Abstract:
A printer including a printhead which is displaceable at the level of a print surface in a main scanning direction and in a sub-scanning direction with respect to a substrate situated in the print surface, and a supply unit for storing the substrate which is disposed on a core and delivering the substrate for transport to the print surface, said supply unit including support means for supporting the core at its end portions so that the core, while being supported, can rotate with the unwinding of substrate from the core, said support means containing an element for exerting a force on the core in the axial direction during the rotation thereof, whereby the core is pressed against a stop which occupies a substantially fixed position in the axial direction with respect to the print surface.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a printer containing a printhead which is displaceable at the level of a print surface in a main scanning direction and in a sub-scanning direction with respect to a substrate situated in the print surface, and a supply unit for storing the substrate and delivering the substrate for transport to the print surface, which unit includes means to support a core in the vicinity of its ends, such that the core, while being supported, can rotate to unwind the substrate rolled on said core. 
   A printer of this kind is generally known from the prior art. In this printer, the substrate is stored on a roll, particularly a core on which 50 to 200 meters of a specific type of substrate are rolled. To print the substrate, the substrate is unwound from the core in the printer as required, and the free end is transported to the print surface. There at least a part of the image for printing is formed by moving the printhead with respect to the substrate in the main scanning direction, typically perpendicularly to the direction of transport of the substrate, and simultaneously energising the printhead imagewise. Thus a strip can be printed on the substrate in the width of the writing part of the printhead. After one or more relative movements of the printhead with respect to the substrate over the strip, the substrate is moved in the sub-scanning direction with respect to the printhead, and this is typically the direction of transport of the substrate, so that a new part of the substrate comes to lie on the print surface. Thus a following part of the image can be formed by scanning the printhead over this new part of the substrate. In this way, a part of the substrate in the size of the image for printing can be completely scanned and provided with all the sub-images. These sub-images together form the image for printing. After the image has been printed, the printed part of the substrate is often cut off from the rest of the substrate so that it can be used separately. The printhead can be equipped with one or more individually activatable print modules. A typical example is an inkjet printhead which comprises four print modules, one for each of the colors cyan, magenta, yellow and black. One problem with this known printer is the incidence of visible connection errors between the different sub-images. For example, sub-images which should just adjoin one another frequently overlap or do not have a common border over their complete length. Such faults may have a very disturbing effect on the printed image, particularly in the case of a color printer. It is known from the prior art to use print strategies to mask these faults. A disadvantage of this is that the possibilities of masking are limited and frequently it is not possible to allow for an arbitrary variation in connection errors. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide a printer which counteracts the above disadvantages. To this end, a printer has been developed wherein the supply unit also includes an element for exerting a force on the core in the axial direction during the rotation thereof, the core being pressed against a stop, which stop, in the axial direction, occupies substantially a fixed position with respect to the print surface. 
   It will be apparent that in this way many connection faults, for example faults in the main scanning direction and in the sub-scanning direction, can be prevented. Since the stop occupies a substantially fixed position in the axial direction with respect to the print surface, the core pressed against the element, and the substrate wound on the core, will also occupy substantially a fixed position with respect to the print surface, at least in the axial direction. It has been found that in this way the incidence of connection faults between the sub-images can be very considerably suppressed. 
   In one embodiment, the element is resilient in the said axial direction. The advantage of this is that the core can be easily placed in the supply unit between the element and the stop, while a pressure application force is at the same time provided in the axial direction. 
   In another embodiment, in which the support means are distributed over a first and second support member, the stop is a part of the second support member. This embodiment has the advantage that the risk of connection faults is further reduced. 
   In another embodiment, the supply unit comprises two elements for exerting a force on the core in the axial direction. This embodiment offers the advantage that a force can be exerted on the core on both sides of the core shaft. As a result, a greater force can be exerted in the axial direction, thus reducing the risk of inducing an oscillation of the core. This further reduces the risk of connection faults. 
