Equipment for coating surfaces with particles

Abstract

This disclosure relates to a coating apparatus for applying a single layer of particles to a receiving surface. The apparatus includes a pressurized air source; a dressing chamber partially defined by the receiving surface, into which air is supplied from the air source; an air return path for returning air from the dressing chamber to an air inlet of the air source to form an air circulation loop; a metering device for introducing particles to be coated onto the receiving surface into the air circulation loop; and a controller for adjusting the metering device to maintain the particle concentration at a predetermined limit. A method for applying a particle layer is also provided, as well as a printing system benefiting from this coating apparatus.

Description

[0001] Related applications

[0002] This application claims priority under the Paris Convention from UK application No. 2005159.5, filed on 7 April 2020, and UK application No. 2020722.1, filed on 30 December 2020. This application also relates to the concurrently filed international application entitled "Apparatus for Coating a Surface with Individual Particles," No. PCT / IB2021 / 052775 (Agent No. LIP 17 / 014PCT). As fully set forth herein, the entire disclosure of all the foregoing applications is incorporated herein by reference for all purposes. Technical Field

[0003] This disclosure relates to an apparatus for coating a receiving surface with particles and the control thereof. Background Technology

[0004] In some types of printing, a film supported by a carrier is transferred to a substrate (e.g., paper, cardboard, plastic film, etc.) by applying pressure and / or heat in the desired pattern. An example of this is found in heat transfer typewriters, where a ribbon carries an ink film that is transferred to paper by applying heat.

[0005] The problem with using traditional coating carriers (whether sheets, rolls, or ribbons) is that the method is wasteful and therefore expensive. This is because when the film coating must be discarded, only a small portion of it has been used (e.g., for printing text), while the majority of the film coating remains on the carrier.

[0006] In WO2016 / 189512, the applicant disclosed a printing apparatus capable of mitigating the aforementioned disadvantages. The latter disclosure... Figure 1 Copy this as an attached image. Figure 1 And now I will briefly describe it.

[0007] Figure 1An annular intermediate transfer member (ITM) is shown, having an outer surface 12 serving as an imaging surface 12. The ITM is described as a drum 10 in WO2016 / 189512, but it can alternatively be an annular belt. As the drum 10 rotates clockwise as indicated by the arrow, it passes beneath a coating apparatus or particle dispenser 14, where it acquires a fine-particle coating, which, if desired, is appropriately configured for particles to form a monolayer downstream of the coating apparatus. In the exemplary example of WO2016 / 189512, after exiting the coating apparatus 14, the imaging surface 12 passes beneath an imaging station 16, where selected areas of the imaging surface 12 are exposed to laser radiation, causing the particle coating on the selected areas of the surface 12 to become sticky. The surface 12 then passes through an impression or transfer station 18, where a printing substrate 20 is compressed between the drum 10 and an impression cylinder 22. This causes the selected areas of the particle coating on the imaging surface 12, which have become sticky due to exposure to laser radiation in the imaging station 16, to transfer from the imaging surface 12 to the substrate 20. The area on the imaging surface corresponding to the sticky region transferred to the substrate is cleared and thus exposed due to the transfer of particles. The imaging surface 12 can then complete its cycle by returning to the coating apparatus 14, in which a new particle coating is applied only to the cleared area, while the previously coated particles are transferred from the cleared area to the substrate 20 in the imprint station 18.

[0008] Although surface 12 is referred to as the imaging surface in the above-described printing system, it may alternatively be referred to as the donor surface 12 in any industrial application, where the coated particles (or a portion thereof) are ultimately donated by the surface (e.g., transferred from the surface), and in relation to the coating apparatus 14, it may also be referred to herein as the receiving surface.

[0009] Figure 1 A comprehensive description of this can also be found in WO2016 / 189513, which relates to coating equipment; therefore, only the parts relevant to this disclosure will be discussed in more detail below.

[0010] This disclosure specifically relates to a coating apparatus that can be used in place of the coating apparatus described particularly in WO2016 / 189513. However, it should be emphasized that the coating apparatus of this disclosure may have other applications and is not limited to use in the apparatus described in WO2016 / 189512. For example, the manner in which selected areas become particle-free is not important to the content of this disclosure, and as an example, the transfer of particles to the substrate may instead be the result of an adhesive material being applied to selected areas of the substrate, or the result of applying heat to the donor surface by means other than laser radiation and / or from a side facing or below the particle coating (e.g., via a thermal printhead located on the rear side of the ITM). Therefore, in an offset printing system benefiting from a coating apparatus according to the teachings of this invention, the imaging station that enables the transfer of particles to the substrate may be an imaging station that applies energy to selected particles on the ITM, or an imaging station that selectively (e.g., by applying an adhesive) modifies areas of the substrate, the modified areas being adapted to separate selected particles from the ITM in the respective areas.

[0011] Figure 1 The coating apparatus 14 includes a plurality of nozzles 1401 aligned with each other along the axis of the ITM 10, and thus only one nozzle is visible in the cross-section of the figure. The spray 1402 from the nozzles is confined within a bell-shaped housing 1403, the lower edge 1404 of which is shaped to fit snugly against the donor surface 12, leaving only a narrow gap between the bell-shaped housing 1403 and the drum 10. The nozzles 1401 can be connected to a common supply rail 1405, which supplies a pressurized fluid carrier, typically air, to the nozzles 1401, in which fine particles to be coated on the donor surface 12 are suspended. Excess spray from the nozzles 1401 is confined within a collection chamber 1406 formed by the internal space of the housing 1403. This excess spray is extracted in this figure via an outlet pipe 1407, which connects to a suitable suction source indicated by the arrow, and can be recycled back to the nozzles 1401 if desired.

[0012] For the coating apparatus 14, it is important to achieve an effective seal between the housing 1403 and the donor surface 12 to prevent jet fluid and fine particles from escaping through the narrow gap that must be substantially maintained between the housing 1403 and the donor surface 12 of the drum 10. Figure 1 The diagram illustrates different ways to achieve this seal.

[0013] The simplest form of the seal is a scraper 1408. This seal is in physical contact with the donor surface and, if used on the outlet side of the housing 1403, that is, on the downstream side of the nozzle 1401, may leave scratches on the applied coating. Therefore, if such a seal is used, it is preferably located only upstream of the nozzle 1401 and / or at the axial end of the housing 1403. As used herein, the terms "upstream" and "downstream" refer to multiple points on the donor surface 12 as it passes through the coating apparatus.

