Patent Description:
In certain types of printing, a film supported by a carrier is transferred to a substrate (e.g., paper, cardboard, plastic films etc.) by application of pressure and/or heat in a desired pattern. One example of this is found in thermal transfer typewriters, where a ribbon carries an ink film that is transferred to paper by the application of heat.

A problem in using a conventional film coated carrier, be it a sheet, a web or a ribbon, is that the process is wasteful, and therefore expensive. This is because, at the time that it has to be discarded, only a small proportion of the film coating will have been used (e.g., for printing a text) and most of the film coating will remain on the carrier.

In <CIT>, the present Applicant disclosed a printing apparatus capable of mitigating the foregoing disadvantage. <FIG> of the latter publication is reproduced herein as <FIG> of the accompanying drawings and will now be described briefly.

<FIG> shows an endless intermediate transfer member (ITM) having an outer surface <NUM> serving as an imaging surface <NUM>. The ITM is described in <CIT> as being a drum <NUM> but it may alternatively be an endless belt. As the drum <NUM> rotates clockwise, as represented by an arrow, it passes beneath a coating apparatus, or particle dispenser, <NUM> where it acquires a coating of fine particles, the dispenser being suitably configured for the particles to form a monolayer downstream of the coating apparatus, if so desired. In the illustrative example of <CIT>, after exiting the coating apparatus <NUM>, the imaging surface <NUM> passes beneath an imaging station <NUM> where selected regions of the imaging surface <NUM> are exposed to laser radiation, which renders the particle coating on the selected regions of the surface <NUM> tacky. Next, the surface <NUM> passes through an impression, or transfer, station <NUM> where a printing substrate <NUM> is compressed between the drum <NUM> and an impression cylinder <NUM>. This causes the selected regions of the particle coating on the imaging surface <NUM> that have been rendered tacky by exposure to laser radiation in the imaging station <NUM> to transfer from the imaging surface <NUM> to the substrate <NUM>. The regions on the imaging surface corresponding to the tacky areas transferred to the substrate consequently become exposed, being depleted by the transfer of particles. The imaging surface <NUM> can then complete its cycle by returning to the coating apparatus <NUM> where a fresh particle coating is applied only to the depleted regions from which the previously applied particles were transferred to the substrate <NUM> in the impression station <NUM>.

Though the surface <NUM> is termed an imaging surface in the above-described printing system, it may alternatively be referred to as a donor surface <NUM> in any industrial application where the coated particles (or part thereof) end up being donated by (e.g., transferred from) the surface, and as far as the coating apparatus <NUM> is concerned it can also be referred herein as a receiving surface.

A comprehensive description of <FIG> is also to be found in <CIT> which is concerned with the coating apparatus and only components of relevance to the present disclosure will be therefore discussed in greater detail below.

The present disclosure is in particular concerned with a coating apparatus that can be used to replace that described inter alia in <CIT>. It should be stressed, however, that the coating apparatus of the present disclosure may have other applications and is not restricted to use in an apparatus as described in <CIT>. For example, the manner in which selected regions become particle-depleted is immaterial to the present disclosure and, as an example, the transfer of particles to the substrate may alternatively be the result of an adhesive substance being applied to selected regions of the substrate or the result of heat being applied to the donor surface by means other than laser radiation and/or from a side either facing the coating of particles or beneath it (e.g., by a thermal print head located on the rear side of an ITM). Thus, in offset printing systems benefiting from a coating apparatus according to the present teachings, the imaging stations rendering the particles transferrable to a substrate can either be imaging stations applying energy to selected particles on the ITM or imaging stations selectively modifying regions of a substrate (e.g., by application of an adhesive), the modified regions being adapted to detach selected particles from the ITM in the corresponding regions.

The coating apparatus <NUM> in <FIG> comprises a plurality of spray heads <NUM> that are aligned with each other along the axis of the ITM <NUM> and only one can therefore be seen in the section of the drawing. The sprays <NUM> of the spray heads are confined within a bell housing <NUM>, of which the lower rim <NUM> is shaped to conform closely to the donor surface <NUM> leaving only a narrow gap between the bell housing <NUM> and the drum <NUM>. The spray heads <NUM> can be connected to a common supply rail <NUM> which supplies to the spray heads <NUM> a pressurized fluid carrier, typically air, having suspended within it the fine particles to be used in coating the donor surface <NUM>. The surplus spray from the spray heads <NUM>, which is confined within a plenum <NUM> formed by the inner space of the housing <NUM>, is extracted in the present illustration through an outlet pipe <NUM>, which is connected to a suitable suction source represented by an arrow, and can be recycled back to the spray heads <NUM>, if so desired.

It is important for the coating apparatus <NUM> to be able to achieve an effective seal between the housing <NUM> and the donor surface <NUM>, in order to prevent the spray fluid and the fine particles from escaping through the narrow gap that must essentially remain between the housing <NUM> and the donor surface <NUM> of the drum <NUM>. Different ways of achieving such a seal are shown schematically in <FIG>.

The simplest form of seal is a wiper blade <NUM>. Such a seal makes physical contact with the donor surface and could score the applied coating if used on the exit side of the housing <NUM>, that is to say if used on the side downstream of the spray heads <NUM>. For this reason, if such a seal is used, it is preferred for it to be located only upstream of the spray heads <NUM> and/or at the axial ends of the housing <NUM>. The terms "upstream" and "downstream" as used herein are referenced to points on the donor surface <NUM> as it passes through the coating apparatus.

