HK1246250B - Indirect printing system - Google Patents

Description

Indirect printing system

Field of the Invention

The present invention relates to an indirect printing system having an intermediate transfer member (ITM) in the form of an endless conveyor belt for transporting an ink image from an imaging station, where the ink image is deposited on an outer surface of the ITM by at least one print bar, to an impression station, where the ink image is transferred from the outer surface of the ITM to a print substrate.

Background of the Invention

An example of a digital printing system as set out above is described in detail in WO 2013/132418, which discloses the use of water-based inks and ITMs having a hydrophobic outer surface.

In indirect printing systems, the ITM is typically wrapped around a support cylinder or drum, and such mounting ensures that the distance of the ITM from the printbar does not change at the imaging station. However, in cases where the ITM is a driven, flexible, endless conveyor that passes over drive and tension rollers, it is useful to take steps to ensure that the ITM does not swing up or down or otherwise shift as it passes through the imaging station and its distance from the printbar remains fixed.

In WO 2013/132418, the ITM is supported on a flat table in an imaging table and it is suggested to use negative air pressure and transverse conveyor belt tension to keep the ITM in contact with its support surface. In some systems, using such a configuration can create a high level of resistance on the ITM as it passes through the imaging table.

WO 2013/132418 also teaches that, to help guide the conveyor belt smoothly, friction can be reduced by passing the conveyor belt over an ink roller adjacent to each print bar, rather than sliding the conveyor belt over a fixed guide plate. The ink rollers do not need to be precisely aligned with their respective print bars. They can be located slightly (e.g., a few millimeters) downstream of the printhead's ejection position. Friction is used to keep the conveyor belt taut and substantially parallel to the print bars. To achieve this, the underside of the conveyor belt has high friction properties, and when the conveyor belt passes under the print bars, the lateral tension applied by the guide channel is sufficient to keep the conveyor belt flat and in contact with the ink rollers.

Some systems rely on lateral tension to maintain the conveyor belt in frictional engagement with the ink roller to prevent the conveyor belt from lifting off the ink roller at any point across the conveyor belt. However, in some systems this can (even severely) increase drag on the conveyor belt and wear on the guide channel.

Overview

By supporting the ITM without significantly increasing the drag on the ITM during its passage through the imaging station, it is possible to avoid wobbling of the ITM, thereby maintaining its surface at a fixed predetermined distance from the print bar. This can be accomplished by a plurality of support rollers having a common flat tangential surface and contacting the inner surface of the ITM.

According to an embodiment of the present invention, an indirect printing system is provided that has an intermediate transfer member (ITM) in the form of an endless conveyor belt for transporting an ink image from an imaging station, where the ink image is deposited on an outer surface of the ITM by at least one printbar, to an impression station, where the ink image is transferred from the outer surface of the ITM to a print substrate, wherein the outer surface of the ITM is maintained at a predetermined distance from the at least one printbar within the imaging station by a plurality of support rollers, the plurality of support rollers having a common flat tangential surface and contacting an inner surface of the ITM, and wherein the inner surface of the ITM is attracted to the support rollers such that, with respect to a direction of movement of the ITM, a contact area between the ITM and each support roller is greater on a downstream side of the support roller than on an upstream side. The attraction of the ITM to each support roller is sufficient to cause a section of the ITM located immediately downstream of the support roller to deflect downwardly, away from the common tangential surface of the support rollers.

In some embodiments of the present invention, the inner surface of the ITM and the outer surface of each support ink roller are formed of materials that adhesively bond to each other, and the adhesion between the outer surface of each support ink roller and the inner surface of the ITM acts to prevent the ITM and the support ink roller from separating during operation when the conveyor belt circulates.

The back-up roller may have a smooth or rough outer surface, and the inner surface of the ITM may be formed of or coated with a material that adhesively bonds to the back-up roller surface.

The material on the inner surface of the ITM may be a viscous silicon-based material, which may optionally be augmented with filler particles to improve its mechanical properties.

In some embodiments of the present invention, the attraction between the inner surface of the ITM and the support roller can be initiated by suction. Each support roller can have a perforated outer surface that communicates with a plenum within the support roller that is connected to a vacuum source, such that negative pressure draws the inner surface of the ITM toward the roller. A fixed shield can surround or line a portion of the circumference of each support roller so that suction is applied only to the side of the roller facing the ITM.

In some embodiments of the present invention, the attraction between the support roller and the ITM can be magnetic. In such embodiments, the inner surface of the ITM can be rendered magnetic (in the same manner as a refrigerator magnet) to attract the ferromagnetic support roller. Alternatively, the inner surface of the ITM can be loaded with ferromagnetic particles to attract the magnetized support roller.

Each printbar may be associated with a respective support roller and the support rollers may be positioned relative to the printbars such that, during operation, ink is deposited by the printbars onto the ITM along a narrow band upstream of the contact area between the ITM and the support roller.

