EP0681525A1

EP0681525A1 - Printing process and device.

Info

Publication number: EP0681525A1
Application number: EP94905724A
Authority: EP
Inventors: Erhard Lorch
Original assignee: DLW AG
Current assignee: DLW AG
Priority date: 1993-01-29
Filing date: 1994-01-28
Publication date: 1995-11-15
Anticipated expiration: 2014-01-28
Also published as: HK68397A; CN1117279A; ATE145860T1; WO1994016892A1; EP0681525B1; DK0681525T3

Abstract

A process and device are disclosed for printing patterns, in particular repeating patterns (register rounds) on webs (8) or sheets. The web (8) or sheets are conveyed through at least two printing stations (2a, 2b). A first printing station (2a) prints part of the pattern on the web (8) or sheet and a second printing station (2b) which, in respect of rotary position, is aligned with the first printing station (2a) and prints the remaining part of the register round on the web (8) or sheet.

Description

description

Printing method and printing device

The present invention relates to a method and an apparatus for printing webs or sheets with patterns, in particular with repeating patterns. A preferred method is the printing of PVC floor covering webs.

For printing PVC floor covering sheets with repetitive

The so-called gravure process is generally used for patterns that are referred to as repeats in printing technology. Here, a printing form provided on a cylinder is lowered on the printing elements by etching or engraving. The web to be printed is guided past the rotating printing form by means of a pressure roller. When printing continuous decorations, the circumference of the cylinder corresponds to the length of one repeat of the decor. In the following, a repeat of a decor should not necessarily mean the smallest period of repetition. Rather, a repeat can also have several such periods, especially if that

Decor is very delicate.

Endlessly engraved, copper-coated printing cylinders with a circumference of 1000 mm and 4 m width have been used so far, in particular in the continuous printing of decorations for PVC floor covering webs. As a rule, these printing cylinders are imaged by engraving, and based on the design templates it is necessary to elaborate the report. In general, these printing cylinders can also be provided with new decors, it being necessary to close the printing cylinder first chemically chrome-plating, then turning off the engraving copper, electroplating copper, engraving the new design again, for example using helio-engraving, and then chrome-plating the printing cylinder. Since up to six impression cylinders are required for a design, this means a lot of effort.

In addition, gravure printing on PVC floor covering webs requires long web paths because of the high pressures required and because of the expulsion of solvents in the printing layers and generally drying between the printing units. These boundary conditions have hitherto prevented the use of the known method of illustration gravure in the RGB-CMYK color space. This is because the precondition for this is exact register control in the longitudinal and transverse directions, which is not possible under the technical boundary conditions used hitherto. For this reason, when printing on

PVC sheets worked in the so-called special colors room. In other words, in order to build up an image statement, the individual color separations to be printed were assigned empirically determined colors that were recognizable with the eyes. When converting images in the spot color space, however, the brilliance and sharpness of the printed image are lost. The implementation from design to design requires an interpretation that is carried out empirically from design to design. This in turn means that the printed image never corresponds exactly to the original.

PVC tiles as flooring are nowadays mostly manufactured by manufacturing PVC sheets according to the above-mentioned method and then punching out the tiles. So the tiles are not made individually in sheet printing. Overall, the previous method of depressing PVC webs is perceived as unsatisfactory, since there is a high level of engraving risk for fashionable markets, high development costs and long development times of an average of six months are required, and rapid reactions to changing market conditions are not possible .

In general, the brilliance and sharpness that the consumer desires is not achieved.

It is the object of the invention to provide a device and a method for printing webs with repeating patterns, by means of which the disadvantages of the state of the art in gravure printing are avoided.

This object is achieved by a printing method with the features of claim 1 and by a printing device with the features of

Claim 15 reached.

The invention is particularly preferably suitable for printing webs with repeating patterns (repeats). Also for printing on sheets, ie general cargo and in particular PVC flooring tiles with patterns, which will be explained in more detail below. In general, however, the printing of PVC tiles has the same advantages as for floor covering webs.

The method according to the invention and the device according to the invention are generally suitable for all printing methods, but in particular for offset printing and letterpress printing. Both processes have so far not been considered for the printing of repeating patterns (repeats) or felt to be unsatisfactory. Since the marginal conditions in high pressure and offset printing are similar, in the following only reference is made to offset printing. However, these references should apply to high pressure.

