WO2024194260A1

WO2024194260A1 - Methods and systems for imaging a mask layer

Info

Publication number: WO2024194260A1
Application number: PCT/EP2024/057163
Authority: WO
Inventors: Dirk Ludo Julien DE RAUW
Original assignee: Xsys Prepress Nv
Priority date: 2023-03-17
Filing date: 2024-03-18
Publication date: 2024-09-26
Also published as: NL2034371B1

Abstract

Method for preparing a relief plate and printing with said relief plate on a substrate, such as a corrugated board, comprising the steps: providing a mask layer; obtaining a halftone raster image file comprising imaging pixels and non-imaging pixels at a first resolution R1; wherein a first pitch P1 corresponds to the first resolution R1; imaging the mask layer in accordance with the halftone raster image file and such that imaged spots have a largest dimension smaller than (√2 * P1); preparing a relief plate using the imaged mask layer; transferring ink to the relief plate using an anilox roller; and transferring said ink from the relief plate onto a substrate; wherein the first resolution R1 is smaller than 2000 dpi; wherein the anilox roller has a volume density which is larger than or equal to 5 cm3/m2.

Description

METHODS AND SYSTEMS FOR IMAGING A MASK LAYER

Field of Invention

The field of the invention relates to methods, control modules and computer programs for imaging a mask layer and for controlling the imaging of a mask layer, in particular a mask layer of a printing plate precursor used to obtain a relief plate, and in particular a relief plate suitable for printing on uneven surfaces, such as corrugated board. The field of the invention further relates to systems for treating a printing plate precursor to obtain a relief plate and for printing with such a relief plate on a substrate, in particular a substrate with an uneven surface. The field of the invention relates in particular to printing greyscale images.

Background

Flexographic printing or letterpress printing are techniques which are commonly used for high volume printing. Flexographic or letterpress printing plates are relief plates with image elements protruding above non-image elements in order to generate an image on a recording medium such as paper, cardboard, films, foils, laminates, etc. Also, cylindrically shaped printing plates or sleeves may be used.

Various methods exist for making flexographic or letterpress printing plate precursors. According to conventional methods flexographic or letterpress printing plate precursors are made from multilayer substrates comprising a backing layer and one or more photocurable layers. Those photocurable layers are imaged by exposure to electromagnetic radiation through a mask layer containing the image information or by direct and selective exposure to light, in order to obtain a relief plate.

In flexographic or letterpress printing, ink is transferred from a plate to a print medium. More in particular, the ink is transferred via an anilox roller on the relief parts of the plate, and not on the non-relief parts. During printing, the ink on the relief parts is transferred to the print medium. Flexographic printing is what may be termed as a binary system: when relief areas contact the printed surface, one gets a substantially solid color area. Greyscale images are typically created using halftoning. By greyscale is meant, for a plate printing in a particular color, the amount of that color being reproduced. For example, a printing plate may comprise different half-tone dot regions to print with different densities in those regions. Half-toning is a well-known process wherein gray tones are reproduced by using a relief plate having a plurality of solid dots per unit area and varying either the frequency of the dots per unit area or the size of the dots per unit area or both. In order to increase the amount of ink transferred and to increase the so-called ink density on the substrate, an additional very fine structure is applied to the surface of the printing areas, i.e. to the relief areas. This surface screening is typically obtained by adding the fine structure to the raster image file and then transferred to the corresponding mask used for exposure.

W02021110831A1 in the name of the Applicant describes in figures 1A-1E an example of an existing method for making a relief plate. Figure 1 A shows the content of a raster image file having an image file resolution corresponding with a pixel size p (which corresponds with the pitch) of e.g. 6.35 micrometer. The image file resolution may be e.g. 4000 dpi (= 25400 * 1/p (in micrometer)). Next, the raster image file is manipulated using a surface screen pattern which is illustrated in figure IB. The surface screen pattern is applied in the image region 1 resulting in a modified raster image file which is shown in figure 1C. As shown in figure 1C, the resulting image region 1 ’ contains fewer pixels 4’ to be printed, and the pixels 4’ to be printed are located at a distance d of each other. Based on the modified raster image file of figure 1C, a mask is prepared. More in particular, for every pixel 4’ to be printed, a hole or a transparent region 5 is arranged in the mask. This may be done using a beam of electromagnetic radiation. As shown in figure ID, such a beam will generate a hole 2 which is larger than the size of a pixel 4’. The resulting image on the mask is shown in figure IE. Thus, according to the method illustrated in figures 1A-1E, the surface screening is obtained by changing the original raster image file, e.g. a tiff file, using software, typically a raster image processing technique, wherein typically due to the manipulation a file having a larger size is generated. W02021110831A1 further discloses an improved method for processing a raster image file, comprising the steps: receiving of a raster image file comprising image data for a plurality of pixels, analyzing the image data of the raster image file, determining control data, and optionally at least one new raster image file, based on the analyzed image data, said control data being data for controlling settings of an imaging device so as to change the physical properties of generated imaged features corresponding with one or more of the plurality of pixels; outputting the raster image file and/or the new raster image file, with the control data to an imaging device for imaging a relief precursor. In this way the image quality can be improved. For example, depending on whether the image data includes text and/or a photo and/or a bar code and/or large continuous areas, etc., the imaging may be controlled in a different manner.

In typical printing processes, a high-resolution image, e.g., 4000-6000 dpi, in combination with surface screening is used to obtain good printing results. However, the inventors have noted that when printing greyscales on a substrate with an uneven surface, such as corrugated board, the unevenness may be seen through the printed layer. For example, for corrugated board, the wavy structure can be seen through the printed layer. This effect is called fluting. Summary

The object of embodiments of the invention is to provide methods, control modules, computer programs and systems, which can improve the image quality when printing grey scales, and in particular when printing grey scale images on a substrate with an uneven surface, such as corrugated board or wallpaper.

