Abstract:
A device for printing a print carrier in the printing region makes use, in a force transmission region, of a driven transport drum and non-driven back-pressure rollers or, alternatively, of a non-driven back-pressure conveyor belt. A stationary ink-jet printing head prints, in the printing region, the print carrier that is moved downstream, the ink-jet printing head being disposed axially relative to the transport drum. The printing region amounts preferably to about 1 inch and is at a distance from the force transmission region, the distance of the furthest pixel from the edge of the transport drum, being smaller than the radius of the circumference of the transport drum.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a device for printing a print carrier in a printing region using a stationary ink-jet printing head and with a print carrier moved downstream in the Z-direction of the printing region. The invention is employed in fully electronic digital printing devices in which recording carriers have variable dimensions (i.e. thickness and size). It is particularly suitable for use in voucher printers, franking machines, addressing machines and other mail processing appliances with a transport and printing device for mail items. 
     German Patent DE 25 01 035 C2 discloses a printing drum with a single ink-jet printing head. The printing drum has a transport function and a printing function, and therefore the ink-jet printing head cannot print the entire printing region. Variable information can be printed only by the ink-jet printing head. Regions in which the printing information cannot, in principle, be changed therefore remain in the printing image. 
     Modern franking machines, such as, for example, the thermotransfer franking machine known from U.S. Pat. No. 4,746,234, employ fully electronic digital printing devices. It is consequently possible, in principle, to print any desired texts, codes and special characters in the franking-stamp printing region and any desired advertising block print or one assigned to a cost center. A franking machine, for example T1000 from the applicant Francotyp-Postalia AG &amp; Co., is controlled by a microprocessor control surrounded by a secured housing which has an orifice for feeding a letter. Whenever a letter is fed, a mechanical letter sensor (microswitch) transmits a print demand signal to the microprocessor which generates a printing image and, after billing the mailing value to be franked, triggers printing. The franking imprint contains previously entered and stored mail information for transporting the letter. 
     U.S. Pat. No. 5,467,709 discloses a printing device for an ink-jet franking machine, a franking print being printed onto a mail item by an ink-jet printing head during approximately horizontal letter transport. For printing, the ink-jet printing head is disposed in a stationary manner in a recess behind a guide plate. A rotating transport belt serves as a transport device and is likewise disposed at the side of the guide plate. A back-pressure device with a plurality of rollers is disposed on the other side, opposite the guide plate, so that a fed mail item is clamped between the rollers of the back-pressure device and the rotating transport belt. However, the configuration cannot prevent the print carriers from running askew. Even an insufficiently tensioned transport belt or a not exactly parallel alignment of the axes of those rollers on which the transport belt rotates gives rise to the above-mentioned risk. Due to the multiplicity of rollers of the back-pressure device, the latter is dynamically redundant. 
     German Patent DE 196 05 015 C1, corresponding to U.S. Pat. No. 5,949,444, has already proposed a version of a printing device of a JETMAIL ink-jet franking machine of the applicant Francotyp-Postalia AG &amp; Co., which, in the case of nonhorizontal approximately vertical letter transport, executes a franking print by an ink-jet printing head which is disposed in a stationary manner in a recess behind a guide plate. A rotating transport belt serves as a transport device, with pressure elements for the mail items (letters with a thickness of up to 20 mm, DIN B 4  format) or for franking strips that are configured to be capable of being glued onto packets of any thickness. The print carrier (letter, packet, franking strip) is clamped between the pressure element and the guide plate. 
     The transport and printing device is disposed in the base and is controlled by a meter. A trigger sensor for the printing process is disposed in the base, just in front of the ink-jet printing head recess, for detecting the start of a letter and cooperates with an incremental generator on the drive of the transport belt. By use of a transmitted-light barrier as a trigger sensor (Published, European Patent Application EP 0 901 108 A2), the leading edge of even especially thick mail items is detected unequivocally. Moreover, optical sensors for detecting the build-up of mail items are employed in the base of the JETMAIL. An automatic feed and a dynamic balance are disposed in the CONDORD franking system upstream of the JETMAIL franking machine, thus allowing genuine mixed-mail processing for mail items of widely differing formats, thicknesses and weights. At very high printing speeds, it becomes increasingly more difficult to print a stamping print of relatively high quality that allows mechanical evaluation with a high degree of reliability. A rotating transport belt should therefore have no stretching, even under mechanical and thermal load, and the pressure elements should not allow any slip during the transport of the mail items. Only transport and printing devices that are of complicated constructions and are cost-intensive have hitherto satisfied these requirements. 