A shaft or linear encoder may be associated with one or more of the support rollers to determine the position of the ITM relative to the printbar.

According to some embodiments, each printbar is associated with a respective support roller and the associated support roller is positioned relative to the printbar such that, during operation, ink is deposited by the printbar onto the ITM along a narrow band upstream of the contact area between the ITM and the support roller.

According to some embodiments, a shaft or linear encoder is associated with one or more of the support rollers to determine the position of the ITM relative to the printbar.

According to some embodiments, the indirect printing system includes a plurality of printbars such that a different respective support ink roller is positioned below and vertically aligned with each printbar of the plurality of printbars.

According to some embodiments, for each given printbar of the plurality of printbars, a corresponding vertically aligned support ink roller is disposed slightly downstream of the given printbar.

According to some embodiments, each given support inker roller of the plurality of support inker rollers is associated with a respective rotational speed measuring device and/or a respective encoder for measuring a respective rotational speed of the given support inker roller.

An indirect printing system is disclosed having an intermediate transfer member (ITM) in the form of an endless conveyor belt for transporting an ink image from an imaging station. According to an embodiment of the present invention, the ink image is deposited on an outer surface of the ITM by a plurality of print bars to a stamping station, where the ink image is transferred from the outer surface of the ITM to a printing substrate. The outer surface of the ITM is maintained at a predetermined vertical distance from the print bars within the imaging station by a plurality of support rollers, the plurality of support rollers having a common flat tangential surface and contacting an inner surface of the ITM. The support rollers are configured such that a different respective support roller is positioned below and vertically aligned with each of the plurality of print bars. Each given support roller in the plurality of support rollers is associated with a respective rotational speed measuring device and/or a respective encoder for measuring the respective rotational speed of the given support roller.

According to some embodiments, for each given printbar of the plurality of printbars, a corresponding vertically aligned support ink roller is disposed slightly downstream of the given printbar.

According to some embodiments, the indirect printing system further comprises: a drop deposition control circuit configured to adjust, for each given print bar of the plurality of print bars, a respective ink drop deposition rate DR onto the ITM, the drop deposition control circuit adjusting the ink drop deposition rate based on and in response to a measurement of a rotational speed of a respective support roller aligned perpendicularly to the given print bar.

In some embodiments, the measuring device and/or encoder is attached (ie, directly or indirectly attached) to its corresponding ink roller (eg, via its shaft).

According to some embodiments, for an upstream printing bar and a downstream printing bar that are respectively aligned perpendicularly to an upstream support ink roller and a downstream support ink roller, a droplet deposition control circuit adjusts the corresponding _DRupstream , _DRdownstream deposition rates at the upstream printing bar and the downstream printing bar so that the difference DRupstream-DRdownstream between the corresponding ink droplet deposition rates at the upstream printing bar and the downstream printing bar is adjusted according to a difference function F= _ωupstream * _Rupstream - _ωdownstream * _Rdownstream , wherein: _i.ωupstream is the measured rotational rate of the support ink roller with which the _upstream printing bar is aligned as measured by its associated rotational speed measuring device or encoder; _ii.Rupstream is the radius of the support ink roller with which the upstream printing bar is aligned; iii.ωdownstream _is the measured rotational rate of the support ink roller with which the _downstream printing bar is aligned as measured by its associated rotational speed measuring device or encoder; and _{ii.Rdownstream} is the radius of the support ink roller with which the upstream printing bar is aligned.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be further described by way of examples and with reference to the accompanying drawings, in which:

Figures 1, 3 and 4 each schematically illustrate an image transfer member passing under four print bars of an imaging station; and

FIG. 2 is a section through an embodiment in which the ITM is attracted to the support roller by the application of negative pressure from within the support roller.

FIG. 5 illustrates the conversion of a digital input image into an ink image by printing.

6-8 illustrate a method for printing with upstream and downstream printbars depending on the angular velocity of the support ink roller.

It will be understood that the drawing figures are only intended to explain the principles employed in the present invention and that the components shown may not be drawn to scale.

Detailed Description of the Figures

FIG1 shows an image transfer member (ITM) 20 passing beneath four printbars 10, 12, 14, 16 of an imaging station of a digital printing system, such as that described in WO 2013/132418. The printbars 10, 12, 14, 16 deposit ink droplets on the ITM, which dry and are simultaneously transported by the ITM and transferred to a substrate at an imprint station (not shown). The direction of movement of the ITM from the imaging station to the imprint station, indicated by arrow 24 in the figure, is also referred to as the printing direction. The terms upstream and downstream, as used herein, indicate the relative position of an element with respect to this printing direction.

