Abstract:
A fixing station and a method for fixing toner images on a carrier material. The fixing station has an arrangement for moving a carrier material in a first direction and in a second direction opposite the first direction, a heat radiation source, which heats the carrier material to fix a toner thereon, and a covering device which is arranged between the carrier material and the heat radiation source and is movable parallel to the direction of the carrier material between an opened and closed position, so that while in the closed position, it blocks radiation from reaching the carrier device and in an opened position allows the radiation to heat the carrier material. The method includes delaying the opening and closing of the covering device so that a previously fixed area of the carrier material is not subjected to additional radiation from the heat radiation source.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a fixing station and a method for the uniform fixing of toner images on a carrier material. The fixing station comprises a heating device having at least one heat radiation source emitting radiation toward the carrier material and a covering device with which an undesired incidence of radiation on the carrier material is prevented. 
   In electrographic printers or copiers, the toner image transferred from an intermediate carrier, generally a photoconductor, to the carrier material, generally paper, is fixed, i.e. it is joined to the carrier material in a smear and abrasion resistant way. Nowadays, heat-pressure fixing is commonly used in electrophotography. Without any preheating of the carrier material, for example with the aid of a heating saddle, this type of fixing is limited in its processing speed to about 0.5 m/s to 0.7 m/s. In the duplex printing mode, in which the front side and the rear or back side of a carrier material is printed, the fixing process is relatively difficult since still smearable toner images are applied to both sides. A high fixing quality together with a simultaneous fixing of the front side and the rear side of the carrier material can only be achieved with relatively soft fixing rollers, for example silicone rollers. These fixing rollers have a short life and are uneconomical. In addition, the guiding of the paper is problematic in the case of two opposite fixing rollers in particular when simultaneously fixing the front and the rear side of an endless carrier material. 
   U.S. Pat. No. 6,449,458 B1, whose disclosure is incorporated herein by reference thereto and which claims priority from German 198 27 210, discloses a fixing station for the simultaneous contact-free fixing of the front and the rear side of an endless carrier material with the aid of a heat radiation source. When fixing with the aid of this fixing station, a smearing of the not yet fixed toner images is avoided. In the case of the fixing station known from U.S. Pat. No. 6,449,458 B1, an additional covering device for the interruption of the ray path of the heat radiation source is provided, which covering device can be moved into the ray path between the heat radiation source and the carrier material. With the transport of the carrier material, the covering device is opened at the transporting speed of the carrier material, and after stopping the transport of the carrier material the covering device is closed in the opposite direction at the same speed. 
   Due to the operational sequence, known printers or copiers for printing endless carrier material are not only operated in the continuous printing mode but also in the intermittent printing mode. For example, the toner images of several color separations are transferred successively onto a transfer band, during which operation the transport of the carrier material is stopped. In addition, the transport of the carrier material is interrupted in the so-called start-stop mode, in the case of automatic cleaning processes and in the case of an interruption of the print data stream. 
   It is necessary that there is a smooth transition between a second printed image generated in a second printing process and a printed image generated in a first process. This requires a carrier material transport which, in particular, takes into account periods during which the transfer bands are swiveled to and away from the paper web as well as a predetermined acceleration and deceleration of the travel of the carrier material after every printing process. The acceleration and deceleration of the travel of the carrier material in a printing process are necessary for the synchronization with elements of the image generating unit of the printer or copier, in particular with a character generator, a photoconductor and a transfer element. In doing so, a backward pulling of the carrier material takes place, i.e. a transport in the opposite direction to the direction in which the carrier material is transported during printing. By means of this backward pulling of the paper, the positional displacements of the carrier material during the deceleration of the travel of the carrier material after the first printing process and the acceleration of the travel before the second printing process are compensated. 
   In the already mentioned fixing station, even in the case of a screening-off of the radiation of the heat radiation source as a result of the backward pulling of the carrier material, heat radiation is, however, once again supplied to an already completely fixed area of the carrier material by the opening of the covering device during the start of the carrier material transport in the second printing process. This results in visible differences in the fixed printed image. In addition, the stress on the carrier material varies, in particular additional moisture is extracted from a paper web by the additionally supplied radiation and as a result thereof, the paper web is further stressed. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to provide a method and a fixing station which operates at high process speed and guarantees a high printing quality in particular by means of a uniform fixing. 
   This object is achieved by the inventive method. 
