GB2578118A

GB2578118A - Droplet ejection apparatus and method of printing

Info

Publication number: GB2578118A
Application number: GB1816845.0A
Authority: GB
Inventors: Cruz-Uribe Tony; Trip Renzo; Douaire Maelle; Kuban Lukasz
Original assignee: Xaar Technology Ltd
Current assignee: Xaar Technology Ltd
Priority date: 2018-10-16
Filing date: 2018-10-16
Publication date: 2020-04-22
Also published as: GB201816845D0

Abstract

A droplet ejection apparatus 1 comprising: one or more droplet ejection heads 10, fixedly mounted in the apparatus, each comprising at least one row of nozzles, each row extending along a row direction and configured to eject droplets; and a deposition media transport system which is configured to transport deposition media 30 in a transport direction x past the fixedly mounted one or more droplet ejection heads such that there is a gap G between the deposition media and the one or more droplet ejection heads. The apparatus further includes one or more gas transfer systems 20, 50 which are configured to carry out at least one of extraction of gas upstream, with respect to the transport direction, and injection of gas downstream, with respect to the transport direction, of each row of nozzles of at least a subset of the one or more droplet ejection heads. The at least one of extraction and injection of gas reduces droplet misplacement caused by gas flows in said gap. A method of printing, using such a droplet ejection apparatus, is also provided.

Description

DROPLET EJECTION APPARATUS AND METHOD OF PRINTING

The present disclosure relates to a droplet ejection apparatus which may be used with particular benefit in applications that require printing a high resolution image onto a textured or flexible surface moving at high speeds.

BACKGROUND

Droplet ejection apparatuses are now in widespread usage, whether in more traditional applications such as inkjet printing of images, or in 3D printing of objects, or other rapid fabrication techniques. Amongst other things, inkjet printheads have been developed that are capable of depositing ink directly onto paper, card, ceramic tiles or other media, with high reliability and throughput. In other applications, droplet ejection heads may be used to form elements such as colour filters in LCD or OLED displays used in flat-screen television manufacturing.

Droplet ejection apparatuses and their components continue to evolve so as to meet the requirements of ever more challenging applications, generally improving resolution and throughput at high print quality.

SUMMARY

Aspects of the invention are set out in the appended independent claims, while particular embodiments of the invention are set out in the appended dependent claims.

The following disclosure describes, in one aspect, a droplet ejection apparatus comprising: one or more droplet ejection heads, fixedly mounted in the apparatus, each comprising at least one row of nozzles, each row extending along a row direction and configured to eject droplets; a deposition media transport system which is configured to transport deposition media in a transport direction past the fixedly mounted one or more droplet ejection heads such that 30 there is a gap between the deposition media and the one or more droplet ejection heads; and one or more gas transfer systems which are configured to carry out at least one of extraction of gas upstream, with respect to the transport direction, and injection of gas downstream, with respect to the transport direction, of each row of nozzles of at least a subset of the one or more droplet ejection heads; -1 -wherein said at least one of extraction and injection of gas reduces droplet misplacement caused by gas flows in said gap.

A method of printing, using such a droplet ejection apparatus, is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now directed to the drawings, in which: Figure 1 is a schematic of a test apparatus used to demonstrate a so-called "woodgrain" printing artefact effect that the inventors have observed; Figure 2 is an illustrative print sample showing the aforementioned woodgrain effect; Figure 3 is a schematic of a droplet ejection apparatus according to an embodiment; Figure 4 is a schematic plan view of a droplet ejection head and associated components of the droplet ejection apparatus of Figure 3; Figure 5A-C shows illustrative print samples for different extraction flow rates using the apparatus according to Figure 1 and Figure 3, with Figure SA corresponding to no extraction, and Figures 5B and SC corresponding to progressively increasing extraction flow rates; Figure 6A is a schematic of droplet landing position along the deposition media transport direction in absence of gas extraction; Figure 6B is a schematic of how induced flow in the gap between the head and the media may affect droplet landing position in the droplet ejection direction; Figure 6C is a schematic of how induced flow in the gap may affect droplet landing position in the transverse direction (the transverse direction being defined as perpendicular to the droplet ejection direction and to the media transport direction); Figure 6D shows simulated stream lines propagating in three dimensions inside the gap; Figure 6E is a schematic of how gas extraction in the gap may affect droplet landing position along the media transport direction; Figure 7A is a schematic of a droplet ejection apparatus with a gas extraction and gas injection system according to an embodiment; Figure 7B is an illustration of flow in the gap for the embodiment according to Figure 7A; Figure 8A is a schematic plan view of the lower face of a droplet ejection head and a gas extraction system having more than one inlet; Figure 8B is a schematic plan view of the lower face of a droplet ejection head with internal inlets for gas extraction; Figure 8C shows an example schematic of gas extraction systems for multiple printbars; -2 -Figure 9A is a schematic cross section of a side view of a droplet ejection apparatus having a gas extraction system comprising a mesh; Figure 9B is a schematic plan view of the lower face of a droplet ejection head and gas extract inlet having several openings; and Figure 10 is a schematic side view in cross section of a droplet ejection apparatus having a gas extraction and gas injection system.

In the figures, like elements are indicated by like reference numerals throughout.

DETAILED DESCRIPTION

To assist the reader in appreciating the functionality of the various embodiments that will be presented below with respect to Figures 3-10, reference is firstly directed to a test apparatus shown in Figure 1 and an output therefrom as shown in Figure 2.

More particularly, the test apparatus is a droplet ejection apparatus 1' having a droplet ejection head 10 fixedly mounted within the apparatus and a deposition media transport system, which is configured to transport deposition media 20 in a transport direction x past the fixedly mounted droplet ejection head 10. As may be seen, the head 10 is mounted such that there is a gap G between the deposition media 30 and the head 10.

In some circumstances, a "woodgrain" effect may be experienced when operating the test apparatus. As used herein, the "woodgrain" effect is an unwanted printing artefact thought to be the result of induced air flow into the gap between a droplet ejection head and a deposition media that is being printed upon, due to the relative motion between the droplet ejection head and the media. The air flow can significantly deviate the landing position of the ejected droplets, as well as causing mist and satellites to accumulate in unpredictable locations on the media as well as on the portions of the droplet ejection head surrounding the nozzles. One visual effect may be that of an undulating "woodgrain" pattern, but the effect may result in other irregular patterns appearing visibly in the print. Woodgraining may be particularly experienced in applications that require a higher gap distance G, for example where the surface of the deposition media is rough, flexible or textured, such as with textiles or precursor sheets for packages such as cartons. -3 -

An illustration of a typical woodgrain pattern is shown in the test print sample in Figure 2. The test print sample was achieved by ejecting at full duty (all nozzles printing) using the head arrangement of Figure 1, with a nozzle spacing of 84.7 pm (300 nozzles per inch) and gap distance 3 mm. The media speed for this sample was 80 m/min and drop velocity at 1 mm distance from the nozzle plate was 6.1 m/s. The pattern that might be expected would be uniform. The actual pattern is that of a typical woodgrain effect due to main drop deviation, resulting in dark "veins" forming an irregular pattern across the image along the media transport direction.

