Indirect printing system and related apparatus

Abstract

Some embodiments relate to printing system is described that has an intermediate transfer member (ITM) in the form of a seamed endless belt for transporting an ink image from an image forming station, at which an ink image is deposited on ITM, to an impression station, where the ink image is transferred onto a printing substrate. Two drive members are provided for movement in synchronism with one another. Rotation of the drive members during installation of a new ITM serves to thread the strip through the printing system by pulling the strip from its leading end. Alternatively or additionally, indirect printing system comprising the ITM and an image forming station at which droplets of ink are applied to the ITM to form ink images thereon is disclosed. One or more blowing mechanisms (e.g. associated with the image forming station) are disclosed herein.

Claims

1. A printing system having an intermediate transfer member in the form of an endless belt for transporting an ink image from an image forming station, at which an ink image is deposited on the intermediate transfer member, to an impression station, where the ink image is transferred onto a printing substrate, the endless belt being formed from a strip having leading and trailing edges, the printing system further comprising two grippers that are mechanically coupled to each other for synchronized motion with one another, each one of the two grippers being disposed on a different lateral side of the strip and/or each one of the grippers being disposed on a different lateral side of a leader that is attached to the leading edge of the strip, wherein synchronized longitudinal motion of the two grippers during installation of a new intermediate transfer member is operative to thread the strip through the printing system by pulling the strip from the leading end of the strip.

2. A printing system as claimed in claim 1, wherein a support plate is provided in the printing system for supporting ends of the strip that are to be joined to one another at a seam in order to convert the strip into an endless belt, the support plate not contacting the intermediate transfer member subsequent to installation.

3. A printing system as claimed in claim 1, wherein the leading edge of the strip is V-shaped.

4. A printing system as claimed in claim 3, wherein the leading edge is formed as part of a leader that is separably connected to one end of the strip.

5. A printing system as claimed in claim 4, wherein the end of the strip connected to the leader and the opposite ends of the strip are formed with rebates for receiving a joining tape to form a seam.

6. A printing system as claimed in claim 1, wherein the opposite ends of the strip are joined at a seam while maintaining the thickness of the intermediate transfer member uniform throughout its length.

7. A printing system as claimed in claim 1, wherein each of the first and second grippers are respectively connected to first and second drive members, the drive members having the form of chains or toothed belts.

8. A printing system of claim 1, wherein each of the first and second grippers respectively laterally extend first and second drive members, the drive members having the form of chains or toothed belts.

9. The printing system of claim 1 wherein the lateral formations are provided on each side of the strip.

10. The printing system of claim 9 wherein the first and second grippers are respectively in form-locking engagement with the lateral formations disposed on the first and second sides of the strip.

11. The printing system of claim 10 wherein during operation of the printing system, the belt is maintained under lateral tension by a tensioning force applied via the lateral formations.

12. The printing system of claim 11 wherein (i) a leader is separably connected to one end of the strip; (ii) lateral formations provided on each side of the leader; and (ii) the first and second grippers are respectively in form-locking engagement with the lateral engagements disposed on the first and second sides of the strip.

13. The printing system of claim 9 wherein (i) a leader is separably connected to one end of the strip; (ii) lateral formations provided on each side of the leader; and (ii) the first and second grippers are respectively in form-locking engagement with the lateral engagements disposed on the first and second sides of the strip.

14. A method of installing an intermediate transfer member in a printing system having an intermediate transfer member in the form of an endless belt for transporting an ink image from an image forming station, at which an ink image is deposited on the intermediate transfer member, to an impression station, where the ink image is transferred onto a printing substrate, the method comprising: (a) providing an elongate strip of a length exceeding the circumference of the intermediate transfer member, (b) connecting a leading end of the strip to two grippers that are mechanically coupled to each other for synchronized motion with one another, each one of the two grippers being disposed on a different lateral side of the strip and/or each one of the grippers being disposed on a different lateral side of a leader that is attached to a leading edge of the strip; (c) longitudinally moving each of the grippers in synchronism with one another to thread the leading end of the strip through the stations of the printing system, and (d) joining the ends of the strip to one another to form an endless intermediate transfer member after the strip has been threaded through the printing system.

15. A method as claimed in claim 14, wherein the strip has a separable leader and the ends of the strip are rebated and wherein, after separating the leader from the remainder of the strip, the ends of the strip are joined to one another by adhering a tape within the rebates in the ends of the strip to form a seam while maintaining the uniformity of the thickness of the intermediate transfer member.

