Title: Printing Apparatus and a Method of Printing

Field of the Invention

This invention relates to printing apparatus, and more particularly to apparatus which prints using the projection of droplets of liquid. The invention also relates to a method of printing.

Background to the Invention

A well known and widely used type of printing apparatus is the inkjet printer. Such a printer creates a printed image by firing discrete droplets of ink from a drop firing chamber onto the surface to be printed. The through-put time of an inkjet printer is restricted by the time taken to refill the droplet firing chamber before the next droplet can be emitted. In addition, most of the kinetic energy needed to project the droplets onto the substrate has to be derived from the firing of the droplets from the chamber.

It has been proposed to provide an alternative type of printing apparatus in which the droplets are released from a rotating cylinder onto the substance to be printed. In this case, most if not substantially all of the kinetic energy needed to project the droplets onto the substance is already present by virtue of the rotation of the cylinder, so that the droplets do not need to undergo rapid linear acceleration and changes of acceleration to which a drop being ejected from a conventional inkjet printing head is subjected. The cylinder type of printer also has to potential for providing a greater through-put.

An example of such a printer is shown in WO98/16389, in which droplets are formed on a rotating cylinder having a hydrophilic surface coated with a hydrophobic layer which is perforated to define an array of cavities.

However, droplets released from the cylinder will be projected along a tangent to the cylinder at the point of release. Accordingly, the control of the point of release of each droplet is critical to ensure that the droplet hits the right part of the surface to be printed.

To that end, it is desirable for the droplets held on the cylinder to be at their balancing point, at state at which the droplets are only just held onto the cylinder by means of surface tension in the ink and the adhesive sources between the droplets and a portion of the cylinder with which they are in contact, so that only a small change in energy of a given droplet can release it from the cylinder.

However, droplets which are near to the balancing point will assume a generally well-shaped profile which will therefore give the droplet a greater surface area than a droplet which simply has a convex free surface. This increase in surface area increases the surface tension which tends to resist the release of the droplet, and as a result makes it harder for the balancing point for the droplet to be achieved.

Summary of the Invention

According to a first aspect of the invention, there is provided printing apparatus comprising a rotary member, having an array of formations, each defining a respective liquid accumulation site, liquid feeding means for feeding liquid to the member to form corresponding accumulation of liquid at said accumulation sites, release means for causing droplets to be selectively released from the sites, as the member rotates, to form a predetermined pattern on a substrate, wherein the surface of the member separating the sites is wetable by the liquid applied by the liquid feeding means to enable the formation of a film of liquid from which liquid is drawn to the accumulation sites.

It has been found that the invention enables the balancing point for the accumulations of liquid to be relatively easily and reliably reached so that droplets of ink can be released from the accumulations at reasonably precisely determined times (and positions of the rotary member).

A liquid forming on a continuously wetted rotating surface is subject to two main forces, surface tension, which acts to minimise the surface area of the liquid, and the angular acceleration acting on the mass of the liquid. According to this invention the relative effect of these two forces on the liquid may be modified by local topological features in order to form accumulations of liquid at specific locations and wherein these accumulations require only a small amount of additional energy or change in surface tension to release a free drop from the rotating surface. This additional energy or change in surface tension may be controlled to allow printing to be achieved by releasing drops selectively from a rotating surface.

One advantage of having a continuous liquid film connecting accumulations of liquid rather than discrete islands of liquid on the rotating member is that the pressure and volume of connected accumulations is equalised by the migration of liquid under surface tension between the accumulations, thereby producing accumulations of consistent volume and pressure irrespective of local variations in the deposition of liquid (by the feeding means) onto the surface or local variation in the release of liquid from the surface.

Preferably, surface of the member and the formations are hydrophilic.

Each formation may comprise a protuberance on the surface of rotary member, but preferably comprises a respective well in the rotary member.

It has been found that a pit or well on the surface of the rotating member has a bi-stable profile dependent on size, depth and shape of the well, the angular acceleration of the surface, the density of the liquid and the surface tension of the liquid. This is because up to a certain 'weight' (that is mass times net angular acceleration) of liquid the liquid in a well can be supported by surface tension with a concave surface over the well (which minimises surface area). However, if the weight relative to surface tension of the liquid is increased it reaches a point where the liquid flows out of the well to form a convex surface in the well even through this leads to an increase in the surface area of the liquid and a large change in the volume of liquid in the volume defined by the well. By selecting appropriate topological properties of the well, surface tension of the liquid and angular acceleration, a balancing

point can be achieved, where only a small amount of additional energy or change in surface tension of the liquid can change the well from a concave to a convex state.

The liquid will tend to fill the wells which will constrain the liquid so that there is a mass of liquid which contributes to the initial force of liquid tending to eject a droplet without contributing significantly to the opposing effect of surface tension.

