GB2518148A

GB2518148A - Printing system

Info

Publication number: GB2518148A
Application number: GB1316136.9A
Authority: GB
Inventors: Benzion Landa; Yehoshua Sheinman
Original assignee: Landa Corp Ltd
Current assignee: Landa Corp Ltd
Priority date: 2013-09-11
Filing date: 2013-09-11
Publication date: 2015-03-18
Anticipated expiration: 2033-09-11
Also published as: GB201316136D0; GB2518148B

Abstract

A method and apparatus are disclosed for treating ink droplets having a liquid carrier immediately after their deposition onto a liquid repelling surface of an image transfer member of an indirect printing system; the method comprises blowing a low velocity stream of gas onto the surface in a direction substantially normal to the surface to minimise disturbance of the ink droplets; the temperature and flow rate of the stream are sufficient to cause evaporation of the liquid carrier, and the duration of the exposure of the ink droplet to the stream of gas is sufficient to form a skin on the surface of the droplet while the centre of the droplets remains as a liquid; the skin acts to fix the ink droplets to the surface and prevent bleeding between successively deposited ink droplets. The gas stream is applied by way of a head unit that incorporates a blower (204, fig 2) and a plenum chamber 202 containing diffusers 302 and a heater 304.

Description

PRINTING SYSTEM

FIELD OF THE DISCLOSURE

The present invention relates to printing systems and in particular to indirect systems using liquid inks on intermediate image transfer members. Specifically this disclosure pertains to a method of stabilising ink droplets on the surface of such members and to an apparatus for doing the same.

BACKGROUND

Digital printing techniques have been developed that allow a printer to receive instmctions directly from a computer without the 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 processes. Such printing devices typically directly apply the desired imagc to the 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 connnercial 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 dcctricafly 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 transfcr member) and dricd thereupon before being transferred to thc desired substrate. Such systems allow the distance between the surface of the intermediate image S transfer member (also herein termed the release layer) and the inkjet print bar to be maintained constant and reduces wetting of the substrate, as the ink can be dried on the intcrmcdiatc 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/1B201 3/051716, filed on March 5, 2013.

Among the problems surmounted by such systems was the need to transiently fix aqueous based ink droplets onto silicone based release layers which, being hydrophobic, cause the ink droplets to bead on the transfer member. This results in a small contact area between the droplets and the blanket that renders the ink image unstable during rapid movement and makes it more difficult to remove the water from the ink by heating the transfer member.

One solution proposed in the aforementioned application to alleviate this problem was to evaporate a substantial proportion of the liquid ink carrier at the stage of the image formation onto the transfer member. The rate of such evaporation depending upon temperature, it was generally preferred for that particular purpose to operate the system at elevated temperatures (e.g., above water boiling point and typically up to 160°C).

1-lowever, as the vapours of the ink carrier might over time affect the print bar nozzles, lower temperatures (e.g., above 40°C) were also considered for the image forming station.

Depending on the ink formulations, the composition of the release layer of the transfer member interacting with the jetted droplets and on the temperature of the printing system at this primordial stage of the process, droplets being subsequently jetted may impinge on an already, at least partially, dried previously jetted droplet, or an a still wet droplet. The latter case of wet-on-wet jetting may suffer from some drawbacks such as colour bleeding.

Generally, the mixing of two adjacent printed dots before they dry reduces print quality. If the two adjacent dots are of the same colour, the merging of the dots may result in a loss of resolution. If in addition the two sequentially jetted dots are of dissimilar colours, the undesired mixing of the colours further affects print quality.

A method for reducing or avoiding colour bleeding, in particular on intermediate transfer members, is needed.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method of treating ink droplets having a liquid carrier immediately after their deposition onto a relatively moving liquid repelling surface of an image transfer member at an image forming station, the method comprising blowing a stream of gas onto the surface of the transfer member, wherein the velocity of the stream is selected to minimise disturbance of the ink droplets, the temperature and flow rate of the stream are sufficient to cause partial evaporation of the liquid carrier, and the duration of the exposure of the ink droplet to the stream of gas is sufficient to form a skin on the surface of the droplet while the centre of the droplets remains as a liquid, the skin serving to fix the ink droplets to the surface and prevent bleeding between successively deposited ink droplets.