<FIG> also shows how egress of the fluid within which the particles are suspended from the sealing gap between the housing <NUM> and the drum <NUM> can be prevented without a member contacting the donor surface <NUM>. A gallery <NUM> extending in the present illustration around the entire circumference of the housing <NUM> is connected by a set of fine passages <NUM> extending around the entire rim of the housing <NUM> to establish fluid communication between the gallery <NUM> and the sealing gap.

The gallery <NUM> is connected to a suction source of a surplus extraction system, which may be the same suction source as is connected to the outlet <NUM> or a different one. In this case, the gallery <NUM> serves to extract fluid passing through the gap before it exits the housing <NUM>. The low pressure may also suck off the drum <NUM> any particles that are not in direct contact with the donor surface <NUM>.

<CIT> also describes an embodiment in which the particles are applied to the donor surface by means of a rotating brush or roller interposed between the spray heads <NUM> and the donor surface <NUM>.

The reason that the coating apparatus <NUM> of <CIT> only applies a monolayer of particles to the donor surface <NUM> is that the particles have a greater tendency to adhere to the donor surface than to one another. Hence, particles not in direct contact with the donor surface, can readily be dislodged and prevented from adhering to the donor surface, either by the action of brushes, or by suction, or by blowing away using an air knife like mechanism, or by a combination of such acts.

The coating apparatus <NUM> such as used inter alia in <CIT>, <CIT>, <CIT>, <CIT>, or <CIT>, needs to be capable of applying a monolayer of particles to an endlessly circulating donor surface of an ITM, ensuring that the donor surface leaves the coating apparatus with a uniform monolayer of particles, regardless of the proportion of the donor surface that may still retain a monolayer coating from a previous operating cycle upon arrival at the coating apparatus.

The rate at which particles need to be supplied to the coating apparatus will therefore vary with the extent of depletion of the coating at the transfer station <NUM> and it is an aim of the present disclosure to provide a coating apparatus capable of regulating the supply of particles in order to achieve reliable application of a uniform layer of particles to the donor surface regardless of the rate at which particles are transferred from the donor surface to a substrate.

Other coating apparatuses are known, see for example <CIT>.

In accordance with a first aspect of the present disclosure, there is provided a coating apparatus as hereinafter set forth in more detail and claimed in Claim <NUM> of the appended claims.

The coating apparatus comprises brushes in the application chamber for brushing the receiving surface to leave only a single layer of particles adhering to the receiving surface, each of the brushes and the receiving surface being in relative movement one with the other. A first part of the brushes may be provided to lie adjacent to the nozzles and rotate in a direction to cause the bristles of the brushes adjacent to the nozzles to pass over the receiving surface in a same direction as the movement of the receiving surface. A second part of the brushes may be provided remote from the nozzles.

In some embodiments, at least one brush of the second part of the brushes is rotated in a direction to cause the bristles of said at least one brush remote from the nozzles to pass over the receiving surface in a direction opposite to the movement of the receiving surface.

If desired, a brush deflector may be provided adjacent at least one of the brushes to deflect tips of bristles prior to contact being made between the bristles and the receiving surface.

In accordance with a second aspect of the disclosure, there is provided a method as hereinafter set forth in more detailed and claimed in Claim <NUM> of the appended claims.

In some embodiments, the air stream is delivered (e.g., ejected) by the air source through a nozzle, the nozzle (or array of individual nozzles) extending across the entire width of the receiving surface.

In accordance with a third aspect of the disclosure, there is provided an offset printing system apparatus as hereinafter set forth in more detail and claimed in Claim <NUM> of the appended claims.

When the coating apparatus is incorporated into a printing system, the receiving surface may be a recirculating receiving surface, which can also be referred to as an intermediate transfer member (ITM). In some embodiments, the recirculating receiving surface may be mounted on, or formed by the outer surface of, a rigid drum, while in other embodiments, the recirculating receiving surface may be the outward surface of an endless flexible belt.

In particular embodiments, the layer of particles formed on the receiving surface of the coating apparatus or as a result of the coating method, whether or not further implemented in a printing apparatus or in a printing method, is a monolayer of particles. In another embodiment, the layer or the monolayer formed on the receiving surface or on the ITM is of particles in the sub-micrometer range.

These and additional benefits and features of the disclosure will be better understood with reference to the following detailed description taken in conjunction with the figures and nonlimiting examples.

Some embodiments of the disclosure will now be described further, by way of example, with reference to the accompanying figures, where like reference numerals or characters indicate corresponding or like components. The description, together with the figures, makes apparent to a person having ordinary skill in the art how some embodiments of the disclosure may be practiced. The figures are for the purpose of illustrative discussion and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental understanding of the disclosure. For the sake of clarity and convenience of presentation, some objects depicted in the figures are not necessarily shown to scale.

<FIG>, showing the prior art of <CIT>, has already been described above and need not therefore be described again. The particle dispenser or coating apparatus <NUM> shown in <FIG> serves the same function as the coating apparatus <NUM> in <FIG> but is intended for a printing system that employs a surface of a flexible endless belt <NUM> as the donor surface <NUM>, in place of the rigid drum <NUM> of <FIG>. According to some embodiments of the present teachings, it would alternatively be possible for the coating apparatus to coat a donor surface formed by the surface of a rigid drum.

