Abstract:
A printing device, including at least one electrophotographic printing unit with a transfer medium for the transfer of a toner powder to a substrate in a transfer zone. One or several substrates may be run through the transfer zone by a transport system. The transport system includes a heated housing device for each substrate, having one or several heating elements for the introduction of heat energy into the substrate. A cooling device is provided for the transfer medium of each printing unit, which extracts heat energy from the transfer medium. There can be a number of serially arranged printing units, for the printing of each substrate, each with a different color.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a printing device, having at least one electro-photographic printing unit, to which a transfer medium for transferring a toner powder to respectively one substrate in a transfer zone is assigned, wherein one or several substrates can be conducted through the transfer zone by a transport system.  
         [0003]     2. Discussion of Related Art  
         [0004]     A printing device is known from U.S. Pat. No. 5,988,068. A revolving endless belt is assigned as the transfer medium to an electro-photographic printing unit. A photo-conductor rolls off the latter for transferring an image having toner powder. The toner image can be applied to a substrate. Thus, the substrate is conducted past the transfer medium by a transport system. In this case the transfer medium rolls off the substrate surface to be imprinted. According to U.S. Pat. No. 5,988,068, there are two heating elements for improving the toner transfer. The first heating element heats the substrate to a temperature higher than 60° C. The second heating element acts on the transfer medium at a temperature of more than 160° C. It has thus been shown to be disadvantageous, in particular when printing with ceramic toners, that residues of the toner stick to the transfer medium. Because of the doughy consistency at this temperature, they can only be removed with great difficulty, or not completely. Furthermore, heat is introduced into the electro-photographic printing device during continuous operation. This leads to a worsening of the image quality. Printing is only possible in one color with the printing device known from U.S. Pat. No. 5,988,068.  
         [0005]     However, it is also known to realize a multi-color printing device because several printing devices of basically the same type are arranged one behind the other, wherein the substrate to be imprinted is sequentially conducted to the individual printing devices for applying respective one-color prints. Thus, a toner with a different color is available in each printing device. A substrate imprinted in multi-color exists after passage over the entire printing path. In this case, the introduction of heat into the entire printing device can be very large because of the mutual temperature effects of the individual printing devices, so that the above described problems in connection with printing by ceramic toners can increase.  
       SUMMARY OF THE INVENTION  
       [0006]     It is one object of this invention to provide a printing device of the type mentioned above but with an improved toner transfer from the transfer medium to the substrate. Also, when used in a multi-color printing installation, the printing device is also intended to avoid the known problems, in particular because of the excessive heat introduction.  
         [0007]     This object is attained with a transport system for the one or every substrate each having a receiving device which can be heated, to which one or several heating elements for introducing heat energy into the substrate are assigned. With such an arrangement, the area to be heated is substantially limited to the substrate, so that heating of the printing unit, or the introduction of heat, beyond this is limited. Thus, in an arrangement of several adjoining printing units the mutual thermal influence is also reduced.  
         [0008]     In addition, a cooling device is assigned to the transfer medium of the respective printing unit, which removes heat energy from the transfer medium.  
         [0009]     With the cooling of the transfer medium, following the transfer to the substrate, the toner powder does not adhere to the surface of the transfer medium, but during the transfer is almost completely released. The heat introduction into the printing unit, in particular into the sensitive photo-conductor, is also prevented, or at least minimized to a permissible amount, because of the cooling.  
         [0010]     Heating, which is substantially restricted to the substrate, is achieved if the heating element is arranged on the side of the substrate facing away from the receiving device of the transport system. Thus, the underside of the substrate is substantially heated over its entire surface, so that good heat penetration up to the substrate surface is assured. Excess heating of the surroundings is avoided.  
         [0011]     In order to minimize undesired heat transfer into the receiving device, while at the same time assuring the placement of the heating device at the receiving device in the area of the substrate underside, the receiving device can have an approximately frame-shaped receiving structure for supporting one substrate. Thus a support and fixation of at least areas of the substrate at the receiving device is assured.  