16. A method as claimed in claim 14, wherein each of the first and second grippers are respectively connected to first and second drive members, the drive members being in the form of chains or toothed belts.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a section through a support system of an intermediate transfer member;

(3) FIG. 2 is a perspective view of one end of the support system shown in FIG. 1;

(4) FIG. 3 is a section through one of the guide channels of the intermediate transfer member with one of the drive members running alongside the formations on the lateral edges of the intermediate transfer member;

(5) FIG. 4 is a detail of FIG. 3 drawn to an enlarged scale;

(6) FIG. 5 shows the leading end of a strip that is threaded through a printing system to form an intermediate transfer member; and

(7) FIG. 6 shows the manner in which the ends of a strip may be joined at a seam while maintaining the thickness of the intermediate transfer member uniform throughout its length.

(8) FIG. 7, which is a schematic representation of a printing system of the invention;

(9) FIG. 8 is a perspective view of a print bar of the system of FIG. 7 carrying a plurality of print heads and fitted with a blowing mechanism, according to an embodiment of the present invention;

(10) FIG. 9 is a perspective view of the main body of the blowing mechanism of FIG. 8;

(11) FIG. 10 is a section through the main body of FIG. 9;

(12) FIG. 11 is a view of the blowing mechanism of FIG. 8, from the side opposite to that of FIG. 8, with the support bar and all but one of the print heads removed;

(13) FIG. 12 is a graphic representation of the difference in the flow-rate provided at each print bar along the print direction of the system;

(14) FIG. 13 is a perspective view of an assembled manifold, or blowing mechanism, secured to a print bar of the system of FIG. 7, according to another embodiment of the present invention;

(15) FIG. 14 is an exploded view of the manifold of FIG. 13 while still secured to the print bar;

(16) FIG. 15 shows a section through the manifold of FIG. 13 and part of the manifold when viewed from below;

(17) FIG. 16 is an exploded view showing the block of the manifold of FIG. 13 and plates secured to its underside to define at least one outlet for discharge of gas flow; and

(18) FIG. 17 is a similar exploded view to that of FIG. 16 but showing the manifold of FIG. 7 from the side facing to the print bar.

(19) It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate identical components but may not be referenced in the description of all figures.

DETAILED DESCRIPTION OF THE DRAWINGS

First Subsection: Apparatus for Threading an Intermediate Transfer Member (ITM) of a Printing System (See PCT/IB2016/052120)

(20) An intermediate transfer member support system 2010 shown in FIGS. 1 and 2 comprises two lateral frames 2012 and 2014, arranged one on each side of the intermediate transfer member 2018, and various support, drive and tensioning rollers 2016 extending transversely between the two frames 2012 and 2014 to constrain the intermediate transfer member to follow a closed loop and pass through the various stations of the printing system. The remainder of the printing system and more details of the intermediate transfer member support system are well documented in the prior art and need not therefore be repeated in the present context. Essentially, in the region designated 2020 the intermediate transfer member passes beneath an image forming station where ink is deposited on the intermediate transfer member to form an image. The intermediate transfer member rotates clockwise as viewed to carry the ink image to the region designated 2022, where the wet image is heated to evaporate the ink carrier and leave behind a tacky resin film. Along its lower run, the intermediate transfer member passes through one or more impression stations where it is pressed against a substrate, so that the tacky film separates from the intermediate transfer member and is transferred to the surface substrate.

(21) The intermediate transfer member support system 2010 is designed to ensure that the intermediate transfer member remains substantially flat and taut as it passes the regions 2020 and 2022 and as it approaches the nip of the impression stations. In order to achieve this objective, the edges of the intermediate transfer member 2018 have formations 2030 in the form of zipper teeth, shown in FIGS. 3 and 4, that have a greater thickness than the transfer member 2018 and are received in C-shaped guide channels 2032 secured to the lateral frames 2012 and 2014. The guide channels 2032 are shown in FIG. 1 and their position in the support system is shown in FIG. 2. Further details of the formations 2030 and the guide channels 2032 are disclosed WO2013/132418 and are shown in FIGS. 11 and 12 of that publication. It is to be understood that the guide channels may have any alternative cross section shape, as long as the lateral formations and the guide channels are compatible and suitable to guide the intermediate transfer member and maintain it under lateral tension while the printing system is operating.

(22) The intermediate transfer member 2018 is installed by threading a strip 2018′ through the printing system and connecting its ends to one another by means of a seam, in order to form a seamed endless loop. Embodiments of the invention, as previously disclosed in PCT/IB2016/052120, are concerned with simplifying this task, which needs to be carried out, not only during manufacture, but from time to time during use as the intermediate transfer member is subjected to wear.

(23) The installation of a new or replacement intermediate transfer member 2018 is made difficult by the fact that its lateral formations 2030 need to be introduced into the guide channels 2032 and, as can be seen in FIG. 1, these need not be continuous. Because they maintain the intermediate transfer member under lateral tension, engagement of the formations 2030 in the guide channels 2032 creates drag and makes threading of the strip 2018′ from which the intermediate transfer member more difficult.