It should be made clear that the coating apparatus of the present invention, an embodiment of which is the apparatus <NUM> shown in <FIG>, is not restricted to coating a donor surface of a thermal printing system. A donor surface is only an example of a receiving surface that may be coated using the coating apparatus. While the terms imaging surface and donor surface are typically associated with more complex apparatuses or systems including a coating apparatus having a receiving surface, for simplicity and unless otherwise clear from context, the terms concerning the surfaces to which particles may be applied or from which they may be transferred can be used interchangeably in the following.

The coating apparatus <NUM> shown in <FIG> overlies the donor surface <NUM>, the direction of movement of the donor surface <NUM> being indicated by an arrow <NUM>. The coating apparatus comprises an air blower <NUM>, serving as a source of pressurized air and supplying nozzles <NUM> with air (under pressure) carrying a suspension of particles, typically dry. The source of pressurized air, which for simplicity can also be referred to as the air source, can alternatively be a device including compressed air. While being referred to as an air source, <NUM> can in fact serve to generate and/or maintain the air stream confined in the coating apparatus and mainly recirculating therein. The "air source" terminology is not intended to imply that only fresh air devoid of particles is fed to the coating apparatus, but that the particles can gain or retain any desirable velocity. While not fully hermetic, the coating apparatus can be considered as a relatively closed system with a controlled volume.

The particles carried in the air circulation loop to be later detailed may be made of any suitable material and, by way of example, may be thermoplastic particles (i.e. comprising or consisting of a thermoplastic polymer), if they are to be rendered tacky by heating. The nozzles can extend across the entire width of the receiving / donor surface and may therefore also be regarded as air knives.

The nozzles <NUM> spray air and suspended particles onto the donor surface <NUM> to form a particle coating on the donor surface <NUM>. A plurality of rotating brushes <NUM> can ensure that particles are brought into contact with every part of the donor surface <NUM> having entered the coating apparatus and that surplus particles are swept off the donor surface <NUM>, to leave substantially only one layer of particles adhering to it. The brushes may have individual bristles, as shown in <FIG>, or groups or tufts of bristles, as shown in <FIG>, arranged, for instance, in staggered rows. Each brush may optionally be associated with a brush cleaner <NUM> that may reduce the amounts of particles that may accumulate on the bristles with time (e.g., by shaking or scraping them off by contact). The brushes are contained within an application chamber <NUM> that is bounded by a partition <NUM> that separates the application chamber <NUM> from a conduit <NUM>. The conduit <NUM> leads to a chamber <NUM> and constitutes with the chamber <NUM> a return path that returns the air and unused particles back to the intake of the blower <NUM> to maintain a desired flow rate. In this way, an air circulation loop is formed for recycling particles not applied to the donor surface <NUM>. Typically, the flow rate of the air recirculating in the circulation loop is higher than the relative speed of the receiving surface, so that any portion of the surface would be exposed to more than one air cycle before exiting the coating apparatus. Without wishing to be bound by any particular theory, it is believed that this relatively higher flow rate of the air propelling the particles in the circulation loop facilitates the formation of a complete (e.g., without voids) layer of particles during transit of the receiving surface through the coating apparatus.

Particles applied to the donor surface <NUM> cause the concentration, or density, of the particles in the circulation loop to decrease. It is therefore necessary to replenish the particles from a tank (not shown) by means of a dosing device <NUM> regulated by means of an electronic controller (not shown). The dosing device <NUM> should be capable of introducing metered quantities of particles into the air circulation loop, because if the particle concentration is too low, then the donor surface <NUM> may not be fully coated with particles. Conversely, if the concentration of particles is too high, it would be difficult to ensure that only an even layer of particles is applied to the donor surface <NUM>. Moreover, too high a concentration of particles may lead to a safety or health hazard (e.g., the concentration exceeding the Lower Explosive Limit (LEL) of the particles). It is therefore vital to maintain the concentration of particles within a predetermined range to ensure safe, uniform and efficient coating of the donor surface <NUM>. The predetermined range (or limits of the range) may depend upon the intended use of the coating apparatus or of a system implementing it, the rate of depletion of the particles from the receiving surface, the extent of loss of particles to the walls of the coating apparatus or to any part thereof (e.g., on the brush bristles or on a filtering system of its recirculation loop) and like considerations. Hence, the predetermined limits desirable for any particular case can be readily ascertained by a person skilled in the use of such apparatus. If the coating apparatus and the layer of particles formed thereby are used, for instance, in a printing system and if it is not always the same image that is to be printed, the dosing device <NUM> cannot be controlled to meter particles at a fixed rate and instead the dosing rate needs to be matched to the density of the image to be printed. In digital printing systems the images to be printed can differ from one image to a subsequent one or from one print job (printing a same first image) to a subsequent print job (printing a same second image). Such changes, which can be rapid and/or frequent, may challenge coating apparatuses to be implemented therein.

Simply put, the dosing device serves to add in the air circulation loop a controlled quantity of new / fresh particles to the particles unbound to the receiving surface in a previous cycle of the recirculating air stream, replacing at least part of the depleted particles. As mentioned, the particles can be depleted "intentionally" in view of their removal from the receiving surface to serve their intended purpose, or inadvertently in view of losses to walls and parts of the coating apparatus.

In <FIG>, the dosing device <NUM> is depicted downstream of a sensor of particle concentration <NUM> and upstream of an air source <NUM> promoting the recirculation of particles unbound to the receiving surface <NUM> in the air circulation loop. In its illustrated location, the dosing device may mask, on a back plane, components of the coating apparatus allowing the circulation of the air stream and particles therein, or any device associated with a desired treatment of the foregoing.