Multiple print bars can be used to print in multiple colors, such as CMYK in the case of the four print bars shown in the figure, or to increase printing speed when printing in the same color. In either case, accurate registration is required between the ink drops deposited by the different print bars, and to achieve this, it is necessary to ensure that the ITM lies in a well-defined plane when the ink is deposited onto its surface.

In the illustrated embodiment, cylindrical support rollers 11, 13, 15, and 17 are positioned immediately downstream of the respective print bars 10, 12, 14, and 16. A common horizontal plane, spaced a desired predetermined distance from the print bars, is tangential to all of the support rollers. The rollers 11, 13, 15, and 17 contact the underside of the ITM 20, that is, the side facing away from the print bars.

To ensure that ITM 20 does not wobble as it passes over ink rollers 11, 13, 15, and 17, the ink rollers in FIG1 may have a smooth, polished surface, and the underside of the ITM may be formed from or coated with a soft, uniformly coated silicone-like material that adhesively bonds to the smooth surface. Such materials are known and widely used commercially, for example, in children's toys. For example, a pattern made of such material can adhere to a vertical glass plate when pressed against it.

Due to the viscous contact between the ITM 20 and the ink rollers 11, 13, 15, and 17, it will be seen in the figure that the ITM is deflected downward from a notionally horizontal tangential plane on the downstream side, or exiting the side of each ink roller 11, 13, 15, and 17. Therefore, the contact area 22 between the ITM 20 and each ink roller 11, 13, 15, and 17 is primarily located on the downstream side, or exiting the side of the ink roller. The tension applied to the ITM in the printing direction ensures that the ITM returns to the desired plane before reaching the subsequent print bar 10, 12, or 14.

The adhesion of the ITM 20 to the support rollers relies on ensuring that the ITM does not lift off the rollers. Since the rollers are supported on bearings and rotate freely and smoothly, the only resistance on the ITM, other than the force required to overcome the resistance of the bearings and maintain the momentum of the support rollers, is the relatively small force required to separate the sticky underside of the ITM from each of the support rollers 11, 13, 15, and 17.

The area where the ITM contacts the uppermost point of each ink roller 11, 13, 15, and 17, as well as the area immediately upstream of each ink roller, lies in a conceptual tangential plane and can be aligned with the print bars 10, 12, 14, and 16. However, if any foreign matter, such as dust particles, were to adhere to the sticky underside of the ITM 20, it would cause the upper surface of the ITM to bulge upward as it passes over the supporting ink rollers. For this reason, it is preferred to position the print bars 10, 12, 14, and 16 upstream of the vertical axial plane of the ink rollers 11, 13, 15, and 17, that is, offset upstream from the area where the ITM contacts the ink rollers.

If there is too much viscous adhesive between the ITM 20 and the back-up rollers 11, 13, 15, and 17, it may cause drag and wear of the ITM 20. The degree of drag can be reduced by suitable selection of the hardness of the adhesive material or by modifying the roughness of the back-up rollers 11, 13, 15, and 17.

The attraction between the ITM 20 of FIG1 and the support rollers 11, 13, 15, and 17 can rely on magnetism rather than viscosity. In such an embodiment, the inner surface of the ITM 20 can be rendered magnetic so as to be attracted to the ferromagnetic support rollers 11, 13, 15, and 17. Alternatively, the inner surface of the ITM 20 can be loaded with ferromagnetic particles so as to be attracted to the magnetized support rollers 11, 13, 15, and 17.

2 schematically illustrates another alternative embodiment in which the attraction between the inner surface of the ITM 120 and the support roller assembly is the result of negative pressure being applied to the inner surface of the ITM 120 by the support roller assembly while the outer surface of the ITM 120 is at atmospheric pressure.

The illustrated support roller assembly includes a support roller 111a, surrounded by a fixed shield 111b for a substantial portion of its circumference. The roller 111a has a perforated surface and is hollow, with an internal plenum 111c connected to a vacuum source. The shield 111b functions to prevent the vacuum in the support roller 111a from dissipating and to concentrate all suction in the arc of the support roller 111a adjacent to and facing the inner surface of the ITM 120. A seal is provided between the support roller 111a and the shield 111b to prevent air from entering the plenum 111c through anything other than the exposed arc of the support roller 111a.

As an alternative to a shield 111b surrounding the outside of the support ink roller 111a, it would be possible to provide a fixed shield lining the inside of the support ink roller 111a.

FIG3 shows the same system as shown in FIG1 , including printbars 10, 12, 14, and 16 having (i) centers with positions labeled PB_Loc _A , PB_Loc _B , PB_Loc _C , and PB_Loc _D , respectively, where PB is an abbreviation for "printbar" and Loc is an abbreviation for "location," and (ii) thicknesses labeled THKNS _A , THKNS _B , THKNS _C , and THKNS _D. The distances between adjacent printbars are labeled Distance _AB , Distance _BC, and Distance _CD .

The 'center' of the printbar is a vertical plane oriented in the cross-printing direction.

