EP1476307A1

EP1476307A1 - Printing device and method, in which the thickness of the humidifying agent layer is measured and reduced

Info

Publication number: EP1476307A1
Application number: EP03708105A
Authority: EP
Inventors: Manfred Wiedemer
Original assignee: Oce Printing Systems GmbH and Co KG
Current assignee: Canon Production Printing Germany GmbH and Co KG
Priority date: 2002-02-19
Filing date: 2003-02-17
Publication date: 2004-11-17
Also published as: DE10206944A1; WO2003070466A1

Abstract

The invention relates to a method and a device for creating a printed image on a carrier material (40). According to said method, the surface of a pressure bearer (10) is coated with a layer of humidifying agent and ink-receptive areas and ink-repellent areas are created in a structuring process, in accordance with the structure of the image to be printed. The layer thickness of the humidifying agent layer is determined and depending on the layer thickness that has been determined, an energy source (94) for reducing the quantity of humidifying agent is controlled in order to reduce the thickness of the humidifying agent layer to a target thickness.

Description

Method and device for printing, the thickness of the dampening solution layer being measured and reduced

The invention relates to a method and a device for producing a print image on a carrier material, in which ink-attracting and ink-repellent areas are produced on the surface of the print carrier in accordance with the structure of the print image to be printed, the ^• ink-repellent areas having a layer of an ink-repellent Medium are provided, on the surface of the printing medium, ink is applied, which adheres to the ink-attracting areas and which is not accepted by the ink-repellent areas, and in which the color distributed on the surface is printed on the substrate.

In the prior art, waterless offset printing processes are known, the non-printing areas of which are fat-repellent and therefore do not accept any printing ink. The printing areas, on the other hand, are fat-absorbing and absorb the greasy ink. Depending on the structure of the printed image to be printed, ink-attracting and ink-repelling areas are distributed on the printing plate. The printing plate can be used for a variety of transfer printing processes. A new plate with ink-accepting and ink-repellent areas must be created for each print image.

From US-A-5, 379, 698 a method is known, which is called the direct aging method, in which in the printing device on a multilayer, silicone-coated film, by selectively burning away the silicone cover layer

Print template is created. The silicone-free areas are the ink-attracting areas that accept printing ink during the printing process. A new film is required for each new print image.

In the standard offset process, which works with water, hydrophobic and hydro- phile areas generated according to the structure of the printed image to be printed. Before the ink is applied, a thin film of moisture is applied to the print carrier using application rollers or spray devices, which wets the hydrophilic area of the print carrier. The ink roller then transfers ink to the surface of the print carrier, which, however, only wets the areas not covered by the moisture film. After coloring, the color is finally transferred to the carrier material.

In the known offset printing process, multilayer processless thermal printing plates can be used as the printing medium, cf. e.g. WO00 / 16,988th In accordance with the structures of the print image to be printed, a hydrophobic layer on the surface of the print carrier is removed by partial burning away and a hydrophilic layer is exposed. The hydrophilic layer can be wetted with an ink-repellent dampening solution. The hydrophobic areas are ink-accepting and can absorb ink during the printing process. A new printing plate must be used to create a new print image.

Furthermore, a method is known from ÜS-A-6, 016, 750, in which a color-attracting substance is deposited from a film by means of a thermal transfer process, transferred to the hydrophilic surface of the print carrier and solidified in a fixing process. In the printing process, the remaining hydrophilic areas are wetted with ink-repellent dampening solution. The ink is then applied to the surface of the print carrier, which, however, only adheres to the areas provided with the ink-attracting substance. The colored print image is then transferred to the carrier material. To create a new print image, a new film with the color-attracting substance is necessary. In the standard offset printing or planographic printing method, the wetting of the printing plate is achieved with the ink-repellent fountain solution by a specific roughening and patterning of the plate surface ^'. The resulting increase in surface area and porosity creates microcapillaries and leads to an increase in the effective surface energy and thus to a good wetting or spreading of the dampening solution. As a further measure, wetting-promoting substances are added to the dampening solution in offset printing. These lower the surface tension of the dampening solution, which also leads to an improved wetting of the surface of the print carrier. In this context, reference is made to the literature Teschner, H.: Offsettechnik, 5th edition, Fellbach, Fachschriften-Verlag 1983, pp. 193 - 202 and p. 350.

