Abstract:
An apparatus for transporting individual sheets past an exposure station for exposing the individual sheets, including a roller that has parallel spaced annular grooves and has plurality of parallel endless belts on each side of the exposure station, which belts respectively overlie the grooves for pressing the individual sheets against the roller. Guiding fingers extend into the roller parallel annular grooves for lifting the individual sheets from the roller.

Description:
FIELD OF THE INVENTION 
     The invention is for an apparatus for transporting individual sheets past a device for exposing of or printing on the individual sheets. 
     BACKGROUND OF THE INVENTION 
     In patent DE 196 36 235 A1, an apparatus for recording information on photographic material, which can be developed thermally, is described. In this apparatus, a heated roller is provided around which a sheet is deflected and which roller rotates each individual sheet by about 180°. In the first part of the deflection, the individual sheet is fixed to the roller by means of four pressure rollers. Between these pressure rollers there is an exposure gap through which a laser beam writes on the individual sheet to expose it. After the exposure, the pressure function is taken over by an endless belt, which is looped around the roller in this second region of the deflection. The endless belt has the task of holding the sheet in close contact with the roller so that the best possible transfer of heat can take place between the roller and the sheet. 
     Patent DE 30 00 887 A1 discloses an ink jet printer in which individual sheets are detached from a roller by guiding fingers engaging parallel annular grooves. 
     OBJECT OF THE INVENTION 
     It is an object of the invention to provide an apparatus using a roller for transporting individual sheets past an exposure gap. The apparatus includes corresponding means for guiding the individual sheets around the roller in such a manner that a continuous, jerk-free movement of the individual sheets is assured even in the case of different formats such as sheet thickness and especially of different widths of the sheets and jamming or tilting of the sheets is prevented. 
     In a preferred embodiment, a plurality of endless parallel belts guide each individual sheet, irrespective of its size, without slippage in close contact with the roller. The roller has annular grooves into which guiding fingers extend to engage below the front edge of the individual sheet so that the sheet is lifted from the surface of the roller without a jerking motion and can be supplied to a subsequent transporting unit. 
     In the apparatus, the space in which an exposure unit is located is heated during a prolonged operation to a temperature, typically of about 35° C. The individual sheets, which are of a light-sensitive material, are usually cut off from rolls in light-tight cassettes or the sheets are housed in light-tight cassettes when they are not being exposed. The cassettes and the light-sensitive materials in them are at normal room temperature, typically of about 20° C. 
     Due to the contact with the roller, which can become heated by heat from the exposure device or other sources, the paper heats up. It was found that individual sheets are heated up far less rapidly at places at which they do not come into contact with the roller. In the present invention, this is accomplished by providing annular grooves in the roller at places over which the sheets run and, as a result, also are sensitized less. This would normally express itself by dark stripes in the finished image. 
     The effect of the dark stripes is avoided in the present invention by positioning the annular grooves opposite endless belts which also become heated. In this way, each place of the paper is in contact at least with an endless belt or with the surface of the roller so that the individual sheets are heated at least from one side. As a result, temperature differences over the area of the light-sensitive layer of the sheet are minimized and the stripes in the image are avoided. 
     The endless belts are constructed as flat belts so that the pressure on the sheet is distributed over a relatively large area. The flat belts are somewhat broader than the annular grooves so that the light-sensitive sheet lies smoothly on the surface of the roller and cannot be pressed into the annular grooves. It has turned out that the heating of the paper from the upper side by the belts and the underside by the roller has far less of an effect then the absence of heating from one side. The overlapping between the roller grooves and the endless belts does not have a harmful effect at the lateral edges of the annular grooves and is not visible in the finished image. 
     In order to be able to ensure good contact between the roller and the individual sheet, several endless belts are provided before as well as after the exposure station and loop the roller in each case at an angle larger than 40 degrees of the 360 degree surface of the roller. 
     It has also turned out to be advantageous to dimension the diameter of the roller so that it is not less than 150 mm. At a smaller diameter, the internal stresses, for example, of the photographic paper would exert a force on the belts so that a reliable contact between the paper and the roller could no longer be assured. 
