Device for coating surfaces with individual particles

Abstract

The present disclosure relates to a coating apparatus for applying a monolayer of particles to a receiving surface. The apparatus includes a pressurized air source, an application chamber partially bounded by a receiving surface to which an air flow is delivered by the air source, an air return path for returning air from the application chamber to an air inlet of the air source to form an air circulation loop, and a dosage device for introducing particles to be coated onto the receiving surface into the air circulation loop, wherein a particle deflector is positioned in the path of the air flow to break up agglomerated particles carried by the air flow before coating the receiving surface with the particles. Also provided are methods for applying a layer of particles, as well as printing systems that benefit from the coating apparatus.

Description

[Technical Field]

[0001] [Related Applications] This application claims Paris Convention priority of UK Application No. 2005159.5, filed April 7, 2020, and UK Application No. 2020721.3, filed December 30, 2020. This application is also related to concurrently filed International Application No. PCT / IB2021 / 052774 (Agent Ref. LIP 17 / 013 PCT), entitled "Apparatus For Coating a Surface With Particles." The entire disclosures of all of the foregoing applications are incorporated herein by reference for all purposes as if fully set forth herein.

[0002] SUMMARY The present disclosure relates to an apparatus for coating a receiving surface with individual particles. [Background technology]

[0003] In one type of printing, a film supported by a carrier is transferred to a substrate (e.g., paper, cardboard, plastic film, etc.) by the application of pressure and / or heat in a desired pattern. An example of this is found in thermal transfer typewriters, where a ribbon carries an ink film that is transferred to paper with the application of heat.

[0004] The challenge with using traditional film-coated carriers, whether they be sheets, webs, or ribbons, is that the process is wasteful and therefore expensive, because by the time they must be discarded, only a small portion of the film coating has been used (e.g., printed text), and most of the film coating remains on the carrier.

[0005] In patent document 1 (WO 2016 / 189512), the applicant disclosed a printing apparatus that can alleviate the above drawbacks. Figure 1 of the above publication is reproduced in this specification as Figure 1 of the accompanying drawings and will be briefly described below.

[0006] FIG. 1 shows an endless intermediate transfer member (ITM) having an outer surface 12 that serves as the imaging surface 12. The ITM is described in the '661 patent as being a drum 10, but could alternatively be an endless belt. As the drum 10 rotates clockwise, as indicated by the arrow, it passes under a coating device or particle dispenser 14, which provides a coating of particulates, suitably configured to cause the particles to form a monolayer downstream of the coating device as needed. In the illustrative example of the '661 patent, after exiting the coating device 14, the imaging surface 12 passes under an imaging station 16, where selected areas of the imaging surface 12 are exposed to laser radiation, which tackifies the particle coating on the selected areas of the surface 12. The surface 12 then passes through an impression or transfer station 18, where a print substrate 20 is compressed between the drum 10 and an impression cylinder 22. This transfers the selected areas of the particle coating on the imaging surface 12, which were made tacky by exposure to laser radiation at the imaging station 16, from the imaging surface 12 to the substrate 20. Areas on the imaging surface corresponding to the tacky areas transferred to the substrate are consequently exposed and depleted by the transferred particles. The imaging surface 12 can then return to the coating apparatus 14 where a fresh particle coating is applied only to the depleted areas where the previously applied particles were transferred to the substrate 20 at the impression cylinder station 18 to complete the cycle.

[0007] Surface 12 is referred to as an imaging surface in the printing system described above, or may alternatively be referred to as a donor surface 12 in any industrial application in which coated particles (or portions thereof) are ultimately donated by (e.g., transferred from) the surface, and may also be referred to herein as a receiving surface insofar as coating apparatus 14 is concerned.

[0008] A comprehensive description of FIG. 1 can also be found in WO 2016 / 189513 A1 on coating apparatus, so only the components relevant to the present disclosure will be described in more detail below.

[0009] The present disclosure relates in particular to a coating apparatus that can be used to replace the coating apparatus described, inter alia, in U.S. Patent No. 6,277,999. However, it should be emphasized that the coating apparatus of the present disclosure may have other applications and is not limited to use with an apparatus such as that described in U.S. Patent No. 6,277,999. For example, the manner in which selected regions become particle-depleted is not critical to the present disclosure; by way of example, particle transfer to the substrate may instead be the result of an adhesive substance applied to selected regions of the substrate, or the result of heat applied by means other than laser radiation and / or to the donor surface from either the side facing or below the particle coating (e.g., by a thermal print head positioned behind the ITM). Thus, in an offset printing system that benefits from a coating apparatus according to the present teachings, the imaging station that enables particles to be transferred to the substrate may either be an imaging station that applies energy to selected particles on the ITM, or an imaging station that is adapted to selectively modify regions of the substrate (e.g., by applying an adhesive) such that the modified regions release selected particles from the ITM in the corresponding regions.

[0010] The coating apparatus 14 of FIG. 1 includes multiple spray heads 1401 aligned along the axis of the ITM 10, so only one is visible in cross section. The spray 1402 of the spray heads is confined within a bell housing 1403, whose lower rim 1404 is shaped to closely conform to the donor surface 12, leaving only a narrow gap between the bell housing 1403 and the drum 10. The spray heads 1401 can be connected to a common supply rail 1405, which supplies the spray heads 1401 with a pressurized fluid carrier, typically air, containing suspended particulates used in coating the donor surface 12. Excess spray from the spray heads 1401, confined within a plenum 1406 formed by the interior space of the housing 1403, is extracted through an outlet pipe 1407, represented in this illustration by an arrow, connected to a suitable suction source and can be recycled back to the spray head 1401 as needed.

[0011] It is important that the coating apparatus 14 be able to achieve an effective seal between the housing 1403 and the donor surface 12 to prevent spray fluid and particulates from leaking through the narrow gap that must be contained approximately between the housing 1403 and the donor surface 12 of the drum 10. Various ways of achieving such a seal are shown schematically in FIG.

[0012] The simplest form of seal is a wiper blade 1408. Such seals come into physical contact with the donor surface and may scratch the applied coating if used on the outlet side of the housing 1403, i.e., downstream of the spray head 1401. For this reason, if such seals are used, they are preferably located only upstream of the spray head 1401 and / or at the axial end of the housing 1403. As used herein, the terms "upstream" and "downstream" refer to points on the donor surface 12 as it passes through the coating apparatus.

[0013] 1 also shows how particle-laden fluid can be prevented from escaping through the sealing gap between the housing 1403 and the drum 10 without any member contacting the donor surface 12. In this illustration, galleries 1409 extending along the entire periphery of the housing 1403 are connected by a set of micro-passages 1410 extending along the entire rim of the housing 1403, establishing fluid communication between the galleries 1409 and the sealing gap.