From US-A-5, 067, 404 a printing method is known in which a fountain solution is applied to the surface of the printing format. The dampening solution is vaporized by selective application of radiation energy in the image areas. The water-free areas later form the ink-bearing areas, which are guided past a development unit and are colored by means of a color steam. Energy-intensive partial evaporation processes are required to generate the structured dampening solution film.

Reference is also made to the patent documents WO 97/36746 and WO 98/32608. In the method described in WO 97/36746, the dampening agent is generated by evaporating a discrete volume of water that condenses on the surface of the pressure medium. According to WO 98/32608 and the US-A-6, 295, 928 resulting therefrom, a continuous ice film is applied and structured. In both cases, locally high thermal energy must be used for structuring. The aforementioned documents ÜS-A-5, 067, 404, WO

98/32608 (US-A-6,295,928) and WO 97/36746 of the same application rin are hereby incorporated by reference into the rich Offenbarungsbe- ^'of the present patent application.

DE-A-10132204 (not previously published) by the same applicant describes a CTP process (computer-to-press process), it being possible for multiple structuring processes to be carried out on the same surface of the print carrier. The surface of a print carrier is coated with an ink-repellent or ink-attracting layer. In a structuring process, ink-attracting areas and ink-repellent areas are created in accordance with the structure of the printed image to be printed. The color-attracting areas are then colored with color. Before a new structuring process, the surface of the print carrier is cleaned and again with an ink-repelling or ink-accepting

Layer coated. A dampening solution layer or an ice layer is used as the layer. This patent document DE-A-10 132 204 is hereby incorporated by reference into the disclosure content of the present patent application.

It is an object of the invention to provide a method and a device which ensures a high-quality printing result with reduced structuring effort.

This object is achieved for a method by the features of claim 1.

According to the invention, a dampening solution layer is applied to the surface of the print carrier. The layer thickness of the dampening solution layer is measured using a layer thickness measuring device. Depending on the determined layer thickness, an energy source is controlled so that the thickness of the dampening solution layer is reduced to a target thickness. The invention ensures a consistently thick dampening solution layer, with the aim being a relatively thin layer. A thin layer thickness, for example in the range of 1 .mu.m allows the ink-repellent areas and ink-attracting areas in the dampening solution layer to be structured with a relatively low selective thermal energy. Accordingly, the hardware expenditure for the structuring is reduced and a constant printing quality is guaranteed.

According to a further aspect of the invention, a device is specified by means of which the described method can be implemented. Advantageous exemplary embodiments are defined in the dependent claims relating to the method and the device.

It should be noted that the term ink-repellent or ink-absorbing layer often occurs in the further description. This layer is adapted to the color to be applied. For example, in the case of a water-containing dampening solution layer and an oil-containing ink, the dampening solution layer is ink-repellent. However, if the ink contains water, this dampening solution layer is ink-absorbing. In practice, predominantly oil-based inks are used, so that a water-containing dampening solution layer is ink-repellent.

Embodiments of the invention are explained below with reference to the drawing. It shows:

FIG. 1 shows a basic illustration of a printing device in which a surfactant layer is applied,

Figure 2 schematically shows a cross section through the

Print carrier before and after structuring by a laser beam,

FIG. 3 shows an exemplary embodiment in which a hydrophilized layer is structured, Figure 4 shows an embodiment in which a ^'coated hydrophilic layer is patterned,

Figure 5 shows a schematic cross section through the

Print carrier before and after structuring the hydrophilic layer,

FIG. 6 shows an exemplary embodiment in which the hydrophilization takes place by means of a corona discharge,

Figure 7 shows a cross section through an isolated

Electrode,

FIG. 8 shows an arrangement with a plastic

Printing support,

FIG. 9 shows an example of an indirect corona discharge, and

Figure 10 shows a printing device with a control of the dampening solution layer thickness.

1 shows a basic illustration of a printing device which is constructed similarly to that described in US Pat. No. 5,067,404 by the same applicant. A print carrier 10, in the present case an endless belt, is passed through a pretreatment device 12, the one

Bucket roller 14 and an application roller 16 contains. The scoop roller 14 is immersed in a liquid contained in a container 13 which contains a wetting-requiring substance. This substance, which contains surfactant, is applied in a molecular layer thickness to the surface of the print carrier 10 via the application roller 16. The layer thickness is typically less than 0.1 μm. The surface of the print Carrier 10 is then guided in the direction of arrow Pl to a dampening unit 18 which, via a scoop roller 20 and an application roller 22, applies an ink-repellent or ink-absorbing fountain solution, for example water, from a fountain solution reservoir 24 to the surface of the print carrier 10.

