Thin film thermal transfer printing and coating

Abstract

A method of printing is disclosed which comprises providing a recirculating blanket including a donor surface, and cyclically: (i) applying to the donor surface a uniform continuous solid thermoplastic polymer ink film, (ii) applying energy to heat selected regions of the polymer film, (iii) pressing the donor surface against a substrate to cause only the selected regions of the polymer film to adhere thereto, (iv) separating the substrate from the donor surface, so as to leave remaining on the donor surface only the regions of the film that have not been transferred to the substrate, (v) removing any polymer film remaining on the donor surface, and (vi) recycling the removed polymer for re-application to the donor surface. Also disclosed are a method and an apparatus for coating a surface with a thin film of a thermoplastic polymer.

Description

[0001] THIN FILM THERMAL TRANSFER PRINTING AND COATING CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims Paris Convention priority from Great Britain patent applications Nos. GB 2500904.4 and GB 2500905.1, filed on January 22, 2025, the contents of which are incorporated by reference in their entirety as if fully set forth herein.

[0003] FIELD

[0004] The present disclosure relates to a method and apparatus for coating a surface with a thin film and the processes enabled thereby, and more particularly to thermal transfer printing with thermoplastic polymers.

[0005] BACKGROUND

[0006] In thermal transfer printing, a polymer film supported by a carrier is selectively heated and pressed against a printing substrate. The heated regions are rendered sufficiently tacky by the heating so that they transfer from the carrier to the substrate to form an ink image.

[0007] This method has been used extensively in typewriters, where the carrier was a disposable ribbon. The use of a ribbon as the polymer film carrier is however very wasteful, as the ribbon could only be used once, on account of the voids created in the polymer film by the typed text and correspondingly transferred ink.

[0008] In EP 3548292, the present Applicant has previously proposed a method of thermal transfer printing onto selected regions of a surface of a substrate, which method comprises the steps of providing a transfer member having an imaging surface, coating the imaging surface of the transfer member with individual particles formed of, or coated with, a thermoplastic polymer, removing substantially all particles that are not in direct contact with the imaging surface to leave a uniform monolayer particle coating on the imaging surface, applying energy to selected regions of the coated imaging surface to heat and render tacky the particles within the selected regions, and pressing at least a portion of the coated imaging surface and at least a corresponding portion of the substrate surface against one another, either during or after application of energy, to cause transfer to the surface of the substrate of only the regions of the particle coating that have been rendered tacky.

[0009] The printing system proposed in EP 3548292 has been found to suffer from certain disadvantages. In particular, the production of powdered polymer is costly, and handling of veryfine powders requires great care. Over time and repeated application of particles to replenish regions previously depleted by transfer, the imaging surface suffered wear and required replacement. Moreover, since the population of particles coating the imaging surface could vary from one cycle to another, as transferred particles are replaced by fresh ones, the density of the transferred image could vary in an increasingly discernable manner. Such changes, including a ghosting phenomenon (z.e., cross contamination of printing images), typically became visibly detectable more rapidly when the particles did not share a narrow size distribution. Such problems could be avoided by cleaning the entire imaging surface between operating cycles, however constant wiping or brushing of the imaging surface to remove surplus particles caused it to wear prematurely.

[0010] JP S59-110662 U describes a method of printing onto a substrate, which comprises: providing a recirculating belt, at least a region of an outer surface of which serves as a donor surface, cyclically repeating the steps of (i) applying to the donor surface a liquid coating of a heat-sensitive ink and allowing the liquid to cool to form a uniform solid film on the donor surface, (ii) heating the ink film to melt the ink, (iii) pressing the donor surface against the substrate to cause the molten ink to adhere to the substrate, and (iv) subsequently replenishing regions of the donor surface depleted from thermal ink in a previous printing cycle with fresh thermal ink for a subsequent cycle. Replenishment of the ink layer is achieved by rolling contact between the belt and a heated engraved cylinder partly immersed and rotating in a tank of molten ink, so that a steady pool of molten ink is formed upstream of the nip of ink recoating.

[0011] OBJECT

[0012] The aim of the present disclosure is to provide an apparatus and method for thermal printing that can advantageously be less wasteful of raw materials, more cost effective and capable of providing more uniform print quality, thanks inter alia to a coating apparatus and method capable of coating a surface with a thin and uniform continuous film of thermoplastic material.

[0013] SUMMARY

[0014] According to a first aspect of the present disclosure, there is provided a method of printing onto a substrate, which comprises providing a recirculating blanket mounted on a drum or looped as an endless belt, at least a region of an outer surface of which serves as a donor surface, and cyclically performing the steps of(i) applying to the donor surface at a film application station a uniform continuous solid ink film consisting of a thermoplastic polymer,

[0015] (ii) applying energy to heat selected regions of the polymer film,

[0016] (iii) pressing the donor surface against the substrate during, or after, the application of energy, to cause only the selected regions of the polymer film, that are rendered tacky by the applied energy, to adhere to the substrate,

[0017] (iv) separating the substrate from the donor surface, so as to leave remaining on the donor surface only the regions of the film that have not been transferred to the substrate,

[0018] (v) removing any polymer film remaining on the donor surface at a cleaning station, and (vi) recycling the polymer of the removed film by returning the polymer from the cleaning station to the application station for re-application to the donor surface, the polymer having a chemistry that is not changed by any energy applied during any of steps (i) to (v).

[0019] As the layer of polymer that is applied to the donor surface in EP 3548292 is made of discrete particles, one cannot avoid the presence of gaps or voids between the individual particles. By contrast, in the present disclosure, the applied film is continuous.

[0020] In JP S59-110662 U, no attempt is made to clean the donor surface before a fresh film is applied. Instead, the donor surface is merely immersed in a pool of molten polymer with the aim of the polymer remaining on the donor surface from one cycle melting and becoming part of the molten polymer in the pool. However, because of the short contact time, the cooling by the donor surface and the surface tension of the polymer film on entering the pool, this does not occur in practice and results in the solidified ink coating on the belt being uneven, which in turn adversely affects the printing quality.

[0021] By contrast, in the present disclosure, the donor surface is cleaned before any attempt is made to apply a fresh film for the next operating cycle. As the cleaning of the donor surface is driven by a thermo-rheological mechanism and the fact that the polymer film to be removed has a temperature different than the temperature of the cleaning surface operating at the cleaning station, so as to have distinct viscosities on each enabling detachment from the donor surface and attachment to the cleaning surface, the cleaning step (v) can be carried out in two ways. The magnitude of the temperature difference enabling complete cleaning depends, inter alia, on the dimension (e.g., thickness) and chemical composition of the polymer film - and its physical properties (e.g., softening temperature) and can be empirically determined by routine experimentation. Typically, the difference in temperatures of the cleaning and polymer surfaceis at least 20 °C, at least 30 °C, or at least 40 °C; said difference generally not exceeding 60 °C, or being no more than 50 °C.

[0022] In one case, the cleaning surface can be a surface of a heated roller or of a heated belt configured to contact the donor surface following step (iv), the polymer film to be removed being at a relatively lower temperature on the donor surface. For illustration, the polymer film can be relatively solid at a temperature lower than the Tg of the heat-sensitive ink, and the cleaning surface can be set to have a temperature higher than the Tm of the ink. Alternatively, the donor surface can be heated following transfer of the selected regions, for instance by applying thermal energy from a back side of the blanket opposite to the donor surface, the polymer film to be removed being at a relatively higher temperature (e.g., > ink Tm) than the cleaning surface, which can be the surface of a cooled roller or of a cooled belt (e.g., having a temperature < ink Tg).

[0023] To avoid wastage of the ink, the material of the polymer film removed in step (v) is recycled in step (vi) for re-application onto the donor surface in a subsequent cycle.

[0024] In embodiments of the above-described printing method, the ink remaining on the donor surface from the preceding cycle can be fully removed without directly contacting the donor surface, thereby avoiding wear of the donor surface.

[0025] In some cases, the polymer film applied on the donor surface has a thickness not exceeding 2 micrometers (pm), 1 pm, 750 nanometers (nm) or 500 nm.

[0026] This small thickness of the film in some cases, can be achieved by providing a series of rollers in rolling contact with one another, applying molten polymer to the first of the rollers in the series to form a film thereon, and transferring a progressively thinned part of the film from the first to the last of the rollers in the series and from the last of the rollers in the series (i.e., the applicator roller) to the donor surface. If the rollers in each pair in contact with one another operate at different temperatures, for instance, one below the glass transition (Tg) or softening (Ts) temperature of the polymer and the other above its melting (Tm) or softening (Ts) temperature, the polymer film splits during transfer between rollers, with a liquid part of the film adhering to the hotter roller and a solid part adhering to the cooler roller. Conceivably, a film of molten polymer may split between adjacent rollers even if both are independently maintained at a predetermined temperature above Tmor Tsof the ink, as long as the two rollers differ in one or more properties, such as in their material composition, their surface energy, their hardness or their surface temperature, one being relatively hotter than the other, the differencein viscosities and / or adherences on each causing the split and determining its extent.

[0027] As the heat-sensitive ink may display a progressive transition from a relatively solid phase at a temperature below melting or softening temperature to a relatively more liquid phase at a higher temperature, the terms liquid and solid are to encompass the intermediate softened phase respectively following or preceding the temperatures at which transition from solid (or relatively more viscous) to liquid (or relatively less viscous) occurs. The heat-sensitive ink can be applied to the donor surface whilst not yet fully solid (z.e., at maximal viscosity), at least the underneath side of the film solidifying (z.e., its viscosity increasing) upon contact with the surface or the film hardening across its entire thickness at the latest by the time energy is selectively applied to render the selected regions of the polymer film tacky.

[0028] In some cases, a cooling step (a) can be introduced between steps (i) and (ii), so that the polymer film of a heat-sensitive ink is sufficiently solid for the energy being applied to be sufficiently selective. This cooling step may be achieved by rolling contact between the donor surface and film thereon and one or more cooling rollers or belts adapted to lower the temperature of the film below its melting or softening point.

[0029] Steps (ii) and (iii) may, in some cases, be performed simultaneously, energy being applied to the donor surface from a side of the drum or belt opposite to the donor surface, while the donor surface is in contact with the substrate.

[0030] If the polymer film is uneven after its initial application, a levelling device may be used to render it more even. In some cases, this leveling step (b) may be achieved by heating the film above its melting or softening point and relying on surface tension to flatten the film. If a levelling step (b) including heating of the film is included after step (i) and before step (ii), then in some cases, the method may further include a cooling step (a) after levelling step (b) and before step (ii).

[0031] While the thickness of the film should ideally be identical over the entire ink layer, small variations can be tolerated. The tolerable variations are dictated by the sought print quality, which may depend inter alia on the energy being applied to render selected regions of the polymer film tacky. In some cases, the thickness of the ink layer (whether levelled or not) may vary by up to 20%, this degree of uniformity enabling transfer of tacky regions without differences visually detectable on the printed substrate. In other cases, the uniform solid polymer film of a heat-sensitive ink may have a thickness variability of less than 15%, less than 10%, or less than 5%. Typically, the variability is calculated with respect to a predeterminedsought thickness, the actual thickness being either 10% smaller or 10% larger, the difference between the tolerable extremes being in such case of 20%. Alternatively, the variability can be determined by measuring the average thickness of the ink layer and its average roughness (e.g., by suitable microscopy analysis), and by calculating (in percentage) the ratio between the roughness and the thickness.

[0032] Advantageously, at the time energy is applied to it, the polymer film can have a top surface substantially free of defects that may adversely affect print quality. The uniform polymer film is deemed defect-free if substantially devoid of ribs, ripples, cracks, orange peels appearance, moire patterns, or like surface defaults. In absence of such defects, the ink films are relatively glossy, as can be ascertained by naked eyes or with a suitable glossmeter or profilometer.

[0033] A relatively glossy polymer film of heat-sensitive ink can have, for instance, a gloss value of 50 or more, 75 or more, or 100 or more, as measured at an angle of 85°; and / or an average roughness Ra of 100 nm or less, 50 nm or less, 25 nm or less. Preferably, this defect-free uniformity of the polymer film applies to both sides of the film, the side of the film in contact with the donor surface becoming the top surface once transferred to the printing substrate. As the selected regions of the polymer film transferred to the printing substrate are relatively thin and relatively smooth, they may faithfully follow the topography of the surface to which they are transferred ensuring a high quality of printing.

[0034] According to a second aspect of the disclosure, there is provided an apparatus for printing onto a substrate conveyed by an impression cylinder or an impression belt, the printing apparatus comprising:

[0035] (i) a recirculating blanket, mounted on a drum or looped as an endless belt, at least a region of an outer surface of which serves as a donor surface,

[0036] (ii) a film application station for applying a uniform continuous solid thermoplastic polymer film onto the donor surface,

[0037] (iii) a writing device for selectively applying energy to heat regions of the polymer film, (iv) an impression station at which the donor surface is pressed against the substrate during, or after, the application of energy, to cause the selected regions of the polymer film, that have been rendered tacky by the applied energy, to adhere to the substrate, the substrate subsequently separating from the donor surface downstream of the impression station so as to transfer the adhered regions of the polymer film from the donor surface to the substrate,(v) a donor surface cleaning station at which any polymer film remaining on the donor surface after transfer of the selected regions to the substrate is removed from the donor surface, and

[0038] (vi) a recycling system which returns removed polymer to the film application station for re-application to the donor surface in subsequent operating cycle.

[0039] The cleaning surface operating at the cleaning station by contacting the donor surface can be of a temperature-controllable roller or endless belt circulating about two or more cylinders, the cleaning surface being set, in operation of the apparatus, to have a temperature different from a temperature of the polymer film of heat-sensitive ink remaining on the donor surface downstream of the impression station. The cleaning surface can be of a heated roller or of a heated belt, when the polymer film to be removed has a relatively lower temperature, or can be of a cooled roller or cooled belt, when the polymer film to be removed has a relatively higher temperature, for instance when the apparatus further comprises heating devices capable of increasing the temperature of the film remaining on the donor surface following transfer. For illustration, the polymer film remaining on the donor surface at the end of a printing cycle can be heated by infrared (IR) lamps positioned to irradiate a back side of the blanket in a region opposite the relatively cooler cleaning surface.

[0040] According to a third aspect of the disclosure, there is provided a printing assembly comprising two or more printing apparatus as introduced above and further detailed hereinafter.

[0041] As can be appreciated, the printing method and apparatus disclosed above rely on the formation of a uniform continuous solid polymer film coating which can thereafter be selectively heated for transfer of a desired image to a printing substrate. In the absence of such selective writing step, the coating apparatus can serve to directly apply or entirely transfer a similarly uniform and continuous film of polymer to a surface of a substrate to be coated thereby.

[0042] A known method of applying a thermoplastics coating is that of dip coating, which involves immersing the surface into a bath containing the polymers or resins dispersed or dissolved in a volatile solvent, but this results in both sides of the surface being coated. Spray coating, in which a similar composition is jetted onto the surface, allows the material to be more selectively applied to only one side of the surface. These relatively simple methods typically suit thermoplastics that can tolerate solvents (e.g., not deforming or degrading therewith) or low curing temperatures, but may require additives in the composition and / or surfacepreparation (e.g., priming or plasma treatment) for better adhesion. The thickness of the coating achievable by such methods depends on numerous variables (e.g., the viscosity of the coating composition, the speed a surface is withdrawn from immersion, the duration and flow rate of spraying, the volume of the sprayed droplets, etc.). Typically, they are used to form coatings having a thickness in a range of a few micrometers (e.g., 5 pm) to hundreds of micrometers.