[0014] Gallery 1409 is connected to a suction source for the excess extraction system, which may be the same suction source connected to outlet 1407 or a different suction source. In this case, gallery 1409 functions to extract fluid that passes through the gap before exiting housing 1403. Low pressure can also suck particles that are not in direct contact with donor surface 12 out of drum 10.

[0015] The '499 patent also describes an embodiment in which the particles are applied to the donor surface by a rotating brush or roller interposed between the spray head 1401 and the donor surface 12 .

[0016] The reason that the coating apparatus 14 of the '661 patent merely applies a monolayer of particles to the donor surface 12 is because particles have a greater tendency to adhere to the donor surface than to each other. Thus, particles that are not in direct contact with the donor surface can be easily removed and prevented from adhering to the donor surface by either brush action, or suction, or blowing away using an air knife-like mechanism, or a combination of such actions.

[0017] Coating apparatus 14, such as those used in, inter alia, Patent Document 1, Patent Document 3 (WO 2018 / 100412), Patent Document 4 (WO 2018 / 100528), Patent Document 5 (WO 2018 / 100530), or Patent Document 6 (WO 2019 / 234597), applies a monolayer of particles to the endlessly circulating donor surface of the ITM and must ensure 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 when it arrives at the coating apparatus. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] International Publication No. 2016 / 189512 Brochure [Patent Document 2] International Publication No. 2016 / 189513 Brochure [Patent Document 3] International Publication No. 2018 / 100412 Brochure [Patent Document 4] International Publication No. 2018 / 100528 Brochure [Patent Document 5] International Publication No. 2018 / 100530 Brochure [Patent Document 6] International Publication No. 2019 / 234597 Brochure Summary of the Invention [Problem to be solved by the invention]

[0019] Therefore, the rate at which particles should be fed to the coating apparatus varies depending on the degree of coating depletion at transfer station 18, and it is an object of this disclosure to provide a coating apparatus that can adjust the particle feed to ensure that a uniform layer of particles is applied to the donor surface regardless of the rate at which the particles are transferred from the donor surface to the substrate. [Means for solving the problem]

[0020] According to a first aspect of the present disclosure, there is provided a coating apparatus for applying a layer of particles to a receiving surface, the coating apparatus comprising: a pressurized air source; an application chamber partially bounded by the receiving surface into which an air flow is delivered by the air source through a nozzle; an air return path for returning air from the application chamber to an inlet of the air source to form an air circulation loop; and a dose dispensing device for introducing particles to be coated on the receiving surface into the air circulation loop, wherein a particle deflector is positioned in the path of the air flow delivered through the nozzle, the deflector serving to break up agglomerated particles carried by the air flow before coating the receiving surface with the particles.

[0021] In some embodiments, the coating apparatus further includes brushes within the application chamber for brushing the receiving surface so as to leave only a monolayer of particles adhering to the receiving surface, and the brushes and the receiving surface move relative to each other. A first portion of the brush may be positioned adjacent to the nozzle and configured to rotate such that the bristles of the brush adjacent to the nozzle rotate in a direction that passes over the receiving surface in the same direction as the movement of the receiving surface. A second portion of the brush may be configured away from the nozzle.

[0022] In some embodiments, at least one brush of the second portion of brushes is rotated in a direction that causes the bristles of said at least one brush away from the nozzle to pass over the receiving surface in a direction opposite to the movement of the receiving surface.

[0023] Optionally, a brush deflector may be provided adjacent at least one of the brushes to deflect the tips of the bristles before contact is made between the bristles and the receiving surface.

[0024] According to a second aspect of the present disclosure, there is provided a method for applying a layer of particles to a receiving surface, the method comprising the steps of: providing an application chamber partially bounded by the receiving surface; blowing an air flow into the application chamber with an air source; returning the air from the application chamber to an inlet of the air source to form an air circulation loop; and introducing particles to be coated on the receiving surface into the air circulation loop, wherein a particle deflector is positioned in the path of the air flow, the deflector serving to break up agglomerated particles carried by the air flow before coating the receiving surface with the particles.

[0025] In some embodiments, the air stream is delivered (eg, jetted) by an air source through a nozzle, the nozzle (or an array of individual nozzles) extending across the entire width of the receiving surface.

[0026] According to a third aspect of the present disclosure, there is provided a printing apparatus including a coating apparatus for applying a layer of particles to a receiving surface; an imaging station for applying energy to selected areas on the receiving surface to enable transfer of the particles coated thereon to a substrate; and an impression cylinder station where only the particles to which energy has been applied at the imaging station are transferred from the receiving surface to the substrate to form an image on the substrate, the coating apparatus comprising: a pressurized air source; an application chamber partially bounded by the receiving surface to which an air flow is delivered by the air source; an air return path for returning air from the application chamber to an air inlet of the blower to form an air circulation loop; and a dosage device for introducing particles to be coated on the receiving surface into the air circulation loop, wherein a particle deflector is positioned in the path of the air flow delivered through the nozzle, the deflector contributing to breaking up agglomerated particles carried by the air flow before coating the receiving surface with the particles.

[0027] When the coating apparatus is incorporated into a printing system, the receiving surface may be a recirculating receiving surface and may also be referred to as an intermediate transfer member (ITM). In some embodiments, the recirculating receiving surface may be attached to or formed by the outer surface of a rigid drum, while in other embodiments, the recirculating receiving surface may be the outward-facing surface of an endless flexible belt.

[0028] In certain embodiments, the layer of particles formed on the receiving surface of the coating apparatus or as a result of the coating method is a monolayer of particles, whether or not further implemented in a printing apparatus or method. In other embodiments, the layer or monolayer formed on the receiving surface or ITM is of particles in the sub-micrometer range.