The term "disturbance" is intended to include both movement of the ink droplets relative to the surface of the transfer member and change in the shape of the ink droplets.

Though the invention can be applied to all inks having liquid carriers and to all surfaces that repel the liquid carrier, it will be described below with reference to examples where the inks have an aqueous carrier typically comprising a colouring agent (e.g., pigments or dyes) and a polymeric resin, and the surface on which the ink droplets are deposited is hydrophobic.

While not wishing to be bound by theory, it is believed that the fixing of the ink droplets is the result of the modification of the surface energy of the ink droplets. Because of the evaporation of the liquid carrier from the surface of the droplets, the skin is expected to have a higher concenfration of pigment/resin than the liquid ink at the centre of the droplets. The adhesion forccs between the pigment/resin of the skin and the release layer are believed to contribute to the transient fixing of the droplets to the transfer member.

The afore-mentioned method is applied at the time the ink droplets are deposited to form an ink image on the outer surface of an intermediate transfer member at a station of a printing system hereinafter referred to as an "image forming station", its elements also said to form an "image forming system".

In some embodiments of the invention, the gas temperature may lie within the range of 100°C to 250°C, or within 100°C to 220°C, or within 100°C to 200°C, or within 130°C to 200°C, or within 105C to 130C The temperature of the surface of the transfer member at the image forming station can be below the evaporation temperature of the ink carrier. In some embodiments, the temperature of the release layer may be at least 5°C, or at least 10°C, or at least 20°C, or at least 30°C, or at least 40°C below the evaporation temperature of the ink carrier.

The gas stream may be blown onto this outer surface byway of a diffuser positioned at a distance of at least 4 millimetres (mm), or at least 5 mm, or at least 6 mm, or at least 7 mm, or at least 8 mm, or at least 9 mm. In some embodiments, the distance between the surface of the transfer member and the diffuser is no more than 15 mm, or no more than 14 mm, or no more than 12 mm, or no more than 10 mm. In other embodiments, the distance between the release layer and the closest point on the difThser is of 4 to 15 mm, or between 6 and 14 mm, or between 8 and 12 mm, or between 9 and 11 mm, or between 9 and 10 mm.

In some embodiments, the stream of gas is blown onto the surface of the transfer member in a direction substantially normal to the surface of the transfer member.

The gas blown onto the release layer may conveniently be air. The volumetric gas flow rate is proportional to the area of the diffuser. For a diffuser having an area of 0.17 m2 the volumetric gas flow rate may be at most 0.2 cubic meters per second (m3/scc), or at most 0.3 m3/sec, or at most 0.5 m3/sec.

The gas blown onto the release layer may be jetted at the rate of between 20 m/scc and 50 mlsec or between 30 mlsec and 40 nilsec.

It is noted that there is a correlation between the gas flow rate, the distance between the diffuser and the release layer, and the geometrical configuration of the diffuser.

Persons skilled in the art to which this invention pertains will readily know how to adjust one of the afore-mentioned parameters as a function of changes in the others.

In some embodiments, the temperature of the transfer member at a location immediately before the first print bar is less than the evaporation temperature of the liquid ink carrier.

The temperature of the transfer member at a location immediately before the first print bar is between 40°C and 95°C, or between 60°C and 95°C, or between 75°C and 95°C. The temperature of the transfer member at a location immediately after the last print bar is between 40°C and 95°C, or between 60°C and 95°C, or between 75°C and 95°C.

The temperatures immediately before the first print bar and immediately after the last print bar, may be the same or different. The first and last print bars are defmed relatively to the printing direction or movement of the transfer member, the first bar being upstream and the last one downstream.

The gas stream may be blown onto the release layer of the transfer member at a temperature such that the tempcraturc of the surface underneath the gas stream shall remain substantially unchanged. In some embodiments, the temperature of the surface of the release layer of the transfer member following impact of the gas stream being blown thereupon is within at most 10°C, at most 5°C, or at most 4°C, or at most 3°C, or at most 2°C. or at most 1°C, from the temperature of the surface before gas stream impact.