[0043] Thinner coatings (e.g., from several nanometers to a few micrometers) can be obtained by spin coating similar liquid composition, this method, however, is restricted to flat surfaces and requires more complex equipment. In any event, the need to include solvents in the coating composition requires them later to be removed, a matter having not only consequences on the duration of the process and its cost, but also environmental drawbacks.

[0044] Known processes that apply a thermoplastic coating without using solvents result in coatings having a thickness to be measured in tens or hundreds of micrometers. Such processes include applying the thermoplastic materials in relatively dry form, such as by powder coating followed by melting of the powder particles and hot melt coating, which involves applying a molten thermoplastic material directly onto the surface, then cooling it in a controlled manner to solidify without stress or defects.

[0045] With a view to enabling application of thinner solvent-free thermoplastics coatings, there is provided in accordance with a fourth aspect of the disclosure, a method of coating a surface which comprises:

[0046] (i) providing an applicator roller,

[0047] (ii) applying a coating of the thermoplastic material to the applicator roller, the thermoplastic material being in a molten state when in contact with the applicator roller, and

[0048] (iii) rolling the applicator roller over the surface while maintaining the temperature of the applicator roller above a predetermined temperature, corresponding to the melting or softening temperature at which the thermoplastic material becomes flowable, and the temperature of the surface below said predetermined temperature,

[0049] whereby cooling of the thermoplastic material on the applicator roller by contact with the surface causes a more viscous film of the thermoplastic material to be formed and to adhere to the surface to be coated, while the applicator roller remains coated with less viscous molten thermoplastic material.According to a fifth aspect of the disclosure, there is provided an apparatus for coating a surface with a film of a thermoplastic material, which comprises:

[0050] (i) an applicator roller,

[0051] (ii) a supply station for applying a coating of the thermoplastic material to the applicator roller,

[0052] (iii) a feeder for providing the thermoplastic material to the supply station, and

[0053] (iv) a coating station at which the applicator roller is rolled over the surface to be coated while, in operation of the apparatus, the temperature of the applicator roller is maintained above a predetermined temperature, corresponding to the melting or softening temperature at which the thermoplastic material becomes flowable, and the temperature of the surface to be coated is maintained below said predetermined temperature,

[0054] whereby cooling of the thermoplastic material on the applicator roller by contact with the surface causes a more viscous film of the thermoplastic material to be formed and to adhere to the surface to be coated, while the applicator roller remains coated with less viscous molten thermoplastic material.

[0055] According to a sixth aspect of the disclosure, there is provided a coating assembly comprising two or more coating apparatus as introduced above and further detailed hereinafter.

[0056] Features of different embodiments of the foregoing aspects and additional benefits of the present disclosure are set out in the following detailed description taken in conjunction with the figures, non-limiting examples and clauses, and in the appended dependent claims.

[0057] BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Systems and methods will now be described further, by way of example, with reference to the accompanying drawings, where like reference numerals or characters (or last digits thereof) indicate corresponding or like components. The description, together with the figures, makes apparent to a person having ordinary skill in the art how some embodiments of the disclosure may be practiced. The figures are for the purpose of illustrative discussion, and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental understanding of the disclosure. For the sake of clarity and convenience of presentation, some objects depicted in the figures are not necessarily shown to scale.

[0059] In the Figures:Figure 1 is a schematic representation of an exemplary printing system;

[0060] Fi ure 2 is a graph showing the temperature of the donor surface at different stages during each operating cycle of an exemplary printing process;

[0061] Figure 3 A is a schematic representation of an exemplary assembly of printing systems;

[0062] Figure 3B is a schematic representation of an exemplary assembly of coating systems;

[0063] Figure 4 shows in the form of a flowchart various steps that can be employed in one exemplary printing method;

[0064] Figure 5 shows thermo-rheological curves of representative inks or thermoplastic coatings, their dynamic viscosity being plotted along the Y-axis as a function of temperature plotted along the X-axis, Fig. 5 A being typical of a crystalline thermoplastic material, Fig. 5B being typical of a semi-crystalline thermoplastic material, and Fig. 5C being typical of an amorphous thermoplastic material;

[0065] Figure 6 shows exemplary close-up pictures of images (including text) printed in a printing system implementing a coating method according to present teachings;

[0066] Figure 7 schematically illustrates the challenges of cleaning a donor surface of a present printing system as compared to a conventional offset printing system, with Fig. 7A showing in top view an ink image formed on the intermediate transfer member of a conventional offset printer, Fig. 7B illustrating how ink residues may remain thereon after transfer, Fig. 7C showing a top view of a uniform solid polymer film of a heat-sensitive ink formed on the present donor surface and Fig. 7D illustrating the ink film depleted from the selected regions having transferred, z.e., the negative image remaining to be cleaned; and

[0067] Figure 8 is a schematic representation of an exemplary printing system including a particular recycling assembly.

[0068] DETAILED DESCRIPTION

[0069] A coating apparatus as taught herein can be used in diverse industries, as will be illustrated in the following with respect to the field of thermal transfer printing.

[0070] Overview of a printing system

[0071] Figure 1 shows a thermal transfer printing system 10 which comprises a drum 12 covered with a blanket 14 of which the outer surface 16 is a donor surface. The donor surface can be made of a hydrophobic material having low surface energy, such as a silicone elastomer, so thatthe film to be formed thereon can be released and cleanly removed therefrom. The drum 12 rotates counterclockwise, as viewed in the drawing, causing the donor surface 16 to pass sequentially and cyclically through various stations.

[0072] At the 3 o’clock position, a film application or coating station 18, which will be described in more detail below, applies a film 15 of a polymeric ink to the donor surface 16. At least downstream of the nip at which the ink is applied, this surface 16 is held at a temperature lower than the temperature of the ink applied thereto, on account of the underlying layers of the blanket acting as heat sink dissipating the heat perceived by the donor surface within the nip. The ink is applied in a liquid (or relatively less viscous) state but solidifies on contacting the donor surface 16 to leave a film 15 having a thickness not exceeding 2 pm, 1 pm, 750 nm or 500 nm. Though the donor surface is hydrophobic, a liquid polymer film can be applied to it without beading, as the liquid is not aqueous and furthermore because it rapidly increases in viscosity upon contact with the relatively colder donor surface which enables the hardening of the applied polymer.

[0073] The surface of film 15 should ideally be smooth and even on application (e.g., by applicator roller 66), but, in practice, this may not prove to be the case. If it proves necessary, the surface of the applied ink film may be rendered more level by passing it through a levelling station 20, this optional station being shown at the 1 o’clock position in Figure 1. Here, the film may contact one or more heated rollers 22, or may be heated by a radiant heater 24, to raise its temperature above its melting or softening point to enable the surface tension of the film to flatten out any unevenness. The heated rollers 22 can be replaced by heated belts of any other heated surface enabling conduction of thermal energy to a film contacted thereby. While radiant heater 24, is shown in this figure as being internal to the drum 12, it may alternatively be disposed externally to is, so as to face the donor surface and the film thereon, replacing or assisting the heated roller(s) or belt(s) 22.

[0074] The film is next carried by the drum 12 to the nip 31 of an impression station 30, shown at the 9 o’clock position in the drawing. At the nip, the reverse side of the blanket 14 (z.e., the side opposite donor surface 16), can be selectively irradiated by means of one or more laser lights 32 arrayed on and individually emitted from a source 34 and forming therewith a writing device 33. This results in only the irradiated regions of the ink film being heated to a temperature at which they will adhere to the material of the substrate, the ink being herein described as “tacky” when in this state of transferability from one surface to another. Without wishing to be bound by a particular theory, the transfer of selected regions of a heated film arebelieved to be governed by the balance between three forces, which at the impression station would include: first the adhesion of the film to the donor surface (which is weakened by heating of the ink), second the cohesion of the ink film and third the adhesion of the film to the printing substrate, which is typically colder than the heated ink film and at a temperature at which the thermoplastic material is no longer flowable. Following writing, the cohesion of the ink film and its adhesion to the substrate exceed the adhesion of the film to the blanket donor surface, resulting in transfer of the film or selected regions thereof. These regions alone are therefore transferred to a substrate 36 that is pressed at the nip 31 of the impression station 30 against the blanket 14 by an impression drum 38. To enable selective heating of the polymer film by laser irradiation, drum 12 and the rear side of blanket 14 need be transparent to the wavelength of the laser lights (ie., not hampering their progression so they may heat the ink film or a radiation absorbing layer positioned under the donor surface). Alternatively, the heating energy can be selectively conveyed by conduction, the writing device 33 being for instance a thermal print head, in which case the drum 12 and the rear side of blanket 14 need not be transparent to light but capable of thermal conductivity.

[0075] Though the writing device 33 in Figure 1 has been shown as coinciding with the impression station, it may instead precede the impression station, provided that the thermoplastic material of the ink has a sufficiently long open time. In such a case, the writing device 33 may alternatively be arranged on the opposite side of the blanket 14 and the latter need not be transparent to radiation. The location of writing device 33 is referred herein as the writing station regardless of its disposition with respect to the impression station and / or the side of the blanket from which energy is applied. As explained, the writing station can be an irradiating or a heating station, depending on the elements elected to selectively “write” on (heat) the polymer film of heat-sensitive ink to render it tacky where desired.

[0076] A reader interested in gaining more details about non-limiting examples of writing devices that can be suitable in writing stations for the present printing systems is referred to WO 2016 / 189510, WO 2016 / 189511 and WO 2018 / 100412. For instance, the writing device may include individually controllable laser emitting elements (e.g., vertical-cavity surfaceemitting lasers; VCSELs), the beams emitted thereby being capable of selectively forming tacky dots on the ink film, which upon transfer to a substrate form corresponding typically thinner dots of approximately even thickness and / or relatively defined contour.

[0077] While passing from the coating station 18, or from the levelling station 20, when present, to the impression station 30, the temperature of the film can be reduced by means of one ormore cooling rollers 28 constituting a cooling station 26, so that, when it arrives at the nip 31, the temperature of film 15 is slightly below the point at which the ink becomes tacky. The energy of the laser light (or thermal print head, as may be the case) is then sufficient to render the film 15 tacky but regions of the film not (selectively) exposed to energy (e.g., radiation or conduction) remain relatively solid and do not transfer to the substrate. It should be clear that in the cooling station 26, the cooling rollers 28 can alternatively be cooling belts or the cooling surfaces of any suitable heat sinks capable of dissipating the heat that might accumulate at the stations travelled by the donor surface. The cooling of the donor surface, dissipating superfluous heat when and where needed, may contribute to having the temperature of the base of the blanket distal from the surface remaining relatively stable during the process. Likewise, in the writing station 33 energy sources other than laser light, such as a thermal print head, may alternatively be used to selectively melt the regions of the film to be transferred to the substrate 36

[0078] After thermal printing has taken place, a positive ink image 40 will have been transferred to the substrate 36, and a negative ink image 42 will remain on the blanket 14, both being illustrated as dashes on their respective support downstream of the impression station 30. Were this negative ink image 42 to be allowed to reach the coating station 18, it would leave a latent image on the blanket 14 and eventually result in the ink film 15 not being level when it reaches the impression station 30 during the next operating cycle. This cross-contamination between cycles can result in undesirable ghost images.

[0079] To avoid this problem, prior to returning to the 3 o’clock position for subsequent recoating, the blanket 14 passes through a donor surface cleaning station 50 which comprises a cleaning surface 53, illustrated in present figure as the outer surface of a heated roller 52, which could alternatively be a heated belt. The cleaning surface 53 (e.g., of heated roller 52) renders the surface of the negative image 42 remaining on the blanket 14 sufficiently tacky to attach to the cleaning surface 53 (e.g., of heated roller 52), the temperature of heated roller 52 maintaining the cohesivity of the remaining film and, as the donor surface 16 has low surface energy, the negative film adheres more strongly to the roller 52 than to the donor surface 16 and therefore peels away cleanly from the donor surface 16, hence from the blanket 14.

[0080] In the case of a cleaning station 50 comprising a heated cleaning surface, the polymer film enters the cleaning station at a temperature lower than the cleaning surface and is rendered tacky thereby. As an alternative, the polymer film arrives at the cleaning station at a temperature higher than the cleaning surface, similar thermo-rheological mechanisms governing its removalfrom the donor surface and its transfer to the cleaning surface. In this case, a heating device can be used to render the polymer film tacky upon entry into the cleaning station, the cleaning surface being relatively cooler, or actively cooled to maintain a temperature difference enabling removal of the film. For instance, IR lamps (better seen in Figure 8 as heating elements 140) can be disposed to irradiate the ink film / negative image remaining on the donor surface, said heating being optionally performed from the reverse side of the blanket 14 opposite the donor surface 16, typically in an area of the blanket aligned with the cleaning surface 53, which can be of a cooled roller or a cooled belt.

[0081] Thus, more generally, a cleaning station 50 suitable for a printing apparatus 10, or 100 to be later detailed, in which a coating apparatus 180 according to the present teachings can be employed comprises a temperature-controlled cleaning surface 53 and optionally a device capable of regulating the temperature of the blanket 14 so as to adjust the temperature of polymer ink film 15 remaining thereon following impression. In operation of the printing apparatus, the temperature of the cleaning surface 53 can be a first temperature Taand the temperature of the ink film 15 (typically essentially the same as the temperature of the donor surface 16) can be a second temperature 7* different from Ta, one of Taand 7* being greater than the softening or melting temperature of the polymer film and the other smaller.

[0082] Advantageously, the cleaning roller, whether heated 52 or cooled, can be made of a rigid material (e.g., a metal or alloy), so that scrapping its outer surface to remove (and optionally recycle) the ink residues detached from blanket 14 do not wear it as rapidly as a donor surface made of an elastomer, were it to be cleaned directly. This, however, is not essential and the cleaning surface 53 can be of a flexible belt, rather than of a rigid roller as shown in the figure. The cleaning surface of the cleaning belt can also be heated or cooled, as desired.

[0083] Figure 7 schematically illustrates the challenges of present cleaning station 50 in order to properly maintain the donor surface 16 of blanket 14 free of ink residues that could hamper the print quality that printing system 10 or 100 could achieve. Figure 7C is a top view of the uniform continuous solid polymer film 15 before being exposed to the energy applied by writing device 33. An exemplary positive image 40 to be formed thereon is indicated by peripheral dotted lines shaped as the letter A. Figure 7D illustrates the negative image 42 remaining on the blanket following transfer of the positive image 40, the selected regions rendered tacky and having transferred exposing the donor surface 16 in the depleted regions. As this method of transfer printing can be assimilated to a subtractive approach of forming an image, the matter remaining to be cleaned on the blanket can be significant, the area of the remaining ink filmfrequently, if not always, exceeding the surface of the ink having transferred. The terms positive and negative images are provided with respect to the ink image present on the transfer members, the printed images thereafter transferred directly to a printing substrate being mirror images of the same.