In some applications, it is desirable to use a droplet ejection apparatus, such as that shown in Figure 1, in which the droplet ejection head is rigidly mounted and the media to be printed is passed underneath it. This is often referred to as single pass printing. In this apparatus set up, the moving media creates the induced air flow in the gap, with a profile that is characteristic to the single pass printing situation. Particularly in high gap distance applications, in which the gap may span several millimetres, and which may additionally require high resolution nozzle density and/or high throughput (high media speed) and thus high drop ejection frequency, the induced air flow needs to be carefully managed to maintain high print quality.

An alternative arrangement is that of a scanning droplet ejection apparatus, in which the droplet ejection head is moved back and forth orthogonally to the media transport direction. In this arrangement, the head travels at high speed relative to the surrounding (stationary) air, causing the air to flow around the head. As part of this flow of air around the head, some air flow is induced in the gap between the deposition media and the head, with a velocity profile characteristic to the scanning application. It will later be apparent that the conditions of air flow in such an arrangement differ from those of the single pass set up.

More generally, whether the head is moved with respect to substrate, or the substrate is moved with respect to the head, a velocity difference exists between the head and the substrate, which gives rise to air flow around the head, and/or in the gap between the head and the substrate.

It should be understood that references above and herein to "air" around the head, or in the gap between the head and the substrate, apply equally to any gas that forms the environment -4 -around the head and/or between the head and the substrate. The environment may be the ambient air atmosphere, or may be an imposed/controlled gas environment, provided for example by encasing the apparatus in a chamber that, in use, contains a desired gas (e g. an inert gas such as helium or argon).

It has surprisingly been found by the inventors that, for a droplet ejection apparatus in which the droplet ejection heads are fixedly mounted, extracting air upstream and/or injecting air downstream of the nozzles, with respect to the media transport direction significantly reduces or completely removes the visual impact of the "woodgrain" effect. This will now be explained in more detail.

In this regard, reference is first directed to Figures 3-5, which are schematic illustrations of a droplet ejection apparatus (Figure 3) according to an illustrative embodiment and close ups of its droplet ejection head and associated components (Figure 4 and 5).

Starting with Figure 3, droplet ejection apparatus 1 has a droplet ejection head 10 fixedly mounted within the apparatus, and a deposition media transport system which is configured to transport deposition media 30 in a transport direction x past the fixedly mounted droplet ejection head 10. As may be seen, head 10 has nozzles 12 extending along a row direction which, in operation, eject droplets onto the media 30. As Figure 3 shows, the head 10 is mounted such that there is a gap G between the deposition media 30 and the head 10. The apparatus 1 further comprises a gas transfer system 20, of which exemplary constituent features are described in greater detail below. Note that, in Figure 3, the gas transfer system 20 is configured to extract gas upstream of the nozzles 12a, 12b, with respect to the media transport direction. The inventors have found that, in some cases, this may result in low or negligible levels of misplacement of droplets caused by air flows in the gap.

While the apparatus 1 of Figure 3 is shown to have a gas transfer system 20 that is configured to extract gas upstream of the nozzles with respect to the media transport direction, other arrangements may include a gas transfer system that is configured to inject gas downstream, rather than upstream, of the nozzles, with respect to the media transport direction. In yet further arrangements, there may be more than one gas transfer system, for example separate gas transfer systems configured for, respectively, extracting gas upstream and injecting gas downstream. Such separate systems may, for instance, be operable to extract gas at the same -5 -time (or potentially at different times) during operation. It is also envisaged to have more than one gas transfer system for extracting gas and/or more than one gas transfer system for injecting gas; for example, for an apparatus having a plurality of heads, multiple gas transfer systems may be provided to assist with transferring gas in the vicinity of all of the heads.

The apparatus 1 of Figure 3 is shown for simplicity to have only one head 10, but in other embodiments there may be multiple heads, for example provided in an array. Such an array may include one or more rows of heads, each such row of heads extending generally in a row direction (which may be perpendicular to the media transport direction), with consecutive rows of heads (where there is more than one row) being offset in the media transport direction.

The media transport system, which is not illustrated in detail in Figure 3, may for example comprise a moving web in which flexible media are transported by rotating rollers.

Alternatively, the media transport system could comprise a moving rigid plate of a flatbed printer. In another embodiment, the media transport system may include a belt on which the media rests. More generally, the media transport system may, for example, include one or more motors that drive moveable elements, such as rollers, drive belts or the like.

Referring once more to Figure 3, in more detail the gas transfer system 20 comprises a conduit 21. Moreover, in the particular embodiment shown, the gas transfer system 20 is configured to extract gas upstream of the nozzles, with respect to the media transport direction. Such extraction may be achieved for example by use of a pump 22 moving gas in direction of the arrow as shown. In the embodiment shown, the pump 22 is an integral part of the gas transfer system. In other examples, however, the pump may connect externally to the gas transfer system. Furthermore, the gas transfer system 20 is shown to be a separate component to the droplet ejection head 10; this is also not an essential feature and in other examples the gas transfer system may be integral to the droplet ejection head 10. For instance, each head may be provided with one or more dedicated conduits that are integrated with the head; in such embodiments, each head may be provided with a dedicated pump, also integrated with the head, or each head may connect externally to one or more pumps.

It will further be appreciated that the apparatus 1 of Figure 3 may be supplied without ejection heads so that the user may implement his own choice of droplet ejection heads -6 -Accordingly, in some embodiments the apparatus may include one or more fixed mounts to which the heads may be mounted. Each mount may, for example, include one or more receiving features (such as apertures, slots, grooves, sockets or the like) for receiving corresponding connector features on one (or more) of the heads and/or one or more connector features (e.g. pins, rails, plugs or the like) for connecting with corresponding receiving features on one (or more) of the heads. In some embodiments, the apparatus 1 may include one or more rigid mounting bars, which provide the fixed mounts. Such rigid mounting bars may assist with installation and/or maintenance of the apparatus, as multiple heads may be installed or interchanged rapidly by the addition or substitution of a mounting bar that has already been stocked with heads.

