Abstract:
An inkjet printer containing a print zone and an inkjet printhead for printing a substrate in the print zone, a supply unit for storing and delivering the substrate for printing, and a transport unit for transporting the substrate from the supply unit to the print zone, in which printer the supply unit contains a number of substrate holders, each holding a roll on which a substrate web is wound, each substrate holder being operatively connected to a sensor for detecting the end of the web in the substrate holder corresponding to that sensor, wherein the transport unit includes a first transport means for engaging and transporting the substrate emerging from the supply unit and a downstream, second transport means for further transporting the substrate and positioning the same in the print zone.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an inkjet printer for printing a substrate. The inkjet printer has a print zone and an inkjet printhead for printing the substrate in the print zone. In particular, the present invention relates to an inkjet printer for printing a substrate which is unwound from a roll.  
         [0002]     The transport of a substrate unwound from a roll is more complex than the transport of sheets of a substrate. One reason for this is that the substrate has fewer degrees of freedom during the positioning thereof in the print zone. The trailing edge of the substrate is connected to the substrate that is still wound on the roll. There are therefore fewer possibilities of correcting faults occurring during the transport of the substrate or, for example, originating from inaccurate positioning of the roll in the printer supply unit. Another disadvantage of unwinding a substrate from a roll is the fact that the end of the roll cannot be predicted with high accuracy. It is therefore possible that at the beginning of printing of an image there appears to be sufficient substrate available to completely image the image on the substrate, but, in fact, during the printing it may be found that the amount of substrate still present on the roll is in fact insufficient to completely image the image on the substrate. It is known to stop printing as soon as it is found that a roll is completely unwound. However, since the distance between the roll and the print zone is often relatively considerable, a relatively large part of the substrate is not used in this case.  
       SUMMARY OF THE INVENTION  
       [0003]     The object of the present invention is to provide an inkjet printer which obviates the disadvantages of the prior art. For this purpose a printer is provided which contains a dedicated transport unit for the transport of the substrate from a supply unit provided with a number of rolls of substrate, to the print zone, the transport unit also providing accurate positioning of the substrate. Since the transport unit is provided with two transport means for the substrate, it is possible to stop the substrate in this unit over a certain length. As a result it is an effective method for accurately positioning the substrate. In this way, substantially the entire length of the substrate situated on the roll can be effectively used. Also, a sensor is utilized so that the end of the web on the roll can be accurately determined. Using the current printer, however, there is no need immediately to stop printing. Since the transport unit includes two transport means and whenever the roll is empty the web can be stopped in a very defined manner, at least as long as the end of the web has not yet passed the first upstream transport means. The passing of the web can be accurately predicted if the distance between the sensor and the first transport means is known. Since the first transport means is also situated downstream of the supply unit, relatively close to the print zone, the requirement of accurate transport and positioning of the substrate can be fulfilled for practically the entire substrate length.  
         [0004]     In one embodiment, the supply unit has a transit path for the substrate, the holders being arranged along the path in the downstream direction. This embodiment has the advantage that the same transit path can be used for the transport of the substrate from each of the holders. As a result, there are fewer variables in the transport of various substrates, so that the transport is more reproducible. This benefits the accuracy of the transport and positioning of the substrate.  
         [0005]     In another embodiment, in which the first and second transport means are driven, the speed at which the second transport means is driven is greater than or equal to the speed at which the first transport means is driven if the substrate is engaged by both means. One important advantage of this embodiment is that the substrate is maintained in tension between the transport means. As a result, it is a simple matter to obtain accurate transport and positioning, and this contributes to a further improvement of the present invention.  
         [0006]     In yet another embodiment, the transport unit is provided with a guide element to guide the substrate from the first to the second transport means. It has been found that as a result of the presence of the guide element, the incidence of creases, folds and other deformation of the substrate can be greatly reduced. This contributes to further increasing the accuracy of transport of the substrate through the printer and improvement of the positioning of the substrate in the print zone.  