Preferably, the rotary member comprises a cylinder, the wells being provided in the circumferential surface of the cylinder.

Each well may to advantage be provided with a respective protuberance for defining an exit point for a liquid droplet expelled from the well. This reduces any uncertainty in the positions at which droplets are released from the wells and thus also improves the accuracy with which the released droplets are projected onto the substrate.

The bi-stable transition of the liquid in the well from concave to convex may or may not be sufficient to cause the liquid contained in a well to be released from the surface, depending on the volume of liquid displaced, the rheological properties of the liquid and the excess energy put into the system. In some circumstances the liquid may simply come out of the well onto the adjacent surface. However, a protuberance placed within the well (and preferably having a height higher than the surrounding surface) may make it energetically favourable for liquid displaced from the well to travel along the protuberance and for a droplet of liquid to be released from the protuberance. A protuberance of this kind has the additional advantage of constraining the surface location of release of the droplet.

Preferably, each protuberance extends radially from the centre of the base of its associated well.

Preferably, in this case, each well is substantially annular.

Preferably, each protuberance is cylindrical.

Each well may conveniently be of the order of 45μm in diameter and 5μm deep, the associated protuberance being 5-10μm high. Alternatively, where the well does not have an associated protuberance, the well may be deeper and narrower, for example 20μm in diameter and 20 μm deep.

The rotary member may to advantage include further protuberances spaced from the wells for providing exits sites for any excess liquid applied to the rotary member by the liquid feeding means.

These further protuberances may act as 'valve protuberances' that provide preferential droplet release sites so that any excess liquid deposited onto the surface is quickly removed. This may be achieved for example by these 'valve protuberances' being taller than the protuberances associated with the liquid accumulation sites, excess liquid thereby being drawn preferentially to these sites where the excess liquid accumulates to form droplets that are released spontaneously from the surface. In this way the volume (and therefore internal pressure) of the liquid remaining on the surface is maintained at a consistent level at the point during the rotation of the member where droplets are to be selectively released, thereby improving the consistency of volume of released droplets and the quality of printing. These additional valve protuberances may be evenly distributed over the surface of the rotating member, alternatively they may be concentrated where it is convenient to collect the excess liquid, for example evenly distributed around the circumferences at the ends of a cylinder, where printing takes place from accumulation sites over the central section of the cylinder.

This feature enables the liquid feeding means continuously to feed ink without overloading the surface of the cylinder. In addition, wells which are already filled and from which ink has not been discharged in the previous revolution can have at least some of the ink therein replenished so that the ink does not dry or become unduly viscous.

The liquid feeding means may to advantage be operable to project liquid onto the rotary member. For example, the feeding means may comprise one or more spray heads for spraying liquid onto the rotary member.

Such feedings means are believed to be particularly effective at providing a continuous film of liquid on the cylinder in a controlled manner.

Preferably, the apparatus includes receiving means for receiving excess liquid applied to the rotary member by the liquid feeding means, before the formations pass the droplet release means.

In this case, the receiving means may comprise a receptacle positioned adjacent, but radially spaced from, the portion of the course of the formations of the rotary member after the ink feeding means and before the droplet release means.

Thus, the accumulation sites are loaded with liquid by the liquid feeding means, then move into registry with the receptacle, before acted on (selectively) by the droplet release means.

Preferably, the rotary member comprises a cylindrical layer of fused silica.

Preferably, the rotary member also carries a metallic coating, for example of gold, etched or perforated to define the wells (and preferably) also the protuberances.

Preferably, the droplet release means comprises one or more sources of electromagnetic radiation for energising the liquid at the selected accumulation sites.

In one example, the source comprises a laser.

According to a second aspect of the invention, there is provided printing apparatus comprising a rotary member having an array of wells, each defining a respective liquid accumulation site, liquid feeding means for filling the wells and release means for causing droplets of liquid to be released from selected accumulation sites to form a predetermined pattern on a substrate, where in the surface defining each well attracts said liquid so as to enable the well to be substantially completely filled by the liquid.

According to a third aspect of the invention, there is provided a method of printing comprising the steps of rotating a rotary member having an array of formations, each defining a respective liquid accumulation site, applying liquid to the area of the member on which the accumulation sites are carried, thereby to create thereon a film of liquid that spans the spaces between the sites, allowing liquid to be drawn through the film to the sites to form accumulations of liquid, causing droplets to be released selectively from the sites such that the droplets are projected towards a substrate, spaced from the member, to form a predetermined pattern of droplets on the substrate.

Preferably, each accumulation site comprises a respective well, and the surface of each well and the area of the rotary member between each well are hydrophilic.

