Patent Grant
7002613
Priority to German Patent Application No. 102 41 911.6, filed Sep. 6, 2002 and hereby incorporated by reference herein, is claimed.
The present invention relates to a method for printing an image on a printing substrate, a number of portions of fluid printing ink being produced on a printing-ink carrier by inputting energy, and the fluid printing ink being transferred to the printing substrate. Moreover, the present invention relates to a device for inputting energy to a printing-ink carrier, including a number of individually controllable laser light sources which have a modular design consisting of subarrays and are disposed in an array, and further including a printing-ink carrier with which is associated an axis of rotation and on the surface of which can be produced a number of image spots of the laser light sources.
Digital or variable printing methods are printing methods that allow different contents or subjects to be transferred to a printing substrate from copy to copy or from print to print. Generally known digital printing methods are, for example, electrophotography or ink jet printing. Besides, however, there are also approaches to transfer images, texts, subjects or the like, to printing substrates in a variable manner using fluid printing inks, also liquid pigmented printing inks. Some approaches of that kind have already been documented in detail in the literature.
For example, German Patent No. 42 05 636 C2 describes a method and a device for variable printing by means of which meltable printing inks are applied to a printing-form carrier, such as a cylinder, and in which printing ink that is solid at room temperature and meltable through the addition of heat is applied to the printing-form carrier as a continuous viscous film and subsequently solidified there by cooling. The solidified film is then exposed to the radiation of a laser or of a laser line on a dot-by-dot or pixel-by-pixel basis, the printing inks being liquefied in the irradiated regions and, while still in the liquid state, transferred to a printing substrate where they cool down again.
Moreover, German Patent Application No. 36 25 592 A1 describes a variable printing method, a so-called “heat transfer recording method”. In this context, a printing ink exhibiting delayed solidification is applied to and solidified on a cylinder as the printing-ink carrier, or the cylinder itself is composed of solid printing ink. After that, the solid ink located on the cylinder is locally softened by energy radiation, for example, of a laser. The softened spots can then be transferred to a printing substrate. After the transferal, the remaining ink layer is scraped off in a thickness which corresponds to the layer thickness that has been transferred to the print carrier.
Another variable printing method, a so-called “suction pressure method” is described in PCT Patent Application No. WO 00/40423. A printing-ink carrier features depressions as the printing regions, whereas non-printing regions are at a constant level. Prior to printing, the entire surface of the printing-ink carrier is inked, that is, flooded with ink, as follows: Prior to receiving printing ink, the air located in the depressions is selectively heated in a controlled manner, expelling it from the depressions due to the strong temperature dependence of its volume. When the entries to the depressions are then closed by the printing ink and the remaining air in the depressions is subsequently cooled, then the air will contract as it cools, thus suctioning printing ink into the depressions. The greater the temperature variation in the depressions, the stronger is this effect. By controlling the temperature in the depressions, it is, in principle, possible to control the received quantity of printing ink. Prior to each new printing cycle, the printing-ink carrier can be imaged anew or differently by means of a thermal image, that is, by selectively radiating energy into the depressions. Prior to transferring the printing ink to a printing substrate, the printing ink is removed from the non-printing regions using a wiper, a doctor blade, or the like, thus leaving printing ink only in the depressions. Ink transfer from the depressions to the printing substrate is accomplished by high contact pressure and the adhesion forces between the printing substrate and the ink.
European Patent Application No. 0 947 324 A1 discloses a printing method and an associated device. Using the light-hydraulic effect, pressure pulses are introduced into an ink layer on a printing-ink carrier by means of a laser light source in such a manner that a portion of printing ink is detached and transferred to a printing substrate.
Another variable printing method and a device for carrying out the method are described in German Patent No. 197 46 174 C1. A printing-form carrier is provided with depressions which can be filled with printing ink. A number of portions of printing ink are selected or produced through the action of a digitally controlled energy beam. The ink transfer takes place due to adhesion forces when the printing ink that is expelled from a depression contacts a printing substrate.
All these approaches have the common requirement that in order to produce an image spot, a certain amount of energy must, if possible, be coupled into a narrowly defined spatial region of a printing-ink carrier that is correlated with the printing dot to be produced, possibly in a contact-free manner. The energy form used here is mostly laser light in the ultraviolet, visible, or infrared spectral ranges because of the high spectral power density, directionality and other properties. Since all individual spots of an image to be printed must be produced during imaging with preferably as short a duration as possible, the total power of the required energy source is relatively high.
