Abstract:
The invention relates to a method according to which print data determine the image elements of a printing format to be printed on a substrate. According to the inventive method, the surface tension of a printing liquid ( 30, 34 ) is influenced depending on the printing date that pertains to the respective image element.

Description:
BACKGROUND OF THE INVENTION 
   The invention is directed to a method wherein print data define the picture elements of a print image to be printed onto the carrier material. Water-based or solvent-based, chromatic fluids are employed as a printing fluid. The carrier material, for example, is white paper or plastic film. The print data contain one or more bit places per picture element. For example, the value one in a bit place indicates that a black picture element is to be printed. The value zero in a bit place indicates that no printing fluid is to be applied on the picture element. The picture element retains the color of the carrier material. 
   European Letters Patent EP 0 756 566 B1 discloses a thermoelectric printing unit for transferring an ink onto a recording medium. The printing unit contains a printing drum with print elements arranged matrix-like that respectively contain a depression for the acceptance of ink. The ink is introduced into the depressions from the outside. A heating element, with the assistance of which the ink is expelled upon vapor formation dependent on the print data, is located in each depression. 
   U.S. Pat. No. 4,275,290 discloses a thermoelectric ink printing unit wherein ink is heated in depressions, whereupon surface tension and volume change. The ink flows into widened portions arranged opposite a recording medium. A meniscus forming thereat inks the recording medium. 
   Further, U.S. Pat. No. 4,675,694 discloses a thermoelectric ink printing unit wherein solid ink is heated. After becoming molten, the ink expands and moistens a recording medium in character-dependent fashion. 
   DE-A1-19718906, which does not enjoy prior publication, likewise discloses a thermoelectric ink printing unit having a hollow drum with depressions arranged thereon in matrix-like fashion. A gas bubble is generated in the ink via a laser, whereupon the ink expands and moistens a recording medium. 
   SUMMARY OF THE INVENTION 
   An object of the invention is to specify a further method for transferring printing fluid onto a carrier material. Moreover, a printer device and a printing drum are to be recited that are suitable for the implementation of the method. 
   According to the method and system of the invention for transferring printing fluid onto a carrier material, with print data defining picture elements of a print image to be printed onto the carrier material. A surface tension of a prescribed volume of a printing fluid is influenced when printing a picture element dependent on the print data belonging to the picture element wherein without significant change in volume, the printing fluid has either a first surface tension which moistens the carrier material or has a second surface tension deviating for the first surface tension, the printing fluid having the second surface tension not touching the carrier material. 
   The invention proceeds on the basis of the perception that, given a modification of the surface tension of a fluid that adjoins a solid body, a wetting angle defined by the boundary surface tension between the surface of the fluid and the seating surface and by the seating surface itself likewise changes. When the fluid is located in a vessel, then the change of the wetting angle forces a change in curvature on the surface of the fluid. The change in curvature results in at least sub-areas of the surface moving by a specific differential distance, for example rising or lowering. The differential distance is dependent on the vessel size and amounts, for example, to 10 μm through 30 μm given a print resolution of 600 dpi (dots per inch). When the carrier material lies against an acceptance unit for transporting the printing fluid for the individual picture elements or when the carrier material is arranged at a distance from the printing fluid that corresponds to the differential distance, then, dependent on the surface tension given a large wetting angle or great curvature, a moistening and thus an inking of the carrier material occurs when the printing fluid advances up to the carrier material. When, however, the wetting angle or the curvature is small, then the printing fluid does not reach the carrier material, and the carrier material retains its base color in the region lying opposite the printing fluid. 
   According to this principle, the surface tension of a printing fluid is influenced in the inventive method when printing a picture element, being influenced dependent of a print datum belonging to the corresponding picture element. The carrier material to be printed is arranged at a distance from the printing fluid where printing fluid having a first surface tension moistens the carrier material and where printing fluid having a second surface tension deviating from the first surface tension does not moisten the carrier material. The variation of the surface tension to be implemented in the inventive method requires far less energy than the acceleration of a drop of ink. In the inventive method, the printing fluid—after the moistening of the carrier material—proceeds to the carrier material due to the adhesion effect between carrier material and printing fluid. 