(24) To mitigate this problem, in the disclosed embodiment, two endless drive members, in the form of chains 2050 passing over sprockets 2052 are provided alongside the edges of the intermediate transfer member 2018, preferably within the enclosed cross section of the C-shaped channels 2032. Reference to chains passing over sprockets is intended herein to include the equivalent of toothed belts passing over toothed pulleys. Each of the chains 2050 has a respective laterally projecting gripper 2054, most clearly shown in FIG. 4, that engages the formations 2030 of the intermediate transfer member strip 2018′ at its leading end in a form locking manner. After the gripper 2054 has engaged the formations on the leading edge of the strip 2018′, the two endless chains 2050 are driven to pull the strip 2018′ by its leading end, in the direction shown by the arrows in FIG. 3, through the various stations of the printing system. To ensure that the two chains 2050 move in synchronism, two of their drive sprockets 2052 are mounted on a common axle that is connected either to a crank for manual operation, or to a suitable motor if threading is automated.

(25) The intermediate transfer member 2018 may in some locations have to be threaded through a gap that is as narrow as 2 mm, while its width may typically be about 1,000 mm. Sagging of the leading edge of the strip 2018′ being threaded through the printing system can therefore cause a problem. This is mitigated by forming a V-shaped cut-out 2060 in the leading edge of the strip in the manner shown in FIG. 5.

(26) Once the strip has been pulled around the entire printing system, and its leading end comes round to the end of the support system shown in FIG. 2, the opposite ends of the strip are laid over a support plate 2062 and joined to one another to form a seam, which may, as shown in FIG. 5, extend along an inclined line 2064. After the seam has been formed, a tension roller is extended to maintain the intermediate transfer member under the desired longitudinal tension and space it from the grippers 2054 that remain stationary during operation of the printing system.

(27) To form the seam, the two ends of the strip 2018′ may be laid one over the other on the support plate 2062 and cut along the inclined line 2064. While retained on the support plate 2062 with the cut edges in abutting relation, an adhesive tape may be laid over the two ends to create the desired join. Such a method of forming the two ends results in a seam having an increased thickness and in order that it should not create a sudden change in tension as it passes over drive rollers or through an impression station, the cut line 2064 may be inclined in the manner illustrated in FIG. 5.

(28) It is alternatively possible to form a seam without increasing the thickness of the intermediate transfer member and this is shown in FIG. 6. In this case, the strip 2018′ is made to a predetermined length and is attached to a separable leader at its leading end. The leading 2019 and trailing 2021 ends of the strip 2018′ proper, would be ground down to form rebates 2070 for receiving a tape 2075 that is as wide as the two rebates 2072. The tape 2072 is secured to the two ends of the strip 2018′ by a layer 74 of adhesive.

(29) In an embodiment where the thickness of the strip 2018′ is about 550 μm, the rebates in the ends of the strip may have a depth of 200 μm. The rebates can then accommodate a tape having a thickness of about 150 μm leaving 50 μm for the thickness of the adhesive layer 2074.

(30) The separable leader should be made of a flexible material so that it may follow the path of the intermediate transfer member but it may be made of a stiffer material than the strip 2018′ so as to have less tendency to sag. The leader may, as shown in FIG. 5, have a V-shaped leading edge and it should have formations that are continuous with the formations on the sides of the strip 2018′.

(31) It is understood that in addition to the image forming station and the impression station above-mentioned, a printing system may further comprise a treatment station and/or a cleaning station to respectively treat and/or clean the intermediate transfer member, a drying station to evaporate liquid carrier out of the ink image, a cooling or a heating station to modify the temperature of the intermediate transfer member along its path, a finishing station for further processing the printed substrate, and so on. All such stations need not be considered in detail in the present context.

Second Subsection: An Indirect Printing System (See U.S. application Ser. No. 16/244,145)

(32) The present subsection relates to an indirect inkjet printing system, and more specifically to an indirect inkjet printing system including a blowing mechanism for preventing condensation on the ink-heads.

(33) The principles, uses and implementations of the present subsection herein may be better understood with reference to FIGS. 7-17. Upon perusal of the description and figures present herein, one skilled in the art is able to implement the invention without undue effort or experimentation. In the figures, like reference numerals refer to like parts throughout.

(34) Before explaining at least one embodiment in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth herein. The invention is capable of other embodiments or of being practiced or carried out in various ways. The phraseology and terminology employed herein are for descriptive purposes and should not be regarded as limiting.

(35) Additional objects, features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings. Various features and sub-combinations of embodiments of the invention may be employed without reference to other features and sub-combinations.

(36) It is to be understood that both the foregoing general description and the following detailed description, including the materials, methods and examples, are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed, and are not intended to be necessarily limiting.

(37) Reference is now made to FIG. 7, which is a schematic representation of a printing system of the invention. The printing system 800 of FIG. 7 comprises an ITM formed of an endless belt 810 that cycles through an image forming station 812, a drying station 814, and an impression station 816.