[0029] These and additional advantages and features of the present disclosure will be better understood by reference to the following detailed description taken in conjunction with the drawings and non-limiting examples. [Brief explanation of the drawings]

[0030] Some embodiments of the present disclosure will now be further described, by way of example, with reference to the accompanying drawings, in which like reference numerals or characters indicate corresponding or similar components. The description taken in conjunction with the drawings will make apparent to those skilled in the art how embodiments of the present disclosure may be practiced. The drawings are for illustrative purposes only, and no attempt is made to show structural details of the embodiments in more detail than is necessary for a fundamental understanding of the present disclosure. For clarity and convenience of illustration, some of the objects shown in the figures may not necessarily be drawn to scale. [Figure 1] As mentioned above, an apparatus for thermal transfer printing on a substrate is known from patent document WO2016 / 189512. [Figure 2] 1 is a cross-sectional view of an embodiment of the present disclosure, with the cross-sectional plane parallel to the direction of movement of the receiving surface. [Figure 3] 3 is an optical density sensor that can be used in the embodiment of FIG. 2. [Figure 4] FIG. 1 is a schematic diagram of a nozzle spraying particles onto a particle deflector. [Figure 5] 3 is a cross-sectional view of the coating apparatus of FIG. 2 taken perpendicular to the direction of movement of the receiving surface, illustrating the tendency of the side edges of the receiving surface to be deflected by suction. [Figure 6] FIG. 6 is a view similar to FIG. 5, illustrating the effect of a liquid layer between the receiving surface and the underlying support surface. [Figure 7] 3 is a detail view of FIG. 2 drawn to scale showing a nozzle with a particle deflector located upstream of two brushes, each having a brush cleaner and bristle deflector, respectively. [Figure 8] FIG. 2 shows a partial view of a brush with individual bristles arranged in staggered rows. [Figure 9] 9 is a view similar to FIG. 8 showing a brush having tufts of bristles arranged in staggered rows. DETAILED DESCRIPTION OF THE INVENTION

[0031] Figure 1, which illustrates the prior art of Patent Document 2 (WO2016 / 189513), has already been described above and therefore need not be described again. The particle dispenser or coating apparatus 102 shown in Figure 2 performs the same function as the coating apparatus 14 of Figure 1, but is directed to a printing system that employs the surface of a flexible endless belt 136 as the donor surface 108 instead of the rigid drum 10 of Figure 1. According to some embodiments of the present teachings, a coating apparatus could alternatively coat the donor surface formed by the surface of a rigid drum.

[0032] It should be clear that the coating apparatus of the present invention, an embodiment of which is apparatus 102 shown in Figure 2, is not limited to coating a donor surface in a thermal printing system. A donor surface is only one example of a receiving surface that can be coated using a coating apparatus. While the terms imaging surface and donor surface are typically associated with more complex apparatus or systems that include a coating apparatus having a receiving surface, for simplicity and unless clear from the context, terms relating to surfaces to which particles can be applied or transferred from can be used interchangeably hereinafter.

[0033] The coating apparatus 102 shown in FIG. 2 is superimposed on a donor surface 108, the direction of movement of which is indicated by arrow 114. The coating apparatus includes an air blower 104, which functions as a pressurized air source and supplies air (under pressure) to the nozzle 106 to carry the particle suspension, typically a dry particle suspension. The pressurized air source, which may also be referred to as an air source for simplicity, can alternatively be a device containing compressed air. Although referred to as an air source, 104 may actually function to generate and / or maintain an air flow that is confined within the coating apparatus and primarily recirculates within the coating apparatus. The term "air source" does not imply that only particle-free fresh air is delivered to the coating apparatus, nor does it imply that particles can attain or maintain any desired velocity. While not completely sealed, the coating apparatus can be considered a relatively closed system having a controlled volume.

[0034] The particles conveyed in the air circulation loop, described in more detail below, can be made of any suitable material, for example, thermoplastic particles (i.e., comprising or consisting of a thermoplastic polymer) if they become sticky upon heating. The nozzle can extend across the entire width of the receiving / donor surface and can also be considered an air knife.

[0035] Nozzles 106 spray air and suspended particles onto donor surface 108, forming a particle coating thereon. A plurality of rotating brushes 110 ensures that particles contact all portions of donor surface 108 that enter the coating apparatus and that excess particles are swept from donor surface 108, leaving substantially only a single layer of particles adhering to the surface. The brushes may have individual bristles, as shown in FIG. 8, or groups or tufts of bristles arranged, for example, in staggered rows, as shown in FIG. 9. Each brush may optionally be associated with a brush cleaner 115 that can reduce the amount of particles that may accumulate on the bristles over time (e.g., by shaking or scraping particles off by contact). The brushes are contained within application chamber 112, which is bounded by a partition 113 that separates application chamber 112 from conduit 116. Conduit 116 leads to chamber 118, which together form a return path for returning air and unused particles to the inlet of blower 104 to maintain a desired flow rate. In this manner, an air circulation loop is formed to recycle particles not applied to donor surface 108. Typically, the flow rate of the recirculating air in the circulation loop is faster than the relative velocity of the receiving surface, so that any portion of the surface is exposed to more than one air cycle before exiting the coating apparatus. Without wishing to be bound by theory, it is believed that this relatively high flow rate of air propelling the particles in the circulation loop promotes the formation of a complete (e.g., void-free) layer of particles as the receiving surface passes through the coating apparatus.

[0036] Particles applied to the donor surface 108 reduce the particle concentration or density in the circulation loop. Therefore, it is necessary to replenish particles from a tank (not shown) by a dose dispensing device 120 regulated by an electronic control device (not shown). The dose dispensing device 120 must be able to introduce a metered amount of particles into the air circulation loop because if the particle concentration is too low, the donor surface 108 may not be completely coated with particles. Conversely, if the particle concentration is too high, it will be difficult to ensure that only a uniform layer of particles is applied to the donor surface 108. Furthermore, too high a particle concentration can lead to safety or health hazards (e.g., a concentration above the lower explosive limit (LEL) of the particles). Therefore, it is important to maintain the particle concentration within a predetermined range to ensure safe, uniform, and efficient coating of the donor surface 108. The predetermined range (or range limits) may depend on considerations such as the intended use of the coating apparatus or the system implementing it, the rate of particle depletion from the receiving surface, and the degree of particle loss to the walls of the coating apparatus or any part thereof (e.g., brush bristles or the filtering system of its recirculation loop). Thus, the predetermined limits desirable in any particular case can be easily ascertained by those skilled in the art of using such apparatus. If the coating apparatus and the particle layer formed thereby are used, for example, in a printing system where the printed image is not always the same, the dosage device 120 cannot be controlled to meter particles at a constant rate; instead, the dosage rate must be matched to the density of the image being printed. In digital printing systems, the printed image may vary from image to image or from one print job (printing the same first image) to a subsequent print job (printing the same second image). Such variations, which may be rapid and / or frequent, can pose challenges for the coating apparatus.

[0037] Briefly, the dose dispensing device functions to add a controlled amount of new / fresh particles within the air circulation loop to particles that were not bound to the receiving surface in the previous cycle of the recirculating air stream, replacing at least a portion of the depleted particles. As previously mentioned, particles can be "intentionally" depleted to account for their removal from the receiving surface to serve an intended purpose, or inadvertently depleted to account for losses to the walls and parts of the coating apparatus.