In principle, dampening solutions other than water can also be used. The dampening solution layer can also be applied by other methods, for example by steaming or spraying. The pressure-active surface of the print carrier 10 is completely provided with this dampening solution layer. The dampening solution layer typically has a layer thickness of less than 1 μm.

The generally ink-repellent dampening solution layer is then structured by an image forming device 26. In the present case, laser radiation 28 is used for this. In this structuring process, ink-attracting areas and ink-repellent areas are created in accordance with the structure of the printed image to be printed. The structured dampening solution layer then arrives at an inking unit 30, which uses the rollers 32, 34, 36 to transfer ink from a storage container 38 onto the surface of the print carrier 10. The oil-based ink adheres to areas without water-based dampening solution. It is pointed out that the ink can also be transferred to the surface of the print carrier 10 by spraying, knife coating or condensing.

When the print carrier 10 is transported further, transfer printing takes place onto a carrier material 40, generally a paper web. For transfer printing, the carrier material 40 is passed between two rollers 42, 44. In the transfer printing process, a rubber blanket cylinder (not shown) and further intermediate cylinders can be connected between the roller 42 and the printing medium 10, which cause ink splitting, as is known per se from the field of offset printing processes. When the print carrier 10 is transported further, the surface of the print carrier 10 is cleaned in a cleaning station 46. The color residues and the residues of the surfactant layer are removed. The cleaning station 46 contains a brush 48 and a wiper lip 50 which are brought into contact with the surface of the print carrier 10. Cleaning can also be supported by using ultrasound, high-pressure liquid and / or steam. Cleaning can also be carried out using cleaning fluids and / or solvents.

A new application of the wetting-promoting substance, e.g. a surfactant application, a dampening solution application and a new structuring take place. In this way, a new print image can be printed each time the print carrier 10 rotates. However, it is also possible to print the same print image several times. The cleaning device 46, the device 12 and the device 26 are then switched to inactive. The print image that is still present in color residues is then recolored and re-printed by the inking unit 30. In this operating mode, a large number of identical print images can be printed.

FIG. 2 schematically shows a cross section through the print carrier 10 before and after the structuring with the aid of the laser beam 28. According to the invention, the wetting is promoted by applying a wetting-promoting substance to the print carrier surface 10. This happens within the printing cycle before the ink-repellent fountain solution is applied. Due to its physical and chemical properties, the wetting-requiring substance can be applied to the surface as an extremely thin layer of a few molecular layers, preferably less than 0.1 μm. This layer is sufficient to promote wetting with the ink-repellent dampening solution on its free surface, so that it can also be applied as a very thin layer 54, preferably less than 1 μm. The further one The printing process is not affected by the small amount of the wetting-promoting substance, in this case a surfactant layer 52. It can be easily removed by the cleaning process integrated in the printing cycle.

Advantages arise above all in the area of digital planographic printing or offset printing, ie a planographic printing process or offset printing process with changing printing information from printing cycle to printing cycle. The wetting-requiring layer 52 makes it possible to dispense with the otherwise usual roughened, porous printing plate surface. Instead, a smooth surface of the print carrier 10 is possible, which can be cleaned with significantly less effort. A fast and stable cleaning process is essential for such a digital flat printing process or offset printing process and is a decisive factor for its effectiveness. Accordingly, the surface of the print carrier 10 has a roughness that is smaller than the roughness used in the standard offset printing method. The average roughness depth R ₂ is typically less than 10 μm, preferably less than 5 μm. Expressed as the mean roughness value R _a , the roughness value is in the range less than 2 μm, preferably less than 1 μm.

A change in the molecular or atomic structure of the material of the print carrier as well as a permanent wetting-promoting layer firmly anchored to the surface of the print carrier is not necessary. The additionally applied wetting-requiring substance proposed here, for example the surfactant layer 52, develops its wetting-required action even in the smallest amounts. Accordingly, their influence on the properties of the print carrier 10 is negligible in many ways. Another advantage results from the fact that it is now possible to dispense with the wetting-promoting additives that are usually present in offset printing in fountain solutions. According to FIG. 2, the dampening solution layer 54 and the surfactant layer 52 are removed by the laser beam 28 in accordance with the required image structure. These areas are then colored with ink by the inking unit 30. Because of the very smooth surface of the print carrier 10, cleaning is made easier, the surfactant layer 52 being completely removed again. Furthermore, the wear on the surface of the print carrier 10 is reduced.

