GB2518150A - Drying apparatus for an indirect printing system - Google Patents

Abstract

A drying apparatus (108, fig 1) suitable for use in an indirect printing system comprises a housing 220 for mounting opposite a section of the surface of an intermediate transfer member 102; the housing 220 has a plenum chamber 222 connectible to a supply of gas under pressure 224, which chamber 222 is bounded by a wall 230 of the housing 220 having an inner side facing the plenum chamber 222 and an outer side facing the surface of the intermediate transfer member 102 on which ink droplets are deposited; the wall 230 is formed with openings (308, fig 6) to allow jets of gas to escape from the plenum chamber 222 and to be directed onto the surface of the intermediate transfer member 102 in order to dry the ink droplets; at least one exhaust channel (320, fig 6) is recessed into the outer side of the wall 230 for enabling gas that has already contacted the surface of the intermediate transfer member 102 to escape from between the housing 220 and the intermediate transfer member 102 without obstructing the path of gas being discharged from the openings (308). The apparatus preferably includes a heater and openings (308) are preferably in the form of nozzles (300, fig 6). The apparatus may re-circulate the gas that has contacted the intermediate transfer member 102 via exhaust gas collection points 244, 242.

Description

DRYING APPARATUS FOR AN INDIRECT PRINTING SYSTEM

FIELD OF THE DISCLOSURE

The present invention relates to printing systems and in particular to indirect systems that deposit an image formed of droplets of liquid inks onto an intermediate transfer member. Specifically this disclosure pertains to a drying apparatus for drying the ink droplets as they are bcing transportcd by the intermediate transfcr member.

BACKGROUND

Digital printing techniques have been developed that allow a printer to receive instructions directly from a computer without thc need to prepare printing plates. Amongst such printing devices are colour laser printers, which use dry toners and the xerographic process, and the widely used inkjet printers, which use liquid inks and rely on inlcjet or bubble jet proccsscs. Such printing dcviccs typically directly apply thc desired image to thc final printing substrate (e.g., paper). In general, the resolution of such processes is limited.

For instance, liquid inks may wick into fibrous substrates requiring the use of substrates specially coated to absorb the liquid ink in a controlled fashion or to prevent its penetration below the surface of the substrate. Such coated substrates may not address all issues associated with direct printing and may even create their own problems. For instance, if the surface of the substrate remains wet following the application of the ink, and such an event is not obviated by the use of all coated substrates, additional costly and time consuming steps are needed to dry the ink, so that it is not later smeared as the substrate is being handled, for example stacked or wound into a roll. Furthermore, excessive wetting of the substrate causes cockling and makes printing on both sides of the substrate (also termed perfecting or duplex printing) difficult, if not impossible.

In commercial settings, there exist additional printing systems, some relying on indirect or offset printing techniques. In such processes, an intermediate image of the final desired pattern (e.g., a mirror image) is typically formed on an image transfer member (e.g., a blanket or a drum) and transferred therefrom to the final printing substrate. The intermediate image can be, as in NP-Indigo printers, an electrostatic image produced on an electrically charged image bearing cylinder by exposure of compatible oil-based inks to laser light, the ink image being then transferred by way of a blanket cylinder onto paper or any other substrate. Though such systems are better suited for high quality digital printing the use of oil-based inks has raised environmental concerns.

The present Applicant has recently disclosed a printing system and process wherein inks having an aqueous carrier are jetted onto an intermediate transfer member (also called an image transfer member) and dried thereupon before being transferred to the desired substrate. Such systems allow the distance between the surface of the intermediate image transfer member (also herein termed the release layer) and the inlcjet print bar to be maintained constant and reduces wetting of the substrate, as the ink can be dried on the intermediate image transfer member before being applied to the substrate. Consequently, the final image quality on the substrate is less affected by the physical properties of the substrate and benefit from various other advantages as a result of the image remaining above the substrate surface. More details on such systems are disclosed in International Patent Application No. PCT/1B20 13/051716, filed on March 5, 2013 which is imported herein by reference in its entirety.

The present invention is concerned with the apparatus used to dry the ink droplcts as they are bcing transported by the image transfer member bcfore they are applicd to the substrate. In particular, the invention seeks to provide an apparatus that can effectively and efficiently dry the ink droplets without altering the ink image.