     There have also already been proposed more simply constructed transport and drive devices without back-pressure device (German Patent DE 196 05 014 C1) or with a back-pressure device (International Patent Disclosure WO 99/44174) in the vicinity of the printing region of at least one ink-jet printing head. In International Patent Disclosure WO 99/44174, the latter is disposed downstream of a pair of draw-in rollers in the transport direction, the upper roller being driven and the lower back-pressure roller being sprung. A further pair of rollers downstream of the ink-jet printing head, near the ejection, likewise exerts a force on the print carrier. The printing region is at a distance from the force transmission region of one of the pairs of rollers of more than one radius of the respectively driven roller. Although, in principle, the printing information can be changed in all regions as a result of digital printing, the print is nevertheless of lower quality, the higher the selected transport speed. In particular, when two ink-jet printing heads are employed, an offset may occur in the printing image (correspondence error) along a printing length in the transport direction, thus making the mechanical evaluation of the printing image more difficult. The force action of the further pair of rollers downstream of the ink-jet printing head, near the ejection, leads to a different path length and therefore, in the case of two ink-jet printing heads offset to one another, to the correspondence error in the printing image. The print quality demanded within the framework of current programs of the mail service providers, for example the information-based Indicia program of the USPS, could therefore be achieved only at the expense of a low printing speed. Another disadvantage is the small thickness of the print carriers that can be printed by such a simply constructed printing device. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a device for printing a print carrier which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which makes it possible, even at very high printing speeds, to have a print of higher quality. Printing is to take place directly onto print carriers that have a thickness of up to 10 mm. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a printing device containing a driven transport drum and a non-driven back-pressure device resilient in a Y-direction and disposed opposite the driven transport drum. The driven transport drum and the non-driven back-pressure device define a force transmission region there-between and exert a transport force on a print carrier in the force transmission region. An ink-jet printing head is disposed axially relative to the driven transport drum in an X-direction. The print carrier is printed on in a printing region by the ink-jet printing head and the print carrier is moved downstream in a Z-direction in the printing region. The printing region is at a distance from the force transmission region, and the X-direction is orthogonal to the Z-direction and orthogonal to the Y-direction. 
     A rotating transport drum serves as the transport device and is disposed on the same side of the guide plate as the ink-jet printing head. The resilient back-pressure device is disposed on the other side, opposite the transport drum, so that a fed print carrier (i.e. letter) is clamped between the transport drum and the back-pressure roller of the back-pressure device. The clamping location is referred to hereafter as the force transmission region, because the force for transporting in the transport direction is transmitted to the print carrier there. None of the back-pressure rollers of the back-pressure device is disposed opposite the ink-jet printing head. The ink-jet printing head is at a distance, or the one hand, from the transport drum in the axial direction and, on the other hand, from the contact surface between the guide plate and print carrier, both distances being minimized. The printing region is offset relative to the force transmission region in the X-direction and is located at the edge of the printing drum. For printing, the at least one ink-jet printing head is disposed in a stationary manner behind a guide plate in a recess which starts at the edge of the printing drum. The imprint is applied contactlessly to the print carrier in the printing region during the transport of the print carrier in the Z-direction. The printing device may be configured for approximately horizontal, inclined or vertical transport of the print carrier in the Z-direction. For example, in a franking machine with the at least one ink-jet printing head, a franking print of high print quality is printed onto a moved mail item in the printing region. Although the transport drum is very large, so that even relatively thick mail items are properly picked up and transported, in the device according to the invention the furthest pixel of the printing region is nevertheless nearer to the force transmission region than corresponds to the radius of the transport drum. 
     The device for printing a print carrier in the printing region makes use, in the force transmission region, of a driven transport drum and non-driven back-pressure rollers or, alternatively, a non-driven back-pressure conveyor belt of the back-pressure device. 