   In a development of the inventive method, the first surface tension is greater than the second surface tension. The curvature of the surface deriving given the first surface tension is greater than the curvature deriving given the second surface tension. A central sub-area of the printing fluid thus projects farther out given the first surface tension than given the second surface tension. 
   In a next development of the inventive method, the first surface tension has a first value at which the surface of the printing fluid arcs outward. The second surface tension, in contrast, has a value at which the surface of the printing fluid is flat or even arcs inward. The direction of the arc is thereby seen proceeding from the inside of the fluid. The differential distance given this development is very large, so that it is possible to conduct the carrier material past at a greater spacing from a vessel for the acceptance of the printing fluid. An abrasion of the carrier material and a wear at the edges of the vessel are thus avoided. When the printing fluid arcs inward at the second surface tension, then the carrier material can be placed against the edge of a vessel for the acceptance of the printing fluid. 
   In one development of the inventive method, the surface tension is varied in that the temperature of the printing fluid is varied. The heating of the fluid usually leads to a reduction of the surface tension. Photoflash lamps, laser beams or laser diodes are employed as heat sources. When fluid additive such as, for example, tensides contained in the printing fluid evaporate given variation of the temperature, then this leads to an increase in the surface tension. Tensides are surface-active substances that reduce the surface tension. An increase in the surface tension consequently arises when these fluid additives are removed. An evaporation of the tensides can already be compelled due to a relatively small temperature change. The surface tension rises more sharply due to the removal of the fluid additives than it drops due to the heating. In this opposed process, thus the increase in the surface tension dominates, this leading to an increase in the wetting angle and, thus to an increase of the curvature on the surface of the printing fluid. 
   In another development, the surface tension is varied due to a variation of the ionization in the printing fluid. The ionization can be varied by introducing ionized particles or by means of electrical fields as well. The variation of the ionization also enables the use of heat-sensitive printing fluids. 
   In one development of the inventive method, the surface tension of a prescribed volume of the printing fluid is varied. The printing fluid to be employed per picture element can be exactly prescribed with the assistance of the prescribed volume. In a next development, the volume is dimensioned such that is corresponds to the printing fluid volume to be applied onto a picture element having the color of the printing fluid. All of the prescribed printing fluid is thus employed. This leads to a thrifty printing event. Collecting printing fluid that is not needed is eliminated. 
   When, in another development, the volume is prescribed by the capacity volume of a depression, then the filling of the volume is simple since the printing fluid runs over the edge of the depression as soon as the depression has been filled with printing fluid. The quantity of fluid to be employed per picture element is exactly prescribed by the capacity volume of the depression and is independent of the printing speed. Since, following a stripping of fluid residues projecting beyond the depression, the printing fluid is topically limited by the edge of the depression, the boundaries of the picture elements can be precisely prescribed. The depression forms a vessel that is very well-suited for producing an optimally great differential distance on the surface of the printing fluid given a change of the surface tension. 
   In a next development of the inventive method, the depressions are arranged in matrix-like fashion, preferably on a drum-shaped surface. The resolution of the printer device is prescribed by the spacing and the diameter of the depressions, i.e. the plurality of picture elements to be printed per unit of area. 
   In a development of the inventive method, the surface tension is influenced due to the action of a radiation source directed through the opening of the depression into the inside of the depression. This development is based on the perception that the surface tension changes with a certain inertia. It is thus possible to first set the surface tension and to subsequently transport the printing fluid to the carrier material. The surface tension remains unmodified during the transport, so that the carrier material is moistened or remains unmoistened dependent on the surface tension. In this development, the radiation of the radiation source reaches the surface of the fluid without having to pass through the fluid first. The direct irradiation of the surface results in fluid additives located at the surface of the fluid being influenced with a lower amount of energy. For example, the fluid additives are tensides that evaporate given a slight increase in temperature. In this development, the radiation source is arranged outside the vessel for the printing fluid. This results in no built-in parts being needed in the material of the vessel for the delivery of the energy. 
