Abstract:
A method of transferring printing ink from an intermediate carrier to a printing-ink receiver selected from the group consisting of a further intermediate carrier and a substrate, wherein the printing ink adheres either in a granular state or in an at least partially liquid state to the intermediate carrier includes, with respect to the first-mentioned state of the printing ink, melting the printing ink at a side thereof facing away from the intermediate carrier before transferring the printing ink to the printing-ink receiver and, with respect to the second-mentioned state of the printing ink, reducing the adhesion of the printing ink to the intermediate carrier with a separating agent at one time at least before and during transfer to the printing-ink receiver; and a device for performing the method.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for transferring solid or liquid printing ink from an intermediate carrier, such as a transfer cylinder, to a further intermediate carrier or a substrate, such as paper, as well as to a device for performing the method. 
     In order to be able to transfer liquid printing ink from a cylinder of a printing press onto a further cylinder and a substrate, respectively, the adhesion of the printing ink to the second cylinder or the substrate, a characteristic which is based upon physical interfacial effects, must be greater than the adhesion thereof to the original cylinder. Upon the transfer of liquid ink, however, a cracking or splitting of the liquid film occurs, so that a portion thereof remains on the original cylinder, and it is virtually impossible to achieve anything even remotely close to 100% transfer of liquid printing ink. 
     The same problem exists in the transfer of solid printing ink which is in a granular state, such as toner powder, for example. To be sure, electrostatic transfer techniques which achieve a transfer efficiency of approximately 95 to 98% maximum have become known heretofore; however, this is only for applications using non-conducting toner. For production presses with an output of many thousands of sheets per hour, this is not sufficient, however, because it would be necessary permanently for the cleaning devices to be replaced and to be cleaned outside the printing press, respectively. 
     In an article entitled “Offset Quality Electrophotography” in the “journal of Imaging Science and Technology”, Volume 37, No. 5, September/October 1993, p. 458 (hereinafter referred to as “OQE”), various xerographic techniques which are suitable for transferring conductive toner even when the humidity is quite high are described. In one of the techniques, the toner is transferred under pressure onto a substrate and is simultaneously fixed. A further technique calls for a thermal or heat transfer in two temperature stages. In addition, combinations of transfer under pressure and thermal transfer are described therein. Such a combination is presented on page 459 of the aforementioned publication. The toner is transferred by pressure from a first cylinder onto an intermediate cylinder and then by thermal transfer onto paper which runs between the intermediate cylinder and a heated impression cylinder. An efficiency of 95% is supposed to be achieved in the case of transfer by pressure, while an efficiency of 100% is attained in the case of thermal transfer. 
     In the periodical “The Seybold Report on Publishing Systems”, Volume 24, No. 20, page 20, left-hand column (hereinafter referred to as the “Seybold Report”), a transfer system is described wherein an image is transferred to paper through the intermediary of two belts. One of the belts accepts the toner in a distribution corresponding to the printed image. The image is then transferred to the other belt, which is heated. The latter belt is not hot enough to melt the toner, but the heat thereof is sufficient to cause the toner particles to adhere to one another. The heated belt then transfers the image onto the paper, which is preheated, the image being fixed by means of a hot pressure roller. Consequently, no subsequent melting or fixing of the toner is required. 
     In the first step, the transfer from a first belt to a second belt, it is not possible at all to achieve a transfer efficiency of 100%. The first belt is, in fact, teflon-coated, and basically exerts at least small adhesion forces on the toner, so that, in the first step at least, a transfer efficiency of less than 100% is to be assumed, as is similar to the situation with the technique described in the aforementioned “Offset Quality Electrophotography” article. 
     In both of the aforedescribed techniques, the toner is thus not transferred in its entirety from the cylinder and from the first belt, respectively. Particularly with regard to recently developed printing techniques, e.g., the printing technique described in the “Seybold Report”, it is necessary, however, for the remaining printing ink to be entirely removed before new printing ink is applied to the first belt and to the first cylinder, respectively, in order to attain a perfect print. This may be very difficult and costly, particularly if the printing ink is to be reused. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method and a device for transferring printing ink wherein transfer techniques are used which virtually always have a transfer efficiency of 100%, not only when transferring ink onto paper, but also when transferring ink onto an intermediate carrier. 
     With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a method of transferring printing ink from an intermediate carrier to a printing-ink receiver selected from the group consisting of a further intermediate carrier and a substrate, wherein the printing ink adheres in a granular state to the intermediate carrier, which comprises melting the printing ink at a side thereof facing away from the intermediate carrier before transferring the printing ink to the printing-ink receiver. 