According to a first aspect, there is provided a method for preparing a relief plate and printing with said relief plate on a substrate, and in particular an uneven substrate such as corrugated board. The method comprises the steps: providing a mask layer; obtaining a halftone raster image file comprising imaging pixels and non-imaging pixels at a first resolution Rl; wherein a first pitch Pl corresponds to the first resolution Rl; imaging the mask layer in accordance with the halftone raster image file and such that imaged spots have a largest dimension (D) smaller than (A/2 * Pl), preferably smaller than Pl; preparing a relief plate using the imaged mask layer; transferring ink to the relief plate using an anilox roller; and transferring said ink from the relief plate onto a substrate. Preferably, the first resolution Rl is smaller than 2000 dpi, more preferably smaller than 1700 dpi, even more preferably smaller than 1500 dpi. Preferably, the anilox roller has a volume density which is larger than or equal to 5 cm3/m2, more preferably larger than or equal to 7 cm3/m2. Preferably, the volume density is below 20 cm3/m2.

For printing on corrugated board typically an anilox roller is used with a rather high volume density, and the inventors have discovered that when using a relatively low first resolution Rl instead of the high resolutions used in the prior art, in combination with a spot size resulting in a fine surface structure of hills on the dots, improved printing results can be obtained. This is believed to be because the imaged spots do not fully overlap and the corresponding fine hills on the relief plate are sufficiently large to be covered in such way by ink from the anilox roller that an uneven structure of the substrate can be effectively hidden, at least to an appropriate extent. The specified volume densities will typically allow having an appropriate number of hills per cell of the anilox roller, i.e., not too little hills and not too many hills, with enough relief between the hills, resulting in a good ink transfer. In addition, because of the rather low resolution, the imaging can be done in a faster manner. This counter-intuitive approach is especially useful for printing grey scales on substrates with an uneven surface.

It is noted that the pitch corresponds with the distance between the centers of adjacent pixels of a row and that this distance is typically equal to the pixel size. The imaging settings used for the imaging are chosen such that imaged spots have a largest dimension (D) smaller than (^2 * Pl), which results in that, after exposing a relief precursor through the imaged mask layer and developing the exposed relief precursor, a surface structure of hills surrounded by valleys is generated on a printing relief corresponding with a dot of a halftone area.

Preferably, the anilox roller has a line count which is smaller than 300 lines per centimeter (Ipcm), preferably smaller than 200 lines per centimeter. Stated differently, the cells of the anilox roller are relatively large, and the pitch Pa corresponding with the line count is typically significantly larger than the first pitch Pl corresponding with the first resolution. For example, a line count of 200 Ipcm corresponds with a pitch Pa = 10.000/200 micron = 50 micron, whilst a resolution of 1500 dpi corresponds with a first pitch Pl = 25.000/1.500 micron = 16.9 micron. Preferably, the line count corresponds with an anilox pitch Pa, and, during the step of obtaining, the first resolution R1 is chosen so that Pa is larger than 1,5*P1, more preferably between 2*P1 and 10*Pl.

Line count measures the finesse of the pattern on the anilox roller. It measures how many cells are engraved per centimeter or inch. In Europe the standard is Ipcm or lines per centimeter. In America, the standard is Ipi or lines per inch. Typically, the measuring takes place in a direction in which one counts the most cells per centimeter.

Preferably, the imaging is done such that a largest dimension of the imaged spots is smaller than Pl and/or larger than Pl/3. In such embodiments the image spots will not overlap and can generate an appropriate fine surface structure for printing a solid area.

Anilox rolls have on their surface a plurality of ink metering cells. These cells are typically arrayed in regular patterns of a predetermined frequency (expressed in lines per centimeter, Ipcm) and of uniform depth and shape. Typically, they are created by engraving a cylinder face of a roller by a mechanical process or by laser. The amount of ink delivered by the anilox roll is controlled by the size of the cells.

Optionally, a sampling pattern is superimposed on the halftone raster image file prior to or during the imaging of the mask layer, so that only a portion of the imaging pixels is imaged. Although, for some application this may be preferred, in other applications good results may be obtained when no sampling pattern is superimposed prior to or during the imaging of the mask layer.

It is further noted that a sampling pattern may be applied in some areas of the half-tone raster image file and not in others. For example, above a threshold tonal value, a sampling pattern may be added while for tonal values below the threshold tonal value no sampling pattern is superimposed. Thus, prior to the imaging, a sampling pattern may be superimposed on pixels of the halftone raster image file to obtain a sampled raster image file in which a portion of the imaging pixels of the image file is changed into non-imaging pixels, and wherein the imaging is based on the sampled raster image file. Also in that case, a sampling pattern may be applied in some areas of the halftone raster image file and not in others. Typically, because of the rather low first resolution, it will be preferred to use a rather dense sampling pattern such as a checkerboard pattern. Instead of a sampled raster image file, it is also possible to superimpose the sampling pattern during imaging: the sampling pattern is then added “on the fly” during the imaging.

The sampling pattern may be a repetition of a block in which one or more imaging pixels are combined with one or more non-imaging pixels. For example, the sampling pattern may be any one of the following or a combination thereof: a single pixel pattern, such as a single pixel checkerboard pattern, a pattern for which each imaging pixel is surrounded by eight non-imaging pixels; a multiple pixel pattern, such as a multiple pixel checkerboard pattern where e.g. a cluster of four imaging pixels or four non-imaging pixels corresponds with a case of the checkerboard; a line pattern; a dash pattern (such as interrupted lines); a circle pattern; a grid pattern.

Optionally, prior to imaging and based on the halftone raster image file or the sampled raster image file if a sampling pattern is used, a modified raster image file may be generated so that it has at least two bits per pixel. The at least two bits indicate for each pixel whether the pixel is one of the following: a non-imaging pixel, an imaging pixel to be imaged with a first imaging setting, an imaging pixel to be imaged with a second imaging setting different from the first imaging setting, optionally an imaging pixel to be imaged with a third imaging setting different from the first and second imaging setting. The imaging is then done based on the modified raster image file. These features integrate an indication of the imaging setting to be used into a single modified raster image file. During the imaging of the mask layer, it is then only necessary to extract information from this modified image file without having to refer to other files and/or without the need for having multiple raster image files. Thus, by generating such a modified raster image file, the imager can be instructed in a convenient manner, whilst allowing to vary the imaging settings. In this way the imaging settings can be changed during the imaging in a convenient manner.

Preferably, the anilox roller has a pattern of hexagonal cells.