Attention is next directed to Figure 4, which is a schematic view from below of the apparatus 1 shown in Figure 3 The section A-A' shown in Figure 3 is indicated with a dashed line in Figure 4.

As is apparent, in the particular embodiment shown, the droplet ejection head 10 includes several rows of nozzles 12a-12d. This may, for example, be provided by discrete parts, such as individual silicon MEMS (micro-electro-mechanical systems) chips. However, this is by no means essential and in other embodiments the head 10 could simply have one or more rows of nozzles provided by a common part, or there may be discrete actuator components having multiple nozzle rows each; the operation of the gas transfer system 20 in such embodiments would nonetheless have broadly the same effect.

Such a head with multiple nozzle rows comprised in more than one discrete parts may, for example, be used in silicon MEMS heads where the silicon chips that contain the nozzles and actuators are fragile, and it is not economical or possible to manufacture them in one piece extending along the entire head. To achieve a continuous line of printed pixels on the media along the entire head from multiple chips, nozzle rows that are spaced apart in the media transport direction (e.g. nozzle rows 12a and 12b, or nozzle rows 12c and 12d) are driven with a time delay between them so that the ejected droplets land along the same pixel row on the media.

Note, also, that the particular head 10 shown in Figure 4 comprises several nozzle plates 1 lad, which have respective rows of nozzles 12a-d, each extending along the row direction. The -7 -nozzle plates may be described as being arranged in a "staggered" format. The row direction may be aligned with the transverse direction (the transverse direction being defined as perpendicular to the droplet ejection direction and to the media transport direction), or may be arranged at an angle relative to the transfer direction.

Of course, other arrangements are possible in order to provide rows of nozzles configured to deposit droplets in the same pixel row on the media. Moreover, the operation of the gas transfer system 20 in such embodiments should be understood to have broadly the same effect.

The inventors carried out a series of tests where a droplet ejection head generally as shown in Figure 3 and 4 and configured as a printhead printed samples with and without its gas transfer system 20 being activated (causing upstream gas extraction in the manner described above). The media was paper, with the deposition media transport system being a moving web. The air was extracted using a separate extraction system mounted upstream of the nozzles to one face of a plate 40 that also presented a mount for the printhead on the opposite face of the plate, as shown schematically in Figure 3.

The experiments were carried out in an air-filled environment, with the gas extracted being air. Print tests were carried out with one row of nozzles at a media speed of 15-80 m/min and a drop ejection frequency of 12-63 kHz. The drop volume and velocity were respectively 2.9 pl at 6.1 m/s and 7.9 pl at 7.6 m/s. The ink was an aqueous black Nazdar ink with a viscosity of 9 mPas. Within the row of nozzles, neighbouring nozzles were spaced apart by 84.6 [im along the nozzle row direction. In addition, alternate nozzles were offset by an offset distance of 84.7 p.m in the media transport direction, so that the direct line distance between neighbouring nozzles was 120 p.m.

In close-ups of the printed images, shown in Figures SA-C, the resulting dots on the media from individual nozzles can be discerned along the media transport direction. Figure SA is a close up of Figure 2 and shows the undulating output per nozzle on the media that represents the woodgrain effect without air extraction.

For the test prints of Figures 5B and SC, extraction of air was added at different rates of extraction. In these experiments, the extraction conduit 21 had a rectangular inlet 25 of -8 - 1.5 mm width along the media transport direction and a length extending in the row direction y and slightly beyond the ends of the nozzle array. The inlet 25 was spaced a distance of 20 mm from the first nozzle row in the media transport direction. Figure 5B shows the effects of air extraction of 720 1/min using the arrangement of Figures 3 and 4. The woodgrain effect can still be observed, although perhaps to a lower degree. For the sample shown in Figure 5C, a higher air extraction rate of 860 1/min was used, and the woodgrain effect is no longer visible. All nozzles deposited droplets in a straight line along the media transport direction. Figures 5B and SC therefore show that the woodgrain effect may be significantly reduced, or even substantially removed, by using the gas transfer system to extract gas (in this case, air) upstream of the nozzles of the printhead in the arrangement shown in Figure 4.

Further experiments were carried out with this apparatus, again in an air environment. In a simple experimental design, three main properties of the apparatus were adjusted to result in different combinations of media speed, drop volume and air extraction rate.

Table 1 shows the results for different experiment conditions of a visual inspection of printed samples. The samples were achieved with one row of nozzles printing at full duty (all nozzles printing) with a nozzle spacing, in the row direction, of 84.7 pm (or 300 npi, 'nozzles per inch') and a gap distance of 3 mm. In Table 1, for simplicity, "Low" and "High" correspond to the following conditions: * Drop volume "Low": 2.9 pl and velocity of 6.1 m/s; * Drop volume "High": 7.9 pl and velocity of 7.6 m/s; * Media speed High was 80 m/min and Low 20 m/min; * Gas flow (air extraction) High was 860 Fmin and Low 720 Umin.

The conduit inlet dimensions were 102 mm x 1.5 mm, and the distance from the downstream conduit edge to the nearest nozzle, in the direction of media transport, was 19.4 mm. -9 -

Sample Line Media Drop Air Woodgrain distance speed volume flow perceived? (dpi) (m/min) (p1) 1 600 High High High No 2 600 High High Low No 3 600 High Low High No 4 1200 High Low High No 600 High Low Low Yes 6 1200 High Low Low Yes 7 600 Low High High No 8 1200 Low Low High No 9 1200 Low Low Low No 600 Low Low Low No 11 600 Low Low None Yes 12 1200 Low Low None Yes 13 600 High Low None Yes 14 1200 High Low None Yes 600 Low High None Yes 16 600 High High None Yes

Table 1

Addressing first the instances in which woodgrain pattern was observed, in samples 5, 6 and 11-16, it is apparent that without air extraction, for all combinations of drop volume and media speed, a woodgrain pattern could be observed by visual inspection (samples 11-16). This included "High" drop volume and "High" media speed, or "Low" drop volume and "Low" media speed.

Adding a "Low" air extraction flow, for combinations with "Low" drop volume and "High" media speed, woodgrain was still observed (sample 5, 6), but disappeared where media speed and drop volume were both "Low" (sample 9, 10) or where media speed and drop volume were both "High" (sample 2). For "High" extract flow, "Low" drop volumes in combination with "High" media speed displayed no perceptible woodgrain pattern (sample 3, 4).

-10 -These results suggest that woodgrain may progressively be removed by increasing drop volume and/or the air extract flow rate at higher media speeds. It may therefore be expected that even higher extract flows will likely manage the appearance of woodgrain also for lower drop volumes.