[0038] 2, the dose dispensing device 120 is shown downstream of the particle concentration sensor 122 and upstream of the air source 104 that facilitates the recirculation of particles not bound to the receiving surface 108 in the air circulation loop. In its illustrated position, the dose dispensing device may obscure at its rear the components of a coating apparatus that allow the airflow and circulation of particles therein, or any device associated with the desired processing described above.

[0039] For example, although not shown in the figures, the particles may optionally be fed to the administration device through one or more pre-treatment devices designed to at least partially remove (e.g., filter) such particles and / or at least partially reduce the size of any agglomerates thereof, so that smaller aggregates, smaller clusters, or even individual particles may be entrained by the air circulation loop after the dose administration device. Similarly, the recirculated air (including the particles therein) may be "treated" to control its temperature, relative humidity, and / or electrostatic charge.

[0040] To regulate the rate at which particles are metered by the dose dispensing device 120, its controller can receive a signal from a particle density sensor. While other forms of such a sensor (e.g., an electrostatic sensor) can be used, the embodiment depicted in Figure 2 employs an optical density sensor (ODS) 122 located in the air circulation loop to monitor the concentration of particles. To improve the accuracy of the particle density sensor (e.g., ODS 122), it is desirable to position its sensing portion in an area with less turbulence in the airflow, such as immediately in front of the dispensing device 120.

[0041] A suitable optical density sensor design is shown schematically in Figure 3. The sensing portion of the sensor includes a light source in the form of a light-emitting diode (LED) 124 that emits white light and a light-sensing element in the form of, for example, a photoresistor 126. As particles 144 flow through the sensor 122 between the LED 124 and the photoresistor 126, the signal output from the photoresistor 126 decreases. The more light is blocked, the lower the output signal. The sensor 122 is shown with a second light-sensing element in the form of a second photoresistor 127 to measure scattered light instead of transmitted light. The scattered light-sensing photoresistor 127 also generates an output signal, but unlike the output of the photoresistor 126, it increases with particle density. Because the output signals of the two photoresistors 126, 127 are complementary, only one of them is needed. If both signals are present, the output signal of the scattered light 127 sensor can be used to verify or combine with the output from the photoresistor 126 to improve the signal-to-noise ratio. If both the photoresistor 126 and the scattered light sensor 127 have low readings, this indicates possible fouling of the LED 124 and / or the photoresistors 126, 127, so the sum of the output signals of the two photoresistors can be used to check the operation and cleanliness of the sensors. It should be noted that although the particles are shown in the diagrams for simplicity by circles to suggest spherical particles, they can take any other shape, and the present coating apparatus and method is also applicable to particles having non-spherical shapes such as flakes, rods, irregular or amorphous chunks, etc.

[0042] To reduce particle buildup on the light source 124 or light-sensing elements 126, 127, the sensor 122 may have air channels 128 leading to each of its three elements to keep them clean. Air can be blown into the channels 128, or if the sensor 122 is placed in an area of ​​the recirculation path under negative pressure, ambient air can be sucked through the channels 128. The output signal of the light-sensing elements (e.g., photoresistors) can then be used by a controller to regulate the amount of particles 144 metered into the air circulation loop by the dosing device 120.

[0043] As an alternative to adjusting the dose dispensing device 120 based on direct measurement of particle concentration in the application chamber of the air recirculation loop, it may be adjusted by the printing system based on measured or predicted consumption of particles. Particle consumption can be measured by viewing the print output of the printing system and analyzing the output signal of an optical device such as a camera or optical density scanner, or it can be predicted by analyzing input signals applied to the printing system. The two methods of adjusting the dose dispensing device, which can be considered "feedback" control and "feedforward" control, need not be mutually exclusive and can be combined to further reduce the time delay in realizing changes in particle delivery achieved by the dose dispensing device.

[0044] Feedback assessment of particle consumption typically involves measuring the optical density (OD) of the printed image. The optical density of various points on the printed image can be measured 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 light reflected from the image. OD measurements can be performed before printing the target image using a reference calibration image (e.g., a 100% coverage "solid patch" color), or they can be performed for the intended target image of each print job, with the controller's goal being to set the dosing device so that the measured OD matches the intended target OD of the image. Conventional proportional-integral-derivative (PID) controllers can be used for relatively long print jobs where the printing process remains relatively constant. However, other controllers that can better account for changes in press operating conditions may be suitable. Such controllers, adapted to maintain particle concentrations in the coating chamber within predetermined limits, are known to those skilled in the art of printing, particularly in the control of digital printing processes, and will not be described in further detail herein.

[0045] The feedforward prediction of particle consumption can be based on an analysis of the images intended to be printed during a particular print job. Controllers adapted for such preventative methods adapted to maintain particle concentrations in the coating chamber within predetermined limits are known in the printing industry and need not be further detailed here.

[0046] The control system can incorporate predictive adjustments of the delivery device, providing initial settings and feedback adjustments as needed to compensate for deviations between prediction and actual.

[0047] With regard to the above-mentioned control of the dosage dispensing device, it should be noted that a printing system implementing a coating apparatus according to the present teachings can be considered more buffered than, for example, a printer that relies on ink jets. While excessive ink deposition, or conversely, insufficient jetting, can easily lead to poor print quality, a coating apparatus typically contains particles in its air circulation loop in an amount not only sufficient to form or replenish a monolayer, but also capable of easily accommodating increased particle consumption; such excess particles are not wasted on the print substrate but are recycled until transfer is required. This makes the print quality obtained using current coating apparatus less dependent on immediate changes in operating conditions and also reduces ink waste.

[0048] Readers interested in details of such offset printing systems, which include 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 enable them to be transferred to a substrate, and an impression cylinder station where only those particles to which energy is applied at the imaging station are transferred from the ITM to the substrate to form an image on the substrate, and in which the coating apparatus of the present teachings may be advantageously implemented, are referred to, inter alia, U.S. Patent Nos. 5,629,997, 5,729,979, 5,733,987, 5,749,967, or 5,759,022, filed by the same applicant (WO2016 / 189512, WO2018 / 100412, WO2018 / 100528, WO2018 / 100530, or WO2019 / 234597).

[0049] 2, the air blower 104 may comprise a chamber housing a large fan configured to output a large volume of air and particle mixture at low pressure. The fan blows particles 144 (both those already in the recirculation loop and new particles from the dose delivery device 120) through a feed chamber 130 that leads to a nozzle 106 in the application chamber 112.