   In another embodiment, the two elements are spaced apart by an amount substantially equal to the diameter of the core. The force can thus be exerted on the edge of the core, distributed over two opposite points on the edge. This is a simple matter of providing adequate and reliable pressing of the core against the stop. In one embodiment, the elements are resiliently mounted wheels. In this case the core can rotate freely over the wheels while they exert a force on the core in the axial direction. As a result of the possibility of free rotation, the frictional force between the elements and the core during the rotation thereof are minimised. The incidence of irregularities in the speed at which the substrate is unwound can thus be prevented. 
   In one embodiment, the supply unit includes a drawer containing the support means and the printer includes another element for exerting force on the drawer in a direction parallel to the core wherein the drawer is pressed against a stop, which stop in the said direction occupies substantially a fixed position with respect to the print surface. In this embodiment, a core with a substrate wound thereon can easily be placed in the supply unit by pulling out the drawer. In this way, the support means located in the drawer are exposed and are easily accessible for insertion of a new roll of substrate. After insertion, the drawer can be closed, whereupon the substrate can be unwound for transport to the print surface. Since this drawer can move with respect to the other parts of the printer, particularly the print surface, in this embodiment the drawer is pressed against a stop which occupies substantially a fixed position with respect to said print surface, at least in a direction parallel to the core. In this way, the drawer itself also occupies substantially a fixed position with respect to the print surface so that connection faults originating from uncertain positioning of the drawer with respect to the print surface can be prevented. In a further embodiment, the drawer is equipped for simultaneously holding two cores, each provided with a substrate rolled thereon. 
   In one embodiment the core is provided with an external toothing in the vicinity of one end of said core and the supply unit is provided with a driveable gearwheel for engaging the said toothing if the core is supported. This embodiment makes it a simple matter to drive the core for rotation, for example for unwinding or rewinding of the substrate. The support means prevent the core from being carried by the drivable gearwheel at the location of the toothing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be explained further with reference to the following drawings, wherein: 
       FIG. 1  is a diagram showing a printer according to the present invention; 
       FIG. 2  is a diagram showing a drawer as a supply unit for the printer; 
       FIG. 3  diagrammatically illustrates a support member of a holder of the drawer; and 
       FIG. 4  is a diagram showing a drawer and the means for positioning it in the sub-scanning direction with respect to the print surface. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a diagram showing a printer according to the present invention. The printer is provided with a supply unit  10  which serves for storage and delivery of the substrate for printing. In addition, the printer includes transport unit  30  which transports the substrate from the supply unit  10  to the print unit  40 . Unit  30  also ensures accurate positioning of the substrate in the print zone formed between the print surface  42  and the inkjet printhead  41 . In this embodiment, print unit  40  is a conventional engine comprising printhead  41  which is constructed from a number of loose sub-heads, each for one of the colors black, cyan, magenta and yellow. A printhead of this type is described in detail in European patent application EP 1,378,360. Printhead  41  has only a limited print range so that it is necessary to print the image on the substrate in various sub-images. For this purpose, the substrate is transported in increments in each case in the transit direction (subscan direction) so that a new part of the substrate can be printed in the print zone. In the example illustrated, the substrate  12  originates from core  11  containing a roll of substrate, which roll is situated in the supply unit  10 . The roll is received in drawer  3  of the supply unit. A web of substrate is wound on the core  11  of the roll and has a length of 200 meters. To accommodate the roll in the printer the drawer  3  is provided with a holder (not shown) to support the core in the surroundings at its ends. As a result the roll can be rotatably accommodated in the drawer. The holder comprises two support members received in side plates of the drawer, the support members being brought into co-operative connection with the ends of the roll. In this embodiment the supply unit is provided with a second drawer  4  to receive a following roll containing a core  21  on which a substrate  22  is wound. The substrate  22  can also be delivered by the supply unit for printing. The drawers can be pushed out of the supply unit  10  in the indicated direction F for the withdrawal of the rolls and/or insertion of new rolls. For the transport of the substrate, core  11  is operatively connected to transport means  15 , which in this case comprises a pair of rollers between which a transport nip is formed. A sensor  17  is mounted upstream of transport means  15  to determine whether there is still substrate on the roll in the relevant holder. The holder is provided with transport means  25  for the transport of a substrate originating from the other roll. Upstream of this means the supply holder is provided with sensor  27  which has the same action as sensor  17 . The supply holder is provided with guide elements  16  and  26  to guide the substrates  12  and  22  respectively to the transport unit  30 . Transit path  13  is located downstream of these guide elements. This transit path is used both for the transport of substrate  12  and the transport of substrate  22 . 