SUM MARY OF THE INVENTION

According to the present invention, there is provided a drying apparatus for use in an indirect printing system to dry ink droplets deposited onto a surface of an intermediate transfer member by means of onc or more print bars, the apparatus being formed of at least one drying unit that comprises a housing for mounting opposite a section of the surface of the intermediate transfer member, the housing having a plenum chamber connectible to a gas supply, which plenum chamber is bounded by a wall of the housing having an inner side facing the plenum chamber and an outer side facing the surface of the intermediate transfer member on which the ink droplets are deposited, the wall being formed with openings to allow jets of gas to escape from the plenum chamber and to be directed onto the surface of the intermediate transfer member in order to dry the ink droplets, wherein at least one exhaust channel is recessed into the outer side of the wall for enabling gas that has already contacted the surface of the intermediate transfer member to escape from between the housing and the intermediate transfer member without obstructing the path of gas being discharged from the openings.

The drying apparatus is located along the path of the intermediate transfer member at a station also termed the drying station. As detailed hereinbelow, the drying apparatus may comprise several individual drying units. Unless the context dictates otherwise, these terms may be used interchangeably.

In some embodiments of the invention, the openings in the wall are formed by individual nozzles each having a tubular body projecting from the outer side of the wall, spaces between the tubular bodies of the nozzles defining multiple exhaust channels.

In some embodiments of the invention, a heater is provided within the plenum chamber to heat the gas directed onto the surface of the intermediate transfer member. The heater may be a radiator generating heat electrically, or a heat exchanger through which a heated fluid flows during use.

In some embodiments of the invention, the heater is external to the plenum chamber, and heats the gas prior to arrival of the gas at the plenum chamber.

To ensure a uniform rate of gas flow through all the openings, gas may conveniently enter into the plenum chamber from the source of gas under pressure through a diffuser.

In some embodiments, the gas supply comprises at least one is a blower and at least some of the gas escaping through the exhaust passage(s) passes by way of a collection passage to an intake side of the blower. The term "blower" is used herein to include any form of gas (e.g., air) mover, for example a fan or a compressor. In this case, the gas used to dry the ink droplets is at least partially recycled. Such recycling ensures that hot vapor-laden gas is not discharged into the ambient atmosphere. Aside from avoiding undesired heating of the printing system, this reduces or prevents vapor condensation that can also create problems in the printing system. A further advantage is that the heat energy in the gases is not wasted, thereby reducing the amount of energy that has to be consumed to dry the ink droplets.

If a large proportion of the gas is recirculated, it may become saturated with vapour after some time. To avoid this problem, it is possible, in some embodiments, to provide a condenser to allow some of the vapour to be condensed and extracted. As an alternative, or in addition, it is possible reduce the proportion of gas that is recirculated by allowing some of the used gas to escape and admitting some fresh charge of gas during each cycle.

In some embodiments, the housing is surrounded by a cowling or a jacket to define an exhaust gas collection passage therebetwccn.

In some embodiments, the housing or the cowling is surrounded by an isolating material, e.g., ceramic walls, to reduce or prevent heat dissipation from the plenum chamber to the surroundings.

In a printing system, the ink droplets deposited by the last of the print bars of an image forming station can readily be dislodged if excessive gas pressure is applied by the heating apparatus. However, as they dry out progressively, the ink droplets are able to withstand higher gas pressures for more effective drying. The optimum gas pressure and temperature is therefore not the same over the entire length of the heated section of the intermediate transfer member.

In some embodiments of the invention, the ink drying apparatus may be formed of a plurality of units, each as set forth above, staggered along the length of the section of the intermediate transfer member along which the ink droplets are to be dried, the units being independently controllable. In this way, it is possible to arrange for the pressure of the gas jets emitted by heating apparatus nearer the image forming station to be less than the pressure of the gas jets emitted by heating apparatus more remote from the image forming station. The temperature of the heated gas emitted by the different apparatus may further be graduated to achieve a desired profile in both the temperature of the ink droplets and the temperature of the surface of the intermediate transfer member on which they are deposited. A drying unit nearer the image forming station may be referred to as an upstream drying unit in relation to a more remote unit that can be termed a downstream unit.