In the following figures, functionally identical elements are identified in the same way. Figures 3, 4 and 5 show a further embodiment of the invention. In FIG. 3, in contrast to the exemplary embodiment according to FIG. 1, a hydrophilic layer with a molecular layer thickness is structured before the ink-repellent or ink-attracting layer is applied to the usable surface of the print carrier. In the present example, a steam device 60 is used, which acts on the surface of the pressure carrier 10 with hot steam. The surface of the print carrier 10 is provided with an SiO 2 coating. After

Steam treatment, the pressure carrier 10 is dried by a suction device 62. The hot water vapor creates a hydrophilic molecular structure on the outer surface, e.g. SiOH.

After the subsequent structuring by the structuring device 26 by means of laser radiation 28, hydrophilic areas and hydrophobic areas are created in accordance with the structure of the printed image to be printed. Through the downstream dampening unit 18, the entire usable surface of the printing medium 10 is brought into contact with a dampening solution layer, the dampening solution accumulating only on the hydrophilic areas, so that ink-attracting areas and ink-repellent areas are formed in accordance with the structuring carried out. The inking unit 30 then applies the ink, the oil-containing ink accumulating in areas without water-containing dampening solution. Then this is followed by the transfer of the printed image onto the carrier material ^■ 40.

After the print carrier 10 has been transported further, its surface is cleaned in a cleaning station 46. The paint residues as well as the residues of any wetting-promoting substance are removed. A new structuring process can then take place.

In the present example according to FIG. 3, the hydrophilic layer is structured on the surface of the print carrier 10 in accordance with the print image. The hydrophilic layer is extremely thin and is only a few nanometers, typically less than 4 nm. It can therefore be structured with very little energy expenditure during a printing cycle, the hydrophilic molecular layer disappearing. The dampening solution is then applied, which creates a moisture film only on the non-hydrophilic areas. Inking and transfer printing takes place according to the known principles of planographic printing or offset printing described. After cleaning, in which, in addition to the color residues, the hydrophilic layer can also be removed, but does not necessarily have to be removed, the printing cycle can begin again. ^• The hydrophilic layer is regenerated or reapplied and then the hydrophilic layer is structured according to the new image data.

In the example according to FIG. 3, the hydrophilic layer is generated by activating the surface of the print carrier and by a suitable change in the outer molecular

Surface structure. For example, this can be made possible by using chemical activators, reactive gases and / or a suitable energy supply. In addition to the use of water vapor as in the example according to FIG. 3, a hydrophilic SiOH structure can also be formed on the surface by the action of hot water and by bases, such as NaOH. For this purpose, the print carrier is coated with an Si02 Layering. It is also possible for the print carrier to pass through an activator bath in order to produce a hydrophilization of the surface. It is also possible to apply an activator via a nozzle system. Another possibility is to generate the hydrophilic layer by flaming the surface of the print carrier 10. Here too, wetting-promoting surface structures are created in a molecular layer thickness.

An advantageous arrangement is the combination of hydrophilization with cleaning. For example, Both the cleaning and the hydrophilizing effect of a hot water jet or a hot water vapor jet can be used. The cleaning and the creation of the hydrophilic layer are then carried out in a single process step.

Another variant is shown in FIG. To create the hydrophilic layer, a wetting-promoting substance is applied to the surface of the print carrier. For example, the pretreatment device 12 described in the embodiment according to FIG. 1 can be used. With the help of the scoop roller 14 and the application roller 16, a liquid can be applied from the container 13 which contains a wetting-promoting substance, e.g. contains a surfactant, are applied in a molecular layer thickness. Here too, the layer thickness is typically less than 0.1 μm. Alcohols can also be considered as a further wetting-promoting substance. The application can alternatively be done by scraping, spraying and vapor deposition.

Due to the very thin hydrophilic layer with a molecular layer thickness, this hydrophilic layer can be partially removed by local thermal energy supply. Due to the small layer thickness, the energy consumption can be low. In addition to the load used in FIGS. Laser radiation 28, laser diodes, LEDs, LED combs or heating elements can also be used.