     In accordance with another aspect of the invention, there is provided a method of transferring printing ink from an intermediate carrier to a printing-ink receiver selected from the group consisting of a further intermediate carrier and a substrate, wherein the printing ink adheres in an at least partially liquid state to the intermediate carrier, which comprises reducing the adhesion of the printing ink to the intermediate carrier with a separating agent at one time at least before and during transfer to the printing-ink receiver. 
     In accordance with a further mode, wherein the intermediate carrier has an outer elastic layer containing the separating agent, the method according to the invention includes pressing the intermediate layer and the printing-ink receiver against one another so as to drive the separating agent to the surface of the elastic layer. 
     In accordance with an added aspect of the invention, there is provided a device for transferring printing ink from an inking unit via an intermediate carrier to a printing-ink receiver selected from the group consisting of a further intermediate carrier and a substrate, comprising a heat source disposed opposite a surface of the intermediate carrier, the surface extending between the inking unit and the printing-ink receiver. 
     In accordance with an additional aspect of the invention, there is provided a device for transferring printing ink from an inking unit, comprising an intermediate carrier for receiving printing ink from the inking unit and for transferring the printing ink to a printing-ink receiver selected from the group consisting of a further intermediate carrier and a substrate, the intermediate carrier having at least one of the properties consisting of being permeable to a separating agent and having a storage capacity for a separating agent. 
     In accordance with yet another aspect of the invention, there is provided a device for transferring printing ink from an inking unit of a printing press to a substrate being transported through the printing press, comprising a first intermediate carrier adjoining the inking unit and having a surface to which printing ink is transferrable from the printing unit, and a second intermediate carrier disposed so as to be in contact with the first intermediate carrier and the substrate and having a surface extending between the first intermediate carrier and the substrate for transferring printing ink from the first intermediate carrier to the substrate, comprising a first heat source disposed opposite the surface of the first intermediate carrier, and a second heat source disposed opposite the surface of the second intermediate carrier. 
     In accordance with yet a further aspect of the invention, there is provided a device for transferring printing ink from an inking unit of a printing press to a substrate being transported through the printing press, comprising a first intermediate carrier adjoining the inking unit and having a surface to which printing ink is transferrable from the printing unit, and a second intermediate carrier disposed so as to be in contact with the first intermediate carrier and the substrate and having a surface extending between the first intermediate carrier and the substrate for transferring printing ink from the first intermediate carrier to the substrate, comprising a first heat source disposed opposite the surface of the first intermediate carrier, and the second intermediate carrier being provided with at least one of a second heat source disposed opposite the surface of the second intermediate carrier and a capability for being permeable to a separating agent at a side thereof facing the printing ink. 
     In accordance with yet another aspect of the invention, there is provided a device for transferring printing ink from an inking unit of a printing press to a substrate being transported through the printing press, comprising a first intermediate carrier adjoining the inking unit for receiving thereon printing ink transferred from the printing unit, and a second intermediate carrier disposed so as to be in contact with the first intermediate carrier and the substrate for transferring printing ink from the first intermediate carrier to the substrate, both the first intermediate carrier and the second intermediate carrier being permeable to a separating agent at a side thereof facing the printing ink. 
     In accordance with another feature of the invention, the device in a printing press having a plurality of inking units includes a plurality of first intermediate carriers and a second intermediate carrier, each of the first intermediate carriers, respectively, adjoining one of the inking units and the second intermediate carrier, the second intermediate carrier being disposed so as to contact all of the first intermediate carriers and a substrate being transported through the printing press, and a heat source disposed opposite a surface of the second intermediate carrier, the surface extending between at least one of the first intermediate carriers and the substrate. 
     In accordance with a further feature of the invention, the device in a printing press having a plurality of inking units includes a plurality of first intermediate carriers and a second intermediate carrier, each of the first intermediate carriers, respectively, adjoining one of the inking units and the second intermediate carrier, the second intermediate carrier being disposed so as to contact all of the first intermediate carriers and a substrate being transported through the printing press, the second intermediate carrier being permeable to a separating agent at a side thereof facing the printing ink. 
     In accordance with an added feature of the invention, the device in a printing press having a plurality of inking units includes a first intermediate carrier and a second intermediate carrier, each of the plurality of inking units adjoining the first intermediate carrier and the second intermediate carrier, the second intermediate carrier being disposed so as to contact the first intermediate carrier and a substrate being transported through the printing press, and a heat source disposed opposite a surface of the second intermediate carrier, the surface extending between the first intermediate carrier and the substrate. 