In a further aspect of the invention, there is provided a method using the drying apparatus herein disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a printing system; Figure 2 is a perspective view of a drying unit of an apparatus of the invention; Figure 3 shows a transverse section through the drying unit of Figure 2; Figure 4 is a longitudinal section through the drying unit in Figure 2; Figure 5 is a perspective view from above of the wall of the drying unit in Figure 4 that faces the surface of the intermediate transfer member on which an ink image is transported; Figure 6 a perspective view from below of the wall shown in Figure 5; Figure 7 is a perspective view of one of the gas jets shown in Figure 6; and Figure 8 is a partial perspective view from below of an altemative lower wall of the drying unit.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Though thc invention can bc used to dry any type of liquid deposited on an optionally fast moving surface, it will be described below with reference to examples where the liquid is the carrier of ink droplets deposited on the outer surface of an intermediate transfer member of an indirect printing system. The present drying apparatus can be deployed in printing systems in which the inks and the release layer have respective properties allowing the ink droplets to wet the outer surface of the transfer member or conversely in which the release layer has liquid repelling properties toward the inks being used. The following examples may refer in particular to the drying of liquid inks having an aqueous carrier typically comprising a colouring agent (e.g., piguients or dyes) and a polymeric resin, these inks having been jetted on a repelling hydrophobic surface of an intermediate image transfer member, but the invention need not be limited to such particular embodiments.

In Figure 1, there is shown schematically a printing system 100 having an intermediate transfer member 102 in the form of a blanket having a release surface (e.g., a hydrophobic release layer repelling aqueous inks) guided over various rollers of a blanket conveyor system 122 to travel in an endless loop through different stations. The invention is equally applicable to printing systems wherein the intermediate transfer member is a drum, the specific designs of the various stations then being accordingly adapted.

At an image forming station 104, print bars 106 deposit droplets of inks onto the outer surface of the blanket 102 to form an ink image that is a minor image of the image to be printed. The inks of the different bars 106 are usually of different colours and all the inks have particles of resin and colouring agent in an aqueous carrier, apart from some transparent inks or varnishes which may be not contain a pigment.

Though the image forming station illustrated in Figure I comprises eight print bars, an image forming station may comprise fewer or more print bars. For instance, an image forming system may have three print bars each jetting Cyan (C), Magenta (M) or Yellow (Y) inks, or four print bars with the addition of a Black ink (K).

After leaving the image forming station 104, the blanket 102 passes through a drying station 108 where the ink droplets are dried and rendered tacky before they reach impression stations 110 where the ink droplets are transferred onto sheets 112 of substrate.

Each impression station 110 includes an impression cylinder 110a and a pressure roller liOb which have between them a nip within which the blanket 102 is pressed against a substrate. In the illustrated embodiment, the substrate is formed as sheets 112 that are transferred from an input stack 114 to an output stack 116 by a substrate transport system 118. The substrate transport system 118, which is generally conventional and need not thercfore be described in detail in the present context, may comprise a perfecting system to allow double-sided, or duplex, printing. Two impression stations 110 are provided to enable printing on both sides of the substrate, one impression station being positioned upstream and the other downstream of the perfecting system. Alternatively, the printing systems in which the present invention may be used can have a single impression station and different perfecting means, if any.

It should be mentioned that the invention is equally applicable to printing systems designed to print on a substrate in the form of a continuous web instead of individual sheets. In such eases, the substrate transfer system is accordingly adapted to convey the substrate from an input roller to a delivery roller.

After passing through the impression stations 110, the blanket 102 in Figure 1 passes through an optional cleaning and/or conditioning station 120 before returning to the image forming station 104. The purpose of a cleaning station would be to remove any ink that may still be adhering to the blanket 102 while a condition station would be operative to apply a conditioning agent, to assist in fixing the ink droplets to the outer surface of the blanket 102. For blankets having a hydrophobic silicone based release surface, which are made hydrophobic to assist in a clean transfer of the tacky ink image to the substrate at the impression station(s) 110, the conditioning agent may be polyethyleneimine cPEI).

The present invention is concerned with the drying station 108 which may be made up of several drying units staggered along the length of the blanket 102. One of the units will now be described with reference to Figures 2 to 8.

The drying unit 108 in Figure 2 comprises a diffuser body 210 and two blowers 212, 214 arranged one at each axial end of the diffuser body 210. Each blower 212, 214 has two outlets 216 supplying air under pressure to two plenum chambers 222 (to be described below by reference to Figures 3 and 4) and two inlets 218 that receive gas (herein termed S exhaust gas) that has already been applied by diffuser body 210 to the surface of the intermediate transfer member 102. The intermediate transfer member 102 passes beneath the diffuser body 210, moving from left to right in Figure 3 (more generally from the image forming station to the impression station). By providing blowers at both ends of the diffuser body 210, it is possible to ensure a more even distribution of the gas over the width of the intermediate transfer member 102.