   In a next development, the surface tension is modified with the assistance of a temporally and topically drivable radiation source. When the radiation source is clocked according to a timing clock, then the surface tension can be successively set for various picture elements. When a plurality of radiation sources are arranged next to one another, then the surface tensions of various picture elements can be simultaneously set. Given a combination of a temporally and topically driven radiation source, the printing speed can be increased upon employment of reasonable clock rates when, for example, radiation sources for exposing the picture elements of two or more lines are arranged behind one another and are simultaneously actuated. 
   In one development of the inventive method, the printing fluid for all picture elements initially has a lower surface tension that is raised dependent on the print data. The increase in the surface tension can be realized in a simple way, for example by evaporating tensides contained in the printing fluid or by introducing ions into the printing fluid. In this development, the surface tension need not be reduced during printing. However, methods are also applied wherein the printing fluid for all picture elements initially has a higher surface tension and is then reduced dependent on the print data when certain printing fluids are employed for which the reduction of the surface tension is easier to implement than the increase of the surface tension. 
   The inventive printer device serves for the implementation of the inventive method and the developments thereof. The technical effects recited above thus also apply to the printer device. 
   In one development of the inventive printer device, a unit for modifying the surface tension contains a radiation source that generates thermal radiation and/or electromagnetic radiation and/or a particle beam. When the unit for modifying the surface tension is arranged outside the receptacle unit for the printing fluid, then this receptacle unit can be constructed in a simple way. The invention is also directed to a printing drum for the application of a printing fluid. Depressions for the acceptance of the printing fluid are arranged in matrix-shaped fashion on the printing drum. The printing drum is free of devices allocated to individual depressions for influencing a physical property of the printing fluid in the respective depression. This means that there are no heating elements or similar elements for delivering energy within the printing drum. The printing drum can be homogeneously made of a uniform material. Regions of the surface of the printing drum in which no depressions lie can be coated with a hydrophobic coating in order to prevent a wetting with printing fluid at these locations. 
   Exemplary embodiments of the invention are explained below on the basis of the enclosed drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a portion of a printing drum; 
       FIG. 2  illustrates a printing unit of a printer; 
       FIG. 3A  shows an irradiation device for varying the surface tension of a printing fluid; 
       FIG. 3B  shows a print perspective view of rows of ceramic cells; and 
       FIG. 4  shows an irradiation unit working according to the scanning principle for varying the surface tension of the printing fluid. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     FIG. 1  shows a longitudinal section along the surface  8  of a printing drum  10 . A plurality of depressions are arranged in matrix-like fashion in the surface  8  of the printing drum  10 ,  FIG. 1  showing two depressions  12  and  14  thereof. The depressions are arranged next to one another in a row direction. Neighboring depressions  12 ,  14  have a spacing A from one another that defines the resolution of the printer. A plurality of rows of depressions are arranged behind one another in column direction  18 , whereby neighboring depressions within a column also have the spacing A from one another. The depressions are all identically constructed, so that only the structure of the depression  12  shall be explained below. 
   The depression  12  is designed as a conoidal frustum-shaped recess (see contour  20 ) and thus has circular cross-sections. The axis of the conoidal frustum lies in the direction of the normal of the surface  8 . The conoidal frustum-shaped contour  20  tapers with increasing distance from the surface  8  of the printing drum  10 . A bottom surface  24  of the depression  12  has a smaller diameter than the aperture  26  of the depression  12  lying on the surface of the printing drum  10 . The circumference of the aperture  26  lies on a circle and determines the shape of the picture elements to be printed. 