For instance, while not shown in the figure, the particles may optionally be fed to the dosing device via one or more preliminary treatment devices designed to at least partially remove (e.g., filter out) and/or to at least partially reduce in size the agglomerates such particles may form, so that smaller agglomerates, smaller clusters or even individual particles can be entrained by the air circulation loop following the dosing device. Similarly, the recirculating air (including the particles therein) may be "treated" to control its temperature, its relative humidity, and/or its electrostatic charge.

To regulate the rate at which particles are metered by the dosing device <NUM>, its controller may receive a signal from a particle density sensor. While it would be possible to use other forms of such sensors (e.g., an electrostatic sensor), the embodiment described in <FIG> employs an optical density sensor (ODS) <NUM> situated in the air circulation loop to monitor the concentration of particles. To improve the accuracy of the particle density sensor (e.g., ODS <NUM>), its detecting part is desirably positioned in a region when the air flow is less turbulent, such as immediately preceding the dosing device <NUM>.

The design of a suitable optical density sensor is schematically illustrated in <FIG>. The detecting part of the sensor comprises a light source, in the form of a light-emitting diode (LED) <NUM>, that emits white light, and a light sensing element, for example in the form of a photoresistor <NUM>. When particles <NUM> flow through the sensor <NUM> and in between the LED <NUM> and the photoresistor <NUM>, the signal output from the photoresistor <NUM> decreases. The more light that is blocked, the lower the output signal. The sensor <NUM> is shown as having a second light sensing element, in the form of a second photoresistor <NUM>, to measure scattered light, instead of transmitted light. The scattered light sensing photoresistor <NUM> also generates an output signal but, unlike the output of the photoresistor <NUM>, it increases with particle density. As the output signals of the two photoresistors <NUM>, <NUM> are complementary, only one of them is needed. When both are present, the output signal of the scattered light <NUM> sensor may be used to validate the output from the photoresistor <NUM> or combined with it to improve the signal to noise ratio. The sum of the output signals of the two photoresistors may be used to check operation and cleanliness of the sensor, because if both the photoresistor <NUM> and the scattered light sensor <NUM> have a low reading, this would indicate probable fouling of the LED <NUM> and/or the photoresistors <NUM>, <NUM>. It is noted that while the particles are for simplicity illustrated in the figure by circles suggesting globular particles, they may assume any other shape and the present coating apparatus and method are applicable to particles also having non-spherical shapes, such as flakes, rods, irregular or amorphous chunks etc..

To reduce deposition of particles on the light source <NUM> or light sensing element(s) <NUM>, <NUM>, the sensor <NUM> may have air channels <NUM> leading to each of its three elements for keeping them clean. Air may either be blown into the channels <NUM> or, if the sensor <NUM> is located in a region of the recirculation path under negative pressure, ambient air may be sucked through the channels <NUM>. The output signals of the light sensing elements (e.g., of the photoresistors) can then be used by a controller to adjust the quantity of particles <NUM> metered into the air circulation loop by the dosing device <NUM>.

As an alternative to regulating the dosing device <NUM> in dependence upon a direct measurement of the particle concentration in the application chamber of the air recirculation loop, it may do so in a printing system based upon the measured or predicted consumption of the particles. The particle consumption can be measured by analysis of the output signal of an optical device, such as a camera or an optical density scanner viewing the printed output of the printing system or it may be predicted by analysis of the input signal applied to the printing system. The two methods of regulating the dosing device, which can be viewed as a "feedback" and a "feed-forward" control, need not be mutually exclusive and can be combined to further reduce any time delay in the implementation of a modification in the feeding of particles achieved by the dosing device.

Feedback assessment of particle consumption typically includes measuring the optical density (OD) of a printed image. Optical density of various points of a printed image can be measured by using a densitometer or scanning densitometer during the printing process. Optical density measurements are performed by illuminating the printed image with a light source and measuring the intensity of the light reflected from the image. The OD measurements can be made ahead of printing a desired image, using a reference calibration image (e.g., <NUM>% coverage "solid patch" color), or can be done on the intended desired image of each print job, the purpose of the controller being to set the dosing device so that the measured OD matches the intended target OD of the image. A conventional proportional-integral-derivative (PID) controller can be used for relatively long print jobs during which the printing process remains relatively constant. But other controllers, more able to account for changes in operating conditions of the press can be suitable. Such controllers, adapted to maintain the particle concentration in the application chamber within predetermined limits, are known to persons skilled in the field of printing, in particular in control of digital printing processes, and shall not be further detailed herein.

Feedforward prediction of particles consumption can be based on the analysis of the image intended for printing during a particular print job. Controllers adapted for such preemptive methods adapted to maintain the particle concentration in the application chamber within predetermined limits are known in the printing industry and need not be further detailed herein.

A control system can combine predictive regulation of the dosing device, which provides for initial settings, and feedback adjustments, as necessary to compensate for actual deviations from prediction.

With respect to aforesaid control of a dosing device, it is to be noted that a printing system implementing a coating apparatus according to the present teachings can be considered more buffered than presses relying for instance on ink jetting. While an excess deposition of ink, or conversely an insufficient jetting, may readily translate into a deficient print quality, a coating apparatus typically includes in its air circulation loop an amount of particles not only sufficient to form or replenish a monolayer, but also capable of readily adapting for an increased consumption of particles, the particles in such excess not being wasted on the printing substrate, but recycled until needed for transfer. This render the print quality that can be obtained using a present coating apparatus less dependent upon immediate changes in operating conditions, the waste of ink being also reduced.