   A substrate leaving the supply unit  10 , substrate  12  in this example, is engaged by transport means  31  of the transport unit  30 . The transport means  31  transports the substrate via a guide element  33  on to the second transport means  32  of the transport unit  30 . The transport means  32  engages the substrate, and transports it on to the print unit  40 . Thus the printer is configured to print substrate  12 . For adapting a configuration to print substrate  22 , it is necessary in this case to wind substrate  12  back on the core  11  so that the free end finally leaves transit path  13 . Roller pair  15  then continues to hold the substrate  12  fast. Substrate  22  can then be spooled over guide element  26  by the drive of the roller pair  25  until nip  31  is reached whereupon the latter takes over the drive for the substrate and spools the substrate onto nip  32  for finally reaching the print surface  42 . The printer is then configured to print substrate  22 . 
   The guide elements  16  and  26  are, in this example, rollers extending parallel to the transport means  15  and  31 ;  25  and  31  respectively. They are basically stationary rollers (i.e., they cannot rotate about their axial axis). The guide elements are so disposed in the supply unit that they can each rotate, at least through a limited angle, about an axis. In the drawing, the rotational axis  18  of element  16  is shown, and also rotational axis  28  of element  26 . These rotational axes are perpendicular to the axes of the guide elements and intersect the middle of these elements. 
   Guide element  33  of transport unit  30 , which element extends substantially parallel to the transport means  31  and  32 , is also disposed so that it can rotate about an axis perpendicular to the axial direction of the said element. The axis is shown by reference  34  and intersects the middle of guide element  33 . Since element  33  in this embodiment is a co-rotating roller, the substrate remains substantially stationary with respect to the surface of this guide element. Element  33  is also suspended so that it can rotate about axis  35  which extends parallel to the bisector  36  of the angle 2α over which the substrate is fed from transport means  31  to transport means  32 . The axis  35  intersects the middle of the substrate web at a distance of about 1 meter from the guide element itself. 
   Guide element  33  is movable from a first position as it is situated in  FIG. 1 , to a second position in which the center of this element coincides with location  37 . In the first position, the distance over which substrate  12  extends between transport means  31  and transport means  32  is at a maximum. In the second position this distance is minimal. Use is made of this during the transport of the substrate to print unit  40 . Since the substrate must in each case be moved over a relatively small distance (typically 5 to 10 cm), it is advantageous for this to take place relatively rapidly. However, the mass inertia of roll  11 , certainly when it is provided with a maximum quantity of substrate, is relatively high. For this reason, displacement while maintaining the configuration shown for the transport means and guide elements would take a considerable amount of time relatively speaking. To counteract this problem, transport means  31  is accelerated much more slowly than transport means  32 . However, in order to ensure sufficient supply of substrate to transport means  32 , the guide element  33  is moved in the direction of location  37 . 