[0050] Particles 144 can, in some circumstances, be attracted to one another and stick together, thereby forming larger agglomerated particles or clusters. This is a problem when the particles are very small (e.g., a few microns in diameter or less) or when the particles are wet. If an airflow containing such agglomerated particles is applied directly to the donor surface 108, it can result in an uneven coating.

[0051] Without wishing to be bound by theory, it is believed that if there are relatively large particles (e.g., agglomerates or clusters) applied to the donor surface 108, the brush 110 may remove them more easily than the relatively small particles, causing holes or discontinuities in the particle coating, or if the brush does not remove the larger clusters, it may be considered a multilayer due to the agglomeration of smaller individual particles. Furthermore, over time, larger particles tend to be replaced by smaller particles, resulting in a change in the impact that such particles (or changes in particle size distribution) have on the intended product. Returning to the use of particle layers or coating devices to apply them in a printing system for illustration, changes in particle size distribution within the population applied to the donor surface affect the appearance of the print. For example, variations in the applied particle population may alter the optical density or gloss of the printed image (which depends on the layer height / thickness, which itself depends on the average particle size).

[0052] To address this issue, to the extent that it may occur with the particles to be applied and / or their size distribution, FIG. 4 shows a particle deflector 132 positioned below the nozzle 106. The particle deflector 132 can be made of any material that is hard enough to break down agglomerated particles into smaller particles 144 that are better suited to the intended application (e.g., printing). The smaller particles may be individual particles or smaller agglomerates of particles. One example of a suitable material for the deflector would be a steel alloy. The particle deflector can have any shape capable of producing smaller particles and significantly redirecting the airflow. The particle deflector 132 is used to cause the cloud of smaller particles 144 in the application chamber 112 to settle primarily onto the rotating brush 110 but also onto the donor surface 108. Applying a cloud of particles 144 rather than spraying them directly onto the donor surface 108 results in, among other things, one or more of: narrowing the particle size distribution; forming a more uniform particle coating than would be possible without the deflector; more effective removal of excess particles 144 from the donor surface 108 by the brush; and reducing, slowing, or preventing damage that some particles may cause to the donor surface upon impact. For example, particles made of a material with a bulk hardness greater than the surface of the transfer member may damage it with a relatively low impact force, while a relatively high impact force may make any particle abrasive to the donor surface.

[0053] It is believed that the particle population may have a narrower size distribution due to the breakup of agglomerated particles, and that constant recycling of particles during operation of the coating apparatus may prevent reagglomeration and / or provide at least a partial size reduction effect, thereby reducing the occurrence of size variations observed over time in the absence of a deflector, thereby reducing or eliminating secondary effects that such variations may have on the final product (e.g., inconsistencies in optical density or gloss of the print).

[0054] In a series of experiments performed under similar conditions, except for the presence or absence of a deflector in the path of particles circulating along the airflow, the presence of a deflector dramatically reduced the proportion of agglomerates on the donor surface, resulting in patches of "multilayers"—a mosaic of smaller particles forming a monolayer (as assessed by microscopy and image analysis). As a result of the reduced amount of relatively large particles applied, fewer voids formed in the applied particle coating after removal of excess particles. While this effect evolves with the number of cycles, for illustration, a device that would achieve approximately 40% monolayer coating, approximately 50% multilayer coating, and 10% voids after 10 cycles without a deflector may achieve improved results with the use of a deflector, increasing the relative coverage to over 90% monolayer coating and reducing the amount of both multilayer patches and voids, each to less than 5% of the coated area.

[0055] Regardless of its benefits to the population of particles applied to the donor surface by the coating apparatus of the present teachings, the deflector may alternatively or additionally function to protect the donor surface from the potentially harmful effects that direct application may have on the surface.

[0056] The application chamber 112 typically contains multiple brushes 110 or rollers. The brushes 110 closest to the nozzle 106 apply particles 144 to the donor surface 108. As seen in FIG. 2 , the three right-most brushes 110 closest to the nozzle 106 may rotate in a direction that causes their bristles to pass in the same direction as the donor surface travel. The rotational speed of these brushes 110 can be such that their linear velocity is greater than the velocity of the donor surface 108. For example, the donor surface 108 may travel at 2 m / s, and the linear velocity at the radial end of the brush 110 may travel at 5 m / s, thereby creating a skid of 3 m / s, allowing the bristles of the brush 110 to wipe the particles 144 onto the donor surface 108. The skid speed may need to be tailored to the particles being applied and the donor surface to be coated, but a skid between 2 m / s and 5 m / s is believed to be suitable for many applications. A higher skid may improve the efficiency of particle application and / or removal, but may also increase the risk of donor surface wear. In the above example, the brush was considered to have a faster speed than the donor surface, but this should not be construed as a limitation; instead, the relative speed skid can be achieved by the donor surface having a faster speed than the brush. The skid does not need to be the same for each brush that can contact the donor surface.

[0057] Brushes 110 located further away from the nozzle 106 may serve the purpose of removing excess particles 144 from the donor surface 108, leaving only a monolayer. In FIG. 2, the four brushes 110 on the left represent particle removal brushes. The particle removal brushes may rotate in a direction that causes the bristles to pass in the opposite direction to the movement of the donor surface at a higher relative speed than the other brushes, and may have wiper blades (e.g., brush cleaners 115) or the like to remove particles 144 from the bristles, thereby cleaning them and making them more effective. In some embodiments, the particle removal brushes may be replaced by an air knife or any similar device capable of removing excess particles so that the donor surface at its exit from the coating apparatus is coated with approximately a monolayer of particles.

[0058] The above description of brushes is intended as an example only. It will be understood that there may be any number of brushes 110 (and indeed nozzles 106), and that they may rotate in different directions and / or at different relative speeds than those described above. Furthermore, while the described brushes 110 are identical in structure, it should be noted that in some embodiments, the brushes may differ to better suit their role. For example, the stiffness and / or chemical composition of the bristles may be altered to suit the task being performed. Thus, the properties of the bristles may vary, but the important thing is only to ensure that the bristles do not damage the donor surface while performing their intended task.

[0059] In one embodiment, the action of the brush and its respective bristles can be further tuned by physical factors to further facilitate the application of particles to the donor surface and / or the removal of excess, and / or to further mitigate potential damage to the particle coating or donor surface. Without wishing to be bound by any particular theory, the bristles can be considered to contact the underlying horizontal donor surface or particles thereon with a force that includes vertical and horizontal components, each having a different magnitude along the bristles as they contact the movable ITM. The vertical force can have an undesirable effect of detaching particles loosely attached to the donor surface (e.g., relatively large particles), while the horizontal component of the bristle force provides a gentler swiping effect. The vertical force can be considered to cause a whipping effect of the bristles. Preferably, the horizontal force should be tuned high enough to remove excess particles not directly contacting the donor surface, yet low enough so as not to disturb the underlying monolayer of particles in contact with the donor surface.