Preferably, the first resolution R1 is between 700 and 1500 dpi. According to a second aspect, there is provided a method for imaging a mask layer for obtaining a relief plate for printing on a substrate. The method comprises the steps: providing of a mask layer; obtaining a halftone raster image file comprising imaging pixels and non-imaging pixels at a first resolution Rl; increasing the resolution of the halftone raster image file to obtain an increased- resolution raster image file comprising increased-resolution imaging pixels and increased-resolution non-imaging pixels at a second resolution R2 higher than the first resolution Rl; superimposing a sampling pattern having a repetition rate equal to the first resolution Rl on the increased-resolution raster image file so that only a portion of the increased-resolution imaging pixels is imaged; imaging the mask layer in accordance with the increased-resolution raster image file with the superimposed sampling pattern.

Although this method may be slower than the method of the first aspect, it has the advantage that “normal” resolutions may be used for R2 and hence that existing sampling patterns may be used. Further, the same effects may be achieved as with the method of the first aspect.

It is noted that the sampling pattern may be applied prior to the imaging, wherein a sampled increased-resolution raster image file is generated and used for the imaging, or the sampling pattern may be applied “on the fly” during the imaging. It is noted that the sampling pattern may be applied in some areas of the half-tone raster image file and not in others. For example, above a threshold tonal value, a sampling pattern may be added while for tonal values below the threshold tonal value no sampling pattern is superimposed. Typically, because of the rather high second resolution, it will be preferred to use a sampling pattern being a repetition of a block with more non-imaging pixels than imaging pixels.

It is further noted that by starting from a low-resolution raster image file with resolution Rl and next increasing the resolution to a resolution R2, interference/Moire problems can be reduced or avoided as compared to a case where one would start from a high resolution raster image file having a resolution R2.

As for the first aspect, for printing on corrugated board, the inventors have discovered that when using a relatively low first resolution Rl, in combination with a resolution increase and the use of a sampling pattern, improved printing results can be obtained. This is believed to be because the imaged spots generate hills on the relief plate which are sufficiently large to be covered in such way by ink from the anilox roller that an uneven structure of the substrate can be effectively hidden, at least to an appropriate extent. This counter-intuitive approach is especially useful for printing grey scales on substrates with an uneven surface. Preferably, the second resolution R2 is a multiple of the first resolution Rl. More preferably, the second resolution R2 is at least four times higher than the first resolution Rl.

Preferably, the imaging of the mask layer in accordance with the increased-resolution halftone raster image file is performed such that imaged spots have a largest dimension (D) larger than P2, more preferably larger than (A/2 * P2). In other words, the imaged spots are “boosted” to a size causing adjacent imaged spots to partially or fully overlap. In this way adjacent spots may form one larger hill instead of individual smaller hills which may in some cases result in better edges of the printing features.

The sampling pattern superimposed on the high-resolution raster image file is a repetition of a block in which one or more imaging pixels are combined with one or more non-imaging pixels. Preferably, R2 is at least three times higher than Rl , and the sampling pattern is formed by a repetition of a block of one or more adjacent imaging pixels surrounded by at least eight non-imaging pixels. For example, when R2=3*R1, the sampling pattern may be a repetition of a block with a single imaging pixel surrounded by eight non-imaging pixels, and when R2=4*R1, the sampling pattern may be a repetition of a block with four adjacent imaging pixel (forming a square) surrounded by twelve nonimaging pixels. It is noted that the imaging pixels may be centered in the bock as in the examples, but may also be arranged in a decentered way, e.g. when R2=3*R1, the sampling pattern may be a repetition of a block with a single imaging pixel in a corner and eight non-imaging pixels filling the remainder of the 3x3 pixel block.

Optionally, prior to imaging and based on the increased-resolution raster image file with the sampling pattern, a modified increased-resolution raster image file is generated so that it has at least two bits per pixel, said at least two bits indicating for each pixel whether the pixel is one of the following: a non-imaging pixel, an imaging pixel to be imaged with a first imaging setting, an imaging pixel to be imaged with a second imaging setting different from the first imaging setting, optionally an imaging pixel to be imaged with a third imaging setting different from the first and second imaging setting, wherein the imaging is done based on the modified raster image file.

The method according to the second aspect may further comprise preparing a relief plate using the imaged mask layer; transferring ink to the relief plate using an anilox roller, and transferring said ink from the relief plate onto a substrate. Similarly as for the first aspect, preferably, the anilox roller has a volume density which is larger than or equal to 5 cm3/m2, preferably larger than or equal to 7 cm3/m2. Preferably, the anilox roller has a line count which is smaller than 300 lines per centimeter, preferably smaller than 200 lines per centimeter. For example, the line count may correspond with an anilox pitch Pa, and, during the step of obtaining, the first resolution R1 is chosen so that Pa is larger than 1,5*P1, more preferably between 2*P1 and 10*Pl.

The preferred features set out below may be used both in methods of the first aspect and in methods of the second aspect.

Preferably, the obtaining of a halftone raster image file is the result of raster image processing a nonraster image file, such as a pdf or a ps file, in order to obtain directly the half-tone raster image file having the first resolution Rl. In this way Moire/interference effects can be reduced or avoided.

The imaging may be done such that all imaged spots have substantially the same dimensions and/or shape; or such that the dimensions and/or shape of the imaged spots are changed according to a regular or irregular pattern.

Such different dimension and/or shape may be achieved by controlling the imaging settings, and in particular by controlling any one or more of the following imaging settings: an intensity value to be used for generating an imaged feature corresponding with an imaging pixel, e.g. an intensity value for controlling a beam used for the imaging of the imaging pixels of a halftone area, a time interval to be used for generating an imaged feature corresponding with an imaging pixel, e.g. an on-time value for controlling a beam used for the imaging of the imaging pixels of a halftone area, a beam diameter value or beam shape value for controlling a beam used for the imaging of the imaging pixels, a number of passes used for the imaging of the imaging pixels of a halftone area, an indication of an exposure head of a plurality of exposure heads to be used for generating an imaged feature or a group of imaged features corresponding to a pixel or a group of pixels of the imaging pixels.

Preferably, the imaging comprises ablating the mask layer, preferably with a laser. Preferably, the method further comprises, prior to the imaging or after the imaging, providing the mask layer on a photopolymerizable layer of a relief precursor.

