27 of the target data matrix have approximately the same point density as the pixels 25 of the output data matrix. As a result, printing speed can be increased with almost unchanged optical density.

The setting of the pixels in the target data matrix can be done in binary form or via gray scale modulation. Then, when the inkjet printing device uses binary inkjet printheads, pixels in the target data matrix, all of which have the same drop size, are set or not, depending on whether nozzle positions hit pixels in the output data matrix during the scan. If, on the other hand, an inkjet printing device is used whose inkjet printheads can modulate gray values, then when a nozzle position encounters a pixel in the output data matrix, the gray value in the target data matrix is set, which is the ratio of the areal coverage of an ink drop and the imaginary pixel area closest to this position.

Also according to the second variant of the present invention, which can be obtained by using the reference to Fig. 6 to 8 described method to compensate for different distances of the nozzles from the printed material to be printed, as with reference to Fig. 5 described, transformed or transformed.

The conversion of output data into target data described above is performed in real time, so that a speed change of the printing speed in the inkjet printing device can be taken into account.

A side effect of the method described is that at a lower printing speed than the maximum printing speed, a higher optical density can be achieved. Especially when printing black and white graphics or when printing texts, this side effect has a positive effect on the print quality. However, if this side effect is perceived as disturbing, then the target data matrix can be thinned out by deleting pixels, in such a way that if only one pixel of the target data matrix has multiple pixels, only the best placed pixels are selected.

LIST OF REFERENCE NUMBERS

10

nozzle row

11

jet

12

line

13

print direction

14

position

15

square

16

parallelogram

17

Output data matrix

18

pixel

19

pixel

20

data matrix

21

Data matrix

22

target matrix

23

data matrix

24

Output data matrix

25

pixel

26

screen ruling

27

pixel

28

screen ruling

Claims (12)

1. Method for printing on a print substrate, wherein the print substrate is printed on by means of a printing-forme-based printing method, in particular with an offset printing method, and also in-line with the printing-forme-based printing method by means of an ink-jet printing method without a printing forme, wherein an ink-jet printing device has, for carrying out the ink-jet printing method, at least one ink-jet print head which comprises at least one nozzle row (10) of nozzles (11) which are arranged side by side and by way of which printing ink can be directed onto the print substrate to be printed on, and wherein the or each ink-jet print head is aligned in such a way relatively to the print substrate to be printed on that the or each nozzle row of the or each ink-jet print head is inclined in relation to a direction of transport of the print substrate, characterised in that output data, in particular an output matrix, of a print image to be printed by means of the ink-jet printing device are converted into target data, in particular a target matrix, for activating the ink-jet printing device in real time as a function of a current printing speed of the printing-forme-based printing method and thus of the ink-jet printing method, as a function of a current drop frequency of the or each ink-jet print head of the ink-jet printing device and as a function of a current angle of inclination of the or each nozzle row (10) of the or each ink-jet print head with respect to the direction of transportation of the print substrate before transmission of the data to the ink-jet printing device.
2. Method according to claim 1, characterised in that the current printing speed of the printing-forme-based printing method and thus of the ink-jet printing method is detected using measurement techniques and is thus a variable input quantity for the conversion of the output data into the target data for activating the ink-jet printing device.
3. Method according to claim 1 or 2, characterised in that the current drop frequency is either the same, when a continuous ink-jet printing device is used for all the nozzles of the or each ink-jet print head, or variable when a drop-on-demand ink-jet printing device is used for the nozzles of the or each ink-jet print head.
4. Method according to one or more of claims 1 to 3, characterised in that the current angle of inclination is selected in such a way that given a maximum printing speed and given a maximum drop frequency precisely still a geometrical area coverage of 100% ensues.
5. Method according to one or more of claims 1 to 4, characterised in that the conversion of the output data into the target data is effected by way of a transformation in such a way that an output-data matrix is scaled and sheared in the printing direction and also transversely to the printing direction.
6. Method according to claim 5, characterised in that a scaling factor for scaling the output-data matrix transversely to the printing direction is determined from the current angle of inclination, namely from a ratio of the expansion of the print image transversely to the printing direction when the ink-jet printing device is inclined to the expansion of the print image transversely to the printing direction when the ink-jet printing device is not inclined.
7. Method according to claim 5 or 6, characterised in that a scaling factor for scaling the output-data matrix in the printing direction is determined from the current printing speed and the current drop frequency.
8. Method according to one or more of claims 5 to 7, characterised in that a shearing angle for shearing the output-data matrix is determined from the current angle of inclination.
9. Method according to one or more of claims 1 to 4, characterised in that the conversion of the output data into the target data is effected in such a way that an output-data matrix is scanned step by step as a function of the current angle of inclination, the current printing speed and the current drop frequency, wherein when one or more nozzle positions of the ink-jet printing device meets or meet with an image point in an output-data matrix, a corresponding image point is set in a target-data matrix.
10. Method according to claim 9, characterised in that in this case in order to modulate grey values in the target-data matrix, that grey value is set that comes closest to the ratio of the area coverage of a drop and an image-point area at this position.
11. Method according to claim 9 or 10, characterised in that a step width of the scanning of the output-data matrix is dependent upon the current printing speed and the current drop frequency, with the step width being greater, the greater the printing speed is.
12. Method according to one or more of claims 1 to 11, characterised in that when the ink-jet printing device is inclined relatively to an arched guiding element for the print substrate to be printed on and the nozzles of the or each nozzle row of the or each ink-jet print head are at a differing distance from the print substrate that is to be printed on, the conversion of the output data into the target data is effected in such a way that running-time differences of the printing ink to the print substrate caused by the differing distance of the nozzles from the print substrate to be printed on are offset or compensated for, wherein for this the output data of the print image to be printed are adapted in such a way that such print-image information that is associated with nozzles at a greater distance from the print substrate to be printed on is displaced to an earlier position in the printing direction in relation to such print-image information that is associated with nozzles at a shorter distance from the print substrate to be printed on.

Images