Without being bound by any particular theory, the inventors consider the following to be a plausible explanation for how the extracted air upstream of the nozzles may affect the air flow in the gap.

In the case of a static droplet ejection head that is rigidly mounted within the apparatus while the media to be printed is transported past the head's nozzles underneath it, the air flow created by the moving media in the direction of the media transport direction is largest near the surface of the media and smallest near the nozzle plate. As the droplets travel from the nozzles towards the media, they slow down as a result of air resistance. Accordingly, the droplets are subjected to the induced flow caused by the movement of the media at a point when they have already slowed down significantly, which makes them more susceptible to being deviated significantly by the induced flow. Therefore, the induced flow effect typically becomes particularly problematic for increased gap sizes (e.g. gap sizes of several millimetres), where droplets have to travel a significant distance to the media and slow down appreciably before fully experiencing the induced flow.

Figure 6A illustrates the effect of induced gas flow caused by movement of the media on droplet landing position in the absence of gas extraction through the conduit 21 of the gas transfer system 20. The induced gas flow in the gap G that is created by the media 30 moving in the media transport direction x is indicated by arrows in Figure 6A, with longer arrows indicating stronger gas flow. As can be seen, the droplets ejected by the head 10 are, as a result of the gas flow induced by the media motion, displaced in time from the position immediately beneath the nozzle upon ejection to the landing position on the media after travelling through the gap.

The displacement caused by the gas flow induced by the moving substrate, from here on referred to as primary flow, is considered to be uniform across the drop ejection head. This part of the displacement can be managed by knowledge of the media speed and coordinating -11 - the timing of ejection accordingly. In addition, secondary flows and the interaction thereof with the primary flow can lead to an additional displacement of the droplet, in the direction of media transport, x, or in the row direction. This part of the deviation is not possible to control by conventional measures, e.g. through timing, since the amount of deviation is non-uniform across the drop ejection head and largely unpredictable. Hence, the inventors consider that the characteristic woodgrain pattern may be due to the interaction of the induced flow caused by the movement of the media with other air flows that may be present during operation of the apparatus.

One example of such an additional air flow is that which may be caused by the ejection of droplets from the head. Each droplet, moving for example at 6m/s, drags air downwards with it, causing a downward columnar air flow. Such air columns may combine in the row direction to form a "curtain" of air around a group of neighbouring nozzles. This may particularly be the case for high resolution heads, which have smaller nozzle spacing and the columns are more densely packed.

The "air curtain" and the flow associated with it, may be exacerbated the faster the droplets are, and the higher the frequency at which the droplets are repeatedly ejected. The flow induced by the droplets impinging on the substrate, and the interaction of the air curtain with the flow induced by the moving substrate, leads to the formation of circulating eddies. This is schematically shown in Figure 6B. The strength and the extent, in the droplet ejection direction, of these eddies depend on droplet velocity and volume (weight). Eddies also occur upstream of the drop curtain and their extent depends on the flow rate of the induced flow caused by the movement of the media.

In addition, the "air curtain" is thought to present a barrier to the oncoming flow induced by the moving substrate. A contribution to the woodgrain pattern results from a flow around the edges of the droplet deposition head that finds a path of least resistance around the droplet curtain and than interacts with the droplets with a significant flow component along the row direction.

Furthermore, the upstream gap edge 27 of the head 10 and its associated components facing the media is also thought to create eddies as result of the induced air flow caused by the -12 -movement of the media. These eddies may be dragged into the gap by the induced air flow and cause further disruption to the flow in the gap.

It is further thought that the induced flow caused by the movement of the media may break through the "air curtain" set up by the droplet curtain, and, upon passing the droplet curtain, will form eddies with a circulating motion crossing the media transport direction. This is shown schematically in Figure 6C. This may occur in particular at weak points or gaps in the droplet curtain. Weaknesses in the droplet curtain, and therefore gaps in the barrier presented by the "air curtain", may occur due to non-uniformities across a row of nozzles, for example due to some nozzles producing lower droplet volumes and/or slower droplets compared to their neighbours, or due to nozzles that are non-active due to image information.

Taken alone or in combination, the different sources of eddies or vortices introduce flow components in the gap that are perpendicular to the droplet ejection direction and perpendicular to the media transport direction, and it is these components that cause the perceived "woodgrain" pattern shown in Figure 2. Woodgrain may not only be due to a deviation of the main droplet, where the main droplet signifies a droplet of or near a desired target volume. The various flows can also give rise to mist or satellites and cause these to form visible variations in density perceived as woodgrain.

To further illustrate the woodgrain effect, in Figure 6D simulated stream lines propagating in three dimensions inside the gap show the interaction of the primary air flow set up by the substrate with secondary flows like those caused by the droplet curtain at a time instance just after initiating printing. The flow is from left to right. The location of the drop curtain is indicated by the dashed line "A". Eddy-like structures can be observed at the extremities of the drop curtain, indicated by -B". The ingress of air just downstream of the drop curtain associated with these eddies is believed to be responsible for drop deviation in the cross-print direction. In time, these disturbances travel inwards towards the centre of the drop curtain, leading to vortex flows indicated by "D". It is vortices such as these that are thought to be responsible for the characteristic woodgrain pattern. The extent "C" of these structures in the print direction is related to the substrate speed. The simulation of Figure 6D was a result of substrate speed of 1 m/s, initial drop velocity 15 m/s and drop size 17 gm, at a gap size of 3 mm and a drop ejection frequency of 23 kHz.

-13 -The combined airflows due to media and droplet motion thus are thought to result in a woodgrain pattern, which in one manifestation may be associated with strong visible veins as illustrated in Figure 4, while ink mist and satellite deviation/accumulation is thought to result in a weaker pattern of smaller scale.

Without being bound by a particular theory, extraction of gas, such as air, upstream of the nozzles is thought to reduce the effect of the flow induced by the moving media. The air conduit presents a path of lower resistance compared to the gap, and becomes an alternative and preferred path for the air to take as it is drawn by the moving substrate. It may thus reduce the interaction of the primary flow associated with the moving media and secondary flows, that leads to the formation of eddies. As a result it may thus reduce or substantially remove deviation in landing position (misplacement) of droplets ejected from the nozzles both along and across the media transport direction. The droplet trajectory in the media transport direction x may be affected as shown in Figure 6E, and induced air is thought to be removed sufficiently to prevent any from breaking through the air curtain created by the droplets, or going around it and interfering with the droplets in such a way.