     In accordance with an additional feature of the invention, the device in a printing press having a plurality of inking units includes a first intermediate carrier and a second intermediate carrier, each of the plurality of inking units adjoining the first intermediate carrier and the second intermediate carrier, the second intermediate carrier being disposed so as to contact the first intermediate carrier and a substrate being transported through the printing press, the second intermediate carrier being permeable to a separating agent at a side thereof facing the printing ink. 
     In accordance with yet another feature of the invention, the intermediate carrier contacting the substrate has an outer elastic layer containing the separating agent therein. 
     In accordance with yet a further feature of the invention, the separating agent is an ink-repelling liquid. 
     In accordance with yet an added feature of the invention, the ink-repelling liquid is silicone oil. 
     In accordance with yet an additional feature of the invention, the intermediate carrier is one of a rotating transfer cylinder and a belt revolving around a cylinder. 
     In accordance with still another feature of the invention, the device includes an impression cylinder journalled opposite a side of the substrate whereon a surface of the one of the rotating transfer cylinder and the belt revolving around a cylinder rolls. 
     In accordance with still a further feature of the invention, the heat source is formed so as to concentrate radiation on one of the intermediate carrier and the substrate. 
     In accordance with still an added feature of the invention, the heat source is one of a laser, an array of lasers and an array of laser diodes. 
     In accordance with a concomitant feature of the invention, the device includes a control device for generating control signals connected to the one of the laser, the array of lasers and the array of laser diodes, the control signals corresponding to a distribution of the printing ink on the printing-ink receiver. 
     It is thus a more specific objective of the invention, when, in the aforementioned method, the printing ink adheres in a granular state to an intermediate carrier, to effect an initial melting of the printing ink at a side thereof facing away from the intermediate carrier, before the printing ink is transferred to another intermediate carrier or to the substrate. 
     It is a further specific objective of the invention, when, in the aforementioned method, the printing ink adheres in an at least partially liquid state to the intermediate carrier, to reduce the adhesion of the printing ink to the intermediate carrier by applying a separating agent before and/or during the ink transfer to the other intermediate carrier or to the substrate. 
     In an embodiment of the invention, the intermediate carrier has an outer elastic layer, such as a rubber layer, the separating agent being contained in the layer and the separating agent being driven to the surface of the rubber layer when the intermediate carrier and either the further intermediate carrier or the substrate, as the case may be, are pressed against one another. In a further embodiment relating to the use of a separating agent, the intermediate carrier has a hard outer layer, such as a porous layer formed of sintered material or ceramic, the outer layer being capable of effectively storing a separating agent therein. The loss of separating agent occurring during operation can be compensated for by a corresponding feeding device, or the separating agent is added to the ink, so that an equilibrium in the supply of separating agent is attained during operation. 
     An intermediate carrier is understood to mean a device having a surface whereon a printed image in the form of an ink distribution is created, further transported and subsequently destroyed or removed, i.e., for example, a rotating transfer cylinder or a belt revolving around a cylinder, the ink being transferred either onto a further intermediate carrier or onto a substrate. 
     A transfer efficiency of virtually 100% is achieved both in the case of the transfer of solid ink according to the invention, i.e. transfer by a melting start-up, the printing ink being predominantly solid, and also in the case of the transfer of liquid inks assisted by a separating agent. 
     In the case of the start-up melting of a granular printing ink from outside, assurance is provided, firstly, that the ink particles adhere to one another. Secondly, the adhesion of the printing ink to the intermediate carrier is not increased by the start-up melting, because the start-up melting takes place only on the outer surface of the ink film, while, on the side facing the intermediate carrier, the printing ink continues to adhere only at individual points to the intermediate carrier. Therefore, coherent ink islands on the intermediate carrier can easily and completely be removed therefrom. Thirdly, the start-up or initial melting of the printing ink from outside results in a stronger adhesion thereof to the following intermediate carrier or substrate, thereby additionally ensuring the complete transfer of the printing ink. 
     Otherwise than in the process known from the “Seybold Report”, according to the invention of the instant application, the solid printing ink does not undergo start-up or initial melting from the inside, but rather, on the outside. Whereas the conventional technology makes the stripping or removal removal of the printing ink from the intermediate carrier more difficult, the stripping or removal forces required in accordance with the invention continue to remain small and, moreover, the adhesion on the target carrier is improved. Consequently, in the method for transferring solid ink, according to the invention, the transfer method is in a plurality of respects more reliable than in the conventional process. With little design effort and with little expenditure of energy, a transfer efficiency of 100% is attained, the contact-pressure forces required for transfer onto a substrate such as paper being small, so that the paper is treated more gently. 