To image a two-dimensional surface of a printing substrate in a variable printing method, the printing substrate is usually moved relative to the image-producing device in one of the directions defining the surface while the image is being produced. In principle, a relative movement in the second unfolding direction, a so-called “scanning”, can be carried out as well. Alternatively, the image can be produced temporally and spatially parallel over the entire width of the image, which is also referred to as “page-wide”.
A clear disadvantage of scanning is the fact that only a limited maximum speed is achievable. An exact synchronization of the movements of the deflecting mirror and of the paper transport at extremely different speeds can only be achieved with great effort; for example, it is required to use piezoelectric mirrors. As a rule, a large installation space is needed. If only a small amount of time is available for each energy input, the energy must be coupled in rapidly, which requires a high power density of the laser light source. The risk of damage to optical components increases, but also the possibility of an unwanted modification of involved materials, such as the printing ink itself. The high power density must be modulated very rapidly. For a page width of 34 cm, 600 dpi, and a printing speed of 1 m/s, over 200 MHz are required. Through the use of a plurality of laser light sources, such as a line of laser light sources, the requirements in terms of power, modulation frequency, and scanning speed are, in fact, reduced, but the coupling-in of two light beams into a polygon scanner is technically already very difficult to implement. For example, fifty light beams, each modulated at 4 MHz, are to be considered extremely difficult.
Page-wide arrays or arrangements of light-emitting diodes (LED), as are widespread, for example, in electrophotographic printing presses, can produce only several milliwatts of optical power in a region of 40 micrometers ×40 micrometers, the size of a printing dot at 600 dpi, due to their unfavorable radiation characteristic. This optical power is insufficient for most of the variable printing methods. Moreover, due to the always low quantum efficiency, a multiple of the optical power must be dissipated as waste thermal power. Increasing the efficiency by special geometries or using cavity LEDs has not helped so far either.
In the context of variable printing methods, it is also known, for example, from PCT Patent Application No. WO 00/12317 to use page-wide arrays or arrangements of fibers or optical waveguides by means of which light is conducted from one or more remote light sources, typically a laser light source, to a printing-ink carrier. Due to the required high positional accuracy over very long periods of time, the positioning effort for such an arrangement of fibers is very high. The assignment of the individual channels during assembly requires considerable effort. Moreover, the cost of a fiber coupling of a laser and of the required optical waveguide length in the range of several meters that is needed for each channel for the connection between the laser and the printing press is so high that a device for inputting energy to a printing-ink carrier in a digital printing press would be uneconomical.
Considering the disadvantages of the prior art, it is an object of the present invention to provide a method for printing an image on a printing substrate, including a powerful energy source, and a device for inputting energy to a printing-ink carrier. In particular, a device for inputting energy is intended to be equipped with a separate light source for each line to be imaged and to be able to write lines densely. The device is also intended to have a high output power and sufficient resolution and depth of focus. Moreover, the device is intended to be comparatively inexpensive to manufacture and maintain and to have a high reliability.
According to the present invention, in a method for printing an image (or a text or subject) on a printing substrate, a number of portions of fluid printing ink are produced on a printing-ink carrier by inputting energy. An energy input is produced on the printing-ink carrier by a number of image spots of an array of individually controllable VCSEL light sources. The fluid printing ink is transferred to the printing substrate. In particular, the fluid printing ink can be liquid.
A portion of fluid printing ink is the amount of printing ink which produces an image spot and has a suitable viscosity to be absorbed on and/or in the printing substrate.
The array of VCSEL light sources can, in particular, be a VCSEL bar having a number of individually controllable VCSEL light sources or an arrangement of a number of such VCSEL bars. A plurality of image spots can be produced on the printing-ink carrier simultaneously and/or spatially parallel. The method according to the present invention can also be referred to as a variable or digital method for printing. In particular, a temporary or transient intermediate image of fluid printing ink can be produced on the printing-ink carrier by inputting energy. The printing-ink carrier can be an intermediate image carrier. In this situation, the printing ink of the temporary intermediate image is transferred to the image carrier by impression. Typical printing substrates are paper, cardboard, paperboard, organic polymer film, or the like. Printing substrates can also be referred to as image carriers.
In other words, in the context of the inventive idea, an array of individually controllable VCSEL light sources, in particular, VCSEL bars, is used or employed in a variable or digital printing method.