[0060] While the normal force can be modified by selecting the appropriate brush distance from the surface or bristle length and physicochemical properties, as discussed above, it can also be attenuated by placing physical obstacles in the path of the bristles on either side of the point where they would contact the ITM in the absence of such interference. In FIG. 7, an example of such an obstacle is shown in the form of a triangular-section deflector rod 150, although it may have other cross sections. Such obstacles to the normal component of the bristle force are referred to herein as bristle or brush deflectors, elements positioned at a height h above the donor surface to allow the particle coating to pass untouched. Bristle or brush deflectors form an angle α with the donor surface on the edge that deflects the bristles, in order to reduce their normal effect. The deflector edge that forms the angle with the donor surface need not be directly below the axis of rotation of the brush with the bristles, but can be offset a distance X therefrom. Those skilled in the art can readily understand how to vary the magnitudes of α, h, and X (in addition to any other preset parameters, such as bristle stiffness and length, and brush distance from the donor surface) in a bristle deflector to tailor the vertical and horizontal bristles' velocity and force to the intended purpose, operating conditions, and particles being applied to a given ITM. In its simplest form, the brush or bristle deflector can be shaped as a surface inclined relative to a plane approximately parallel to the donor surface and a distance (height) h away from the donor surface. The bristle deflector can be shaped, for example, as a triangle whose edges follow the contour of the donor surface, although this example should not be construed as limiting, and all other shapes of deflectors capable of deflecting the bristles to sufficiently alter the vertical component of the bristle's force against the donor surface are encompassed.

[0061] Regardless of the shape, size, and location relative to the expected unaltered contact point (e.g., upstream and / or downstream of the point where the bristles would initially contact the brush deflector), such brush deflectors should advantageously be made of a material that is compatible with the bristles. Compatible, in this context, means that the bristle deflector does not damage the bristles upon contact with it, either physically (e.g., reducing or preventing scratches, cuts, or breakage) or chemically (e.g., selecting the materials of the bristles and their deflectors so that they do not adversely affect each other), or in any other way detrimental to the function of the brush and its deflector (if present). For example, the brush deflector should not affect the ability of the bristles to attract, retain, and / or dispense particles, as is the case with different brushes with different functions. By way of example, the brush deflector may be made of a metal or alloy, or coated with an elastomer, that is stiff enough to bend the impacting bristles but not so stiff as to damage them.

[0062] 2 can be at least partially replaced by a flexible member circulating over two or more rollers, the flexible member having bristles on its outer surface facing the donor surface that can act as detailed above for more conventional brushes, where the bristles are typically attached to a single, more rigid roll. Without wishing to be bound by theory, it is believed that a circulating member with such bristles can replace at least two brushes while also providing a bristle deflection effect over a larger area of ​​the corresponding donor surface.

[0063] 2, the coating chamber 112 is partially bounded by the donor surface 108. The gap that exists between the walls of the coating chamber 112 and the donor surface should not allow particles 144 to escape. Aside from the fact that such particles are not recycled, they may pose health and even safety risks.

[0064] To prevent leakage of air and particles from the coating chamber 112, the coating chamber 112 is surrounded by a bell housing 141 that defines a suction chamber 142 that surrounds the coating chamber 112 on all sides. As represented by arrow 146, the suction chamber 142 is connected to a pump and filter unit 134 that reduces the pressure within the suction chamber 142 to below atmospheric pressure and below the pressure within the coating chamber 112. Thus, particle-carrying air is constantly sucked into the coating apparatus 102, as represented by arrow 148, instead of escaping into the ambient atmosphere. In another embodiment, particles captured by the suction chamber of the bell housing, which may accumulate over time on the filter unit, can alternatively or additionally be delivered to the dose administration device 120, allowing previously filtered particles to be reintroduced into the air circulation loop.

[0065] By applying a low enough pressure in the vacuum chamber 142 to ensure that particles 144 do not escape, the donor surface can be sucked against the bell housing 141. However, it is important that the donor surface 108 and the bell housing should not come into contact with each other.

[0066] The tension in belt 136 can be used to ensure that donor surface 108 does not come into contact with the laterally extending edge of bell housing 141 as it enters and exits coater 102 because belt 136 can tension the rollers at this point. However, the laterally extending edges of donor surface 108 cannot be supported in this manner along the entire length of the coater, and as shown in FIG. 5 , the low pressure in suction chamber 142 can cause the side edges of donor surface 108 to flap and be drawn into contact with the sides of bell housing 141 that are parallel to the direction of belt / donor surface movement. This contact not only accelerates wear and can rupture donor surface 108, but can also damage the coater and / or particle layer.

[0067] In some embodiments, the belt 136 may further include protruding formations along its side edges that can engage with the side tracks as the belt travels beneath the coating device. When engaged with their respective tracks, these lateral formations can place the belt under lateral tension, at least in the area facing the suction chamber. Additionally or alternatively, these formations can force the belt to follow a desired path, at least in the segment of the path corresponding to the coating device or at least its suction chamber. The lateral formations may be (a) multiple formations spaced apart along the length of the belt, or (b) a continuous formation along the entire length of the belt's side edges, optionally having a greater thickness than the belt. In one embodiment, the formations are (a) made of a material with a low coefficient of friction to ensure smooth movement of the formation within the lateral tracks, and / or (b) made of a material with lubricating properties, or containing an agent with lubricating properties, or coated with a coating agent with lubricating properties.

[0068] However, belt 136, whose surface serves as donor surface 108, may not be formed from a material strong enough to withstand the degree of tension required to prevent contact between the side edges of the belt and the longitudinally extending sides of bell housing 141. For example, belt 136 may need to be formed from a relatively thin, or in certain embodiments, transparent, material to allow light to pass through its backside before reaching donor surface 108.

[0069] The donor surface 108 can be prevented from being sucked against the coating apparatus housing in a variety of ways, such as by providing rollers along the sides of the housing. Figures 5 and 6 show two currently recommended solutions.

[0070] 5, a fiber-reinforced silicone blanket 138 serves to provide a support surface for belt 136, which carries the donor surface. Blanket 138 moves over a different set of rollers independently of belt 136, but the movements of belt 136 and blanket 138 are synchronized. Blanket 138 is under sufficient tension and is stiff enough to prevent it from being sucked against bell housing 141.