Brief Description of the Drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 is a cut-away side view of an embodiment of printing apparatus in accordance with the invention;

Figures 2-4 are cut-away side views of part of a cylinder which is a component of the apparatus of Figure 1, showing steps in the formation of a droplet ejected from the cylinder surface;

Figures 5 and 6 are similar views of an alternative type of cylinder which may be used in apparatus in accordance with the invention;

Figure 7 is a perspective view of part of another embodiment of printing apparatus in accordance with the invention, the view showing an alternative form of droplet release means to that used in the embodiment shown in Figure 1.

Detailed Description

It should be noted that the figures are diagrammatic and are not drawn to scale. Relatively dimensions and proportions are shown exaggerated or reduced in size, for sake of clarity and convenience. The printer shown in Figure 1 comprises an elongate cylinder 1 which is rotatable about its longitudinal axis 2, that axis being perpendicular to the plain of Figure 1. In use, the cylinder is rotated by a suitable drive (not shown) in the direction indicated by the arrow 4 (i.e. clockwise as viewed in Figure 1).

The circumferential surface of the cylinder 1 carries an array of annular wells such as the well 6. The array has many thousands of wells arrange regularly over the circumferential surface of the cylinder, but for the sake of simplicity only four wells are shown in Figure 1. Since all the wells are identical, only the well 6 will be described in any detail (with reference to Figures 2 — 4).

In use the wells are loaded with ink by means of liquid feeding means comprising a reservoir of ink and a linear array of spray heads 10 which extend longitudinally relative to the cylinder 1 so that spray from the heads will be applied to substantially the entire length of the cylinder. The liquid feeding means also includes a pump (not shown) for supplying the ink to the spray heads under pressure.

Underneath the cylinder 1 there is provided an opened topped receptacle in the form of a tank 12. The tank 12 receives excess ink sprayed by the head 10 onto the cylinder 1, and this ink can be re-circulated by means of a further pump 14 supplying the ink collected in the tank 12 to the reservoir 8.

Positioned downstream of the tank 12 is droplet release means 14 for causing droplets to be released from selected wells and thus be projected (at see for example droplet 16) onto a substrate 18 positioned above the cylinder 1 and moving relative to the cylinder 1 in the direction indicated by the arrow 20.

The droplets released from the cylinder travel along a trajectory which approximately lies on a tangent to the circumferential surface 3 of the cylinder 1 at the point of the release of the

droplet concerned. The droplet velocity is approximately equal to the surface velocity of the cylinder and is typically 2m/s or more.

In the present embodiment, the droplet release means 14 comprises a linear array of lasers which are controlled by means of a controller 22 that is connected to the array so as to be able to selectively to activate the lasers and is also connected to an angular position sensor 24 having a fixed component mounted on the printer frame (not shown) in which the cylinder 1 is supported and the moveable component mounted on the cylinder 1. Thus the controller 22 can monitor the rotation of the cylinder and determine the necessary time of activation of any given laser in order to release a droplet from the required site.

The selection of wells from which droplets are to be released enables a predetermined pattern of droplets to be formed on the moving substrate 18. Those wells from which droplets aren't released carry their load of ink away from the release means 14 for possible re-use in a subsequent revolution of the cylinder 1. As this happens, however, the loaded wells will pass through the region being sprayed by the spray head 10, and this will tend to counteract possible changes in ink composition caused by drying.

In this example, the diameter of the cylinder is 12mm, the cylinder is 100mm long and it is envisaged that the cylinder will be rotated at a speed of 40,000 rpm.

With reference to Figures 2-4, the circumferential surface of the cylinder 1 comprises a substrate 26 of fused silica coated with a layer of gold 28. The gold layer is etched to define the array of wells. Since the wells are all identical, only the well 6 will be described in detail.

That well comprises a cylindrical recess 30 of a diameter of 45μm and a depth of 5μm, from which a central cylindrical protuberance 32 (formed from the gold layer) project radially. In this example, the protuberance 32 has a diameter of 15μm and a height (i.e. radial dimension relative to the axis of the cylinder) of lOμm. The well 6 is thus annular. The well 6 is separated from its neighbouring wells by part of the layer 12 which therefore defines an upper surface 34 of gold between the wells.

Thus the whole of the exposed circumferential surface of the cylinder 1 (i.e. the gold and exposed silica surfaces) is hydrophilic so that when the ink, which is acquiesce, is sprayed onto the cylinder, a continuous film of ink 36 is formed. The surface tension in the film of ink is such as to cause the ink to flow over the surface of the cylinder so as to reduce the total expose surface area of the ink film. This results in the ink filling the wells as is shown in Figure 2 in the case of the well 6.