While conventional semiconductor lasers are edge emitters, i.e., the light propagates perpendicular to the surface of the pn junction and emerges from the gap surfaces of the chip in a perpendicular direction, surface-emitting laser diodes (VCSEL light sources, VCSEL laser diodes, vertical cavity surface emitting lasers) emit light perpendicular to the wafer surface. The resonator axis is parallel to the area of the pn junction. In the context of this description of the method and device according to the present invention, the term “VCSEL light source” can be understood to mean all diode lasers whose emission direction is perpendicular to the active zone. These can be, in particular, surface emitters whose resonator length is short compared to the thickness of the active zone, surface emitters whose resonators are extended monolithically, or surface emitters having an external or a coupled resonator (also referred to as NECSELs). Moreover, a VCSEL light source can be a diode laser whose resonator is essentially parallel to the active zone and is provided with a diffracting or reflecting structure which couples out the laser radiation perpendicular to the active zone.
The functionality and a number of properties of a VCSEL light source can be tested already on the wafer or immediately after manufacture. Due to the extended emitter surface, the radiation is emitted with a small divergence angle, in particular, compared to conventional edge-emitting semiconductor lasers. It generally applies to VCSEL light sources that the active length of the resonator can be very short, typically only several micrometers, and that highly reflecting resonator mirrors are required in order to obtain low threshold currents. The required mirrors can be grown epitaxially. Using an extremely short resonator, often below a length of 10 micrometers, a large longitudinal mode distance is achieved, which promotes single-mode emission above the laser threshold. However, single-mode emission is not necessarily required in the context of the inventive idea because multi-mode VCSEL light sources can be used as well. Using a rotationally symmetric resonator, a circular near-field is obtained, as well as a small beam divergence due to the relatively large diameter. The beam quality and the shape of the emitted light beam are largely determined by the size of the output facet. By selecting the proper size (diameter limitation), a VCSEL generates the fundamental mode (Gaussian beam), which, due to the high depth of focus, is advantageous for a controlled energy input for imaging. For high optical output power, larger output facet diameters can be advantageous. Moreover, the design of the laser allows simple monolithical integration of two-dimensional arrays of VCSEL laser diodes. Finally, it is possible to test the lasers directly on the wafer disk after manufacture.
The typical layered structure of a surface-emitting laser is known to one skilled in the art and can be gathered from relevant literature. In this respect, see, for instance, K. J. Ebeling “Integrierte Optoelektronik” [Integrated Optoelectronics], Springer Publishing House, Berlin, 1992. This document is incorporated into this disclosure by reference. Arrays of VCSEL light sources can be manufactured as two-dimensional arrangements. For example, European Patent Application No. 0 905 835 A1 describes a two-dimensional array of VCSEL light sources which can be addressed or controlled individually. To increase the achievable output power and to force the laser to oscillate in the fundamental mode, U.S. Pat. No. 5,838,715 discloses a special resonator shape for a VCSEL layer structure.
For a resolution of 600 dpi, a typical resolution in variable printing methods, lasers having a beam quality inferior to diffraction-limited quality are already sufficient. VCSELs having 90 mW of output power can be focused to 40 micrometers ×40 micrometers (which corresponds to 600 dpi). The luminous intensity on the exit facet of a VCSEL is only a fraction of that occurring on the exit surface of an edge-emitting semiconductor laser, so that the risk of facet destruction is reduced. The reliability of VCSEL light sources compared to edge-emitting semiconductor lasers is, in principle, much higher. The increased reliability is particularly advantageous if the intention is for a device for inputting energy to a printing-ink carrier to be used in a printing method using a plurality of light sources.
In a preferred embodiment of the inventive method for printing an image on a printing substrate, the number of portions of fluid printing ink are produced by melting or softening solid printing ink on the printing-ink carrier on a dot-by-dot basis. In a special embodiment, the printing ink can exhibit delayed solidification during cooling. In other words, the melting point is at a higher temperature than the solid point. Due to the solidification delay, the printing ink remains in the liquid state until it is printed on the printing substrate by contact.
In an alternative embodiment of the printing method according to the present invention, the number of portions are produced by suctioning fluid printing ink into depressions on a dot-by-dot basis upon cooling of the volumes of the depressions that were heated by the energy input. Subsequently, the fluid printing ink is printed on the printing substrate. In other words, the printing method includes steps of a suction pressure method.