In the example according to FIGS. 3 and 4, a new structuring can take place per revolution of the print carrier 10, as a result of which a new print image is printed per revolution. However, it is also possible, as in the example according to FIG. 1, to print the same print image a number of times, the existing print image being inked and re-printed by the inking unit 30. The devices for the restructuring are then switched to inactive.

FIG. 5 shows a cross section through the print carrier 10 before and after the structuring by the laser beam 28 for the example according to FIG. 4. The surface of the print carrier 10 is very smooth, as is the case with the previous examples. The thin surfactant layer 52 is structured by the laser beam 28, i.e. hydrophilic areas 68 and hydrophobic areas 64 are generated. The dampening unit 18 applies a thin, water-containing moist film only to the hydrophilic areas. The areas 64 are then colored by the inking unit 30 with an oil-containing ink which is repelled by the dampening solution 54 in the area of the hydrophilic areas 68.

The following exemplary embodiments according to FIGS. 6 to 9 describe the hydrophilization of the surface of the print carrier 10 by exposure to free ions. These exemplary embodiments can also be combined with the example according to FIG. 3.

In order to ensure good wetting with the generally ink-repellent dampening solution film, the surface energy of the printing medium 10 must be at least as high as the surface tension of the dampening solution film. This means that the value of the contact angle between the surface of the printing medium 10 and the dampening solution has a value below 90 °. must take. In practice it is necessary that a contact angle of <25 ° has to be achieved in order to produce the required liquid film with a thickness of approx. 1 μm. This places high demands on the surface energy of the print carrier, which, especially when considering the extremely high surface tension value of water, namely 72 mN / M, as the basis of the ink-repellent fountain solution. Plastic print media or metallic print media cannot do this without further measures, such as roughening, applying surfactants, generating microcapillaries etc. For example, the contact angle from water to polyimide or polycarbonate is approximately 75 °. Even metal surfaces, which in their purest form have very high surface energies and therefore the smallest contact angles, show relatively hydrophobic behavior under normal ambient conditions. This is essentially due to the oxidation layer effective on metal surfaces, which always forms under normal conditions. Even the slightest contamination has a negative impact on the desired surface energy. Contact angles of over 70 ° can often be found in practice.

In the example according to FIG. 6, a corona treatment of the surface of the print carrier 10 is carried out for hydrophilization. A high voltage generator 70 generates an alternating voltage in the range of 0 to 30 kV, preferably in the range of 15 to 20 kV, at a frequency of 10 to 40 kHz, preferably in the range of 15 to 25 kHz. An output terminal of the high voltage generator 70 is connected to an insulated electrode 72. In the present case of a metallic pressure carrier 10, the other output connection is placed on a sliding contact 74 which is connected to the pressure carrier 10.

The relatively high voltage on the electrode 72 leads to the ionization of the air. A corona discharge arises, the surface of the printing medium 10 being bombarded with free ions becomes. In the case of a plastic surface, this leads not only to a cleaning effect, which typically removes organic contaminants such as grease, oil, wax, etc., but also to the formation of free radicals on the surface, which form highly hydrophilic functional groups in connection with oxygen. These are mainly carbonyl groups (- C = 0-), carboxyl groups (HOOC-), hydroperoxide groups (H00-) and hydroxyl groups (HO-). In the case of metallic printing media, the focus is on the cleaning effect, whereby degreasing the surface and removing the oxide layer increases the surface energy and thus reactivates the hydrophilic properties of metals. In this way, contact angles to water of up to less than 20 ° can be achieved with plastic surfaces and with metal surfaces. The corona treatment changes the physical surface properties of the support beforehand, but not its mechanical properties. There are no visible changes, for example with a scanning electron microscope. By varying the level of the voltage or the frequency of the high-voltage generator, the effect on the surface of the print carrier 10 can be influenced and matched to the respective carrier material. The hydrophilization can be improved by adding process gases, preferably oxygen or nitrogen.

In FIG. 6, as in the example according to FIG. 1, a dampening solution is applied to the hydrophilized surface of the pressure carrier 10 in the dampening unit 18; This is followed by structuring with the aid of laser radiation 28. The structured dampening solution layer is colored by the inking unit 30 and the color is later printed onto the carrier material 40. Color residues are removed in the cleaning station 46. Since the surface of the print carrier 10 is also very smooth, as in the previous examples, the cleaning process can be carried out easily and with high effectiveness. The cyclical printing process can then start again. Alternatively, a restructuring can also be omitted. len and the previous print image is colored again and ^• reprinted.