In some embodiments, THKNS _A =THKNS _B =THKNS _C =THKNS _D , although this is not limiting, and in other embodiments, printbar thickness may vary. In some embodiments, the printbars are evenly spaced such that distance _AB =distance _BC =distance _CD , although this is not limiting, and in other embodiments, the distance between adjacent printbars may vary.

In some embodiments, each printbar is associated with a respective support ink roller, positioned below and vertically aligned with the support ink roller.

For the purposes of the present disclosure, when a support ink roller 13 is 'vertically aligned' with an associated printbar 12, the center of the support ink roller 13 may be precisely aligned (i.e., in the printing direction indicated by 24) with the centerline PB_LOC _B of the associated printbar 12. Alternatively, if there is a 'slight' horizontal displacement/offset between the center of the support ink roller 13 and the center of the associated printbar 12 in the printing direction (e.g., a downstream offset of the support ink roller relative to its associated printbar), the printbar 12 and support ink roller 13 are still considered to be 'vertically aligned' with each other.

FIG3 illustrates the horizontal displacement/offset between each print bar 10, 12, 14, 16 and its corresponding supporting roller 11, 13, 15, and 17 in the printing direction, namely, offset _A , offset _B , offset _C , and offset _D. However, because the print bars and supporting rollers are 'vertically aligned,' this displacement/offset is at most 'slight.' The terms 'slight' or 'slightly shifted/offset' (used interchangeably) are defined below.

In a non-limiting example, all support rollers have a common radius - this is not a limitation, and embodiments are also contemplated where the radii of the support rollers are different.

In one specific example, the radius of each support ink roller 11, 13, 15 and 17 is 80 mm, the center-to-center distance between adjacent pairs of print bars (Distance _AB = Distance _BC = Distance _CD ) is 364 mm, the thickness of each print bar (THKNS _A = THKNS _B = THKNS _C = THKNS _D ) is 160 mm, and the offset distance between the center of the print bar and the center of its associated ink roller (Offset _A = Offset _B = Offset _C = Offset _D ) is 23 mm.

Printbars 10 and 16 are 'end printbars,' each having only a single neighbor—printbar 10's neighbor is printbar 12, and printbar 16's neighbor is printbar 14. Conversely, printbars 12 and 14 are 'interior printbars,' having two neighbors. Each printbar is associated with a nearest neighbor distance—for printbar 10, this is distance _AB ; for printbar 12, this is MIN(Distance _AB , Distance _BC ), where MIN indicates the minimum; for printbar 14, this is MIN(Distance _BC , Distance _CD ); and for printbar 16, this is distance _CD .

For the purposes of the present disclosure, when the support roller is 'slightly displaced/offset' from its associated printbar, this means that the ratio α between (i) the offset/displacement distance "offset" defined by the centers of the support roller and the printbar and (ii) the nearest neighbor distance of the printbar is at most 0.25. In some embodiments, the ratio α is at most 0.2, or at most 0.15, or at most 0.1. In the specific example described above, the ratio α is 23/364 = 0.06.

In some embodiments, to achieve accurate alignment between ink drops deposited by different printbars, the position of the ITM must be monitored and controlled not only in the vertical direction but also in the horizontal direction. Due to the adhesive nature of the contact between the ink roller and the ITM, the angular position of the ink roller can provide an accurate indication of the horizontal position of the surface of the ITM and, therefore, the position of the ink drop deposited by the previous printbar. Shaft encoders may therefore be suitably mounted on one or more of the ink rollers to provide position feedback signals to the controller of the printbars.

In some embodiments, the length of the flexible conveyor belt or portions thereof may fluctuate over time, where the magnitude of the fluctuation may depend on the physical structure of the flexible conveyor belt. In some embodiments, the conveyor belt may stretch and contract unevenly. In these cases, the local linear velocity of the ITM at each print bar may vary due to the stretching and contraction of the conveyor belt or ITM in the printing direction, depending on the print bar. Not only may the degree of stretch be uneven along the length of the conveyor belt or ITM, but it may also fluctuate instantaneously.

Registration accuracy may depend on accurate measurement of the corresponding line speed of the ITM under each print bar. For systems where the ITM is a cylinder or flexible conveyor belt with temporally constant and spatially uniform stretch (and therefore constant shape), it may be sufficient to measure the ITM speed at a single location.

However, in other systems (e.g., when the ITM stretches or contracts spatially non-uniformly and in a temporally fluctuating manner), the linear velocity of the ITM at PB_Loc _A under the first printbar 10 may not match the linear velocity of the ITM at PB_Loc _B under the second printbar 12. Thus, if the linear velocity of the ITM at the downstream printbar 10 exceeds the linear velocity of the ITM at the downstream bar 12, this may indicate that the blanket is locally extending (i.e., increasing its local stretch) at a location between the two printbars 10, 12. Conversely, if the linear velocity of the ITM at the downstream printbar 10 is less than the linear velocity of the ITM at the downstream bar 12, this may indicate that the blanket is locally contracting at a location between the two printbars 10, 12.