   According to the inventive method, after moving the carrier material in the second direction of movement, the covering device is opened during the subsequent movement of the carrier material in the first direction of movement in a delayed manner such that at least an already completely fixed area of the carrier material is not again subjected to the radiation of the heat radiation source. As a result, the area which has already been fixed, is not fixed once again. However, a following toner image which has not already been completely fixed, is again fixed due to the opening of the covering device. 
   Thus, from a visual standpoint, there results a uniform surface of the fixed toner material, which is of importance for the visual impression of a print page in particular when a toner image presented on a print page has been fixed differently in at least two areas. Then, the luster and the optical density of the toner image surface vary and are visible when the print page is viewed. As a result, the print image of a print page is not of high-quality for a viewer. By means of the method according to the invention, a uniformly fixed toner image or, respectively, print image, is generated. 
   A second aspect of the invention relates to a fixing station for fixing toner images on a carrier material. The fixing station comprises a heating device having at least one heat radiation source which emits radiation toward the carrier material. A drive means is provided which conveys the carrier material in a first direction of movement and in a second direction of movement substantially opposite to the first direction of movement. The fixing station includes a covering device which can be moved substantially parallel to the first and second directions of movement of the carrier material and is movable into the ray path between the heat radiation source and the carrier material. 
   During the transport of the carrier material in the first direction of movement, the covering device is opened so that toner images on the carrier material can be fixed. The covering device is closed when the carrier material is conveyed in the second direction of movement. After transport of the carrier material in the second direction of movement, the covering device is opened during the subsequent transport of the carrier material in the first direction of movement in a delayed manner such that the radiation is at least not incident on an already completely fixed area of the carrier material. 
   What is achieved by this inventive fixing station is that the toner image is uniformly fixed on the carrier material, and print images of high-quality can be generated. Further, the carrier material, in particular a paper web, is not unnecessarily stressed. 
   For the purposes of promoting an understanding of the present invention, reference will now be made to the preferred embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated devices and/or method, and such further applications of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates. Embodiments of the invention are shown in the figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 and 1   a  are schematic longitudinal cross-sectional views of a fixing station having a covering device of the blind-type; 
       FIG. 2  is a speed-distance diagram illustrating the transport speed of the blind according to  FIG. 1 ; 
       FIG. 3  is a speed-time diagram illustrating the speed of the paper web and the speed of the blind during the incomplete opening of the blind; 
       FIG. 4  is a speed-time diagram according to  FIG. 3  illustrating the speed curves in the case of a completely opening blind; 
       FIG. 5  is a speed-time diagram according to  FIGS. 3 and 4  illustrating a transition area between complete and incomplete opening of the blind; 
       FIG. 6  is a speed-time diagram, which is similar to that illustrated in  FIG. 5 , illustrating the borderline case between complete and incomplete opening; and 
       FIG. 7  is another speed-time diagram illustrating the complete opening of the blind. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a longitudinal cross-section of the fixing station according to the invention. This fixing station is used in a high-performance printer which prints a paper web  10  on both sides. In the illustration according to  FIG. 1 , only the upper part of the fixing station is shown, with which heat energy for fixing toner images is applied to the top side of the paper web  10 . An identical device shown in  FIG. 1   a  is provided for the underside of the paper web  10  for fixing toner images on the underside. The fixing station includes two heat radiation sources  12 ,  14 , which are implemented as foil radiators. 
   In this embodiment, such a foil radiator includes 50 μm thick strips which acquire a temperature of &lt;900° C. when a current is applied. The advantage of a foil radiator is that it has a low heat capacity and can thus be heated up quickly and likewise cools down quickly. Other heat radiation sources which can be used are ceramic panel radiators, in which the heating coil is embedded in a ceramic member. Quartz radiation sources in which the coiled filament is mounted in quartz tubes can likewise be used. 
   Between the heat radiation sources  12 ,  14  and the paper web  10  a covering device  16  is provided which can be moved into the ray path between the radiation source  12  and the paper web  10 . In the present case, the covering device  16  comprises strip-shaped lamellae  24  to  40  which are put together in the manner of a blind  16 . As a result, the blind  16  is flexible in the direction of movement of the paper web  10  and can be deflected at deflection rollers  18 ,  20 ,  22 . One of the deflection rollers  18 ,  20 ,  22 , e.g. the deflection roller  22 , is driven by a drive unit (not illustrated). 