[0014] Figure 1 It is also shown how to prevent fluid with suspended particles from flowing out of the sealed gap between the housing 1403 and the drum 10 without any component contacting the donor surface 12. In this description, a corridor 1409 extending around the entire circumference of the housing 1403 is connected by a set of narrow channels 1410 extending around the entire edge of the housing 1403 to establish fluid communication between the corridor 1409 and the sealed gap.

[0015] The channel 1409 is connected to the suction source of the residue extraction system, which can be the same as or a different suction source connected to the outlet 1407. In this case, the channel 1409 is used to extract fluid passing through the gap before the fluid leaves the housing 1403. Low pressure can also extract any particles from the drum 10 that are not in direct contact with the donor surface 12.

[0016] WO2016 / 189513 also describes an embodiment in which particles are applied to the donor surface by means of a rotating brush or roller placed between the nozzle 1401 and the donor surface 12.

[0017] The reason why the coating apparatus 14 of WO2016 / 189513 applies only a single layer of particles to the donor surface 12 is that particles are more likely to adhere to the donor surface than to each other. Therefore, particles that do not come into direct contact with the donor surface can be easily removed, and adhesion to the donor surface is prevented by the action of a brush, by suction, by blowing away using an air-blade mechanism, or by a combination of these measures.

[0018] For example, the coating equipment 14 used in WO2016 / 189512, WO2018 / 100412, WO2018 / 100528, WO2018 / 100530 or WO2019 / 234597 needs to be able to apply a single layer of particles to the continuously circulating donor surface of the ITM, ensuring that the donor surface leaves the coating equipment with a uniform single layer of particles, regardless of the proportion of the donor surface that still retains a single layer of coating from the previous operating cycle when it reaches the coating equipment.

[0019] Therefore, the rate at which particles need to be supplied to the coating apparatus will vary depending on the degree of evacuation of the coating at the transfer station 18, and the object of this disclosure is to provide a coating apparatus that can regulate the supply of particles in order to reliably apply a uniform particle layer to the donor surface, regardless of the rate at which the particles are transferred from the donor surface to the substrate. Summary of the Invention

[0020] According to a first aspect of this disclosure, a coating apparatus is provided for applying a particle layer to a receiving surface, the apparatus comprising a pressurized air source, a dressing chamber, an air return path, a metering device, and a controller, the dressing chamber being partially defined by the receiving surface, an airflow being delivered to the dressing chamber by the air source, the air return path being configured to return air from the dressing chamber to the air inlet of the air source to form an air circulation loop, the metering device being configured to introduce particles to be coated onto the receiving surface into the air circulation loop, and the controller being configured to adjust the metering device to maintain the particle concentration in the dressing chamber within a predetermined limit.

[0021] According to a second aspect of the invention, a method for applying a particulate layer to a receiving surface is provided, wherein, as set forth in more detail below and as claimed in claims 20 and 21 of the appended claims, such adjustment can be based, in particular, on direct measurement of the concentration of particles in the air circulation loop.

[0022] When coating equipment is incorporated into a printing system, the receiving surface can be a recyclable receiving surface, which may also be referred to as an intermediate transfer element (ITM). In some embodiments, the recyclable receiving surface may be mounted on a rigid drum or formed from the outer surface of a rigid drum, while in other embodiments, the recyclable receiving surface may be the outer surface of an annular flexible strip.

[0023] In certain embodiments, whether further implemented in a printing apparatus or a printing method, the particle layer formed on the receiving surface of the coating apparatus or as a result of the coating method is a monolayer of particles. In another embodiment, the layer or monolayer formed on the receiving surface or on the ITM is submicron-scale particles.

[0024] In some embodiments, a sensor may be provided to measure the particle concentration in the air circulation loop, and the controller may adjust the metering device based on the sensor’s output signal.

[0025] Instead of this direct measurement of particle concentration, the amount of particles consumed by the printing system incorporating the coating equipment can be determined by optical analysis of the printed substrate or by predicting particle consumption based on instructions received from the imaging station of the printing system. Therefore, according to other aspects of the invention, a printing system as described in more detail below and claimed in claims 21 and 22 of the appended claims is provided.

[0026] These and additional benefits and features of this disclosure will be better understood by referring to the following detailed description taken in conjunction with the accompanying drawings and non-limiting examples. Attached Figure Description

[0027] Some embodiments of this disclosure will now be further described by way of example with reference to the accompanying drawings, wherein like reference numerals or symbols indicate corresponding or identical components. This description is consistent with the accompanying drawings. Figure 1 The accompanying drawings are intended to make it clear to those skilled in the art how some embodiments of this disclosure can be practiced. The drawings are for illustrative purposes and do not attempt to show structural details of the embodiments in more detail than necessary for a basic understanding of this disclosure. For clarity and convenience, some objects depicted in the drawings are not necessarily shown to scale.

[0028] In the attached diagram:

[0029] Figure 1 An apparatus for performing thermal transfer on a substrate, as described above from WO2016 / 189512, is shown;

[0030] Figure 2 The cross-sectional view according to an embodiment of the present disclosure shows that the cross-sectional plane is parallel to the direction of travel of the receiving surface;

[0031] Figure 3 It is possible Figure 2 The optical density sensor used in the embodiments;

[0032] Figure 4 This is a schematic diagram of a nozzle that sprays particles onto a particle deflector.

[0033] Figure 5 It is a passage perpendicular to the direction of travel of the receiving surface. Figure 2 A cross-sectional view of the coating equipment shows the tendency of the lateral edge of the receiving surface to deflect due to suction;

[0034] Figure 6 It is similar to Figure 5 The diagram shows a view of the effect of the liquid layer between the receiving surface and the underlying supporting surface;

[0035] Figure 7 yes Figure 2 The view is detailed, drawn at magnification, and shows a nozzle with a particle deflector located upstream of two brushes, each brush having a brush cleaner and a bristle deflector respectively.

[0036] Figure 8 A partial view of a brush with individual bristles arranged in staggered rows is shown; and

[0037] Figure 9 Is with Figure 8 A similar view shows a brush with bristle clusters arranged in staggered rows. Specific Implementation

[0038] The prior art shown in WO2016 / 189513 Figure 1 As already described above, there is no need to describe it again. Figure 2 The particle dispenser or coating device 102 shown is Figure 1 The coating equipment 14 in the middle serves the same function, but is used in a printing system that employs the surface of the flexible annular belt 136 as the donor surface 108, instead of Figure 1 Rigid drum 10. According to some embodiments of this teaching, alternatively, it is possible for the coating apparatus to coat the donor surface formed by the surface of the rigid drum.