(38) In the image forming station 812 four separate print bars 822 incorporating one or more print heads, that use inkjet technology, deposit aqueous ink droplets of different colors onto the surface of the belt 810. Though the illustrated embodiment has four print bars each able to deposit one of the typical four different colors (namely Cyan (C), Magenta (M), Yellow (Y) and Black (K)), it is possible for the image forming station to have a different number of print bars and for the print bars to deposit different shades of the same color (e.g. various shades of grey including black) or for two print bars or more to deposit the same color (e.g. black).

(39) At least one print bar, and in the illustrated embodiment each print bar, has mounted thereon a blowing mechanism 823. The blowing mechanism 823 is adapted to blow gas through the body of the blowing mechanism 823 and into the gap between the print heads of the print bar and the ITM 810, and in the print direction, so as to prevent condensation of vapor from the jetted ink drops onto the print heads. The blowing mechanism 823 may also prevent or reduce satellite drops, which may cause distortion of the images. Two embodiments of blowing mechanisms 823 are described hereinbelow with reference to FIGS. 8-11 and with reference to FIGS. 13-17.

(40) Following each print bar 822 in the image forming station, an intermediate drying system 824 is provided to blow hot gas (usually air) onto the surface of the belt 810 to dry the ink droplets at least partially, to leave a tacky film having the ability to adhere to the substrate when transferred thereonto in the impression station.

(41) In the impression station 816, the belt 810 passes between an impression cylinder 820 and a pressure cylinder 818 that carries a compressible blanket 819. Sheets 826 of substrate are carried by a suitable transport mechanism (not shown in FIG. 1) from a supply stack 828 and passed through the nip between the impression cylinder 820 and the pressure cylinder 818. Within the nip, the surface of the belt 810 carrying the ink image, is pressed firmly by the blanket 819 on the pressure cylinder 818 against the substrate 826 so that the ink image is impressed onto the substrate and separated neatly from the surface of the belt. The substrate is then transported to an output stack 830.

(42) Belt 810 typically includes multiple layers, one of which is a hydrophobic release layer, as described, for example, in WO 2013/132418 and in WO 2017/208152, which is herein incorporated by reference in its entirety.

(43) Reference is now made to FIG. 8, which is a perspective view of a print bar 10, similar to print bars 822 of the system of FIG. 7, carrying a plurality of print heads 16 and fitted with a blowing mechanism, 25, similar to blowing mechanism 823 of FIG. 7, according to an embodiment of the present invention, to FIG. 9, which is a perspective view of the main body of the blowing mechanism of FIG. 8, to FIG. 10, which is a section through the main body of FIG. 9, and to FIG. 11, which is a view of the blowing mechanism of FIG. 8, from the side opposite to that of FIG. 8, with the support bar and all but one of the print heads removed.

(44) The print bar 10 shown in FIG. 8 carries a plurality of print heads 16 and is mounted above an intermediate transfer member 12 (shown in FIG. 11), similar to ITM 810 of FIG. 7. Typically, the print bar 10 spans most of the width of ITM 12. The print heads 16 are spaced from the ITM 12 by a small gap, and jet ink onto the ITM 12 as it moves to form an ink image. The ink image is subsequently dried and impressed on a substrate. Thereafter, the ITM is cleaned and returned to the print bar 10 to commence a new printing cycle.

(45) The ITM 12 is an endless belt that moves in the direction of the arrow 14 in FIG. 11, also termed the “print direction”, while the print bar 10 remains stationary.

(46) According to the embodiment of the present invention illustrated in FIGS. 8-11, the print bar 10 includes a rigid support bar 18 onto which the print heads 16 are mounted. The print heads 16 are precisely aligned in relation to one another, so as to ensure the quality of printing. In the printing system, the support bar 18 is suspended from a rail by a carriage 20, which is movable in a direction perpendicular to the direction of movement of the ITM, also termed the “cross-print direction”. Such movement is required in order to provide access to the print bar 10 for the purpose of replacement and maintenance thereof.

(47) As discussed hereinabove in the background section, two problems arise during operation of inkjet printing systems, that can affect the quality of the image ultimately printed onto the substrate.

(48) One problem is caused by the fact that the ITM 12 is heated in order to dry the ink image and, at the time of jetting of the ink, the temperature of the surface of the ITM 12 may be near the boiling point of the solvent or carrier in the ink, which is preferably water. The carrier in the ink therefore starts to evaporate as soon as a droplet contacts the ITM 12 and the emitted vapor tends to condense on the cooler surface of the print heads 16. Such condensation can block nozzles of the print heads, and may drip onto the ITM 12 to create defects and/or distortions in the final printed image.

(49) The other problem is caused by the structure and operation of print heads 16. Ideally, to print a dot onto the ITM 12, a print head 16 should jet a single droplet from a nozzle. In practice, oftentimes after a droplet has been released, smaller droplets, termed satellites, are ejected from the nozzle. Because the ITM 12 is in motion relative to the print heads 16, these satellite droplets do not land on the same point on the ITM 12 as their parent droplet, and create a blurred shadow or otherwise distort the image.