     In accordance with an added feature of the invention, the driven transport drum has an edge and a radius of a circumference. And a distance of a furthest pixel in the printing region from the edge of the driven transport drum is smaller than the radius of the circumference of the driven transport drum. 
     In accordance with an additional feature of the invention, a housing having a slit-shaped orifice formed therein is provided. The driven transport drum and the non-driven back-pressure device are disposed in the housing. The driven transport drum and the back-pressure device exert the transport force on the print carrier in an area of the slit-shaped orifice of the housing. The slit-shaped orifice has a depth extending in the X-direction orthogonally to the Z-direction for the print carrier moved downstream, so that the force transmission region and the printing region are disposed within the slit-shaped orifice. The driven transport drum has a bearing axle running parallel to the X-direction and has an orifice formed therein. The ink-jet printing head has nozzles and an ink container disposed in the X-direction with the ink container at least partially in the orifice of the driven transport drum in such a way that the nozzles are located at the edge of the driven transport drum for emitting ink drops on demand, opposite to the Y-direction, onto a surface of the print carrier in the printing region. And the non-driven resilient back-pressure device exerts a spring pressure in the Y-direction. 
     In accordance with another feature of the invention, the non-driven back-pressure device has non-driven back-pressure rollers functioning as resilient devices. 
     In accordance with a further feature of the invention, the non-driven back-pressure device has at least one non-driven back-pressure conveyor belt functioning as a resilient device. 
     In accordance with a further added feature of the invention, a motor is disposed in the housing. A guide plate against which the print carrier rests and has a guide plate orifice formed therein is disposed on one side of the slit-shaped orifice of the housing. The driven transport drum exerts the transport force on the print carrier in a transport direction through the guide plate orifice in the guide plate when the motor is activated. A feed deck is disposed opposite the guide plate, on another side of the slit-shaped orifice of the housing. The feed deck has feed deck orifices formed therein for receiving the resilient devices of the non-driven back-pressure device. The feed deck orifices include a first orifice and a second orifice and the guide plate orifice and the first orifice of the feed deck are located opposite one another. 
     In accordance with another added feature of the invention, the guide plate has a further guide plate orifice formed therein for ink-jet printing from the nozzles of the ink-jet printing head. The further guide plate office is disposed next to the guide plate orifice so as to be offset in the X-direction. 
     The further guide plate orifice has a size corresponding to the printing region. 
     In accordance with further additional feature of the invention, a driven draw-in roller is provided. The guide plate has a further guide plate orifice formed therein for receiving the driven draw-in roller, and the further guide plate orifice is disposed next to the guide plate orifice. The guide plate orifice for the driven transport drum is offset to the further guide plate orifice in the Z-direction. 
     In accordance with another additional feature of the invention, a non-driven draw-in roller is disposed in the second orifice of the feed deck. The first orifice and the second orifice of the feed deck are disposed next to one another such that the first orifice is offset to the second orifice in the Z-direction. 
     In accordance with an added feature of the invention, the guide plate has a further guide plate orifice formed therein for detecting the print carrier and is disposed next to the guide plate orifice for the driven transport drum. The guide plate orifice is offset to the further guide plate orifice in the Z-direction. 
     In accordance with an additional feature of the invention, the second orifice of the feed deck is provided for detecting the print carrier and is disposed next to the first orifice of the feed deck such that the first orifice is offset to the second orifice in the Z-direction. 
     In accordance with another feature of the invention, the non-driven back-pressure device has a first fixed bearing axle, a resilent rocker mounted pivotably about the first fixed bearing axle, a first axle fastened on the resilent rocker, and a first tension spring acting on the resilent rocker. The non-driven back-pressure rollers are mounted rotatably on the first axle and act on the print carrier through the first orifice in the feed deck by a spring force of the first tension spring. 
     In accordance with a further feature of the invention, a pilot control mechanism having a front and disposed upstream of the non-driven back-pressure device is provided. The pilot control mechanism has a second fixed bearing axle, a sprung rocker disposed at the front along the transport path and is mounted pivotably about the second fixed bearing axle, a second axle fastened to the sprung rocker, a second tension spring acting on the sprung rocker, and a non-driven draw-in roller mounted rotatably on the second axle. An upper draw-in roller is disposed above the non-driven draw-in roller and the non-driven draw-in roller acts the upper draw-in roller or on the print carrier through the second orifice in the feed deck by a spring force of the second tension spring. A lifting rod is provided for coupling the sprung rocker to the resilent rocker in such a way that a movement of the sprung rocker caused by a thickness of the print carrier is transmitted at least partially to the resilent rocker being a rear rocker. 