[0039] It should be clear that the coating equipment of the present invention (the embodiment of which is) Figure 2 The embodiment shown (102) is not limited to the donor surface of the thermal printing system. The donor surface is merely an example of a receiving surface that can be coated using a coating apparatus. Although the terms imaging surface and donor surface are often associated with more complex apparatus or systems that include coating apparatus with receiving surfaces, for simplicity, and unless clearly apparent from the context, the terms relating to surfaces to which particles can be coated or from which particles can be transferred are used interchangeably below.

[0040] Figure 2 The coating apparatus 102 shown covers a donor surface 108, the direction of movement of which is indicated by arrow 114. The coating apparatus includes a blower 104, which acts as a pressurized air source and supplies (under pressure) air carrying (typically dry) suspended particles to nozzles 106. The pressurized air source, for simplicity, may also be referred to as an air source, and alternatively, may be a device comprising compressed air. Although referred to as an air source, 104 can actually be used to generate and / or maintain an airflow confined within and primarily recirculated within the coating apparatus. The term "air source" is not intended to imply that only fresh air, free of particles, is supplied to the coating apparatus, but rather that particles can be acquired or maintained at any desired velocity. While not completely sealed, the coating apparatus can be considered a relatively closed system with a controlled volume.

[0041] The particles carried in the air circulation loop (described in detail later) can be made of any suitable material, and if they become sticky upon heating, they can be, for example, thermoplastic particles (i.e., containing or composed of thermoplastic polymers). The nozzle can extend across the entire width of the receiving / donor surface and can therefore also be used as an air knife.

[0042] Nozzle 106 sprays air and suspended particles onto donor surface 108 to form a particulate coating on donor surface 108. Multiple rotating brushes 110 ensure that particles contact every portion of the donor surface 108 that has entered the coating apparatus and sweep away excess particles from the donor surface 108, leaving essentially only a single layer of particles adhering to it. The brushes may have, for example, an arrangement of staggered rows... Figure 8 Multiple individual bristles as shown, or as Figure 9 Multiple bristle groups or tufts are shown. Each brush may optionally be associated with a brush cleaner 115, which can (e.g., by shaking or by contact scraping) reduce the amount of particles that may accumulate on the bristles over time. The brushes are contained within a dressing chamber 112, which is defined by a partition 113 separating the dressing chamber 112 from a conduit 116. The conduit 116 leads to a chamber 118 and forms a return path with the chamber 118, which returns air and unused particles to the inlet of the blower 104 to maintain the desired flow rate. In this way, an air circulation loop is formed for recirculating particles not coated onto the donor surface 108. Typically, the air recirculated in the loop flows at a higher velocity than the relative velocity of the receiving surface, such that any part of the surface will be exposed to more than one air cycle before leaving the coating apparatus. Without being bound by any particular theory, it is believed that this relatively high flow rate of air propelling the particles in the circulation loop is conducive to the formation of a complete (e.g., void-free) particle layer on the receiving surface during the coating process.

[0043] The particles applied to the donor surface 108 cause a decrease in the concentration or density of particles in the circulation loop. Therefore, it is necessary to replenish particles from a tank (not shown) via a metering device 120 regulated by an electronic controller (not shown). The metering device 120 should be able to introduce a metered quantity 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 is difficult to ensure that only a uniform layer of particles is applied to the donor surface 108. Furthermore, excessively high particle concentrations can lead to safety or health hazards (e.g., concentrations exceeding the lower explosive limit (LEL) of particles). Therefore, maintaining the particle concentration within a predetermined range is crucial to ensure safe, uniform, and effective coating of the donor surface 108. The predetermined range (or the limits of the range) may depend on the intended use of the coating equipment or the system implementing the coating equipment, the rate at which particles are emptied from the receiving surface, the extent of particle loss to the walls of the coating equipment or any part thereof (e.g., on the brush bristles or on the filtration system in its recirculation loop), and similar considerations. Therefore, a person skilled in the art using such equipment can readily determine the predetermined limits required for any particular situation. If the coating equipment and the resulting particle layer are used, for example, in a printing system, and if the same image is not always to be printed, the metering device 120 cannot be controlled to meter the particles at a fixed rate; instead, the metering rate needs to be matched to the density of the image to be printed. In digital printing systems, the image to be printed can differ from one image to the next, or from one printing job (printing the same first image) to the next printing job (printing the same second image). Such rapid and / or frequent changes can pose challenges to the coating equipment to be implemented.

[0044] In simple terms, a metering device is used to add a controlled number of new / fresh particles to the air circulation loop, replacing particles that were not bound to the receiving surface in a previous cycle of the recirculating airflow, thereby replacing at least partially emptied particles. As mentioned above, particles may be "intentionally" emptied, considering their removal from the receiving surface for their intended purpose, or unintentionally emptied, considering potential damage to the walls and components of the coating equipment.

[0045] exist Figure 2 In this embodiment, the metering device 120 is depicted downstream of the particle concentration sensor 122 and upstream of the air source 104, thereby facilitating the recirculation of particles not bound to the receiving surface 108 in the air circulation loop. In its illustrated location, the metering device may be located on a base plate to shield components of the coating equipment that allow airflow and the circulation of particles therein, or any device associated with the aforementioned desired treatment.

[0046] For example, although not shown in the figures, particles may optionally be supplied to the metering device via one or more pretreatment devices designed to at least partially remove (e.g., filter out) and / or at least partially reduce the size of agglomerates that such particles may form, such that smaller agglomerates, smaller clusters, or even individual particles can be entrained by the air recirculation loop following the metering device. Similarly, recirculated air (including particles therein) may be “treated” to control its temperature, its relative humidity, and / or its electrostatic charge.

[0047] To regulate the rate at which particles are metered by the metering device 120, its controller can receive a signal from a particle density sensor. While other forms of this sensor (e.g., electrostatic sensors) can be used, Figure 2 The embodiment described herein employs an optical density sensor (ODS) 122 located in the air circulation loop to monitor particle concentration. To improve the accuracy of the particle density sensor (e.g., ODS 122), it is desirable that its detection portion be located in a region of less turbulent airflow, such as close to the metering device 120.

