Abstract:
A sheet handling device for wide format sheets comprising a sheet support plate having a top surface containing suction holes which are connected to at least one suction chamber, said at least one suction chamber being divided into compartments that are connected, though an opening, to a suction device adapted to create a subatmospheric pressure in the compartments, wherein at least one internal wall between adjacent ones of said compartments defines a flow restriction orifice, and at least one of said compartments is directly connected to the suction device, and at least another one of the compartments is indirectly connected to the suction device through the orifice.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a sheet handling device for wide format sheets including a sheet support plate, said plate having a top surface containing suction holes which are connected to at least one suction chamber, said at least one suction chamber being divided into compartments that are connected to a suction device adapted to create a subatmospheric pressure in the compartments.  
         [0002]     In the copying and printing industry, a sheet support element with suction holes is frequently used for supporting an image receiving sheet and at the same time ensuring that the sheet lies perfectly flat on the support element. For example, in an ink jet printer, a sheet, e.g. a sheet of paper, is advanced over a sheet support plate while the image is being printed. The sheet is held on the sheet support plate due to the subatmospheric pressure in the suction chamber which is connected to suction holes facing the bottom side of the sheet. A certain subatmospheric pressure is required to hold the sheet sufficiently flat on the support plate.  
         [0003]     When sheets of different width are to be printed, the smaller sheets do not cover the sheet support element completely. Therefore, some of the suction holes are not covered. When the suction chamber is made of one large compartment extending over the whole area of the sheet support element, an air flow through the uncovered suction holes leads to an increase of the pressure in the suction chamber. This results in the sheet not being held firmly on the support element. If, on the other hand, a suction device of higher power is used to compensate for the increased air flow into the suction chamber, the suction will be too strong when all suction holes are covered by a large sheet, and the advance of the sheet over the support element is impeded.  
         [0004]     From the European patent application EP 0 997 308 A2 a media hold down unit is known that comprises two or more vacuum chambers, a first vacuum chamber being directly connected to a vacuum source, while the other vacuum chambers are each connected to the vacuum source via separate bypass channels. The sizes of the chambers correspond to different widths of sheets that are to be handled. If one of the chambers is not covered by a sheet, an air flow is generated from this chamber through the bypass channel. The air flow affects the uniformity of the pressure inside the first vacuum chamber. Since the bypass channel and the first vacuum chamber are connected at an entrance into a conduit leading to the vacuum source, the uniformity of the pressure provided at the first chamber is affected only to a reduced extent. However, only a limited number of different sheet widths can be handled. Furthermore, for each partition of the support plate an extra bypass channel is needed and requires extra space.  
       SUMMARY OF THE INVENTION  
       [0005]     It is an object of the present invention to provide a sheet handling device in which sheets of different sizes can be held down on a sheet support element with appropriate suction pressure, the sheet handling device being of simple and space saving construction.  
         [0006]     According to the present invention, this object is achieved by a sheet handling device of the type indicated above, wherein at least one internal wall between adjacent ones of said compartments defines a flow restriction orifice, and at least one of said compartments is directly connected to the suction device, and at least another one of the compartments is indirectly connected to the suction device through said orifice.  
         [0007]     The flow restricting orifices restrict the air flow from a compartment that is not covered by a sheet. Thus, the necessary subatmospheric pressure can be maintained in those compartments that are covered by the sheet. When, however, a sheet of larger size covers also the suction holes for which the flow path is restricted by the orifices, the flow rate will be reduced substantially. Thus, the effect of the flow resistance of the orifices will also be reduced, and the suction effect will be essentially uniform over the whole area of the sheet. Furthermore, no extra space is needed for bypass channels and the like. Another advantage is that the size of the compartments can be chosen by arranging the internal walls at appropriate positions in the suction chamber. Therefore, when manufacturing the sheet handling device, the sheet support place is readily adaptable to the required sizes of the compartments. Furthermore, since the internal walls do not require extra space outside of the suction chamber, the number of compartments may easily be increased. Therefore, the sheet handling device can readily be adapted to handle sheets of more than two sizes.  