When gas or air is injected downstream of the nozzles with respect to the media transport direction, it is thought that a positive pressure may be built up downstream of the nozzles, and therefore downstream of the "air curtain" created by the ejected droplets. Such positive pressure is thought to reduce the pressure differential between locations upstream of the droplet curtain and locations downstream of the droplet curtain, thereby counteracting the induced air flow or at least preventing forceful eddies from forming. Therefore extracting upstream and/or injecting gas downstream has the effect of reducing or removing the effects of the induced air flow caused by the movement of the media, and the formation of eddies within the gap.

Figure 7A shows an apparatus according to a further embodiment, that provides injection of gas downstream of the nozzles, in addition to extraction of gas upstream of the nozzles Using both injection of gas downstream and extraction of gas upstream may, in some applications, allow the user to reduce the undesired flows upstream and downstream in order to reduce or remove the woodgrain effect. As may be seen from Figure 7A, in addition to the gas transfer system 20 upstream of the nozzles configured to extract gas, a second gas transfer system 50 configured to inject gas is mounted downstream of the nozzles, with -14 -respect to the media transport direction x, in the apparatus L Gas may be injected into the gap G, with a portion of the injected gas flowing in a direction opposite to the media transport direction x, and thereby reducing the net flow in the gap. In such an arrangement, the gas extraction and injection system may work cooperatively to more effectively remove the unwanted disturbances causing woodgraining.

The gas transfer system 50 for injecting gas and the gas transfer system 20 for extracting gas may both form a part of the same gas transfer system, which is configured such that the gas extracted from the gap through extraction conduit 21 may be passed to an injection conduit 51 having an outlet downstream of the nozzles, with respect to the media transport direction.

Alternatively, the gas transfer system 50 for injecting gas may be physically and/or operationally separate from the gas transfer system 20 for extracting gas; for example, the two systems may have separate pumps and/or conduits 51.

The flow directions for the extraction and injection system are further illustrated in Figure 7B, which is a plan view of the gap area and looking towards the ejection head. Gas flow into the gas extraction conduit 21 and out of the gas injection conduit 51 is indicated by the arrows. In Figure 7B, the length of the arrows are not intended to suggest relative flow rates, since these will depend on a combination of factors and the pattern of flow rates may therefore be complex.

The above embodiments are described in terms of one gas transfer system upstream and/or downstream, purely for illustrative purposes. It may be envisaged that the or each gas transfer system may comprise several subsections, for example four subsections, wherein each subsection may be addressed separately by a respective pump for example, so as to extract gas upstream of only some of the nozzles of the array. This is shown by way of example in Figure 8A, where the gas transfer system is divided into four subsections with inlets 25a-d and respective conduits 21a-d (not shown). In Figure 8A, each inlet is shown to approximately address one of the nozzle plates l la-d, although this is by no means essential.

In the example shown in Figure 8A, each inlet 25a-d may be thought of as addressing a subset of the row of nozzles formed within the head 10. For example, the row of nozzles may extend the entire length of the head, along the row direction y.

-15 -By having individual gas transfer systems arranged along the row direction, it may be possible to balance out edge effects due to the side edges of the droplet ejection head or array, causing additional eddies and/or induced air leaking out near the edges of the head and causing a different pressure profile in the gap. It will also be possible to adjust for nozzle plates being located closer to the inlet of the gas transfer system than others, as is the case in the example shown in Figure SA. Furthermore, it will in principle be possible to dynamically adjust the flow rate in each subsection, and therefore the flow velocity profile in the gap along the row direction, in response to, for example, image data.

Alternatively, multiple subsections of a given gas transfer system may be sewed by a common pump -potentially a single common pump serving all the subsections of the gas transfer system.

In the case of a single gas transfer system in which multiple subsections are sewed by a common pump, the flow rate in each subsection may be adjusted to achieve a similar effect to that of having several gas transfer systems arranged along the row direction. For example, baffles may be arranged within the conduit along the row direction. The baffles may be static to adjust the flow rate profile into the conduit inlet permanently, or they may be individually operable.

In yet another example, individual conduits arranged along the row direction may share the same pump and may similarly be baffled, statically or dynamically.

Furthermore, it is not necessary that the gas transfer system is separate from the head, as is shown in Figure 3. In an alternative arrangement, each head may be fitted with its own gas transfer system, which may comprise a pump, or which might simply comprise gas transfer conduit 21 and/or 51 that is connectable to a pump external to the head. Providing a gas transfer system as part of a head may, for example, assist in locating the gas extraction conduit(s) close to the nozzles of that head. A possible consequence is that the air flow in the gap near the nozzles can be influenced more easily and/or the flow rate may be reduced to achieve a similar effect to the external gas transfer system. It may also be appropriate to arrange more than one gas transfer system (whether for extraction or injection of gas, or both) or respective conduit(s) within the droplet ejection head, for example to allow groups of nozzle rows to have an upstream extraction inlet at substantially the same distance to the -16 -nozzle location. An example arrangement of a droplet ejection head having integral gas transfer conduits for the extraction of gas according to this principle and associated mounting component is shown by way of illustration in Figure 8B.

In plan view of the lower surface of the head 10, two neighbouring nozzle plates 11 a, c and 11 b, d share respective extraction conduits 21e and f (not shown) having inlets 25e and 251 In such an arrangement, the distance between the nozzles of a specific nozzle plate 11 and the corresponding gas extraction inlet 25 may be substantially the same. The gas flow near each nozzle row can further be adjusted by applying the principles of separate gas extraction systems, where for example the two conduits have separate flow control, by way of separate pumps or controllable baffles in each conduit, or by arranging subsections of conduit along the row direction as discussed above.

In some arrangements, one or more droplet ejection heads may be mounted to one or more fixed mounts within the droplet deposition apparatus. The droplet deposition apparatus may be supplied with one or more fixed mounts which are configured to receive one or more droplet deposition heads. In some arrangements, the one or more fixed mount(s) may be provided by a rigid mounting bar, or printbar, which in turn is fixedly mounted to the apparatus.

Figure 8C is a plan view of a rigid mounting bar 40 to which a gas transfer systems 20 is mounted on one of the mounting bar faces. On the opposite face, the mounting bar provides a mount for each of the droplet ejection heads 10. In this example, several heads 10 are mounted side by side to span the width of the media, for example, along the row direction y.