[0048] Figure 3 A suitable optical density sensor design is schematically illustrated. The sensor's detection section includes a light source in the form of a light-emitting diode (LED) 124 emitting white light, and a photosensing element, for example, in the form of a photoresistor 126. As particles 144 flow through sensor 122 and between LED 124 and photoresistor 126, the signal output from photoresistor 126 decreases. The more light is blocked, the lower the output signal. Sensor 122 is shown as a second photosensing element in the form of a second photoresistor 127 to measure scattered light instead of transmitted light. Scattered light sensing photoresistor 127 also produces an output signal, but unlike the output of photoresistor 126, it increases with particle density. Since the output signals of the two photoresistors 126, 127 are complementary, only one of them is needed. When both are present, the output signal of the scattered light sensor 127 can be used to verify the output from photoresistor 126 or combined with it to improve the signal-to-noise ratio. The sum of the output signals of the two photoresistors can be used to check the operation and cleanliness of the sensor, because if both photoresistor 126 and the diffuse light sensor 127 have low readings, this will indicate possible fouling of LED 124 and / or photoresistors 126, 127. It should be noted that although particles are represented in the figures by circles indicating spherical particles for simplicity, they can take on any other shape, and the coating apparatus and method of the present invention are applicable to particles that also have non-spherical shapes, such as flakes, rods, irregular or amorphous blocks, etc.

[0049] To reduce particle deposition on the light source 124 or the (multiple) photosensitive elements 126, 127, the sensor 122 may have an air passage 128 leading to each of its three elements to keep them clean. Air may be blown into the passage 128, or ambient air may be drawn in through the passage 128 if the sensor 122 is located in a region of the recirculation path under negative pressure. The controller can then use the output signal of the photosensitive element (e.g., a photoresistor) to regulate the amount of particles 144 metered into the air circulation loop by the metering device 120.

[0050] As an alternative to adjusting the metering device 120 based on a direct measurement of the particle concentration in the dressing chamber of the air recirculation loop, it can be done in the printing system based on measured or predicted particle consumption. Particle consumption can be measured by analyzing the output signal of an optical device (e.g., a camera or optical density scanner) of the printing output of the printing system, or predicted by analyzing the input signal applied to the printing system. The two methods of adjusting the metering device, which can be viewed as "feedback" and "feedforward" control, do not need to be mutually exclusive and can be combined to further reduce any time delay in the implementation of modifications to particle supply achieved through the metering device.

[0051] Feedback assessment of particle consumption typically involves measuring the optical density (OD) of the printed image. During the printing process, the optical density of each point in the printed image can be measured using a densitometer or scanning densitometer. Optical density measurement is 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 using a reference calibration image (e.g., a 100% coverage "solid" color) before printing the desired image, or on the desired image for each printing job. The controller aims to set the metering device so that the measured OD matches the desired target OD of the image. Conventional proportional-integral-derivative (PID) controllers are suitable for relatively long printing jobs during which the printing process remains relatively constant. However, other controllers that better account for variations in operating conditions, such as pressure, are also suitable. Such controllers, suitable for maintaining the particle concentration in the dressing 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 here.

[0052] Feedforward prediction of particle consumption can be based on analysis of the image intended to be printed during a particular printing job. Controllers suitable for this preferred method, designed to maintain the particle concentration in the dressing chamber within predetermined limits, are known in the printing industry and need not be elaborated upon here.

[0053] The control system can combine predictive and feedback regulation of the metering device, where predictive regulation provides the initial settings and feedback regulation is necessary to compensate for the actual deviation from the prediction.

[0054] Regarding the above-described control of the metering device, it should be noted that printing systems implementing coating equipment according to this teaching can be considered more buffered than printing presses that rely on, for example, inkjet printers. While excessive ink deposition or conversely insufficient jetting can easily translate into suboptimal print quality, coating equipment typically includes a quantity of particles in its air circulation loop. This quantity of particles is not only sufficient to form or replenish a single layer but also readily adaptable to increased particle consumption. Such excess particles are not wasted on the printing substrate but are recycled until transfer is required. This makes the print quality obtained using this coating equipment less dependent on immediate changes in operating conditions and also reduces ink waste.

[0055] Readers interested in obtaining more details about such offset printing systems shall refer in particular to WO2016 / 189512, WO2018 / 100412, WO2018 / 100528, WO2018 / 100530 or WO2019 / 234597 of the same applicant, which include an intermediate transfer unit (ITM), a coating apparatus, an imaging station and an impression station, wherein the coating apparatus is used to coat a particle layer onto the ITM, the imaging station is used to apply energy to selected particles on the ITM so that the particles can be transferred to a substrate, and at the impression station, only the particles for which energy has been applied in the imaging station are transferred from the ITM to the substrate to form an image on the substrate, wherein the coating apparatus according to the present teachings can be advantageously implemented.

[0056] like Figure 2 As best shown, blower 104 may include a chamber housing a large fan configured to output a high volume of air and particulate mixture at low pressure. The fan blows particles 144 (those already present in the recirculation loop and new particles from metering device 120) through feed chamber 130 leading to nozzle 106 in dressing chamber 112.

[0057] In some cases, particles 144 can attract and adhere to each other, resulting in larger agglomerated particles or clusters. This is a problem when encountering very small particles (e.g., no more than a few micrometers in diameter) or damp conditions. Applying an airflow containing such agglomerated particles directly to the donor surface 108 may result in an uneven coating.

[0058] While not wishing to be bound by theory, it is believed that if relatively large particles (e.g., agglomerates or clusters) are present on the donor surface 108, the brush 110 may remove them more easily than relatively smaller particles, thus causing porosity or discontinuity in the particulate coating; or if the brush does not remove the larger clusters, they may be considered multilayered due to the agglomeration of smaller individual particles. Furthermore, over time, larger particles tend to be replaced by smaller particles, resulting in variations in the effect such particles can provide to the intended product (or variations in particle size distribution). To illustrate the use of particulate layers or coating equipment for applying particulate layers in a printing system, variations in the size distribution of particles in the group applied to the donor surface can subsequently affect the appearance of the printed matter. For example, variations in the applied particle group can 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).

[0059] To address this issue, in terms of the extent that it may occur along with the particles and / or their size distribution that people are trying to apply, Figure 4 A particle deflector 132 is shown located below nozzle 106. Particle deflector 132 can be made of any material that is hard enough to break down aggregated particles into smaller particles 144 more suitable for the intended use (e.g., printing). The smaller particles can be individual particles or smaller aggregates of particles. An example of a suitable material for the deflector is a steel alloy. The particle deflector can have any shape capable of producing smaller particles and significantly redirecting the airflow. Using particle deflector 132 causes a cloud of smaller particles 144 in dressing chamber 112 to settle primarily on the rotating brush 110, but also on the donor surface 108. The application of this cloud, rather than directly spraying particles 144 onto the donor surface 108, results in one or more of the following: a narrower particle size distribution; the formation of a more uniform particle coating than without the deflector; more efficient removal of excess particles 144 from the donor surface 108 by the brush; and reduction, delay, or prevention of damage to the donor surface that some particles might cause upon impact. For example, particles made of a material with an overall hardness greater than that of the transfer component surface may scratch the transfer component even under relatively low impact forces, while relatively high impact forces may cause any particles to abrade the donor surface.