         [0012]     In accordance with a preferred embodiment, it is possible to realize a multi-color printing device with a plurality of printing units arranged one behind the other. In this case, single-color printing is performed in each printing unit, wherein toners of different color are employed in the different printing units.  
         [0013]     A plurality of substrates can be arranged one behind the other can be conducted by the transport system through the transfer zones of each one of the printing units. Thus it is possible to achieve a particularly high throughput, in particular if the transport system moves on the respective substrate, or arrangement of substrates, continuously.  
         [0014]     In one embodiment, each substrate is received in a separate receiving device, upstream in the transport direction, of the first of the printing units arranged one behind the other. Thereafter the substrate, or the sequence of the substrates, is conducted in the respective receiving devices one after the other to the individual printing units. Following the last one of the printing units arranged one behind the other, each of the imprinted substrates can be removed again from the respective receiving device.  
         [0015]     In this case, after having been taken out of the receiving device, the substrate can be transferred to a transfer unit or a sorting unit, for a partial or a fully automatic handling of the imprinted substrates.  
         [0016]     In connection with a multi-color printing device in particular, the transport system can have a conveying arrangement which transports every receiving device along a guidance arrangement. Here, the conveying arrangement can have at least one toothed belt, continuously revolving endless belts, or like conveying elements. Such a conveying arrangement causes an advancing movement of the respective receiving device. In this case, the guidance device can be an arrangement of guide rods or guide rails, or similar guide elements.  
         [0017]     In one embodiment, the guidance arrangement can form a closed track, or a conveying circuit, for conveying the receiving devices. A particularly compact printing device can be realized with such an arrangement. During this the receiving devices can have exactly guiding linear bearings, which leave the guide elements in the area of the course changes of the conveying circuit and, after passing through the course changes, are caught again in the conically designed guide elements and are centered.  
         [0018]     In a particularly advantageous embodiment, a cleaning device for the receiving devices can be arranged at the closed track or at the conveying circuit. It is thus possible to dependably remove the dirt accumulation on the receiving devices, which is unavoidable, in particular when printing.  
         [0019]     The cleaning device can be preferably arranged following the last of the printing units arranged one behind the other in the conveying direction of the receiving devices. Following the removal of the respective substrate, the soiled receiving device is introduced into the cleaning installation. After passing through the cleaning installation, the cleaned receiving device is again available for receiving a substrate to be imprinted. For this purpose, the cleaning device can be arranged upstream of the first of the printing units arranged one behind the other in the conveying direction of the receiving devices.  
         [0020]     In accordance with one embodiment of this invention, the transfer medium has a lower temperature in the transfer zone formed with the substrate, at least in the area of the contact surface, than the surface of the substrate. In this case, a heat flow can take place at most from the substrate to the transfer medium. Then the cooling device draws off this heat, at least the largest part of it, in a controlled way. In this case, the transfer medium can be embodied as a transfer roller or transfer belt containing at least a portion of the cooling device.  
         [0021]     Alternatively or in addition, the substrate can also be placed on a conductive support of the receiving device. In connection with negatively charged toners, the support is positively charged. Correspondingly negatively with positively charged toners. The voltages of the charges can be reduced so that negative field effects, such as occur with toner transfers generated purely by electrostatic fields alone, no longer occur.  
         [0022]     For achieving a particularly good toner transfer it is possible to move the substrate by the transport system past or beyond the transfer medium synchronously with the circumferential speed of the transfer medium. It is thus possible to apply a voltage to each receiving device in the transport system which, with respect to the transfer medium, is opposite to the charge of the toner.  
         [0023]     A particularly effective heating of the substrate can take place if the substrate can be charged by an infrared radiation device and/or a heating element embodied as a hot-air blower and/or by charging with heat energy. In this case, the temperature should be set as a function of the toner used. Tests with ceramic toners having a proportion of solid materials, such as pigments, or glass flit, of 50 to 70% have shown that a surface temperature of the substrate of between 220° C. and 150° C. is particularly advantageous. After being transferred, the toner powder should melt and adhere to or on the substrate. If the toner powder melts completely, a subsequent fixation can possibly become unnecessary.  