[0084] For comparison, conventional offset printing systems typically have an additive approach of forming an image on an intermediate transfer member. The ink due to form the desired image is selectively deposited, for instance as ink droplets or as toner particles. Figure 7A shows in top view an ink image formed on the intermediate transfer member of a conventional offset printer, the outline of the shape being filled with discrete dark spheres representing the selectively deposited ink. The ink image can then be cured and impressed onto a printing substrate. Ideally, the ink would be entirely transferred, however in practice ink residues may remain on the intermediate member after transfer, such as shown in Figure 7B. The situations depicted in Figure 7B illustrating the extent of cleaning typically needed in the prior art (removal of residues of positive images) as compared to Figure 7D illustrating present cleaning needs (removal of films of negative images) emphasize the importance of constantly cleaning the blanket from films remaining at each cycle in the present printing process, such cleaning steps being preferably only intermittently used in conventional processes when suffering from the presence of discrete sporadic residues.

[0085] The skilled person will also appreciate that a traditional blanket cleaning device suited for removal of sporadic residues of a positive image, as typically the case in the art illustrated in the previous figures, would be confronted with significantly more demanding challenges when the material to be removed from the negative image forms continuous film(s) requiring that the cleaning device overcomes not only the adhesivity of the material to the donor surface, but also the cohesivity of relatively larger areas of polymer film. Hence, a conventional cleaning device (which may furthermore be deleterious to the ink residues in order to remove them) is unsuited for cleaning the donor surface in present method or apparatus.

[0086] Notably, while in conventional processes the residual material removed from the intermediate transfer member following transfer printing is typically discarded, this need not be the case in the printing method according to present teachings. Traditional residues cannot be re-used, as the inks leading to their remaining presence are generally irreversibly transformed during the printing process (e.g. , being cured by ultraviolet radiation) and / or during the cleaning process (e.g., being removed from the transfer member with detergents or other materials modifying the ink composition). In the present disclosure, and on the contrary, the polymermaterial removed from the donor surface at the cleaning station by the cleaning surface can be recycled, avoiding waste hence having clear commercial benefits.

[0087] The ink of the negative image removed from the donor surface 16 is, in turn, removed from the cleaning surface 53 (e.g., from heated roller 52) by a scraper or doctor blade 54 and can be collected in a reservoir 56. From the collecting reservoir 56, the ink can be recirculated by a back-feeding system 57 to a second (feeding) reservoir 58 from which ink can be supplied, if needed together with fresh ink provided from a stock (not shown) to the coating station 18.

[0088] Any and all devices (e.g., 54, 56, 57, 58) selected and disposed to return the polymer removed from the donor surface by the cleaning surface for re-application to the donor surface in subsequent operating cycles can be referred to as a recycling system 55.

[0089] As the polymer material can be detached from the cleaning surface serving for its removal from the donor surface as a relatively solid material, it can be conveyed as such to the ink feeding system, the surrounding temperature being set to allow the material to remain solid inbetween its removal and feeding. Alternatively, the removed polymer material can be detached from the cleaning surface as a relatively liquid material, or can be detached as a relatively solid material that can be melted prior to or during being conveyed to the ink feeding system, the surrounding temperature then selected to allow the material to remain liquid in-between its removal and feeding.

[0090] In one case, the devices or sub-systems enabling the recycling are spatially arranged one with respect to another such that the polymer material removed from the donor surface at the cleaning station (during the cleaning step) can be directly recycled to the ink feeding system. Alternatively, the recycling of the materials can require intermediate devices or steps, the removed polymer material being then “indirectly” recycled to the ink feeding system. For illustration, the removed material, if solid, can be conveyed by a recycling conveyor, and if liquid, be conveyed through recycling pipes, optionally with the assistance of a pump when gravitational flow of the material is inappropriate or insufficient, or through intermediate cylinders conveying the molten material by contact. The latter recycling option is illustrated in Figure 8.

[0091] Figure 8 schematically illustrates how material removed at the cleaning station can be recycled for a new cycle of application as a polymer film on the donor surface at the coating station. For clarity, only part of impression drum 38 is shown in printing apparatus 100, the impression station 30 being represented by an arrow facing the writing station 33. Optional stations of the printing apparatus which have been detailed with reference to Figure 1 andprinting apparatus 10 are omitted in present drawing. Conversely, in Figure 8, there are depicted elements of embodiments of the printing apparatus previously unshown. For instance, four cylinders 130 are depicted to enable the rotation of drum 12 which can be made of glass, upon which a transparent blanket 14 is mounted, its outer surface forming the donor surface 16.

[0092] Alternative number of cylinders 130 can be used, one of them being a drum-driving cylinder and the other(s) drum-guiding cylinders.

[0093] Also shown in Figure 8 are two optional heating devices 140 that may locally heat the blanket and the material resting on its donor surface in the segments coming into contact with the applicator roller 66 and / or with a cleaning roller 102. Each heating device 140 can be, for illustration, an IR lamp equipped with reflective surfaces enabling the heat to converge where desired, the heating devices being in present drawing shown disposed internally to the drum 12, on the rear side of the blanket 14. For clarity, the polymer ink film 15 formed by coating or residues thereof following impression are not shown in this figure.

[0094] Reverting to the printing process, at the end of a particular cycle, following printing and transfer of selected regions of the ink film from the donor surface 16 to the substrate, any remaining polymer film can be removed from the donor surface by the cleaning roller 102, by enabling a thermo-rheological mechanism as previously explained (e.g., the cleaning surface 53 of roller 102 being heated, or on the contrary cooled in the event a heating device 140 is used to render the remaining polymer tacky). The removed material can then transfer to an intermediate recycling roller 104 and therefrom to the anilox roller 60. Besides by contact between the outer surfaces of these rollers, the recycled material can be conveyed back to the anilox thanks to the presence of a doctor blade or scrapper 106 and a levelling wiper 107 suitably disposed one in relation with the other and a segment of the cylinders so as to form a volume entrapped between the walls of these devices, constituting a reservoir 108 into which ink being detached from one of the cylinders with blade 106 can be reutilized by being wiped on an adjacent cylinder by wiper 107, the latter component also preventing egress of the entrapped recycled material. Though two such arrangements exist in the illustrative figure, it will be described only where better seen with respect to the nip between cleaning roller 102 and intermediate recycling roller 104 where the scrapper 106 of cleaning surface 53 is shown as a relatively thin horizontal line coming into contact with the surface of the cleaning roller 102 detaching material into volume 108 further bordered by a wiper 107 shown as a relatively thicker vertical line, which can readily reapply some of the material to intermediate recycling roller 104 which will convey it downstream to the anilox roller 60. This reservoir 108, or theone located closer to the anilox, can additionally communicate with a supply of fresh heatsensitive ink (not shown in the figure). This series of components including the cleaning roller 102, the intermediate recycling roller 104, the scrappers 106, the wipers 107 and the reservoirs 108 formed there-between can be said to constitute a recycling assembly or suite 110.

[0095] The components that can now convey the recycled material from the ink reservoir 108 shared with the recycling assembly (optionally supplemented with fresh ink) back to the donor surface of the blanket include an anilox roller 60, an intermediate roller 62 and an applicator roller 66, as previously described, and can be said to constitute an ink film forming assembly or suite 120. The rollers constituting the film forming assembly can have different lengths, the diameter of the dosing and applicator rollers (respectively, 60 and 66) being the same and the diameter of the intermediate coating roller 62 being different (smaller or larger).

[0096] The recyclability of the ink material used in present printing process stems from its thermoplastic properties, and the operating conditions of the process avoiding its degradation, the properties of the polymer (other than its viscosity at the different stations) remaining essentially the same from one cycle to another. In other words, the heat-sensitive ink selected to form the films as herein disclosed have a chemistry that is not changed or irreversibly altered by the process, for instance by the energy applied during any of the steps previously described. Recycling of the ink, however, might not suffice, and fresh ink may be provided to the feeding system from an external stock (e.g., a cartridge of thermoplastic material) to compensate for the ink consumed by printing.

[0097] It will be appreciated that though the described embodiment of these thermal transfer printing systems 10 or 100 comprises a drum 12 to support the blanket 14 and convey its donor surface 16 through the afore-mentioned stations (e.g., 18, 30, 50, and optional 20 and 26), the blanket can alternatively be an endless belt circulating about suitably disposed guide rollers, one of them driving the motion of the belt. In such a case, backing surfaces may be positioned on the rear side of the blanket at least in regions of the belt facing stations of the printing system acting by contact with the donor surface. For illustration, backing surfaces can be cylinders, or plates made of or coated with a material having low friction to enable smooth displacement of the belt. Backing surfaces may be named according to the region of the apparatus where they provide support to the blanket or station they are facing. For illustration, if the blanket is looped as an endless belt circulating about suitably located backing surfaces, the one positioned at the nip of the impression station can be referred to as an impression backing surface.As can be appreciated, when the present coating apparatus 180 is used to coat a surface of a 3-dimensional object, the above-described printing apparatus 10 can be modified to become a coating system 200 comprising a coating apparatus 180 (including a feeder 58 adapted to supply the polymer material for the coating, a supply system 68 capable of transferring it, an applicator 66 and a coating station 18 at which the polymer film coating is applied to a surface). In one case, a component of the coating system following the path of blanket 14 as described in a printing apparatus can serve to convey the coating substrates 136 to be directly coated on a surface thereof by the coating station 18. In another case, the blanket 14 can act as an intermediate transfer member 114 for the film coating 15, the selective writing station 33 of the printing apparatus being replaced in the coating system 200 by a non-selective heating station 133 including a heating device generating heat enabling the transfer of the entire coating 15 to a surface of objects conveyed by impression drum 38 (which can be replaced by a belt 138 supported at least at the impression station 30 by a backing surface). A supported surface of an impression drum 38 or of an impression belt 138, both capable of supporting a substrate against which a heated polymer coating can be pressed for transfer, can be referred to more generally as an impression surface.

[0098] While a cleaning station 50 can be superfluous in such a coating system, a levelling station 20 and / or a cooling station 26 can still be beneficial, as they may further increase the uniformity and quality of the film coating. The absence of stress on film 15 (other than the splits, to be further detailed hereinbelow, enabling its formation by the coating apparatus 180) and / or the reduction (or elimination) of such strains in present method is believed to be pivotal to the quality of the coating outer surface. For instance, the coating apparatus 180 and the coating system 200 in which it may operate are configured to avoid longitudinal tensions (e.g., stretching of the film), or like distortions of the polymer film in any other direction that would damage the uniformity of the coating surface (e.g., its high gloss and / or low roughness), thin coatings of thermoplastic materials as envisioned herein being particularly sensitive to adverse deformations.

[0099] Overview of the film application system

[0100] The coating station 18 is designed to apply a film of very small thickness to the blanket 14, which could alternatively be a surface of an object carried thereby when used solely for coating, the film 15 ideally having a thickness less than a few microns (micrometers), often not exceeding 750 nanometers or half a micron. In order to achieve this aim, the coating station 18 may be the donor surface contacting part of a more complex coating apparatus 180 which mayfurther comprise an anilox roller 60 onto which a layer of the polymer is applied from a feeding system, represented schematically by the reservoir 58 in Figure 1, and surplus polymer is removed by a doctor blade in a conventional manner to leave a film of the heat- sensitive ink on the (dosing) roller 60. For convenience, the exemplary coating apparatus 180 discussed in connection with Figure 1 is surrounded in the drawing by a dotted line.

[0101] Anilox rollers 60 have a patterned outer surface, the geometry and dimensions of the patterns determining inter alia the amount of ink material that may be loaded on the roller surface (e.g., as expressed in cm3 / m2). These patterns may, for example, be slashed, slanted, curved, dotted, ART, quadragene (such as diamonds), hexagonal, tri-helical, etc. The center points of such shapes or their orientation may furthermore follow any desirable alignment angle such as 30°, 45° or 60°, (as may be seen for instance in honeycomb patterns) and the number of anilox cells in such patterns (e.g., per line or over the entire surface) may vary. For illustration, the anilox roller may have a line spacing per unit length of 250 lines per centimeter (L / cm) or more, 300 L / cm or more, 350 L / cm or more, the line spacing typically not exceeding 500 L / cm, this however depending on cell geometry. The nominal volume of ink that may be carried on the surface of the anilox roller, may depend on the desired operation conditions of the printing system and the type of matters to be printed, and can be for example of 2.5 cnf / m2or more, 3.0 cmVm2or more, 3.5 cm3 / m2or more, and 4 cm3 / m2or more. While anilox rollers with nominal volume of up to 50 cm3 / m2exist, for the present purposes the anilox roller may have a nominal volume of only up 20 cm3 / m2, up to 15 cm3 / m2, or up to 10 cm3 / m2.

[0102] The pattern (typically produced by laser engraving of the surface of the anilox roller) may additionally be selected in view of the viscosity of the fluids to be conveyed in its recessed areas. Anilox rollers, or their outer surfaces if provided as anilox sleeves mounted on support shafts, are typically made of (and / or coated / plated with) mechanically resilient materials, such as metals or ceramics. For illustration, the anilox roller can have a core made of wear-resistant, corrosion-resistant, and high-temperature resistant steel, its outer surface being made of ultra-hard metal plating or ceramic layer; and the pattern can consist of 30° hexagons with thin bridges. The hexagonal anilox cells can have a line spacing of 360 L / cm and the roller a nominal volume of 4.8 cm3 / m2as shall be used in Example 3 to illustrate printing according to present teachings using the coating apparatus according to present teachings.

[0103] The doctor blade shaving excess ink polymer from the surface of the roller can include segments having more than one orientation with respect to the axial length of roller 60 or be provided as separate blades each having a particular orientation. For illustration, one blade canbe substantially parallel to the axis of rotation of the roller and have a length similar to the width of the roller surface, and two additional blades can be disposed near the lateral edges of the roller each with an angle enabling redirecting the molten ink supplied by the feeding system towards a centerline on the roller surface (to avoid spilling of molten ink out of anilox surface). The blades are positioned at an angle to the surface of the anilox so that its cells can be efficiently filled, the flexibility of the blades and their shapes (usually including a round tip) being selected in accordance with the patterns elected for the anilox roller.

[0104] The feeding system 58 can be a progressive cavity extruder heating and pushing the ink from a hopper to which it can be supplied in relatively solid form through a nozzle towards the surface of roller 60, by which time the ink is molten so as to properly coat its surface. The system is set to feed the anilox surface at a pace substantially enabling replenishing the amount of ink being consumed by the positive image 40. The temperature at which the feeding system operates is selected in accordance with the ink (enabling its melting by the time dispensed through the nozzle) and the temperature on roller 60. The dispensing nozzle (be it a slit elongate orifice or an array of individual nozzles that may be similarly used) has a length commensurate with the width of the anilox roller, albeit slightly shorter to avoid spillage and waste of ink by overflow to the sides of the roller. The feeding system is not limited to the one exemplified herein and can be any alternative device known to the skilled persons, such as a manifold which can deliver the ink to the anilox roller.

[0105] While the feeding system 58 can be set to provide a fresh volume of molten ink in response to its consumption during a particular print job, other considerations may be taken into account for its control. Advantageously, the feeding rate can be steady, sufficiently high to feed the anilox cells and prevent their starvation, while being low enough to avoid overflow and waste of material to the sides of the anilox roller 60.