     This applies not only when the printing ink is transferred directly onto a substrate such as paper, which assists transfer due to the strong capillary action thereof, but also in the case of a multistage process wherein the printing ink is transferred onto the substrate via a further intermediate carrier. The complete transfer of a latent image, developed on a first intermediate carrier, onto a second intermediate carrier and from there onto a substrate imposes contradicting requirements on the two processes. First of all, the affinity of the printing ink to the second intermediate carrier must be greater than the affinity thereof to the first intermediate carrier; secondly, the affinity of the printing ink to the substrate must be greater than the affinity thereof to the second intermediate carrier. Therefore, the second intermediate carrier must initially accept the printing ink before then releasing it again. Because, in accordance with the invention, the stripping or removal of the printing ink from the first intermediate carrier after the development thereof is facilitated, a lesser affinity is required on the second intermediate carrier than if the stripping or removal were not assisted, in order for the ink to be transferred completely onto the second intermediate carrier. The low adhesion forces on the second intermediate carrier make it easier for the printing ink subsequently to be transferred completely onto the substrate. 
     If the start-up melting method according to the invention is likewise used for transfer onto the substrate, then the method of transfer and final fixing require overall considerably less heat energy than in conventional techniques, wherein the printing ink is melted by heating the intermediate carrier or by preheating the paper. The paper does not dry out during printing and is treated more gently. 
     If the printing ink is on the intermediate carrier in an at least partially liquid state or has been brought to such a state, then, in accordance with the invention, a separating agent is used in order to ensure 100% transfer. The printing ink may be either an ink which is liquid at normal temperature, or a meltable ink which is solid at normal temperature and is kept at a temperature above the melting temperature. In the latter case, for example, the intermediate carrier is provided with a heated outer layer of high thermal conductivity and low thermal capacity and with an insulating layer lying therebelow in order to keep the heat losses low. 
     The use of a separating agent is particularly advantageous if the intermediate carrier is in the form of a rubber-covered transfer cylinder which transfers the liquid printing ink onto a substrate. An elastic layer of the transfer cylinder formed, for example, of rubber or a similar material and being able huggingly to adapt to an uneven substrate surface in order to ensure uniform ink transfer without having to exert excessive pressure, serves simultaneously as the carrier for the separating agent which, in the preferred embodiment, is silicone oil. The capacity of the elastic layer to absorb the separating agent may be based on diffusion and/or on the penetration of the separating agent into micropores of the elastic layer. When the elastic layer is pressed against the substrate, the silicone oil is driven out, so that the printing ink is repelled from the surface of the transfer cylinder. At the same time, the printing ink is driven into the substrate surface, so that the complete transfer of the ink is achieved in an especially simple manner. 
     In the case of more modern inking units, particularly in the case of digital inking units, it is necessary, for technical reasons, that the printing ink be applied initially to a first intermediate carrier which has a hard surface. If the printing ink is liquid, in this case, provision may be made for the first intermediate carrier to be formed with micropores through which a separating agent may be pressed, before and/or during the transfer of the printing ink, onto a second intermediate carrier, the separating gas being, in this case, not restricted to a liquid only, but also possibly being a gas. If the printing ink, conversely, is originally solid, then the solid-ink transfer according to the invention is performed on the first intermediate carrier, while either the solid-ink transfer or the liquid-ink transfer according to the invention is performed on the second intermediate carrier. 
     A printing press with one or more printing units according to the invention includes a transport device, such as a conventional conveyor with chains and grippers or a transport belt, the transport device conveying substrates consecutively through the in-line printing units, the substrates being pressed against the corresponding intermediate carriers by means of impression cylinders. 
     Furthermore, due to the high transfer efficiency, simplified constructions for multicolor printing presses are possible. 
     The heat sources used in the various embodiments for start-up or initial melting or for fixing may be, for example, infrared radiators which concentrate the radiation onto the intermediate carrier or substrate. The location on the intermediate carrier at which the radiation is concentrated should be as close as possible to the location at which the printing ink is transferred onto the further intermediate carrier or substrate, so that, on the travel path of the printing ink to the transfer location, as little heat as possible flows onto the intermediate carrier and the printing ink need not be heated to a considerably higher temperature than is required for transfer, respectively. 
