Abstract:
A sheet handling device for a printer or copier which includes a print surface for supporting a first surface of a sheet, a feed plate having an edge adjacent to the print surface, and a feed mechanism for feeding the sheet to the print surface through a gap between the edge of the feed plate and the print surface, wherein the edge of the feed plate has notches arranged such that at the edge, the notches provide space for the sheet at a second surface of the sheet.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a sheet handling device for a printer or copier containing a print surface for supporting a first surface of a sheet, a feed plate having an edge being adjacent to the print surface, and a feed mechanism for feeding a sheet to the print surface through a gap between the edge of the feed plate and the print surface. 
   In printers in which paper sheets or similar image receiving sheets are used as recording media, a tendency of the paper to cockle may sometimes constitute a serious problem. The cockling phenomenon is related to the fact that paper and similar materials tend to absorb humidity from ambient air and to expand and contract in accordance with their humidity content. Typically, the expansion and contraction is not isotropic and is particularly pronounced in a direction in which the fibers of the paper are predominantly oriented. When there exists a gradient in humidity within the paper, then the more humid portion of the paper will expand more than the drier portion, which inevitably leads to the production of cockles or wrinkles. 
   Once cockles have developed in the paper during the transport of the paper towards the sheet support plate, a further expansion or contraction of the paper may lead to an expansion of the cockles, so that the height of the cockles also grows. 
   In a typical setup of an ink jet printer, especially a large format printer, the paper is intermittently advanced over a flat sheet support plate, while a carriage moves back and forth across the paper, and ink jet printheads mounted on the carriage are energized to eject droplets of ink onto the paper to form a printed image. Since the carriage moves with relatively high velocity, the ink droplets ejected onto the paper undergo a certain aberration and are deposited on the paper in a somewhat dislocated position. The amount of dislocation is proportional to the flight distance of the ink droplets. Thus, when cockles are present in the paper, the flight distance is non-uniform and, accordingly, the dislocation of the spots of ink on the paper also becomes non-uniform, so that the quality of the printed image becomes deteriorated. The larger the height of the cockles, the more pronounced is the deteriorating effect. 
   When the ink jet print printheads are positioned very close to the surface of the paper to minimize the dislocation, the printheads might even touch large cockles or bumps of the paper, so that the quality of the printed image is also deteriorated. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a sheet handling device which feeds a sheet to a print surface in basically a flat, low or not at all cockled configuration, and to provide a printer containing such sheet handling device. 
   According to the present invention, this object is achieved by a sheet handling device of the type indicated above, wherein the edge of the feed plate contains notches being arranged such that, at the edge, the notches provide space for the sheet at a second surface of the sheet. 
   The notches are separate from each other and are arranged to guide the sheet mainly at those parts of the edge that are between the notches. Thus, the notches govern the positions at which cockles or wrinkles develop. By adapting the size and the positions of the notches to the material and thickness of the sheets, to their tendency to produce cockles or wrinkles, and to the humidity gradients and temperatures that are to be expected, the notches may be arranged to favor certain smaller cockle sizes over larger cockle sizes. 
   At a conventional feed plate with an edge that forms a straight line, an expansion of the sheet material at a certain region of the sheet might lead to the development of a large bump. However, the feed plate of the present invention will regulate the forming of cockles, and the expansion of the material of the sheet will be distributed over several smaller bumps or cockles. Thus, the height of the cockles or bumps is considerably reduced. 
   Generally, the height of the cockles is related to their lateral extension. By reducing the lateral extension and thus the height of the cockles, the disadvantages of cockling mentioned above are reduced. 
   Preferably, the notches are arranged in a regular pattern, whereby the effect of the notches is uniformly distributed. For example, a repeat distance of the notches may be the same for all neighboring notches, so that a cockle size corresponding to the repeat distance is favored. 
   Preferably, the feed mechanism includes sheet transport rollers that are distributed over the width of the feed plate. For example, the sheet transport rollers are accommodated in slots of the feed plate. 
   In a preferred embodiment, the sheet transport rollers and the notches are positioned such that, at lateral positions of the sheet transport rollers, there is a larger distance between neighboring notches than an average distance. Thus, the flattening effect of the transport rollers is accounted for which suppress the occurrence of cockles at the positions of the transport rollers and thereby favors the development of cockles between the positions of the transport rollers. 