         [0007]     In a further embodiment, the guide element can be moved from a first to a second position such that the distance over which the substrate extends from the first transport means to the second transport means is greater when the guide element is in the first position. The advantage of this embodiment is that it is possible to obviate the problem that the mass inertia of the roll, particularly if it has been only slightly unwound or not unwound at all, is often greater than that of the transport means. As a result of this relatively high mass inertia, considerable power is required to combine a sudden acceleration of the transport means with the same acceleration of the roll, or at least an acceleration where the speed at which the substrate is unwound from the roll is equal to the speed at which the substrate is transported by the transport unit. The present embodiment can provide a solution to this problem by gradually moving the guide element to the second position on the sudden start up of the second most downstream transport means. As a result, the acceleration of the roll does not have to completely follow the acceleration of the transport means. Although a residue is, in fact, built up during the unwinding of the substrate, this can be compensated for by letting the unwinding take place longer than the transport by the second transport means.  
         [0008]     In still a further embodiment of the present invention, each substrate holder is provided with its own sensor. This embodiment simplifies printer control because each holder has a dedicated sensor. In addition, it is now possible to dispose the sensors relatively close to each of the rolls at a distance which is also equal for each substrate holder. Here again, a simplification of the printer control is obtained.  
         [0009]     In another embodiment, the printer has a control unit to determine, after a sensor detects the end of the web of the substrate, which image still to be printed can be completely imaged on the substrate without the end of the web passing the first transport means in the downstream direction. As indicated hereinbefore, for accurate positioning of the substrate in the print zone it is important for the substrate to be engaged by both the first and second transport means. In this embodiment it is possible to check if this is the case, for example on completion of the image currently being printed. If not, then the part of the image in which the substrate is no longer engaged by the first transport means, may possibly have print artefacts as a result of inaccurate positioning. It is then possible to decide whether to immediately stop the printing of that image and reprint the image on a following substrate. In this way no valuable ink is lost. If the image that is currently being printed can still be completed while both transport means engage the substrate, then it would already have been possible to check, if the next image for printing is already known and whether the following image can still be completely printed on condition that the substrate remains engaged by both transport means. If not, then it can be decided not to print the next image. In this manner optimum use of the substrate is possible without wasting ink. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will now be explained in detail with reference to the following drawings, wherein:  
         [0011]      FIG. 1  is a diagram of a printer according to a specific embodiment of the present invention;  
         [0012]      FIGS. 2   a  and  2   b  show a guide element that can be used as a guide for the substrate;  
         [0013]      FIGS. 3   a ,  3   b  and  3   c  show another embodiment of the guide element; and  
         [0014]      FIGS. 4   a  and  4   b  are diagrams showing the speeds at which the substrate is transported through the transport nips  32  ( FIG. 4   a ) and  31  ( FIG. 4   b ). 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]      FIG. 1  is a diagram of a printer according to the present invention. This printer is provided with the supply unit  10 , which serves for the storage and delivery of the substrate for printing. In addition, this printer includes a transport unit  30  which transports the substrate from the supply unit  10  to the print engine  40 . Unit  30  also provides accurate positioning of the substrate in the print zone formed between the print surface  42  and the inkjet printhead  41 . In this embodiment, print engine  40  is a conventional engine which includes printhead  41 , which is constructed from a number of separate sub-heads, each of one of the colors: black, cyan, magenta and yellow. Printhead  41  has only a limited printing range so that it is necessary to print the image on the substrate in different sub-images. To this end, the substrate is transported in increments in each case so that a new part of the substrate can be printed in the print zone. In the example illustrated, the substrate  12  comes from a roll  11  from the supply unit  10 . A web of the substrate is wound on this roll, the web having a length of 200 meters. To accommodate the roll in the printer, the supply unit is provided with a holder (not shown) to rotatably receive the roll. This holder consists of two parts mounted in side plates of the printer, which parts are brought into co-operative connection with the ends of the roll. In this embodiment, the supply unit is provided with a second holder to receive roll  21 . Another substrate  22  is wound on this roll and can also be delivered by the supply unit for printing. For the transport of the substrate, roll  11  is operatively connected to transport means  15 , which in this case includes a pair of rolls between which a transport nip is formed. More particularly, means  15  is a set of two shafts each extending in a direction substantially parallel to roll  11 , on which a number of roll pairs are mounted, each forming a transport nip for the substrate. In an alternative embodiment, only one roll pair is mounted on the shafts substantially coinciding with the middle of the web  12 .  