         [0024]     Particularly effective heating of the substrate can occur by metal tape heating or metal foil heating, in which a temperature is created whose wavelength is exactly matched to the absorption maximum of the substrate and the plastic matrix of the toner. A further advantage of the metal heating and metal foil heating is in the low mass of the heat conductors, and thus in a very steep characteristic heating and cooling line.  
         [0025]     On the one hand, the print medium can be of a thermoplastic matrix, in which organic or inorganic color pigments and/or small glassy flow particles can be embedded, for coloring.  
         [0026]     In another case the plastic matrix is of a mixture of hardening and binder resins, or of polymers which, at temperatures &gt;160° C., react to form thermosetting, such as a rule spatially cross-linked, structures, in which organic or inorganic color pigments can also be embedded.  
         [0027]     Furthermore, other additives, such as particles capable of conducting, or mechanically resistant materials can also be contained, which later on result in an electrically conductive coating or a scratch-protection layer.  
         [0028]     It can be necessary to keep the substrate temperature as low as possible and match it to the substrate to be imprinted. This is of particular importance with highly temperature-sensitive plastic substrates or with glass which is less resistant to temperature differences. It is thus necessary to match the plastic matrix of the print media so that the softening point of the matrix is also lowered. This is of particular interest if, in the case of additives such as ceramic pigments or small glassy flow particles, the softening temperature rises with an increase in the proportion of the solid materials in the plastic matrix.  
         [0029]     A reduction of the softening temperature in case of an increase in the proportion of the solid materials occurs, for one, by the addition of polymer additives, such as wax, or by the use of a different, lower melting plastic matrix.  
         [0030]     For achieving control of the substrate temperature, a temperature sensor, for example a pyrometer, can be assigned to the substrates, or to each one of the substrates, and the heating element and/or the transport system can be controlled by a control device as a function of the signal emitted by the temperature sensor.  
         [0031]     It is thus possible to regulate the temperature by acting on the transport system via the advancement time of the substrate, or its loitering time in areas outside of the transfer zone. Thus, for example in case of an arrangement of several printing units in series, the heat output of the heating elements can be reduced when the substrate is not located in a transfer zone, for example in the area between two adjoining printing units.  
         [0032]     The regulation can occur so that the substrate always enters a transfer zone at a constant surface temperature. During transfer, the substrate surface should be uniformly heated.  
         [0033]     To achieve an effective temperature of the transfer medium, one or several liquid-cooled contact rollers of the cooling device can roll off on the transfer medium, and/or a conditioned air flow can be directed on the surface of the transfer medium.  
         [0034]     It is also possible for the transfer medium to be embodied as a transfer roller which contains at least a part of the cooling device. In this case, the cooling device can also have one or several Peltier elements. Alternatively or in addition, the transfer roller can also be water-cooled or air-cooled.  
         [0035]     If the cooling device removes heat energy from the transfer medium downstream of the transfer zone and upstream of the photo-conductor of the printing unit, viewed in the transport direction of the transfer medium, then the introduction of heat into the photo-conductor is dependably prevented. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]     This invention is described in greater detail in view of an exemplary embodiment represented in the drawings, wherein:  
         [0037]      FIG. 1  shows a single-color printing device with a printing unit, in a schematic lateral view; and  
         [0038]      FIG. 2  shows a multi-color printing device in a schematic lateral view, with six printing units in accordance with  FIG. 1  arranged in series, and with a cleaning device, which circulates in a printing circuit, for receiving devices for substrates to be imprinted. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0039]     A printing device with an electro-graphic printing unit  10 . 1  is shown in  FIG. 1 . It has a roller-shaped photo-conductor  30  and has a uniform charge on its surface, by a charging corona  32 . This charge is partially removed, corresponding to the pattern to be printed, by an adjoining LED-writing head or a laser arrangement  34 , so that a lateral charge image is created. A developer unit  36  applies toner powder to the discharged areas of the photo-conductor surface. The toner image thus developed is transferred in a transfer zone to a transfer medium  12 . The transfer medium  12  has a base roller body  12   a . A resilient, electrically semi-conducting intermediate layer  12   b  is applied to the base roller body  12   a . It can comprise, for example, silicon, EPDM or polyurethane. An anti-adhesion coating  12   c  is arranged indirectly or directly above the intermediate layer  12   b  and forms the roller surface.  