The mask layer can be a separate layer, which is applied to the relief precursor, typically following the removal of a protective layer that may optionally be present, or an integral layer of the precursor, which is in contact with the relief layer or one of the optional layers above the relief layer, and is covered by a protective layer that may possibly be present. The mask layer can also be a commercially available negative which, for example, can be produced by means of photographic methods based on silver halide chemistry. The mask layer can be a composite layer material in which, by means of image-based exposure, transparent layers are produced in an otherwise non-transparent layer, as described, for example in EP 3 139 210 Al, EP 1 735 664 B 1, EP 2987 030, Al EP 2 313 270 B 1. This can be carried out by ablation of a non-transparent layer on a transparent carrier layer, as described, for example, in U.S. Pat. No. 6,916,596, EP 816 920 Bl, or by selective application of a non-transparent layer to a transparent carrier layer, as described in EP 992 846 Bl, or written directly onto the relief-forming layer, such as, for example, by printing with a non-transparent ink by means of ink-jet, as described, for example, in EP 1 195 645 Al.

Preferably, the mask layer is an integral layer of the relief precursor and is located in direct contact with the relief-forming layer or a functional layer which is arranged on the relief-forming layer, which is preferably a barrier layer. Furthermore, the integral mask layer can be imaged by ablation and in addition removed with solvents or by heating and adsorbing/absorbing. For example, this layer may be heated and liquefied by means of selective irradiation by means of high-energy electromagnetic radiation, which produces an image-based structured mask, which is used to transfer the structure to the relief precursor. For this purpose, it may be opaque in the UV range and absorb radiation in the visible-IR range, which leads to the heating of the layer and the ablation thereof. Following the ablation, the mask layer also represents a relief, typically with lower relief heights, for example in the range from 0.1 to 5 pm. In an exemplary embodiment, the optical density of the mask layer in the UV range from 330 to 420 nm and/or in the visible-IR range from 340 to 660 nm lies in the range from 1 to 5, preferably in the range from 1.5 to 4, particularly preferably in the range from 2 to 4. The layer thickness of the laser-ablatable mask layer is generally 0.1 to 5 pm. Preferably, the layer thickness is 0.3 to 4 pm, particularly preferably 1 pm to 3 pm. The laser sensitivity of the mask layer (measured as the energy which is needed to ablate a 1 cm2 layer) may be between 0.1 and 10 mJ/cm2, preferably between 0.3 and 5 mJ/cm2, particularly preferably between 0.5 and 5 mJ/cm2.

Preferably, the step of preparing the relief plate using the imaged mask layer comprises exposing the photopolymerizable layer of the relief precursor through the mask layer and developing the relief precursor to obtain the relief plate. The relief plate may be a flexographic plate, a letter press plate, and it may be a flat or cylindrical plate.

According to another exemplary embodiment, the method further comprises detecting at least one interior area and at least one edge area in the image file, and a different imaging setting may be used for the at least one interior area and the at least one edge area, wherein the at least one interior area and the at least one edge area may comprise an interior area and an edge area of a dot of a halftone area.

The raster image file may be a 1 BPP (1 bit per pixel) file or a multi-level image file with multiple bits per pixel (e.g. such that a pixel can have various grey levels). The raster image file may have any one of the following file formats: TIFF, LEN, JPEG, JPG, BMP, JDF, PNG, etc.

In exemplary embodiments, a raster image processing (RIP) module converts a source image file, such as a pdf file or a ps file, into a raster image file which corresponds with the raster image file having the first resolution R1 mentioned above. The RIP module is a component used in image processing which produces a raster image file also known as a bitmap, which is a pixel-based format. The source image file may be a page description in a high-level page description language such as PostScript, Portable Document Format, XPS or another bitmap. In the latter case, the RIP applies either smoothing or interpolation algorithms to the input bitmap to generate the output bitmap. Raster image processing is the process of turning e.g. vector digital information such as a PostScript file into a high-resolution raster image file. Usually the RIP module is implemented either as a software component of an operating system or as a firmware program executed on a microprocessor. The RIP module may further have a layout function. When a plurality of small images needs to printed, those images may be grouped according to print patterns. This grouping may also be done by the RIP module.

Optionally, the raster image file may include raster image processed data, i.e. bitmap data, as well as vector coordinates corresponding to image patches. Alternatively, the raster image file with raster image processed data may be part of a job file which further comprises vector coordinates. The raster image processed data, i.e. the bitmap data, and associated vector coordinates corresponding to the image patches may be used as an input for processing the raster image processed data to automatically create one or more raster image processed image patches. One or more register marks may be attached to each such image patch, and the one or more image patches with register marks and the corresponding vector coordinates for each image patch may be stored in a processing file for creating a printing plate. Also, mounting device information, a bar code and other information may be associated thereto, and saved to the processing file. In one embodiment, bitmap information may be stored in a first layer of a template file with the vector coordinates for each image patch stored in a second layer of the template file. In another embodiment, bitmap information may be stored in a first file and the vector coordinates for each image patch may be stored in a second file associated with the first file.

According to another aspect there is provided a relief plate obtained by the method of any one of the previous embodiments.

According to another aspect there is provided a computer program or computer program product or digital storage means comprising computer-executable instructions to control the method of any one of the previous embodiments, when the program is run on a computer.

According to a further aspect, there is provided a system for treating a relief precursor, comprising an imager configured to image a mask layer; an exposure means configured to expose the relief precursor through the imaged mask layer, a developing means configured to remove at least a part of non-exposed material from the relief precursor, and a control module to control the imager in accordance with the method of any one of the previous embodiments. The imager may be a device which selectively removes parts of a mask layer, changes the transmission of a mask layer or selectively adds a non-transparent material to a substrate layer or relief precursor. Preferably, the imager removes parts of a mask layer or changes the transmission of a mask layer and this may be achieved by using beams of electromagnetic radiation. Most preferably the imager removes parts of a mask layer by ablation wherein beams of electromagnetic radiation are employed. Preferably, the wavelength of the beams of electromagnetic radiation is in the range of 700 nm to 12.000 nm.

Optionally, the system further comprises any one or more of the following: at least one transport system configured to transport the relief precursor, a storage device, a drying system, a post-exposure device, a cutting device, a mounting station, a heater. Optionally the system may comprise a printing station wherein the obtained relief plate is supplied with ink and transfers ink to a substrate.

Optionally, the system further comprises an anilox roller configured to apply ink on the obtained relief plate. The anilox roller may have any one of the features disclosed above in connection with the method.