     In accordance with a further added feature of the invention, the first tension spring has a spring constant being substantially higher than that of the second tension spring. 
     In accordance with a further additional feature of the invention, the sprung rocker and the resilent rocker are each formed of two angle levers including a first angle lever and a second angle lever each having legs including a first leg and a second leg. The sprung rocker and the resilent rocker each have a first spacer piece connected between the first leg of the two angle levers, a second spacer piece connected between the first leg and the second leg of each of the two angle levers, and a bolt fastened to the second leg of the first angle lever and the second leg of the second angle lever for spring suspension. 
     In accordance with another added feature of the invention, the lifting rod has a first hole and a second hole formed therein at opposite ends of the lifting rod. The sprung rocker has a further bolt disposed along the transport path and is held in the first hole at one end of the lifting rod. The bolt of the rear rocker is disposed along the transport path and is held in the second hole at the other end of the lifting rod. 
     In accordance with an added feature of the invention, the non-driven back-pressure device has a sprung long rocker, first axles fastened on the sprung long rocker, and supporting rollers mounted rotatably on the first axles. The non-driven back-pressure conveyor belt runs on tile supporting rollers. The non-driven back pressure device has second axles fastened on the sprung long rocker and deflecting rollers for the non-driven back-pressure conveyor belt are mounted rotatably on second axles. The non-driven back pressure device has a sprung short rocker, a third axle fastened on the sprung short rocker, and a non-driven lower draw-in roller mounted rotatably on the third axle. The non-driven back pressure device has a common fixed bearing axle and the sprung long rocker and the sprung short rocker are mounted pivotably about the common fixed bearing axle. The non-driven back pressure device has a feed deck with a first orifice and a second orifice formed therein, a first compression spring and a second compression spring. The non-driven back-pressure conveyor belt acts on the print carrier through the first orifice in the feed deck by a spring force of the first compression spring, and the non-driven lower draw-in roller acting on the print carrier through the second orifice in the feed deck by a spring force of the second compression spring. 
     In accordance with an additional feature of the invention, the first compression spring has a spring constant substantially higher than that of the second compression spring. 
     In accordance with another feature of the invention, the locking nuts fasten the second axles to the sprung long rocker. 
     In accordance with a further feature of the invention, at least one friction covering rests annularly against a circumference of the driven transport drum. 
     In accordance with another added feature of the invention, the driven transport drum has a bearing axle. A worm wheel, a drive belt, and a driving wheel driven by the drive belt are disposed near one end face of the driven transport drum on the bearing axle of the driven transport drum. A motor driving the driven transport drum is provided. The motor has a motor axle and a worm pinion disposed on the motor axle and engages the worm wheel. A driven wheel and an upper draw-in roller coupled to the driven wheel which is driven by the drive belt, are provided. 
     In accordance with yet another feature of the invention, the driving wheel and the driven wheel are toothed-belt wheels, and the drive belt is a toothed belt. 
     In accordance with a further feature of the invention, an encoder disk, which can be sensed by a light barrier in an encoder, is disposed on the motor axle. 
     In accordance with a concomitant feature of the invention, the driven transport drum has an end face with markings disposed on the end face, near a circumference of the driven transport drum to be read by an encoder. 
     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 embodied in a device for printing a print carrier, 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view through a front side of a printing device with a transport and back-pressure device, in a first variant, according to the invention; 
     FIG. 2 is a perspective, rear view of the printing device from above; 
     FIG. 3 is a perspective, front view of the printing device from below; 
     FIG. 4 is a sectional view through the front side of the printing device with the transport and back-pressure device, in a second variant; and 
     FIG. 5 is a sectional view from below of the back-pressure device according to the second variant. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a section through the front view of a printing device  1  with a transport and back-pressure device in a first variant. Such a printing device may be employed in a franking machine, an addressing machine or another mail processing appliance with mail-item transport. The printing device  1  is disposed in a housing  4  with an orifice  3  for the mail-item feed. 