[0071] 5 relies on the fact that both blanket 138 and belt 136 carrying donor surface 108, when made from silicone materials, tend to stick to each other. This effectively strengthens donor surface 108 and prevents it from being sucked against bell housing 141.

[0072] Instead of relying on the tendency of surfaces (e.g., silicone) to stick together, the embodiment of Figure 6 relies on surface tension. In this embodiment, a liquid (e.g., oil) film 140 is applied between the belt 136 and the blanket 138, which serves to prevent the two from separating. Because oil is a lubricant, a stationary lubricated plate could be provided in place of the blanket 138, but a recirculating blanket is preferred.

[0073] As discussed above and shown in FIGS. 8 and 9 , the brush 110 may have rows of individual bristles or tufts of bristles, which may be staggered to ensure that the bristles contact the entire donor surface. Those skilled in the art of brush manufacturing will understand the parameters that control the density of the bristles in a brush and, more importantly, the density of the bristle tips on the brush's exterior surface that contact the donor surface or particles thereon. While these need not be detailed herein, the diameter of each bristle, its shape, the number of bristles in a bundle (if present), the distance between the joined ends of each bristle or bundle in a row, the distance between adjacent rows, the angle each row forms with the brush's axis of rotation, and similar considerations can affect the density of the bristle tips on the brush's exterior surface. Advantageously, this density should be sufficient to allow the brush to have a generally uniform effect on the donor surface or particles thereon. Conversely, the bristles or rows thereof cannot be spaced apart in such a way that their tips cannot reach areas of the donor surface.

[0074] Depending on, among other things, their dimensions and the material from which they are formed, bristles can exhibit varying hardness / flexibility / ability to acquire, hold, and / or dispense particles, which can be selected and adapted to the particles and the donor surface to be coated therewith, and / or the distance between the axis of rotation of the brush to which the bristles are attached and the donor surface. While the length of the bristles should be sufficient to impact the donor surface to apply and / or remove particles, and short enough to avoid damaging the donor surface or particle coating, the range of this acceptable length (including the physical distance spanned by the bristles) can depend on the considerations discussed above.

[0075] By way of illustration, all other bristle parameters being similar, including the excess length of the bristle impacting the donor surface, a relatively long bristles rotating about an axis farther from the donor surface will apply less pressure than a relatively short bristles rotating about an axis closer to the donor surface. While reduced pressure is beneficial as far as donor surface wear is concerned, it may be insufficient to remove excess particles. Similarly, all other parameters being similar, a bristles with a relatively large diameter will apply more pressure than a bristles with a relatively small diameter. While this increased pressure is beneficial for removing excess particles, it may also damage the donor surface.

[0076] In some embodiments, the bristles are made of durable natural or synthetic materials. Suitable natural materials can be of plant or animal origin, including animal hair, fur, down, plumes, or feathers. Synthetic materials can be made to mimic the previous examples of natural materials, but can also be plastic materials, such as nylon. Preferably, the bristles are made of a material that allows for adequate transport of particles coated by the device on the donor surface. In other words, the bristles should be able to sufficiently attract / retain particles and remove excess, albeit to a lesser extent than the donor surface, while also being able to release particles to prevent accumulation or other saturation processes that would result in inefficiency. Since the brushes of the coating device do not need to be identical, the bristles of each can be different as well.

[0077] The bristles may have any suitable shape and cross-section. In some embodiments, the bristles have a cylindrical shape and the diameter of the bristles is at least 5 μm, at least 10 μm, or at least 15 μm. In some embodiments, the diameter of cylindrical bristles is at most 100 μm, at most 75 μm, or at most 50 μm. In further embodiments, the diameter of cylindrical bristles is between 5 μm and 100 μm, between 10 μm and 75 μm, or between 10 μm and 50 μm. In other embodiments, the cross-section perpendicular to the length of the bristles is not an ideal circle, but rather an ellipse or polygon, in which case the appropriate "bristle diameter" can be approximated by the maximum length of the cross-section at the upper limit and the minimum length of the cross-section at the lower limit. For example, taking a bristles having a rectangular cross section, its long side may in some embodiments not exceed 100 μm, 75 μm, or 50 μm, and its short side may be at least 5 μm, at least 10 μm, or at least 15 μm.

[0078] In some embodiments, the bristles have an overall length of at least 7 mm, at least 8 mm, or at least 9 mm. In some embodiments, the overall length of the bristles is at most 20 mm, at most 17.5 mm, or at most 15 mm. In further embodiments, the overall length is between 7 mm and 20 mm, between 8 mm and 17.5 mm, or between 9 mm and 15 mm.

[0079] In some embodiments, the bristles have a total length that exceeds the shortest distance between the end of the bristles attached to the brush and the donor surface, with the difference between the two values ​​(excess length) being at least 200 μm, at least 500 μm, or at least 1 mm. In some embodiments, the excess length of the bristles compared to the shortest distance is at most 5 mm, at most 4 mm, or at most 3 mm. In further embodiments, the excess length is between 200 μm and 5 mm, between 500 μm and 4 mm, or between 1 mm and 3 mm. While too much excess length may damage the donor surface, a sufficiently long length may improve the swiping effect of the bristles. In other words, it may facilitate the application of particles and the removal of excess portions that do not directly contact the donor surface.

[0080] In some embodiments, the bristles are attached to the brush base or other suitable support as a bristle bundle. Because bundled bristles tend to diverge relative to their attachment point at the base, the bundle can be characterized by the dimensions of the attachment point. For example, in the case of a bristle bundle attached to the brush via a cylindrical recess in the brush support, the bundle can be defined by the diameter of the recess into which their bases are inserted and secured to the brush. In such embodiments, bundle diameters of less than 3.0 mm, less than 2.5 mm, or less than 2.0 mm have been found to be suitable.

[0081] For illustrative purposes, the present disclosure has been described with reference to specific embodiments and generally associated methods, but modifications and permutations of the embodiments and methods will be apparent to those skilled in the art based on applicant's disclosure herein. It is to be understood that the present disclosure is not limited to the specific embodiments described herein. It is intended to embrace all such alternatives, modifications, and alterations and to be bound only by the spirit and scope of the disclosure and any changes that come within the meaning and range of equivalents thereof.

[0082] It will be understood that certain features of the present disclosure that are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the present disclosure that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or with any other described embodiment of the present disclosure, as appropriate. Certain features described in the context of various embodiments should not be construed as essential features of those embodiments, unless the embodiments are inoperable without those elements.

[0083] Unless otherwise stated, the use of the word "and / or" between the last two elements of a list of alternatives indicates that selecting one or more of the alternatives is appropriate and may be performed.

[0084] The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." An embodiment described as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments and / or as excluding the incorporation of features from other embodiments.