     Ideally, the flat belts have a width of between 8 and 12 mm, whereas the annular grooves have a width of 3 to 5 mm. An overlapping between the roller surface on the one side and the endless belts on the other of 2 to 4 mm should be maintained on either side of the annular grooves. In this way, it is ensured that the flat belt is guided to overlie on the edges of the annular groove without running the risk that the paper will be pressed into an annular groove. On the other hand, the overlapping is so slight that there is no danger of a local temperature peak due to heating from both sides. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details and advantages of the invention are described in conjunction with the description of an example, which is illustrated in detail by means of the drawings, in which: 
     FIG. 1 is a perspective view of a laser exposure unit for photographic paper in accordance with the invention; 
     FIG. 2 is a cross-section through the apparatus of FIG. 1; 
     FIG. 3 shows an enlarged detail of FIG. 2; 
     FIG. 4 is a view of a further detail of the apparatus shown in FIG. 1; and 
     FIG. 5 is a view of a sheet leaving the mechanism. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the exposure device shown in FIG. 1, a print roller  2  is mounted in a frame  1 . The invention is described with respect to a laser exposure device, but other types of such devices can be used. The print roller  2  is driven by a motor  3 . Between the motor  3  and the print roller  2  there is a friction gear  4 , which converts a fast rotation speed of the motor output into a slower and uniform speed of the print roller  2 . 
     On each of the entry and exit sides of an exposure gap  5 , a plurality of belt units  6  are provided. Each belt unit includes an endless belt. There are a plurality of the units  6  along the length of the roller  2  on each side of the exposure gap  5  to engage a sheet being fed across its width. The endless belts of the unit  6  act to hold the individual sheets in contact with the print roller  2 . 
     For reasons of clarity, only one belt unit and belt component  7  is shown in FIG.  1 . The details of the belt units  6  on the opposite sides of the exposure gap are shown in FIG.  2 . Again, there are a plurality of the units  6  along the roller length. 
     Each unit  6  has an endless belt  7  stretched to lie around a part of roller  2  and extends around two fixed deflection rollers  8   a  and  8   b . Each of the deflection rollers  8  is mounted on a support  9  extending across frame  1 . 
     The deflection roller  8   a  of each belt unit  6  is disposed on one side of the exposure gap  5  in the transporting (feeding) direction of the individual sheets. As seen in FIGS. 2 and 3, the surface of the roller  8   b  of the upper unit forms with the print roller  2  a wedge-shaped intake gap  18 , which can accept individual sheets, the front edge of which sheet can deviate slightly from a nominal position. 
     A tensioning roller  10  acts on the belt  7  of a unit  6  with a defined tension so that the belt always presses with the same force on the surface of the roller irrespective of whether there is a thick or a thin sheet, or even no paper on the print roller  2 . Each tensioning roller  10  is mounted on a common support  11  shown extending across frame  3 , and each acts upon its corresponding belt  7  in the tensioning direction against the support  11  by a spring  12  (see FIG.  2 ). 
     The individual belts  7  do not have a driving mechanism and are moved over the print roller  2  as the print roller is rotated. 
     The exposure gap  5  is bounded across its width by two comb-like guide plates  13 ,  14 , which dip with their prongs  15  through the belts  7  across the roller length. Both guide plates  13 ,  14  are movable relative to roller  2  so that the position of free ends of the legs  16  can be alternately moved from generally parallel to the surface of the roller, as shown by the upper leg  16  of FIG. 3, to a position inclined from the roller, as shown by the lower leg  16  of FIG.  3 . This is accomplished by a mechanism (not shown) that can be driven, for example, by a solenoid. The width of the gap  17  between the print roller  2  and each of the legs  16  when parallel to the roller corresponds to the thickest paper sheet that is to be processed. 
     As seen in FIG. 4, the print roller  2  has a plurality of parallel annular grooves  23 . At the ejection gap of the lower belt units  6 , to which the arrow  20  in FIG. 2 points, the guiding element  21  shown in FIG. 4 is provided. The guiding element  21  has guiding fingers  22 , which fit in the annular grooves  23  of roller  2 . Preferably, the guiding fingers  22  are disposed so that there is no contact with the print roller  2 . Consequently, there is no frictional effect, which could affect the rotation of the roller  2 . The fingers  22  lift the sheet from the roller at the exit point  20  (FIG.  2 ). This adds in feeding the individual sheets to a transporting unit (this is not shown here and is independent of the roller  2 ). 
     As seen in FIG. 5, the guiding fingers  22  engage a sheet leaving the roller  2  at the exit  20  at an acute angle beneath the front edge of the individual sheets and lifts the sheet jerk-free from the surface of the roller  2 . The number of guiding fingers  22  are preferably made equal relative to each side of the center of the roller  2 . 