     A transport direction for a fed mail item is marked by an arrow and runs downstream from left to right in the Z-direction. The mail item, when being transported, comes to rest against a guide plate  2 . A transport and drive device  5  is located on one side of the guide plate  2 . A driven draw-in roller  71  is likewise located on the same side of the guide plate  2 . A motor  13  generates the drive force required. Orifices  9  and  11  for the driven draw-in roller  71  and for a driven transport drum  51  are disposed in the guide plate  2  so as to be offset in the Z-direction. 
     A nondriven back-pressure device  6  and a nondriven pilot-control mechanism  7  are disposed on the other side, opposite the guide plate  2 . A feed deck  8  limits the thickness of the fed mail item to 10 mm. Orifices  81 ,  82  are disposed, offset in the Z-direction, in the feed deck  8 . The orifices  11 ,  81  and  9 ,  82  are located opposite one another. The orifice  81  is provided for back-pressure rollers  61 ,  62  of the back-pressure device  6 , which are mounted on a resilient rocker  66 , and the orifice  82  is provided for a draw-in roller  72  of the pilot-control mechanism  7 , the draw-in roller being mounted on a resilient rocker  76 . The rockers  66 ,  76  are formed preferably in each case from two angle levers which are mounted pivotably at a fixed location and on which a spring  63 ,  73  acts in each case. The angle levers are coupled to one another via a lifting rod  77 . 
     For a highly accurate transmission of the drive force to the print carrier, the transport drum  51  is employed, inside which there is sufficient room for accommodating ink containers (cartridges) of two ink-jet printing heads  21 ,  22  (FIG.  2 ). The printing device thereby has a highly compact construction. The two ink-jet printing heads  21 ,  22 , which are disposed with their ink containers in the transport drum  51  and in FIG. 1 are concealed by the latter, are disposed exchangeably. For example, ½-inch bubble-jet printing heads can be used and can be taken out in the Y-direction after they have first been moved in the X-direction. The X-direction and Y-direction are orthogonal to the transport direction (Z-direction). 
     A bearing axle  516  of the transport drum  51  is parallel to the X-direction. A worm wheel  517  is fastened on the bearing axle  516  and is connected to a motor axle  131  of the direct-current motor  13  via a worm pinion  16  and a coupling sleeve  15 . The ink-jet printing head  21 ,  22  (see FIG. 2) is disposed in the X-direction, in such a way that its nozzles are located at the edge of the transport drum  51 , in order to emit ink drops on demand, opposite to the Y-direction, onto a surface of the print carrier in the printing region. Nozzles of the two ink-jet printing heads  21 ,  22  are oriented in the direction of the print carrier orthogonally to the transport direction and orthogonally to the bearing axle  516  of the transport drum  51 . 
     There is provision for disposing an orifice  111 , not shown, in the guide plate  2  next to the orifice  11 , so as to be offset in the X-direction, for ink-jet printing from the nozzles of the ink-jet printing head  21 ,  22 , the orifice  111  having a size corresponding to the printing region. 
     The compact device, in which the nozzles of the ink-jet printing heads  21 ,  22  are disposed so near to an edge  511  of the transport drum  51  that virtually no distortions can occur in the printing image, is, of course, advantageous. A further advantage is that the transport drum  51  can be manufactured with high precision in an uncomplicated way, thus giving rise to a uniform transport of the print carriers (i.e. mail items) under the ink-jet printing heads  21 ,  22 . 
     A non-illustrated main circuit board for control controls the printing device. Markings are applied to an end face  512  of the transport drum  51 , near a circumference of the transport drum  51 , and are distributed over the circumference. The markings are, for example, reflecting dashes that are detected by a reflex-light barrier of an encoder  52  and are converted by the microprocessor of the above-mentioned control into printing pulses in the ratio 1:1. Fluctuations in the transport speed therefore have no influence on a printing image produced. 
     Alternatively to this, the encoder  52  may be disposed on the motor axle  131 . 
     FIG. 2 shows a perspective rear view of the printing device  1  from above. The mail-item feed again takes place in the Z-direction. The transport drum  51  is set in motion by the motor  13 , via the worm wheel  517  fastened on the bearing axle  516 , when a non-illustrated sensor for mail-item detection emits a signal and the microprocessor establishes a printing demand. 