         [0008]     Preferably, the compartments are arranged symmetrically. Thus, the maximum distance of a compartment from the suction means is reduced as compared to, e.g., a configuration in which the suction device is connected to a first compartment in a chain of interconnected compartments. With this preferred arrangement, smaller sheets may be passed over the first compartment in a middle section of the sheet support plate instead of being fed at a lateral edge of the sheet support plate.  
         [0009]     In a preferred embodiment, at least two groups of compartments are arranged in the direction of advance of the sheet, and only one group has a compartment that is directly connected to the suction device, whereas the compartments of the other group or groups are connected indirectly through flow restricting orifices. This helps to reduce the air flow into the suction chamber when a sheet covers only part of the support plate in the transport direction of the sheet. For example, when a new sheet is printed and advanced onto the sheet support plate, the sheet may already cover a compartment of the first group while the leading edge of the sheet has not yet reached a neighboring compartment of the second group.  
         [0010]     One or more hollow spaces may be provided in the sheet support plate in order to reduce the weight of the plate and also its heat capacity, in the case that the temperature of the plate must be controlled, and yet to obtain a sufficient thickness and stability of the support plate. Then, the suction holes are preferably drilled in such positions that they lead into the hollow space which itself is connected to the compartments of the suction chamber by a number of apertures. As a result, an additional air flow would take place from one compartment to the other through the hollow space. It is therefore preferable that the internal wall separating these compartments comprises, for each of said hollow spaces, a protrusion extending through the aperture into said hollow space, so as to restrict the air flow through the hollow space.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]     Preferred embodiments of the present invention will now be described in conjunction with the drawings, in which:  
         [0012]      FIG. 1  is a schematic perspective view of a hot-melt ink jet printer;  
         [0013]      FIG. 2  is a partial cross section of a sheet support plate in the printer shown in  FIG. 1 ;  
         [0014]      FIG. 3  is an enlarged section of a suction chamber of the sheet support plate, the section being taken along the line III-III in  FIG. 4 ;  
         [0015]      FIG. 4  is a partial cross section along the line IV-IV in  FIG. 3 ; and  
         [0016]      FIG. 5  is a partial cross section of a modified embodiment of the sheet support plate. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     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 . A number of transport rollers  16  are rotatably supported in a cover 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 paid out through a gap formed between an edge of the cover plate  18  and the surface of the sheet support plate  14 .  
         [0018]     A carriage  22  which includes a number of 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 . In each pass of the carriage  22 , a number of pixel lines are printed on the sheet  12  by means of the printheads which eject droplets of hot melt ink onto the sheet in accordance with image information supplied to the printheads. For the sake of simplicity, guide and drive means for the carriage  22 , ink supply lines and data supply lines for the printheads, and the like, have not been shown in the drawing.  
         [0019]     The top surface 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 via a tube  27  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  and is thereby held in a flat condition, especially in the area which is scanned by the carriage  22 . Thereby, a uniform distance between the nozzles of the printheads and the surface of the sheet  12  is established over the whole width of the sheet, and a high print quality can be achieved.  
         [0020]     The droplets of molten ink that are jetted out from the nozzles of the printheads have a temperature of 100° C. or more and cool down and solidify after they have been deposited on the sheet  12 . Thus, while the image is being printed, the heat of the ink must be dissipated with a sufficient rate. On the other hand, in the initial phase of the image forming process, the temperature of the sheet  12  should not be too low, because otherwise the ink droplets on the sheet  12  would be cooled too rapidly and would not have time enough to spread-out. For this reason, the temperature of the sheet  12  is controlled via the sheet support plate  14  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 .  
         [0021]     The sheet support plate  14 , which has been shown in cross-section in  FIG. 2 , is made of a material, such as a metal, having a relatively high heat conductivity and also a relatively high heat capacity. A number of elongated cavities  34  are formed in the interior of the plate  14  so as to extend in parallel with one another and in parallel with the direction (B) of travel of the carriage  22  between opposite ends of the plate  14 , where they are connected to the tubes  32  through suitable manifolds. Each cavity  34  is delimited by a top wall  36 , a bottom wall  38  and two separating walls  40 . The top walls  36 , together, define the top surface  42  of the plate  14  which is machined to be perfectly flat.  