In another embodiment, the blowers may be located remotely/away from the diffuser body, and connected to the plenum chamber via outlet ducts. The outlet duets can be made of a variety of materials capable of withstanding the temperature of the circulated gas, including metals (e.g., aluminum, steel, etc.), and of polymers (e.g., silicones, rubbers, etc.) In another embodiment, a single blower is connected to the diffuser body. The outlet of the blower communicates with the plenum chamber at both of the axial ends by way of a Y-connector, i.e., a connector with one port on one end and two ports on the second end.

The diffuser body 210, as best shown in the section of Figure 3, is formed of two box-shaped housings 220 arranged side by side. Each housing defines a plenum chamber 222 that communicates with a passage 224 receiving compressed gas from one of the outlets 216 of each of the two blowers 212, 214. Gas, normally ambient air, blown into the passages 224 passes through inclined perforated diffuser plates 226 (see Figure 4) before entering the plenum chambers 222. A heat exchanger, or a heating e'ement, 228 disposed within each plenum chamber 222 heats the gas before it exists through openings in the lower wall 230 of the plenum chamber 222.

As can be seen from Figure 3, the two housings 220 are spaced apart from one another and are surrounded by a cowling 240. A first exhaust gas collection passage 244 is defined between the two housings 220 and a second exhaust gas collection passage 242 is defined between the housing 220 shown on the right in Figure 3 and the cowling 240. Each of the two exhaust gas collection passages 242, 244 is thus arranged downstream (in the direction of movement of the intermediate transfer member) from a respective one of the housings 220. Each collection passage is connected to the suction side of the two blowers 212, 214 by way of a common conduit 250 arranged above the two housings 220.

As mentioned beforehand, the blowers may alternatively be remotely located. In such an embodiment, a connector, connecting between the exhaust gas collection passage and thc suction side of the blower, is provided.

In operation, the diffuser body 210 receives gas from the pressure side of the blowers 212, 214, evens out the flow by passing the gas through the inclined diffuser plates 226, heats the gas as it passes through the heat exchangers 228 in the plenum chambers 222 and then blows the gas through openings in its lower wall 230 onto the surface of the intermediate transfer member. The gas blown onto the droplets of ink transported by the intermediate transfer member 102 dries them and is used to extract the released vapour in the immediate vicinity of the droplet. Because of the relative movement of the intermediate transfer member 102, the gas tends naturally to migrate downstream and on reaching the end of each of the two housings 220, the gas is drawn into one or other of the two exhaust collections passages 242, 244 and is returned to the suction side of the blowers 212, 214 after passing through the conduit 250.

If 100% of the air passing through the blowers 212, 214 were to be recycled, it would be necessary to incorporate a condenser in the air recirculation circuit to extract the moisture, otherwise the gas could become saturated with vapour. In the disclosed embodiment, not all the exhaust gases are collected into the passages 242 and 244 and each blower takes in fresh air from the ambient atmosphere through an opening 215 the area of which is controllable by a sliding shutter that allows the proportion the recirculated gas to be varied.

The wall 230 of each of the plenum chambers 222 needs to be designed in such a manner as to allow jets of gas to be directed onto the surface of the intermediate transfer member 102 with a minimal component of the gas velocity extending parallel to the surface of the intermediate transfer member 102. This is because the ink droplets may not be firmly fixed to the surface of the intermediate transfer member and could be dislodged by a high component of gas velocity parallel to the surface. If the side of the wall 230 facing the intermediate transfer member were to be made flat and with holes to allow gas to escape from the plcmim chamber 222, each gas jet emitted from one of the holes would be laterally deflected after impinging on the surface of the intermediate transfer member to flow parallel to the surface of the intermediate transfer member. The vapour laden exhaust gases would therefore risk dislodging the ink droplets and would also interfere with and deflect gas jets that are being emitted from surrounding holes.

The wall 230, which will now be described by reference to Figures 5 to 7, it designed to mitigate the above problem. The wall 230, as shown in Figures 5 and 6, is formed of a perforated metal sheet with a nozzle 300, as shown in Figure 7, inserted into each hole and projecting from the underside of the wall 230. The nozzle 300 has a taper 306 at its lower S end terminating in a hole 308 from which a gas jet escapes. At its upper end, each nozzle 300 has an enlarged diameter collar 302 that is an interference fit in the hole pre-drilled into the wall 230 and a flange 304 that acts to effect a seal and provide a positive stop to assist in insertion of the nozzles 300 into the wall 230.