Often, several printbars are mounted one after the other to facilitate deposition of different colours within close succession, or to increase the amount of ink that may be deposited onto the media using the same ink in more than one printbar. This is illustrated in Fig SC by showing a second mounting bar 40 of identical arrangement located along the media transport direction x. The gas transfer systems 20, here shown as one configured to extract gas, may be shared or separate, such that inlets 25 may or may not share a common pump. Depending on the arrangement of the mounting system for heads and gas transfer, each gas transfer system and row of heads may be mounted to a separate mounting plate, or may be mounted to the same mounting plate 40 as shown. The order of the mounting plate, gas transfer system and -17 -heads 10 is also not essential; in some embodiments, the gas transfer system may be configured to provide the mounts for the printhead; in alternative arrangements, the gas transfer system may be integral to the mounting bar. It will be appreciated that one or more further gas transfer systems configured to inject gas downstream of the nozzles may be applied to each of the embodiments of Figures 8A-C using the same principles for arranging the gas transfer systems 20 for extracting gas upstream of the nozzles, with respect to the media transport direction, x.

In the arrangement shown in Figure 8C, the rows of heads 10 may be thought of as a subset of nozzles addressed by the gas transfer system 20. Each gas transfer system 20 may be divided into subsections, each having an inlet 25 arranged along the row direction. In this case, each head 10 may be thought of as a subset of the row of nozzles extending along the mounting plate and across the several heads in the row.

Attention is now drawn to the configuration of the gas transfer system. Each gas transfer system is arranged with one or more conduits through which the gas is channelled away from or towards the gap between the droplet ejection head and the media. Each conduit has an inlet or outlet in the vicinity of the row of nozzles through which gas exits or enters the gap, depending on whether the conduit is an extraction or an injection conduit. The rate of flow of gas near the inlet or outlet is greatly dependent on the area of the opening it presents to the flow of gas. In addition, the effect of this flow rate at the nozzle location is further affected by the extent of overlap in the row direction (or in the direction "transverse" to the media transport direction) of the area associated with the inlet or outlet with a specific subset of the droplet deposition head.

Each of the gas transfer systems, for example when configured to extract gas upstream of the nozzles, and associated extraction conduits, may be configured such that gas may be extracted at a greater rate with increasing distance downstream across the extraction conduit inlet(s), with respect to said transport direction. Likewise, when configured to inject gas downstream of the nozzles, associated injection conduits of gas transfer systems may be configured such that gas may be injected at a greater rate with increasing distance upstream across the conduit outlet(s), with respect to said transport direction.

-18 -More specifically, an extraction conduit may be configured such that there is a decrease of impedance to the flow of gas therethrough with increasing distance downstream across the conduit inlet, with respect to said transport direction. Likewise, an injection conduit may be configured such that there is a decrease in the impedance to the flow of gas therethrough, with increasing distance upstream across the conduit outlet, with respect to the media transport direction.

In some embodiments, to manage the flow distribution, the inlet and/or outlet of the conduits may be configured with a mesh that extends across the area of the inlet or outlet, where the mesh has openings through which extraction or injection occurs. Such an arrangement is shown in Figure 9A for a droplet ejection apparatus 1 having a gas transfer system 20 configured to extract gas and having an extraction conduit 21. The inlet of the conduit 21 is covered with a mesh 24. The mesh 24 has openings through which gas is extracted from the gap between the head 10 and the media 30. Provision of a mesh over an increased conduit inlet area, in comparison to that of Figure 3 for example, extending over the inlet of the extraction conduit 21 increases the area within the gap over which the induced air flow due to the media speed may be reduced. The filter mesh in turn reduces the total area over which air is extracted through the openings, to maintain a similar extraction rate per unit area. In this way, as indicated by the air flow arrows, fast moving induced air may be removed more efficiently by the openings in the conduit that are furthest away from the upstream edge 27 of the gap, thereby reducing the amount of induced air that meets the droplet curtain.

According to current theory, and as indicated by the flow rate of induced air in Figure 6A, the flow rate decreases linearly from the surface of the media to the lower surface of the droplet ejection head. Induced air flow in Figure 9A is indicated by dashed arrows, where the arrows in close proximity to the media 30 represent fast moving induced air, while the arrows further away from the media, along the droplet ejection direction, represent slow flow of induced air. Note that the air flows in the gap in a direction opposite the media transport direction are not shown in this illustration, for simplicity.

Alternatively, a similar effect to that of the mesh may be achieved by arranging separate ducts within the extraction conduit. To achieve a varying rate of extraction, the area of the inlets of the ducts may be suitably adjusted, or flow restrictions such as baffles may be used -19 -where extraction of gas is achieved by a shared pump. Alternatively, separate pumps for each duct may be used.

In Figure 9A, the inlet is shown to be the widest part of the extraction conduit 21. This does not indicate a necessity for the conduit to widen towards the inlet in order to accommodate a mesh or in order to extract different rates of induced air flow at different locations in the mesh. However in some arrangements, a widening conduit that widens towards the inlet may be beneficial, and overall a wider inlet in the direction of media transport, x, may be beneficial in removing a high volume of fast moving induced air more effectively.

For example, the area of the openings may be adjusted depending on their location along the direction of media transport, x, such that openings positioned closer to the nozzles have a relatively larger area, with respect to other openings in the mesh, through which gas can be transferred. Such an opening having a relatively larger area compared to any other openings of the mesh would pose a lower impedance to air flow compared to that of inlets of relatively smaller area, and therefore may allow extraction of a larger volume of fast flowing induced air. Alternatively, or additionally, a decrease in impedance of some of the openings may/will allow a faster rate of extraction in the mesh by being able to run the pump at higher speed and apply a higher vacuum to the conduit.

The same principles described above apply equally to the injection outlet. A mesh arranged to extend across the injection outlet may have openings that pose a lower impedance to the flow of gas with increasing distance upstream, with respect to the media transport direction (i.e. a lower impedance at distances closer to the nozzles). Such openings of lower impedance may for example inject air through a larger associated area relative to other openings of the mesh.

Returning to a gas transfer system for the extraction of gas, an example of an inlet of extraction conduit 21 having different sized openings 26 is shown in Figure 9B. Figure 9B illustrates how the area of openings may be adjusted by dividing the inlet area into three elongate parallel openings 26a,b,c, where opening 26a upstream from opening 26c, with respect to the media transport direction, x, is smaller in area with respect to opening 26c. The area of opening 26b is intermediate to the areas of openings 26a and 26c. In this way, the impedance to the flow of gas decreases along the media transport direction. The dashed -20 -arrows again indicate the flows of induced air, with longer arrows representing a higher speed of induced flow. By "mesh" any suitable division or variation of the area and/or flow impedance of the inlet of conduit 21 of may be envisaged, whether by elongate slots or arrays of openings having mechanical separation of equal thickness between the openings, or by openings for which the mechanical separation between them is varied so as to vary the impedance to the flow of gas. It will therefore be appreciated that Figure 9B merely illustrates one example of how to achieve a division of area and/or flow impedance of the inlet 21. It will further be understood that the same principle applies to the outlet of injection conduit 51 and any division or variation of the outlet area and/or outlet flow impedance, such that the impedance to the flow of gas is reduced across the opening in a direction opposite to the media transport direction.