The offset printing process is known per se. In offset printing, a plate-like printing form is applied around a plate cylinder. From a purely mechanical point of view, it is not possible for the two ends of the printing form to touch each other seamlessly. In this respect, seams have so far always been seen when printing rapport by means of offset printing. For the application of the method according to the invention or the device according to the invention to the gravure printing process, it should be noted that it would generally also be possible to clamp printing forms prepared for the gravure printing process onto a cylinder. However, the printing surfaces in the gravure printing process must be wiped off with a doctor blade. In this respect, the opening of gravure forms

Cylinder to solve the mechanical problems with the squeegee.

By means of the method according to the invention or the device according to the invention, the offset printing method can be used advantageously when printing webs with repeating patterns, the so-called continuous printing. Provided that the two printing units can be matched exactly, that is to say in the range from at least 1/100 mm to in the range 1/1000 mm, continuous printing according to the invention can be achieved using the offset method without visible seams.

With the method according to the invention or with the device according to the invention, it is possible for the first time to print PVC floor covering webs in the RGB-CMYK color space. This is now a brilliance and to achieve a sharpness of the print that was previously not possible in the spot color space by gravure printing. Furthermore, the amount of engraving is significantly reduced. In the offset printing process, flat printing forms are used which are clamped onto plate cylinders. Engraving plate or foil-like printing forms is much easier than engraving gravure cylinders. In addition, the handling of these flat printing forms is much easier than that of the heavy gravure cylinders, the weight of which is approximately 2000 kg. As a result, the development costs for designs are lower and the development times are shorter, so that the method according to the invention can also be used in fashionable markets. Of particular advantage, however, is the use of the method according to the invention when using a so-called direct imaging offset printing machine, in which, instead of the dampening units, exposure units or imaging units are built which directly image a printing plate from a digital data stock (cf. offset Praxis 9/1991, pages 26, 28).

According to a preferred embodiment, the first printing unit prints a part, preferably half, of the repeat or pattern, that is to say that

Half of a period of the pattern. The repeat does not necessarily have to form the smallest period of the pattern. The second printing unit, through which the web to be printed runs, is so matched to the rotational position of the first printing unit and to the distance from it that the remaining part, that is to say preferably the second half of the repeat or the pattern, is matched to that Web prints.

The two printing units virtually form a unit like a single conventional offset printing unit. If one therefore designs in multicolor printing, a set of two offset printing units must be provided for each color. The division of the repeat is preferably chosen differently for each individual color. This achieves a higher level of uniformity, since the slightest misalignments that may occur do not appear to be effective, since they only affect part of the

Impact overall pressure.

According to a particularly preferred embodiment, the repeat or pattern is divided within surface areas of constant density. This method is somewhat more complex than the simple division of the repeat or pattern by half mentioned above. Because the print template is to be examined for the density and the dividing line is to be laid depending on the density. The dividing line often does not run perpendicular to the longitudinal direction of the web but follows the surface areas of constant density according to the motif of

Print template. This procedure can result in several equally favorable division options for a motif. In this case, it is particularly preferable to divide it approximately in the area of the center of the repeat or pattern, for reasons of uniform stress.

It is particularly advantageous if the division is carried out within areas of the lowest density, that is to say in the so-called “lights”. Conventionally, halftones are reproduced by so-called amplitude-modulated screening. The size of concrete pressure points of the grid corresponds to the tonal value: the lighter an image area in the original, the smaller the pressure point that reproduces the tonal value of this image area in the reproduction. In the so-called "lights", these halftone printing dots have the smallest possible size, but are always present. In this respect, a dividing line preferably runs between the halftone printing dots. However, this will often not be possible due to the mechanical boundary conditions of the print, since the grid dimension, ie the distance between the print points, is smaller than the adjustment accuracy of the printing press. If you place the dividing line in areas of lowest density, that is, in the so-called "lights", some of the halftone dots are cut accordingly. Nevertheless, the dividing line can be seen far less than in cases in which not only the smallest but also larger pressure points are cut, as will regularly be the case when the repeat is divided in half.

However, the preferred embodiment of dividing the repeat or pattern within the lowest density areas is particularly advantageous if the so-called frequency-modulated halftone reproduction is used instead of the amplitude-modulated halftone reproduction. This corresponds to the halftone reproduction in black and white photography. Silver grains are stochastically distributed in the film or photo paper. These silver grains are either exposed or not, so they are either black or white. The value of the semitone results from the density of exposed or unexposed grains. The tonal value does not result from a single grain, but from the density of exposed or unexposed grains per image zone. This principle of halftone reproduction in black and white photography can also be transferred to printing, in particular offset printing, by using suitable data processing technology using, for example, pseudostochastic methods. Surprisingly, it has been shown that in the case of frequency-modulated halftone reproduction in offset printing, generally coherent image zones with the lowest possible density exist, ie without any blackening. If one now places the division of the repeat according to the invention in those image zones of lowest density, the division is quasi "invisible".

