FIELD OF THE DISCLOSURE

The present disclosure relates to an indirect inkjet printing system. BACKGROUND

There has previously been proposed by the present applicant, see for example WO2013/132418, a printing system in which, at an image forming station, an aqueous ink is jetted onto an endless belt or drum that serves as an intermediate transfer member (ITM). 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 image forming station to commence a new printing cycle.

Certain problems have been encountered during operation of such a printing system to which the solution has been found to be the blowing of a gas (air) stream through the gap traversed by the ink droplets from jetting nozzles of print heads mounted on a print bar to the surface of the ITM. These problems are briefly explained below:

First, the ITM is operated at an elevated temperature and the ink droplets start evaporating on impacting the ITM. The released water vapour then condenses on the cooler print heads and forms droplets, which eventually drip onto the ITM to damage the printed image. Preventing such condensation requires a fast gas stream and, because of the turbulence that it creates, such a stream can only be applied intermittently during periods when no jetting of ink is taking place, such as between pages or between print runs.

Second, when a droplet is jetted by a printing nozzle, it is often followed, a short time after it has separated from the printing nozzle, by a much smaller droplet, referred to as a satellite. Being emitted sequentially, the droplets and their satellites do not fall on the same point on the ITM and therefore result in some image dots on the substrate having a faint shadow caused by their satellites. To overcome this problem, it has been proposed to blow a constant steady laminar stream through the gap between the ITM and the print heads. The effect of this stream is to carry all droplets in the direction of movement of the ITM. However, because of their size, the smaller satellites are more strongly affected by the gas stream than the larger droplets and if the stream speed is carefully selected, the large droplets and the satellites merge into one another on reaching the surface of the substrate.

In the following description, the laminar stream for avoiding satellites is referred to as the low speed stream and the turbulent stream for dislodging condensation from the jetting heads is referred to as the high speed stream. Furthermore, the sources for supplying these two gas streams will be referred to as high pressure and low pressure supplies but the terms "low" and "high" are used only to distinguish the stream and supplies from one another.

The present disclosure seeks to provide a manifold that is capable of delivering both types of gas stream into the small gap at the image forming station between the print heads and the ITM.

SUMMARY

According to the present disclosure, there is provided a manifold for introducing gas into a gap between a print head and an intermediate transfer member (ITM) of an indirect inkjet printing system, the manifold having a first gas flow path terminating in a first discharge mouth for delivering a continuous low speed gas stream and a second separate gas flow path terminating in a second discharge mouth, vertically spaced from the first discharge mouth, for intermittently delivering into the gap a high speed gas stream.

The invention is predicated on the realisation that even though the gap between the print heads and the ITM is very small, typically one 1mm to 2 mm, one needs to use two separate discharge mouths for the two gas streams and different gas flow paths must be used to conduct the two gas streams, because the two gas flow paths must fulfil different criteria.

In the case of the gas flow path supplying a low speed steady gas stream, it is important for it to be designed to produce streamlined flow that is even across the full width of the print bar carrying the different print heads.

In the case of the high speed gas flow, on the other hand, the flow should not be streamlined. Furthermore, equal distribution across the width of the print bar is not only inessential, but it is undesirable. A high speed gas flow causes a drop in pressure and if the pressure is dropped across the entire width of the print bar at the same time, it can cause the ITM to lift off its support surface.

In some embodiments of the invention, therefore, the gas flow path conducting the high speed gas is divided into a plurality of discrete branches and high speed gas is not made to flow through all the branches simultaneously.

Thus while the entire mouth delivering low speed gas may be connected to a common single plenum chamber of the manifold that is connected at all times during use to a source of gas at relatively low pressure, the mouth delivering high speed gas may be divided into regions each connected to a different respective plenum chamber that is only intermittently connected to a relatively high pressure gas supply.

In some embodiments, the manifold may comprise a block that, in use, is directly secured to a print bar that carries the print heads.

Each of the branches conducting high speed gas may comprise a plenum chamber connected to a supply of gas at high pressure and a buffer chamber intermittently connected to the latter plenum chamber by way of a respective valve, each of the buffer chambers being connected to a respective region of the second discharge mouth of the manifold.