The temperature of the gas stream needs to be sufficiently elevated to achieve the desired film skin on the surface of the ink droplets during the timc period the droplets arc exposed to the stream. The temperature of the gas stream needs to be sufficiently low not to affect the temperature of the surface of the transfer member in an undesired manner. As mentioned, the temperature of the transfer member may vary between the first and last print bars. Such variations may be desired for instance to accommodate the ink compositions being sequentially deposited on the transfer member, in some embodiments, the temperature of the release layer of the transfer member immediately following the last print bar can be higher than the temperature immediately preceding the first print bar. In such embodiments, the gas stream may have accordingly increasing temperatures suitable to achieve the desired temperature gradient on the surface of the intermediate transfer member.

In some embodiments, the temperature of the gas stream is at most 40°C, or at most 60°C, or at most 80°C, or at most 110°C, or at most 150°C above the evaporation temperature of the ink carrier. In some embodiments, the temperature of the gas stream is at most 150°C, or at most 200°C, or at most 250°C.

In some embodiments, the ink droplets are exposed to the stream of gas for a time period of 25 milliseconds to 600 milliseconds (ms) and the ink droplet is first exposed to the stream of gas within 30 ms, or within 120 ms, or within 240 ms, or within 480 ms, or within 960 ms of its deposition on the surface.

In accordance with a second aspect of the invention, there is provided a printing system comprising a movable image transfer member having a liquid repelling surface, a plurality of print bars each for depositing onto the relatively movable surface droplets of an ink having a liquid carrier, a drying system for drying ink droplets as they are transported on the surface, and an impression station at which the ink droplets are transferred onto a substrate after passing through the drying system, characterised by a stabilising apparatus for fixing the ink droplets on the surface of the image transfer member, the stabilising apparatus comprising a source of gas, and a plenum chamber connected to the source of gas and positioned immediately after at least one print bar in the direction of movement of the surface, the plenum chamber having an outlet opening positioned to direct a gas stream onto the ink droplets, the temperature and flow rate of the gas and the exposure time of the ink droplets to the gas stream being such as to dry the surface of the droplets to form a skin while the centre of the droplets remains liquid, the skin serving to fix the ink droplets to the surface and prevent bleeding between successively deposited ink droplets.

In some embodiments, the plenum chamber of the stabiising apparatus of the indirect printing system has an outlet opening positioned to direct a gas stream onto the ink droplets in a direction normal to the plane of the surface of the image transfer member passing underneath.

In some embodiments, the printing system according to the present invention can implement the afore-mentioned method.

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 shows a perspective view of a head unit of a droplet stabilising apparatus positioned between the print bars of the printing system in Figure 1; Figure 3 is a cut-away view of the head unit in Figure 2; Figure 4 is a vertical section through the head unit of Figures 2 and 3 in a plane normal to the direction movement of the transfer member; Figure 5 is a section through a first embodiment of an external diffuser that forms part of the head unit shown in Figures 2 to 4, the cross section being taken along the line X-X shown in Figure 6; Figure 6 is a plan view of the diffuser shown in Figure 5; Figure 7 is a perspective view of the diffuser illustrated in Figures 5 and 6; Figure 8 is a detail of Figure 5 drawn to an enlarged scale illustrating a protrusion of the diffuser; Figurc 9 is a plan view similar to that of Figure 6 showing an alternative construction of an external diffuser; Figure 10 is a section through the diffuser of Figure 9 taken along the line X-X; and Figure 11 is a detail of Figure 10 drawn to an enlarged scale illustrating a countersunk hole of the diffuser.

DETAILED DESC14TPTION OF THE ILLUSTRATED EMBODIMENTS In Figure 1, there is shown schematically a printing system 100 having an IS intermediate transfer member 102 in the form of a blanket having a hydrophobic outer surface guided over various rollers of a blanket conveyor system 122 to travel in an endless loop. While circulating through the loop, the blanket passes through various stations. The invention is equally applicable to printing systems wherein the intermediate transfer member is a drum, the specific designs of the various stations being accordingly adapted.