(50) One solution to these problems has been proposed in the Applicant's earlier PCT application PCT/IB2016/053049, which is incorporated by reference as if fully set forth herein. In this solution, two different gas streams are introduced into the gap between the print heads 16 and the ITM 12. A low speed, uniform and laminar gas stream is blown in the print direction, so as to push the jetted droplets in the print direction. Under the force of the blown low-speed air, the smaller satellite droplets travel further in the print direction than their parent droplets, and thus land on the same point on the ITM 12, even though the parent droplets were released from the nozzles at an earlier time. A second, high speed turbulent gas stream is passed through the gap between the print heads 16 and the ITM 12, in the print direction, to remove vapor from the vicinity of the print heads 16, and prevent condensation thereon.

(51) The Applicant has surprisingly found that a single gas stream, provided to mitigate the problem of condensation, also assists in mitigating the problem of satellite droplets, as described herein.

(52) According to the embodiment of FIGS. 8-11, a blowing mechanism 25 is secured to the support bar 18, the blowing mechanism including a main body 30, which may be formed by extrusion, as shown in detail in FIGS. 9 and 10. As best seen from the cross-section of FIG. 9, the main body 30 includes three chambers 31, 38 and 40 that extend along the entire length of main body 30. However, in some embodiments, chambers 38 and 40 may be obviated.

(53) In some embodiments, the chamber 31 is subdivided into three compartments 32, 34 and 36 that are connected to one another by narrow slots 41 in partition walls that separate them. The compartment 32 is connected to a gas supply and is formed with ribs 33 and grooves that are used to retain perforated baffle plates 44 and 46. The compartment 34 also has grooves for retaining a third baffle plate 48. The baffle plates 44, 46 and 48 are formed separately from the main body 30 and are slid into the grooves formed in the walls of the compartments 32 and 34 before the ends of the all the chambers are capped. The compartment 36 communicates with a gas outlet 50, also termed a mouth, that is positioned immediately upstream of the gap between the support bar 18 and the ITM 12, so as to emit gas into the gap in the print direction.

(54) The convoluted flow path created by the slots 41 and the perforated baffle plates 44, 46 and 48, results, when a gas stream flows through chamber 31, in an even pressure along the length of the blowing mechanism 25, so that the rate of flow of gas out of the outlet 50 and into the gap between the print heads 16 and the ITM 12 is uniform across the width of the ITM 12, and is not affected by the position, along the length of the print bar, of the connection to the gas supply.

(55) In some embodiments, the main body 30 further includes chambers 38 and 40, defining a second gas flow path, separate from the gas flow path through chamber 31 and outlet 50. Chambers 38 and 40 are upper and lower chambers that serve to provide high pressure or high speed turbulent gas streams, in the print direction. The upper chamber 38 is connected to a high pressure gas supply.

(56) In some embodiments, the lower chamber 40 may be subdivided, by partitions inserted after extrusion of the main body 30, into separate sections in the cross-print direction that can each independently provide a gas stream to only part of the width of the ITM 12. An outlet for delivering the high pressure gas stream is formed by small holes 56 that communicate with the lower chamber 40, the outlet being divided into regions each communicating with a respective one of the transverse sections of the lower chamber 40. As such, a high speed gas stream is not provided at the same time over the entire width of the ITM 12, preventing the ITM from lifting. Additionally, because gas flows only beneath part of the print bar 10 at any one time, the power requirement placed on the high pressure gas supply is reduced.

(57) The upper chamber 38, which is connected to the high pressure gas supply, is connected to each of the transverse sections of the chamber 40 by a respective conduit 52 (see FIG. 11). A respective valve 54 controls the flow of gas between the chamber 38 and the transverse sections of the chamber 40, so that, during operation, the sections of the lower chamber 40 may be pressurized intermittently. The holes 56 drilled into the blowing mechanism 25 allow the pressurized gas to escape from the transverse sections of the lower chamber 40 and to flow over the top surface of the outlet 50 into the gap between the ITM 12 and the print heads 16.

(58) In use, a gas supply is connected to the chamber 31, and gas passes through the three compartments 32, 34 and 36, as well as through the baffles 44, 46 and 48, before exiting as a constant uniform laminar stream through the outlet 50 and entering the gap between the ITM 12 and the print heads 16.

(59) As described in further detail hereinbelow with respect to FIG. 12, the speed of the gas stream may be dependent on the position of the print bar along the print direction, such that the speed of the gas stream at the fourth print bar, illustrated in FIG. 7 as the K print bar, is higher than the speed of the gas stream at the first print bar, illustrated in FIG. 7 as the C print bar. Additionally, in some embodiments, the speed of the gas stream is higher than the speed of the ITM in order to cause satellites to catch up with their parent droplets, but should not be so high as to cause satellites to overtake their parent droplets nor sufficiently high to cause turbulence and distortion of the image being printed. The pressure supplied to the chamber 31 needs therefore to be regulated so as to provide an ideal gas flow rate, as described hereinbelow with reference to FIG. 12.