[0106] Regardless of the feeding system, the molten ink generally first accumulates upstream of the doctor blade(s) forming a continuous pool from which the polymer is drawn into a film. Typically, the film formed by feeding system 58 on roller 60 will have a thickness of several microns, it being difficult to achieve a thinner film by this method alone. The pattern on the surface of the anilox roller 60 may serve to ensure a sufficient dose of molten polymer is present on this first roller feeding those further downstream within the coating apparatus till the coating station, such first roller being also referred to as a dosing roller 60. This film is next progressively reduced in thickness by transferring it from one roller to the next in a series of rollers, the illustrated embodiment having one intermediate roller 62 and an applicator roller66. Each time the film is transferred between a pair of rollers, its thickness is reduced because the transfer in each case is only partial. The rollers (e.g., 60 and 62) supplying to the applicator roller 66 a serially thinned film of polymer together form a supply station or system 68.

[0107] The proportion of the film transferred between adjacent rollers can be controlled by regulating the temperatures of the rollers and / or other properties of their respective outer surfaces. In Figure 1, each roller is marked with an “H” (for hot) or a “C” (for cold) to indicate if its temperature is above the melting or softening point of the polymer or below that temperature. It will be seen that at each contact point between two adjacent rollers, the rollers are at different temperatures. Thus, a temperature gradient is set up within the film passing at any nip between adjacent rollers of the coating apparatus including at its terminal the coating station, so that one side of the film is solid (or relatively solid) while the other is liquid (or relatively liquid). The surface energies of the rollers can also be selected such that the solid part of the film adheres to one of the rollers while the molten part adheres to the other, thereby resulting in the film being split into two films of reduced thickness. The relative thicknesses of the two split films will depend on the temperatures of the two rollers and the temperature (or range of temperatures) at which a particular heat-sensitive ink transit from a solid to a liquid phase, including via intermediate softening phase when relevant.

[0108] Additionally, or alternatively, the relative thickness of the films being split at any particular nip between a pair of adjacent rollers may also depend on the pressure applied at the nip and on the materials forming their respective outer surfaces (affecting e.g., their relative surface energy or relative hardnesses). Advantageously, the intermediate roller 62 of a coating apparatus 180 or of an ink film forming assembly 120 can have a relatively soft outer surface as compared to its adjacent rollers (e.g., 60 and 66), the hardness of intermediate (coating) roller 62 being between 40 and 70 Shore A, or between 50 and 70 Shore A.

[0109] Thus, in Figure 1, the surface of the molten film on the roller 60 solidifies on contact with the roller 62 and the solidified partial layer transfers to the roller 62. This reduced thickness film can optionally be ironed by heated rollers 64 to level it still further and / or enhance its continuity before it reaches the 9 o’clock position on the roller 62 as a solid layer and is contacted by the hot applicator roller 66. The surface of the film is then melted, and the film is split to leave a solid portion on the roller 62 and transfer the liquid portion to the applicator roller 66. This is the liquid film that is then applied to the donor surface 16 of blanket 14, the film being split once again during the transfer from the applicator roller 66 to the blanket 14.For this purpose, and while as a rule true for all rollers being internally heated (e.g., 52, 64) or cooled (e.g., 28, 52), it may be desired that applicator roller 66 be made of a material having a thermal conductivity high enough for proper heat transfer to the layer of polymer coated thereon, so that the split of the ink film to the blanket may take place. Materials suitable for the applicator (or any other thermally-controllable roller or belt) can have a thermal conductivity of 100 Watts per meter per degree Kelvin (W / m»K) or more, 150 W / m»K or more, 200 W / m»K or more, 250 W / m»K or more, or 300 W / m»K or more. While materials with higher thermal conductivity exist, the applicator typically need not exceed 600 W / m»K or even 500 W / m»K.

[0110] Additionally, or alternatively, the surfaces to be heated or cooled can be selected according to the thermal diffusivity of the materials (« - alpha) of which they are formed. Suitable material can have a thermal diffusivity a of 25 mm2 / sec or more, 30 mm2 / sec or more, or 35 mm2 / sec or more. Usually, the thermal diffusivity a does not exceed 125 mm2 / sec.

[0111] For illustration, the applicator roller can be made of aluminum, brass or copper, said metals or alloys thereof, being advantageously relatively mechanically gentle with respect to the blanket. If desired, applicators made of such illustrative materials can be further coated with 5 pm to 100 pm of chrome, nickel or silver, which can optionally be polished or alternatively treated if required to achieve a desired smoothness. For instance, a suitable applicator can be made of brass (e.g., CuZmo (70% Cu, 30% Zn) or CuZm? (63% Cu, 37% Zn)), providing suitable thermal diffusivity, with a 30 pm coat of nickel (Ni) to enable smoothness and hardness of the outer surface due to contact the blanket.

[0112] It will be appreciated that though the described embodiment of this particular type of coating apparatus 180 including coating station 18 comprises a series of only three rollers (60, 62, 66), the series may comprise a larger number of rollers, their number being preferably (but not necessarily) an odd integer. All rollers involved in the transfer of the polymer film to the last of the rollers, namely to the applicator roller 66, regardless of their number and disposition upstream of the applicator and downstream of the feeding system can be said to constitute a supply station or system. Besides being typically maintained at different temperatures, adjacent rollers in a pair along the path leading to the donor surface can be made of materials having different hardness. For example, hotter rollers may have an outer surface comprising or consisting of a metal, with a hardness of 60 HRB or more on Rockwell B scale, while cooler rollers may have a relatively soft outer surface comprising or consisting of an elastomer (e.g., made of silicone), with a hardness of 70 Shore A or less.Preferably, the surface of the rollers should be smooth. In some cases, the average roughness (Rd) of the roller surfaces is of 0.1 gm or less, 0.05 gm or less, or 0.025 gm or less, which can also be selected by having finishing grades N2, N1 and NO.

[0113] It is to be noted that while the first roller 60 and the last roller 66 of the series may have a same diameter, the intermediate roller 62 of the coating suite must have a different diameter, this difference enabling taking corrective actions as the film proceeds from its feeding and formation on a first roller towards its application to the donor surface. While in Figure 1 the diameter of intermediate (e.g., silicone coated) roller 62 is schematically represented as being larger than the diameter of rollers 60 and 66, this should not be construed as limiting, and its diameter can conversely be smaller.

[0114] Similarly, while all rollers of a coating apparatus 180 may have a similar length, in a direction transverse to rotation, this is not essential. It may even be advantageous to have an intermediate (coating) roller 62 slightly shorter than the first (dosing) roller 60, being preferably shorter than the blade forming the film thereon. The alignment of centerlines on their surfaces enables the intermediate roller 62 to pick up an ink film from central regions formed on anilox 60 by the blade, such regions being typically more uniform than the side regions of the film. The application roller 66 can have any length between the lengths of upstream 60 and 62, and different therefrom, the length of the applicator being commensurate with (but typically slightly shorter than) the width of the blanket.

[0115] For illustration, assuming that the intermediate roller 62 has a length of X mm, ironing rollers 64 optionally levelling the ink layer on its outer surface or applicator roller 66 picking up a split part of the ink layer may each have a length of X + 2 mm, or more, whereas the first roller 60 can have a still longer length of X + 10 mm, or more. Considering now elements downstream of the coating station 18, blanket 14 may have a width of X + 4 mm or more, a drum 12 (or backing surface of a belt) supporting it having a longer length, e.g., of X + 10 mm, or more. Heating or cooling elements, when present to modify the temperature of the polymer ink film as it circulates with the blanket, typically having a length exceeding the width of the blanket to ensure even temperatures upon exit of the relevant stations (e.g., 20, 26). The cleaning roller 52 also has a length exceeding the width of the blanket to be cleaned thereby by 6 mm or more, e.g., the cleaning surface having a length of X + 10 mm, or more. Usually, all aforesaid rollers and parts of the printing apparatus are aligned to their respective centerlines, the excess length any one may have with respect to a proximal one being typically equally split between edge margins.While not shown in the figure, the rollers of the coating apparatus 180 upstream of coating station 18 are urged into contact with the blanket 14 at coating station 18 using a suitable actuator, such as a pneumatic piston (e.g., capable of applying at least 2 kgf / cm, at least 5 kgf / cm, at least 10 kgf / cm, or at least 20 kgf / cm) or by lead screws. Advantageously, the pressure is so applied that it is substantially the same at all points along the lengths of any of the afore-described nips, so that a layer passing through the nips can have essentially a similar thickness over the entire surface downstream of the nip (z.e. , with a variability of less than 20%). Heterogenous pressure at any particular nip may also lead to a diversion of the (non-uniform) layer to the side(s) of the roller(s), resulting in waste and need for time-consuming cleaning of the coating apparatus.

[0116] If the coating apparatus 180 is constituted inter alia by a series of cylinders, such as by rollers 60, 62 and 66 as shown in Figure 1 or Figure 8, the pressure applied at each nip between each pair of adjacent rollers and ultimately between the applicator 66 at the coating station 18 and the drum 12 (or a coating backing surface in case of a belt) need not be the same. For illustration, a first actuator may be capable of urging the intermediate roller 62 of the ink film forming assembly in contact with anilox roller 60 at a first pressure Pl, a second actuator may be able to urge the foregoing set of rollers into contact with the applicator roller 66 at a second pressure P2 between rollers 62 and 66; and a third actuator can be configured to yield a third pressure P3 between applicator roller 66 and the donor surface of the blanket (as supported by drum 12 or a corresponding backing surface).

[0117] While the first and second pressures Pl and P2 can be selected so as to only satisfy intimate contact between adjacent rollers and the formation of uniform layers of materials being transferred at each nip, Pl and P2 optionally being the same, the third pressure P3 is subjected to additional considerations for its selection. Besides enabling close contact and the formation of a uniform layer of polymer ink 15 on the donor surface 16 or on the surface of the object contacted thereby, P3 should be sufficiently low to avoid or reduce wear of the blanket 14 (also if serving as intermediate transfer member 114 in a coating system), while being high enough to permit a sufficient heat flow to the polymer ink and underlying donor surface at the nip so it can completely coat the donor surface 16 or an object disposed on the blanket conveying it, preferably in a defect-free manner as early as of the nip of coating between the applicator and the blanket. As can be appreciated, the blanket being relatively compressible, in particular as compared to applicator roller 66, the magnitude of P3 may also control the width of the nip between the two surfaces, the area of the nip facilitating a desirable control of fluid dynamicproperties of the ink layer from the applicator and upon the donor or object surface.

[0118] To ensure that a layer of thermoplastic material passing through the various nips herein described have essentially a similar thickness and even face over the entire surface to which it is transferred, attention should be given to the rotational speed of each roller, so that slippage between any two adjacent rollers is avoided. In other words, the method, and the apparatus enabling its implementation should be substantially devoid of skid at any line of rolling contact (the adjacent rollers having essentially the same surface velocity) each nip involving a compressible surface (e.g., a roller or drum coated with an elastomer) forming a surface having the length of the line of rolling contact and a minor width depending on the materials of the rollers forming the nip, the thickness of the compressible material and the pressure applied one against the other. Typically, the width of a nip can vary between 3 and 10 mm, or 4 and 8 mm.

[0119] Notably, in absence of skid or slippage between adjacent rollers, longitudinal tension on the films being split and transferred at each nip can be avoided, this advantage of present teachings being believed to preserve or increase the quality of the coating.

[0120] While the coating apparatus 180 (including any supply and feeding stations upstream of the coating station 18, and their exemplary constituents) has been described as adapted to apply a film of thermoplastic material to a recirculating blanket, as may be used in the printing industry, this should not be construed as limiting. The coating apparatus can alternatively be used as an independent device configured to directly coat any surface which may benefit from the presence of a thin film of thermoplastic material. In such a case, the blanket serves no longer as an intermediate transfer member 114 of a coating system 200, but can be viewed as a mere substrate conveyor.

[0121] For illustration, thin coatings of thermoplastic films (whether directly applied or not) can be beneficial to packaging materials, e.g., improving their durability, their appearance, their resistance or impermeability to substances they may contact (rendering the surface water-impermeable or greaseproof) or surfaces of electronic or medical devices, to name a few of the diverse industries where the coating apparatus can be independently implemented. By way of example, coating photovoltaic modules with protective thermoplastic layers is known to extend the lifespan of solar panels.

[0122] In such a case, the surface to be coated (e.g., one face of an object) can be considered a coating substrate, which can be brought into contact with the application roller 66 of the coating apparatus under conditions similar to those herein detailed for the blanket. Briefly, the coatingsubstrate can be transported, at least in a region of transfer, over a support mechanism providing sufficient compressibility during coating, so that the surface of the coating substrate can be in intimate contact with the application roller to allow efficient split of the thermoplastic film. The support mechanism should, if necessary, enable heating of the coating substrate to a temperature adapted for transfer of the ink film from the applicator to the surface of the substrate, the difference in temperatures of the two surfaces allowing to control to some extent the split therebetween. For qualitative transfer, it is believed that substrates having smooth surfaces can be advantageous.

[0123] If the coating apparatus 180 is to be used for the coating of substrates having more challenging (e.g., rough) surfaces, it may alternatively be designed to include an intermediate transfer member 114, operating as described for blanket 14. While the support mechanism can be sufficiently compressible to ensure contact between the surfaces of the coating substrate and the application roller at the coating station 18, this contact might not be sufficiently intimate for the ink film on the applicator 66 to follow minute variations in surface topography, as might be present in more demanding coating substrates. This lack of sufficiently intimate conformability can be resolved when using an intermediate transfer member 114, as suitable ones can be configured to have conformational properties, in addition to basic compressibility. In contrast with the printing apparatus, the coating system 200 including the coating apparatus 180 and an intermediate transfer member 114 can then further comprise a heating device 133 non-selectively heating the entire polymeric film passing at the impression nip, so that the segment of the film exposed to said heating may completely transfer to the coating substrate 136. This non-selective heater replaces the writing device 33 and renders the cleaning station 50 superfluous.

[0124] As similar principles apply to a coating apparatus 180 when independently operated and when operated as part of a more complex printing system 10 or 100 or coating system 200, such as herein taught, the considerations herein detailed in the latter case shall not be repeated. For illustration, the teachings pertinent to the thermoplastic materials and the temperatures controlling their transfer, are similar when the compositions are to serve as ink compositions in printing systems or as coating compositions in coating systems. Hence, the terms herein referring to “ink(s)”, as used for illustration in ink compositions, heat-sensitive inks or ink films, should be understood to refer to corresponding coating compositions, heat-sensitive thermoplastic materials or coatings, depending on context. Likewise, if a coating system includes an intermediate transfer member 114 operating as described for blanket 14, its outersurface upon which the coating is applied before being transferred to a coating substrate operates as described for donor surface 16.

[0125] Cyclic thermo-:

[0126]

[0127] behaviors

[0128]

[0129] the

[0130]

[0131] The surface temperature of the blanket will vary at different stages in the printing or coating process wherein the blanket serves as intermediate transfer member to a surface of an object later contacted thereby. These variations of temperatures on the donor surface depend on the architecture of the printing or coating system and their operational conditions, such a variation being illustrated by the graph of Figure 2 for a printing system comprising a cleaning surface being of a heated roller. Typically, these fluctuations of temperatures are most significant on the donor surface and gradually diminish with the distance from the surface, the more distal layers of the base of the blanket displaying a more stable temperature along the process. Such an average process temperature, as could be measured for instance in the blanket base at 100 pm beneath the donor surface, is depicted by a dotted line in the drawing.