The technical benefits set out above for embodiments according to the first and second aspect apply mutatis mutandis for embodiments of the system. Brief description of the figures

The accompanying drawings are used to illustrate presently preferred non-limiting exemplary embodiments of methods, control modules and systems of the present invention. The above and other advantages of the features and objects of the invention will become more apparent and the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

Figure 1 illustrates schematically an exemplary embodiment of a method for imaging a mask layer.

Figure 2A illustrates schematically a system for printing with an anilox roller; Figure 2B illustrates some parameters characterizing an anilox roller; and Figure 2C illustrates different types of anilox rollers.

Figures 3 and 4 illustrate schematically other exemplary embodiments of methods for imaging a mask layer.

Figures 5A-E illustrate exemplary embodiments of imaged spots which may be obtained using the embodiments of Figures 1 , 3 and 4.

Figure 6 illustrates schematically another exemplary embodiment of a method for imaging a mask layer.

Figure 7A, 7B and 7C illustrate printing results when using respectively a method of the prior art (Figure 7A), a method according to Figure 1 (Figure 7B) and a method according to Figure 3 (Figure 7C).

Figure 8 illustrates a schematic view of an exemplary embodiment of a system for producing a relief printing plate/sleeve.

Figure 9 illustrates a schematic view of an exemplary embodiment of a control module arranged downstream of a RIP module.

Figure 10 illustrates a schematic view of another exemplary embodiment of a control module.

Figure 11 illustrate schematically other embodiments of methods for imaging a mask layer. Figure 12A illustrates an embodiment which starts with a high-resolution file and images the pixels using a spot dimension D of 14 micron. Figure 12B illustrates an embodiment which starts with a high-resolution file, reduces the resolution thereof, and then uses a spot dimension D of 14 micron. Figure 12C illustrates an embodiment which starts with a low-resolution raster image file.

Detailed description of embodiments

Figures 1 and 2A-2C illustrate a first embodiment of a method for preparing a relief plate and printing with said relief plate on a substrate, and in particular on an uneven substrate such as corrugated board. First the plate is imaged as illustrated in Figure 1. The method comprises the steps of providing a mask layer, obtaining a halftone raster image file comprising imaging pixels and nonimaging pixels at a first resolution Rl, see part 1 of Figure 1, and imaging the mask layer in accordance with the halftone raster image file and such that imaged spots have a largest dimension D smaller than (A/2 * Pl), preferably smaller than Pl, wherein a first pitch Pl corresponds to the first resolution Rl, see part 2 of Figure 1. In the example of Figure 1, the file contains a circular solid area but it will be understood that the file may contain any image. On the right sight of Figure 1, six imaging pixels of the file are represented to illustrate more clearly the first pitch Pl and the dimension of the spots D. Preferably, the first resolution Rl is smaller than 2000 dpi, preferably smaller than 1700 dpi, more preferably smaller than 1500 dpi. For example, the first resolution Rl is between 700 and 1500 dpi.

In the example Rl = 1270 dpi, so that the first pitch Pl = 25400/1270 micron = 20 micron.

After the step of imaging the mask layer, a relief plate is prepared using the imaged mask layer, in accordance with well-known methods, typically comprising a UV exposure of a relief precursor through the mask, and the development of the exposed relief precursor, e.g. , by washing or thermally developing. Next the finished relief plate P may be used for printing, as shown in Figure 2A.

In the exemplary embodiment of Figure 2A, the printing plate P is mounted on a mounting cylinder 30 and brought in contact with an anilox roller 10, so as to transfer ink to the relief plate P. The anilox roller 10 picks up ink from an ink reservoir 20 and transfers the ink to the relief plate P. Next the ink is transferred from the relief plate P onto a substrate S. Preferably, the anilox roller 10 has a volume density which is larger than or equal to 5 cm³/m², more preferably larger than or equal to 7 cm³/m², for example between 5 cm³/m² and 10 cm³/m². The top image of Figure 2B illustrates a cell which is filled up with ink. The volume density corresponds with the total volume for a certain surface area of the anilox roller 10. Preferably, the anilox roller has a line count (which is smaller than 300 lines per centimeter (Ipcm), preferably smaller than 200 lines per centimeter, for example between 160 and 260 Ipcm. The line count determines the pitch Pa of the anilox roller as follows: Pa = 10.000 /Ipcm. The pitch Pa of the anilox roller is illustrated in Figure 2B. Figure 2C illustrates some possible anilox rollers which may be used in embodiments of the invention. For example, the anilox roller may have a pattern of hexagonal cells, see the first two images, or a pattern of square cells, see the last image.

Preferably, during the step of obtaining the raster image file, the first resolution R1 is chosen so that Pa is larger than 1,5*P1, more preferably between 2*P1 and 10*Pl.

Figures 3 and 4 illustrate two further exemplary embodiment for imaging a mask layer. The examples are similar to the example of Figure 1 with this difference that a sampling pattern is superimposed on the halftone raster image file. This pattern may be added prior to (Figure 3) or during the imaging (Figure 4) of the mask layer. Preferably, the sampling pattern is a repetition of a block in which one or more imaging pixels are combined with one or more non-imaging pixels. In the example, the sampling pattern is a single pixel checkerboard pattern, but the sampling pattern may be any one of the following or a combination thereof: a single pixel pattern, such as a single pixel checkerboard pattern, a pattern for which each imaging pixel is surrounded by eight non-imaging pixels; a multiple pixel pattern, such as a multiple pixel checkerboard pattern; a line pattern; a dash pattern; a circle pattern; a grid pattern.

Figure 6 illustrates another exemplary embodiment of a method for imaging a mask layer for obtaining a relief plate for printing on a substrate. The method comprises the steps of providing of a mask layer, obtaining a halftone raster image file comprising imaging pixels and non-imaging pixels at a first resolution R1 (in the illustrated example 6 pixels of which 5 are imaging pixels, are shown), see step 1 , increasing the resolution of the halftone raster image file to obtain an increased-resolution raster image file comprising increased-resolution imaging pixels and increased-resolution nonimaging pixels at a second resolution R2 higher than the first resolution R1 , see step 2, superimposing a sampling pattern having a repetition rate equal to the first resolution R1 on the increased-resolution raster image file so that only a portion of the increased-resolution imaging pixels is imaged, see step 2, and imaging the mask layer in accordance with the increased-resolution raster image file with the superimposed sampling pattern, see step 3. Preferably, the second resolution R2 is a multiple of the first resolution Rl. In Figure 6, two possible second resolutions R2 are shown: in the example on the left R2 is four times Rl, and in the example on the right R2 is three times Rl. This implies that for the example on the left 1 pixel with resolution Rl is replaced with 4x4 pixels with resolution R2. In this example, the superimposed sampling pattern consists of a repetition of a block of 2x2 imaging pixels surrounded by twelve non-imaging pixels. For the example on the right one pixel with resolution R1 is replaced with 3x3 pixels with resolution R2. In this example, the superimposed sampling pattern consists of a repetition of a block of one imaging pixel surrounded by eight nonimaging pixels.