FIG. 7 shows the insulated electrode 72. A metallic core 76 is surrounded by a ceramic jacket 78. With such a structure, electrical flashovers are prevented. This is particularly advantageous if metal is used as the printing medium 10. Alternatively, the insulation can also be created by a plastic jacket.

Figure 8 shows the structure of a printing medium 10 made of plastic. An electrode plate 80 is arranged on the side of the print carrier 10 which lies opposite the electrode 72. The electrode 72 can be designed without insulation.

FIG. 9 shows a hydrophilization process with an indirect corona treatment. The output connections of the high-voltage generator 70 are connected to two electrodes 82, 84, which are arranged above the pressure carrier 10. The electrical discharges generated by the high voltage between the two electrodes 82, 84 generate ions which are directed to the surface of the pressure carrier 10 by an air stream or process gas stream and which develop the wetting-promoting effect here. A blower 86 is used to generate the flow.

Alternatively, a low-pressure plasma treatment can also be used, which increases the surface energy on the surface of the print carrier 10. In this case, a high-voltage discharge is generated under vacuum conditions, for example in the range from 0.3 to 20 mbar, by which process gas is ionized and brought into the plasma state. This plasma comes into contact with the surface of the print carrier 10. The effect of the plasma can be compared with the effect of the corona treatment. With the aid of the hydrophilization process described in FIGS. 6 to 9, a considerable increase in the surface energy is achieved, which enables the ink-repellent fountain solution to be applied very thinly. The layer thickness is typically in the range of 1 μm.

Various advantages result from the described hydrophilization process. There is no need for the roughened porous printing plate surface as in the standard off-pressure printing process. Instead, a very smooth surface is possible, the roughness range of which is very low, for example in a range of the average roughness value R _a <1 μm. This enables a quick and stable cleaning process for the surface. Neither a permanent change in the molecular or atomic structure of the material of the print carrier nor a permanent wetting-promoting layer firmly anchored to the print carrier is necessary for the described printing process. The described hydrophilization process enables the print carrier to be optimized with regard to further requirements without taking the surface energy into account.

The described hydrophilization process also makes it possible to dispense with the wetting-promoting additives used in offset printing for fountain solutions. A further application of additional wetting-promoting substances is no longer necessary. This avoids a relatively complicated process control and reduces the additional expenditure on consumables. Another advantage is the cleaning effect of the hydrophilization process. It supports the cleaning process necessary for the digital printing process and thus further reduces the hardware expenditure required.

Figure 10 shows another embodiment. In offset printing and in particular in the digital processes, for example according to US-A-5, 067, 404 and US-A-6, 295, 928 by the same applicant, the constant and precisely defined plays Thickness of the dampening solution layer on the surface of the print carrier plays a crucial role for the stability and the efficiency of the printing process. According to the example according to FIG. 10, a printing device is described which permits and monitors a defined, controllable and controllable very thin application of the dampening solution. In the standardized offset printing process, a dampening unit consisting of a number of rotating rollers is generally used to apply the dampening solution. Together with a roughened or porous printing plate with good water content, the result is a water film that is sufficiently stable for standard offset printing. The amount of dampening solution and the thickness of the dampening solution layer can be adjusted, for example, by the delivery of certain rollers to one another or the speed of the scoop roller. The storage effect of the dampening system and also that of the pressure plate leads to a strongly delayed reaction to adjustment measures. However, the roughened, strongly water-storing pressure plates are absolutely necessary for the generation of a sufficiently stable water film. It is also known from the prior art to produce a very thin film of water by cooling the printing plate and the consequent condensation of the atmospheric moisture on the printing plate. However, the thickness of the water film is strongly dependent on the ambient conditions, such as air humidity and temperature, and can hardly be kept constant over a long period.

In the exemplary embodiment according to FIG. 10, a structure is used which is similar to the structure described in the aforementioned DE-A-101 32 204, which realizes a CTP process (computer-to-press process).

The printing device shown in FIG. 10 allows different printing images to be generated on the same surface of the cylindrical printing medium 10. The printing device contains the inking unit 30, with a plurality of rollers, through which the oil-containing ink is transferred from the reservoir 38 to the surface of the printing medium 10. The colored surface before the print carrier 10 transfers the ink to a rubber • cylinder 90. From there. the color on the paper web 40, which is pressed by the impression cylinder 42 against the blanket cylinder 90.