     Located upstream are the driven draw-in roller  71 , on the same side of the guide plate  2  as the transport drum  51 , and the draw-in roller  72  of the additional pilot-control mechanism  7  for drawing in print carriers (letters, mail items) up to 10 mm. The upper draw-in roller  71  is coupled to a driven wheel  711  which is driven via a drive belt  519  which is set in motion by a driving wheel  518  on the bearing axle  516  (FIG.  3 ). The driving wheel  518  and the driven wheel  711  are, for example, toothed-belt wheels and the drive belt  519  is a toothed belt. The non-driven draw-in roller  72  is mounted rotatably on an axle  761  of the front rocker  76  of the pilot-control mechanism  7 . The mutually parallel legs that carry the axle  761  of the draw-in roller  72  are connected fixedly via a spacer piece  768 . The rocker  76  is mounted pivotably about a fixed bearing axle  766 . Any influence exerted on the printing image by the pair of draw-in rollers is ruled out in that the pressure forces and coefficients of friction between the transport drum  51  and a back-pressure roller  61  are configured to be at least one order of magnitude higher than those with respect to the pairing of the draw-in rollers  71 ,  72 . To increase the coefficients of friction, the transport drum  51  has, on its outer surface, at least one annular friction covering  513 ,  514  conforming to the circumference of the transport drum  51 . The back-pressure rollers  61 ,  62  are mounted rotatably on an axle  661  of the rear rocker  66  of the back-pressure device  6 . The mutually parallel legs of the rocker  66 , the legs carrying the axle  661  of the rollers  61 ,  62 , are connected via a spacer piece  668 . The rocker  66  is mounted pivotably about a fixed bearing axle  666 . The transport drum  51  and the sprung back-pressure rollers in the vicinity of the printing region makes it possible to dispense with further devices for transport downstream in the region of the print-carrier ejection. This rules out the situation where the further device of transport leads to a print offset. 
     The two ½-inch ink-jet printing heads  21 ,  22  project with their ink containers into the orifice  515  of the transport drum  51 . This ensures that the 1-inch printing region is very near to the force transmission region. The correspondence side of the ½-inch ink-jet printing head  21 ,  22  lies in the X-direction and is configured in a particular predetermined way. Corresponding contacting units  211  and  221  are adapted to the correspondence side of the ½-inch ink-jet printing heads  21 ,  22  for electronic signal conversion and mechanical connection. 
     FIG. 3 illustrates a perspective front view of the printing device from below. Advantageously, the rockers  66  and  76  are formed in each case from two angle levers  662 ,  663  and  762 ,  763  mounted pivotably at a fixed location. The two angle levers  662 ,  663  and  762 ,  763  of each of the rockers  66  and  76  are parallel to one another. They are fixedly connected to one another with their first legs, which in each case carry the axles  661  and  761  of the rollers  61 ,  62  and  72 , via a spacer piece  668  and  768  and with their second legs  664 ,  665  and  764 ,  765  via a spacer piece  667  and  767 . A bolt  65  or  75  is fastened for spring suspension to the latter legs  664 ,  665  and  764 ,  765  respectively. Advantageously, the rockers  66  and  76  can be produced as a plastic molding by the injection-molding method. 
     The rockers  66  and  76  in each case pivot about the fixed bearing axles  666  and  766 . However, the pivoting of the rocker  76  takes place by virtue of the thickness of a print carrier, but that of the rocker  66  takes place by virtue of the coupling of the latter to the front rocker  76  via a lifting rod  77 . The springs counteracting the respective differing outward pivoting of the rockers  66  and  76  are configured as tension springs  63  and  73 , the tension springs acting on the legs  664 ,  665  and  764 ,  765  of the angle levers  662 ,  663  and  762 ,  763  and being fastened to a bolt  65  or  75  for spring suspension. The tension springs  63  and  73  are fastened at the other end to a fixed bolt  64  and  74  respectively for spring suspension. The fixed bolts  64  and  74  are an integral part of a supporting frame, not shown, or of the housing  4  or are fastened to the latter in a way known per se. 