     FIG. 2  diagrammatically illustrates an alternative embodiment of the drawer  3 . In this case, the drawer is provided with two holders to receive two individual cores. The first holder contains a first pair of support members  50  and  51 . The second holder includes a second pair of support members  60  and  61 . In the drawing, the core  11  is received in the first holder. When this drawer is in use in a printer the core present therein will be provided with a substrate wound thereon (not shown). To unwind the substrate, the core is rotatably accommodated in the holder. Roller pair  15 , of which only one roller is visible in the drawing, also forms part of the drawer. The roller illustrated is mounted on shaft  19  which can be driven by gearwheel  20 . 
   The distance between the support members is such that a user can easily place a roll in the holder by bringing the core ends substantially to coincide with the positions of the two support members. After the roll has been placed in the holder, it is automatically brought by means of a number of resilient elements into a substantially fixed position with respect to the print surface. This is shown in greater detail in  FIG. 3  and the accompanying description. 
     FIG. 3  is a detail of the support member  50 , the core  11  being omitted for the sake of clarity. Support member  50  includes two support rollers  65  and  66  which rotatably support the core. During the core rotation, either for the unwinding of the substrate or for winding it on the core, the core rotates over these rollers. The rollers are not driven under these conditions. 
   To drive the core, member  50  is provided with a gearwheel  70 . The teeth of this gearwheel engage in an external toothing of the core. In this way, by driving the gearwheel  70 , the core is forced to rotate. In the printer in accordance with the embodiment illustrated here, gearwheel  70  is only driven when the substrate has to be rewound on to the core, for example when the roll in question is to be replaced by a roll with a different substrate. During unwinding of the substrate, the core is driven passively: the roller pair  15 , in this case, forms a transport nip and pulls on the substrate. As a result the core  11  will rotate and the substrate can be unwound from the core. Member  50  is also provided with two resiliently mounted wheels  75  and  76 . These wheels are spaced apart an amount exactly equal to the diameter of the core. Upon the rotation of the core these wheels will co-rotate over the end edge of the core. The suspension of these wheels is such that they exert a force on the core in the axial direction so that it is pushed in the direction of support member  51 . The latter member also includes two such wheels (not shown), although they are not resiliently suspended. These wheels therefore form a stop for the core. In addition, these wheels occupy substantially a fixed position with respect to the print surface  42 . As a result, the core and hence the substrate thereon, will also be able to occupy a fixed position with respect to said print surface. 
     FIG. 4  diagrammatically shows a top plan view of a drawer  3  and a means for positioning it in the sub-scanning direction with respect to the print surface  42 . Print surface  42  is connected to the printer frame, which frame in this case includes side panels  80  and  82 . These panels are provided with projections  81  and  83 , respectively. Drawer  3  is provided with two positioning elements  85  and  95 . These elements are provided with indentations  86  and  96  respectively to co-operate with the projections  81  and  83 , respectively. Element  95  is fixedly connected to drawer  3  by the use of connecting elements  97  and  98 . Element  95  is movably connected with respect to the drawer  3 . For this purpose a rotational spindle  100  is provided which is mounted on the drawer and which is rotatably enclosed by a hole in element  85  coinciding with said spindle. On the other side, element  85  is provided with a slot  100  which accommodates a spindle  105  fixed on the drawer. This embodiment makes it possible for element  85  to rotate, at least through a small angle, about spindle  100 . At the indentation  86  the element is provided with a spring  106  which is fixed against an upright edge  107  of the drawer  3 . In this way the element is pushed in the indicated direction S. 
   When the drawer is pushed inwards in the indicated direction B (which is the opposite direction to the direction F shown in  FIG. 1 ), element  85  will be forced, in the opposite direction to S, as soon as element  85  comes into contact with projection  81 . On further insertion of the drawer in the direction B element  85  will be increasingly forced further in this direction. As soon as the position of the projections  81  and  83  coincides with that of the indentations  86  and  96 , however, element  85  will be pushed in the direction S under the influence of spring  106 . As a result, the drawer  3  will be pushed in the direction of side panel  82  so that the drawer will occupy substantially a fixed position with respect to the print surface  42 . 
   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.