   An all-around sidewall of the depression  12  is obliquely arranged relative to the surface  8  of the printing drum  10 . The filling of a chromatic ink  30  is facilitated by the conoidal frustum-shaped design of the depression  12 . In addition to conoidal frustum-shaped depressions having a circular cross-section, depressions with an elliptical or a polygonal cross-section are also employed. 
   When the ink  30  is situated within the depression, it is held within the depression  12  by capillary forces. The capillary forces are greater than the force of gravity exerted on the ink  30 , so that the ink  30  also remains within the depression  12  when the aperture  26  is directed down, i.e. toward the center of the earth. After the ink  30  has been filled in, the surface  32  thereof has a surface tension that leads to a concave curvature, i.e. the surface  36  of the ink  30  is arced inward. The surface  32  is in a condition I wherein a wetting angle RI has a value of approximately 45°. The wetting angle lies between a vector V 1  of the surface tension on the surface of ink  30  and the sidewall  28 . The vector V 1  begins at the edge of the depression  12 , i.e. at a location at which the boundary between fluid  30  and sidewall  28  or surface  8  lies. 
   The volume capacity of the depression  12  is selected such that exactly that quantity of ink  30  that is required for printing a single picture element can be held therein. How a condition II of the surface  36  of the ink influences the printing event shall be explained below on the basis of a printing fluid  34  within the depression  14 . The ink  34  also had an inwardly arced, i.e. concave, surface after being filled into the depression  14 . The surface tension of the ink  34 , however, was increased as a result of one of the techniques explained below on the basis of  FIGS. 2 through 4 , as a result whereof the surface  36  is arced outward in convex fashion. A wetting angle RII between a surface tension vector VII and the sidewall of the depression  14  has a value somewhat above 90°. The vector VII begins at the sidewall of the depression  14  and proceeds in the direction of the surface tension of the surface  36 . The starting point of the surface tension vector VII lies at the boundary between printing fluid  34  and the sidewall of the depression  14 . A middle region  38  of the surface  36  projects beyond the surface  8  of the printing drum  10  by a distance B. When the depression  14  is conducted past paper to be printed at a distance that is smaller than the distance B, then a wetting of the paper occurs. The adhesion forces between paper and printing fluid  34  are greater than the capillary forces between printing fluid  34  and depression  14 . All of the printing fluid  34  is therefore sucked from the depression  14  and inks a region on the paper that is provided for a picture element. 
     FIG. 2  shows a printing unit  50  of a printer. A printing drum  10   a  rotates counter-clockwise—see arrow  52 . The devices explained below are successively arranged along the rotational direction of the printing drum  10   a.    
   At the beginning of a revolution of the printing drum  10   a , the depressions extending in the longitudinal direction of the printing drum  10   a  for printing a line are free of printing fluid—see position P 1 . Ink  56  is filled into the depressions of a row at an inking station  54 . The inking station  54  contains a scoop drum  58  whose axis proceeds parallel to the axis of the printing drum  10   a . At position P 2 , the surface of the scoop drum  58  touches the surface of the printing drum  10   a . The scoop drum  58  turns in a direction opposite the printing drum  10   a —see arrow  60 . The lower part of the scoop drum  58  immerses into the ink  56  held by a reservoir  62 , so that the surface of the scoop drum  58  is moistened with ink when it reaches the position P 2 . As a result of the capillary forces, the ink  56  is sucked from the surface of the scoop drum  58  into the depressions  12 ,  14  of the printing drum  10   a  that are located at the position P 2 . 
   A doctor blade  64  with which the surface of the printing drum  10   a  is swept so that no ink remains on the surface of the printing drum  10   a  outside the depressions is located at a position P 3 . After being swept with the doctor blade  64 , the ink in all depressions has a respectively inwardly arced surface. 