   The strip-shaped lamellae  24  to  40 , which form the blind  16 , are clamped between two rotating toothed belts  42  (only one toothed belt  42  is visible in  FIG. 1 ). By a forward or backward movement of the toothed belts  42 , the blind  16  can be moved into the ray path between the radiation source  12  and the paper web  10  in order to shield the paper web  10  from heat radiation being incident thereon. The length of the blind  16  is dimensioned so that, in its closed condition, it shields the paper web from the entire radiation-emitting area of the heat radiation sources  12  and  14 . As an alternative to the toothed belts  42 , two wire cables or chains can be used. 
   The transport mechanism with toothed belt or wire cable drive is provided on both sides outside the radiation area of the heat radiation sources  12 ,  14 . The deflecting device formed by the deflection rollers  18 ,  20 ,  22  is thus compact and requires only little space. 
   The blind  16  is subjected to relatively high temperature differences. In the opened condition, it is at almost room temperature, and in the closed condition the covering device  16  can acquire a temperature of up to about 600° C. Owing to the changes in length as a result of the differences in temperature, a tensioning device (not illustrated) is provided at least at one deflection roller  18 . This tensioning device creates a constant tension within the toothed belt  42  so that the blind  16  is likewise tensioned. The tensioning device can, for example, be implemented by a belt tightener with a permanently applying spring force. In order to compensate for a change in length as a result of the differences in temperature in a direction transverse to the paper web  10 , the deflection rollers  18 ,  20 ,  22  are arranged axially adjustable. 
   The individual lamellae  24  to  40  consist of a high-temperature resistant material, for example sheet steel, having a typical thickness of 0.1 to 0.3 mm. Materials in the form of bands or plates or cloths having a low heat conduction, such as glass fibers, silicate fibers or ceramic fiber paper can also be used, and guarantee that the heat load on the paper web  10  is as low as possible in the closed condition. For stabilization purposes, the lamellae can be applied to a temperature resistant tear-proof support grid. The afore-mentioned fiber products can also be used together with metals; in this case, the fiber products serve for an additional heat insulation. 
   In other embodiments, the blind  16  or, respectively, the lamellae  24  to  40  are wound up like a coil, i.e. the rotating deflection device illustrated in  FIG. 1  is replaced by a take-up reel and a take-off reel, onto which the blind or, respectively, the band is wound on and off. 
   The lamellae  24  to  40  can be formed of a relatively inflexible material, for example ceramics or hollow steel profiles. Such hollow profiles, which in turn can be assembled of U-profiles, are preferably flushed with air for cooling. 
   The movement of the blind  16  depends on the operating state of the paper web  10 . When the paper web  10  is stopped, the blind  16  is closed at the speed v R  according to the relation: v R =−v P , where v P  is the transport speed of the paper web  10 . This means that even in the case of a sudden standstill of the paper web  10 , for example as a result of a paper jam or as a result of an operation-dependent stop of the paper web  10 , the fixing process of the section of the paper web  10  present under the heat radiation source  12  will be continued for as long as heat radiation would have been applied thereto during normal transport. Thus, despite the stoppage of the paper web  10 , the section present under the heat radiation sources  12 ,  14  is still sufficiently exposed in order to fix the toner images. 
   When the paper web  10  is conveyed further at the speed v P , the covering device  16  is opened in the same direction at the speed v R . Thus, the relation: v R =v P  applies. As a result, the correct amount of heat radiation required for fixing is applied to the section of the paper web  10  newly arriving under the heat radiation source  12 . The preceding section of the paper web  10  is not overexposed. 
   The covering device  16  according to  FIG. 1  is flexible. In an alternative embodiment, it is, however, possible to use a rigid plate, which, if necessary, can be moved into the ray path between the heat radiation source  12  and the paper web  10  by means of a drive mechanism. 
   The heat radiation source  12  has a preferred radiation temperature in the range of 500° C. to 800° C. Its maximum radiation intensity is at a wavelength of &gt;2 μm. 
   During printing in an electrographic printer so-called start-stop-processes occur for various reasons, in the case of which the paper transport has to be stopped for a certain amount of time; for example in the case of an interruption of the electronic data stream, in the case of necessary cleaning processes in the printing unit or in the case of specific paper transport movements. 