         [0022]     Between each pair of two separating walls  40 , which delimit to adjacent cavities  34 , a hollow space  44  is formed. The hollow spaces  44  extend parallel to each other and in parallel with the direction (B) of travel of the carriage  22  between opposite ends of the plate  14 . The suction holes  24  pass through the top wall  36  into the hollow spaces  44 . The hollow spaces  44  are connected to the suction chamber  26  via apertures  60  having the form of slits extending in the direction of the hollow spaces.  
         [0023]      FIG. 3  schematically shows the interior of the suction chamber  26  in a horizontal cross-section. The suction chamber  26  comprises internal walls  62  extending in the sheet advance direction (A) and internal walls  64  extending in the reciprocating direction (B) of the printheads.  
         [0024]     The internal walls  62  and  64  divide the suction chamber  26  into two groups of compartments  66 A,  66 B,  66 C and  68 A,  68 B,  68 C, respectively. The blower  28  is directly connected to the compartment  66 C in the middle of the first group of compartments through an opening  69  and the tube  27  ( FIG. 1 ). The compartment  68 C is connected to its neighboring compartment  66 C via three flow restricting orifices  70  of circular cross section in the wall  64 . The compartments  66 B and  68 B are connected to the compartments  66 C and  68 C, respectively, through orifices  70  in the walls  62 . The outermost compartments  66 A and  68 A are only connected to their neighboring compartments  66 B and  68 B within the same group of compartments via orifices  70  in the walls  62 . However, the neighboring compartments  66 A and  68 A are not directly connected to each other. Also, the neighboring compartments  66 B and  68 B are not directly connected to each other, the compartments  66 A,  66 B,  68 A,  68 B and  68 C are indirectly connected to the suction device through the orifices  70  and the compartment  66 C.  
         [0025]     When the sheet  12  is in the position of  FIG. 1 , it covers approximately the rectangular area of the first group of compartments  66 A,  66 B and  66 C. The suction holes  24  which are connected to the second group of compartments  68 A,  68 B,  68 C are not covered by sheet  12 , so that air flows through the corresponding suction holes. However, the air flow to the compartment  66 C is restricted by the flow restriction orifices  70 . Thereby, the necessary subatmospheric pressure can be maintained in the compartments  66 A,  66 B and  66 C.  
         [0026]      FIG. 4  shows a cross sectional view of the sheet support plate  14  along the line IV-IV in  FIG. 3 . Unlike in  FIG. 2 , the cross section is through one of the walls  62 . The orifice  70  connecting the compartments  66 A and  66 B is shown.  
         [0027]     The walls  62  comprise a plurality of protrusions  72  extending upwards through the apertures  60  ( FIG. 2 ) in the bottom wall  38  into the hollow spaces  44  up to the bottom of the top wall  36 . Accordingly, air flow through the hollow spaces  44  is reduced by the protrusions  72  of the flow restricting members  62 .  
         [0028]     When a sheet  74  ( FIG. 1 ) of small width is advanced over the sheet support plate  14 , the sheet  74  covers, for example, only the area of the compartments  66 C and  68 C. The non-covered suction holes  24  which lie in the area of the compartments  66 A,  68 A,  66 B, and  68 B, permit an air flow into the hollow spaces  44 . Then, the protrusions  72  restrict the air flow through the hollow spaces  44  from the area of the compartment  66 A to the area of the compartment  66 B, for example, and from the area of the compartment  66 B to the area of the compartment  66 C. At the same time, the orifices  70  restrict the air flow from the compartment  66 A to the compartment  66 B and from the compartment  66 B to the compartment  66 C. Thereby, the required subatmospheric pressure can be maintained in the compartment  66 C.  
         [0029]      FIG. 5  shows a modified embodiment in a cross sectional view along a line that is parallel to the direction B. In this embodiment, the width of the suction chamber  26  is reduced as compared to the width of the sheet support plate  14 . The outermost compartments  66 A,  68 A have a reduced size but nevertheless are connected to a similar surface area of the sheet support plate  14  as described in connection with FIGS.  1  to  4 . This is achieved by the hollow spaces  44  connecting the respective suction holes  24  via the apertures  60  to the compartment  66 A, for example. Thereby, the top surface  42  with the suction holes  24  can have a width that is larger than the width of the underlying suction chamber  26 .  
         [0030]     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.