     Best suited for the concentration of the radiation are lasers, the radiation of which is converted at the point of incidence into heat. Through a suitable choice of the radiation wavelength, it is possible to ensure that the radiation be absorbed with a higher efficiency by the printing ink and with a lower efficiency by the intermediate carrier or substrate, respectively, so that the intermediate carrier and the substrate are heated as little as possible. Even more selective heating is possible with the aid of an array of lasers or of laser diodes, which are controlled in conformance with the transferred printing image, in order to heat only those locations which bear the printing ink. The information required for generating such a heating pattern is known from the control of the imaging heads. Given a corresponding resolution of the laser-diode array, it is possible to implement heat transfer to pixel accuracy and, with the aid of the gray-value information, it is possible to take into account the respective ink-film thickness. Consequently, the supply of heat can be measured so that, upon the transfer of the printing ink to the paper, the printing ink has the same temperature overall, irrespective of other parameters, thereby assuring a reliable transfer of the ink. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as a method and device for transferring printing ink, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a basic diagrammatic side elevational view of a first embodiment of a printing unit for effecting a solid ink transfer; 
     FIG. 2 is a view like that of FIG. 1 of a second embodiment of a printing unit for effecting a liquid ink transfer; 
     FIG. 3 is a view like those of FIGS. 1 and 2 of a third embodiment of a printing unit for effecting a two-stage ink transfer, namely a combination of solid ink transfer and liquid ink transfer; 
     FIG. 4 is a view like those of FIGS. 1,  2  and  3  of a fourth embodiment of a printing unit for effecting a two-stage liquid ink transfer; 
     FIG. 5 is a view like those of FIGS. 1 to  4  of a first embodiment of a multicolor printing press for effecting a two-stage ink transfer; 
     FIG. 6 is a view like those of FIGS. 1 to  5  of a second embodiment of a multicolor printing press for effecting a two-stage ink transfer; 
     FIGS. 7 and 8 are diagrammatic and schematic side elevational views of a printing unit showing various heat sources suitable for the embodiments of FIGS. 1 to  6 ; and 
     FIG. 9 is a flow chart depicting the operation, in accordance with the invention, of a control device shown in FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings and, first, particularly to FIG. 1 thereof, there is shown therein a printing unit with a transfer cylinder  1 , a diagrammatically represented inking unit  2  and an impression cylinder  3 , as well as a substrate  4 . A heat source  5  is disposed opposite the surface of the transfer cylinder  1  and between the inking unit  2  and the substrate  4 , and a heat source  6  is disposed opposite the printed side of the substrate  4 . 
     During the operation of the printing unit shown in FIG. 1, the transfer cylinder  1  and the impression cylinder  3  rotate in the directions indicated by the curved arrows respectively associated therewith, while a non-illustrated transport apparatus conveys the substrates  4  consecutively through the nip located between the transfer cylinder  1  and the impression cylinder  3 , in the direction of the straight arrow. 
     At the inking unit  2 , ink particles  7  are applied to the transfer cylinder  1  in a distribution corresponding to a printing image which is to be formed. The ink particles  7  are represented in the drawing as spheres of like size; in practice, however, they are irregularly shaped. While the ink particles  7  are being further transported by the transfer cylinder  1 , they are melted from the outside by a heat source  5  which is directed thereon, so that cohesive ink islands  8  are formed. In the ink islands  8 , the printing ink adheres, as in the ink particles  7  beforehand, only at respective points to the transfer cylinder  1 , as represented diagrammatically in FIG.  1 . As long as this point-wise adhesion is not markedly changed, the intensity of the melting is not critical. What is important is that the plasticity in the ink islands  8  should increase from the inside to the outside thereof, and that the outer surface of the ink islands  8  should not yet be completely melted. Due to the glasslike melting behavior of conventional solid inks, it is relatively easy to satisfy these conditions. 
     Between the transfer cylinder  1  and the impression cylinder  3 , the substrate  4  is pressed onto the transfer cylinder  1 , the printing ink being transferred to the substrate  4 . Because the printing ink has become melted on the side thereof facing away from the transfer cylinder  1 , the ink islands  8  are cohesively and easily transferred to the substrate  4 . The impression or pressure forces between the substrate  4  and the transfer cylinder  1  can consequently be kept small. The printing ink is then fixed by means of the heat source  6 , it being sufficient, in many cases, to heat the ink islands  8  at the surface in order to smooth finish them. Alternatively or additionally, the substrate  4  may be preheated by a suitable non-illustrated device before the substrate  4  passes the transfer cylinder  1 . 
     FIG. 2 shows a printing unit with a transfer cylinder  21 , an inking unit  22  and an impression cylinder  23 , as well as a substrate  24 . A heat source  25  is disposed opposite the surface of the transfer cylinder  21  and between the inking unit  22  and the substrate  24 . 
     The transfer cylinder  21  is a rubber-covered cylinder with an outer rubber jacket  26 . The material of the rubber jacket  26  has an absorption capacity for a silicone oil, with which it is quite saturated. 