   Due to the rotation of the printing drum  10   a , the depressions of a row filled with ink  56  are subsequently transported to a position P 4  at which an exposure device  70  alters the surface tension in selected depressions. The exposure device  70  contains a tubular photoflash  72  whose longitudinal axis is arranged parallel to the longitudinal axis of the printing drum  10   a . A reflector  74  that extends along the photoflash lamp  72  and has an arcuate cross-section is located at that side of the photoflash lamp  72  facing away from the printing drum  10   a . The photoflash lamp  72  is located approximately in the focus of the reflector  74 . The exposure device  70  also contains a row of ceramic cells  76  arranged next to one another whose transparency can be varied with the assistance of a control voltage. Exactly one ceramic cell  76  is located opposite each depression when exposing a row of depressions at the position P 4 . The ceramic cells  76  are a matter of transparent, ferro electric ceramic laminae. Such ceramic laminae are known from optoelectronics. For example, European Letters Patent EP 0 253 300 B1 discloses such ceramic laminae as PLZT elements. However, optoelectronic elements that work according to the Kerr principle are also employed. 
   The exposure device  70  is controlled by a drive device  78  dependent on printing data  80  that define the picture elements of the print image to be printed. A first output line  82  of the drive device  78  carries a clock signal  84  that clocks the photoflash lamp  72  synchronously with the rotation of the printing drum  10   a , so that each row of depressions that is moved past the position P 4  is irradiated exactly once by the photoflash lamp  72 . 
   Output lines  86  lead from the drive device  78  to individual ceramic cells  76  of the row of ceramic cells  76 . The drive unit  78  drives the ceramic cells  76  such that a ceramic cell  76  under observation is light permeable when the depression lying opposite the corresponding ceramic cell contains ink that is to be employed for printing at a position P 5  given the next pass. The light coming from the photoflash lamp  72  can then proceed through the corresponding ceramic cell  76  and onto the ink. Tensides that are situated on the surface of the ink are evaporated due to the photo-energy. The result is that the surface tension of the ink rises and the wetting angle increases. When, in contrast, the ink situated in a specific depression is not to be employed for printing a picture element, then the ceramic cell  76  lying there opposite is blacked out with the assistance of the drive device  78 , so that no light from the photoflash lamp  72  can impinge the depression. The surface tension and the wetting angle of the ink remain unmodified. 
   As explained above with reference to  FIG. 1 , there are depressions at the position P 4  after the passing of a row of depressions wherein the surface of the printing fluid has the condition I. The surface of the ink has the condition II in other depressions. 
   A transfer printing zone  92  is located at the position P 5  between the printing drum  10   a  and a transport roller  90 . The longitudinal axis of the transport roller  90  lies parallel to the axis of the printing drum  10   a . The transport roller  90  is turned in a direction opposite the printing drum  10   a  by a transport mechanism (not shown), see arrow  94 . Continuous form paper is transported in a conveying direction  98  between printing drum  10   a  and transport roller  90 . The continuous form paper  96  lies against the surface of the transport roller  90 . 
   Continuous form paper  96  and the surface of the printing drum  10   a  have the same velocity in the region of the transfer printing zone  92 , so that they are at rest relative to one another. That surface of the continuous form paper  96  facing toward the printing drum  10   a  has a spacing from the surface of the printing drum  10   a  in the transfer printing zone  92  that is smaller than the spacing B, see  FIG. 1 . The spacing B assures that no abrasion will arise at the continuous form paper  96  and at the printing drum  10   a . In another exemplary embodiment, the continuous form paper is pressed against the printing drum  10   a  by a soft pressure roller. In the region of the transfer printing zone, the continuous form paper  96  is printed at locations that lie opposite depressions that have a high surface tension and, thus, have a great curvature at the surface, condition II. 