A reader interested in gaining more details on such offset printing systems comprising an intermediate transfer member (ITM), a coating apparatus for applying a layer of particles to the ITM, an imaging station for applying energy to selected particles on the ITM to render the particles transferrable to a substrate, and an impression station at which only particles to which energy is applied in the imaging station are transferred from the ITM to a substrate to form an image on the substrate, wherein a coating apparatus according to the present teachings can be advantageously implemented is referred inter alia to <CIT>, <CIT>, <CIT>, <CIT>, or <CIT> of the same Applicant.

As best seen in <FIG>, the air blower <NUM> may comprise a chamber that houses a large fan configured to output a high volume of air and particle mixture at a low pressure. The fan blows the particles <NUM> (both those already present in the recirculation loop and the new particles from the dosing device <NUM>), through a feeding chamber <NUM> that leads to the nozzles <NUM> in the application chamber <NUM>.

The particles <NUM> may, in some circumstances, attract and adhere to one another, thereby creating larger, agglomerated particles or clusters. This is a problem encountered when the particles are very small (e.g., have a diameter of no more than a few microns) or when they are damp. If an air stream containing such agglomerated particles is applied directly to the donor surface <NUM>, it may result in an uneven coating.

While not wishing to be bound by theory, it is believed that, if there are relatively larger particles (e.g., agglomerates or clusters) being applied to the donor surface <NUM>, then the brushes <NUM> may either remove them more easily than the relatively smaller particles, causing holes or discontinuities in the particle coating, or if the brushes do not remove the larger clusters then they may be considered multilayer due to the agglomeration of smaller individual particles. In addition, as time goes by, larger particles tend to be replaced by smaller particles causing a variation in the effect such particles (or the change in particle size distribution) may provide to intended product. Reverting for illustration to the use of the layer of particles or coating apparatus applying them in a printing system, a change in the distribution of the size of the particles in the population applied to the donor surface, may in turn affect the appearance of the printed matter. For instance, variations in the applied population of particles may alter the optical density or the glossiness of a printed image (this being dependent on layer height/thickness which is itself dependent on mean particle size).

With a view to addressing this problem, to the extent that it may occur with the particles one seeks to apply and/or with their size distribution, <FIG> shows a particle deflector <NUM> positioned under the nozzle <NUM>. The particle deflector <NUM> may be manufactured from any material that is sufficiently hard to break up the agglomerated particles into smaller particles <NUM> better suited for the intended use (e.g., printing). The smaller particles may be individual particles or may be smaller aggregates of particles. One example of a material suitable for a deflector would be a steel alloy. The particle deflector may have any shape capable of yielding smaller particles and of largely redirecting the air stream. The use of the particle deflector <NUM> causes a cloud of smaller particles <NUM> in the application chamber <NUM> to settle predominantly on the rotating brushes <NUM> but also on the donor surface <NUM>. The application of a cloud rather than jetting particles <NUM> straight onto the donor surface <NUM> results inter alia in one or more of: a narrowing of the distribution of particle size, the formation of a coating of particles being more uniform than in the absence of a deflector, the brushes more effectively removing excess particles <NUM> from the donor surface <NUM> and the reduction, delaying or prevention of damage that some particles may cause to the donor surface upon impact. For instance, particles made of a material having a bulk hardness greater than the surface of the transfer member may scratch it even at relatively low impact force, while relatively high impact force may render any particle abrasive to the donor surface.

As the population of particles may be narrowed in its size distribution by the breaking up of agglomerated particles and as the constant recycling of particles in operation of the coating apparatus may prevent re-agglomeration and/or provide at least a partial size reducing effect, it is believed that the deflector can reduce the occurrence of size variations observed over time in its absence, hence decreasing or eliminating any secondary effect such variations may have on the end-product (e.g., disparity in optical density or gloss inconsistency in printed matter).

In a series of experiments run under similar conditions, except for the absence or presence of a deflector in the path of the particles circulating along the air flow, the presence of a deflector dramatically decreased the proportion of aggregates on the donor surface, such aggregates yielding patches of "multilayers" in a mosaic of smaller particles which form the monolayer (as assessed by microscopy and image analysis). The reduction in the amount of relatively larger particles being applied resulted in turn in fewer voids being formed in the applied particle coating following removal of surplus particles. Such effects evolve with the number of cycles but, for illustration, an apparatus that would achieve after <NUM> cycles about <NUM>% monolayer coating, about <NUM>% multilayer coating and <NUM>% voids in absence of a deflector, might display an improved outcome with a deflector, the relative coverage increasing to above <NUM>% monolayer coating, with a drop both in the amounts of multilayer patches and voids to be each of less than <NUM>% of the coated area.

Regardless of its benefits on the population of particles being applied to the donor surface by a coating apparatus according to the present teachings, the deflector may alternatively or additionally serve to protect the donor surface from any deleterious effect direct application may have to the surface.