In a further embodiment of the printing method according to the present invention, the number of portions of fluid printing ink are produced by detachment from a layer of printing ink. The portions of fluid printing ink are transferred to the printing substrate due to the energy input in a contact-free manner. In other words, the further embodiment of the method according to the present invention uses the light-hydraulic effect.
In another alternative embodiment of the printing method, the number of portions of fluid printing ink are produced by expelling from depressions in the printing-ink carrier. The portions of fluid printing ink are transferred to the printing substrate upon contact (preferred) or in a contact-free manner.
Also related to the inventive idea is a device according to the present invention for inputting energy to a printing-ink carrier, including a number of individually controllable laser light sources which have a modular design consisting of subarrays and are disposed in an array, and further including a printing-ink carrier with which is associated an axis of rotation, and on the surface of which can be produced a number of image spots of the laser light sources. The subarrays of laser light sources are VCSEL bars. The VCSEL bars can be accommodated on imaging modules. In the case of simultaneous triggering (when the light sources are switched on simultaneously), rows, i.e., lines and/or columns, of image spots of the VCSEL bars are located on the printing-ink carried such that they are inclined with respect to the axis of rotation.
At this point, it should be mentioned that it is known from the literature, such as from U.S. Pat. No. 5,477,259, that an array of light sources can be made up of individual modules of subarrays. These are typically rows, that is, one-dimensionally arranged laser diodes which are fixed to a holding element side-by-side, forming a two-dimensional array of light sources. The array of light sources disclosed in U.S. Pat. No. 5,477,259 is located on the intersection points of a parallelogram grid.
In particular, the printing-ink carrier can be an intermediate image carrier. The array can be regular and/or one-dimensional or two-dimensional (preferred), preferably Cartesian. It is particularly advantageous if the laser light sources on the VCSEL bars are arranged on the intersection points of a regular Cartesian, two-dimensional grid, so that an inclination with respect to the axis of rotation has a uniform effect on all light sources. It is also worth mentioning that in a two-dimensional arrangement, it is possible to leave larger spaces between the individual VCSEL light sources, channels and emitted light beams, so that collimation is simplified. Unlike edge-emitting semiconductor lasers, the beam diameter and the divergence angle of a VCSEL light source in both lateral directions perpendicular to the propagation direction of the emitted light are equal, so that collimation and focusing can be accomplished using relatively simple optics arranged downstream, such as microlens arrays, in particular, a microlens for one or more emitted light beams.
In a preferred embodiment of the device according to the present invention for inputting energy to a printing-ink carrier, the inclination angle between the unfolding direction of the row of image spots of the VCSEL bars and the axis of rotation or the complementary angle of the inclination angle is selected such that the projected spots of the image spots on a line parallel to the axis of rotation have even spaces between neighboring spots.
For a two-dimensional, regular Cartesian arrangement of n×m image spots, with the direction in which the n image spots are located having an inclination angle α with the perpendicular to the axis of rotation, it applies that a row of projected image spots has regular or even spaces between neighboring spots if tan α=1/n. If the two-dimensional arrangement is Cartesian, but has a spacing a of neighboring image spots in the direction of the n image spots, as well as a spacing b of neighboring image spots in the direction of the m image spots, then it applies that tan α=b/na.
In a further development of the device according to the present invention, the printing-ink carrier is illuminated by the laser light sources from its underside. In other words, the printing-ink carrier can be transparent so that the laser light can penetrate it up to the printing ink, or the printing-ink carrier is designed in such a manner that it is able to absorb the energy of the laser light at least partially and impart it to the printing ink.
In an advantageous embodiment of the inventive device for inputting energy, the VCSEL bars are staggered in at least two substantially parallel rows.