   After the depressions are transported past the position P 5 , there are depressions in which ink  56  is still situated. The ink  56  was removed from other depressions when printing in the transfer printing zone  72 . A cleaning station  100  is located at a position P 6 . The cleaning station  100  contains a cleaning drum  102  whose longitudinal axis lies parallel to the longitudinal axis of the printing drum  10   a . The cleaning drum turns in a direction opposite the printing drum  10   a , see arrow  104 . The surface of the cleaning drum  102  and the surface of the printing drum  10   a  touch at the position P 6 . The surface of the cleaning drum  102  is fabricated of an absorbent material which absorbs ink  56  from the depressions in which ink has remained. Ink that has previously been in the depressions on the printing drum  10   a  is squeegeed from the cleaning drum  102  with the assistance of a doctor blade  106 . The ink that has been squeegeed off runs into a collecting basin  108  arranged under the doctor blade  106 . After being transported past the position P 6 , the depressions on the transfer printing drum  10   a  are again in their original condition, as was explained above for the position P 1 . An interconnecting feeder  110  via which the ink dripping down from the doctor blade  106  returns into the reservoir  62  is located between the collecting basin  108  of the cleaning station  100  and the reservoir  62  of the inking station  54 . An ink circulation for ink that was not used is thus closed via the interconnecting feeder  110 . 
     FIG. 3A  shows a second exemplary embodiment for an exposure device  70   a  that is employed instead of the exposure device  70 . The exposure device  70   a  likewise contains a photoflash lamp  72   a  and a reflector  74   a  that have the same structure as the photoflash lamp  72  or the reflector  74 . However, four rows of ceramic cells  76   a ,  76   b ,  76   c  and  76   d  are arranged between photoflash lamp  72   a  and printing drum  10   a  in the exposure device  70   a .  FIG. 3A  shows a side view onto the rows of ceramic cells  76   a  through  76   d  that are arranged in the light path between photoflash lamp  72   a  and printing drum  10   a , so that the light coming from the photoflash lamp  72   a  successively passes through ceramic cells  76   a  through  76   d  of different rows. What is referred to as a self-focusing lens  120  is situated between the row of ceramic cells  76   a  and the printing drum  10   a . Such lenses are manufactured of gradient fibers and are known by the trade name SELFOC (also see EP 0 253 300 B1). 
     FIG. 3B  shows a front perspective view of the rows of ceramic cells  76   a  through  76   d  lying behind one another. Ceramic cells  76   a  through  76   d  lying behind one another are respectively offset by a quarter length of a ceramic cell relative to one another. As a result of this offset, printing drums  10   a  can also be exposed wherein neighboring depressions have a very small spacing A. The terminals of the ceramic cells contained in the rows of ceramic cells  76   a  through  76   d  are connected to the drive device  78 , so that individual ceramic cells can be separately driven. The arrangement of the ceramic cells  76   a  through  76   d  shown in  FIGS. 3A and 3B  enable a higher printing speed or a higher resolution of the printing event given an unaltered printing speed. 
     FIG. 4  shows an exposure unit  70   b  working according to the scanning principle that is employed instead of the exposure unit  70 . A laser  200  driven by the drive unit  78  emits a laser beam  202  that impinges a polygonal mirror  204 . The polygonal mirror  204  turns in a counter-clockwise direction along its longitudinal axis, see arrow  204 . Upon rotation of the polygonal mirror  204 , the laser beam  202  successively impinges lateral faces  206  of the polygonal mirror  205 . Due to the rotation of the polygonal mirror  204 , the laser beam  202  is successively reflected by different lateral faces  206  of the polygonal mirror  204  and sweeps across the printing drum  10   a  along a principal scan direction  208  in a row direction of the depressions. The drive unit  78  drives the laser  200  such that the laser beam  202  impinges depressions to which picture elements to be presented black are allocated. When sweeping across depressions to which white picture elements are allocated, the laser beam  202  is blacked out. 
   A motion in a secondary scan direction, see arrow  52 , is generated due to the rotation of the printing drum  10   a , so that the next row with depressions is irradiated given incidence of the laser beam  202  onto the next lateral face  206  of the polygonal mirror. 
   While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.