At an image forming station 104, print bars 106 deposit droplets of inks onto the hydrophobic outer surface of the blanket 102 to form an ink image. 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 not contain a pigment. The method and apparatus herein-disclosed arc particularly relevant to ink compositions having film forming abilities, Le., being capable of selectively forming a skin on the surface of a droplet under the desired operating conditions.

Though the image forming station illustrated in Figure 1 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).

The blanket 102 then 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 arc transfcrred onto shccts 112 of substratc. Each imprcssion station 119 includcs an impression cylinder llOa and a prcssurc roller ilOb which havc between them a nip within which the blanket 102 is pressed against a substrate. In the illustrated embodiment, the substrate is formed as shcets 112 that arc transfcrrcd 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 therefore be described in detail in the present context, may comprise a perfecting system to allow double-sided, or duplex, printing. Two imprcssion stations 110 arc providcd to cnablc printing on both sidcs of thc substrate, onc impression station bcing positioned upstream and the other downstream of the perfecting system.

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 cases, 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 192 in Figure 1 passes through an optional cleaning and/or conditioning station 120 before returning to the image forming station 104. The purpose of the station 120 is to remove any ink that may still be adhering to the blanket 102 and/or to apply a conditioning agent, to assist in fixing the ink droplets to the outer surface of the blanket 192. For blankets having a silicone based outer surface, the conditioning agent may be polyethyleneimine (PEI). The outer surface of the transfer member 102 is made hydrophobic to assist in a clean transfer of the tacky ink image to the substrate at the impression station(s) 110.

A problem in such a printing system, with which the present invention is concerned, is the stabilising of the droplets deposited on the surface of the intermediate transfer member, such as the blanket 192, by the print bars 106. It has previously been proposed to operate the intermediate transfer member at a sufficiently high temperature for evaporation of the carrier in the ink droplet to commence on impact. Aside from the high energy consumption to maintain the blanket 102 at such a high temperature (in the region of 140°C to 160°C) it was found that evaporated carrier would condense on the print bars 106 and cause blockage (e.g., clogging) of the ink jet nozzles. If the blanket 102 is operated at lower temperatures, on the other hand, the ink droplets which have been flattened into a disc upon impact with the surface of the blanket 102 tend to bead up into globules and then move around on the surface of the moving blanket. A thither problem is that the droplets remain liquid when they reach the next print bar 106 and the ink droplets of the next print bar blend and merge with the previously deposited ink drops causing "colour bleeding".

Thus for example if a blue droplet lands near a yellow one instead of creating a clear blue/yellow boundary, the two droplets merge into a large green droplet. The purpose of the stabilisation apparatus proposed in the present invention is therefore to fix the droplet on the intermediate transfer member and prevent separately deposited droplets from merging into and blending with one another.

This stabilisation of the droplets is achieved in Figure 1 by means of stabilising apparatus in the form of head units 130 positioned after all but the last of the print bars 106. The head units 130, which will be described in morc detail below with reference to the remaining drawings, have plenum chambers connected to a source of gas, usually air, at a controlled temperature and pressure and each has an outlet opening positioned to direct a gas stream onto the ink droplets. The gas stream travels in a direction normal to the plane of the hydrophobic surface to avoid displacing the ink droplets. The temperature and flow rate of the gas and the exposure time of the ink droplets to the gas stream are set to dry only the surface of the droplets to form a skin while the centre of the droplets remains liquid. The integrity of the skin that is formed is sufficient to fix the ink droplets to the hydrophobic surface and prevent bleeding between successively deposited ink droplets.

The ink droplets of the last print bar 106 do not need to be stabilised because they enter next the drying station 108 and there is no subsequent deposition of ink droplets to cause any bleeding to take place.

The design of the head units, as described below, is intended to achieve as even a distribution of the stream of gas as possible, because flow rate and pressure differentials will tend to cause drops to migrate across the hydrophobic outer surface of the blanket 102 or to experience undesired shape deformation, such as departing from the impinging flattened disc shape.