[0132] In Figure 2, the location on the blanket is plotted along the X-axis in angular degrees, measured from the 3 o’clock position in Figure 1, and its surface temperature in degrees centigrade is plotted on the Y-axis. The temperature shown by way of example in Figure 2 assumes a polymer having a softening point above 90°C, this being the temperature of the blanket surface 16 upon contact with cleaning roller 52 (see section 80 in the graph) the temperature of roller 52 being typically above perceived by the blanket, and being set according to a softening of the ink sufficient for its detachment.

[0133] The section of the graph designated 70 at the 0° / 360° position is the result of hot melted polymer being applied to the blanket at the nip with the applicator roller 66. The peaks in the region designated 72 are the result of the film being heated by the rollers 22 of the levelling station 20, three rollers being used in the present case, each one contributing one the peaks. The steep transition shown at 74 is the result of the film being contacted by the first of the cooling rollers 28, while the peaks in the region 76 are caused by the remaining cooling rollers 28 of the cooling station 26. The brief blip 78 approximately at the 180° position is the result of the cooling of the film by contact with the substrate 36 at the nip 31 of the impression station 30. Following gradual heating by the drum having at equilibrium a substantially constant temperature, the surface is heated in the region 80 by the roller 52 of the cleaning station 50 before returning to the starting 3 o’clock position.As the surface 16 of the blanket 14 is designed to permit release, so that selected regions irradiated or heated by the writing device could transfer to a printing substrate in due time, care should be given to avoid working at temperatures that may inadvertently promote release of the ink film from the blanket during its circulation and at any station other than the impression station. Particular attention should be paid when the blanket surface might be inadvertently non-specifically enabling release even if at temperature below Tg or Ts of the polymeric film, in which case its temperature should be set to prevent non-specific release.

[0134] If the process were considered now from the standpoint of the dynamic viscosity of the ink film on the donor surface, the plot would be an inversed image from the one monitoring the changes in temperatures. Namely, in region 70 when being applied to the blanket 14 by applicator roller 66, the hot melted polymer typically has its lowest viscosity. At the temperature of application (77, e.g., being greater than the melting temperature Tmof the ink), the viscosity of the ink is typically lower than 105millipascal seconds (mPa.s, 1 mPa.s being equal to 1 cps), being generally lower than 104mPa.s, or even lower than 103mPa.s. In region 72, the heating rollers 22 of the levelling station 20 may each decrease the viscosity of the ink after it naturally increased upon inherent cooling and solidification of the film. As the result of the film being contacted by the cooling rollers 28, the steep transition at 74 and following peaks in the region 76 would appear as dramatic increases in viscosity so that upon entry at the impression station 30 where the film is to be selectively heated, it is relatively solid displaying at the temperature of transfer to the substrate (77, e.g., being lower than the glass transition temperature Tgof the ink) a dynamic viscosity higher than 107, 108or 109mPa.s. Preferably, to be energy efficient and enable clean transfer of irradiated dots at impression, the difference in temperature between 77 and 77 should be narrow enough (e.g., not exceeding 60°C, 50°C or 40°C).

[0135] Since the drum has at equilibrium a substantially constant temperature, higher than the one experienced by the film during impression, the viscosity of the negative film remaining on the donor surface in the case of selective printing gradually decreases until the surface is heated in the region 80 by the roller 52 of the cleaning station 50 to a temperature 77 being sufficiently high for the residual film on the blanket to be tacky enough to adhere to roller 52, preferably as a cohesive film, while being sufficiently low to avoid melting of the residual film on the blanket or on the cleaning roller, which may promote splitting of the film, a phenomenon to be avoided for efficient cleaning.

[0136] In other words, the temperature 77 provided to the film with the assistance of the cleaning roller 52 of station 50 is intermediate to 77 and 77, without being necessarily midway. If the inkwere to have a transition temperature of softening Tsin a rubbery state between a liquid and a solid phase, then T? can be selected to be within 15°C, within 10°C, or within 5°C of Tsdepending on the steepness of the transition from a solid to a liquid ink.

[0137] While described for a cleaning surface being of a heated roller, similar principles can be exploited for the same purpose when the cleaning surface is alternatively of a relatively cooled roller, the temperature of the ink film on the donor surface being raised following impression and prior to, or upon entry, into the nip of cleaning, to establish a similar gradient of temperatures between the donor and cleaning surfaces, albeit in an opposite direction. For avoidance of doubt, a cooled or heated temperature of a cleaning surface is considered with regards to temperatures characterizing the polymer film and the heat-sensitive ink forming it, or the temperature such an ink film may have on the donor surface upon entry into the cleaning station. Hence, while a heated cleaning roller can have in absolute terms an elevated temperature (e.g., > 90°C) greater than the Tm of the ink, a cooled cleaning roller is not necessarily “cold”, but only colder than the blanket in the cleaning station (e.g., at a temperature lower than the Tg of the ink, for instance at 50°C).

[0138] Preferably, the temperatures selected for the donor and cleaning surfaces at the cleaning station should take into account the next stations in the apparatus (or next steps in the method) each surface would need to comply with so as to remain energy efficient (e.g., not heating or cooling more than needed, if it were to require a superfluous energy-consuming cooling or heating thereafter).

[0139] As mentioned, a coating process not requiring the cleaning of the donor surface when the blanket serves as intermediate transfer member 114, the temperature of the blanket (and its transient cooling at 78) following thermal transfer of the entire coating film would gradually increase to its temperature upon entry into the coating station as shown at 360° position, without any significant drop or rise as previously needed for its temperature-controlled cleaning (see region 80).

[0140] Printing and coating assemblies

[0141] Athermal transfer printing system 10 or 100, in which present coating apparatus 180 can be utilized, can form on a substrate 36 a positive image made of the heat-sensitive ink applied by coating station 18, the ink being clear or colored. Considering colored inks, for illustration, typical printing systems require at least three primary colors, cyan, magenta, and yellow (CMY), to enable the formation of images spanning the entire color gamut, some printingsystems often adding black (K) ink to the series. As a printing system 10 or 100, such as respectively illustrated in Figure 1 or 8 but including the alternatives herein disclosed, would enable transfer of a single ink / color to the printing substrate, additional printing systems can be combined to form a printing assembly.

[0142] Figure 3A schematically illustrates a printing assembly 300 in which four thermal transfer printing systems designated 10a to lOd are disposed along a surface of a same impression drum 38, a substrate 36 sequentially passing through the printing systems, each capable of thermally transferring to the substrate an ink image having a different color. For instance, printing systems 10a to lOd may be configured to each apply one of CMYK inks. Each thermal transfer printing system 10a to lOd comprises a respective coating station 18a to 18d, each applying a respective ink film on a dedicated blanket 14a to 14d. Each ink film is selectively heated using a respective writing device 33a to 33d. The tacky regions of each film can then transfer from their respective donor surfaces to the substrate conveyed by the impression drum. Four impression stations 30a to 30d (marked for clarity within the drum, though the nip of impression is on the opposite side) are positioned along the path travelled by the substrate 36. If needed, cooling stations are disposed between the impression stations, so that a temperature of a substrate printed at a first station does not adversely affect proper printing at a following station. A substrate transport mechanism 90 is partially illustrated by roller 90a enabling the substrate fed by an upstream substrate feeding system (not shown) to follow the impression drum in its direction of rotation and by roller 90b enabling the printed substrate to be carried away from the impression drum, for instance to a downstream printed substrate collecting system (not shown). The printing assembly may further include a perfecting system, flipping the side of the substrate being printed, in which case additional printing systems (similar to 10a to lOd) can be disposed along the same impression drum to print on the reverse side of the substrate. For clarity, not all stations of the printing systems are represented in this figure, a cleaning station, or optional levelling, cooling, and recycling systems being omitted.

[0143] If the printing assembly is capable of printing different colors on both sides of a same substrate, but even if being intended for printing on a single side, each printing system (or each series of two or more printing systems) can be associated with a different impression drum. For avoidance of doubt, as each printing system may thermally transfer a different ink which can diverge from another not only by color but also by thermo-rheological behavior, each printing system can be configured to have a specific regimen of temperatures at the temperature-controlled stations, the temperatures set at each station being adapted to the properties of their respective inks.

[0144] Similarly, a single coating apparatus would apply (directly to a face of a coating substrate 136 or via an intermediate transfer member 114) only a single layer of a single polymer film to the coating substrate. Figure 3B schematically illustrates a coating assembly 400 in which four thermal transfer coating apparatuses designated 180a to 180d are disposed along a surface of a same impression drum 38, a coating substrate 136 sequentially passing through the coating devices, each capable of thermally transferring to the substrate an ink film having a different color and / or a different property (e.g., being made of distinct polymers and / or including different materials). For instance, coating apparatuses 180a to 180d may be configured to each apply one of four films having different properties sought for layers of a coating of stacked films (in analogy to CMYK inks as might be required for a printing assembly). Each thermal transfer coating apparatus 180a to 180d comprises a respective coating station 18a to 18d, each applying a respective ink film on a dedicated blanket serving as intermediate transfer members 114a to 114d. Each polymer film is heated using a respective heating device 133a to 133d. The tacky segments of the films can then transfer from their respective donor surfaces to the coating substrate conveyed by the impression drum. Four impression stations 30a to 30d are positioned along the path travelled by the coating substrate 136. If needed, cooling stations are disposed between the impression stations, so that a temperature of a substrate coated at a first station does not adversely affect proper transfer coating by a different layer at a following transfer coating nip. Though the substrate to be sequentially coated by layers of films transferred at each of the impression stations appear as being carried upon an impression drum 38, as suitable for flexible objects, the coating substrate can alternatively be carried through the thermal transfer coating stations by a belt 138, supported by backing surfaces at least at the impression stations, as necessary for more rigid coating substrates 136.

[0145] Not all coating apparatuses 180 of a coating assembly 400 need to operate in a similar manner, for instance one may directly apply a layer of film coating to a surface of a coating substrate 136, while another may coat the same substrate indirectly, first forming the film coating on an intermediate transfer member 114, then heating it at heating station 133 for transfer at impression station 30.

[0146] The combination of a single coating apparatus 180 with an intermediate transfer member 114, a heating device 133, an impression station 30 facing an impression surface of a drum 38 or of a belt 138 forming an impression nip 31 to which the substrate is conveyed by a substratetransport mechanism 90 can be said to form a coating system 200. Such a coating system 200 is schematically surrounded by a dashed line in the figure, the coating assembly 400 being in present illustration formed of four coating systems sharing a same impression drum 38 or impression belt 138 and a same substrate transport mechanism 90. For clarity, not all stations of the coating systems are represented in this figure, for instance, optional levelling and cooling stations are omitted.

[0147]

[0148] stations

[0149] The printing assembly 300, or even a single printing system 10 or 100, as well as a coating assembly 400, or a single coating system 200, can be preceded and / or followed by conventional printing or coating sub-systems respectively suitable for the apparatuses according to the present teachings. A preceding station, z.e., one preceding the coating of a surface by a film of ink, may for instance treat the surface of the printing or coating substrate and / or of the donor surface to facilitate the transfer of selected regions of the film from the donor surface to the substrate and / or to enhance its adherence thereto. A suitable pre-treatment can be chemical (e.g. , applying a chemical substance to the desired surface(s)) or physical (e.g, a corona treatment modifying the surface energy of the desired surface(s)).

[0150] A station following the thermal transfer printing of an ink image or thermal transfer coating of a film to the substrate may for instance be a finishing station to apply an overcoat on the printed image or transferred coat (e.g, protecting it and / or modifying its appearance), cut the substrate, trim its margins, or bind or assemble individual printing sheets together (e.g., into brochures), to name a few standard actions that may take place following printing or coating of a surface.

[0151] Preceding and finishing stations to printing or coating systems, or assemblies thereof, are known to the skilled person and need not be further detailed herein. Advantageously, they should be selected so that the speed of printing or coating by the system or assembly is not slowed down by their presence. In some cases, the speed of printing or coating enabled by the present teachings is up to 1 meter per second (m / s), up to 2 m / s, up to 5 m / s or up to 10 m / s.

[0152] The printing method

[0153] Figure 4 shows in the form of a flowchart how the present printing method can be practiced. The rationale and operating conditions (e.g., temperatures, dynamic viscosities) detailed with respect to the printing system apply mutatis mutandis to the method and for brevity will not be all repeated. For clarity of the figure, optional steps previously detailed, suchas those preceding the application of the uniform continuous film of a heat-sensitive thermoplastic ink by the applicator, such as thanks to gradual splitting of polymer films at nips of a coating apparatus; following coating of the donor surface, such as levelling and / or cooling of the film; or preceding or following the printing process, are omitted from the flow chart In a first (coating) step designated SOI, the method comprises applying to a donor surface 16 of a circulating blanket 14 a uniform continuous film 15 of a heat- sensitive ink. The application can be achieved by splitting a layer of molten ink between a relatively hotter surface (e.g., of applicator 66) and a relatively cooler one (e.g., of the blanket mounted on a drum 12, or circulating about a backing surface facing the coating station 18 if the blanket is an endless belt). As the blanket proceeds to a subsequent station, for the performance of a following step, the ink film may self-level and passively cool down to form a solid uniform film. However, such formation may require active performance of intermediate steps, such as the levelling of the film so it may be uniform (or more uniform than upon its application). As levelling can be performed by heating the film (with or without contact therewith), the method may further require the cooling of the film, so it be sufficiently solid for the performance of the following step.

[0154] In a second (writing) step designated S02, energy is applied to selected regions of the uniform solid polymer film to render the selected regions sufficiently tacky for subsequent transfer to a printing substrate upon impression. The energy selectively applicable by a writing station 33 can be in the form of radiation (e.g., laser beams) or conduction (e.g., thermal print heads).

[0155] In a third (impression) step designated S03, the donor surface and the substrate are pressed one against the other to cause the tacky selected regions of the polymer film to adhere to the substrate. The impression can take place concomitantly with the application of energy, if applied from a rear side of the blanket substantially at the nip of impression. This can be required when working with heat-sensitive inks having substantially a zero open time. Alternatively, this step can take place after the application of energy. For illustration, a writing device 33 can be positioned upstream of the impression station 30, being by way of example a laser-based device facing the donor surface and exposing the ink film to radiation from its outermost surface. In this case, the ink film 15 should remain sufficiently tacky for transfer during the time between laser exposure and impression, the ink having, in other words, an open time with a suitable duration in view of the distance between the writing and impression stations and the speed of the donor surface between the two.In a fourth (transfer) step designated S04, the substrate is separated from the donor surface, so as to transfer the selected regions of the polymer film having adhered thereto from the donor surface to the substrate, considered following transfer of the ink image a printed substrate.

[0156] In a fifth (cleaning) step designated S05, following transfer of an ink image to a printed substrate and prior to commencement of a new cycle, the donor surface, which carries residual background areas of the film not transferred to the substrate, is brought into contact with a cleaning surface 53 (e.g., of cleaning roller 52 of cleaning station 50). The surface of the cleaning roller is set at a temperature enabling its adherence to any ink film still overlying the donor surface, enabling its detachment therefrom, by peeling it away. The cleaning surface can be of a heated or cooled roller (or belt), when the donor surface is on the contrary relatively cooler or relatively hotter.