     The operation of the printing unit shown in FIG. 2 is similar to that of the printing unit in FIG.  1 . Unlike the inking unit  2  of the printing unit in FIG. 1, however, the inking unit  22  of FIG. 2 does not apply individual ink particles to the transfer cylinder  21 , but rather, a liquefied printing ink which is normally solid at room or normal temperature. Ink islands  27  formed in accordance with the printed image are kept liquid by the heat source  25  or by heating the transfer cylinder  21  as the ink islands  27  travel to the substrate  24 . 
     The rubber jacket  26  of the transfer cylinder  21  becomes slightly compressed at the impression cylinder  23 , so that previously absorbed silicone oil is expelled from the rubber jacket  26 , breaking the adhesion between the printing ink and the transfer cylinder  21 . Consequently, no cracking of the printing ink occurs, and the printing ink is transferred in its entirety to the substrate  24 . 
     After the pressure on the rubber jacket  26  has been released, the majority of the silicone oil is re-absorbed by the rubber jacket  26 . A non-illustrated conventional oil-feeding device is provided to compensate for any losses. 
     If necessary, the substrate  24  in this embodiment can be preheated by a suitable non-illustrated device before the substrate  24  passes the transfer cylinder  21 . 
     FIG. 3 shows a printing unit with a first transfer cylinder  30 , a second transfer cylinder  31 , an inking unit  32  and an impression cylinder  33 , as well as a substrate  34 . A heat source  35  is disposed opposite the surface of the first transfer cylinder  30  and between the inking unit  32  and the second transfer cylinder  31 , and a heat source  36  is disposed opposite the surface of the second transfer cylinder  31  and between the first transfer cylinder  30  and the substrate  34 . The second transfer cylinder  31  is provided with a silicone oil-containing rubber jacket  37 , as was described in connection with FIG.  2 . 
     During the operation of the printing unit shown in FIG. 3, the first transfer cylinder  30 , the second transfer cylinder  31  and the impression cylinder  33  rotate in the directions represented by the respective curved arrows associated therewith, while a non-illustrated transport device conveys substrates  34  consecutively through the nip between the second transfer cylinder  31  and the impression cylinder  33  in the direction of the straight arrow. 
     At the inking unit  32 , ink particles  38  are applied to the first transfer cylinder  30  in accordance with a latent printed image generated in an otherwise non-described manner by a conventional diagrammatically represented imaging head  39 . Transfer of the printing ink to the second transfer cylinder  31  is accomplished in the same manner as was described in connection with FIG.  1 . The printing ink is completely melted on the second transfer cylinder  31  by means of the heat source  36  and/or by heating the transfer cylinder  31 , and is then transferred to the substrate  34  in the same manner as was described in connection with FIG.  2 . 
     With the arrangement shown in FIG. 3, it is further possible to dispense with a separating agent if the transfer of ink from the second transfer cylinder  31  to the substrate  34  is likewise effected in a manner similar to that in FIG. 1, i.e., by melting the outside of the printing ink by means of the heat source  36 , while the inside of the printing ink is chilled on the transfer cylinder  31 . The increased adhesion of the printing ink to the second transfer cylinder  31  due to the preceding melting is at least balanced by the considerably greater adhesion of the printing ink to the substrate  34 . In this case also, complete transfer of the ink is possible, if necessary or desirable, by suitable additional measures, such as by preheating the substrate  34 . Instead of the combination of solid-ink and liquid-ink transfer depicted in FIG. 3, a two-stage transfer of more-or-less solid printing ink is realized. 
     FIG. 4 shows a printing unit with a first transfer cylinder  40 , a second transfer cylinder  41 , an inking unit  42  and an impression cylinder  43 , as well as a substrate  44 . The first transfer cylinder  40  has a jacket  45 , which is permeable to a separating agent fed thereto by a suitable feeding device  46 . In the embodiment of FIG. 4, the feeding device  46  is diagrammatically represented as being inside the jacket  45  of the transfer cylinder  40 , but it may also be disposed adjacent to the outer surface thereof in order to keep the quantity of separating agent stored in the porous jacket  45  constant during operation. If a separating agent is added to the printing ink itself, equilibrium is automatically attained during operation. The second transfer cylinder  41  has a silicone oil-containing rubber jacket  47 , as was described in connection with FIG.  2 . 
     The printing unit shown in FIG. 4 operates in a manner similar to that of the printing unit shown in FIG.  3 . Unlike the inking unit  2  in FIG. 1, however, the inking unit  42  of FIG. 4 does not apply individual ink particles, but rather, a liquefied printing ink to the first transfer cylinder  40 . In conformity with a latent printed image generated by an imaging head  48 , ink islands  49  are formed on the first transfer cylinder  40 , the ink islands  49  being kept liquid, e.g., by heating the transfer rollers  40  and  41 , as the ink islands  49  travel farther on their way via the second transfer cylinder  41  to the substrate  44 . 