Turning to Figure 10, a droplet ejection apparatus 1 of a further example implementation has both a gas transfer system 20 for the extraction of gas and a gas transfer system 50 for the injection of gas. In these gas transfer systems, a mesh 24 extends across both the inlet and outlet of; respectively, the extraction conduit 21 and injection conduit 51. The dashed arrows indicate the expected locations within the meshes 24, with respect to the media transport direction x, for extracting or injecting different portions/volumes of the induced air flow in the gap. Note that the air flows in the gap in a direction opposite the media transport direction are not shown in this illustration, for simplicity.

In Figures 9 and 10, extraction of gas occurs upstream of all the nozzles, but this is not essential. In other embodiments, extraction might be upstream of only a subset of the nozzles. The same applies to the injection of gas in Figure 10, which is not essential to occur downstream of all the nozzles but may only occur downstream of a subset of the nozzles.

In a further aspect, an additional although optional component that may further enhance the gas transfer effect and reduce the occurrence of eddies near the perimeter of the gap is that of a gap extender. Such a gap extender 61 is illustrated in the droplet ejection apparatus 1 of Figure 7A. Droplet ejection head 10 comprises a generally planar surface in which the corresponding nozzles 12 are formed; and further has a gap extender 61 located upstream of the head 10. The gap extender has a generally planar surface that lies in substantially the same plane as the generally planar surface of the head. One of the functions of the gap extender is that of prolonging the extent of the gap upstream of the gas extraction conduit 21 -21 -so as to move the occurrence of any eddies due to the upstream gap edge 27 away from the air curtain. In addition, any discontinuities within or near the surface that present an obstruction to the flow of gas within or near the gap are likely to cause eddies that disrupt the flow in the gap. Therefore it is preferable that the lower surface of the head 10 and its associated components facing the media are generally planar, without such disruptions.

Disruptions could be edges, gaps or protrusions etc. In this light the leading edge of the gap extender should be designed for smooth flow as it splits the impinging air stream, thereby reducing the formation of eddies.

A gap extender may work equally with other arrangements that employ gas extraction upstream of the nozzles with respect to the media transport direction, for example with the one shown in Figures 3 and 4. Returning briefly to Figures 9A-B and 10, the widening conduits ending in an inlet and/or outlet may be thought of as acting as gap extenders in addition to having a mesh extending across the inlet and/or outlet. The wider inlet, as seen in the media transport direction, moves the upstream gap edge 27 further away from the nozzles. Similarly, a wider outlet, in the media transport direction, moves the downstream gap edge further away from the nozzles. In other examples, where the mounting plate to which the head is mounted is located upstream of the nozzles, for example upstream of the inlet, a gap extender may be presented by the lower surface of the mounting plate. Some benefit of the gap extender may also be experienced in the arrangements shown in Figures 3 and 8, where the mounting plate 40 is located between the inlet and the nozzles.

A gap extender may also be beneficial downstream of the outlet of the gas injection conduit 51. This is optional, and is shown as gap extender 62 in Figure 7A. The presence of a gap extender 62 downstream of the outlet may increase the flow resistance of the gap downstream of the injection conduit and thereby reduce the relative flow resistance in the gap upstream of the outlet. The gap extender therefore may facilitate a higher injected flow rate into the gap in a direction opposite the media transport direction, and may be expected to keep the flow within the gap more stable by moving the occurrence of eddies away from the droplet curtain.

The presence of a visible woodgrain pattern may be particularly felt at gap distances G higher than a conventional 1 mm, for example at gap distances G of 2.5 mm to 10 mm. In some applications, such as packaging or textiles, the gap distance may be 3 mm. In yet other applications where the media is a highly textured surface, the gap distance may need to be -22 -higher than 3 mm. In addition, a contributing property is that of high media speed. A high media speed may range from 30 m/min to 120 m/min, or from 50 m/min to 100 m/min, or from 80 m/min to 100 m/min The woodgrain pattern may be exacerbated by other factors such as drop volume and the rate of flow or extraction/injection of gas upstream/downstream of the nozzles with respect to the media transport direction. The skilled person will, with little experimental burden, be able to adjust flow rates of extraction and/or injection of gas, such as air, to find an improved condition that reduces the appearance of woodgrain patterns for a given application that dictates the specific parameters of gaps distance, media speed and drop volume, for example.

Detailed embodiments have been described above, together with some possible modifications and alternatives. As those skilled in the art will appreciate, a number of additional modifications and alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein.

Reference to a gas transfer system is intended to address air or other gas environments of the printer. It will be apparent to the skilled person that the gas used within the printer environment does not alter the essence of the described embodiments.

Careful adjustment of the distance of the extraction or injection conduit to the nozzle row(s), gas flow rates and droplet ejection properties in combination is expected to lead to improved results for given media speeds, throw distance and the resolution (nozzle distance) of the droplet ejection head.

The media surface property may further alter the conditions for which reduced woodgrain effect may be obtained. For example, rough media surfaces and/or absorbent surface such as those of textile may be less affected than smooth and less absorbent surfaces such as board used in packaging for example. It will be apparent to the skilled person that the actual combination of gap size, air extraction, droplet volume and velocity, media speed, drop ejection frequency etc. will be specific to the media, the required image resolution and the native resolution/nozzle spacing of the droplet ejection head.

-23 -The pump may be shared between more than one head. Each head may comprise one or more gas transfer systems, each with its own pump, or one or more gas conduits to be connected to, for example, a pump external to the head. In some implementations having both gas extraction upstream and gas injection downstream of the nozzles, with respect to the media transport direction, the one or more extraction conduit may communicate with and transfer gas to the one or more injection conduit such that the extracted gas is channelled from the inlet via the injection conduit out of the injection outlet.

In the example of Figure 3, the head 10 and the gas transfer system 20 are mounted to opposing faces of a mounting plate 40. However, in alternative embodiments, the gas transfer system may be mounted to the mounting plate and the head may be mounted to the gas transfer system.

Figure 4 shows the extraction conduit 21 as extending the length and up to the ends of the array provided by the four nozzle rows of nozzles 12a-d. This may result in uneven air flow at the ends of the array. Thus, in alternative embodiments, the conduit may extend beyond the ends of the array to alleviate or remove this effect.