Preferably, the imaging of the mask layer in accordance with the increased-resolution halftone raster image file, see step 3, is performed such that imaged spots have a largest dimension D larger than a second pitch P2, more preferably larger than (A/2 * P2), wherein the second pitch P2 corresponds to the second resolution R2. In the example on the left, this results in four overlapping spots for each imaging pixel with resolution Rl. In the example on the right this results in one big spot for each imaging pixel with resolution Rl.

As for the example of Figure 1 , the method may further comprise preparing a relief plate using the imaged mask layer, transferring ink to the relief plate using an anilox roller, and transferring said ink from the relief plate onto a substrate, see also the discussion of Figure 2A above. Preferably, the anilox roller has a volume density which is larger than or equal to 5 cm3/m2, preferably larger than or equal to 7 cm3/m2, and a line count which is smaller than 300 lines per centimeter, preferably smaller than 200 lines per centimeter. Preferably, the first resolution Rl is chosen so that the pitch Pa of the anilox roller is larger than 1,5*P1, more preferably between 2*P1 and 10*Pl.

In the exemplary embodiments of Figure 1, 3, 4 and 6, optionally, prior to imaging and based on the halftone raster image file, a modified raster image file may be generated so that it has at least two bits per pixel, said at least two bits indicating for each pixel whether the pixel is one of the following: a non-imaging pixel, an imaging pixel to be imaged with a first imaging setting, an imaging pixel to be imaged with a second imaging setting different from the first imaging setting, optionally an imaging pixel to be imaged with a third imaging setting different from the first and second imaging setting, wherein the imaging is done based on the modified raster image file. The imaging setting may indicate for example the dimension D or the shape of the spot to be imaged. Using such modified raster image files makes it possible to use different imaging settings in different parts of the image in a convenient manner.

Figure 7A, 7B and 7C illustrate printing results on corrugated board when using respectively a method of the prior art, a method according to Figure 1 (but with Rl = 846 dpi) and a method according to Figure 3, using the same anilox roller having a line count of 600 lines per inch, or 236 lines per centimeter. In the prior art embodiment of Figure 7A, a rather high-resolution image (R1 = 2540 dpi) was used, and the unevenness of the corrugated board can be seen through the printed layer. Indeed, the wavy structure can be seen through the printed layer. This effect is called fluting. The printing results shown in Figures 7B and 7C are clearly improved and show no or reduced fluting. When using a relatively low first resolution R1 instead of the high resolution used in the prior art, in combination with a spot size resulting in a fine surface structure of hills on the dots, improved printing results can be obtained. This is believed to be because the imaged spots do not fully overlap and the corresponding fine hills on the relief plate are sufficiently large to be covered in such way by ink from the anilox roller that an uneven structure of the substrate can be effectively hidden, at least to an appropriate extent. In that manner more ink is transferred to the rough substrate covering the unevenness. Thus, the image quality can be improved when printing grey scales, and in particular when printing grey scale images on a substrate with an uneven surface, such as corrugated board or wallpaper.

It is further noted that by starting from a low-resolution raster image file with resolution R1 and next increasing the resolution to a resolution R2, interference/Moire problems can be reduced or avoided as compared to a case where one would start from a high-resolution raster image file having a resolution R2. This is illustrated in Figures 12A-C. Figure 12A shows the result of starting with a high resolution file and imaging the pixels using a spot dimension D of 14 micron. This will avoid Moire problems but will suffer from the fluting effect, cf. Figure 7A. Figure 12B shows the result of starting with a high-resolution file, reducing the resolution thereof, and then using a spot dimension D of 14 micron. This will reduce the fluting problems but suffers from Moire problems. Figure 12C illustrates an embodiment of the invention where one starts with a low-resolution raster image file, having a resolution R1 below 2000 dpi, here 800 dpi. This implies that a non-raster image file, such as a pdf or ps file, is raster image processed directly into a raster image file having a resolution Rl. Next, the pixels are imaged using a spot dimension D of 14 micron. This will reduce both Moire problems as well as fluting effect problems, see also Figures 7B and 7C.

Figures 5A-5E show further examples of how the shape and/or size of the image spots may be controlled. Figure 5 A corresponds with what is shown in Figures 1 , 3 and 4, where the diameter D of the spots is well below Pl. In the example of Figure 5B, D is equal to Pl so that the imaged spots touch each other, and in the example of Figure 5C, D is larger than Pl so that the imaged spots overlap. In the example of Figure 5D, the imaging was done such that the dimensions and/or shape of the imaged spots are changed according to a regular or irregular pattern. Further, as is shown in Figure 5E, the position of the imaged spots may also be changed with respect to one another. All pixels of a row and/or column may be aligned as in Figures 5A-2C or some pixels may be shifted away from a central position, as in Figure 5E.

Preferably, the imaging is done such that all imaged spots have substantially the same dimensions and/or shape; or such that the dimensions and/or shape of the imaged spots are changed according to a regular or irregular pattern. Also, the dimension and/or shape may be changed in function of the greyscale value.

Preferably, the imaging comprises ablating the mask layer, preferably with a laser. Prior to the imaging or after the imaging, the mask layer may be arranged, typically fixed, on a photopolymerizable layer of a relief precursor. The step of preparing the relief plate using the imaged mask layer typically comprises exposing the photopolymerizable layer of the relief precursor through the mask layer and developing the relief precursor to obtain the relief plate.

Figure 8 illustrates a system to produce a relief printing plate or sleeve from a relief precursor. The system comprises a control module 100, an imager 110, an exposure means 120 and a developing means 130. After the mask layer on the relief precursor is imaged by the imager 110 using a raster image file and/or imaging instructions generated by the control module 100, the precursor is exposed to electromagnetic radiation in the exposure means 120, through the imaged mask layer so that a portion of the photosensitive layer 16 is cured. The electromagnetic radiation may have a wavelength in the range of 200 to 2000 nm, preferably it is ultraviolet (UV) radiation with a wavelength in the range of 200 to 450 nm.