         [0016]     Upstream of means  15  is a sensor  17 , by means of which it is possible to determine whether there is still substrate on the roll situated in the associated holder. As soon as the roll is used up, the end of the web will pass the sensor, and this is detected by the sensor. For the transport of a substrate originating from roll  21 , the supply holder is provided with transport means  25 . Upstream of the transport 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 . Downstream of these guide elements, there is a transit path  13 . This transit path is used for both the transport of substrate  12  and the transport of substrate  22 .  
         [0017]     A substrate leaving the supply unit  10 , in this example substrate  12 , is engaged by transport means  31  of the transport unit  30 . This transport means transports the substrate via guide element  33  on to the second transport means  32  of the transport unit  30 . The transport means  32  engages the substrate, transports it to print engine  40  and ensures good positioning of the substrate in the print zone between the print surface  42  and the printhead  41 . The transport means  31  and  32  extend substantially parallel to the rolls  11  and  21 , and have a length such that the substrate can be engaged over substantially its entire width.  
         [0018]     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 substantially stationary rollers, i.e., they do not rotate about their axial axis. For the substrate  12  illustrated, this means that during transport, the substrate slides over element  16  and is at the same time fed in the direction of transport means  31 . When this configuration is used it has been found that movement of the substrate at the guide element in a direction parallel to the direction in which the element extends is possible. In other words, the substrate can, in this way, make a lateral movement with respect to the direction in which the substrate is transported. The reason that a lateral movement of this kind is possible in this configuration is associated with the fact that the substrate makes a sliding movement with respect to the guide element. As a result, the required frictional force to set the substrate in motion initially with respect to the guide element is already overcome and practically no force is needed to move the substrate laterally over the guide element.  
         [0019]     The guide elements are so disposed in the supply unit that they can each rotate, at least through a limited angle, about an axis substantially perpendicular to the direction in which the guide elements extend (i.e., the axial direction of the guide elements). In the Figure, 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 center of said elements. As a result of this rotation combined with the possibility of moving the substrate laterally, the substrate has been found to have very good guidance from the supply unit  10  to nip  31  of the transport unit  30 . As a result, despite the fact that the transport means  15  and  31 ;  25  and  31  respectively are not perfectly parallel, it is nevertheless possible to transport the substrate without any damage thereto.  
         [0020]     Guide element  33  of transport unit  30 , which element extends substantially parallel to the transport means  31  and  32 , is also so disposed that it can rotate about an axis perpendicular to the axial direction of said element. This axis is shown by reference  34  and intersects the center of guide element  33 . Since element  33 , in this embodiment, is a co-rotating roller, the substrate is substantially stationary with respect to the surface of said guide element. As a result, lateral movement of the substrate at the guide element is made difficult. In order that such a movement can be made possible, element  33  is suspended so that it can rotate about axis  35 , which axis  35  extends parallel to the bisector  36  of the angle  2   a  over which the substrate is fed from transport means  31  to transport means  32 . This axis  35  intersects the center of the substrate web at a distance of about 1 metre from the guide element itself. On the rotation of element  33  about this axis, the substrate makes a substantially lateral movement. The possibility of rotation of guide element  33  over the axes of  34  and  35  ensures flexible and accurate transport of the substrate from transport means  31  to transport means  32 , even though the two means do not extend 100% parallel to one another.  