The spaces 320 between the individual nozzles 300 act as channels that allow the exhaust gases to escape without again contacting the surface of the intermediate transfer member 102. After each gas jet contacts the surface of the intermediate transfer member 102, it is reflected upwards into the spaces 320 and flows towards the exhaust gas collection passages while travelling in a plane that is further removed from the surface of the intermediate transfer member 102 that the plane of the holes 308. The exhaust gases do not therefore exert a force tending to disturb the ink droplets and they do not pass beneath any of the holes 308 of the surrounding nozzles to interfere with or deflect their gas jets.

Instead, all the channels defined between the individual nozzles are maintained at a negative pressure by their connection to a nearby collection passage 242, 244 and act an extension of the collection passages 242, 244 to remove the exhaust gases efficiently from the space between the wall 230 and the surface of the intermediate transfer member 102.

The manufacture of the wall 230 in Figures 5 to 7 is labour intensive in view of the need to assemble the separately formed nozzles 300 into the holes in the wall 230. An alternative construction of the wall 230' shown in Figure 8 is formed by machining a solid plate. In this case, recessed channels 320' are milled or otherwise machined into the underside of the wall 230? around the holes 308' through which the gas jets are discharged.

It is preferred for each hole 308' to be surrounded by recessed channels 320' on all sides, but it is alternatively possible for holes to arranged in groups without there being a recessed channel between adjacent holes in a group so long as there is a route for the exhaust gas from each hole 308' to a nearby recessed channel without passing through the gas stream emitted by another hole 308'.

The drying apparatus 108 is formed of several units as described above arranged one after the other in the direction of motion of the intermediate transfer member 102. In an embodiment of the invention, the different units are controlled independendy. In this way, units deployed nearer the image forming station 104 may operate with low pressure to avoid disturbing the jetted ink droplets before they are well fixed to the surface of the intermediate transfer member 102, whereas units mounted further from the image forming station 104 may operate at higher pressure. It is also possible to vary the temperature S between the different units to achieve a desired temperature profile both of the ink droplets and the surface of the intermediate transfer member 102.

In some embodiments, the velocity of the gas exiting the nozzles is no more than 45 mis, or no more than 40 mIs, or no more than 35 mIs. The velocity of the gas exiting the nozzles can be no less than 15 mis, or no less than 20 m's, or no less than 25 mIs. The velocity of the gas exiting the nozzles can lie between 15 mis and 45 mis, or between 20 rn1s and 40 mis, or between 20 rn/s and 35 mis, or between 25 m's and 35 mis.

In some embodiments, the temperature of the gas is no more than 350°C, or no more than 325°C, or no more than 300°C, or no more than 275°C. The temperature of the gas may be no less than 200°C, or no less than 225°C, or no less than 250°C. The temperature of the gas may lie between 200°C and 350°C, or between 250°C and 350°C, or between 250°C and 325°C, or between 250°C and 300°C, or between 200°C and 300°C, or between 225°C and 300°C.

In some embodiments, the temperature gradient is such that the difference between the temperature of the gas at the first upstream drying unit and at the last downstream drying unit is 50°C or less, or less than 40°C, or less than 30°C, or less than 20°C. In another embodiment, the temperature of the gas at the first upstream drying unit and at the last downstream drying unit is substantially the same.

Inks suitable for use in the method and apparatus herein disclosed are capable of being jetted from an inkjet print bar (e.g., having sufficiently low viscosity to be propelled from print bar nozzles without clogging) and of being neatly transferred from the intermediate transfer member to the substrate under the conditions of use (e.g., temperature and pressure), once most or substantially all of their carrier and, if present, other volatiles are removed therefrom prior to the transfer.

Conditions that may affect the exact composition of inks that may benefit from the present teachings include, but are not limited to, one or more of the following factors: a) the composition of the release layer with which the ink is due to interact; b) the temperature of ink jetting; c) the temperature of the transfer member at the image forming station; ci) the temperature of the gas blown through the dispensers of the drying apparatus; and e) the temperature of transfer.

Non-limiting examples of ink compositions for which the present invention can be suitable are water based inks as disclosed in co-pending PCT Application No. PCT/1B2013!051755, ified on March 5,2013.

Intermediate transfer members that may be used in connection with such water based inks, though a priori repelling them, may comprise a silicone material. Under suitable conditions, a silanol-, sylyl-or silane-modified or terminated polydialkylsiloxane silicone material and amino silicones have been found to work well. However the exact formulation of the silicone is not critical as long as the selected material allows for release of the image from the transfer member to a final substrate. Further details of non-limiting examples of release layers and intermediate transfer members for which the present invention can be suitable are disclosed in co-pending PCT Applications Nos. PCT/1B2013/051743 and PCT/1B2013/051751, filed on March 5, 2013.