The dampening unit 18 transfers dampening solution, e.g. Water, from the fountain solution reservoir 24 onto the surface of the print carrier 10. Before the application of the fountain solution layer, the surface of the print carrier 10 can be brought into a more hydrophilic state using wetting agents and / or surfactants or by a corona and / or plasma treatment. as has already been described above. In the further course, the dampening solution layer is selectively removed by supplying energy by means of a laser beam 28 and the desired image structure is created. As mentioned, the inking 30 then takes place at the ink-attracting areas of the structuring. After structuring, the color can be solidified using a fixing device 92.

Two operating modes are also possible in this example. In a first operating mode, a large number of printing processes take place before the surface is structured again. The print image located on the print carrier 10 is inked and re-printed once per print, i.e. the printed image is colored several times. In a second operating mode, a new print image is applied to the surface of the print carrier. Before that, the previously structured ink-repellent layer and the paint residues must be removed, for which the cleaning station 46 is provided. This cleaning station can be pivoted towards the print carrier 10 according to arrow P2 and pivoted away from it again. Further details of the construction of the printing device according to FIG. 10 are described in the aforementioned DE-A-101 32 204.

Seen in the transport direction Pl, an energy source 94 is arranged after the dampening unit 18, the thermal energy to the Dampening film releases on the surface of the printing medium 10. This energy reduces the thickness of the dampening solution layer. Seen in the direction of transport, the energy source is followed by a layer thickness measuring device 96. This layer thickness measuring device 96 determines the current thickness of the dampening solution film and emits an electrical signal corresponding to the thickness to a control 98. The controller 98 compares the measured actual thickness with a predetermined target thickness. In the event of a deviation between the target and actual values, the energy source 94 is controlled in such a way that the thickness of the dampening solution layer is reduced to the desired target thickness.

The layer thickness measuring device 96 can, for example, operate in a contactless manner using the triangulation method, the transmission method or the capacitive method. One or more IR lamps, radiant heaters, laser systems, laser diodes or heating elements can be considered as energy source 94.

The interaction of the energy source 94, the layer thickness measuring device 96 and the control 98 can be such that only a monitoring function is carried out. If the layer thickness exceeds or falls below a predetermined target value, a corresponding warning signal is emitted and the energy supply for the energy source 94 is then reset. The energy source 94, the layer thickness measuring device 96 and the controller $ 8 can, however, also be combined to form a control loop in which the energy source 94 is controlled in such a way that, in the event of a control deviation between the actual value and the desired value of the layer thickness, this control deviation is minimized and preferably is regulated to zero.

The energy source 94 can be controlled by the control using an analog voltage control or digitally by pulse modulation, as is indicated by the signal sequence 100. According to the example according to FIG. 10, a thickness-constant dampening solution film is produced in a first process step over the usable width of the print carrier 10, and the layer thickness is reduced in a defined manner in a subsequent second step. The result is an even dampening solution layer with a defined and very small thickness. The subsequent structuring can thus be carried out with minimal energy and with a constant result. Overall, the print quality is increased. The advantages of the printing device shown are that an immediate reaction to a change in the layer thickness of the dampening solution layer can take place, that a known and defined thickness of the dampening solution layer can be set and that extremely thin dampening solution layers can be produced. Furthermore, the structuring energy required, in particular for digital printing processes, can be minimized.

Numerous further variations of the exemplary embodiments described above are possible. For example, both an endless belt and a cylinder can be used as the print carrier. The transfer printing onto the carrier material can take place directly or with the interposition of a rubber blanket cylinder or further intermediate cylinders for color splitting. The layer thickness control according to the example according to FIG. 10 can also be used for the other examples. Likewise, for the examples according to FIGS. 1 to 9, the applied color can be fixed using a fixing device. Furthermore, the cleaning station 46, the dampening unit 18 and the image generation device can be switched inactive and active, for example by pivoting. LIST OF REFERENCE NUMBERS