The application chamber <NUM> typically houses multiple brushes <NUM> or rollers. The brushes <NUM> positioned nearest to the nozzles <NUM> apply the particles <NUM> to the donor surface <NUM>. As seen in <FIG>, the three brushes <NUM> furthest to the right and nearest to the nozzles <NUM> may rotate in a direction to cause bristles to pass over in the same direction as movement of the donor surface. The rotational speed of these brushes <NUM> can be such that the linear speed of the brushes <NUM> is greater than the speed of the donor surface <NUM>. For example, the donor surface <NUM> may travel at <NUM>/s, and the linear speed at the radial edge of the brush <NUM> may travel at <NUM>/s, thereby creating a skid of <NUM>/s and allowing the bristles of the brushes <NUM> to wipe particles <NUM> on to the donor surface <NUM>. While skid speed may need to be adapted to the particles being applied and the donor surface due to be coated thereby, skid between <NUM>/s and <NUM>/s is deemed suitable for many applications. While a higher skid may improve the efficiency of application and/or removal of particles, it may also increase the risk of wear of the donor surface. Though in the above example the brushes were deemed to have a greater speed than the donor surface, this should not be construed as limiting and alternatively the skid in relative speed can be achieved by the donor surface having a greater speed than the brushes. The skid need not be the same for each brush the donor surface can be contacted by.

The brushes <NUM> positioned further away from the nozzles <NUM> may serve the purpose of removing excess particles <NUM> from the donor surface <NUM> to leave only a single layer. In <FIG>, the four brushes <NUM> on the left represent the particle removing brushes. The particle removing brushes may rotate in a direction to cause bristles to pass over in opposite direction to the movement of the donor surface and at a higher relative speed than the other brushes and may also have a wiper blade (e.g., brush cleaner <NUM>) or equivalent for removing particles <NUM> from the bristles, thereby cleaning them and making the bristles more effective. In some embodiments, a particle removing brush can be replaced by an air knife or any similar device able to eliminate superfluous particles to that a donor surface on its exit of the coating apparatus is substantially coated by a monolayer of particles.

The above description of the brushes is merely intended as an example. There may be any number of brushes <NUM> (and indeed nozzles <NUM>) and it will be appreciated that they may rotate in different directions and/or at different relative speeds than those described above. Further, although the brushes <NUM> described are the same in construction, it should be noted that in some embodiments the brushes may differ to better suit their role. For example, the stiffness and/or the chemical composition of the bristles may vary to suit the task being performed. The nature of the bristle may therefore vary, it being only important to ensure that they do not damage the donor surface while performing their desired task.

In one embodiment, the action of the brushes and their respective bristles can be additionally adjusted by a physical element to further facilitate application of the particles to the donor surface and/or removal of surplus, and/or to further reduce any damage they may cause to the particle coating or to the donor surface. Without wishing to be bound to any particular theory, a bristle can be viewed as contacting an underneath horizontal donor surface or particles thereon with a force comprising a vertical component and a horizontal one, each having a different magnitude along the bristle as it contacts the movable ITM. The vertical force may have such an impact so as to undesirably detach particles loosely attached to the donor surface (e.g., relatively larger ones), whereas the horizontal component of the bristle force provides a gentler swiping effect. The vertical force can be viewed as causing a whipping effect of the bristle. Preferably the horizontal force should be adjusted to be sufficiently high to remove excess particles not directly contacting the donor surface, while being small enough to leave the underlying monolayer of particles contacting the donor surface undisturbed.

While the vertical force can be modified by selecting a proper distance of the brush from the surface or bristle length and physicochemical properties as previously described, it may also be attenuated by placing a physical obstacle in its path on either side of a point a bristle would have contacted the ITM in absence of such interference. In <FIG>, one example of such an obstacle is shown in the form of a deflector rod <NUM> of triangular section, but it may have other cross sections. Such hindrance to the vertical component of a bristle force is termed herein bristle or brush deflector, the element being positioned at a height h above the donor surface, allowing the untouched passage of the particle coating. The bristle or brush deflector forms an angle a with the donor surface on the edge deflecting the bristle, so as to reduce its vertical impact. The edge of the deflector forming an angle with the donor surface is not necessarily exactly underneath the axis of rotation of the brush comprising the bristle but can be at an offset distance X therefrom. A skilled person can readily appreciate how to vary the magnitudes of a, h and X in the bristle deflector (in addition to any other pre-set parameters such as the bristle rigidity and length, and the distance of the brush from the donor surface), to adjust the vertical and horizontal velocities and force of the bristles to suit the intended purpose, operation conditions and the particles being applied to a predetermined ITM. In its simplest form, a brush or bristle deflector can be shaped as a surface tilted with respect to a plane substantially parallel to the donor surface and at a distance (height) h therefrom. The bristle deflector can, for instance, be shaped as a triangle an edge of which following the contour of the donor surface, but this example should not be construed as limiting and all other shapes of deflectors capable of deflecting the bristles so as to satisfactorily modulate the vertical component of their force on the donor surface are encompassed.

Regardless of shape, dimensions and positioning with respect to a prospective point of unaltered contact (e.g., upstream and/or downstream of a point the bristle would have first contacted in absence of the brush deflector), such brush deflector should advantageously be made of a material compatible with the bristles. By compatible, it is meant in this context that the bristle deflector would not damage the bristles due to contact with it, nor be damaged by them, neither physically (e.g., reducing or preventing abrasion, cuts or breakage), nor chemically (e.g., selecting the materials of the bristles and their deflector so as to avoid adverse reactions one with the other), nor in any other way deleterious to the functionality of the brush and its deflector, if present. For instance, the brush deflector should not affect the ability of the bristle to attract, retain and/or donate the particles, as the case may be for different brushes having different roles. By way of example, the brush deflector may be made of a metal or alloy, or be coated with an elastomer, sufficiently hard to bend the impinging bristles, but not excessively harder than the bristles to an extent it would damage them.