(60) The Applicants have found that a single gas flow, provided continually during printing through outlet 50, may be used to solve both problems described above, without damaging the quality of the resulting image.

(61) However, in some embodiments, it may be advantageous to provide separate gas flows to deal with each of the problems. In such embodiments, a low speed continuous gas flow may be provided during printing via outlet 50, in the print direction, for dealing with the satellite droplets, and a second high speed gas flow may be provided via chambers 38, 40, and outlet 56, in the print direction, to prevent condensation on the print heads 16.

(62) In such embodiments, the chamber 38 is connected to a high pressure gas supply, and the valves 54 are opened intermittently to allow high pressure gas to flow into a section of the lower chamber 40 and escape through the holes 56. In some embodiments, adjacent valves 54 are not operated simultaneously so that at no time does gas flow at high speed over the entire width of the ITM 12. In some embodiments, the valves 54 are only operated at times when ink is not being jetted onto the ITM, that is to say only between pages or between print jobs, so as to prevent turbulence of the high speed gas stream from adversely affecting the quality of the printed image.

(63) In some embodiments, the speed of the gas required to dislodge droplets condensing on the print heads 16 via chambers 38 and 40 is significantly in excess of the speed of the constant gas flow that is used to push the satellite droplets to merge with their parent droplets. In such embodiments, the optimum pressure and gas flow rate can be determined empirically as the only requirement that it needs to meet is to ensure that it is high enough to avoid droplet condensation on the print heads 16. The optimum speed will depend on certain factors, such as the temperature of the gas and the time available between pages.

(64) As illustrated in FIG. 7, a printing system typically includes multiple print bars 822, which are arranged in the print direction of the ITM, such that a first print bar (illustrated as the C print bar) is the first to apply ink to the ITM, a second print bar (illustrated as the M print bar) then applies additional ink to the ITM, and so on, until the last print bar (illustrated as the K print bar) applies ink to the ITM, such that the ink applied by all the print bars together forms the image being printed.

(65) The Applicants have found that condensation on print heads of print bars which are downstream along the print direction is greater and more problematic than condensation at print bars which are the first to print. For example, in the system of FIG. 7, condensation would be more problematic on print heads of the Key print bar than on print heads of the Cyan or Magenta print bars. Stated differently, the further the print bar is from the beginning of printing or from the first print bar, the greater the problem of condensation.

(66) The Applicants have discovered that this problem may be remedied by adjusting the flowrate and/or air speed provided by the blowing mechanisms 25 based on the position of the print bar, such that the flow rate through the blowing mechanisms in increased the further down stream the print bar is located.

(67) As such, according to embodiments of the teachings herein, a first flow rate FR.sub.1 is provided by a blowing mechanism 823 disposed on a first print bar 822 (C print bar of FIG. 1), a second flow rate FR.sub.2 is provided by a blowing mechanism 823 disposed on a second print bar 822 (M print bar of FIG. 1), a third flow rate FR.sub.3 is provided by a blowing mechanism 823 disposed on a third print bar 822 (Y print bar of FIG. 1), and a fourth flow rate FR.sub.4 is provided by a blowing mechanism 823 disposed on a fourth print bar 822 (K print bar of FIG. 1).

(68) According to embodiments of the disclosed technology, the further downstream the print bar, the greater the flow rate provided by the blowing mechanism disposed on the print bar, such that, FR.sub.1<FR.sub.2<FR.sub.3<FR.sub.4. If the size of the outlet through which the air stream is provided (outlet 50 in FIGS. 8-11) is equal in all the print bars, the air speed provided in each of the print bars increases along the print direction according to the location of the print bar, such that the air speed provided at the second print bar is greater than the air speed provided at the first print bar, the air speed provided at the third print bar is greater than the air speed provided at the second print bar, and so on.

(69) In some embodiments, if more than four print bars are included in the printing system, the same principles may be applied to the additional print bars as well.

(70) However, if the flow rate or air speed is too high, this may result in distortion of the image, for example by causing satellite droplets to engage the ITM beyond the parent droplets, or by pushing the main droplets out of their intended position. As such, in some embodiments, there may be an upper threshold for the flow rates provided by the blowing mechanisms. In some such embodiments, particularly when the impression station includes more than four print bars, the flow rates provided by some of the blowing mechanisms may be equal to one another, and equal to the upper threshold. For example, in embodiments in which the printing system includes seven print bars, the flow rate at the sixth print bar and the seventh print bar may be the same flow rate, and may be equal to the upper threshold. In some embodiments, the difference between the flow rate through a blowing mechanism of one print bar (e.g. FR.sub.2) and the flow rate through a blowing mechanism of a second, adjacent upstream print bar (e.g. FR.sub.1), is in the range of 70-220.