10 print carriers

12 pretreatment device 13 container

14 scoop roller

16 applicator roller

18 dampening system

20 scoop roller 22 applicator roller

24 fountain solution reservoir

26 image forming device

28 laser beam

30 inking unit 32, 34,

36 rollers

38 storage containers

40 carrier material

42, 44 rollers 46 cleaning station

48 brush

50 wiper lip

52 surfactant layer

54 dampening solution layer 60 steam device

62 suction device ^*

64 hydrophobic areas

68 hydrophilic areas

70 high voltage generator 72 electrode

74 sliding contact

76 metallic core

78 ceramic jacket

80 electrode plate 82, 84 electrode

86 blowers

90 blanket cylinders 92 fixing device

94 energy source

96 coating thickness measuring device

98 control

100 signal sequence

Pl direction of transport

P2 directional arrow

Claims

Expectations

1. Method for generating a printed image on a carrier material (40),

in which the surface of a print carrier (10) is coated with a dampening solution layer,

in a structuring process, ink-attracting areas and ink-repellent areas are generated in accordance with the structure of the printed image to be printed,

paint is applied to the surface which adheres to the ink-attracting areas and which is not accepted by the ink-repellent areas,

the applied color is subsequently transferred to the carrier material (40),

before a new structuring process, the surface of the print carrier (10) is cleaned and again coated with a dampening solution layer,

the layer thickness of the dampening solution layer is determined,

and in which, depending on the determined layer thickness, an energy source (94) for reducing the amount of dampening solution is activated in order to reduce the thickness of the dampening layer to a desired thickness.

2. The method according to claim 1, in which a control takes place in such a way that the actual value of the dampening solution layer thickness is determined, an actual value-target-value-value comparison is carried out and, depending on the comparison, the energy source ( 94) is controlled to reduce the amount of dampening solution so that the control deviation between the actual value and the target value is minimized.

Method according to one of the preceding claims, in which a dampening unit (18) with rollers is used to apply the dampening solution layer.

Method according to one of the preceding claims, in which the surface of the pressure carrier (10) has an average roughness depth R _z less than 10 μm or an average roughness value R _a less than 2 μm.

5. The method according to any one of the preceding claims, in which a layer thickness measuring device (96) is used to determine the layer thickness, which works according to the triangulation method, the transmission method or the capacitive method.

6. The method according to any one of the preceding claims, wherein the energy source (94) for reducing the amount of dampening solution comprises a heat source or radiation source.

7. The method according to claim 6, in which IR lamps, radiant heaters, laser systems, laser diodes or heating elements are used as the heat source or radiation source.

8. The method according to any one of the preceding claims, in which radiation is used for structuring.

9. The method according to claim 8, wherein the radiation from a laser system, a laser, laser diodes, LEDs or a laser diode array is used.

10. The method according to any one of the preceding claims, wherein the pressure carrier (10) is a cylindrical surface or an endless belt.

11. Device for generating a print image on a carrier material (40), where funds are provided, ^• by the

the surface of a print carrier (10) is coated with a dampening solution layer,

the applied color is subsequently transferred to the carrier material (40),

prior to a new structuring process, the surface of the print carrier (10) is cleaned and again coated with a dampening solution layer,

the layer thickness of the dampening solution layer is determined,

and by the function of the determined thickness of an energy source (94) is driven to reduce the quantity of dampening solution to the thickness of the fountain solution layer to reduce ^'to a desired thickness.

12. The device as claimed in claim 11, in which regulation takes place in such a way that the actual value of the dampening solution layer thickness is determined, an actual-value-target-value comparison is carried out and, depending on the comparison, the energy source ( 94) is controlled to reduce the amount of dampening solution so that the control deviation between the actual value and the target value is minimized.

13. Device according to one of the preceding claims, in which a dampening unit (18) with rollers is used to apply the dampening solution layer.

14. Device according to one of the preceding claims, in which the surface of the pressure carrier (10) has an average roughness depth R _z less than 10 μm or an average roughness value R _a less than 2 μm.

15. Device according to one of the preceding claims, in which a layer thickness measuring device (96) is used to determine the layer thickness, which works according to the triangulation method, the transmission method or the capacitive method.

16. Device according to one of the preceding claims, wherein the energy source (94) for reducing the amount of dampening solution comprises a heat source or radiation source.

17. Device according to claim 16, in which IR lamps, radiant heaters, laser systems, laser diodes or heating elements are used as the heat source or radiation source.

18. Device according to one of the preceding claims, in which radiation is used for structuring.

19. Device according to claim 18, in which the radiation from a laser system, a laser, laser diodes, LEDs or a laser diode array is used.

20. Device according to one of the preceding claims, wherein the pressure carrier (10) is a cylindrical surface or an endless belt.