This embodiment in conjunction with frequency-modulated halftone reproduction thus represents a particularly preferred embodiment of the invention.

According to a preferred embodiment, a drying device is provided between the first and the second printing unit, by means of which the pressure applied by the first printing unit is dried before entering the second printing unit. The drying device is preferably of the radiation type. A UV drying device is particularly advantageous.

Another particularly preferred embodiment consists in that the printing is generally carried out in the so-called wet-on-wet process with radiation-drying inks, only one radiation-drying device being provided at the end of the printing section, that is to say downstream of the last printing unit.

The method according to the invention is preferably carried out with a device in which the printing units have a direct imaging unit instead of the dampening units, which e.g. by means of the contactless

Electroerosion or, using laser techniques, can directly image printing forms provided for this purpose. The imaging unit is able to receive and process digital print data directly. The printing units are preferably compared with one another with the aid of a reference pattern, which is printed or otherwise provided, for example, on the web or sheet by a reference pattern identification device. The reference pattern is advantageously located at the edge of the web or sheet in an area that is generally cut off after printing.

The reference pattern is preferably printed on with an upstream reference printing unit in a printing process which allows the reference pattern to be printed continuously. For example, the reference pattern can be printed using the gravure printing process, the reference pattern being engraved endlessly on a gravure cylinder. However, all other printing methods are also suitable as far as they allow an uninterrupted continuous printing of the reference pattern. However, the reference pattern does not necessarily have to be provided by printing. It is also conceivable to punch or emboss or also cut the pattern, for example by means of a laser cutting device or a water jet cutting device.

The adjustment takes place in all printing units that follow the device with which the reference pattern was applied. The reference pattern is preferably such that the following printing units can be adjusted not only in the longitudinal direction, that is to say the web direction, but also in the transverse direction of the web or sheet. The subsequent printing units are then adjusted taking into account the read reference pattern at least in the longitudinal direction of the web or sheet, but preferably also in the transverse direction of the web or sheet. A precise adjustment or adjustment of the printing units to one another is essential in the method according to the invention at least when large webs are printed with large masses, such as PVC floor covering webs. Due to the mechanical boundary conditions there is the susceptibility to incorrect adjustments with the

Time is particularly long, which, if readjustments are omitted, lead to blurring and color shifts in the print.

The accuracy of the adjustment depends, among other things, on the web speed during printing, on the spacing of the elements of the reference pattern and on the scanning frequency of the reading device in the following printing units. The speed when printing PVC floor covering webs is about 50 m per minute, which is very slow compared to the printing of paper like newspapers. In this respect, when using the scanning frequencies of opto-electronic reading devices that are possible today, the spacing of the elements of the reference pattern is very small, e.g. in the micrometer range. With the use of high-precision adjustment devices in the following printing units, high-precision transitions at the seams between the partial repeats, with multi-color printing a high register accuracy of the color components, which are in part superimposed, and high sharpness, color purity and brilliance during printing can be achieved.

Although the inventive method and the inventive

The device is particularly suitable for printing on PVC floor covering webs, but it is also possible to apply the invention to printing on gift paper, toilet paper and the like. In general, it is of course also possible to use the method according to the invention to be used in the printing of carpets or textiles, in which cases, however, the requirements for the transitions between the rapports will generally not necessarily require the application of the invention.

A direct imaging machine currently on the market allows the processing of sheets or repeat parts up to max. 504 mm x 340 mm. A commonly used PVC flooring tile size is 500mm x 500mm. In this respect, the invention is particularly preferably also suitable for printing tiles of this standard size, each of the two printing units printing a part of the tile pattern, a maximum tile size of 504 mm × 680 mm being conceivable.

Since the media to be printed are preferably floor coverings, the following is noted regarding the question of resistance to treading. In general, a distinction is made between floor coverings and so-called calender coverings. The coating layers are generally printed directly and then a protective layer is also applied in the coating process. A running layer is usually laminated onto the calender coverings, the lower layer of which can generally also consist of regenerated material. In the case of calender coverings, either the lower layer can therefore be printed, a transparent film being subsequently laminated on. Alternatively, it is possible to print on the film and then laminate the printed side onto the underlayer.

The following is a preferred embodiment of the invention described with reference to the accompanying drawing.