When ink is to be released from the well, the latter is subjected to a short laser pulse of light energy of 1064nm wavelength and 50ns duration to deposit 50μj of energy into the ink in the well. The effect of this pulse is to heat up the ink accumulated in the well. Depending on ink composition, this may cause a reduction in surface tension, an expansion of liquid and/or such bubbles to form in the liquid. Any of these three mechanisms can cause the energy balance of the liquid in the well to be disturbed to an extent sufficient for a drop 36 to form over the protuberance 32 and then be released under the influence of the centripetal acceleration of the cylinder surface to create a free drop 38 (Figure 4). As can be seen from a comparison of Figures 2 and 3, the formation of the droplet 36 will increase the overall surface area of ink over the well, but this is at least in part counteracted by the fact that the ink is still part of the continuous film from which further ink can be drawn into the well and the droplet 36. As can be seen from Figure 4, the ink film dips into the well 6 immediately after the droplet has been released. However, since this configuration has a higher surface area than is the case when the well is filled with ink (see Figure 2), surface tension in the ink will cause the well to be filled with more ink from the film.

To facilitate the action of the laser, the ink may have infrared absorbing properties or contain infrared absorbing additives such as infrared absorbing dyes.

The ink accumulated in the well 6 is generally in either of two stable states, either the well is full, minimising effective surface area of the liquid, or the accumulation sites define by the well substantially empties, leaving only a thin film of liquid on the surfaces of the site. The other characteristic of the site is that the liquid empties preferentially from a fixed location,

i.e. central protuberance 32, at the centre of the well and not simply in the area surrounding the accumulation.

Figures 5 and 6 show an alternative type of well 40 for a modified version of the invention. The well is, again, formed as a cylindrical recess created by etching a coating of gold 42 on a fused silica substrate 44. In this case, however, the well does not have a central protuberance. To reduce the uncertainty over the area from which the drop is released, the well is deeper and narrower than the well shown in Figures 2-4. In this example, the well is 20 microns in diameter and 20 microns deep. Figure 5 shows the well when fully charged with ink, whilst Figure 6 shows a well just after having been emptied. As before, the film of ink will rapidly draw further ink in the well to fill the latter.

It will be appreciated that other modifications may be made to the printer without departing from the scope of the invention.

In particular, the droplet release means could be constituted by means other than a linear array of lasers. For example, the droplets could be released by a single laser producing a beam along an axis which can be swept longitudinally over the surface of the cylinder (by either pivoting or by reciprocal transitional movement of the laser). Alternatively, the arrangement shown in Figure 7 could be used. In this Figure, components which correspond to those shown in Figure 1 are denoted by the reference numerals of Figure 1 raised by 100. In this case, the droplet release means comprises a single laser of 114 which is controlled by a controller 122 that monitors the angular position of the cylinder 101 by means of a sensor 124. The controller also monitors the angular position of a rotating prism and 50 by means of a further sensor not shown. The prism is interposed in the path of the beam from the laser 114 to the surface of the cylinder 101 and is operable to transmit firing signals to the pulsed laser 114 so that the output light 152 from the laser 114 is incident on the prism when the latter is at a position at which the light reflected along the path 154 onto a selected site (in this case 156). Figure 7 also shows a data store 158 that contains data concerning the desired pattern of drops to be deposited on the substrate and instructs the controller 122 accordingly. -A similar data store is used in the arrangement shown in Figure 1 but has not

been shown in the drawing. In all respects other than the droplet release means, the embodiment of printer which is shown in part in Figure 7 is identical that of Figure 1.

A further refinement to the invention would be to localise the heating of the ink, either to the base layer of the drop or to the surface layer of the drop. Base layer heating may be accomplished either with a buried heater or with a laser energy absorbing layer at the base of the well. For example, an infra red laser may be used in conjunction with an infra red absorbing layer at the base of the well preferentially to heat the liquid at the base to form bubbles leading to an outward expansion of the liquid and/or a direct acceleration of the liquid at the accumulation site. ' Alternatively, the outermost layer of the liquid may be preferentially heated by the action of laser energy which is highly absorbed by the liquid, as would be the case if, for example, the laser was operable to emit an ultra violet beam. Heating of the outermost layer of the liquid may in itself reduce surface tension sufficiently to trigger drop ejection and/or may trigger a chemical reaction which reduces a change in surface tension, perhaps of a thermal or photo initiated surfactant. Alternatively, the liquid may comprise a non-miscible liquid layer that is highly absorbing to laser energy, heating of this layer causing a reduction of surface tension of the liquid system and thus achieving droplet release.

In addition, the cylinder 1 may carry additional radial protuberances, each of 10 microns diameter and 5 microns height which are not situated in any wells and which provide exit sites for excess ink (which has been sprayed on to the cylinder surface) as those further protuberances pass through the tank 12.