(71) In some embodiments, the difference between the flow rates provided by adjacent blowing mechanisms decreases the further downstream the print bars are positioned. For example, FR.sub.2-FR.sub.1>FR.sub.4-FR.sub.3.

(72) In some embodiments, the ratio between the flow rate through a blowing mechanism of one print bar (e.g. FR.sub.2) and the flow rate through a blowing mechanism of a second, adjacent upstream print bar (e.g. FR.sub.1), is in the range of 1.5-1.1.

(73) In some embodiments, in which the printing system includes four print bars, the flow rate at the first print bar (FR.sub.1) is in the range of 400-450 L/min, the flow rate at the second print bar (FR.sub.2) is in the range of 600-650 L/min, the flow rate at the third print bar (FR.sub.3) is in the range of 720-780 L/min, and the flow rate at the fourth print bar (FR.sub.4) is in the range of 820-870 L/min.

(74) As seen in FIG. 12, which is a graphic representation of the difference in the flow-rate provided at each print bar along the print direction of the system, in a printing system including four print bars having a distance of 750 mm between each pair of adjacent print bars, the flow-rate required in order to take care of condensation increases as the print bar is further downstream in the print direction.

(75) FIG. 12 shows a graph illustrating one example of air flow rates at which little or no condensation was observed. The results illustrated in FIG. 12 are also summarized in Table 1.

(76) TABLE-US-00001 TABLE 1 Distance of print bar from the Air Print bar printing beginning Flowrate speed number point (mm) (L/min) (m/s) 1 0 430 2.4 2 750 630 3.5 3 1500 750 4.2 4 2250 840 4.7

(77) An additional embodiment of a blowing mechanism is now discussed with respect to FIGS. 13 to 17. FIG. 13 is a perspective view of an assembled manifold, or blowing mechanism, secured to a print bar of the system of FIG. 7, according to another embodiment of the present invention. FIG. 14 is an exploded view of the manifold of FIG. 13 while still secured to the print bar. FIG. 15 shows a section through the manifold of FIG. 13 and part of the manifold when viewed from below. FIG. 16 is an exploded view showing the block of the manifold of FIG. 13 and plates secured to its underside to define at least one outlet for discharge of gas flow. FIG. 17 is a similar exploded view to that of FIG. 16 but showing the manifold of FIG. 7 from the side facing to the print bar.

(78) FIG. 13 shows a print bar 110 that is, in use, positioned immediately above the surface of an ITM having the form of a constantly recirculating endless belt. As described in WO2013/132418 and WO 2017/208152, an aqueous ink is jetted onto the surface of the ITM by print heads (not shown) mounted on the print bar 110. The resulting ink image is transported by the ITM to an impression station and during its transportation it is dried to leave behind a tacky ink residue. At the impression station, the ink residue is transferred onto a substrate and the ITM surface then returns to the print bar 110 to commence a new printing cycle.

(79) The print bar 110 forms part of a carriage (not shown) that is supported by rollers 112 from a gantry to allow the print bar to be moved in a direction transverse to the direction of movement of the ITM between a deployed position in which it overlies the ITM and a parked position away from the ITM where servicing of print heads can take place.

(80) A set of individual print heads (not shown) is secured to one side of the print bar 110, while a manifold 114 of the present disclosure is secured to its opposite side. The purpose of the manifold 114 is to deliver into the narrow gap between jetting nozzles of the print heads and the surface of the ITM two different gas streams. The first is a constant low speed laminar gas stream that is uniform across the width of the ITM, to cause main droplets and their satellites to merge on the surface of the ITM. The second is an intermittent high speed turbulent gas stream, to dislodge any condensation that may collect on the nozzle plates of the print heads. The second gas stream is intermittent because, being turbulent, it can only take place at times when no ink image is being formed on the ITM, so as to avoid image distortion. Furthermore, the drop in pressure caused by the high speed gas stream can lift the ITM off its support surface if applied across the entire width of the ITM at the same time and it is therefore divided in the illustrated embodiment into four separately controllable branches that can be delivered sequentially, or two at a time.

(81) Referring to FIG. 14, the manifold 114 is formed of a rectangular block 116 having various channels machined into its opposite sides. The channels on one side are sealed by the a cover and on the other side by a closure plate 118 to form different plenum chambers for gas, usually air, under two different pressures for delivery of the low and high speed streams. The figure also shows a protective cover plate 120 and a sponge layer 122 to prevent condensation on the cover surface. A top plate 124, a bottom plate 126 and a spacer 128, best seen in the exploded views of FIGS. 16 and 17, are secured to the underside of the block 16 to define the mouths of the manifold from which the two different gas streams are discharged.