As previously explained, the release layer may be treated prior to reaching the image forming station with a conditioning agent that may partly reduce the hydrophobic effect of some of the afore-mentioned silicone-based release layers. Further details on conditioning solutions suitable for printing processes wherein water-based inks are jetted onto hydrophobic surface of transfer members and which may be used in printing systems for which the present invention can be suitable are disclosed in co-pending PCT Application No. PCT/1B2013!000757, ified on March 5, 2013.

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

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 members, components, elements or parts of the subject or subjects of the verb. As used herein, the singular form "a", "an" and "the" include plural references unless the context clearly dictates otherwise. For example, the term "an impression station" may include more than one such station.

Claims (16)

1. CLAIMS1. A drying apparatus for use in an indirect printing system to thy ink droplets deposited onto a surface of an intermediate transfer member by means of one or more print bars, the apparatus being formed of at least one drying unit that comprises a housing for mounting opposite a section of the surface of the intermediate transfer member, the housing having a plenum chamber connectible to a supply of gas under pressure, which plenum chamber is bounded by a wall of the housing having an inner side facing the plenum chamber and an outer side facing the surface of the intermediate transfer member on which the ink droplets are deposited, the wall being formed with openings to allow jets of gas to escape from the plernim chamber and to be directed onto the surface of the intermediate transfer member in order to dry the ink droplets, wherein at least one exhaust channel is recessed into the outer side of the wall for enabling gas that has already contacted the surface of the intermediate transfer member to escape from between the housing and the intermediate transfer member without obstructing the path of gas being discharged from the openings.
2. 2. A drying apparatus as claimed in claim 1, wherein the openings in the wall are formed by individual nozzles each having a body projecting from the outer side of the wall, spaces between the bodies of the nozzles defining multiple exhaust channels.
3. 3. A drying apparatus as daimed in claim 2, wherein the wafl of the p'enum chamber facing the intermediate transfer member is formed with a holes and a separately formed nozzle body is firmly mounted within each hole to project from the underside of the wall.
4. 4. A drying apparatus as daimed in claim 1, wherein the wafl of the p'enum chamber facing the intermediate transfer member is formed with holes for the discharge of jets of gas and channels are machined into the side of the wall facing away from the plenum chamber around the gas discharge holes.
5. 5. A drying apparatus as claimed in any preceding claim, wherein a heater is provided to heat the gas directed onto the surface of the intermediate transfer member.
6. 6. A drying apparatus as claimed in claim 5, wherein the heater is a heat exchanger through which a heated fluid flows during use.
7. 7. A drying apparatus as claimed in any prcceding claim, wherein a diffuser is arranged in the path of the gas entering into the plenum chamber from the source of gas under pressure.
8. 8. A drying apparatus as claimed in any preceding claim, wherein at least some of the gas escaping through the exhaust passage(s) passes by way of a collection passage to an intake side of the supply of gas under pressure.
9. 9. A drying apparatus as claimed in claim 8, wherein only a proportion of the gas supplied by the blower under pressure is derived from recycled exhaust gases.
10. 10. A drying apparatus as claimed in any preceding claim, wherein, the housing is surrounded by a cowling or a jacket to define an exhaust gas collection passage therebetween.
11. 11. A drying apparatus as claimed in any preceding claim, formed of a plurality of independently controllable drying units staggered along the length of the section of the intermediate transfer member along which the ink droplets are to be dried.
12. 12. A method of drying ink droplets deposited onto a surface of an intermediate transfer member of an indirect printing system, the drying being performed with an apparatus according to any preceding claim.
13. 13. A method according to claim 12, wherein the ink droplets have an aqueous liquid carrier.
14. 14. A method according to any of claims 13 and 14, wherein the surface of the intermediate transfer member has repelling properties toward the ink droplets.
15. 15. A method according to any of claims 12 to 14, wherein the drying apparatus is formed of a plurality of independently controllable drying units and wherein the pressure of the gas at the first upstream unit is lower than the pressure of the gas at the last downstream unit.
16. 16. A method according to any of claims 12 to 15, wherein thc drying apparatus is fbrmed of a plurality of independently controllable drying units and wherein the temperature of the gas at the first upstream unit is different than the temperature of the gas at the last downstream unit.