     During the transfer of the printing ink from the first transfer cylinder  40  to the second transfer cylinder  41  and from the second transfer cylinder  41  to the substrate  44 , use is made of a separating agent, respectively, as was described in connection with FIG. 2, care being taken, however, by suitable constructive measures that the first transfer cylinder  40  be provided with a hard surface which can be written on by the imaging head  48 . 
     Although the foregoing embodiments of the invention for transferring printing ink in liquid form have been described with reference to a liquefied printing ink which is solid at normal or room temperature, they are basically also suitable for transferring printing ink which is liquid at normal or room temperature. 
     FIG. 5 shows four first transfer cylinders  50 , a second transfer cylinder  51 , an impression cylinder  52 , as well as a substrate  53 . The four first transfer cylinders  50  are disposed in-line or in tandem at the circumference of the second transfer cylinder  51 , and an inking unit  54  and an imaging head  55  are disposed at the circumference of each of the first transfer cylinders  50 . Furthermore, a heat source is disposed at the circumference of each of the transfer cylinders  50  and  51 . 
     During the operation of the printing unit shown in FIG. 5, the first transfer cylinders  50 , the second transfer cylinder  51  and the impression cylinder  52  rotate in the directions represented by the respective curved arrows associated therewith, while a non-illustrated transport device conveys substrates  53  consecutively through a nip between the second transfer cylinder  51  and the impression cylinder  52  in the direction of the straight arrow shown associated therewith. 
     The printing inks are transferred in a manner similar to that described in connection with FIG. 3 or, not represented in this connection, in a manner similar to that described in connection with FIG. 4, all four printing inks being transferred during one revolution of the second transfer cylinder  51 . 
     FIG. 6 shows a first transfer cylinder  60 , a second transfer cylinder  61 , an impression cylinder  62 , as well as a substrate  63 . The circumference of the second transfer cylinder  61  is four times as large as the circumference of the first transfer cylinder  60 . Four inking units  64  and an imaging head  65  are disposed at the circumference of the first transfer cylinder  60 . Furthermore, a heat source is disposed at the circumference of each of the transfer cylinders  60 ,  61 . 
     During the operation of the printing unit shown in FIG. 6, the first transfer cylinder  60 , the second transfer cylinder  61  and the impression cylinder  62  rotate in the directions represented by the curved arrows associated therewith, while a non-illustrated transport device conveys substrates  63  consecutively through a nip formed between the second transfer cylinder  61  and the impression cylinder  62  in the direction of the straight arrow. 
     The printing inks are transferred in a manner similar to that described in connection with FIG. 3 or, not represented in this connection, in a manner similar to that described in connection with FIG. 4, a respective printing ink being transferred to the transfer cylinder  61  upon each revolution of the transfer cylinder  60 . 
     As is apparent from FIG.  5  and FIG. 6, respectively, the embodiments illustrated therein are multicolor printing presses with a two-stage ink transfer, wherein several transfer cylinders are dispensed with. 
     Infrared radiators, for example, are taken into consideration as the heat sources used in the aforedescribed embodiments. Further, particularly suitable heat sources are described in connection with FIGS. 7 and 8. 
     FIG. 7 shows diagrammatically a transfer cylinder  70 , a substrate  71  being transported in a direction represented by the arrow associated therewith and a laser  72  for melting the outside of the printing ink which is being transferred from the transfer cylinder  70  to the substrate  71 . The laser  72  is, for example, a carbon-dioxide laser, directing the radiation therefrom along the broken line into the nip between the transfer cylinder  70  and the substrate  71 , whereat it is absorbed by the printing ink and converted into heat. At the point formed by the converging surfaces of the transfer cylinder  70  and the substrate  71 , a manifold or multiple reflection occurs in a direction towards the transfer location, so that the radiation energy is guided very closely to the transfer location. The radiation is uniformly distributed over the length of the nip between the transfer cylinder  70  and the substrate  71  by means of lenses or mirrors, and/or a plurality of lasers  72  are provided along the length of the transfer cylinder  70 . 
     At least in the case of the hereinafore-described transfer of ink by melting, the radiation is concentrated as intensely as possible, i.e., it is concentrated in a linear region, which is as narrow as possible, along the transfer cylinder  70 . This applies also, or especially, if the radiation, assisted by reflection, is not directly introduced into the nip between the transfer cylinder  70  and the substrate  71 , but rather, impacts before the transfer location, albeit as slightly as possible. With the aid of lasers, it is possible to attain a linear radiation area with a width within the range of micrometers, so that the printing ink passing the irradiated area is subjected only very briefly to radiation energy. Assurance is thereby provided that the printing ink is actually heated only on the outer surface thereof and is therefore melted only on the surface thereof. 