In Figure 2, there is shown a stabilising apparatus 200 comprising of a head unit 202 and a gas supply unit 226. Such apparatus was previously schematically illustrated as head unit 130 in Figure 1. The gas supply unit comprises a blower 204, a pressure duct 210 and a suction duct 208. The blower 204 causes a stream of gas to be drawn in via the suction duct 208 and expelled into the pressure duct 210. In this embodiment, the preferred gas flow ratc through thc blower is 0.1 to 0.5 m3!see, in particular 0.1 to 0.3 m3/sce.

As seen from the cut-away view of Figure 3, the head unit 202 comprises three different chambers, namely an inlet chamber 306, an outlet chamber 308 and a heating chamber 310. The opening of the inlet chamber 306 forms a seal with the pressure duet 210 and the opening of the outlet chamber 308 seals against the suction duct 208.

The gas that enters the inlet chamber 306 flows through an internal diffuser 302 in order to equalize the flow rate in the heating chamber 310. To maximise efficiency, it is desirable for the gas to have an equalized flow rate in the heating chamber 310 so that exchange of energy between the gas and a heater 304 should be as uniform as possible. In the illustrated embodiment heater 304 is a radiator, but other forms of heaters may be acceptable. The heater is configured such that the gas is heated to a temperature of at least 100°C, or at least 120°C, or at least 140°C. In another embodiment of the invention the heater is configured such that the gas is heated to a temperature of no more than 130°C, or no more than 150°C, or no more than 170°C, or no more than 200°C, or no more than 220°C, or no more than 25 0°C. In another embodiment of the invention, the heater is configured such that the gas is heated to a temperature between 100°C and 250°C, or between 100°C and 220°C, or between 120°C and 220°C, or between 140°C and 220°C, or between 130°C and 200°C, or between 100°C and 180°C, or between 100°C and 150°C.

The heated gas is discharged via an external diffUser 300 arranged at an outlet of the heating chamber 310 facing the surface of the intermediate transfer member, illustrated by blanket 102 in Figure 1. The distance between the surface of the intermediate transfer member and the surface of the diffuser 300 to the surface is at least 4 mm, or at least 5 mm, or at least 6 mm, or at least 7 mm, or at least 8 mm, or at least 9 mm. In some embodiments, the distance between the surface of the transfer member and the diffuser is no more than 15 mm, or no more than 14 mm, or no more than 12 mm, or no more than 10 mm. In other embodiments, the distance between the release layer and the closest point on the diffuser is of 4 to 15 mm, or between 6 and 14 mm, or between 8 and 12 mm, or between 9 and 11 mm, or between 9 and 10 mm.

The gas discharged through the external diffuser 300 is discharged substantially normal to the surface of the release layer of blanket 102 at the image forming station 104.

As well as ensuring that the droplets are not made to migrate across the surface of the intermediate transfer member, this ensures the flow direction acts in a direction opposing the tendency of the ink droplets to form globules.

Under certain conditions it may be desirable to incline the diffuser 300 at a small angle of up to 300 relative to the surface of the intermediate transfer member so as to create a pressure that varies as a function of the distance between the external surface of the external difThser 300 and the surface of the transfer member. In particular the external diffuser 300 may be inclined so that the diffuser is farther apart from the transfer member on its upstream side. Tn such manner, the flow rate perceived on the surface of the transfer member may be modulated such that at entry point when the jetted droplets are not yet stabilized the perceived flow and resulting pressure is lower, the distance being greater, whereas the perceived flow and pressure would increase as the distance between the surface of the blanket and the inclined dififiser is reduced.

The head unit 202 is connected to sidewalls of the blanket conveyor system 122 by mounting brackets 214 so as to be positioned adjacent to a print bar 106 of the image forming station 104. The mounting brackets 214 are capable of remaining substantially static despite thermal expansion that may occur due to the hot gas contacting the walls of the head unit 202. The brackets are configured so that the external surface of the external diffuser is at the desired set distance from the hydrophobic surface of the intermediate transfer member, such as illustrated by blanket 102.