The electromagnetic radiation changes the properties of the exposed parts of the photosensitive layer such that in the following developing means non-exposed portions of the photosensitive layer are removed by the developing means 130 and a relief printing plate or sleeve is formed. Preferably, the developing is achieved by treatment with liquids (solvents, water or aqueous solutions) or by thermal development, wherein the liquefied or softened material is removed.

Treatment with liquids may be performed by spraying the liquid onto the precursor, brushing or scrubbing the precursor in the presence of liquid. The nature of the liquid used is guided by the nature of the precursor employed. If the layer to be removed is soluble, emulsifiable or dispersible in water or aqueous solutions, water or aqueous solutions might be used. If the layer is soluble, emulsifiable or dispersible in organic solvents or mixtures, organic solvents or mixtures may be used. For thermal development, a thermal development means, wherein the flexible plate is fixed on the rotating drum, may be used. The thermal developing means further comprises assemblies for heating the at least one additional layer and also assemblies for contacting an outer surface of the heated, at least one additional layer with an absorbent material for absorbing material in a molten state. The assemblies for heating may comprise a heatable underlay for the flexible plate and/or IR lamps disposed above the at least one additional layer. The absorbent material may be pressed against the surface of the at least one additional layer by means, for example, of an optionally heatable roll. The absorbent material may be continuously moved over the surface of the flexible plate while the drum is rotating with repeatedly removal of material of the at least one additional layer. In this way molten material is removed whereas non-molten areas remain and form a relief.

The relief printing plate or sleeve may be treated further and may finally be used as a printing plate. Optionally, the system may further comprise a light finisher or any other post-exposure unit. Optionally, a controller may be provided to control the various units of the imaging system. Optionally, one or more pre-processing modules, such as a raster image processing (RIP) module which converts an image file, such as a pdf file, into a raster image process file, may be provided upstream of the control module 100, see also Figure 9 which is discussed below.

Figure 9 illustrates an exemplary embodiment of a control module 100 which is arranged downstream of a raster image processing module 90. The raster image processing (RIP) module 90 converts a source image file, such as a pdf or ps or xps file, into a halftone raster image file (also called bitmap) with a first resolution Rl. Raster image processing is the process of turning e.g. vector digital information such as a PostScript file into a high-resolution raster image file. Usually, the RIP module 90 is implemented either as a software component of an operating system or as a firmware program executed on a microprocessor. The RIP module 90 may further have a layout function. When a plurality of small images needs to be printed, those images may be grouped according to print patterns. This grouping may also be done by the RIP module 90.

Figure 9 shows an embodiment of a control module 100 which may be used in embodiments according to the first aspect, e.g. in combination with the embodiment of Figures 1 , 3 and 4. The control module 100 is configured for receiving the halftone raster image file comprising imaging pixels and non-imaging pixels at a first resolution Rl; optionally for applying a sampling pattern on the halftone raster image file; and for controlling the imager 110 such that imaged spots have a largest dimension smaller than (A/2 * Pl). Figure 10 shows an alternative embodiment of a control module 100 which may be used in embodiments according to second aspect, e.g. in combination with the embodiment of Figure 6. The image source file is raster image processed in RIP 90 to generate a halftone raster image file having a first resolution Rl, e.g. 800 dpi. In the example it is assumed that grey scale areas have to be printed. The control module 100 is configured to increase the resolution to a second resolution R2 and to superimpose a sampling pattern on the increased resolution raster image file. The resulting raster image file is then sent to the imager 110 in order to perform the imaging accordingly.

Figure 11 illustrates imaged spots resulting from an exemplary embodiment of a method for imaging a mask layer. For illustrative purpose some portions of halftone areas (10%, 30% and 70%) are shown, but the skilled person understand that any combination of halftone areas may be present. As illustrated the diameter dl, d2, d3 of the imaged spots 41 may be different depending on the size of the dots of the respective halftone area 34.

For example, for small tonal values, e.g. between 0 and 10%, a first diameter dl may be used so that touching or overlapping imaged spots 41 are obtained, see the top image of Figure 11. For larger tonal values, e.g. between 10 and 50%, a second diameter d2 may be used which is smaller than dl so that the imaged spots 41 are not overlapping, and for even larger tonal values, e.g. between 50 and 99%, an even smaller diameter d3<d2 may be used, see the middle and lower image of Figure 11. Alternatively, a sampling pattern could be used for those even larger tonal values in combination with a larger diameter.

Embodiments of the invention are especially useful for classic amplitude modulated (AM) screens, where the distance Dd between adjacent dots of a halftone area is the same for halftone areas having different tonal values. The tonal value of the halftone area is then determined by the size of a group of clustered imaging pixels corresponding with clustered imaged spots 41 (i.e. the size of a dot). However, the skilled person understands that other embodiments of the invention may be used for frequency modulated (FM) screens or AM and FM screens, where the distance Dd is not constant.

A person of skill in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Herein, some embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. The embodiments are also intended to cover computers programmed to perform said steps of the above-described methods.

Whilst the principles of the invention have been set out above in connection with specific embodiments, it is to be understood that this description is merely made by way of example and not as a limitation of the scope of protection which is determined by the appended claims.

Claims

1. A method for preparing a relief plate and printing with said relief plate on a substrate, and in particular an uneven substrate such as corrugated board, comprising the steps:

- providing a mask layer;

- obtaining a halftone raster image file comprising imaging pixels and non-imaging pixels at a first resolution Rl; wherein a first pitch Pl corresponds to the first resolution Rl;

- imaging the mask layer in accordance with the halftone raster image file and such that imaged spots have a largest dimension (D) smaller than (A/2 * Pl), preferably smaller than Pl ;

- preparing a relief plate using the imaged mask layer;

- transferring ink to the relief plate using an anilox roller; and transferring said ink from the relief plate onto a substrate;

- wherein the first resolution Rl is smaller than 2000 dpi, preferably smaller than 1700 dpi, more preferably smaller than 1500 dpi;

- wherein the anilox roller has a volume density which is larger than or equal to 5 cm³/m², preferably larger than or equal to 7 cm³/m².