Alignment can therefore benefit from obtaining an accurate measurement of the local velocity of the ITM at each printbar. Instead of relying solely on a single ITM-representative velocity value (i.e., as may be done for a cylinder), the "printbar-local" linear velocity of the ITM at each printbar can be measured at a location relatively 'close' to the printbar center PB_LOC.

For example, as shown in FIG4 , a respective device (e.g., a shaft encoder) 211, 213, 215, or 217 can be used to measure the respective rotational speed ω of each support roller. This rotational speed, along with the radius of the support roller, can describe the local linear speed of each support roller. Because the support rollers are aligned perpendicular to the print bar, this rotational speed and the radius of the support roller can provide a relatively accurate measurement of the linear speed of the ITM beneath the print bar.

FIG4 schematically illustrates a rotational speed measuring device. As is known in the art (e.g., in the field of shaft encoders), the rotational speed measuring device 211, 213, 215, or 217 may include mechanical, electrical, optical, magnetic, or any other component for monitoring the rotation of the supporting ink roller. For example, the rotational speed measuring device 211, 213, 215, or 217 may directly monitor the rotation of the ink roller or a rigid object (e.g., a shaft) rigidly attached to the ink roller and rotating in series with the ink roller.

Because the ITM can stretch or shrink locally over time, depositing ink drops based on only a single 'ITM-representative' speed for all printbars can lead to registration errors. Instead, it can be advantageous to measure the linear speed of the ITM locally at each printbar.

To this end, the support roller can be used for multiple purposes - namely, supporting the ITM in a common tangential plane and measuring the speed of the ITM at a location where the ITM is in contact with the support roller (e.g., non-sliding contact - e.g., due to attachment of the inner surface to the support roller - e.g., due to the presence of a viscous material on the inner surface of the ITM).

In order for the support roller to provide an accurate measurement of the linear velocity of the ITM below the print bar, it is desirable to align the support roller vertically with its associated print bar. To this end, it is desirable to position the support roller so that the value of the ratio α (defined above) is relatively small.

In some embodiments, the ratio β between (i) the offset/displacement distance "OFFSET" defined by the centers of the support roller and the print bar and (ii) the thickness TKNS of the print bar is at most 1, or at most 0.75, or at most 0.5, or at most 0.4, or at most 0.3, or at most 0.2. In the example described above, the value of the ratio β is 23 mm/160 mm = 0.14.

In some embodiments, the ratio γ between (i) the diameter of the vertically aligned support roller and (ii) the thickness TKNS of the print bar is at most 2 or at most 1.5 or at most 1.25. In the examples described above, the value of the ratio β is 160 mm/160 mm=1.

In some embodiments, the ratio δ between (i) the diameter of the vertically aligned support roller and (ii) the nearest neighbor distance of the associated print bar is at most 1, or at most 0.75, or at most 0.6, or at most 0.5. In the example described above, the value of the ratio β is 160 mm/364 mm = 0.44.

Figure 5 is a general diagram illustrating any printing process - a digital input image is stored in electronic or computer memory (e.g., as a two-dimensional array of grayscale values), and this 'digital input image' is printed by a printing system to obtain an ink image on an ITM.

Each printbar deposits ink drops onto the ITM at a corresponding deposition rate that depends on (i) the content of the digital input image being printed and (ii) the speed of the ITM as it moves beneath the printbar. The 'deposition rate' is the rate at which ink drops are deposited onto the ITM 20 and has the dimension of 'drops per unit time' (e.g., drops per second).

FIG6 illustrates a method for operating the upstream printbar 14 and the downstream printbar 12, according to some embodiments. In step S205, the angular velocity ω _upstream of the support ink roller 15 is monitored; similarly (e.g., simultaneously), in step S215, the angular velocity ω _downstream of the support ink roller 13 is monitored. In step S251, ink droplets are deposited on the ITM 20 by the upstream printbar 14 at a rate determined (e.g., primarily) by a combination of (i) the digital input image and (ii) ω _upstream . In step S255, ink droplets are deposited on the ITM 20 by the downstream printbar 12 at a rate determined (e.g., primarily) by a combination of (i) the digital input image and (ii) ω _downstream .

It will be appreciated that due to transient fluctuations in the non-uniform stretching of the ITM, the linear speeds of the ITM at the upstream printbar 14 and downstream printbar 12 will not always match. These linear speeds can be approximately and correspondingly monitored by monitoring the linear speeds at (i) the contact location with the upstream support inking roller 15 (i.e., perpendicularly aligned with the upstream printbar 14) and (ii) the contact location with the downstream support inking roller 13 (i.e., perpendicularly aligned with the downstream printbar 12).