   The arrow v P  illustrated in  FIG. 1  indicates a first transport direction of the paper web  10 , in which the paper web  10  is conveyed at the printing speed v P , then, the blind  16  being generally open so that the heat radiation of the radiators  12 ,  14  is incident on the paper web  10  for fixing toner images on the paper web  10 . When the printing process is interrupted, i.e. after a first printing process has been finished and before the beginning of a second printing process, the paper web  10  is pulled backward so that the front edge of a print image generated in a subsequent second printing process is transfer-printed at the rear edge of a print image generated in a first printing process. The distance covered for decelerating the paper web  10  when the first printing process is finished and for accelerating the paper web  10  in the second printing process thus has to be traveled by the paper web  10  in the opposite direction of the arrow v P , i.e. in a second direction of movement after finishing the first printing process. The second direction of movement is thus substantially opposite to the first direction of movement. 
   During the transport of the paper web  10  in the second direction of movement, the covering device  16  is closed. During the subsequent transport of the paper web  10  in the first direction of movement in the second printing process, the covering device  16  is inventively not opened until the paper web  10  has been already conveyed so far that it has already been guided past the point at which the covering device  16  is opened. A following, not yet completely fixed area is, however, subjected to the radiation in order to be completely fixed. 
   For starting and stopping the transport of the paper web  10 , the paper web  10  is linearly accelerated each time. This linear acceleration is also referred to as a ramp acceleration. The ramp acceleration is achieved by a corresponding control of a drive unit of the paper web  10 . Likewise, during opening and closing of the covering device  16 , the same is accelerated with a ramp acceleration to transport speed v R  and is decelerated with the aid of a ramp acceleration having a negative slope. In  FIG. 1 , a first acceleration area of the covering device  16  is referenced by S 1  and a second acceleration area is referenced by S 2 . For closing the covering device  16 , the lamellae  24  to  40  are conveyed in the direction of the arrow P 1  between the heat radiators  12 ,  14  and the paper web  10 . 
   In other embodiments, other ramp accelerations, in particular a sinusoidal acceleration for accelerating the paper web  10  and/or the covering device  16  are used as well. 
   In the acceleration area S 2 , the covering device  16  is accelerated so that the covering device  16  is not driven at full transport speed v R  in this area but rather is only accelerated to that speed in that area S 2 . Following the area S 2 , the covering device  16  is driven at a constant speed in the area S 3  up to the area S 1 . In the area S 1 , the drive speed v R  of the covering device  16  is uniformly reduced until the covering device  16  stands still, i.e. the covering device  16  is decelerated from transport speed to standstill. 
   For the subsequent opening in the second printing process, the covering device  16  is uniformly accelerated according to the ramp function in the area S 1 , as a result whereof the area S 1  is not cleared at a uniform speed. Subsequently, the covering device  16  is driven at a constant speed so that in an area referenced by S 3  and lying between the areas S 1  and S 2  the radiation area is uniformly extended at a constant speed. Subsequently, the covering device  16  is again decelerated in the area S 2  according to a ramp function and comes to a standstill at the end of the area S 2 . 
   In the case of the fixing station according to  FIG. 1 , the heat radiators  12 ,  14  are exclusively arranged in the area S 3  between the areas S 1  and S 2  so that the ray path of the radiators  12 ,  14  is cleared at a constant speed. The first direction of movement of the paper web  10  for fixing toner images on the paper web  10  has the same direction as the opening direction of the covering device  16 . It is therefore possible that the front edge of the lamella  24  forms a fixing edge during the opening of the covering device  16 . A toner image which is present under the lamellae  24  to  40  at the point in time of opening the covering device  16  is not fixed once again since the covering device  16  interrupts the ray path of the heat radiation emitted by the heat radiators  12 ,  14 . 
   A following toner image is, however, completely fixed since the covering device  16  does not impede the ray path of the heat radiation onto this area. The fixing boundary results from the toner image positioned at the beginning of the area S 3  under the lamella  24 , with, as already mentioned, the front edge, i.e. the edge of the lamella  24  forming the outer edge of the covering device  16  continuously clearing the path of the heat radiation of the heat radiators  12 ,  14  in the area S 3 , the opposite paper web  10  being conveyed at the same speed in the opening direction. Only at the end of the area S 3 , at which already the entire ray path of the radiators  12 ,  14  has been cleared, the covering device  16  is uniformly decelerated to standstill with a negative acceleration. 