         [0021]     Guide element  33  is movable from a first position in which it is situated in  FIG. 1 , to a second position in which the center of this element coincides with the 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 at a minimum. Use is made of this fact during the transport of the substrate to print engine  40 . Since the substrate must, in each case, be moved over a relatively short distance, typically 5 to 10 cm, it is advantageous for this to occur relatively quickly. The mass inertia of roll  11 , certainly when it is provided with the maximum quantity of substrate, is relatively high. For this reason, if the configuration of transport means and guide elements as illustrated were maintained, movement would take a considerable amount of time. To counteract this problem, transport means  31  is accelerated much more slowly than transport means  32 . Nevertheless, in order to ensure adequate supply of substrate to transport means  32 , the guide element  33  is moved in the direction of location  37 . As a result, there is no lack of substrate at transport means  32  during its passage to print engine  40 . If the passage by transport means  32  is stopped, the residue at transport means  31  is compensated for by allowing the transport means to continue rotating for some time. In these conditions, the element  33  is moved back to its first position. In this way, prior to the subsequent transport of a part of the substrate requiring printing with print engine  40 , guide element  33  is in the same initial starting position. It has been found that in this way very accurate transport of the substrate is possible. As a result, the various sub-images can match up more satisfactorily and the number of print artefacts can be reduced.  
         [0022]     The provision of accurate transport and particularly accurate positioning of the substrate in the print zone by the control of transport means  32 , is related to the fact that the substrate is engaged by both transport means  31  and transport means  32 . The position of the substrate is more satisfactorily defined as a result. Together with the rotational possibilities of guide element  33 , very accurate transport and positioning of the substrate is obtained with the tension in the substrate not increasing to the extent where under normal circumstances, mechanical damage of the substrate would occur. An important additional advantage of this arrangement is that printing can still be continued on the substrate as long as the end of the web has not passed transport means  31 . The instant at which this happens can easily be determined if the end of the web is detected by means of the sensor  17  or  27  operatively associated with the web. It is then a simple matter to determine what length of the substrate can still be fed on to the print engine  40  before the end of the web passes the means  31 . In this way it is possible to determine whether the image printed at that instant can still be completely imaged on the substrate without the end of the web passing the first transport means. If so, that image will be completed. If not, then it is possible to choose to stop printing. However, when the end of the web passes means  31  the transport and the positioning of the substrate may be accompanied by more errors, and this may result in print artefacts. Too many artefacts can result in the image having to be reprinted. In order to save ink and substrate it is therefore better to stop printing.  
         [0023]     If it is still possible to print the current image on the substrate (without the end of the web passing the means  31 ), it is then possible to determine whether the next image for printing can still be printed on the substrate (without the end of the web passing the means  31 ). If so, that image will be printed. If not, then it is better to print the following image on a new substrate, for example originating from roll  21 .  
         [0024]      FIGS. 2   a  and  2   b  show a guide element  116  which can be used in a preferred embodiment as a guide for the substrate in the supply unit  10  (instead of the guide element  16  and/or  26 ).  FIG. 2   a  is a side elevation of the guide element. This element comprises a bent plate having a part  200  situated upstream of the bend  202 , and a part  201  which is situated downstream of the bend  202 . Part  200  is connected by spot welds  206  to a rigid frame part  205 . The frame part  205  is a U-profile extending over the length of element  116  and connected to the frame of the printer. Part  201  of the plate is much less restricted in its freedom of movement than part  200 . Yoke  210  fixed on the U-shaped profile  205  provides a point of support for part  201 , as seen in the front elevation of element  116 , as shown in  FIG. 2   b . It will be clear from this front elevation that part  201  is substantially free. Since the plate is relatively thin, part  201  is torsionally weak and can at least partially rotate about the axis passing through the center of the yoke  210  and perpendicular to the longitudinal axis of element  116 . In one embodiment, part  201  is provided with slots so that this part has less resistance to torsion.  
         [0025]     If element  116  is placed in the supply unit to replace element  16 , the free end of plate part  200  points towards the transport nip  15  and part  201  is substantially parallel to the transit path  13  of the supply unit. Element  116  is also stationary in the supply unit. As a result of the tension in the substrate, part  201  can be pulled against yoke  210 . As a result, the ends particularly of part  201  can rotate about the axis passing through the center of the yoke, perpendicular to the direction in which element  116  extends. The advantages of this rotational possibility are described under  FIG. 1 .  