Note : The angular velocity of the upstream support roller 15 is ω _upstream , and the angular velocity of the downstream support roller 13 is ω _downstream . The linear velocity of the ITM 20 at the contact point between the ITM 20 and the upstream support roller 15 is labeled LV _upstream ; the linear velocity of the ITM 20 at the contact point between the ITM 20 and the upstream support roller 15 is labeled LV _upstream . The ink drop deposition rate of the upstream printbar 14 is labeled DR _upstream , and the ink drop deposition rate of the downstream printbar 12 is labeled DR _downstream . R _upstream is the radius of the upstream support roller 15; R _downstream is the radius of the downstream support roller 13.

In some embodiments, the ink drop deposition rate DR at any point in the printbar is regulated by an electrical circuit (e.g., a control circuit). For purposes of this disclosure, the term 'circuitry' (or control circuitry, such as a drop deposition control circuit) is broadly intended to include any combination of analog circuitry, digital circuitry (e.g., a digital computer), and software.

For example, the circuitry may adjust the ink drop deposition rate DR based on and in response to electrical input received directly or indirectly from any rotational speed measuring device (eg, a shaft encoder).

For this paragraph, it is assumed that LV _upstream is equal to the linear velocity of the ITM directly beneath the upstream printbar 14 and LV _downstream is equal to the linear velocity of the ITM directly beneath the downstream printbar 12 - this is a good approximation because (i) any horizontal displacement/offset between the upstream printbar 14 and its associated support ink roller 15 is slight at best; and (ii) any horizontal displacement/offset between the downstream printbar 12 and its associated support ink roller 13 is slight at best.

When the upstream and downstream line velocities are matched (i.e., when LV _upstream = LV _downstream ), the difference in the respective ink drop rates at any given time (DR _upstream - DR _downstream ) will be determined primarily (e.g., solely) by the content of the digital input image. Thus, when printing a uniform input image, when the upstream and downstream line velocities are matched, this difference (DR _upstream - DR _downstream ) will be zero, and each printbar will deposit ink drops at a common deposition rate difference DR _upstream = DR _downstream .

However, due to transient fluctuations in the non-uniform stretching of the ITM, there may be periods of mismatch between the upstream and downstream line speeds—i.e., when LV _upstream ≠ LV _downstream . To compensate (e.g., when printing a uniform input image or a uniform portion of a larger input image), the greater the difference between the upstream and downstream line speeds, the greater the difference in ink deposition rate—i.e., as the line speed difference LV _upstream - LV _downstream increases (decreases), the deposition rate difference DR _upstream - DR _downstream increases (decreases).

Assuming no slippage between the ITM 20 and the upstream support roller 15, the magnitude of LV _upstream is the product ω _upstream * R _upstream . Assuming no slippage between the ITM 20 and the downstream support roller 13, the magnitude of LV _downstream is the product ω _downstream * R _downstream . The linear velocity difference LV _upstream - LV _downstream is given by ω _upstream * R _upstream - ω _downstream * R _downstream .

Thus, in some embodiments, the respective ink drop deposition rates at the upstream print bar 14 and the downstream print bar 12 are adjustable such that for at least some digital input images (e.g., uniform images), the difference between the ink drop deposition rates, _DRupstream -DRdownstream, increases (decreases) as _ωupstream * _Rupstream - _{ωdownstream *Rdownstream increases} (decreases).

This is illustrated in FIG. 7 , where (i) steps S205 and S215 are as in FIG. 6 , and (ii) in step S271 , droplets are deposited onto the ITM 20 by the upstream printbar 14 and the downstream printbar 12 such that the ink drop deposition rate difference DR _upstream -DR _downstream is adjusted according to ω _upstream * R _upstream - ω _downstream * R _downstream . In some examples (e.g., when printing a uniform digital input image or a uniform portion of a non-uniform digital image), the ink drop deposition rate difference DR _upstream - DR _downstream is proportional to ω _upstream * R _upstream - ω _downstream * R _downstream . In this example, whenever ω _upstream * R _upstream - ω _downstream * R _downstream increases (decreases), DR _upstream - DR _downstream increases (decreases).

FIG8 illustrates another method for depositing ink droplets on an ITM 20, wherein steps S205 and S215 are similar to those of FIG6-7. In steps S201 and S211, droplets are deposited by the upstream printbar 14 and the downstream printbar 12 ₍ i.e., at respective deposition rates _DRupstream and _DRdownstream ). In steps S221-S225, DRupstream - DRdownstream increases in response to _ωupstream * _Rupstream - _ωdownstream * _Rdownstream increasing. In steps S229 _{and S235} , _DRupstream - DRdownstream decreases in response to ωupstream * _Rupstream - _ωdownstream * _{Rdownstream decreasing} .