     Due to the spring force of at least the first tension spring  63 , the back-pressure rollers  61 ,  62  act on the print carrier through the orifice  81  in the feed deck  8 . Due to the spring force of the second tension spring  73 , the lower draw-in roller  72  can act on the upper draw-in roller  71  or on the print carrier through the Orifice  82  in the feed deck  8 . In this case, the orifice  82  is disposed next to the orifice  81 , the orifice  81  being offset to the further orifice  82  in the Z-direction. The tension spring  63  of the rear rocker  66  and the tension spring  73  of the front rocker  76  exert in each case a sprung back pressure on the print carrier resting against the driven draw-in roller  71 , against the driven transport drum  51  or against the guide plate  2 . The print carrier is, for example, a mail item, not illustrated in FIG.  3 . 
     The legs  664 ,  665  of the angle levers of the rear rocker  66  are coupled to the legs  764 ,  765  of the angle levers of the front rocker  76  via the lifting rod  77 , in such a way that an opening of the front rocker  76  having the draw-in roller  72  results in a lesser opening of the rear rocker  66  having the back-pressure roller  61 ,  62 . A movement of the front rocker  76  caused by the thickness of the print carrier is transmitted at least partially to the rear rocker  66 . For this purpose, a bolt  7671 , of the front rocker  76  along a transport path, is disposed in a hole  771  of the lifting rod  77  at one end of the latter. The bolt  65 , of the rear rocker  66  along the transport path, is disposed in a long hole  772  of the lifting rod  77  at the other end of the latter. The long hole  772  in the lifting rod  77  makes it possible, on the one hand, for the rear rocker  66  having the back-pressure rollers  61 ,  62  to open further and, on the other hand, for the draw-in rollers  71 ,  72  to close. The spring constant of the tension spring  63  is substantially higher than, but at least double, that of the tension spring  73 . For example, a pressing force of 5 to 20 Newton prevails between the draw-in rollers  71 ,  72 , but a pressing force of 10 to 50 Newton prevails between the transport drum  51  and the back-pressure rollers  61 ,  62 , so as not to influence adversely the transport or the printing image by the pilot control. 
     FIG. 4 illustrates a section through the front view of the printing device with the transport and back-pressure device  5  according to a second variant. The transport and drive device  5  corresponds essentially to the device already explained according to the first variant. However, the guide plate  2  and the feed deck  8 , including the orifices therein, have been omitted for reasons of simplification. Another difference is the configuration of the encoder  52  near the motor axle  131 . Fastened on the motor axle  131  is an encoder disk  524  that a light barrier  523  of the encoder  52  senses. Alternatively to this, the encoder  52  may be disposed on the transport drum  51 . The back-pressure device  6  according to the second variant is formed of at least one long rocker  68  mounted pivotably at a fixed location and of at least one short rocker  69  mounted pivotably at a fixed location. The above-mentioned rockers in each case pivot about a fixed bearing axle  696 , in each case counter to a spring force. It should be stressed, as regards the back-pressure device  6  according to the second variant, that this manages without a pilot-control mechanism. Since two annular friction coverings  513  and  514  for increasing the coefficients of friction are disposed on the outer surface of the transport drum  51  in conformity to the circumference of the latter, the back-pressure device  6  is equipped with two long rockers  67 ,  68  which are mounted pivotably at a fixed location and each carry at least one non-driven back-pressure conveyor belt  671 ,  672  and  681 ,  682 . The latter draws even relatively thick mail items into the force transmission region, without the pilot-control mechanism  7 , and then adapts more closely to the mail item resting against the circumference of the transport drum  51  and increases the feeding surface between the mail item and the transport drum  51 . Supporting rollers  6711 ,  6721 ,  674  and  6811 ,  6821 ,  684  are mounted rotatably on axles  675 ,  676  and  685 ,  686 . The back-pressure conveyor belt can be pretensioned by an axle  673  and  683  of the inner deflecting rollers. The axle  673  and  683  can be fixed by a locking nut  6731  and  6831 . 