[0060] Since deflectors can narrow the size distribution of particle groups by breaking down agglomerated particles, and since the continuous circulation of particles during the operation of the coating equipment can prevent re-agglomeration and / or provide at least a partial size reduction effect, it is believed that deflectors can reduce the occurrence of size changes observed over time when they are not present, thus reducing or eliminating any side effects of such changes on the final product (e.g., inconsistencies in optical density or gloss inconsistencies in prints).

[0061] In a series of experiments conducted under similar conditions, the presence of deflectors significantly reduced the proportion of aggregates on the donor surface (as assessed by microscopic examination and image analysis), except for the absence or presence of deflectors in the path of particles circulating along the airflow. These aggregates produced “multilayered” flakes within the smaller particle mosaic forming a monolayer. The reduction in the relatively large amount of coated particles, in turn, resulted in fewer voids in the coated particle coating after the removal of excess particles. These effects varied with the number of cycles; however, for illustration, a device that yielded approximately 40% monolayer coating, approximately 50% multilayer coating, and 10% voids after 10 cycles without deflectors could show the improved results with deflectors, with relative coverage increasing to over 90% for monolayer coatings, and the amount of multilayer flakes and voids decreasing to less than 5% of the coated area each.

[0062] Regardless of its benefit to the population of particles applied to the donor surface by a coating device according to this teaching, the deflector may be used alternatively or additionally to protect the donor surface from any harmful effects that may be present when applied directly to the surface.

[0063] The dressing chamber 112 typically houses multiple brushes 110 or rollers. The brush 110 closest to the nozzle 106 applies the particles 144 to the donor surface 108. Figure 2 As shown, the three brushes furthest to the right and closest to the nozzle 106 can rotate in one direction so that the bristles travel in the same direction as the movement of the donor surface. The rotational speed of these brushes 110 allows the linear velocity of the brushes 110 to be greater than the velocity of the donor surface 108. For example, the donor surface 108 can travel at 2 m / s, while the linear velocity at the radial edge of the brushes 110 can be 5 m / s, resulting in a 3 m / s slip and allowing the bristles of the brushes 110 to rub the particles 144 onto the donor surface 108. While the slip velocity may need to be adapted to the particles being coated and the donor surface thus coated, a slip velocity between 2 m / s and 5 m / s is considered suitable for many coatings. While higher slip velocity can improve the efficiency of particle coating and / or removal, it can also increase the risk of donor surface abrasion. Although in the example above the brush is assumed to have a greater velocity than the donor surface, this should not be interpreted as a limitation, and alternatively, gliding with relative velocity can be achieved by a donor surface having a greater velocity than the brush. The gliding need not be the same for every brush that the donor surface can contact.

[0064] A brush 110 positioned away from the nozzle 106 can be used to remove excess particles 144 from the donor surface 108, leaving only a single layer. Figure 2In the image, the four brushes 110 on the left represent particle removal brushes. These brushes can rotate in one direction so that the bristles move in the opposite direction to the movement of the donor surface and pass over it at a higher relative speed than the other brushes. They may also have wiping blades (e.g., brush cleaners 115) or equivalents for removing particles 144 from the bristles, thereby cleaning them and making the bristles more efficient. In some embodiments, the particle removal brushes can be replaced by an air knife or any similar device capable of removing excess particles, so that the donor surface at the outlet of the coating apparatus is substantially coated with a single layer of particles.

[0065] The above description of the brush is merely illustrative. Any number of brushes 110 (actually nozzles 106) can be present, and it is understood that they can rotate in directions different from those described above and / or at relative speeds different from those described above. Furthermore, although the brushes 110 are structurally identical, it should be noted that in some embodiments, the brushes may differ to better suit their function. For example, the stiffness and / or chemical composition of the bristles may vary to suit the task being performed. Therefore, the properties of the bristles can vary, the only important thing being to ensure that they do not damage the donor surface while performing their intended task.

[0066] In one embodiment, the movement of the brush and its corresponding bristles can be further adjusted by physical elements to further facilitate the application of particles to the donor surface and / or the removal of excess particles, and / or further reduce any damage it may cause to the particle coating or the donor surface. Without being bound by any specific theory, the bristles can be considered as contacting the underlying horizontal donor surface or particles thereon with a force comprising both vertical and horizontal components, the vertical and horizontal components each having different magnitudes along the bristles when the bristles contact the moving ITM. The vertical force can have such an impact that it undesirably separates loosely attached particles (e.g., relatively large particles) from the donor surface, while the horizontal component of the bristle force provides a gentler brushing effect. The vertical force can be viewed as causing a shaking effect in the bristles. Preferably, the horizontal force should be adjusted to be high enough to remove excess particles that are not in direct contact with the donor surface, while being low enough to undisturb the underlying monolayer of particles in contact with the donor surface.

[0067] While, as previously mentioned, the vertical force can be modified by selecting an appropriate distance between the brush and the surface, or by adjusting the bristle length and physicochemical properties, it can also be weakened by placing a physical obstacle in its path on either side of the point where the bristles will contact the ITM, in the absence of such interference. Figure 7In this context, one example of such an obstacle is shown as a deflector 150 with a triangular cross-section, but it can also have other cross-sections. This obstruction of the vertical component of the bristle force is referred to herein as a bristle or brush deflector, which is positioned at a height h above the donor surface, thereby allowing non-contact passage of the particulate coating. The bristle or brush deflector forms an angle α with the donor surface at the edge of the deflecting bristles in order to reduce its vertical impact. The edge of the deflector forming an angle with the donor surface need not be precisely below the axis of rotation of the brush including the bristles, but can be at an offset distance X. Those skilled in the art will readily understand how to vary the magnitudes of α, h, and X in the bristle deflector (as well as any other preset parameters, such as bristle stiffness and length, and the distance between the brush and the donor surface) to adjust the vertical and horizontal speeds and forces of the bristles, thereby adapting to the intended purpose, operating conditions, and particles applied to a predetermined ITM. In its simplest form, the brush or bristle deflector can be shaped as a surface inclined relative to a plane substantially parallel to the donor surface and at a distance (height) h from the donor surface. The bristle deflector can, for example, be shaped as a triangle with edges following the contour of the donor surface; however, this example should not be construed as limiting, and all other shapes of deflectors capable of deflecting the bristles to satisfactorily adjust the vertical component of the force exerted by the bristles on the donor surface are included.