In an embodiment, the two mouths of the manifold are defined by a top plate, a bottom plate and an intervening spacer that are secured to an underside of the block, the first discharge mouth, for the low speed gas, being defined between the top plate and the bottom plate and the second discharge mouth, for the high speed gas, being defined by groves in the upper surface of the top plate and the underside of the block.

The spacer may be shaped to define divergent channels each leading from a respective hole in the block, connected to the single plenum chamber of the first flow path, to the first discharge mouth.

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 perspective view of an assembled manifold secured to a print bar, Figure 2 is an exploded view of the manifold of Figure 1 while still secured to the print bar,

Figure 3 shows a section through the manifold and part of the manifold when viewed from below,

Figure 4 is an exploded view showing the block of the manifold and plates secured to its underside to define the mouths for discharge of the low and high speed gas streams, and

Figure 5 is a similar exploded view to that of Figure 4 but showing the manifold from the side facing to the print bar.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Figure 1 shows a print bar 10 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, an aqueous ink is jetted onto the surface of the ITM by print heads (not shown) mounted on the print bar 10. 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 10 to commence a new printing cycle.

The print bar 10 forms part of a carriage (not shown) that is supported by rollers 12 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.

A set of individual print heads (not shown) is secured to one side of the print bar 10, while a manifold 14 of the present disclosure is secured to its opposite side. The purpose of the manifold 14 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.

Referring to Figure 2, the manifold 14 is formed of a rectangular block 16 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 18 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 20 and a sponge layer 22 to prevent condensation on the cover surface. A top plate 24, a bottom plate 26 and a spacer 28, best seen in the exploded views of Figures 4 and 5, 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.

The single plenum chamber 30 for the low pressure gas used to deliver the low speed gas stream is formed by a single channel seen in Figures 2 and 4 and in section in Figure 3) that extends across the full width of the manifold 14. The plenum chamber 30 is connected to a supply of gas under low pressure (for example 0.5 bar) by a connector 32. Small vertical holes 34 in the manifold block 16 and the top plate 24 (not shown in the block but visible in the top plate 24) allow gas from the plenum chamber 30 to pass to the low speed discharge mouth of the manifold, defined between the top plate 24 and the bottom plate 26 which are separated by the spacer 28 (seen in Figure 4). The spacer 28 has a saw-tooth shaped edge that, together with depressions formed in the top surface of the bottom plate 26, 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.

Gas at high pressure, for example at a pressure of 3 to 6 bar, is fed, through respective connectors 42, into four separate second plenum chambers 40 defined by the block 16 and the cover plate 18. Each of the second plenum chambers 40 is connected by a respective valve 44, and vertical holes (not shown) within the block 16, to a respective buffer chamber 46 that is arranged on the opposite side of the block 16 from the plenum chamber 40. The buffer chambers 46 are closed off by a cover and can be seen in Figures 3 and 5. Pressurised gas from the buffer chambers 46 passes through further vertical holes in the block 16 that open onto grooves in the top plate 24, as best shown in Figure 4. The upper surface of the top plate 24 together with the bottom surface of the block 16 form the second discharge mouth of the manifold 14, from which high speed gas is intermittently delivered into the gap between the print nozzles and the ITM.

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.

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 24 is resisted not by the top plate alone but by a sandwich consisting of the top plate 24, the bottom plate 26 and the spacer 28 between them. This sandwich, which is screwed to the underside of the block 16 can have a combined thickness approaching 4mm and can therefore readily withstand the high pressure in the buffer chamber 46. The low speed gas is discharged from between the top plate 24 and the bottom plate 26 but the latter can readily withstand the low pressure without buckling.

In use, low speed gas is constantly discharged from the mouth defined between the top plate 24 and the bottom plate 26 and the plenum chamber 30 is constantly at the pressure of the low pressure gas supply. The plenum chambers 40, on the other hand are permanently connected to the high pressure gas supply but are isolated from the buffer chambers 46. Intermittently and individually, the second plenum chambers 40 are connected to their respective buffer chamber 46 by briefly opening the associated valves 44. This results in a volume of gas being transferred into the buffer chamber 46 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.

The valves 44 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 46 at the same time.

While the invention has been described by reference to only one embodiment, it will be clear to the person skilled in the art that various modifications may be made to the design of the manifold without departing from the scope of the invention as set out in the appended claims.