[0085] In this disclosure, unless otherwise specified, adjectives such as "substantially," "approximately," and "about" modifying a condition or relationship characteristic of a feature or characteristic of an embodiment of the present technology should be understood to mean that the condition or characteristic is defined within a tolerance acceptable for operation of the embodiment for its intended use or within a variation expected from measurements made and / or measurement equipment used. When the terms "about" and "approximately" precede a numerical value, they are intended to indicate only + / - 15%, or + / - 10%, or + / - 5%, as the case may be, to indicate the exact value. Furthermore, unless otherwise specified, terms (such as numbers) used in this disclosure should be interpreted as having tolerances that may deviate from the exact meaning of the associated term, even without such adjectives, but that enable the invention, or relevant portions thereof, to operate and function as described and as understood by those skilled in the art.

[0086] In the description and claims of this disclosure, the verbs "comprise," "include," and "have," and each of their conjugations, are used to indicate that the object or subject of the verb is not necessarily a complete list of features, members, steps, components, elements, or parts of the subject or subject of the verb.

[0087] As used herein, the singular forms "a," "an," and "the" include plural references and mean "at least one" or "one or more" unless the context clearly dictates otherwise. At least one of A and B is intended to mean either A or B, and in some embodiments can mean A and B.

[0088] Location or motion terms such as "upper," "lower," "right," "left," "bottom," "below," "lowered," "low," "top," "above," "elevated," "high," "vertical," "horizontal," "backward," "forward," "upstream," and "downstream," and grammatical variations thereof, are used herein for illustrative purposes only and may be used to indicate the relative location, position, or displacement of particular components, to indicate first and second components in the view at hand, or both. Such terms do not necessarily indicate, for example, that a "bottom" component is below a "top" component, since orientation, components, or both may be flipped, rotated, moved, diagonally positioned or positioned, horizontally or vertically positioned, or similarly modified in space.

[0089] Unless otherwise stated, when outer boundaries of a range for a feature of an embodiment of the present technology are set forth in the disclosure, it should be understood that in this embodiment, possible values ​​for that feature can include the stated outer boundaries, as well as values ​​between the stated outer boundaries.

[0090] To the extent necessary to understand or complete the disclosure of this disclosure, all publications, patents, and patent applications mentioned herein, including specifically applicant's applications, are expressly incorporated by reference in their entirety as if fully set forth herein.

Claims

1. A coating apparatus for applying a layer of particles comprising a thermoplastic polymer to a receiving surface, comprising: a) a source of pressurized air; b) an application chamber partially bounded by said receiving surface into which an air flow is delivered by said air source through a nozzle; c) an air return passage for returning air from the application chamber to the air source inlet to form an air circulation loop; d) a dose dispensing device for introducing a metered amount of particles to be coated onto the receiving surface into the air circulation loop; a particle deflector positioned in the path of the airflow delivered through the nozzle, the deflector serving to break up agglomerated particles carried by the airflow prior to coating the receiving surface with the particles; Coating equipment.

2. 10. The coating apparatus of claim 1, further comprising brushes within the application chamber for brushing the receiving surface so as to leave only a monolayer of particles adhering to the receiving surface, each of the brushes and the receiving surface moving relative to one another, each of the brushes comprising bristles.

3. 3. The coating apparatus of claim 2, wherein the first portion of the brush is positioned adjacent to the nozzle and is rotatable in a direction that causes the bristles of the brush adjacent to the nozzle to pass over the receiving surface in the same direction as the movement of the receiving surface.

4. 4. A coating apparatus according to claim 2 or 3, wherein the second portion of the brush is provided away from the nozzle.

5. 5. The coating apparatus of claim 4, wherein at least one brush of the second portion of brushes is rotated in a direction that causes the bristles of the at least one brush away from the nozzle to pass over the receiving surface in a direction opposite to the movement of the receiving surface.

6. 6. The coating apparatus according to claim 2, wherein a brush deflector is provided adjacent to at least one of the brushes to deflect the tips of the bristles before contact is made between the bristles and the receiving surface.

7. 7. A coating apparatus according to claim 1, wherein the application chamber is surrounded by a bell housing defining a suction chamber to prevent particles from escaping into the surrounding atmosphere.

8. 8. A coating apparatus according to claim 1, wherein the receiving surface is a recirculating surface of an endless belt, the endless belt having a support surface on the opposite side of the receiving surface.

9. 9. The coating apparatus of claim 8, wherein said support surface is movable with said endless belt.

10. 10. A coating apparatus according to claim 8 or 9, adapted to provide an oil film between the endless belt and the support surface.

11. A method for applying a layer of particles comprising a thermoplastic polymer to a receiving surface, comprising: i) providing a coating chamber partially bounded by said receiving surface; ii) blowing an air flow into the application chamber with an air source; iii) returning air from the application chamber to the air source inlet to form an air circulation loop; iv) introducing into the air circulation loop a metered amount of particles to be coated onto the receiving surface; a particle deflector positioned in the path of the airflow, the deflector serving to break up agglomerated particles carried by the airflow prior to coating the receiving surface with the particles; method.

12. 12. The method of claim 11, wherein the air flow is blown by the air source through a nozzle.

13. 13. The method of claim 11 or 12, further comprising the step of providing brushes within the application chamber for brushing the receiving surface so as to leave only a monolayer of particles adhering to the receiving surface, each brush having bristles, and each brush and the receiving surface moving relative to one another.

14. 14. The method of claim 13, further comprising a brush deflector disposed adjacent at least one of the brushes for deflecting the tips of the bristles of that brush before contact is made between the bristles and the receiving surface.

15. An offset printing system comprising an ITM (intermediate transfer member), a coating device for applying a layer of particles to the ITM, an imaging station for making the particles transferable to a substrate, and an impression cylinder station where only the particles made transferable by the imaging station are transferred from the ITM to the substrate to form an image on the substrate, wherein the coating device is as described in any one of claims 1 to 10.

16. 16. The offset printing system of claim 15, wherein the imaging station applies energy to selected particles on the ITM such that only those particles to which energy is applied are transferred.

17. 17. The offset printing system of claim 16, wherein the applied energy is in the form of electromagnetic (EM) radiation that can be applied from the front or back side of the ITM, or in the form of thermal conduction that can be applied through the ITM.

18. 16. The offset printing system of claim 15, wherein the imaging station modifies selected areas of the substrate upstream of the impression cylinder station so that only particles in areas on the ITM corresponding to the selected areas are transferred.

19. An offset printing system according to any one of claims 15 to 18, wherein the particles are thermoplastic particles.

Images