     FIG. 2  is a speed-distance diagram illustrating the acceleration of the covering device  16  according to  FIG. 1 . Like elements have like reference characters. As already explained in connection with  FIG. 1 , the covering device  16  is uniformly accelerated to the blind speed v R  or, respectively, magnitude-wise to the paper speed v P  in the area S 1 . In the area S 3 , the covering device  16  is driven at a constant speed so that the area S 3  below the heat radiators  12 ,  14  is uniformly opened. Subsequently, in the area S 2 , the covering device  16  is decelerated from the speed v R  to the speed  0 , i.e. to standstill. 
   In the same way as the speed-distance diagram according to  FIG. 2 , there results a speed-time diagram for the acceleration and the deceleration of the covering device  16 . In a first time interval T 1  (not illustrated), the covering device  16  is uniformly accelerated to the speed v R  or, respectively, v P , subsequently driven at a constant speed in a time interval T 3  and uniformly decelerated to a standstill in a time interval T 2 . 
   The fixing behavior of the fixing station according to  FIG. 1  with respect to different operating states is explained below with the aid of the speed-time diagrams illustrated in  FIGS. 3 to 7 . In the following, the blind  16  is generally referred to as covering device. In  FIGS. 3 to 7 , speed curves of the paper web  10  are illustrated in solid lines and speed curves of the covering device  16  are illustrated by means of broken lines. 
   At the time t 1 , after a backward pulling of the paper web  10 , that has been carried out after a preceding printing process, the transport of the paper web  10  is started in the direction of the arrow v P  after the start of a second printing process. In doing so, the paper web  10  is uniformly accelerated up to a time t 2  to the transport speed v P  of 1 m/s and is conveyed further at a constant speed up to the time t 4 . At the time t 3 , the section of the paper web that has been pulled backward during backward pulling of the paper web  10 , has again been conveyed in the direction of the arrow v P . At this point in time, the covering device  16  is already opened so far that the front edge of the lamella  24  has reached the boundary between the areas S 2  and S 3  according to  FIG. 2  and, from the time t 3 , starts to clear the ray path of the heat radiation source  12  and, subsequently, of the heat radiation source  14 . 
   At the time t 4 , the toner image generated in the second printing process has been transferred onto the paper web  10 , and the transport speed of the paper web  10  is uniformly reduced up to the time t 6 . Both the acceleration of the paper web  10  and the reduction of the transport speed of the paper web  10  take place by means of a uniform acceleration, also referred to as ramp acceleration. 
   At the time t 5 , the covering device  16  has been accelerated so much that it has the same transport speed, i.e. opening speed, as the already reduced transport speed v P  of the paper web  10 . The opening speed v R  of the covering device  16  is reduced from the time t 5  up to the time t 6  in the same way as the drive speed v P  of the paper web  10 . Thus, at the time t 6 , the paper web  10  and the covering device  16  stand still. The covering device  16  then at least clears an area of the radiation generated by the heat radiator  12 , i.e. the ray path between the heat radiator  12  and the paper web  10 . As a result, the opposite paper web  10  is subjected to the heat radiation in this area so that a toner image present thereon is fixed. Thus, the covering device  16  remains open in this position up to the time t 7 . 
   At the time t 7 , the covering device  16  is accelerated up to the time t 8  in the direction of the arrow P 1  for closing the opened area of the covering device  16  and subsequently again decelerated up to the time t 9 . The time interval between the times t 7  and t 8  substantially corresponds to the time interval between the times t 3  and t 5 , and the time interval between the times t 8  and t 9  substantially corresponds to the time interval between the times t 5  and t 6 . The acceleration ramps for the acceleration to transport speed and for the reduction of the transport speed have the same slope, however opposite in sign. 
   From the time t 9 , the covering device  16  is at least closed so far that the ray path between the heat radiation sources  12 ,  14  and the paper web  10  is interrupted. From the time t 10 , the backward pulling of the paper web is carried out after the second printing process, the paper web  10  being accelerated in opposite direction to the arrow v P  with the aid of an acceleration ramp illustrated in  FIG. 3 . The further speed curve of the backward pulling of the paper web  10  is not illustrated in  FIG. 3  and the following figures. In the present embodiment, the time interval between the times t 1  and t 2  as well as between the times t 4  and t 6  amounts to approximately 200 ms. The time interval between the times t 3  and t 4  corresponds to the time of the print image generation in the current printing process, i.e. the time required by a character generator for generating a charge image which is to be generated as a print image on the paper web  10  in the second printing process. 