FIG. 1 is a side view of a gravure cylinder according to the prior art, around the entire circumference of which a printing form is provided or which is circumferentially provided with a repeat;

FIG. 2 is a schematic side view of a device according to the invention for printing webs with repeating patterns by two offset printing units;

Figure 3 is a top view of two offset printing forms and a web section printed therewith; and

FIG. 4 is a top view of a web section and shows the rapport division within surface areas of the lowest density.

When printing webs with repeating patterns in intaglio printing according to the prior art, an intaglio cylinder 10 (FIG. 1) is engraved endlessly around its entire circumference, so that the period of the repeating pattern (repeat) is πD, where D is the

Diameter of the gravure cylinder.

A device according to the invention is shown schematically in FIG. Of the two offset printing units (high-pressure units are also suitable), only the plate cylinders 2a, 2b are shown in the side view. The inking unit assigned to the plate cylinders 2a, 2b, the printing cylinder around which the web to be printed is guided, and possibly a rubber cylinder provided between the plate and printing cylinders for the use of indirect printing are not shown. The plate cylinders 2a, 2b carry a printing form 4a and 4b, respectively. The printing forms 4a, 4b each span half of the respective plate cylinder 2a, 2b in this embodiment. Therefore, if one selects the plate cylinders 2a, 2b with the same diameter as the gravure cylinder 10 from the prior art, its printing form 12 and the printing forms 4a, 4b taken together have the same length, so that reports of the same period can be printed.

FIG. 2 also schematically shows a web 8 to be printed with the device according to the invention and a UV drying device 6, which is provided between the two offset printing units and that of the first plate cylinder 2a with the first printing form 4a applied pressure dries before entering the second printing unit with the second plate cylinder 2b, which carries the second printing form 4b.

Alternatively, it is also possible to carry out the printing according to the invention entirely in the so-called wet-on-wet method and only at the end of the printing units, i.e. downstream of the last printing unit, a single radiation drying device, e.g. UV drying type that dries the prints of all printing units.

In this embodiment, to compare the longitudinal and transverse position of the two plate cylinders 2a, 2b or of their printing forms 4a, 4b, a reference pattern 5 (see FIG. 3) is continuously and uninterruptedly applied to the web by a reference pattern identification device (not shown) printed. The printing units with the plate cylinders 2a, 2b each have a matching or adjusting device 7 which reads the reference pattern 5 and on-line in this way to the rotational position and Transverse position of the respective plate cylinder 2a, 2b acts that a high registration accuracy is achieved.

In FIG. 3, the two printing forms 4a and 4b provided on the plate cylinders 2a, 2b, each with a half repeat R, and a web 8 printed with them are shown in detail.

FIG. 3 also shows that the web 8 is provided with the reference pattern 5 at the edge. The reference pattern 5 is shown exaggerated in the drawing. In practice, very finely printed or e.g. read cut or punched reference samples that are barely visible, still read properly.

In this embodiment, both plate cylinders 2a, 2b are provided with an adjusting device 7, which in the position of the plate cylinders 2a, 2b

The transverse and longitudinal directions are readjusted as a function of the reference pattern read. If further pairs of printing units, e.g. are provided for multi-color offset printing, all subsequent printing units should preferably have such an adjustment device 7. In this case, the distribution of the repeat can vary for the individual colors in order to avoid even the slightest misalignment.

FIG. 4 is a top view of another web section 8 'and shows the particularly preferred repeat division along or within surface areas of lowest density. 4 is another one

Form of reference pattern 5 'shown, which allow not only an alignment in the longitudinal but also in the transverse direction of the web 8'. The pattern shown in FIG. 4, which is not unlike a tire tread pattern, would result in a half division of the repeat both within surface areas lowest and highest density. By repositioning the division path within the surface areas of lowest density, as shown in FIG. 4, the repeat division is virtually invisible, in particular when the frequency-modulated halftone reproduction is selected. In this case, the division line leads through image zones of the lowest density without any blackening, so that the division actually becomes invisible.

In the design shown, the advantages of laying the division of the repeat in correspondence with the design are obvious. The designs of e.g. Plastic coverings, however, are generally not lines, but are significantly "softer". Multi-color printing is also mostly used. It has now been shown that in this case too, for each color template, in particular when frequency-modulated halftone reproduction is used, from one side of the web to the other

There are coherent, low-density image zones on the side of the web into which the dividing line can be placed. It has also been shown that these contiguous surface areas have a different position for each color. In multi-color printing, the repeat divisions can thus be made for each in a particularly preferred manner

Place the color differently in the so-called "lights", which makes the repeat divisions invisible.