     If a carbon-dioxide laser is used, the radiation is, in any case, absorbed by the printing ink. However, polymeric coloring agents also absorb shorter-wave light, so that, for example, it is also possible to employ ND-YAG lasers. The shortwave light offers the advantage that the individual ink islands on the transfer cylinder  70  can be heated selectively, with maximum care or protection being given to the substrate  71  and with minimum heating of the transfer cylinder  70 . The latter aspect is particularly important if the transfer cylinder  70  is a developing cylinder which, if there is heating at the transfer location, must subsequently be cooled again in order to ensure proper development. 
     An embodiment of the invention offering selective heating of the ink islands which permits uniform heating of printing ink which is provided both with locally differing distribution, as well as with differing thickness, is described hereinafter with reference to FIG.  8 . 
     FIG. 8 shows diagrammatically a transfer cylinder  80 , a substrate  81  which is transported in the direction of the arrow associated therewith and a laser-diode array  82  for externally melting printing ink which is being transferred from the transfer cylinder  80  to the substrate  81 . The laser-diode array  82  extends along the length of the transfer cylinder  80 , being disposed close to the transfer cylinder  80  and as near as possible to the point of transfer to the substrate  81 . 
     The laser-diode array  82  is controlled by a control device  84 , which receives from a non-illustrated computer of the printing press the very same image information, including the gray values, which is also fed to the s of the printing unit. The control device  84  controls the laser-diode array  82 , taking the time offset into account, so that the ink film on the transfer cylinder  80  is supplied with heat in accordance with the areal distribution and the respective thickness thereof. 
     In this manner it is possible to ensure that the printing ink is at the same temperature throughout at the instant of transfer thereof from the transfer cylinder  80  to the substrate  81 . Except for the heat which flows onto the transfer cylinder  80  on the short path to the printing-ink transfer location, the transfer cylinder  80  does not absorb any other heat. 
     The control device  84  may, for example, be a microprocessor with an integrated memory and may operate as shown in the flow chart of FIG.  9 . In step S  91 , the image data activating the imaging heads of the printing unit are received, while a substrate is being printed. If the pixel resolution of the laser-diode array  82  is not identical with the resolution of the imaging heads, the image data are transformed, in step S  92 , into bitmap data having a format corresponding to the pixel spacing in the laser-diode array  82 , and stored temporarily (step S  93 ). If the pixel resolution of the laser-diode array  82  is identical with the resolution of the imaging heads, the image data may be stored immediately as represented by the broken line. After a time interval during which the location on the substrate which is being printed moves from the imaging heads to the laser-diode array  82 , the image data are read out again line by line in step S  94  and transformed into respective activating signals for the laser-diode array  82 . The laser-diode array  82  activated by the signals irradiates and heats the substrate pixelwise in accordance with the respective ink thickness. These functions may be performed not only by a separate control device  84  but also completely or partly by the printing-press computer. 
     Selective heating is most accurately effected if the pixel resolution of the laser-diode array  82  is identical with the imaging-head resolution. Instead of an array with individually controllable laser diodes at spaced pixel intervals, it is also possible, for example, to employ a single, continuously radiating laser with a scanning mirror and a switchable filter. 
     A laser-diode array  83  constructed and controlled in a manner like for the laser-diode array  82  may be disposed behind the transfer location above the substrate  81 . The laser-diode array  83  uniformly smoothes and fixes, respectively, the printing ink transferred to the substrate  81  in accordance with the distribution of the printing ink on the substrate  81 , without heating the substrate  81  directly. 
     Instead of the laser-diode array  82 , it is also possible to employ one or more continuously operating lasers, such as carbon-dioxide lasers, for example, which, in a manner similar to that described in connection with the transfer cylinder  70 , linearly irradiate the printing ink which has been transferred to the substrate  81 . Due to the brief subjection of the substrates  81  to irradiation as they pass by, the printing ink is initially melted at the surface thereof, which is generally sufficient for the purpose of fixing, yet also prevents too much moisture from leaving the paper. 
     In all of the aforedescribed methods of heating, the laser wavelength and the composition of the printing ink can be matched to one another, so that the printing ink is heated with the maximum possible efficiency whereas the transfer cylinder and the paper, respectively, are heated as little as possible at that time. Consequently, the paper does not dry out.