   The duration of the print image generation in the second printing process according to  FIG. 3  amounts to 77 ms, as a result whereof a print image having a length of 7.7 cm is generated at a printing speed of 1 m/s. At the time t 6 , the covering device  16  is likewise opened by 7.7 cm so that an area of a print image generated in the preceding first printing process or possibly in another earlier printing process, is fixed over a length of 7.7 cm. The print image generated in the second printing process is transferred onto the paper web  10  as a toner image after inking the charge image with toner. However, it is not necessarily fixed on the paper web  10  in the second printing process. In the present embodiment, the fixing station is arranged downstream of a printing unit so that the toner image generated in the second printing process has not yet reached the fixing station after the transfer-printing even given a deceleration travel. The print image generated in the second printing process is, dependent on the length of the print images generated in the following printing processes, only supplied to the fixing station for fixing in one of these following printing processes. However, at least a longitudinal area of a toner image previously generated is fixed on the paper web  10  over the length of the print image generated in the current second printing process. 
     FIG. 4  illustrates the speed-time diagram according to  FIG. 3 , a longer print image being generated in the second printing process according to  FIG. 4  as compared to the one of the second printing process according to  FIG. 3 . The transport of the paper web  10  is started at the time t 1 , the paper web  10  being accelerated to transport speed v P  up to the time t 2  and subsequently being conveyed further at the speed v P  up to the time t 3  and further up to the time t 14 . As in the case of the diagram according to  FIG. 3 , at the time t 3 , the distance traveled during the previously carried out backward pulling of the paper web  10  has again been traveled in the transport direction v P  so that the boundary between the fixed area and the non-fixed area is situated under the front edge of the lamella  24 . From the time t 3  up to the time t 15 , the covering device  16  is accelerated to its transport speed v R . Thus, at the time t 15 , the boundary between the fixed print image and the non-fixed print image is situated at the front edge of the heat radiation source  12  together with the front edge of the lamella  24 . 
   The transport speed v R  of the covering device  16  substantially corresponds to the magnitude of the transport speed v P  of the paper web  10 . The covering device  16  is opened from the time t 15  up to the time t 14  at the transport speed v R , and from the time t 14 , the opening speed v R  of the covering device  16  is reduced in the same manner as the transport speed v P  of the paper web  10 . At the time t 16 , the covering device  16  is not yet fully opened so that only a part of the ray path between the heat radiation sources  12 ,  14  and the paper web  10  has been cleared by the covering device  16 . 
   The covering device  16  remains open in this position up to the time t 17  at which the covering device  16  is accelerated in the direction of the arrow P 1  for closing the covering device  16 . The covering device  16  is accelerated to a transport speed of likewise 1 m/s from the time t 17  up to the time t 18   a  and is driven further at this closing speed up to the time t 18   b . From the time t 18   b  up to the time t 19 , the drive speed of the covering device  16  is uniformly reduced up to standstill. During standstill of the covering device  16  at the time t 19 , the ray path between the heat radiation sources  12 ,  14  and the paper web  10  is completely interrupted. From the time t 20 , a backward pulling of the paper web  10  is carried out after the second printing process, as already described. 
   The time interval between the times t 3  and t 14  substantially corresponds to the time interval required by the character generator for generating a print image in the embodiment according to  FIG. 4 . By the sequence illustrated in  FIG. 4 , the covering device  16  is opened so far that a longitudinal section of the paper web  10  lying underneath and substantially corresponding to the length of the print image generated in the second printing process, is fixed. Thus, the covering device  16  does reach the maximum speed v R  of 1 m/s during opening and closing, however, the covering device  16  is, as already described, not fully opened. The opening covering device  16  is decelerated synchronously to the paper web  10  and comes to a standstill in the resulting stop position. 
   The opened area is not closed until the exposure time, i.e. the required fixing period, has been reached so that the area lying opposite the heat radiation sources  12 ,  14  and in which the ray path between the radiation heat sources  12 ,  14  and the paper web  10  has been cleared, has been fixed. The closing of the covering device  16  takes place with the same speed curve as the opening, however in opposite direction. 
     FIG. 5  illustrates the speed-time diagram according to  FIGS. 3 and 4 , however, in contrast to the diagram according to  FIG. 4 , a print image of a still greater length being generated in the second printing process so that the covering device  16  is completely opened. At the time t 3 , it is started to uniformly accelerate the covering device  16  for opening until it has reached the opening speed v P  of 1 m/s at the time t 15 . At the time t 24 , the almost opened covering device  16  is decelerated until it has the speed of 0 m/sec at the time t 26 . As already described, the paper web  10  is conveyed from the time t 2  up to the time t 24   a  at a constant speed and subsequently uniformly decelerated up to the time t 26   a , i.e. up to the standstill of the paper web  10 . 