     When the apparatus is in operation, the roller  2  and the belts will be heated by the waste heat of the motor  3  and of the laser exposure device. Therefore, during normal operation the roller and the belts are at a higher temperature than the individual sheets coming from outside the housing. 
     FIG. 4 shows the endless belts of the drive units  6  distributed across the length of the roller. There is a flat belt, here designated  27 , near each end of the roller that opposes a part of the surface of the roller  2  that does not have an annular groove  23 . The individual sheets are heated in the regions of these flat belts  27  from the top by the belts and from the bottom by the roller. But this does not lead to greater sensitization of the light-sensitive layer, so that the image quality does not suffer. 
     Other flat belts  27   a  of the drive units  6  are arranged so that each covers a respective annular groove  23 , completely. In order to ensure that these flat belts  27   a  can securely hold a sheet on the surface of the roller and to prevent the side edge of such flat belts  27   a  running into an annular groove  23 , the width of these flat belts  27   a  is selected to be larger than the width of the annular grooves  23 . Ideally, the width of a flat belt  27   a  is about {fraction (21/2)}times the width of an annular groove  23 . 
     For an individual sheet being supplied in the direction of the arrow  19 , an intake gap  18  accommodates the thickness of the individual sheet. The gap is formed between the flat belts  27  of the (first) belt units  6  at the ends of the roller and the print roller  2  surface. The belts  27  at the ends of the roller continue to lie directly on the surface of the roller. Thus, transverse or rotational forces are prevented from acting on the individual sheet so that the latter is transported completely straight and uniformly. 
     Before the leading edge of the individual sheet being fed reaches the deflection rollers  8   a  of the upper (first) belt units  6  on the feed side of the apparatus, which rollers  8   a  are disposed directly in front of the exposure gap  5 , the guide plate  13 , which is also disposed in front of the exposure gap  5 , is positioned so that the legs  16  are approximately parallel to the surface of the roller  2 . On the other hand, the guide plate  14 , which is on the output side of the exposure gap  5 , is positioned so that the legs  16  form a wedge-shaped gap with the surface of the roller (see FIG.  3 ). 
     In this manner, the leading edge of the individual sheet is held by the legs  16  of the first guide plate  13  in close contact with the print roller  2  and is permitted to lift off slightly from the surface of the roller by the fingers  16  of the guide plate  14  only after learning the exposure gap. The leading edge is thereafter captured gently by the fingers  16  of the second guide plate  14  and beyond the wedge-shaped gap once again pressed onto the print roller  2 . A practically jerk-free transport of the leading edge across the exposure gap  5  is made possible by the acute angle of the wedge-shaped gap between the fingers  16  of the plate  14  and the roller  2 . 
     Before the rear, or trailing, edge of the individual sheet reaches the first guide plates  13 , the latter is moved into a raised position, in which a wedge-shaped gap is formed. When now the rear edge reaches this guide plate  13 , it can lift off slowly and jerk-free by a small amount from the print roller  2 , following the relief of tension in the paper. However, since the second guide plate  14 , which is disposed behind the exposure gap  5 , is in the lowered position with its legs  16  parallel to roller  2 , the rear edge of the individual sheet, as soon as the latter has passed by the exposure gap  5 , once again lies almost on the print roller  2 . 
     By means of the inventive arrangement, it becomes possible to expose individual sheets with good image quality and completely without a margin from their leading edge up to the rear edge. 
     At the outlet gap  20 , the front edge of the individual sheet runs gently onto the guiding fingers  22  protruding into the annular grooves  23  and, in this manner, is lifted from the surface of the roller. The individual sheet is supplied over the guiding element  21  to the next transporting device. 
     By the arrangement of the heated flat belts  27   a  lying over the annular grooves  23 , and the heated belts  27  lying over the roller surface at its ends, each individual sheet on its path between the inlet gap  18  and the outlet gap  20  is brought uniformly to the temperature of the roller  2  and of the endless belts  27  and  27   a . By these means, it is reliably avoided that temperature differences become visible in the finished images in the form of stripes. 
     Of course, the invention is not limited only to laser exposing devices for photographic paper. Instead, it develops its advantages also in other devices in which individual sheets are to be inscripted. As an example, reference is made here only to another device, in which individual sheets are inscripted with the help of a line exposing devices, such as an LCD line, an LED line or a line DMD (digital micromirror device). 
     Specific features of the invention are shown in one or more of the drawings for convenience only, as each feature may be combined with other features in accordance with the invention. Alternative embodiments will be recognized by those skilled in the art and are intended to be included within the scope of the claims.