2. The method of claim 1, wherein the anilox roller has a line count which is smaller than 300 lines per centimeter, preferably smaller than 200 lines per centimeter.

3. The method of claim 2, wherein the line count corresponds with an anilox pitch Pa, and wherein, during the step of obtaining, the first resolution Rl is chosen so that Pa is larger than 1,5*P1, more preferably between 2*P1 and 10*Pl.

4. The method of any one of the previous claims, wherein a sampling pattern is superimposed on the halftone raster image file prior to or during the imaging of the mask layer.

5. The method of claim 4, wherein the sampling pattern is a repetition of a block in which one or more imaging pixels are combined with one or more non-imaging pixels.

6. The method of claim 4 or 5, wherein the sampling pattern is any one of the following or a combination thereof: a single pixel pattern, such as a single pixel checkerboard pattern, a pattern for which each imaging pixel is surrounded by eight non-imaging pixels; a multiple pixel pattern, such as a multiple pixel checkerboard pattern; a line pattern; a dash pattern; a circle pattern; a grid pattern.

7. The method of any one of the previous claims, wherein, prior to imaging and based on the halftone raster image file, a modified raster image file is generated so that it has at least two bits per pixel, said at least two bits indicating for each pixel whether the pixel is one of the following: a non-imaging pixel, an imaging pixel to be imaged with a first imaging setting, an imaging pixel to be imaged with a second imaging setting different from the first imaging setting, optionally an imaging pixel to be imaged with a third imaging setting different from the first and second imaging setting, wherein the imaging is done based on the modified raster image file.

8. The method of any one of the previous claims, wherein the anilox roller has a pattern of hexagonal cells.

9. The method of any one of the previous claims, wherein the first resolution R1 is between 700 and 1500 dpi.

10. A method for imaging a mask layer for obtaining a relief plate for printing on a substrate, comprising the steps:

- providing of a mask layer;

- obtaining a halftone raster image file comprising imaging pixels and non-imaging pixels at a first resolution Rl;

- increasing the resolution of the halftone raster image file to obtain an increased- resolution raster image file comprising increased-resolution imaging pixels and increased-resolution non-imaging pixels at a second resolution R2 higher than the first resolution Rl;

- superimposing a sampling pattern having a repetition rate equal to the first resolution Rl on the increased-resolution raster image file so that only a portion of the increased-resolution imaging pixels is imaged;

- imaging the mask layer in accordance with the increased-resolution raster image file with the superimposed sampling pattern.

11. The method of claim 10, wherein the second resolution R2 is a multiple of the first resolution Rl.

12. The method of claim 10 or 11, wherein the second resolution R2 is at least four times higher than the first resolution Rl.

13. The method of any one of the claims 10-12, wherein a second pitch P2 corresponds to the second resolution R2, and wherein the imaging of the mask layer in accordance with the increased-resolution halftone raster image file is performed such that imaged spots have a largest dimension (D) larger than P2, more preferably larger than (A/2 * P2).

14. The method of any one of the claims 10-13, wherein the sampling pattern is a repetition of a block in which one or more imaging pixels are combined with one or more non-imaging pixels.

15. The method of any one of the claims 10-14, wherein R2 is at least three times higher than Rl, and wherein the sampling pattern is formed by a repetition of a block of one or more adjacent imaging pixels surrounded by at least eight non-imaging pixels.

16. The method of any one of the claims 10-15, wherein, prior to imaging and based on the increased-resolution raster image file with the superimposed sampling pattern, a modified increased-resolution raster image file is generated so that it has at least two bits per pixel, said at least two bits indicating for each pixel whether the pixel is one of the following: a non-imaging pixel, an imaging pixel to be imaged with a first imaging setting, an imaging pixel to be imaged with a second imaging setting different from the first imaging setting, optionally an imaging pixel to be imaged with a third imaging setting different from the first and second imaging setting, wherein the imaging is done based on the modified raster image file.

17. The method of any one of the claims 10-16, further comprising preparing a relief plate using the imaged mask layer; transferring ink to the relief plate using an anilox roller having a resolution Ra; and transferring said ink from the relief plate onto a substrate.

18. The method of claim 17, wherein the anilox roller has a volume density which is larger than or equal to 5 cm3/m2, preferably larger than or equal to 7 cm3/m2.

19. The method of claim 17 or 18, wherein the anilox roller has a line count which is smaller than 300 lines per centimeter, preferably smaller than 200 lines per centimeter.

20. The method of claim 19, wherein the line count corresponds with an anilox pitch Pa, and wherein, during the step of obtaining, the first resolution R1 is chosen so that Pa is larger than 1,5*P1, more preferably between 2*P1 and 10*Pl.

21. The method of any one of the previous claims, wherein the obtaining of a halftone raster image file is the result of raster image processing a non-raster image file, such as a pdf or a ps file, in order to obtain directly the half-tone raster image file having the first resolution Rl.

22. The method of any one of the previous claims, wherein the imaging is done such that all imaged spots have substantially the same dimensions and/or shape; or such that the dimensions and/or shape of the imaged spots are changed according to a regular or irregular pattern.

23. The method of any one of the previous claims, wherein the imaging comprises ablating the mask layer, preferably with a laser.

24. The method of any one of the previous claims, further comprising, prior to the imaging or after the imaging, providing the mask layer on a photopolymerizable layer of a relief precursor.

25. The method of the previous claim, wherein the step of preparing the relief plate using the imaged mask layer comprises exposing the photopolymerizable layer of the relief precursor through the mask layer and developing the relief precursor to obtain the relief plate.

26. A relief plate obtained by the method of any one of the previous claims.

27. A computer program or computer program product or digital storage means comprising computer-executable instructions to control the method of any one of the claims 1-25, when the program is run on a computer.

28. A system for treating a relief precursor, comprising an imager configured to image a mask layer, an exposure means configured to expose the relief precursor through the imaged mask layer, a developing means configured to remove at least a part of non-exposed material from the relief precursor, and a control module to control the imager in accordance with the method of any one of the claims 1-25.

29. The system of claim 28, further comprising an anilox roller having a volume density which is larger than or equal to 5 cm3/m2, preferably larger than or equal to 7 cm3/m2, and/or having a line count which is smaller than 300 lines per centimeter, preferably smaller than 200 lines per centimeter.