         [0040]     A cleaning unit  31  is arranged at the photo-conductor  30 , and a cleaning unit  13  at the transfer medium  12 , which remove toner residue with cleaning brushes and suitable strippers and return it to the developer unit  36  via suitable toner return worms.  
         [0041]     A transport system  16  is arranged below the transfer medium  12  and has an approximately frame-shaped receiving device  18 . 1 , which is movable toward the right in the direction of the arrow A, and on which a substrate  14 . 1  can be conveyed. Here, the transport system  16  is arranged so that the transfer medium  12  rolls off on the surface to be imprinted of the substrate  14 . 1 . In the process, the toner powder on the transfer medium is transferred onto the substrate  14 . 1 .  
         [0042]     The transport system  16  comprises a guide rod  52 , along which the receiving device  18 . 1  is guided by engaging guide devices, not represented. The advancing movement in the direction A is provided by a motor-driven toothed belt  50 .  
         [0043]     A panel heating element, or several individual heating elements  20 . 1 , are arranged inside the receiving device  18 . 1  and act on the underside of the substrate  14 . 1  and heat the substrate so that the surface is uniformly heated to a temperature within a range between 160° C. and 170° C. One or several temperature sensors  26  are arranged between the heating elements  20 . 1  and the transfer medium  12 , for monitoring the temperature and transmit a temperature signal to one or several regulators  23 . The regulator  23  reads in a preset value via a control device  24 . The preset value is compared with the temperature signal. In case of a temperature difference, the heating elements  20 . 1  can be readjusted. The heating element  20 . 1  is supplied via a bus bar  25 .  
         [0044]     In a supporting way, it is also possible to regulate the transport speed of the transport system  16  in the area of or near the transfer medium  12 . It is thus assured that the substrate  14 . 1  always enters the transfer zone at an approximately constant surface temperature.  
         [0045]     A sensor unit  38 . 1  is arranged in the area upstream of the transfer medium in the transport direction A of the transport system  16 , which, in the manner of a light barrier, puts the printing unit  10 . 1  into operation as soon as it is detected and the substrate  14 . 1 , or the receiving device  18 . 1  with the substrate  14 . 1  arranged therein, enters the transfer zone. This detection signal can be provided to the control device  24  which, by the regulator  23 , increases the heating output of the heating elements  20 . 1  so that the temperature required or preferred for hot transfer is reached in the substrate  14 . 1 .  
         [0046]     A cooling device  28  is assigned to the transfer medium  12 . It has an air inflow conduit  40 . It is possible to blow a gaseous cooling medium, preferably conditioned air, through this onto the surface of the transfer medium  12 . The air removes heat energy from the transfer medium  12 . The heated fluid flow can then again be drawn off via a return air flow conduit  42 . The return air flow conduit  42  prevents gas flows from being created outside the cooling zone, which could lead to a damage of the toner image maintained on the transfer medium  12  or the photo-conductor  30 .  
         [0047]     In an alternative embodiment, not represented, the cooling device has one or several water-cooled rollers, which are in surface contact with the transfer medium. The rollers are connected with a temperature-regulating unit and withdraw heat energy from the transfer medium. The water coming from the rollers is conducted to the temperature-regulating unit via a circulation system. It is cooled in the temperature-regulating unit and is then again conducted to the rollers.  