[0068] Regardless of the shape, size, and location of the intended unaltered contact point (e.g., upstream and / or downstream of the point where the bristles would first contact without the brush deflector), such a brush deflector should advantageously be made of a material compatible with the bristles. Compatibility, as used herein, means that the brush deflector will not damage the bristles due to contact with them, nor will it be damaged by them physically (e.g., to reduce or prevent abrasion, cutting, or breakage) or chemically (e.g., by selecting materials for the bristles and their deflectors to avoid adverse reactions with each other), or in any other way (if present) that impairs the functionality of the bristles and their deflector. For example, since the situation may be for different brushes with different functions, the brush deflector should not affect the bristles' ability to attract, retain, and / or deliver particles. For example, the brush deflector may be made of metal or alloy, or coated with an elastomer that is stiff enough to bend impacting bristles, but not stiff enough to damage them.

[0069] Figure 2The brush shown can be at least partially replaced by a flexible member circulating on two or more rollers, the flexible member having bristles on its outer surface facing the donor surface, which can function as detailed above for more conventional brushes, where the bristles are typically attached to a single, more rigid roller. Without wishing to be bound by any particular theory, it is believed that such a bristle-bearing circulating member can replace at least two brushes while also providing a bristle deflection effect over a larger corresponding area on the donor surface.

[0070] like Figure 2 As shown, the dressing chamber 112 is partially defined by the donor surface 108. Any gaps between the walls of the dressing chamber 112 and the donor surface must prevent the particles 144 from escaping. Besides the fact that these particles cannot be recycled, they may pose health hazards and even safety risks.

[0071] To prevent air and particles from escaping from the dressing chamber 112, which is surrounded by a bell-shaped housing 141 that defines a suction chamber 142 that surrounds the dressing chamber 112 on all sides. As indicated by arrow 146, the suction chamber 142 is connected to a pump and filter unit 134, which reduces the pressure in the suction chamber 142 to below atmospheric pressure and below the pressure in the dressing chamber 112. Therefore, instead of air carrying particles that would escape into the ambient atmosphere, it is constantly drawn into the coating apparatus 102, as indicated by arrow 148. In another embodiment, particles that may accumulate over time on the filter unit due to the suction chamber of the bell-shaped housing can be alternatively or additionally conveyed to the metering device 120, thereby allowing previously filtered particles to be reintroduced into the air circulation loop.

[0072] Applying a sufficiently low pressure in the suction chamber 142 to ensure that no particles 144 escape would cause the donor surface to be drawn against the bell-shaped housing 141. However, it is necessary that the donor surface 108 and the bell-shaped housing should not come into contact with each other.

[0073] The tension in belt 136 is used to ensure that the donor surface 108 does not contact the laterally extending edge of the bell-shaped housing 141 as it enters and exits the coating apparatus 102, because belt 136 can be tensioned on the rollers at these points. However, the laterally extending edge of the donor surface 108 cannot be supported in this way over the entire length of the coating apparatus, and as... Figure 5 As shown, the low pressure in the suction chamber 142 may cause the lateral edges of the donor surface 108 to wobble and be drawn into contact with the sides of the bell-shaped housing 141, which is parallel to the direction of movement of the tape / donor surface. This contact can accelerate wear on the donor surface 108 and may cause it to tear, as well as potentially damage the coating equipment and / or the particle layer.

[0074] In some embodiments, the belt 136 may further include protruding structures along its lateral edges, which are capable of engaging with lateral tracks, at least when the belt is running below the coating apparatus. When engaged in the respective tracks, such lateral structures can place the belt under lateral tension, at least in the region facing the suction chamber. Such structures 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 suction chamber. The lateral structures may be (a) a plurality of structures spaced apart from each other along the length of the belt, or (b) a continuous structure along the entire length of the lateral edges of the belt, the thickness of which optionally exceeds that of the belt. In one embodiment, the structure is (a) made of a material with a low coefficient of friction to ensure smooth travel of the structure within the lateral tracks, and / or (b) made of a material, or containing an agent, or coated with a coating having lubricating properties.

[0075] However, the strip 136 whose surface serves as the donor surface 108 cannot, in some cases, be formed of a material that has sufficient strength to withstand tension to the extent necessary to prevent contact between its lateral edges and the longitudinally extending sides of the bell-shaped housing 141. For example, the strip 136 may need to be relatively thin, or in certain embodiments, formed of a transparent material to allow light to pass through its rear side before reaching the donor surface 108.

[0076] The donor surface 108 can be prevented from being sucked onto the housing of the coating equipment in a variety of ways, for example by providing rollers along the side of the housing. Figure 5 and 6 Two currently preferred solutions are shown.

[0077] exist Figure 5 In this configuration, a fiber-reinforced silicone rubber blanket 138 provides a supporting surface for the belt 136 that carries the donor surface. The rubber blanket 138 moves independently of the belt 136 on different roller sets, but the movement of the belt 136 and the rubber blanket 138 is synchronized. The rubber blanket 138 is under sufficient tension and has sufficient stiffness to prevent it from being sucked onto the bell-shaped housing 141.

[0078] Figure 5 The embodiments rely on the fact that if both the rubber blanket 138 and the strip 136 supporting the donor surface 108 are made of silicone material, they will tend to adhere to each other. This will effectively reinforce the donor surface 108 and prevent it from being attracted to the bell-shaped housing 141.

[0079] As an alternative to relying on the tendency of surfaces (e.g., made of silicone) to adhere to each other, Figure 6The embodiments rely on surface tension. In this embodiment, a liquid (e.g., oil) film 140 is applied between the belt 136 and the rubber blanket 138, and this serves to prevent the two from separating. Since oil is a lubricant, a fixed lubrication plate can be provided instead of the rubber blanket 138, but a recirculated rubber blanket is preferred.