     FIG. 5 shows a view from below of the back-pressure device  6  according to the second variant. The construction of the long rockers  67 ,  68  is preferably approximately identical. The construction is in each case box-shaped with a middle web carrying a spring guide boss  679 ,  689 , on which engages one side of a compression spring  47 ,  48  which is supported on its other side (in a way not shown) on the bottom or frame of the housing  4 . Only one of the two long rockers  67 ,  68  mounted pivotably at a fixed location has the integrated short rocker  69  which is mounted pivotably at a fixed location and which is likewise supported on the housing bottom or on the frame via a spring guide boss  699  and a compression spring  43 . The short rocker  69  is mounted pivotably on the common fixed bearing axle  696  and carries a lower non-driven draw-in roller  691  (FIG.  4 ). A non-illustrated sensor for mail-item detection is also disposed, upstream of the pair of draw-in rollers  71 ,  691 , in corresponding further orifices in the guide plate  2  and/or in the feed deck  8 . The measurement location is disposed in the vicinity of the transport path upstream of one of the printing heads positioned in the printing position. Disposed at the measurement location in the orifice  3  are rigid light guide elements which are configured as transparent plastic light guides for fixing and focusing an IR light beam and which transmit IR light to a main circuit board  14  of the housing bottom. An interruption in the IR light beam leads to the activation of the motor  13 . 
     The short rocker  69  is mounted pivotably on the common fixed bearing axle  696  in a first box-shaped orifice  6801  of the long rocker  68 . The short rocker has two legs  694 ,  695  and carries the lower non-driven draw-in roller  691  rotatably on an axle  692  disposed between the two legs. A middle piece  693  between the legs  694 ,  695  of the short rocker  69  carries the spring guide boss  699 . 
     The compression spring  43  of the short rocker  69  and the compression springs  47 ,  48  of the long rockers  67 ,  68  bring about a sprung back pressure on a print carrier resting against the driven draw-in roller  71 , against the driven transport drum  51  or against the guide plate  2 . Due to a spring force of the first compression spring  47 ,  48 , the non-driven back-pressure conveyor belt  671 ,  672  or  681 ,  682  acts on the print carrier through the orifice  81  in the feed deck  8 . A spring constant of the first compression spring  47 ,  48  is substantially higher than that of the second compression spring  43 . Due to a spring force of the second compression spring  43 , the non-driven lower draw-in roller  691  acts on the print carrier  12  through the orifice  82  in the feed deck  8  and lays the print carrier against the guide plate  2  or against the driven draw-in roller  71 . The lower non-driven draw-in roller  691  and the driven draw-in roller  71  form a pair or draw-in rollers  71 ,  691  which thus exerts a lower transport force on the print carrier, for example a mail item, not shown. 
     The at least one non-driven back-pressure conveyor belt  671 ,  672  or  681 ,  682  is mounted on rollers in a second box-shaped orifice  6702 ,  6802  or the at least one long rocker  67 ,  68 . The non-driven back-pressure conveyor belt  671 ,  672  is depicted cut away for the purpose of explaining the roller configuration. The roller configuration is identical for each of the long rockers  67 ,  68 . Two outer supporting rollers  6711 ,  6721  and  6811 ,  6821  are mounted rotatably on the axles  676  and  686  and are mounted at a distance from one another by spacer disks  6713 ,  6723  and  6813 ,  6823 . A guide edge  6712 ,  6722  and  6812 ,  6822  of the outer supporting rollers prevents the above-mentioned back-pressure conveyor belt  671 ,  672  and  681 ,  682  from sliding down. A middle support roller  674  or  684  engages into the interspace of the outer supporting rollers located at a distance from one another and is mounted rotatably on the axle  675  and  685 . The inner deflecting rollers  677 ,  678  and  687 ,  688  are mounted rotatably on the axles  673  and  683  and have a reduced diameter, as compared with that of other supporting rollers. This results in the back-pressure conveyor belt  671 ,  672  and  681 ,  682  being guided from the outset along an ascending run on the above-mentioned roller configuration as far as the middle supporting roller  674  and  684  in each case. By virtue of the above-mentioned guidance, an additional pilot-control mechanism may be dispensed with for mail-item thicknesses (letter thicknesses) of below 10 mm. 
     The invention is not restricted to the present embodiment. On the contrary, a number of variants may be envisaged within the scope of the claims. Thus, further different versions of the invention may obviously be developed or employed which, emanating from the same basic idea of the invention, are embraced by the accompanying claims.