[0157] The residual film detached from the donor surface by the cleaning roller can be scraped away from its outer surface, to enable ongoing cleaning of the blanket, but is recycled in a sixth (recycling) step designated S06. Such a step ensures that ink remaining on the blanket after a particular printing cycle may serve again for a subsequent application of a film to the donor surface, the removed material being conveyed back to the applicator by any suitable method, such as illustrated by a recycling system 55 including the back-feeding station 57 or by a recycling assembly 110 previously detailed.

[0158] Similar steps may be used in an indirect coating method, when it is desired to first apply the coating to the donor surface of an intermediate transfer member 114 analogous to blanket 14 and thereafter thermally transfer it to the surface of the three-dimensional object to be coated thereby. Initial step(s) of the method would result in the coating of the donor surface, substantially as described for SOI of the printing method. The second step of the coating method would involve heating the uniform continuous film so the heated segments of the film be sufficiently tacky for transfer to a coating substrate upon impression. As heating at station 133 need no longer be selective, IR lamps, heated plates, rollers or belts can be used. In one case, the impression drum can be heated to a temperature enabling detachment of the coating film from the donor surface and its transfer to the coating substrate. The following steps of the coating method relating to the thermal transfer of the coating film proceed substantially as described for the impression and transfer steps S03 and S04 of the printing method. As explained, the cleaning and recycling steps S05 and S06 of the printing method are not required for a coating method.The consumables

[0159] Consumables suitable for printing or coating methods and systems according to the present teachings include, in addition to the substrates (e.g., printing substrates which can be supplied as individual sheets or as a continuous web), by the transport mechanism 90 being accordingly selected, the main following materials: a) the heat-sensitive ink compositions, b) the blankets (e.g., sleeves or continuous belts, with or without a seam), and optionally c) the chemical substances that may be used in preceding or finishing stations, if present. Such optional substances, often used in liquid form, can serve for pre-treating the substrates or the blankets ahead of ink transfer, for cleaning the printing system or parts thereof (e.g., for removing ink residuals from the blanket during maintenance), for applying an overcoat covering the ink image or the coating transferred to the substrate and like conventional purposes readily appreciated by a person skilled in the art of printing or coating.

[0160] The consumables are selected and adapted to any desired particular configuration and operation of the printing or coating system (and vice versa the operating conditions of the printing or coating system are adapted to the consumables used therein). Generally, the consumables are compatible with the printing method. Having already discussed the compatibility of the ink (in view of its thermo-rheological behavior) with the temperatures being set on various surfaces of the printing or coating system, we shall exemplify now which properties may render a blanket compatible with present method. For instance, if, during the printing or coating process, the donor surface of the blanket is to be exposed to elevated temperature, it should be heat resistant at least to the applied temperature; if the blanket is a sleeve to be mounted on a drum, it should have enough stretchability to enable mounting; if the blanket is a tensioned belt, it should have mechanical resistance at least to the applied tension; if the blanket is a belt in motion, it should include, on the side opposite the donor surface, a layer providing suitable friction or lack thereof with underneath guiding systems; the donor surface shall be wettable by the heat-sensitive ink while being able to release the ink film for transfer to the substrate; and any such considerations allowing use of the consumable under the operating conditions, which a person skilled in printing or coating may readily appreciate.

[0161] Moreover, the consumables should be compatible one with the other. For instance, the ink need be compatible with the blanket and / or with a chemical substance applied thereon (if present). Fundamentally, a material or a chemical composition is compatible with another if it does not prevent its activity or does not reduce it to an extent that would significantly affect the intended purpose. For instance, the inks would not be compatible if, among other things, unableto selectively transfer from the donor surface and / or attach to the surface of the substrate, whether or not any of the aforesaid surfaces is pre-treated. As readily understood, this principle of chemical compatibility of any consumable used herein with any other consumable should preferably guide the selection of all materials necessary for the implementation of the present printing or coating method.

[0162] While consumables suitable for printing or coating methods and systems according to the present teachings are known to the skilled person, and need not be detailed herein, basic guidance shall be provided regarding the ink and the blanket.

[0163] A- The heat-sensitive ink

[0164] As can be readily appreciated from the description of the present printing or coating process, materials suitable for the formation of a thin film of ink by the coating apparatus till application at the coating station can be referred to as thermoplastic materials, thermoplasticity customarily meaning that a material may reversibly melt or soften under the influence of heat, solidifying back upon cooling. With respect to the present teachings, it further signifies melt processibility, that is, the capability of the thermoplastic materials being applied in a molten state to a surface.

[0165] In practice, the shape of the thermo-rheological curve and the transition between the different phases depend on the type of thermoplastic material constituting the ink. If the thermoplastic material is crystalline, the drop in viscosity from a solid to a melt as temperature increases is rather sharp, the transition phase being relatively narrow as illustrated by the exemplary curve displayed in Figure 5A. If the thermoplastic material is semi-crystalline, the drop in viscosity from a solid plateau to a melt plateau as temperature increases is relatively moderate, the transition phase being relatively broader as illustrated by the exemplary curve displayed in Figure 5B. If the thermoplastic material is amorphous, the drop in viscosity as temperature increases fails to provide clearly discernible step-shaped phases, the changes being relatively more monotonic over wide softening temperature ranges as illustrated by the exemplary curve displayed in Figure 5C.

[0166] Temperatures characterizing the thermoplastic materials typically set to define the exact or narrow range of temperatures at which a polymer transitions from one phase to another. The glass transition temperature (Tg), applicable to amorphous polymers, is the temperature at which the thermoplastic materials undergo a transition from glassy to rubbery state. Pure crystalline polymers do not have a glass transition temperature. The melt temperature (Tm), applicable tocrystalline polymers, is the temperature at which a polymer transitions from a solid to a liquid state. Pure amorphous polymers do not have a melting temperature. In between, the thermoplastic materials can be characterized by the temperature at which some property of the raw material reaches an arbitrary value depending on the test being considered (e.g., degree of deformation, flow etc.). This temperature, known as the softening point (Ts), can be determined using standardized tests like the Vi cat Softening Temperature (VST) or the Heat Deflection Temperature (HDT). As thermoplastic materials are not necessarily constituted of a pure and single material, they can be defined by more than one and even all three temperatures. This may be the case for semi-crystalline materials which may display a Tg, a Tsand a Tm, all being displayed for illustration at arbitrary positions along the curve shown in Figure 5B. The temperatures characterizing any particular ink composition can be determined by routine analysis and standard methods such as but can be readily determined by standard methods, using for illustration Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA) or Differential Scanning Calorimetry (DSC), such as described in ASTM E794-06, or ASTM 3418.

[0167] The values assessed for the ink Tg, Tsand / or Tm, as applicable, can serve to set the heating and / or cooling temperatures that should be perceived by the ink film at each pertinent station to enable a printing or coating process as herein described. This matter is illustrated on the curve shown in Figure 5 A where prospective temperatures for 77, 77 and 77, are arbitrarily positioned.

[0168] Thermoplastic materials can be of natural origin (e.g., vegetal resins, cellulosics, polylactic acids) or synthetic (e.g., derived from petrochemicals), they can be homopolymers, copolymers (e.g., random, uniform, block or graft copolymers) or chemical modifications thereof, and may have wide ranges of average molecular weights (MW) that may influence material properties. For illustration, MW distributions typically determine melt flow characteristics, materials having relatively lower MW, such as waxes, having a relatively lower viscosity (higher melt flow at a given temperature) than materials having relatively higher MW (lower melt flow at a same temperature). As thermoplastic substances of each class may have respective advantages, different compounds can be blended in a given ink to produce a compromise of properties from two or more compatible polymers. Alternatively, or additionally, the properties of a thermoplastic substance can be modulated by non-thermoplastic materials, for instance by including compounds acting as plasticizers, which relatively soften the material, or compounds acting as fillers, which may on the contrary relatively toughen the material. Other additives can be present in the thermoplastic materials forming the inks, such as wetting agentsto facilitate their uniform spreading on the surfaces to be coated thereby (e.g., the donor surface of the blanket or the surface of a coating substrate), and surfactants to enable or maintain the homogeneous dispersion of fillers within the thermoplastic materials (e.g., of pigments providing a desired color). Such agents (which may additionally include antioxidants, preservatives, lubricants, etc.) are known to persons skilled in the art of formulating compositions having a thermoplastic behavior, such as hot melt adhesives for example.

[0169] While the thermoplastic materials have been described above mainly in terms of the heatdependent viscosity they may afford to the inks, they may additionally be characterized by and / or chosen for additional properties. For instance, thermoplastic materials can be selected for the cohesivity they display enabling a desirable null, partial (split) or full transfer depending on the nip, the surfaces in rolling contact and the operating conditions (e.g., temperature and pressure) being considered. Furthermore, the thermoplastic materials can be configured to have a relatively low affinity towards the transferring surface, such as the donor surface of the blanket (e.g., made of silicone or other elastomeric surfaces having a relatively low surface energy) and / or to have a relatively high affinity towards the receiving surface, such as the printing or coating substrate to which they are to adhere and transfer. More generally, the thermoplastic materials should be selected and formulated according to the intended process (e.g., having a suitable heat resistance to avoid their degradation, being able to absorb the laser radiation selectively heating the ink for transfer (optionally by including radiation absorbing agents into the ink), etc.) and the intended end-product following transfer (e.g., having a suitable mechanical resistance upon cooling and at temperatures of use, demonstrating for instance scratch resistance, rub resistance, low tack, etc.).

[0170] Thermoplastic materials can be selected from a group comprising vinyl alcohol polymers, polyvinyl butyral, polyacrylonitrile, polyacrylates, polyacetals, polyolefins such as polyethylene, polypropylene, and their copolymers, polycarbonates, polysulfones, polyethers, aromatic polyamides, polyimides, fluoroplastics, polyphenylene oxide, polyphenylene sulfides, polybenzimidazoles, polyphenylquinoxalines, aromatic polyhydrazides, PEEK, ketone-based thermoplastics, thermoplastic polyesters, and thermoplastic polyurethanes. Thermoplastic polymers that may degrade (e.g., ketone based) or undergo significant changes in rheology following their exposure to the temperatures perceived during one cycle as herein taught may not be used in printing systems or method recycling the residual ink from the blanket, back to the feeder. Such materials can be freshly provided to the feeding system from a stock and be used for coating systems.B- The blanket

[0171] Intermediate transfer members, as used in indirect (offset) printing technologies, can be as herein disclosed blankets mounted on a drum or looped to form a continuous endless belt. Regardless of the backing surfaces enabling their motion, the blankets may include one or more seams (to reach a desired length) or may preferably be seamless (e.g., formed as a sleeve), the area of the seam typically detracting from the evenness of the ink film that can be formed thereon, from the uniformity of heating, and in turn from the quality of printing or coating from said area. Blankets comprise a flexible support base, or body, and a donor surface, the base of the blankets being optionally made of more than one layer of materials, each layer typically providing for a sought function to the blanket. For illustration, the base of a blanket can include a compressible layer, to ensure that the donor surface may suitably contact the surface of the substrate, a conformable or conformational layer, to ensure intimate contact with the topography of the substrate, an adhesive layer to bind adjacent layers, a friction layer, to ensure proper attachment to or conversely movement over backing surfaces of the blanket, and like layers. More importantly, the blanket may include a radiation absorbing / thermal conduction blocking layer ensuring that the donor surface (and an ink film formed thereon) can be suitably heated by a heating device or more selectively by a writing device so as to permit thermal transfer of the polymer film or regions thereof. Such a heating-enabling layer can be selected according to the writing or heating device of the particular system, and include, for instance, radiation absorbing agents capable of converting the wavelength of the lasers of a laser-based writing device into heat.

[0172] The donor surface can be the outermost surface of such a heating-enabling layer or can be the outermost surface of a separate layer, dedicated to proper release and transfer of the selected tacky regions of the ink film. In the latter case, the heating-enabling layer can be disposed immediately beneath the release layer. In order to be able to release the polymer film in due time, a donor surface is typically made of a material having a relatively low surface energy, the surface energy of the donor surface being for instance of 25 millinewtons per meter (mN / m) or less, 23 mN / m or less, 21 mN / m or less, or 19 mN / m or less. Typically, the surface energy of a donor surface is at least 8 mN / m, at least 12 mN / m, or at least 16 mN / m.

[0173] Additional layers of the base, if present, can be selected in accordance with the writing or heating device and its position with respect to the blanket (e.g., facing the donor surface or a rear side of the blanket). If the writing or heating device is positioned to heat the blanket from its rear side (as illustrated in Figures 1 and 8), and is laser-based or IR-based, then the base ofthe blanket should enable transmission of the beams till they reach the radiation absorbing layer enabling their conversion into heat. In this case, the base is said to be “transparent” to the radiations. If heating of the donor surface is achieved by conduction of thermal energy applied by an alternative writing or heating device, then the base should be selected to permit the thermal conduction to an extent enabling sufficient (e.g., selective) heating of the ink film. Such a base, however, need no longer be transparent to radiations, and can be referred to as being “opaque”.

[0174] By way of non-limiting example, the blankets can be prepared as sleeves by centrifugal casting elastomers including or consisting of silicone-based polymers in a drum having dimensions (e.g., diameter, length) adapted to the sought sleeves or belt, said materials being suitable to release thin films of thermoplastic materials transiently transferring therefrom.

[0175] Suitable silicone polymers can be selected from liquid silicone resins (LSR), room temperature vulcanization (RTV) silicones, vinyl methyl silicone (VMQ), phenyl silicone rubber (PMQ, PVMQ), fluorosilicone rubber (FMQ, FMVQ), and polydialkyl siloxanes (PDAS) capable, alone or in combination, of providing once cured a mechanically resilient silicone matrix. The silicone polymers can be addition-curable or condensation-curable, depending on their particular chemistry, and can be formulated to include any desirable and suitable additive (e.g., cross-linkers, catalysts, curing inhibitors, etc. . The silicone polymers can also be mixed with elastomers of different chemical families (e.g., polyurethanes) to further tailor the properties of the sleeves as desired. Persons knowledgeable in the art of elastomeric blankets can readily adapt the exact composition of the fluid polymer to be centrifugally cast to the desired properties (e.g., hardness) of the sleeve, the mixing, defoaming, spinning and curing (e.g., temperature) conditions being also derivable from the properties sought of the cured blanket product.

[0176] When the writing or heating device is laser-based or IR-based and a radiation absorbing layer is required, this layer incorporating or not the donor surface, it is selected to absorb electromagnetic (EM) radiation at the wavelength of the laser- or IR- emitting elements. For instance, if the radiation is emitted in any portion of the near infrared (NIR) range within about 800-2,000 nm, then the donor surface or an underlying radiation absorbing layer should absorb over at least such portion of the NIR spectrum. In this way, the heating up of the donor surface assists in the softening of the ink film disposed thereupon, sufficient heating rendering selected regions or exposed segments suitably tacky so as to transfer to a printing substrate. Advantageously, the radiation absorbing material is such that it may absorb over a relativelywide range of wavelengths, compatible with different types of inks, each eventually having a different sub-range, even minute ones, of laser absorbance. Carbon black (CB), which has a broad absorption and is a strong absorber in the NIR region, can be used to provide desired corresponding properties to the energy absorbing layer of the blanket. Incorporation of particles of carbon black into silicone-based layers may also contribute to the thermal conductivity of the donor surface and allow it to be modulated, if and as desired. Preferably, the materials forming a heating-enabling layer, the energy being applied as EM radiation or thermal conduction, and more generally the material forming the blanket, should allow the heat generated by the writing device to dissipate rapidly enough for the heating of the thermoplastic ink film to be time and / or spot specific (e.g., enabling the formation of a desired pixel).