According to some embodiments, for the upstream printing bar 14 and the downstream printing bar 12 that are respectively aligned perpendicularly to the upstream support ink roller 15 and the downstream support ink roller 13, the droplet deposition control circuit adjusts the corresponding _DRupstream , _DRdownstream deposition rates at the upstream printing bar and the downstream printing bar so that the difference DRupstream-DRdownstream between the corresponding ink droplet deposition rates at the upstream printing bar and the _downstream printing bar is adjusted according to the difference function F= _ωupstream * _Rupstream - _ωdownstream * _Rdownstream , _where : _i.ωupstream is the measured rotational rate of the support ink roller 13 aligned with the upstream printing bar as measured by its associated rotational speed measuring device or encoder 213; _ii.Rupstream is the radius of the support ink roller 215 aligned with the upstream printing bar; _{iii.ωdownstream} is the measured rotational rate of the support ink roller 15 aligned with the downstream printing bar as measured by its associated rotational speed measuring device or encoder 215; and _{ii.Rdownstream} is the radius of the support ink roller 15 aligned with the upstream printing bar.

Embodiments of the present invention relate to encoder devices and/or rotational speed measuring devices. The rotational speed measuring device and/or encoder device can convert the angular position or motion of a shaft or spindle into an analog or digital code. The encoder can be an absolute or incremental (relative) encoder. The encoder can include any combination of mechanical (e.g., including gears) (e.g., based on stress and/or based on rheometers) and/or electrical (e.g., conductive or capacitive) and/or optical and/or magnetic (e.g., coaxial or off-axis - for example, including Hall effect sensors or magnetoresistive sensors) technology or any other technology known in the art.

In various embodiments, the measuring device and/or encoder may be attached (ie, directly or indirectly) to its respective ink roller.

It should be understood that certain features of the invention described in the context of separate embodiments for the sake of clarity may also be provided in combination in a single embodiment. Conversely, various features of the invention described in the context of a single embodiment for the sake of simplicity may also be provided in any other described embodiment of the present disclosure, either individually or in any suitable subcombination or where appropriate. Certain features described in the context of different embodiments are not considered essential features of those embodiments, unless the embodiment is ineffective without those elements.

The presently disclosed teachings can be practiced in systems employing water-based inks and ITMs having a hydrophobic outer surface. However, this is not limiting and other inks and ITMs can be used.

Although the present invention has been described with reference to its various specific embodiments presented for illustrative purposes only, such exact disclosed embodiments should not be considered as limiting. Those skilled in the art will envision many other alternatives, modifications and variations of such embodiments based on the applicant disclosure herein. Therefore, it is intended to encompass all such alternatives, modifications and variations and to be defined solely by the spirit and scope of the present invention as defined in the appended claims and any variations within the meaning and scope of its equivalents.

In this specification and the claims of this disclosure, each of the verbs "comprise," "include," and "have," and their conjugations, is used to mean that the object or object of the verb is not necessarily a complete list of features, components, steps, parts, elements, or parts of the object or subject matter of the verb.

Thus, as used herein, the singular forms "a," "an," and "the" include plural references and mean "at least one" or "one or more" unless the context clearly dictates otherwise.

As used herein, when a numerical value is followed by the term "about," the term "about" is intended to indicate +/- 10%.

To the extent necessary to understand or complete the present disclosure, all publications, patents, and patent applications mentioned herein are expressly incorporated by reference in their entirety as if fully set forth herein.

Citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.

Claims (20)