   For example, the distance between neighboring notches varies in a regular pattern. For example, the notches are grouped into pairs, each pair being arranged between the lateral positions of the transport rollers. Additionally or alternatively, the size and/or shape of the notches may vary in a regular pattern. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will now be described in conjunction with the drawings wherein: 
       FIG. 1  is a schematic perspective view of a hot melt ink jet printer; 
       FIG. 2  is a schematic view of a paper sheet, illustrating the occurrence of large cockles after the sheet has been moved past an edge of a conventional feed plate; and 
       FIG. 3  is a schematic view of a paper sheet, illustrating the occurrence of smaller cockles after the sheet has passed an edge of a feed plate of the sheet handling device of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As is shown in  FIG. 1 , a hot melt ink jet printer includes a platen  10  which is intermittently driven to rotate in order to advance a sheet  12 , e.g. a sheet of paper, in a direction indicated by an arrow A over the top surface of a sheet support plate  14 , the top surface forming a print surface  15 . A number of transport rollers  16  that are distributed over the width of the feed plate  18  are accommodated in slots  19  ( FIG. 3 ) of the feed plate  18  and are rotatably supported in the feed plate  18 . The transport rollers  16  intersect the feed plate  18  and form a transport nip with the platen  10 , so that the sheet  12 , which is supplied from a reel (not shown) via a guide plate  20 , is transported along a sheet transport slot that is formed by the feed plate  18  and the print surface  15  of the sheet support plate  14 . The sheet  12  is paid out through a gap formed between an edge  21  of the feed plate  18  and the surface of the sheet support plate  14 . At the edge  21 , the feed plate  18  forms an angle of, for example, less than 10° with the print surface  15 . 
   A carriage  22  which includes a number of hot melt ink jet printheads (not shown) is mounted above the sheet support plate  14  so as to reciprocate in the direction of arrows B across the sheet  12 . Thus, by energizing the printheads, a number of pixel lines of an image are printed in each pass of the carriage  22 . Then, the sheet  12  is advanced by a step of appropriate length in the direction indicated by the arrow A, so that the next pixel lines can be printed. 
   The print surface  15  of the sheet support plate  14  has a regular pattern of suction holes  24  which pass through the plate and open into a suction chamber  26  that is formed in the lower part of the plate  14 . The suction chamber is connected to a blower  28  which creates a subatmospheric pressure in the suction chamber, so that air is drawn-in through the suction holes  24 . As a result, the sheet  12  is drawn against the flat surface of the support plate  14 . 
   The sheet support plate  14  is temperature-controlled in order to control the cooling rate and the solidification of the hot melt ink that has been deposited on the paper. The sheet support plate  14  is temperature-controlled by means of a temperature control system  30  which circulates a temperature control fluid, preferably a liquid, through the plate  14 . The temperature control system includes a circulating system with tubes  32  that are connected to opposite ends of the plate  14 . One of the tubes passes through an expansion vessel  33  containing a gas buffer for absorbing temperature-dependent changes in the volume of the liquid. As will be readily understood, the temperature control system  30  includes heaters, temperature sensors, heat sinks, and the like for controlling the temperature of the fluid, as well as a pump or other displacement means for circulating the fluid through the interior of the sheet support plate  14 . 
   On its way from the guide plate  20 , past the platen  10  and past the feed plate  18  to the print surface  15 , the sheet  12  will inevitably be exposed to ambient air and, as a result, will absorb humidity, especially when the relative humidity (RH) of the ambient air is high. 
   When the humidity content of the paper increases, it tends to expand, in particular in the direction in which the fibers in the paper are predominantly oriented. Typically, this is the direction transverse to the longitudinal direction of the web. When the sheet  16 , after having expanded in this way, reaches, for example, the sheet support plate  14  and is, for example, heated to the temperature of the sheet support plate  14 , part of the water contained in the paper will be evaporated, and the paper shrinks again in the width direction of the sheet. Thus, since a humidity gradient is present in the paper, the accompanying reduction in the width of the sheet leads to the production of cockles. This has been exaggeratedly illustrated in  FIGS. 2 and 3 . 
   Generally, when the sheet  12  comes into contact with the print surface  15 , the sheet might be exposed to a different temperature or a different relative humidity of the ambient air at the sheet support plate  14 . Thus, new cockles may develop, or those cockles which have already been present in the sheet may expand further. 
   To control the distribution and the development of the cockles in the sheet  12  in order to prevent larger cockles from existing, the feed plate  18  contains notches  40 . 
   As a comparative example,  FIG. 2  shows a part of a conventional feed plate  42  having an edge  44  that forms a straight line. Large cockles or bumps  46  may occur in the sheet  12  that is to be printed. 
   Due to the notches  40  provided on the feed plate  18  of the present invention, the occurrence of large bumps  46  is prevented, and smaller cockles  48  ( FIG. 3 ) are favored. This is due to the fact that the notches provide space for the sheet, so that the occurrence of small cockles  48  is favored at the positions of the notches  40 . 
   As is shown in  FIG. 1 , the notches  40  are arranged in a regular pattern. However, as is shown in  FIG. 3 , a smaller distance and a larger distance between neighboring notches can be alternately provided. Thus, the notches  40  are grouped into pairs. Regarding the lateral positions of the slots  19  that accommodate the sheet transport rollers  16  ( FIG. 1 ), the slots  19  are arranged in coincidence with the larger distance between the notches  40 . Thus, each pair of notches  40  is arranged between the lateral positions of neighboring transport rollers  16 . 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.