The mounting brackets 214, and consequently the head units 202, are fixed at locations on the mounting frame of the blanket conveyor system 122 so that the maximum time between an ink droplet impinging on the surface and the initial exposure to the stream of gas is between 20 and 200 milliseconds, or between 20 and 400 ins, or between 20 and 600 ms, or between 20 and 800 ms, or between 20 and 1,000 ms, or between 20 and 1,500 ms or between 20 and 2,000 ms.

The width of the head unit is configured so that the minimum exposure time of the ink droplets to the gas stream is between 25 and 250 milliseconds, or between 25 and 500 ms, or between 25 and 750 ms or between 25 and 1,000 ms.

Because of the exchange of heat between the gas inside the chambers and the chamber walls it may be desirable for safety or maintenance of convenient operating conditions to cover the extemal surfaces of the head units 202 with thermal insulators 212, as shown in Figure 2.

While it may suffice to allow the gas that has been blown over the surface of the intermediate transfer member to disperse into the interior of the printing system, such an approach has several disadvantages. First, hot moist gas released into the ambient atmosphere can result in an unpleasant work environment, second the water vapour can S cause condensation problems which may affect the operation of the printing system and third the release of hot gases into the ambient atmosphere is highly wasteful of energy.

In the illustrated embodiment of the invention, these problems are mitigated by recycling at least part of the gas used to stabilise the ink droplets. Thus it can be seen in Figure 4 that a cowling 320 placed over the head unit 202 defines spaces 322 on opposite sides of the head unit 202 that communicate through openings 324 with the outlet chamber 308 that is itself in communication with the suction side of the blower 204. At its lower end, the cowling 320 has a skirt defining openings 326 that straddle the external diffuser 300. In this way, at least some of the gas is sucked through the openings 326 and returned to the blower 204 for recycling, by way of the spaces 322, the openings 324, the outlet chamber 308 and the suction duet 208.

If all the air blown onto the ink droplets were to be constantly recirculated, it would eventually become saturated with water vapour and would be ineffective in drying the droplets. While it would be possible to incorporate a condenser in the recirculation loop to remove the water vapour, it is possible to allow some of the air to escape through a pipe designated 224 in Figure 2 that leads to the pressure side of the blower 204 and to draw in fresh air through an opening designated 206 in Figure 2 leading to the suction side of the blower.

In order to control the amount of recycled gas the size of the opening 206 may be varied by means of a slidable shutter 2t6. Other methods of controlling the area of the opening 206, such as a remote controllable diaphragm, a leaf shutter, and other techniques may be used to provide the same thnctionality.

The diffuser 300 shown schematically in Figure 2 may take any of several different fonms as described in more detail below.

A first type of external diffuser 500 is shown in Figure 5 to 8. This diffuser comprises a depressed surface 502 from which project an anay of protrusions 504 or nozzles. Each protrusion 504 is hollow and is open on both ends, as can be seen in the detail showm in cross-section to an enlarged scale in Figure 8. The advantage of forming protruding fine nozzles is that the volume surrounding the individual nozzles on the side of the diffuser facing the blanket serves as a return path to allow air that has contacted the ink droplets to be returned to the suction side of the blower.

The diameter of the opening at the end of the protrusion is between I and 4 millimetres, or between 1.5 and 3 mm, or between 1.5 and 2.5 mm. In some embodiments of the invention, the diameter of the opening at the end of the protrusion is no larger than 6 mm, or no larger than 4 mm, or no larger than 3.5 mm, or no larger than 3 mm, or no larger than 2.5 mm. The external diameter of the protrusions preferably may be between 4 and 15 mm, or between 4 and 12 mm, or between 4 and 8 mm, or between 5 and 8 mm. In some embodiments of the invention, the external diameter of the protrusions may be no larger than 12 mm, or no larger than 10 mm, or no larger than 8 mm. In still further embodiments of the invention, the external diameter of the protrusions may be at least 4 mm, or at least 5 millimetres.

To maxiniise the density of the protrusions, they can be arranged in a hexagonal array in which the centre of each protrusion in any row or column lies midway between the centres of its neighbours in the adjacent rows and columns, as best seen in Figure 6.