1. An indirect printing system having an intermediate transfer unit (ITM) in the form of a circulating annular conveyor belt for transporting an ink image from an imaging stage to an impression stage, in which the ink image is deposited on the outer surface of the ITM by at least one printing bar, and in the impression stage, the ink image is transferred from the outer surface of the ITM to a printing substrate, wherein the outer surface of the ITM is maintained within the imaging stage at a predetermined distance from the at least one printing bar by a plurality of support ink rollers having a common flat horizontal tangential surface and contacting an inner surface of the ITM, and wherein the inner surface of the ITM is attracted to the support ink rollers such that, with reference to the direction of movement of the ITM, the contact area between the ITM and each support ink roller is larger on the downstream side of the support ink roller than on the upstream side. 2. The indirect printing system of claim 1, wherein the inner surface of the ITM and the outer surface of each support ink roller are formed of a material adhesively bonded to each other, the adhesion between the outer surface of each support ink roller and the inner surface of the ITM serving to prevent the ITM from separating from the support ink roller during operation as the conveyor belt circulates. 3. The indirect printing system of claim 2, wherein the support ink roller may have a smooth or rough outer surface, and the inner surface of the ITM is formed of or coated with a material adhesively bonded to the surface of the support ink roller. 4. The indirect printing system of claim 3, wherein the material on the inner surface of the ITM is a viscous silicone material. 5. The indirect printing system of claim 4, wherein the viscous silicone material is supplemented with filler particles. 6. The indirect printing system of claim 1, wherein the attraction between the inner surface of the ITM and the supporting ink roller can be lifted by suction. 7. The indirect printing system of claim 6, wherein each support ink roller has a perforated outer surface that is in communication with a pressure ventilation system connected to a vacuum source within the support ink roller. 8. The indirect printing system of claim 7, wherein a fixed shield surrounds or lines a portion of the circumference of each support ink roller such that suction is applied only to the side of the support ink roller facing the ITM. 9. The indirect printing system of claim 1, wherein the attraction between the support ink roller and the ITM is magnetic. 10. The indirect printing system of claim 9, wherein the inner surface of the ITM is magnetic and attracted to the ferromagnetic support ink roller. 11. The indirect printing system of claim 9, wherein the inner surface of the ITM is loaded with ferromagnetic particles and attracted to a magnetized support ink roller. 12. The indirect printing system of any one of claims 1 to 11, wherein each printing bar is associated with a corresponding support ink roller and the position of the associated support ink roller relative to the printing bar is such that, during operation, ink is deposited from the printing bar onto the ITM along a narrow band upstream of the contact area between the ITM and the support ink roller. 13. The indirect printing system of any one of claims 1 to 11, wherein a shaft or linear encoder is associated with one or more of the support ink rollers to determine the position of the ITM relative to the printing bar. 14. The indirect printing system according to any one of claims 1 to 11, comprising a plurality of the printing bars such that different corresponding support ink rollers are located below and perpendicularly aligned with each of the plurality of printing bars. 15. The indirect printing system of claim 14, wherein for each given printing bar of the plurality of printing bars, a correspondingly vertically aligned support ink roller is disposed slightly downstream of the given printing bar. 16. The indirect printing system of claim 14, wherein each given support ink roller of the plurality of support ink rollers is associated with a corresponding rotational speed measuring device for measuring the corresponding rotational speed of the given support ink roller. 17. The indirect printing system of claim 16, further comprising: a droplet deposition control circuit configured to adjust a corresponding ink droplet deposition rate DR on the ITM for each given print bar of the plurality of print bars, the droplet deposition control circuit adjusting the ink droplet deposition rate based on and in response to a measured rotational speed of a corresponding support ink roller aligned perpendicularly to the given print bar. 18. An indirect printing system having an intermediate transfer unit (ITM) in the form of a circulating annular conveyor belt, the intermediate transfer unit for transporting an ink image from an imaging stage to an impression stage, in the imaging stage, the ink image being deposited on the outer surface of the ITM by a plurality of print bars, and in the impression stage, the ink image being transferred from the outer surface of the ITM to a printing substrate, wherein the outer surface of the ITM is maintained within the imaging stage at a predetermined vertical distance from the print bars by a plurality of support ink rollers, the plurality of support ink rollers having a common flat tangential surface and contacting the inner surface of the ITM, the support ink rollers being disclosed such that... Each corresponding support ink roller is located below and perpendicularly aligned with each of the plurality of printing bars, wherein each given support ink roller is associated with a corresponding rotational speed measuring device for measuring the corresponding rotational speed of the given support ink roller; and wherein the system further includes: a droplet deposition control circuit configured to adjust a corresponding ink droplet deposition rate DR on the ITM for each given printing bar, the droplet deposition control circuit adjusting the ink droplet deposition rate based on and in response to the measurement of the rotational speed of the corresponding support ink roller perpendicularly aligned with the given printing bar. 19. The indirect printing system of claim 18, wherein for each given printing bar of the plurality of printing bars, a correspondingly vertically aligned support ink roller is disposed slightly downstream of the given printing bar. 20. The indirect printing system of claim 18, wherein for an upstream printing bar and a downstream printing bar respectively aligned perpendicularly to an upstream support ink roller and a downstream support ink roller, the droplet deposition control circuit adjusts the corresponding deposition rates _{DR_upstream} and _{DR_downstream} at the upstream and _downstream printing bars such that the difference between the corresponding ink droplet deposition rates at the upstream and downstream printing bars is adjusted according to the difference function F = _{ω_upstream} * _{R_upstream} - _{ω_downstream} * _{R_downstream} , where: i. _{ω_upstream} is the measured rotational speed of the support ink roller aligned with the upstream printing bar, the measured rotational speed being measured by a rotational speed measuring device associated with the support ink roller; ii. _{R_upstream} is the _radius of the upstream roller among the plurality of support ink rollers; iii. _{ω_downstream} is the measured rotational speed of the support ink roller aligned with the downstream printing bar, the measured rotational speed being measured by a rotational speed measuring device associated with the support ink roller; and iv. _{R_downstream} is the radius of the downstream roller among the plurality of support ink rollers.