         [0048]     In a further embodiment, not shown, the core of the transfer roller comprises a material with good heat-conducting properties, for example copper, aluminum or ceramic materials, such as SiC or Si3N4 and, if desired, has cooling fins and is cooled by an airflow in the interior of the transfer roller. The core is coated with a 1 to 2 mm thick, flexible, heat-conducting material, such as PTFE, FPM, silicon or PUR plastic, filled with a glass or mineral material, for example.  
         [0049]     A transfer belt with an interior blower is also possible, so that cooling over a large surface with a relatively small air flow is possible.  
         [0050]     It is advantageous if zone heating, not represented, is provided on the receiving device  18 . 1  over the printing width defined by a width of the fed-in substrate. With such zone heating it is possible to regulate the heat output in the edge area of the substrate surface, respectively separately toward the center. This has one advantage that the surface temperature can be better regulated over the printing width, and with it the temperature constancy over the printing width can be improved. For this purpose, respectively individual regulators  23  and temperature sensors  26  are assigned to each zone heating element. In this case, the temperature sensors  26  are advantageously in the form of pyrometers which detect the surface temperature of the substrate  14 . 1 .  
         [0051]     In a schematic lateral view of  FIG. 2 , a multi-color printing device with six printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6  arranged in series, and a cleaning device  54  for receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  for substrates  14 . 1 , . . . ,  14 . 5 ,  14 . 6  circulating in a printing circuit. For the sake of clarity, only three of the six printing units are represented in  FIG. 2 .  
         [0052]     The printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6  are electro-photographic units, such as described in regard to  FIG. 1 . However, alternatively the individual printing units can also be designed differently. The individual printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6  can be activated by a sensor arrangement  38 . 1 , . . . ,  38 . 5 ,  38 . 6  arranged upstream of the assigned printing unit.  
         [0053]     The printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6  are arranged in a modular arrangement on a common support frame  60 . Because of the modular arrangement, the individual printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6  are easily accessible, which assures cost-effective operations, in particular for maintenance, repair, but also for toner changes. The serial arrangement of the six printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6  allows direct full-color printing by the use of different toner colors in the individual printing units, wherein the complete color print can be applied in one passage.  
         [0054]     In this case, operating panels for electric devices, flat glass, plastic panels, light-density laminated plates for the advertising industry, signs, or like media suited for the hot-transfer method are used as substrates  14 . 1 , . . . ,  14 . 5 ,  14 . 6  to be imprinted.  
         [0055]     The substrates  14 . 1 , . . . ,  14 . 5 ,  14 . 6  are conveyed through the multi-color printing device by a transport system  16 . In this case, each substrate  14 . 1 , . . . ,  14 . 5 ,  14 . 6  is received in a heatable receiving device  18 . 1 , . . . ,  18 . 5 ,  18 . 6 . The receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  have one or several heating elements  20 . 1 , . . . ,  20 . 5 ,  20 . 6  for introducing heat energy into the substrate  14 . 1 , . . . ,  14 . 5 ,  14 . 6 . For this purpose, each heating element is arranged on the side of the substrates  14 . 1 , . . . ,  14 . 5 ,  14 . 6  facing away from the printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6 , so that always the underside of the substrates is heated. The heat penetrates the substrate material, so that the surface is also heated.  
         [0056]     As described in view of  FIG. 1 , the heating elements  20 . 1 , . . . ,  20 . 5 , 20 . 6  of the receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  are controlled by an arrangement having a regulator  23 , a control device  24  and one or several temperature sensors  26 . During this, the same temperature conditions are set at all receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  located in the areas of the six printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6 . A bus bar  25  is arranged in this area for this purpose, through which the heating elements  20 . 1 , . . . ,  20 . 5 ,  20 . 6  are provided with electrical current. However, alternatively it is also conceivable to control the individual heating elements  20 . 1 , . . . ,  20 . 5 ,  20 . 6  of the individual receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  separately.  