Implementation of the gas transfer system can be external to the printhead, can apply over the entire or part of the extent of the array of nozzles, may be provided integrally to the head, be provided as a separate system altogether or as part of the mounting plate. It may also be designed to allow retrofitting to the head or the printbar, for example in the form of a hollow sleeve around the cover of the printhead to form the conduits.

The conduits may be arranged such that the gas flow direction is parallel to the droplet ejection direction; however this is not essential. Instead, it may be of benefit to angle the conduit, or at least a part of the conduit near the inlet/outlet of the gas transfer conduit, towards the nozzles. Such arrangements might better direct the gas flow from / into the gap between the droplet ejection head and the media.

-24 -

Claims

CLAIMS1. A droplet ejection apparatus comprising: one or more droplet ejection heads, fixedly mounted in the apparatus, each comprising at least one row of nozzles, each row extending along a row direction and configured to eject droplets; a deposition media transport system which is configured to transport deposition media in a transport direction past the fixedly mounted one or more droplet ejection heads such that there is a gap between the deposition media and the one or more droplet ejection heads; and one or more gas transfer systems which are configured to carry out at least one of extraction of gas upstream, with respect to the transport direction, and injection of gas downstream, with respect to the transport direction, of each row of nozzles of at least a subset of the one or more droplet ejection heads; wherein said at least one of extraction and injection of gas reduces droplet misplacement caused by gas flows in said gap.
2. A droplet ejection apparatus according to Claim 1, wherein each droplet ejection head comprises a respective one or more of said gas transfer systems.
3. A droplet ejection apparatus comprising: one or more fixed mounts, which are configured to receive one or more droplet ejection heads, each comprising at least one row of nozzles, each row extending along a row direction; a deposition media transport system which is configured to transport deposition media in a transport direction past the fixedly mounted one or more droplet ejection heads, such that there is a gap between the deposition media and the one or more droplet ejection heads, and one or more gas transfer systems which are configured to carry out at least one of extraction of gas upstream, with respect to the transport direction, and injection of gas downstream, with respect to the transport direction, of each row of nozzles of at least a subset of the one or more droplet ejection heads; wherein said at least one of extraction and injection of gas reduces droplet misplacement caused by gas flows in said gap.
-25 - 4. A droplet ejection apparatus according to Claim 3, further comprising one or more rigid mounting bars, which provides at least some of, and preferably all of, said one or more fixed mounts.
5. A droplet ejection apparatus according to Claim 1, wherein each of said one or more droplet deposition heads belongs to said subset.
6. A droplet ejection apparatus according to any preceding claim, wherein said at least one of extraction and injection of gas comprises extraction of gas; wherein the one or more gas transfer systems comprise one or more extraction conduits, each extraction conduit having an inlet with an associated area, through which said extraction of gas occurs; wherein each head is configured to eject droplets in an ejection direction, a transverse direction being defined perpendicular to said ejection direction and to said transport direction; 15 and wherein, with respect to said transverse direction, the row of nozzles for each droplet deposition head in said subset overlaps with the area of at least one of said inlets.
7. A droplet ejection apparatus according to Claim 6, wherein said gas transfer systems and said extraction conduits are configured such that gas is extracted at a greater rate with increasing distance downstream, with respect to said transport direction.
8. A droplet ejection apparatus according to Claim 6 or Claim 7, wherein each of at least a group of said extraction conduits is configured such that there is a decrease of the impedance to the flow of gas therethrough with increasing distance downstream, with respect to said transport direction.
9. A droplet ejection apparatus according to Claim 6, wherein the inlet of each extraction conduit is elongate parallel to said transverse direction.
10. A droplet ejection apparatus according to Claim 6 or Claim 9, wherein a mesh extends across each inlet, said mesh providing a plurality of openings through which said extraction occurs.
-26 - 11 A droplet ejection apparatus according to Claim 10 when dependent upon Claim 8, wherein, for each of said group of extraction conduits, the openings are configured to provide said decrease in impedance.
12. A droplet ejection apparatus according to any preceding claim, wherein said at least one of extraction and injection of gas comprises injection of gas; wherein the one or more gas transfer systems comprise one or more injection conduits, each injection conduit having an outlet with an associated area, through which said injection of gas occurs; wherein each head is configured to eject droplets in an ejection direction, a transverse direction being defined perpendicular to said ejection direction and to said transport direction; and wherein, with respect to said transverse direction, the row of nozzles for each droplet deposition head in said subset overlaps with the area of at least one of said outlets.
13. A droplet ejection apparatus according to Claim 12, wherein said gas transfer systems and said injection conduits are configured such that gas is injected at a greater rate with increasing distance downstream, with respect to said transport direction.
14. A droplet ejection apparatus according to Claim 12 or Claim 13, wherein each of at least a group of said injection conduits is configured such that there is a decrease in the impedance to the flow of gas therethrough, with increasing distance downstream with respect to the media transport direction.
15. A droplet ejection apparatus according to Claim 12, wherein the outlet of each injection conduit is elongate parallel to said transverse direction.
16. A droplet ejection apparatus according to Claim 12, wherein a mesh extends across each outlet, said mesh providing a plurality of openings through which said injection occurs.
17. A droplet ejection apparatus according to Claim 16, wherein for each of said group of injection conduits, the openings are configured to provide said decrease in impedance.
-27 - 18. A droplet ejection apparatus according to any one of claims 12 to 15, when dependent upon Claim 6, wherein said extraction conduits communicate with and transfer gas to said injection conduits.
19. A droplet ejection apparatus according to any preceding claim, wherein each head is configured to eject droplets in an ejection direction, a transverse direction being defined perpendicular to said ejection direction and to said transport direction; and wherein the row of nozzles for each of said one or more heads extends parallel to said transverse direction.
20. A droplet ejection apparatus according to any preceding claim, wherein each head comprises a generally planar surface in which the corresponding nozzles are formed; and further comprising a gap extender located either upstream or downstream of said one or more heads and having a generally planar surface that lies in substantially the same plane as the generally planar surface of each head.
21. A droplet ejection apparatus according to any preceding claim, wherein each head is configured to eject droplets in an ejection direction, wherein the gap, in said ejection direction, between said one or more droplet deposition heads and said deposition media transport system is at least 2 mm.
22. A droplet ejection apparatus according to any preceding claim, wherein said deposition media transport system is configured to transport said deposition media at a speed of at least 10 m/min, and preferably at least 20 m/min.
23. A droplet ejection apparatus according to any preceding claim, wherein said deposition media transport system is configured to transport said deposition media at a speed of at most 100 m/min, and preferably at most 80m/min
24. A method of printing, using a droplet ejection apparatus according to any preceding claim.-28 -