The longitudinal pitch, parallel to the direction of motion of thc blanket at the imagc forming station, is between 8 and 9 millimetres, or between 7 and 10 mm, or between 6 and 11 mm. The transverse pitch, perpendicular to the direction of motion of the blanket at the image forming station, is between 7 and 8 mm, or between 6 and 9 mm, or between S and 10mm.

A second type of external diffliser 900, shown in Figures 9 to 11, differs from the diffuser 500 shown in Figures 5 to 8 in three ways. First, there are no protrusions and instead the diffuser surface has a plurality of countersunk holes 902, as shown in the detail of Figure 11, in a surface facing the hydrophobic surface carrying the ink droplets. The second difference is the provision of recessed channels 904 on the face of the diffuser that do not have any holes. These channels 904 act as a low resistance return path for the blower of the gas that has been blown onto the intermediate transfer member, to reduce the suction that the blower needs to apply.

The diameter of the holes 902, as seen in thc cross-section of Figure 11, may be between 1 and 4 millimetres, or between 1.5 and 3 mm, or between 1.5 and 2.5 mm. In some embodiments of the invention, the diameter of the opening is no larger than 6 mm, or no larger than 4 mm, or no larger than 3.5 mm, or no larger than 3 mm, or no larger than 2.5 mm.

The longitudinal pitch between such holes, parallel to the direction of motion of the blanket at the image forming station, may be between 8 and 9 millimetres, or between 7 and 10 mm, or between 6 and 11 mm. The transverse pitch, perpendicular to the direction of motion of the blanket at the image forming station, may be between 7 and 8 mm, or between 6 and 9 mm, or between 5 and 10 mm.

While the provision of spaces or channels adjacent to the protrusions or holes of the external diffuser to collect the return gas is desirable, it would be alternatively possible not to provide such channels and allow sufficient distance between the diffuser and the surface of the intermediate transfer member, or to provide gas return channels in only selected areas of the diffuser.

While persons skilled in the art to which the invention pertains will readily understand that the teachings herein can be applied to the stabilization of droplets of any film forming ink on any repelling surface of an intermediate transfer member of a printing system, examples of ink formulations or transfer members that may benefit from the present invcntion are provided below.

Inks suitable for use in the method and apparatus herein disclosed are a) able of being jetted from an inlcjet print bar (e.g., having sufficiently low viscosity to be propelled from print bar nozzles without clogging); b) capable of film forming (e.g., comprising, in addition to the colouring agent, film forming resins or thermoplastic polymers able to form a skin on the surface of an ink droplet once jetted on the release layer at the temperature of the transfer member and/or of the heated gas); and c) being neatly transferable 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 solvent 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; d) the temperature of the gas blown through the dispensers of the stabilising apparatus; e) the maximal drying temperature; and 0 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 International Patent Application No. PCT/1B2013/051755, filed 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 silanc-modified or tcrminatcd polydialkylsiloxanc 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/1B20 13/051743 and PCT/1B2013/05175l, filedon March 5,2013.

As previously explained, the release layer may be pre-treated 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/IB2Ol3/000757, filed on March 5,2013.

The contents of all of the above mentioned applications of the Applicant are incorporated by reference as if ifilly 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