         [0057]     For a secure conveyance through the multi-color printing device, each substrate  14 . 1 , . . . ,  14 . 5 ,  14 . 6  is supportingly fixed, at least over some areas, on the assigned receiving device  18 . 1 , . . . ,  18 . 5 ,  18 . 6 . In the represented embodiment, each receiving device  18 . 1 , . . . ,  18 . 5 ,  18 . 6  has an approximately frame-shaped receiver for respectively one substrate  14 . 1 , . . . ,  14 . 5 ,  14 . 6 .  
         [0058]     The transport system  16  is designed so that several substrates  14 . 1 , . . . .  14 . 5 ,  14 . 6  can be continuously conducted, one behind the other, through the transfer zones of each one of the printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6 .  
         [0059]     A feed unit  62  is arranged in the transport direction A of the transport system  16  upstream of the first printing unit  10 . 1  of the printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6  arranged one behind the other, in which each substrate to be imprinted is received in a separate receiving device  18 . 1 , . . . ,  18 . 5 ,  18 . 6 , and is sequentially conveyed to the printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6 .  
         [0060]     A removal unit  64  is arranged after the last printing unit  10 . 6  in the transport direction A of the transport system  16 , in which each already imprinted substrate  14 . 1 , . . . ,  14 . 5 ,  14 . 6  can be taken out of the assigned receiving device  18 . 1 , . . . ,  18 . 5 ,  18 . 6 . In addition, following the removal from the receiving device  18 . 1 , . . . ,  18 . 5 ,  18 . 6 , which can take place automatically, each substrate  14 . 1 , . . . ,  14 . 5 ,  14 . 6  can be transferred into a transfer unit or a sorting unit.  
         [0061]     The transport system  16  has a conveying device  50 , which transports each receiving device  18 . 1 , . . . ,  18 . 5 ,  18 . 6  along a guide rod  52 . Alternatively, the receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  can also be guided in a guide rail or like guide element.  
         [0062]     The conveying device  50  has an arrangement of toothed belts driven by electric motors, which are linked, capable of being placed under tension, to each receiving device  18 . 1 , . . . ,  18 . 5 ,  18 . 6 . Alternatively, an arrangement of a continuously revolving belt or like conveying element can be employed. All receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  are uniformly moved with the aid of the conveying device  50 . Thus a closed track, or a transport circuit for the transport of the receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  is formed by the guide arrangement  52 .  
         [0063]     The transport system  16  is laid out so that the guide devices, not represented, of the receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  which engage the guide rod  52  are disengaged at the course-change locations  70 . 1  and  70 . 2  of the closed track, and entry into the linear bearings takes place via cones at the guide rod  52 .  
         [0064]     A cleaning device  54  for removing dirt or printing ink residue from the receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6  is arranged at the closed track or the transport circuit. The cleaning device is arranged, viewed in the transport direction A of the transport system  16 , downstream of the last printing unit  10 . 6  of the printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6  arranged one behind the other, and downstream of the removal unit  64 . Each receiving device  18 . 1 , . . . ,  18 . 5 ,  18 . 6  is introduced into the cleaning device  54  following the removal of each substrate  14 . 1 , . . . ,  14 . 5 ,  14 . 6 . Here, the cleaning device  54  is arranged, viewed in the transport direction A of the transport system  16 , upstream of the feed unit  62  and upstream of the first printing unit  10 . 1 .  
         [0065]     Together with the temperature regulating device at the individual receiving devices  18 . 1 , . . . ,  18 . 5 ,  18 . 6 , the transport system  16  is controlled by a computer arrangement  66  and a controller circuit  68 . In addition, printing software required for the individual printing units  10 . 1 , . . . ,  10 . 5 ,  10 . 6 , along with the appropriate software tools, is also contained in the computer arrangement  66 . It is also possible to perform image processing of the print pattern in the computer arrangement  66 .