In alternative embodiments, the VCSEL bars feature top emitters ((p-side up emitters, p-doped layer up) or bottom emitters (p-side down emitter, p-doped layer down). In other words, in a p-side up embodiment of the inventive device for inputting energy, the light emission occurs at the top side of the device whereas in a p-side down embodiment, the laser radiation used for inputting energy can be emitted through the semiconductor substrate of at least one VCSEL bar of the number of VCSEL bars, preferably all VCSEL bars. In addition or as an alternative to this, in one embodiment of the inventive device for inputting energy, at least one VCSEL bar can include at least one drive electronics of which at least a part is accommodated on the substrate or the wafer of the VCSEL bar and/or of which at least a part is accommodated on a common heat sink together with the VCSEL bar and/or have a common cooling circuit. In addition or as an alternative to this, in one embodiment of the device according to the present invention, at least one VCSEL bar, preferably all VCSEL bars, and a part of its drive electronics can be made from one substrate or on one substrate or from one wafer or on one wafer. In particular, in one embodiment, at least one VCSEL bar, preferably all VCSEL bars, can be accommodated on a surface containing diamond and/or aluminum nitride. In addition or as an alternative to this, in one embodiment, at least one VCSEL bar can be contacted with conductor tracks from two sides. In addition or as an alternative to this, in one embodiment of the inventive device for inputting energy, at least one VCSEL bar, preferably all VCSEL bars, can be deposited on a surface in which or on which conductor tracks for controlling the individual light sources are accommodated. The separately described measures, alone or in cooperation with each other, advantageously permit a compact design of the array of light sources.
A particularly preferred embodiment of the inventive device for inputting energy to a printing-ink carrier features a page-wide array of VCSEL bars. In this context, the projected spots of the image spots on a line parallel to the axis of rotation are dense, which means that the spacing of the imaging spots corresponds to the minimum printing dot spacing or screen ruling of the image, which makes it possible to produce solid areas. In other words, using the special embodiment of the device according to the present invention, it is possible to write, image or place page-wide rows of dense image spots on the printing-form carrier so that a number of portions of fluid printing ink are densely produced over the width of a page.
Embodiments of the inventive device and/or improvements thereof can be employed or used in a particularly advantageous manner in the inventive method and/or improvements thereof described in this specification, in particular, in the specific embodiments addressed in this specification. In other words, a method according to the present invention for printing an image on a printing substrate can be characterized by the generation of an energy input using a device according to the present invention.
Also related to the inventive idea is a printing press that works using a printing method according to the present invention. In particular, depending on the specific embodiment of the inventive method, the printing press can be referred to as a gravure printing press or a planographic printing press. The printing press can be a web-fed press or a sheet-fed press (preferred), in particular, a perfecting press. The printing press can have one or more printing units. In other words, a printing unit or a printing press according to the present invention feature at least one inventive device for inputting energy to a printing-ink carrier.
Further advantages as well as expedient embodiments and refinements of the present invention will be depicted by way of the following Figures and the descriptions thereof. Specifically,
In
Sub
In the embodiment with bottom emitters, the emitters are contacted via conductor tracks that are provided in an electrically insulating substrate, such as a diamond substrate. In the case of bottom emitters, it is advantageously avoided that a number of bonding wires are arranged on the light exit side which could possibly hinder the exit of light. If the n-doped side of the light source is up and the p-doped side of the light source is down, then the substrate surface opposite the p-doped side must be patterned. The substrate itself is attached to a heat sink, preferably to a patterned heat sink, such as a microchannel cooler, so that adequate and efficient heat transfer is provided between the substrate and the heat sink. In this embodiment, the current sources for the VCSEL light sources are situated in the immediate vicinity of the light sources on one or more semiconductor components which can be attached to or accommodated on the same substrate as the VCSELs, or which can be attached to or accommodated on a separate substrate on the same or a different heat sink.
The beam shaping of the laser light emerging from the emitters can be accomplished using micro-optical components (acting on only one or more light beams of the VCSEL bar) and/or macro-optical components (acting on all light beams of the VCSEL bar). Suitable for beamshaping are, in particular, arrays of micro-optical components, such as microlens arrays, where the spacing between the individual components corresponds to the spacing of two laser emitters or a multiple thereof.
Since two neighboring imaging modules 20 or neighboring VCSEL bars cannot be placed close enough to write neighboring lines densely (at 600 dpi 40 micrometers), the two-row arrangement shown in
Alternatively to the situation shown in
In this connection, it should also be mentioned that it is possible to carry out an automatic calibration at regular intervals by means of control unit 814 in order to compensate for deviations of the performance curves of the VCSEL light sources on a bar or in an array that are due to ageing. Since deviations of the performance curves of individual emitters of an array rarely occur in VCSEL light sources on one bar or are insignificant, it is even possible to limit such a calibration to one emitter or a small number of emitters a subarray, respectively. The resulting measured current can be used with sufficient accuracy for all light sources.
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| 102 41 911 | Sep 2002 | DE | national |
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| Number | Date | Country | |
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| 20040046860 A1 | Mar 2004 | US |