[0080] As mentioned above and Figure 8 and 9 As shown, brush 110 may have multiple rows of individual bristles or bristle clusters (tufts), and these rows may be staggered to ensure that the bristles contact the entire donor surface. Those skilled in the art of brush manufacturing will understand that these parameters will regulate 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 with the donor surface or particles thereon. Although not required to be described in detail here, it is readily understood that the diameter of each bristle, its shape, the number of bristles in a tuft (if any), the distance between the constrained ends of each bristle or tuft in a row, the distance between adjacent rows, the angle formed by each row with respect to the axis of rotation of the brush, and similar considerations can all affect the density of the bristle tips on the outer surface of the brush. Advantageously, this density should be sufficient to give the brush a substantially uniform effect on the donor surface or particles thereon. As a contrasting example, the bristles or rows thereof should not be spaced apart from each other in a way that prevents areas of the donor surface from accessing their tips.

[0081] In particular, depending on the size of the bristles and the material forming them, the bristles can exhibit various stiffness / flexibility / capabilities to acquire, retain, and / or deliver particles, which can be selected and adapted to the particle and the donor surface to be coated, and / or adapted to the distance between the axis of rotation of the brush to which the bristles are attached and the donor surface. Although 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 the particle coating, the range of this permissible length (which includes the physical distance to be bridged by the bristles) may depend on the aforementioned considerations as detailed above.

[0082] As an illustration, all other similar bristle parameters include the excess length of the bristles colliding with the donor surface; relatively longer bristles rotating around an axis farther from the donor surface will apply less pressure than relatively shorter bristles rotating around a less distant axis. While the reduced pressure is advantageous in terms of wear on the donor surface, it may not be sufficient to remove excess particles. Similarly, and for all other parameters, bristles with a relatively large diameter will apply greater pressure than bristles with a relatively small diameter. This increased pressure is advantageous for removing excess particles, but may damage the donor surface.

[0083] In some embodiments, the bristles are made of durable natural or synthetic materials. Suitable natural materials can be plant or animal-derived and include, for example, animal hair, fur, down, feathers, or plumes. Synthetic materials can be made to mimic examples of the aforementioned natural materials, but can also include, for example, nylon or plastic materials composed of, for example, nylon. Preferably, the bristles are made of a material capable of properly moving the particles coated by the device onto 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 excess particles, while being able to release them to prevent accumulation or any other saturation process that would render them inefficient. Since the brushes of the coating devices need not be identical, their individual bristles can also differ.

[0084] The bristles can have any suitable shape and cross-section. In some embodiments, the bristles have a cylindrical shape and a diameter of at least 5 μm, at least 10 μm, or at least 15 μm. In some embodiments, the diameter of the cylindrical bristles is at most 100 μm, at most 75 μm, or at most 50 μm. In other embodiments, the diameter of the 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 bristle length is not ideally circular, but elliptical or polygonal, in which case a suitable “bristle diameter” can be approximated by the upper limit of the maximum cross-sectional length and the lower limit of the minimum cross-sectional length. For example, taking a bristle with a rectangular cross-section as an example, in some embodiments, its long side may not exceed 100 μm, 75 μm, or 50 μm, while its short side may be at least 5 μm, at least 10 μm, or at least 15 μm.

[0085] In some embodiments, the bristles have a total length of at least 7 mm, at least 8 mm, or at least 9 mm. In some embodiments, the total length of the bristles is at most 20 mm, at most 17.5 mm, or at most 15 mm. In other embodiments, the total length is between 7 mm and 20 mm, between 8 mm and 17.5 mm, or between 9 mm and 15 mm.

[0086] In some embodiments, the bristles have a total length exceeding the shortest distance between the bristle tip and the donor surface, the difference between these 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 other embodiments, the excess length is between 200 μm and 5 mm, between 500 μm and 4 mm, or between 1 mm and 3 mm. Although excessive excess length may damage the donor surface, sufficiently high excess length can improve the brushing effect of the bristles; in other words, it facilitates the application and removal of particles that do not directly contact the donor surface.

[0087] In some embodiments, bristles are attached as bundles to the brush base or any other suitable support. Since bundles of bristles tend to open at their ends compared to their attachment points at the base, the bundle is characterized by the size of their attachment points. For example, consider a bundle of bristles attached to the brush via a cylindrical recess in a brush support; a bundle of bristles can be defined by the diameter of the recess, into which the bases of the bristles 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 suitable.

[0088] Although this disclosure has been described with reference to certain embodiments and generally associated methods for illustrative purposes, variations and substitutions of these embodiments and methods will be apparent to those skilled in the art based on the disclosure herein. This disclosure should be understood as not being limited to the specific embodiments described herein. The invention is intended to cover all such alternatives, modifications, and variations, and is limited only by the spirit and scope of this disclosure and any changes within its equivalent meaning and scope.

[0089] It should be understood that certain features of this disclosure described in the context of individual embodiments for clarity may also be provided in combination in a single embodiment. Conversely, various features of this disclosure described in the context of individual embodiments for brevity may also be provided individually or in any suitable sub-combination or as suitably as in any other described embodiments of this disclosure. Certain features described in the context of various embodiments are not considered essential features of those embodiments unless the embodiment would be inoperable without those elements.

[0090] Unless otherwise stated, it is appropriate and permissible to use the expression "and / or" between the last two components of the list of options for selection to indicate the selection of one or more of the listed options.

[0091] The word "exemplary" is used herein to mean "serving as an example, illustration, or illustration." Any embodiment described as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments and / or to exclude combinations of features from other embodiments.

[0092] In this disclosure, unless otherwise stated, adjectives such as “substantially,” “approximately,” and “about” that modify one or more features of embodiments of the present technology are to be understood as meaning that the condition or characteristic is limited to an operational tolerance acceptable for the embodiment of the intended application, or to a variation expected from the measurement being performed and / or from the measuring instrument being used. When the terms “about” and “approximately” precede numerical values, they are intended to indicate + / -15% or + / -10% or even only + / -5%, and in some cases, precise values. Furthermore, unless otherwise stated, even without such adjectives, terms (e.g., numbers) used in this disclosure should be interpreted as having tolerances that may deviate from the precise meaning of the relevant terms, but will enable the invention or related portions thereof to operate and function as described, and as understood by those skilled in the art.

[0093] In the specification and claims of this disclosure, the verbs “comprising,” “including,” and “having,” and their variations thereof, are used to indicate a complete list of features, components, steps, parts, elements, or portions of one or more objects of the verb, which are not necessarily the subjects of the verb.

[0094] As used herein, the singular forms “a,” “one,” and “the” include plural references and mean “at least one” or “one or more”, unless the context clearly indicates otherwise. At least one of A and B is intended to mean either A or B, and in some embodiments may mean both A and B.