         [0026]      FIG. 3   a  is a diagram of one embodiment of guide element  33 . In this embodiment element  33  comprises a shaft  300  on which a series of transport wheels  301  are disposed. The substrate is guided over these wheels. Since the shaft is suspended to be freely rotatable, it can co-rotate with the substrate without any mutual difference in speeds. As a result, the frictional force accompanying the transport of the substrate at the roller is practically only dependent on the friction in the mounting of this roller.  
         [0027]     Element  33  is provided with a guide plate  302  bent in the form of a V to assist in guiding the substrate. It should also be clear that the V-shape of the element  302  substantially coincides with the V-shape of the substrate as shown in  FIG. 1 . Shaft  300  is resiliently suspended by leaf springs  305  and  306  which are fixed to be freely rotatable on fixed frame parts  307  and  308  respectively. These leaf springs each form the same angle with the shaft in such a manner that the center lines of the leaf springs have a point of intersection  310  upstream of the roller. Rotational axis  35  intersects this point of intersection.  
         [0028]      FIG. 3   b  shows the suspension of the shaft in greater detail. The leaf spring  305  is fixed on the end of shaft  300 . Leaf spring  305  is, in turn, fixed on shaft  311  which is suspended to be freely rotatable in U-shaped frame part  307 . By means of this suspension it is possible for roller  33  to rotate about the axes  34  and  35 . Although the rotational possibility is finite, it appears to be sufficient to make possible accurate and reliable transport of the substrate between the nips  31  and  32 .  
         [0029]      FIG. 3   c  diagrammatically shows the spring mechanism with which roller  33  is pushed in the indicated direction A. This direction A coincides with the direction extending from the above-mentioned second position that the element  33  can occupy (see  FIG. 1 , location  37 ) to the first position that the element occupies in  FIG. 1 . To this end, the shaft  300  is provided with side panels  315  and  316  which at their end remote from the shaft are provided with elements  317  and  318  respectively. The set of weak springs  322 ,  323  and  324  is fixed to these elements and freely guided over rotatable wheels  320  and  321 . The springs are, to some extent, stretched so that they tend to move the ends of the set of springs to the center thereof, as indicated in  FIG. 3   c . As a result, the elements  317  and  318 , and hence the shaft  300 , are pushed in the indicated direction A.  
         [0030]     Since the construction chosen results in a resistance to the displacement of the roller, a stiffness in respect of the movement of translation is introduced for the roller in principle. During movement of the roller to the second position, the resistance to this movement becomes increasingly greater. The advantage of this resistance is that the movement of the roller takes place more accurately and is more satisfactorily reproducible. By placing a number of long weak springs in series, this resistance remains sufficiently small but very effective.  
         [0031]      FIGS. 4   a  and  4   b  diagrammatically show the speed at which the substrate is transported through the transport nips  32  ( FIG. 4   a ) and  31  ( FIG. 4   b ) during the passage of part of the substrate so that a new strip thereof can be printed using the inkjet printhead  41 .  
         [0032]     Curve  400  in  FIG. 4   a  shows what speed of passage is imposed on the substrate at the nip  32 . A high speed of transit is generated relatively quickly and this is retained for some time and then rapidly drops to zero. Despite the high mass inertia of the roll on which the substrate is wound, this high acceleration can be obtained by moving roller  33  as indicated under  FIG. 1 .  
         [0033]     Curve  401  in  FIG. 4   b  shows the speed of transit imposed on the substrate at nip  31  for the transport of the same length of the substrate. It will be seen that this nip is driven before nip  32  so that the substrate is already partly unwound from roll  11  before nip  32  is driven. It may happen that movement of the roller  33  will enable the web to be tensioned between the means  31  and  32 . The acceleration which is imparted by nip  31  is smaller than that of nip  32 , and the maximum speed of transit that this nip provides is lower. However, the substrate is passed through for a longer time so that ultimately the same length of the substrate passes the nip  31 .  
         [0034]     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.