The brushes shown in <FIG> can be replaced at least in part by a flexible member circulating on two rollers or more, the flexible member having on its outer surface, facing the donor surface, bristles that may act as previously detailed for more conventional brushes wherein the bristles are typically attached to a single more rigid roll. Without wishing to be bound by any particular theory, it is believed that such a bristle-bearing circulating member may replace at least two brushes, while also providing a bristle deflecting effect over a larger corresponding area of the donor surface.

As shown in <FIG>, the application chamber <NUM> is partially bounded by the donor surface <NUM>. Any gap that exists between the walls of the application chamber <NUM> and the donor surface must not allow particles <NUM> to escape. Aside from the fact that such particles would not be recycled, they could present a health hazard and possibly even a safety risk.

To prevent escape of air and particles from the application chamber <NUM>, it is surrounded by a bell housing <NUM> that defines a suction chamber <NUM> that surrounds the application chamber <NUM> on all sides. As represented by an arrow <NUM>, the suction chamber <NUM> is connected to a pump and filter unit <NUM> that lowers the pressure in the suction chamber <NUM> to below atmospheric pressure and below the pressure in the application chamber <NUM>. Thus, instead of air carrying particles escaping to the ambient atmosphere, it is constantly sucked into the coating apparatus <NUM> as represented by the arrows <NUM>. In another embodiment, the particles captured by the suction chamber of the bell housing that may accumulate with time on the filter unit can alternatively or additionally be delivered to the dosing device <NUM>, allowing to reintroduce the previously filtered out particles back to the air circulation loop.

Applying a sufficiently low pressure in the suction chamber <NUM> to ensure that no particles <NUM> escape could cause the donor surface to be sucked against the bell housing <NUM>. However, it is essential that the donor surface <NUM> and the bell housing should not contact one another.

The tension in the belt <NUM> can be used to ensure that no contact takes place with the transversely extending edges of the bell housing <NUM> as the donor surface <NUM> enters and leaves the coating apparatus <NUM>, because the belt <NUM> can be tensioned over rollers at these points. However, the lateral edges of the donor surface <NUM> cannot be supported in this manner over the entire length of the coating apparatus and, as illustrated in <FIG>, the low pressure in the suction chamber <NUM> could cause the lateral edges of the donor surface <NUM> to flutter and to be drawn into contact with the sides of the bell housing <NUM> parallel to the direction of movement of the belt / donor surface. This contact may accelerate wear of the donor surface <NUM> and may cause it to tear, as well as possibly causing damage to the coating apparatus and/or the layer of particles.

In some embodiments, the belt <NUM> may further comprise, along its lateral edges, protruding formations which are capable of engaging with lateral tracks at least when the belt runs underneath the coating apparatus. Such lateral formations when engaged in respective tracks may place the belt under lateral tension, in at least the region facing the sucking chamber. Such formations may additionally, or alternatively, constrain the belt to follow a desired path in at least a segment of the path corresponding to the coating apparatus or at least its sucking chamber. The lateral formations can be (a) a plurality of formations that are spaced from one another along the length of belt or (b) a continuous formation along the entire length of the lateral edge of the belt, the formations optionally having a thickness greater than the belt. In one embodiment, the formations are (a) made of a material having a low friction coefficient to ensure smooth running of the formations within the lateral tracks and/or (b) made of a material, or comprise an agent, or are coated with a coating having lubricating properties.

However, the belt <NUM> of which the surface acts as the donor surface <NUM> cannot in some circumstances be formed of a material having sufficient strength to withstand tensioning to the extent necessary to prevent contact between its lateral edges and the longitudinally extending sides of the bell housing <NUM>. For example, the belt <NUM> may need to be relatively thin or, in particular embodiments, formed of a transparent material to allow light to pass through it from its rear side before reaching the donor surface <NUM>.

It is possible to prevent the donor surface <NUM> from being sucked against the housing of the coating apparatus in a variety of ways, such as by providing rollers along the side of the housing. <FIG> illustrate two of the solutions that are currently preferred.

In <FIG>, a fibre reinforced silicone blanket <NUM> serves to provide a support surface for the belt <NUM> bearing the donor surface. The blanket <NUM> is moved independently of the belt <NUM> over a different set of rollers, but the movements of the belt <NUM> and the blanket <NUM> are synchronised. The blanket <NUM> is under sufficient tension and has sufficient stiffness to prevent it from being sucked against the bell housing <NUM>.

The embodiment of <FIG> relies on the fact that the blanket <NUM> and the belt <NUM> bearing the donor surface <NUM> will tend to stick to one another, if both are manufactured from a silicone material. This will effectively strengthen the donor surface <NUM> and prevent it from being sucked against the bell housing <NUM>.

As an alternative to relying on the tendency of surfaces (e.g., made of silicone) to stick to one another, the embodiment of <FIG> relies on surface tension. In this embodiment, a liquid (e.g., an oil) film <NUM> is applied between the belt <NUM> and the blanket <NUM> and this serves to prevent the two from separating. As oil is a lubricant, it would be possible to provide a stationary lubricated plate instead of blanket <NUM>, but a recirculating blanket is preferred.