(82) The single plenum chamber 130 for the low pressure gas used to deliver the low speed gas stream is formed by a single channel (seen in FIGS. 14 and 16 and in section in FIG. 15) that extends across the full width of the manifold 114. The plenum chamber 130 is connected to a supply of gas under low pressure (for example 0.5 bar) by a connector 132. Small vertical holes 134 in the manifold block 116 and the top plate 124 (not shown in the block but visible in the top plate 124) allow gas from the plenum chamber 130 to pass to the low speed discharge mouth of the manifold, defined between the top plate 124 and the bottom plate 126 which are separated by the spacer 128 (seen in FIG. 16). The spacer 128 has a saw-tooth shaped edge that, together with depressions formed in the top surface of the bottom plate 126, defines diverging channels leading from the above-mentioned vertical holes in the manifold block to the common discharge mouth. The divergent channels guide the gas flowing to the discharge mouth to ensure that it leaves as a laminar gas stream that is uniform over the entire width of the discharge mouth.

(83) Gas at high pressure, for example at a pressure of 3 to 6 bar, is fed, through respective connectors 142, into four separate second plenum chambers 140 defined by the block 116 and the cover plate 118. Each of the second plenum chambers 140 is connected by a respective valve 144, and vertical holes (not shown) within the block 116, to a respective buffer chamber 146 that is arranged on the opposite side of the block 116 from the plenum chamber 140. The buffer chambers 146 are closed off by a cover and can be seen in FIGS. 15 and 17. Pressurized gas from the buffer chambers 146 passes through further vertical holes in the block 116 that open onto grooves in the top plate 124, as best shown in FIG. 16. The upper surface of the top plate 124 together with the bottom surface of the block 116 form the second discharge mouth of the manifold 114, from which high speed gas is intermittently delivered into the gap between the print nozzles and the ITM.

(84) The plates defining the discharge mouth from which the high speed gas is discharged need to be able to withstand the high gas pressure without buckling.

(85) In the illustrated embodiment of the invention, this problem is overcome in that the block 16 itself acts as one side of the high speed gas discharge mouth and the pressure acting on the top plate 124 is resisted not by the top plate alone but by a sandwich consisting of the top plate 124, the bottom plate 126 and the spacer 128 between them. This sandwich, which is screwed to the underside of the block 116 can have a combined thickness approaching 4 mm and can therefore readily withstand the high pressure in the buffer chamber 146. The low speed gas is discharged from between the top plate 124 and the bottom plate 126 but the latter can readily withstand the low pressure without buckling.

(86) In use, low speed gas is constantly discharged from the mouth defined between the top plate 124 and the bottom plate 126 and the plenum chamber 130 is constantly at the pressure of the low pressure gas supply. The plenum chambers 140, on the other hand are permanently connected to the high pressure gas supply but are isolated from the buffer chambers 146. Intermittently and individually, the second plenum chambers 140 are connected to their respective buffer chamber 146 by briefly opening the associated valves 144. This results in a volume of gas being transferred into the buffer chamber 146 and stored there temporarily at high pressure. This volume then escapes through the second discharge mouth of the manifold to cause a turbulent burst of gas flowing at high speed to pass between the printing nozzles and the ITM.

(87) The valves 144 are not all opened simultaneously to avoid lifting the ITM off its support surface. They are instead either operated sequentially, or two at a time. In the latter case, it is preferred not to open the valves of adjacent buffer chambers 146 at the same time.

(88) The contents of all of the above mentioned applications of the Applicant are incorporated by reference as if fully set forth herein.

(89) The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons skilled in the art to which the invention pertains.

(90) It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

(91) Although the present disclosure has been described with respect to various specific embodiments presented thereof for the sake of illustration only, such specifically disclosed embodiments should not be considered limiting. Many other alternatives, modifications and variations of such embodiments will occur to those skilled in the art based upon Applicant's disclosure herein. Accordingly, it is intended to embrace all such alternatives, modifications and variations and to be bound only by the spirit and scope of the appended claims and any change which come within their meaning and range of equivalency.

(92) In the description and claims of the present disclosure, each of the verbs “comprise”, “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of features, members, steps, components, elements or parts of the subject or subjects of the verb.

(93) As used herein, the singular form “a”, “an” and “the” include plural references and mean “at least one” or “one or more” unless the context clearly dictates otherwise.

(94) Unless otherwise stated, the use of the expression “and/or” between the last two members of a list of options for selection indicates that a selection of one or more of the listed options is appropriate and may be made.

(95) Unless otherwise stated, adjectives such as “substantially” and “about” that modify a condition or relationship characteristic of a feature or features of an embodiment of the present technology, are to be understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended.

(96) To the extent necessary to understand or complete the present disclosure, all publications, patents, and patent applications mentioned herein, including in particular the applications of the Applicant, are expressly incorporated by reference in their entirety by reference as is fully set forth herein.