   After the standstill of the covering device  16 , the same is fully opened at the time t 26  and subsequently accelerated to a speed v R  of 1 m/s up to the time t 28   a  for closing in the direction of the arrow P 1 . At this speed v R , the covering device  16  is conveyed up to the time t 28   b , at which the covering device  16  is almost closed. At the time t 28   a , the front edge of the lamella  24  is at the boundary between the areas S 2  and S 3 , and at the time t 28   b  at the boundary between the areas S 1  and S 3 . Likewise, at the time t 15  the front edge of the lamella  24  is at the boundary between the areas S 1  and S 3 , and at the time t 24  at the boundary between the areas S 3  and S 2 . From the time t 28   b , the transport speed of the covering device  16  is uniformly reduced up to the time t 29 , the covering device  16  being closed and standing still at the time t 29 . At the time t 30 , the backward pulling of the paper web  10  is started. 
   In the speed curves of  FIG. 5 , a linear interpolation takes place between a driving of the covering device  16  for complete opening and for incomplete opening. By means of this linear interpolation, however, an exposure error occurs so that not the exact same amount of radiation is supplied to every fixed area of the paper web  10 . 
     FIG. 6  is another speed-time diagram which likewise, as the diagram according to  FIG. 5 , illustrates the speed curve with linear interpolation in the borderline area between incomplete and complete opening of the covering device  16 . Up to the time t 24 , the speed curves correspond to the speed curves illustrated in  FIG. 5 . From the time t 24 , the transport speed v R  of the covering device  16  is uniformly reduced up to the speed 0 m/s in agreement with  FIG. 5 , the covering device  16  then being fully opened at the time t 26 . 
   From the time t 26 , the covering device  16  is uniformly accelerated in the direction of the arrow P 1  up to the time t 28   a , until the covering device  16  has reached a transport speed of v R  of 1 m/s at the time t 28   a . In agreement with  FIG. 5 , the transport speed of the covering device  16  is maintained up to the time t 28   b  and subsequently uniformly reduced up to the time t 29 , until the covering device  16  stands still at the time t 29  and the covering device  16  is completely closed. 
   In contrast to the speed curves according to  FIG. 5 , the paper web  10  is conveyed up to the time t 24   a  at a transport speed v P  of 1 m/s, and subsequently from time t 24   a  up to the time t 28   a , the transport speed v P  is uniformly reduced up to the standstill of the paper web  10 . From the time t 30 , the backward pulling of the paper web  10  takes place, in a way similar to that already described in connection with  FIGS. 3 to 5 . 
     FIG. 7  is a speed-time diagram illustrating the speed curves of the covering device  16  and of the paper web  10  similar to the diagrams  3  to  6 . The speed curves up to the time t 24  substantially correspond to the speed curves illustrated in  FIGS. 5 and 6  up to this time t 24 . From the time t 24 , the transport speed v R  of the covering device  16  is uniformly decelerated up to the time t 26  and stands still at the time t 26  in fully opened position. 
   The paper web  10  is conveyed up to the time t 44  at a transport speed v P  of 1 m/s in the direction of the arrow v P . Subsequently, the transport speed v P  is uniformly reduced up to the time t 46 , with the paper web  10  standing still at the time t 46 . At the time t 44 , the covering device  16  is accelerated in the direction of the arrow P 1  for closing the covering device  16 . At the time t 46 , the covering device  16  has reached a closing speed of v P of 1 m/s, the covering device  16  being driven at a uniform speed v R  of 1 m/s up to the time t 46   b . Subsequently, the speed v R  of the covering device  16  is reduced up to the time t 49 . At the time t 49 , the covering device  16  is completely closed and stands still. At the time t 50 , the backward pulling of the paper web  10  begins and is accelerated. 
   Further, in  FIG. 7  an area is specified by the marked points in time t 42  and t 43  in which area a transfer band is swiveled to the paper web  10  for the transfer of a toner image generated in the second printing process from the transfer band to the paper web  10 . Thus, the transfer band contacts the paper web  10  in the time interval between the time t 42  and the time t 43 . In the remaining time illustrated in  FIG. 7  the transfer band is swiveled away from the paper web  10 . The area  50  between the two vertical broken lines in the diagram indicates an area of arbitrary length so that the speed curve of the paper web  10  and of the covering device  16  illustrated in  FIG. 7  can be arbitrarily extended. Therefore, a time interval of arbitrary length can exist between the two broken lines without the speed curves v R  and v P  before and after this area  50  indicated by the broken lines changing. 