As mentioned previously and shown in <FIG>, the brushes <NUM> may have rows of individual bristles or groups (tufts) of bristles and the rows may be staggered to ensure that the bristles contact the entire donor surface. A person skilled in manufacturing of brushes can appreciate the parameters that would modulate the density of the bristles on the brush, and more importantly the density of the bristle tips on the outer surface of the brush due to contact the donor surface or particles thereon. While they need not be detailed herein, it is readily understood that the diameter of each bristle, its shape, the number of bristles in a bundle (if any), the distance between the bound end of each bristle or bundle in a row, the distance between adjacent rows, the angle each row may form with the axis of rotation of the brush and like considerations, may affect the density of the bristle tips on the outer surface of the brush. Advantageously, this density should be sufficient for the brush to have a substantially uniform action on the donor surface or particles thereon. By way of converse example, the bristles or rows thereof cannot be distanced from one another in a way that would render areas of the donor surface inaccessible to their tips.

Depending inter alia on their dimensions and the materials from which they are formed, bristles may display a variety of stiffness / flexibility / ability to obtain, retain and/or donate particles which can be selected and adapted to the particles and the donor surface to be coated therewith, and/or to the distance between the axis of rotation of the brush to which the bristles are attached and the donor surface. While the length of a bristle should be sufficient to impinge on the donor surface to apply and/or remove the particles and short enough to avoid damaging the donor surface or particle coating, the extent of this permissible length (which include the physical distance to be bridged by the bristle) may depend on the foregoing considerations as previously detailed.

By way of illustration, all other bristle parameters being similar including the excess length of the bristle that would collide with the donor surface, a relatively longer bristle rotating around an axis more distant from the donor surface would apply less pressure than a relatively shorter one rotating around a less distant axis. While diminished pressure is an advantage as far as the wear of the donor surface is concerned, it might become insufficient to remove surplus particles. Similarly, and all other parameters being similar, a bristle having a relatively larger diameter would apply more pressure than a bristle with a relatively smaller one. This increased pressure is an advantage to remove surplus particles, but can be damaging to the donor surface.

In some embodiments, the bristles are made of a durable natural or synthetic material. Suitable natural materials can be of plant or animal origin, and include by way of example animal hair, fur, down, plume or feather. Synthetic materials may be made to mimic the former examples of natural materials, but can also be plastic materials comprising or consisting, for instance, of nylon. Preferably, the bristles are made of a material able to suitably displace the particles being coated by the apparatus on the donor surface. In other words, the bristles should be able to sufficiently attract / retain the particles, albeit to a lesser extent than the donor surface, to remove surplus, while being capable of releasing them to prevent build-up or any other saturation process rendering them inefficient. As the brushes of the coating apparatus need not be the same, their respective bristles may likewise differ.

The bristles may have any suitable shape and cross-section. In some embodiments, the bristles have a cylindrical shape, the diameter of the bristles being of at least <NUM>, at least <NUM>, or at least <NUM>. In some embodiments, the diameter of the cylindrical bristles is at most <NUM>, at most <NUM>, or at most <NUM>. In further embodiments, the diameter of the cylindrical bristles is between <NUM> and <NUM>, between <NUM> and <NUM>, or between <NUM> and <NUM>. In other embodiments, the cross-section perpendicular to the length of the bristle is not an ideal circle but an ellipse or a polygon, in which case a suitable "diameter of the bristle" can be approximated by the maximal length of the cross section for the upper limits and for the minimal length of the cross section for the lower limits. Taking for example a bristle having a rectangular cross section, its longer side might not exceed, in some embodiments, <NUM>, <NUM>, or <NUM>, while its smaller side might be of at least <NUM>, at least <NUM>, or at least <NUM>.

In some embodiments, the bristles have a total length of at least <NUM>, at least <NUM>, or at least <NUM>. In some embodiments, the total length of the bristles is at most <NUM>, at most <NUM>, or at most <NUM>. In further embodiments, the total length is between <NUM> and <NUM>, between <NUM> and <NUM>, or between <NUM> and <NUM>.

In some embodiments, the bristles have a total length in excess of the shortest distance between the end of the bristle bound to the brush and the donor surface, the difference between the two values (the excess length) being of at least <NUM>, at least <NUM>, or at least <NUM>. In some embodiments, the excess length of the bristles as compared to the shortest distance is at most <NUM>, at most <NUM>, or at most <NUM>. In further embodiments, the excess length is between <NUM> and <NUM>, between <NUM> and <NUM>, or between <NUM> and <NUM>. While too high an excess length may damage the donor surface, a sufficiently high one may improve the swiping effect of the bristles, in other words may facilitate application of particles and removal of excess not directly contacting the donor surface.

Claim 1:
A coating apparatus (<NUM>) configured to apply to a receiving surface (<NUM>) a layer of particles (<NUM>) that have a greater tendency to adhere to the receiving surface than to one another, the apparatus comprising:
a) a source (<NUM>) of pressurized air,
b) an application chamber (<NUM>) partially bounded by the receiving surface (<NUM>) into which an air stream is delivered by the air source (<NUM>) through a nozzle (<NUM>),
c) an air return path (<NUM>) for returning air from the application chamber to an intake of the air source (<NUM>) to form an air circulation loop, and
d) a dosing device (<NUM>) for introducing particles to be coated onto the receiving surface into the air circulation loop,
characterized in that
a particle deflector (<NUM>) is positioned in the path of the air stream delivered through a nozzle (<NUM>), the deflector serving to break up agglomerated particles carried by the air stream prior to coating the receiving surface with the particles, and
brushes (<NUM>) are provided in the application chamber (<NUM>) for brushing the receiving surface (<NUM>) to leave only a single layer of particles (<NUM>) adhering to the receiving surface, each of the brushes and the receiving surface being in relative movement one with the other.