[0177] While an intermediate transfer member, if present in an independent coating apparatus, may be subject to less demanding specifications, the heating of the polymer film being non-selective and ongoing during coating, similar principles may guide its selection. For example, a suitable transfer member would comprise a release surface adapted to the thermoplastic polymer intended for coating and the temperature to be applied thereon to effect transfer, the body of the transfer member comprising at least a compressible and a conformational layer.

[0178] The interested reader is referred to WO 2018 / 100541 for non-limiting examples of blankets having transparent or opaque bases and their prospective layered structure and to WO 2017 / 208155 for an exemplary method of manufacturing such blankets, both further illustrating how to implement the present teachings and their various embodiments.

[0179] EXAMPLES

[0180] Example 1 - Ink Compositions

[0181] Ink formulations were prepared by weighing the ingredients to a total amount of 20 g and mixing them jointly for about 10 to 15 cycles in a three-roll mill (such as Model SG6’ by Sailing International Industry) at a temperature between 100°C and 140°C, until a homogeneous blend was obtained. A sample of each formulation was subjected to thermo-rheological analysis using a Discovery Hybrid Rheometer-2 (DHR-2) by TA instruments. The samples were disposed in the viscometer in a gap of 1 mm and tested with a plate spindle having a diameter of 25 mm operated in oscillation time sweep at 1 Hz and 30% strain, the temperature being ramped up between 30°C and 150°C at a pace of 2°C per minute.

[0182] While the plots presented in Figure 5 relate to the specific formulations detailed in the following, they are representative of the three main classes of thermoplastic materials as maybe used printing or coating methods according to present teachings.

[0183] Formulation A illustrates a crystalline thermoplastic material, its thermo-rheological curve being presented in Figure 5A. Formulation A consisted of 40 wt.% rice bran wax (SP-9002 by Strahl & Pitsch), 40 wt.% hydrogenated hydrocarbon C9 resin (HY-9100 by Ecopower), and 20 wt.% cyan pigment (Heliogen® Blue K 7097 by BASF).

[0184] Formulation B illustrates a semi-crystalline thermoplastic material, its thermo-rheological curve being presented in Figure 5B. Formulation B consisted of 40 wt.% hydrogenated hydrocarbon C9 resin (HY-9100), 40 wt.% micro crystalline wax (SP-623 by Strahl & Pitsch), and 20 wt.% cyan pigment (Heliogen® Blue K 7097).

[0185] Formulation C illustrates an amorphous thermoplastic material, its thermos-rheological curve being presented in Figure 5C. Formulation C consisted of 60 wt.% Dynacoll® 7150, 20 wt.% Dynacoll® 7340 (both by Evonik), and 20wt.% cyan pigment (Heliogen® Blue K 7097).

[0186] Example 2 - Blanket

[0187] A blanket was prepared by centrifucal casting against the walls of a drum made of Aluminum 6061, having an average roughness Ra between 0.2 pm and 1.6 pm and an inner diameter of 150 mm. First, a radiation absorbing layer was formed by applying a filtered fluid silicone-based composition comprising dispersed carbon black (CB) particles, the constituents of this first layer and their concentration in weight percent of the entire non-volatile composition being detailed in Table 1. Volatile compounds were used during the preparation of this layer, but as they were eliminated by the time the blanket was cured, their concentration is indicated as not relevant (NR).

[0188] Table 1

[0189]

[0190] The silicone-based composition comprising the dispersed CB was applied within the drum after confirmation that its mixing steps (including vortex and sonication) achieved a homogeneous dispersion of the CB particles. Following its filtration (using a filter having pores of 1 pm) and application to the bottom of the drum, it was subjected to spinning at room temperature (circa 23°C) for 20 minutes at a tangential linear speed of the inner walls of the drum of 30 meter per minute (m / min) until most volatile compounds evaporated, then for 15 more minutes at same speed and 80°C to promote curing, the cured radiation absorbing layer being then allowed to cool down to room temperature under ongoing spinning at 14 m / min for about one hour before the application of the elastomer forming the base of the blanket. The thickness of the radiation absorbing layer so prepared was about 1 pm.

[0191] The base was made of LSR (SL 9508 by KCC Silicones) by mixing parts A and B at a weight ratio of 10: 1 to a final volume adapted to the desired thickness of the base (e.g., 120 g). The LSR mixture was then gradually added to the rear side of the absorbing layer opposite the walls of the drum, the drum being first slowly rotated at a linear speed of about 15 m / min until the LSR was entirely poured in the drum. The speed was then increased to 659 m / min for ten minutes to get a uniform layer, at which time the temperature was raised to 80°C to promote its curing, the drum being then spun at 471 m / min for 90 minutes under this curing temperature. The thickness of the blanket base so prepared was about 2 mm.

[0192] At this stage, the blanket was gently released from the walls of the drum, the surface being then exposed forming the donor surface of the blanket. The blanket was transferred to an oven for post curing at 175°C for 30 minutes. The fully cured sleeve was then mounted on a drum (such as drum 12 illustrated in Figure 1).

[0193] Example 3 - Printing

[0194] Ink formulations prepared as described in Example 1 were tested in a printing system as illustrated in Figure 1, the printing system including a coating apparatus according to present teachings, the blanket 14 and its donor surface 16 being prepared as described in Example 2. The ink was fed to a feeder 58 being a screw drive set at 10 rpm and a heating temperature of 85°C. The molten ink was applied with flexible blades to an anilox cylinder 60 having a diameter of 70 mm and an axial length of 510 mm, the cylinder bearing a hexagonal pattern arranged at an angle of 30° allowing a dosing of about 0.11 ml of molten ink during 1 cycle, per meter length on the surface of the cylinder. The anilox cylinder 60 was maintained at a temperature of 110°C so that the ink remained in molten state at the point of transfer to thedownstream roller 62 having an outer surface made of an elastomer mounted thereon as a sleeve, upon which the ink layer was first levelled to a thickness of about 2 pm. The silicone roller 62 having a diameter of 80 mm, an axial length of 500 mm and an outer surface made of 50 Shore A silicone rubber was maintained at a temperature of 60°C so that the levelled ink was relatively rubbery at the point of transfer to the downstream application cylinder 66. As the application cylinder 66 having a diameter of 70 mm and an axial length of 502 mm, and being made of brass coated with a polished nickel finish, was set to have a temperature of 135°C, the ink layer splat thereto, forming a molten coat having a thickness of about 1 pm. Upon engagement with the blanket 14 set to have on average a temperature of 50°C (varying at each station, such as illustrated in Figure 2), the molten ink disposed on the application cylinder 66 underwent a final split, transferring to the blanket as a relatively solid coat having a thickness of about 0.5 pm with a glossy appearance. The blanket was prepared as a seamless sleeve as previously detailed and mounted on a glass drum having a diameter of 160 mm and an axial length of 504 mm.

[0195] The printing apparatus was operated at a linear speed of 1 m / s and the pressure applied to urge the rollers and blanket into contact was 8 kgf / cm as equally perceived at all nips. The ink coat transferred to the blanket was first levelled by two rollers 22 heated to 140°C, then cooled by contact with four cooling rollers 28 each set to a temperature of 15°C. At the nip 30 between the blanket 14 and the impression cylinder 38, the cooled coat of ink was selectively exposed to laser beams being emitted from a laser device positioned within the glass drum and directing its beams in the direction of the adjacent impression cylinder. At cleaning station 50, the donor surface of the blanket was entirely cleaned from remaining polymer film using a cleaning surface heated to 85°C, the cleaning surface being made of anodized aluminum.

[0196] Various printing substrates were fed to the impression nip, the printed results shown in Figure 6 corresponding to transfer of ink images to a PET foil. Figure 6A is a picture of arrayed individual dots, the diameter of the substantially round ones being about 45 pm, and Figure 6B is a picture showing how individual dots can be combined to form continuous lines on the printed substrate, each having a width of about 7 pm. Figures 6C and 6D illustrate how text having a font height of about 1.2 mm can be printed, the former showing some letters and the latter some numbers. Thickness of ink images so formed was measured by microscopy and found to be about 520 nm with an average roughness of about 50 nm, the variability in ink thickness being in this case of less than 10%. By modifying the applied pressure and the temperature regimen, ink images of different thicknesses, as low as 300 nm, were obtained.In addition to the features that are set out in the appended claims, the present specification has disclosed several further features that are believed to be inventive in their own right, and in order that these features may subsequently be patented separately, by way of divisional applications, they are set out below in the following clauses.

[0197] 1. A method of coating a surface with a film of a thermoplastic material, which comprises:

[0198] (i) providing an applicator roller,

[0199] (ii) applying a coating of the thermoplastic material to the applicator roller, the thermoplastic material being in a molten state when in contact with the applicator roller, and (ii) rolling the applicator roller over the surface while maintaining the temperature of the applicator roller above a predetermined temperature, corresponding to the melting or softening temperature at which the thermoplastic material becomes flowable, and the temperature of the surface below said predetermined temperature,

[0200] whereby cooling of the thermoplastic material on the applicator roller by contact with the surface causes a more viscous film of the thermoplastic material to be formed and to adhere to the surface to be coated, while the applicator roller remains coated with less viscous molten thermoplastic material.

[0201] 2. A method as in clause 1, wherein the thermoplastic material is applied to the applicator roller by a supply system comprised of a plurality of rollers arranged serially in rolling contact with one another, and a feeder for supplying thermoplastic material to the first roller of the series, being the roller furthest from the applicator roller, wherein, in each pair of adjacent rollers, one of the rollers in the pair is maintained above the predetermined temperature and the other below the predetermined temperature, whereby, at a nip between the rollers of each pair, the thermoplastic material on only one side of the nip is molten, the molten thermoplastic material adhering to one of the rollers of the pair and the solid material adhering to the other.

[0202] 3. A method as in clause 2, wherein the first roller of the supply system is an anilox roller on which the thermoplastic material is in a molten state, there being an optionally odd number (including one) of intermediate rollers arranged serially between the anilox roller and the applicator roller.

[0203] 4. A method as in clause 2 or clause 3, wherein rollers maintained at different temperatures during operation have surfaces made of different materials.5. A method as in clause 4, wherein hotter rollers operating above the predetermined temperature have surfaces made of metal and cooler rollers operating below the predetermined temperature have surfaces made of a silicone elastomer.

[0204] 6. A method as in any one of clause 2 to clause 5, wherein a pressure is applied at each nip of each pair of adjacent rollers in an assembly comprising the applicator roller and the supply system including the plurality of rollers arranged serially in rolling contact with one another, the pressure at each nip being controlled by a respective actuator.

[0205] 7. A method as in clause 6, wherein the pressure at at least one the nips differ from the pressure at the other nips.

[0206] 8. A method as in any one of clause 1 to clause 7, wherein following coating of the surface with the film of thermoplastic material, the film is levelled, optionally by heating.

[0207] 9. A method as in any one of clause 1 to clause 8, wherein following coating of the surface with the film of thermoplastic material, and optionally its levelling, the film is cooled by rolling contact with one or more cooling rollers or belts.

[0208] 10. A method as in any one of clause 1 to clause 9, wherein the film of thermoplastic material formed and adhered to the surface has at least one of the following structural features: a) a thickness not exceeding 2 pm, 1 pm, 750 nm or 500 nm;

[0209] b) a gloss of at least 50, at least 75 or at least 100 arbitrary gloss units, as can be determined by a glossmeter; and

[0210] c) an average roughness Ra not exceeding 100 nm, not exceeding 50 nm or not exceeding 25 nm, as can be determined by a profilometer.

[0211] 11. Apparatus (180) for coating a surface with a film of a thermoplastic material, which comprises:

[0212] (i) an applicator roller (66),

[0213] (ii) a supply station (68) for applying a coating of the thermoplastic material to the applicator roller,

[0214] (iii) a feeder (58) for providing the thermoplastic material to the supply station (68), and (iv) a coating station (18) at which the applicator roller (66) is rolled over the surface (16) to be coated while, in operation of the apparatus, the temperature of the applicator roller (66) is maintained above a predetermined temperature, corresponding to the melting or softeningtemperature at which the thermoplastic material becomes flowable, and the temperature of the surface to be coated (16) is maintained below said predetermined temperature,

[0215] whereby cooling of the thermoplastic material on the applicator roller by contact with the surface causes a more viscous film of the thermoplastic material to be formed and to adhere to the surface to be coated, while the applicator roller remains coated with less viscous molten thermoplastic material.

[0216] 12. An apparatus as in clause 11, wherein the supply station (68) comprises an intermediate roller (62) in rolling contact with the applicator roller (66), the intermediate roller bearing, in operation of the apparatus, a coating of the thermoplastic material in a solidified state and being maintained at a temperature below said predetermined temperature, whereby, at a nip between the intermediate roller (62) and the applicator roller (66) the thermoplastic material on the intermediate roller is split, with only the surface of the thermoplastic material on the intermediate roller being melted by contact with the applicator roller to form a molten film that is transferred to the applicator roller.

[0217] 13. An apparatus as in clause 12, wherein the supply station (68) further comprises an anilox roller (60) in contact with the intermediate roller (62), the feeder (58) loading, in operation of the apparatus, the anilox roller (60) with the thermoplastic material in a molten state, the anilox roller (60) being maintained at a temperature above said predetermined temperature, whereby, at a nip between the anilox roller and the intermediate roller the thermoplastic material on the anilox roller is split, with only the surface of the thermoplastic material on the anilox roller solidifying by contact with the intermediate roller to form a solidified film that is transferred to the intermediate roller.

[0218] 14. An apparatus as in clause 13, wherein a doctor blade is provided to remove surplus molten thermoplastic material applied to the anilox roller (60) in operation of the apparatus.

[0219] 15. An apparatus as in clause 13 or clause 14, wherein one or more ironing rollers (64) are provided in contact with the intermediate roller (62) to level, in operation of the apparatus, the solidified coating transferred to the intermediate roller (62) from the anilox roller (60).

[0220] 16. An apparatus as in any one of clause 13 to clause 15, wherein the anilox roller (60) comprises a pattern of anilox cells, the anilox roller being characterized by at least one of the following features:

[0221] a) a line spacing of 250 lines per centimeter (L / cm) or more, 300 L / cm or more, or 350 L / cm or more, the line spacing optionally not exceeding 500 L / cm; andb) a nominal volume of 2.5 cm3 / m2or more, 3.0 cm3 / m2or more, 3.5 cm3 / m2or more, or 4 cm3 / m2or more, the nominal volume optionally being up 20 cm3 / m2, up to 15 cm3 / m2, or up to 10 cm3 / m2.

[0222] 17. An apparatus as in any one of clause 11 to clause 16, wherein the applicator roller (66), and when present the anilox roller (60) and the ironing roller(s) (64), are each independently made of a material having at least one of:

[0223] a- a thermal conductivity of 100 Watts per meter per degree Kelvin (W / m»K) or more, 150 W / m»K or more, 200 W / m»K or more, 250 W / m»K or more, or 300 W / m»K or more, the thermal conductivity of a material forming said roller(s) optionally not exceeding 600 W / m»K or 500 W / m»K;

[0224] b- a thermal diffusivity a of 25 mm2 / sec or more, 30 mm2 / sec or more, or 35 mm2 / sec or more, the thermal diffusivity a of a material forming said roller(s) optionally not exceeding 125 mm2 / sec;

[0225] the material further optionally comprising or consisting of a metal.