   In embodiments other than those illustrated in  FIGS. 1 to 7 , the heat radiators  12 ,  14  at least extend into the areas S 1  and S 2  or they completely overtop them. As a result, the maximum blind speed v R  at the edges of the heat radiators  12 ,  14  has not been reached yet or, respectively, is no longer reached. The radiation period of the paper web in the acceleration areas then has to be adapted accordingly so that a uniform radiation period is achieved. 
   For putting the inventive teaching into practice, it might be necessary in other printers or copiers that due to technically conditioned positional displacements of the toner images and/or the paper web  10  additional and/or varied delay times for opening and/or closing of the covering device  16  have to be provided. In addition, it might be useful to adapt the acceleration curves and, possibly, the speeds in order to truly prevent that areas of the paper web  10  are not fixed. The opening of the covering device  16  during an ongoing transport of the paper web  10  as well as the deceleration of the covering device  16  in the opened position should take place as fast as technically possible. 
   When the covering device  16  is decelerated in the open position, however, the deceleration should take place as fast as possible only in the case of a complete opening of the covering device  16 , in other cases it might be useful to adapt the closing speed exactly to the speed of the paper web  10 . However, both the acceleration to transport speed v R  as well as the deceleration from transport speed v R  are technically limited due to a maximum allowable tensile load on the lamellae  24  to  40  and the power of the drive unit of the covering device  16 . In the illustrated embodiment, both for the paper web  10  as well as for the covering device  16  identical possible acceleration and deceleration curves have been used. In other embodiments, the paper web  10  and the covering device  16  can also be accelerated and decelerated differently. However, the closing of the covering device  16  preferably takes place with an acceleration which, with regard to its magnitude, substantially corresponds to the negative acceleration of the paper web  10  during the deceleration of the paper web  10 . As a result, the drive of the paper web  10  is decelerated and the covering device  16  is simultaneously accelerated in the closing direction. As a result, an identical radiation period is achieved even in the case of a slower movement of the paper web. In general, a slower movement of the paper web requires a shorter radiation distance for an identical radiation period. 
   In printing processes, in which the covering device  16  is only incompletely opened, an acceleration of the covering device  16  to transport speed v R  as fast as possible should likewise take place, which transport speed v R  corresponds magnitude-wise to the transport speed v P  of the paper web  10 . However, the closing of the covering device  16  should take place with the same magnitude of acceleration as the deceleration of the transport speed of the paper web  10 . 
   In practice, a sudden, i.e. jump-wise change of the speed v R  of the covering device  16  is technically not possible. For improving the exactness of the positioning of the area of the paper web  10  fixed during the first printing process and of the area of the paper web  10  fixed during the second printing process, the speed v R  can be increased magnitude-wise for a predetermined time as compared to the speed v P , as a result whereof exposure errors due to acceleration and deceleration influences can at least again be compensated. An area of the paper web  10 , which during the acceleration phase of the covering device  16  already again disappeared under the just opening covering device  16 , is again cleared by the increased transport speed v R  of the covering device  16  so that radiation is again incident on this area. 
   In addition, it is possible to provide the acceleration and deceleration travels of the covering device  16  exclusively in the areas S 1 , S 2 , in which in the case of a movement of the covering device  16  the ray path from the radiation sources  12 ,  14  to the paper web  10  is not influenced. As a result, the limited acceleration capability of the covering device  16  has no influence on the uniform clearance or, respectively, interruption of the ray path. This is at least the case when the acceleration of the covering device  16  to the transport speed of the paper web  10  as well as the deceleration from transport speed to the standstill are to be carried out completely within these areas S 1 , S 2 . The covering device  16  then always has the same speed v R  in the radiation area during opening and closing, as a result whereof an exact area of the paper web  10  is fixed. By means of these exactly determined areas which are either completely fixed or which are substantially not yet fixed, it is possible that the front edge of an area fixed in the second printing process exactly borders on the rear edge of an area fixed in the first printing process so that neither areas which are fixed twice nor areas which are not yet fixed occur. 
   In alternative embodiments, individual sheets are used as carrier material  10  instead of the paper web. 
   While preferred embodiments have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention both now or in the future are desired to be protected.