CLAIMS1. A method of treating ink droplets having a liquid carrier immediately after their deposition onto a relatively moving liquid repelling surface of an image transfer member at an image forming station, the method comprising blowing a stream of gas onto the surface of the transfer member, wherein the velocity of the stream is selected to minimisc disturbance of the ink droplets, the tcmpcraturc and flow rate of the stream arc sufficient to cause partial evaporation of the liquid carrier, and the duration of the exposure of the ink droplet to the stream of gas is sufficient to form a skin on the surface of the droplet while the centre of the droplets remains as a liquid, the skin serving to fix the ink droplets to the surface and prevent bleeding between successively deposited ink droplets.
2. A method as claimed in claim 1, wherein the ink droplets have an aqueous carrier and the liquid repelling surface is hydrophobic.
3. A method as claimed in claim I or 2, wherein the gas temperature lies within any one of the ranges 100°C to 250°C, 100°C to 220°C, 100°C to 200°C, and 130°C to 200°C.
4. A method as claimed in any preceding claim, wherein the temperature of the surface of the image transfer member at the image forming station, is at least 5°C, at least 10°C, or at least 20°C, or at least 30°C, or at least 40°C below the evaporation temperature of the ink carrier.
5. A method as claimed in any preceding claim, wherein the gas blown onto the surface of the image transfer member is air, flowing at a rate of at most 0.5 m3!s, or at most 0.3 m3/s, or at most 0.2 m3/s.
6. A method as claimed in any preceding claim, wherein the gas stream is blown onto the surface of the imagc transfer member by way of a diffuser positioncd at a distance of at least 4 mm, or at least 5 mm, or at least 6 mm, or at least 7 mm, or at least mm, or at least 9 mm from said surface.
7. A method as claimed in any preceding claim, wherein the gas stream is blown onto the surface of the image transfer member by way of a diffuser and wherein the distance between said surface and the diffuser does not exceed 15 mm, or 14 mm, or 12 mm, or 10mm.
8. A method as claimed in any of claims 6 and 7, wherein the diffuser is inclined at up to 300 relative to the surface of the image transfer member and the distance between the surface of the image transfer member and the closest point on the diffuser lies within anyone of the ranges 4 to 15 mm, 6 to 14 mm, 8 to 12 mm, 9 to 11 mm and 9 to 10 mm.
9. A method as claimed in any preceding claim, wherein the image forming station comprises a plurality of print bars including a first upstream print bar and a last downstream print bar, and wherein the stream of gas is blown between adjacent print bars.
10. A method as claimed in claim 9, wherein the temperature of the surface of the image transfer member at a location immediately upstream of the first print bar lies within any one of the ranges 40°C to 95°C, 60°C to 95°C, and 75°C to 95°C.
11. A method as claimed in any of claims 9 and 10, wherein the temperature of the surface of the image transfer member at a location immediately downstream of the last print bar lies within any one of the ranges 40°C to 95°C, 60°C to 95°C, and 75°C to 95°C.
12. A method as claimed in any of claims 9 to 11, wherein the temperature of the surface of the image transfer member at a location immediately upstream of the first print bar differs from the temperature of said surface at a location immediately downstream of the last print bar.
13. A method as claimed in any preceding claim, wherein the temperature of the surface of the image transfer member after exposure to the gas stream does not exceed the temperature of the surface of said member prior to exposure to the gas stream by more than 10°C, 5°C, 4°C, 3°C, 2°C, or 1°C.
14. A method as claimed in any preceding claim, wherein the temperature of the gas stream is at most 150°C, at most 110°C, or at most 80°C, or at most 60°C, or at most 40°C above the evaporation temperature of the ink liquid carrier.
15. A method as claimed in any preceding claim, wherein the temperature of the gas stream is at most 250°C, or at most 200°C, or at most 150°C.
16. A method as claimed in any preceding claim, wherein the ink droplets are first exposed to the stream of gas within 960 milliseconds, or within 480 ms, or within 240 ms, or within 120 ms, or within 30 ms of deposition on the surface.
17. A method as claimed in any preceding claim, wherein the ink droplets arc exposed to the stream of gas for a time period of between 25 and 600 milliseconds.
18. A printing system comprising a movable image transfer member having a liquid repelling surface, a plurality of print bars each for depositing onto the relatively movable surface droplets of an ink having a liquid carrier, a drying system for drying ink droplets as they are transported on the surface, and an impression station at which the ink droplets are transferred onto a substrate after passing through the drying system, the printing system characterised by a stabilising apparatus for fixing the ink droplets on the surface of the image transfer member, the stabilising apparatus comprising a source of gas, and a plenum chamber connected to the source of gas and positioned immediately after at least one print bar in the direction of movement of the surface, the plenum chamber having an outlet opening positioned to direct a gas stream onto the ink droplets, the temperature and flow rate of the gas and the exposure time of the ink droplets to the gas stream being such as to dry the surface of the droplets to form a skin while the centre of the droplets remains liquid, the skin serving to fix the ink droplets to the surface and prevent bleeding between successively deposited ink droplets.
19. A printing system according to claim 18, wherein the ink droplets have an aqueous carrier and the liquid repelling surface is hydrophobic.