[0226] 18. An apparatus as in any one of clause 12 to clause 17, wherein the intermediate roller (62) comprises a surface made of a silicone elastomer, the elastomer optionally having a hardness between 40 and 70 Shore A or between 50 and 70 Shore A.

[0227] 19. An apparatus as in any one of clause 11 to clause 18, wherein the applicator roller (66), and when present the intermediate roller (62), the anilox roller (60) and the ironing roller(s) (64), have each independently a surface having an average roughness (Rd) of 0.1 pm or less, 0.05 pm or less, or 0.025 pm or less.

[0228] 20. An apparatus as in any one of clause 11 to clause 19, further comprising an actuator for urging the applicator roller (66), and each any roller in rolling contact therewith, towards the surface (16) to be coated, the pressure applied at each nip of rolling contact by each respective actuator being substantially uniform over the length of each nip, the pressure applied at the different nips being same or different.

[0229] 21. An apparatus as in any one of clause 11 to clause 20, wherein the applicator roller (66) and any roller in rolling contact therewith, are rotatable with the same surface velocity to avoid skidding between adjacent rollers.

[0230] 22. An apparatus as in any one of clause 11 to clause 21, wherein the surface (16) to be coated is a release surface of an intermediate transfer member (114), the apparatus furthercomprising an impression cylinder (38) or an impression belt (138) the impression surface of which being maintained, in operation of the apparatus, at a temperature above said predetermined temperature, whereby, at a nip between the impression surface and a substrate transport mechanism (90), the thermoplastic material on the release surface entirely transfers to the substrate.

[0231] 23. A coating assembly (400) for printing onto a coating substrate (136), the assembly comprising two or more coating apparatuses (180) each as disclosed in any one of clause 11 to clause 22, the apparatuses serving to apply solid polymer films differing from one another. 24. A coating assembly as in clause 23, wherein the two or more coating apparatuses (180) are as disclosed in clause 22 and are arranged along a same impression cylinder (38) or impression belt (138).

[0232] 25. A coating assembly as in clause 23 or clause 24, further comprising at least one of a preceding and a finishing station.

[0233] 26. A method of printing onto a substrate, which comprises providing a recirculating blanket mounted on a drum or looped as an endless belt, at least a region of an outer surface of which serves as a donor surface, and cyclically performing the steps of:

[0234] (i) applying to the donor surface at a film application station a uniform continuous solid ink film consisting of a thermoplastic polymer,

[0235] (ii) applying energy to heat selected regions of the polymer film,

[0236] (iii) pressing the donor surface against the substrate during, or after, the application of energy, to cause only the selected regions of the polymer film, that are rendered tacky by the applied energy, to adhere to the substrate,

[0237] (iv) separating the substrate from the donor surface, so as to leave remaining on the donor surface only the regions of the film that have not been transferred to the substrate, wherein step (i) comprises

[0238] (a) providing an applicator roller,

[0239] (b) applying a coating of the thermoplastic material to the applicator roller, the thermoplastic material being in a molten state when in contact with the applicator roller, and (c) rolling the applicator roller over the donor surface while maintaining the temperature of the applicator roller above a predetermined temperature, corresponding to the melting or softening temperature at which the thermoplastic material becomes flowable, and thetemperature of the donor surface below said predetermined temperature,

[0240] whereby cooling of the thermoplastic material on the applicator roller by contact with the surface causes a more viscous film of the thermoplastic material to be formed and to adhere to the donor surface, while the applicator roller remains coated with less viscous molten thermoplastic material.

[0241] It is appreciated that certain features of the disclosure, which 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 disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the disclosure. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. None of the specifically disclosed embodiments should be considered limiting, being provided for the sake of illustration. Many other alternatives, modifications and variations of such embodiments will occur to those skilled in the art based upon Applicant’s disclosure herein. Accordingly, it is intended to embrace all such alternatives, modifications and variations and to be bound only by the spirit and scope of the disclosure and any change which comes within their meaning and range of equivalency.

[0242] In the description and claims of the present disclosure, each of the verbs “comprise”, “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of features, members, steps, components, elements or parts of the subject or subjects of the verb. Nevertheless, it is contemplated that the compositions of the present teachings also consist essentially of, or consist of, the recited components, that the methods of the present teachings also consist essentially of, or consist of, the recited process steps, and that the apparatus or system of the present teachings also consist essentially of, or consist of, the recited devices or stations.

[0243] Positional or motional terms such as “upper”, “lower”, “right”, “left”, “bottom”, “below”, “lowered”, “low”, “top”, “above”, “elevated”, “high”, “vertical”, “horizontal”, “front”, “back”, “backward”, “forward”, “upstream” and “downstream”, as well as grammatical variations thereof, may be used herein for exemplary purposes only, to illustrate the relative positioning, placement or displacement of certain components, to indicate a first and a second component in present illustrations or to do both. Such terms do not necessarily indicate that, for example, a “bottom” component is below a “top” component, as such directions, components or bothmay be flipped, rotated, moved in space, placed in a diagonal orientation or position, placed horizontally or vertically, or similarly modified.

[0244] As used herein, the singular form “a”, “an” and “the” include plural references and mean “at least one” or “one or more” unless the context clearly dictates otherwise. As used herein, the term at least one of A and B is intended to mean either A or B, and may mean, in some embodiments, A and B. Unless otherwise stated, the use of the expression “and / or” between the last two members of a list of options for selection indicates that a selection of one or more of the listed options is appropriate and may be made.

[0245] The word “exemplary” is used herein to mean “serving as an example, instance or illustration”. Any embodiment described as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments and / or to exclude the incorporation of features from other embodiments.

[0246] As used herein, unless otherwise stated, adjectives such as “substantially”, “approximately” and “about” that modify a condition or relationship characteristic of a feature or features of an embodiment of the presently disclosed subject matter, are to be understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended, or within variations expected from the measurement being performed and / or from the measuring instrument being used. For example, when the term “about” or “approximately” precedes a numerical value, it may indicate + / -15%, or + / -10%, or even only + / -5%, or any other suitable + / - variation within such ranges, and in some instances may indicate the precise value. Furthermore, unless otherwise stated, the terms (e.g., numbers) used in an embodiment of the presently disclosed subject matter, even without such adjectives, should be construed as having tolerances which may depart from the precise meaning of the relevant term but would enable the embodiment or a relevant portion thereof to operate and function as described, and / or as understood by a person skilled in the art.

[0247] To the extent necessary to understand or complete the present disclosure, all publications, patents, and patent applications mentioned herein, including in particular the applications of the Applicant, are expressly incorporated by reference in their entirety as is fully set forth herein.

[0248] Certain marks referenced herein may be common law or registered trademarks of third parties. Use of these marks is by way of example and shall not be construed as descriptive or limit the scope of this disclosure to material associated only with such marks.

Claims

CLAIMS1. A method of printing onto a substrate, which comprises providing a recirculating blanket mounted on a drum or looped as an endless belt, at least a region of an outer surface of which serves as a donor surface, and cyclically performing the steps of:(i) applying to the donor surface at a film application station a uniform continuous solid ink film consisting of a thermoplastic polymer,(ii) applying energy to heat selected regions of the polymer film,(iii) pressing the donor surface against the substrate during, or after, the application of energy, to cause only the selected regions of the polymer film, that are rendered tacky by the applied energy, to adhere to the substrate,(iv) separating the substrate from the donor surface, so as to leave remaining on the donor surface only the regions of the film that have not been transferred to the substrate,(v) removing any polymer film remaining on the donor surface at a cleaning station, and (vi) recycling the polymer of the removed film by returning the polymer from the cleaning station to the application station for re-application to the donor surface, the polymer having a chemistry that is not changed by energy applied during any of steps (i) to (v).

2. A method as claimed in claim 1, in which the polymer film has a thickness not exceeding 2 micrometers (pm), 1 pm, 750 nanometers (nm) or 500 nm.

3. A method as claimed in claim 1 or claim 2, in which step (i) comprises:(a) applying a coating of molten polymer to the donor surface, and(b) cooling the coating to form a solid film.

4. A method as claimed in claim 3, wherein step (i)(a) comprises providing a series of rollers in rolling contact with one another, applying molten polymer to the first of the rollers in the series to form a film thereon, and transferring a progressively thinned part of the film from the first to the last of the rollers in the series and from the last of the rollers in the series to the donor surface.

5. A method as claimed in claim 4, wherein the rollers in each pair in contact with one another operate at different temperatures, one below and the other above the melting or softening temperature of the polymer, whereby the polymer film is split during transfer between rollers, with a flowable part of the film adhering to the hotter roller and a solid part of the film adhering to the cooler roller.

6. A method as claimed in any one of the preceding claims, wherein steps (ii) and (iii) are performed simultaneously, energy being applied to the donor surface from a side of the blanket opposite to the donor surface, while the donor surface is in contact with the substrate.

7. A method as claimed in any one of the preceding claims, wherein following application of the uniform solid polymer film onto the donor surface in step (i), the solid film is levelled with a levelling device.

8. A method as claimed in claim 7, wherein the levelling device comprises a heating device configured to raise the temperature of the film by application of radiant energy or by physical contact.

9. A method as claimed in any one of the preceding claims, wherein prior to step (iii) the polymer film is cooled by rolling contact with one or more cooling rollers.

10. A method as claimed in any one of the preceding claims, wherein, at the cleaning station, the donor surface is brought into contact with a cleaning surface to which all the polymer film remaining on the donor surface is transferred.

11. A method as claimed in claim 10, wherein the cleaning surface and the polymer film brought into contact therewith have different temperatures, one of the temperatures being optionally lower than a glass transition temperature (Tg) of the heat-sensitive ink and the other temperature being higher than a melting temperature (77M) of the heat sensitive ink.

12. A method as claimed in claim 10 or claim 11, wherein the cleaning surface is of a cooled roller or cooled belt, the temperature of the cleaning surface being lower than the temperature of the polymer film to be removed.

13. Apparatus (10) for printing onto a substrate (36) conveyed by an impression cylinder (38) or an impression belt (138), the printing apparatus comprising:(i) a recirculating blanket (14), mounted on a drum or looped as an endless belt, at least a region of an outer surface of which serves as a donor surface (16),(ii) a film application station (18) for applying a uniform continuous solid polymer film onto the donor surface (16),(iii) a writing device (33) for selectively applying energy to heat regions of the polymer film,(iv) an impression station (30) at which the donor surface (16) is pressed against the substrate (36) during, or after, the application of energy, to cause the selected regions of thepolymer film, that have been rendered tacky by the applied energy, to adhere to the substrate (36), the substrate subsequently separating from the donor surface (16) downstream of the impression station (30) so as to transfer the adhered regions of the polymer film from the donor surface (16) to the substrate (36),(v) a donor surface cleaning station (50) at which any polymer film remaining on the donor surface after transfer of the selected regions to the substrate is removed from the donor surface (16), and(vi) a recycling system (55) which returns removed polymer to the film applicator station (18) for re-application to the donor surface (16) in subsequent operating cycle.

14. Apparatus as claimed in claim 13, in which the film application station (18) is configured to apply a continuous polymer film having a thickness not exceeding 2 pm, 1 pm, 750 nm or 500 nm.

15. Apparatus as claimed in claim 13 or claim 14, in which the film application station (18) is operative to apply a continuous coating of molten polymer to the donor surface, the coating cooling on the donor surface to form the solid polymer film.

16. Apparatus as claimed in any one of claim 13 to claim 15, wherein the film application station (18) is part of a coating apparatus (180) which comprises a plurality of rollers (60, 62, 66) in rolling contact with one another and a feeding station (58) for applying a film of molten polymer to the first (60) of the rollers in the series, the film being progressively thinned during transfer from each roller in the series to the next, prior to being transferred from the last (66) of the rollers in the series to the donor surface (16).

17. Apparatus as claimed in any one of claim 13 to claim 16, further comprising a levelling station (20) for levelling the film applied by the film application station (18).

18. Apparatus as claimed in any one of claim 13 to claim 17, further comprising a cooling station (26) for cooling the film upstream of the writing device (33).

19. Apparatus as claimed in any one of claim 13 to claim 18, wherein the writing device (33) is located at the impression station (30) and serves to apply energy to the donor surface (16) at the nip from a side of the blanket (14) opposite to the donor surface while the donor surface (16) is in contact with the substrate (36).

20. Apparatus as claimed in claim 19, wherein the writing device (33) comprises individually controllable laser emitting elements and the blanket (14) comprises a base transparent to the radiation emitted by the lasers, the donor surface (16) being formed on, or part of, a radiation absorbing layer, the blanket being mounted on a drum (12) having walls transparent to the radiation emitted by the lasers, or being looped as an endless belt circulating over backing surfaces, a backing surface positioned between the writing station and the nip being made of materials transparent to the radiation emitted by the lasers.

21. A printing assembly (300) for printing onto a substrate (36), the assembly comprising two or more printing apparatuses (10) each as claimed in any one of claim 13 to claim 20, the apparatuses serving to apply solid polymer films differing in color from one another.

22. A printing assembly as claimed in claim 21, wherein the two or more printing apparatuses (10) are arranged along a same impression cylinder (38) or impression belt (138).

23. A printing assembly as claimed in claim 21 or claim 22, further comprising a perfecting station for flipping the substrate to enable double-sided printing.

24. A printing assembly as claimed in any one of claim 21 to claim 23, further comprising at least one of a preceding and a finishing station.

25. A method of coating a surface which comprises:(i) providing an applicator roller,(ii) applying a coating of the thermoplastic material to the applicator roller, the thermoplastic material being in a molten state when in contact with the applicator roller, and(iii) rolling the applicator roller over the surface while maintaining the temperature of the applicator roller above a predetermined temperature, corresponding to the melting or softening temperature at which the thermoplastic material becomes flowable, and the temperature of the surface below said predetermined temperature,whereby cooling of the thermoplastic material on the applicator roller by contact with the surface causes a more viscous film of the thermoplastic material to be formed and to adhere to the surface to be coated, while the applicator roller remains coated with less viscous molten thermoplastic material.

26. An apparatus (280) for coating a surface with a film of a thermoplastic material, which comprises:(i) an applicator roller (66),(ii) a supply station (68) for applying a coating of the thermoplastic material to the applicator roller,(iii) a feeder (58) for providing the thermoplastic material to the supply station, and(iv) a coating station (18) at which the applicator roller is rolled over the surface to be coated while, in operation of the apparatus, the temperature of the applicator roller is maintained above a predetermined temperature, corresponding to the melting or softening temperature at which the thermoplastic material becomes flowable, and the temperature of the surface to be coated is maintained below said predetermined temperature,whereby cooling of the thermoplastic material on the applicator roller by contact with the surface causes a more viscous film of the thermoplastic material to be formed and to adhere to the surface to be coated, while the applicator roller remains coated with less viscous molten thermoplastic material.

27. A coating assembly (400) comprising two or more coating apparatus as claimed in claim 26 arranged along a same path followed by the surface of a substrate (136) to be coated thereby.

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