[0095] Positional or motion terms, such as “up,” “down,” “right,” “left,” “bottom,” “below,” “lower,” “lower,” “top,” “above,” “raise,” “high,” “vertical,” “horizontal,” “backward,” “forward,” “upstream,” and “downstream,” and their grammatical variations, may be used herein for illustrative purposes only to show the relative positioning, placement, or displacement of certain components to indicate the first and second components or both in this description. These terms do not necessarily indicate, for example, that a “bottom” component is below a “top” component, as these orientations, components, or both may be flipped, rotated, moved, placed in diagonal orientations or positions, horizontally or vertically positioned, or similarly modified in space.

[0096] Unless otherwise stated, when the outer boundary of the range of features relating to embodiments of the present technology is indicated in this disclosure, it should be understood that, in embodiments, possible values ​​of the features may include the outer boundary and values ​​between the outer boundary.

[0097] To the extent necessary to understand or complete the disclosure of this disclosure, all publications, patents and patent applications mentioned herein, including, in particular, the applicant’s applications, as fully set forth herein, are expressly incorporated herein by reference in their entirety.

Claims

1. A coating apparatus for applying a particulate layer comprising a thermoplastic polymer to a receiving surface, the apparatus comprising: a) Pressurized air source, b) An enclosed dressing chamber, partially defined by the receiving surface, into which airflow is delivered by the air source. c) An air return path for returning air from the dressing chamber to the air inlet of the air source to form an air circulation loop. d) A metering device for introducing a metered quantity of particles to be coated onto the receiving surface into the air circulation loop, and e) A controller for dynamically adjusting the metering device to maintain the concentration of particles suspended in the recirculated air within a predetermined limit.

2. The coating apparatus of claim 1, wherein, The air source is connected to a plurality of nozzles disposed within the dressing chamber.

3. The coating apparatus of claim 2, wherein, The nozzle is shaped as an air knife extending across the width of the dressing chamber.

4. The coating apparatus of claim 2, wherein, The particle deflector is located in the path of the airflow ejected from the nozzle, and the deflector is used to break up agglomerated particles carried by the airflow.

5. The coating apparatus of any one of claims 2 to 4, further comprising a plurality of brushes in the dressing chamber for brushing the receiving surface to leave only a single layer of particles adhering to the receiving surface, each of the plurality of brushes and the receiving surface moving relative to each other.

6. The coating apparatus of claim 5, wherein, A first portion of the plurality of brushes is provided, which is placed flat adjacent to the plurality of nozzles and rotated in one direction so that the bristles of the plurality of brushes adjacent to the plurality of nozzles pass through the receiving surface in the same direction as the movement of the receiving surface.

7. The coating apparatus of claim 6, wherein, A second portion of the plurality of brushes is disposed away from the plurality of nozzles.

8. The coating apparatus of claim 7, wherein, At least one brush of the second portion of the plurality of brushes rotates in one direction such that the bristles of the at least one brush away from the plurality of nozzles pass through the receiving surface in a direction opposite to the movement of the receiving surface.

9. The coating apparatus of claim 5, wherein, The brush deflector is positioned adjacent to at least one of the plurality of brushes to deflect the ends of the bristles before contact is formed between the bristles and the receiving surface.

10. The coating apparatus of any one of claims 1 to 4, wherein, The dressing chamber is surrounded by a bell-shaped shell, which defines a suction chamber to prevent particles from escaping into the ambient atmosphere.

11. The coating apparatus of any one of claims 1 to 4, wherein, The receiving surface is a recirculating surface of an annular belt, and the supporting surface is configured to be supported by the side of the annular belt opposite to the receiving surface.

12. The coating apparatus of claim 11, wherein, The supporting surface is movable together with the annular belt.

13. The coating apparatus of claim 11, wherein, An oil film is disposed between the annular belt and the support surface to allow the belt to adhere to the support surface.

14. The coating apparatus of any one of claims 1 to 4, comprising a sensor located within the air circulation loop to provide an output signal indicating the concentration of particles in the recirculated air, and the controller for adjusting the metering device according to the output signal of the sensor.

15. The coating apparatus of claim 14, wherein, The sensor is located in the chamber with less turbulent airflow, which is located immediately in front of the metering device.

16. The coating apparatus of claim 14, wherein, The sensor is an optical density sensor.

17. The coating apparatus of claim 16, wherein the optical density sensor comprises a light source and a photosensitive element for measuring the amount of light transmitted through the air circulation loop.

18. The coating apparatus of claim 16, wherein the optical density sensor comprises a light source and a photosensitive element for measuring the amount of light scattered by particles in the air circulation loop.

19. The coating apparatus of claim 17 or claim 18, further comprising a channel for introducing air into or removing air from the optical density sensor to prevent particle deposition on the light source and / or photosensing element.

20. A method of applying a particulate layer comprising a thermoplastic polymer to a receiving surface, the method comprising: i) Provides a dressing chamber partially defined by a receiving surface, ii) Blowing air into the dressing chamber using an air source. iii) Allow air to return from the dressing chamber to the air inlet of the air source to form an air circulation loop. iv) Introduce a metered quantity of particles to be coated onto the receiving surface into the air circulation loop, and v) Dynamically control the rate at which particles are introduced into the air circulation loop in order to keep the particle concentration within a predetermined limit.

21. The method of claim 20, wherein, The rate at which particles are introduced into the air circulation loop is controlled based on a measurement of the particle concentration in the air circulation loop.

22. An offset printing system comprising an intermediate transfer unit (ITM), a coating apparatus, an imaging station, and an impression station, wherein the coating apparatus is used to coat a particle layer onto the ITM, the imaging station is used to apply energy to selected particles on the ITM to make the particles transferable to a substrate, and at the impression station, only particles for which energy has been applied in the imaging station are transferred from the ITM to the substrate to form an image on the substrate, wherein the coating apparatus is as described in any one of claims 1 to 13, and the system further comprises an optical device for observing the image transferred to the substrate, and an output image processor for analyzing image data from the optical device, wherein a controller of the coating apparatus is used to adjust the metering device according to a signal from the output image processor.

23. An offset printing system comprising an intermediate transfer unit (ITM), a coating apparatus, an imaging station, and an impression station, the coating apparatus for coating a particle layer onto the ITM, the imaging station for applying energy to particles on the ITM selected according to a signal received from an input image processor to make the particles transferable to a substrate, and at the impression station, only particles for which energy has been applied in the imaging station are transferred from the ITM to the substrate to form an image on the substrate, wherein the coating apparatus is as described in any one of claims 1 to 13, and the controller of the coating apparatus is configured to adjust the metering device according to a signal from the input image processor.

Images