The evaluation of the print template with regard to the density required for the use of this type of repeat division can either be carried out using a suitable sensor system. Since the print templates are now often digitally available in a data processing system, density evaluation is also possible within the data processing system by using suitable algorithms.

Claims

Expectations

1 . Process for printing webs or sheets with patterns, in particular with repeating patterns (repeats), whereby

the web or the sheet is guided through at least two printing units, of which

a first printing unit prints a part of the pattern on the web or sheet and

a second printing unit is aligned with the first printing unit with respect to the rotational position and prints the remaining part of the repeat on the web or the sheet.

2. The method of claim 1, wherein the two printing units each print half of the pattern.

3. The method of claim 1, wherein the division within

Surface areas of constant density.

4. The method according to claim 3, wherein the division takes place within surface areas of lowest density.

Method according to one of claims 1, 3 or 4, wherein for multi-color printing a series of two printing units is provided, each printing one color, and wherein the division of the pattern is different for each color.

6. The method according to any one of claims 1 to 5, wherein between the first and the second printing unit, the pressure applied by the first printing unit is dried by means of a drying device.

7. The method according to any one of claims 1 to 5, wherein radiation-drying colors are used in the wet-on-wet method for printing, and only one radiation drying device is provided, which is arranged after the last printing unit.

8. The method according to any one of claims 1 to 7, wherein for the comparison of the printing units a reference pattern is printed on the web or sheet, preferably at the edge of the web or sheet.

9. The method according to claim 8, wherein the printing of the reference pattern is carried out by a reference pattern identification device connected upstream of the first printing unit.

10. The method according to claim 9, wherein a reference printing unit provided as a reference pattern identification device is designed in such a way that the reference pattern can be printed continuously on the web, preferably in the letterpress or gravure printing method.

1 1. The method of claim 9 or 10, wherein the adjustment is carried out by reading the reference pattern in the following printing unit or printing units and by adjusting the rotational position and / or the transverse position of the following printing unit or of the following printing units taking into account the web position read.

1 2. The method of claim 1 1, wherein the reference pattern is a bar code, the reading and evaluating the cross and the

Longitudinal position of the web allows.

13. The method according to any one of claims 1 to 12, wherein the printing of the webs takes place in the offset printing process or in the high-pressure process.

14. Device for printing webs or sheets with patterns, in particular with repeating patterns (repeats) with

- two printing units through which the web is led, one of which

a first printing unit is provided with a printing form which is arranged on a first cylinder and is illustrated with part of the pattern, and

a second printing unit is provided with a second printing form, which is arranged on a second cylinder and is illustrated with the other part of the pattern, and with

a balancing device which balances the rotational positions of the first and second cylinders in such a way that the prints of the first and the second printing form are connected to one another seamlessly.

15. The apparatus of claim 14, wherein the first printing form is imaged with the half pattern and the second printing form with the other half of the pattern.

16. The apparatus of claim 14, wherein the division of the pattern takes place within surface areas of constant density.

17. The apparatus of claim 16, wherein the division takes place within areas of lowest density.

18. Device according to one of claims 14 to 17, wherein a drying device is provided between the first and the second printing unit, which dries the pressure applied by the first printing unit before entering the second printing unit.

19. Device according to one of claims 14 to 18, wherein four printing unit sets each consisting of a first and a second printing unit are provided for producing four-color prints.

20. Device according to one of claims 14 to 17 or 19, wherein the last printing unit is followed by a radiation drying device.

21. Apparatus according to claim 19 or 20, wherein the division of the repeat is different for each printing unit set.

22. The device according to one of claims 14 to 21, wherein the Printing units for indirect printing each have rubber rollers.

23. Device according to one of claims 14 to 22, wherein each cylinder is assigned an imaging unit, one on the

Cylinder provided, provided for this purpose printing form directly, in particular by contactless electrical erosion.

24. Device according to one of claims 14 to 23, wherein a reference printing unit is provided upstream of the first printing unit, which prints a reference pattern on the web, preferably at the edge, in the high or low pressure method.

25. The apparatus according to claim 24, wherein the printing units which follow the printing unit printing the reference pattern, a reading device for the reference pattern and a device for adjusting their rotational position and / or their transverse position, taking into account the reading from the reference pattern have derived Bahn¬ layer.

26. The apparatus of claim 22, wherein the reference pattern is a bar code.

27. Use of a device according to one of claims 14 to 26 for printing PVC floor covering webs.

28. Use of a device according to one of claims 14 to 26 for printing sheets of film which are suitable for laminating onto plastic flooring tiles.