FIELD

The present disclosure relates to a method and system for printing on a surface of a substrate with a film of a thermoplastic material.

BACKGROUND

The present disclosure is a development of the teachings of WO2016/189512 to the same Applicant, which was published on 1 Dec. 2016 and has a priority date of 27 May 2015. To avoid unnecessary repetition, reference will be made throughout the present disclosure to the latter publication.

Thermal transfer printers are known that employ a ribbon carrying a polymeric ink film. The ribbon is equivalent to the ink ribbon used in a conventional typewriter but the ink is solid ink and is transferred from it onto a substrate (usually paper) not by impact but by means of a thermal print head that heats only the regions of the ribbon from which the ink is to be transferred to the paper. Thermal transfer printers can print in monochrome or in full color, by transferring images successively from colored ribbons.

Such printers achieve printing of high quality but are wasteful, and therefore costly to operate, since the ribbon is generally single-use and when discarded, much of its ink surface has not been transferred to a printing substrate.

WO2016/189512 discloses a printing system and method that operate on the same principle as thermal transfer printers, but in which the single-use ribbon is replaced by a transfer member which, rather than carrying a polymeric ink film, is coated with a layer of thermoplastic or thermoplastic-coated particles, which can be replenished after each transfer cycle, enabling the transfer member to perform multiple printing cycles, significantly reducing waste.

In particular, WO2016/189512 discloses a method of thermal transfer printing onto a surface of a substrate, which method comprises the steps of:

a) providing a transfer member having front and rear sides with an imaging surface on the front side,

b) coating the imaging surface of the transfer member with individual particles formed of, or coated with, a thermoplastic polymer,

c) 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,

d) applying radiation to selected regions of the coated imaging surface to heat and render tacky the particles within the selected regions, and

e) 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 radiation, to cause transfer to the surface of the substrate of only the regions of the particle coating that have been rendered tacky.

To permit continuous printing, following transfer of particles from the selected regions to a first substrate surface, steps b) and c) may be repeated to apply a fresh monolayer coating of particles at least to the selected regions from which the previously applied monolayer coating was transferred to the substrate surface in step e), so as to leave the imaging surface again uniformly coated with a monolayer of particles for printing onto a subsequent substrate surface, as described in steps d) and e). In other words, for printing of subsequent images (which need not be identical from cycle to cycle), steps b) to e) can be sequentially repeated.

WO2016/189512 only teaches rendering the particles tacky by exposing them to EM radiation and the present invention extends the teaching to an alternative method of rendering the particles tacky.

SUMMARY

In accordance with a first aspect of the present disclosure, there is provided a printing system for thermal transfer printing onto a surface of a substrate, the system comprising:

a) a movable transfer member having opposite front and rear sides with an imaging surface on the front side;

b) a coating station adapted to apply to the imaging surface or at least a segment thereof, a monolayer of particles made of, or coated with, a thermoplastic polymer;

c) an imaging station adapted to apply energy via the rear side of the transfer member to selected regions of the particles coated imaging surface to render the particles thereon tacky within the selected regions; and

d) a transfer station adapted to press said imaging surface of said transfer member and said substrate surface, or respective segments thereof, against each other to cause transfer to the surface of the substrate of only the regions of the particle coating that have been rendered tacky;
characterised in that
e) the imaging station comprises a thermal print head in thermal contact with the rear side of the transfer member and operative to apply energy to the particles on the imaging surface by heat conduction through the transfer member.

In accordance with a second aspect of the invention, there is provided a method of thermal transfer printing onto a surface of a substrate, which comprises:

a) providing a movable transfer member having opposite front and rear sides with an imaging surface on the front side,

b) applying to the imaging surface a monolayer coating of particles made of, or coated with, a thermoplastic polymer,

c) applying heat by thermal conduction via the rear side of the transfer member to selected regions of the coated imaging surface to render the particles thereon tacky within the selected regions, and

d) pressing the imaging surface and the surface of the substrate against one another to cause transfer to the surface of the substrate of only the regions of the particle coating that have been rendered tacky;

e) repeating step b) to apply a fresh monolayer coating of particles to the selected regions from which the previously applied monolayer coating was transferred to the substrate surface in step d), to leave the imaging surface again uniformly coated with a monolayer of particles, which can be repeatedly subjected to steps c) to e).

Features of particular non-limiting embodiments of the invention are set out in the appended dependent claims.

The terms “tacky” and “sufficiently tacky” as used herein are not intended to mean that the particle coating is necessarily tacky to the touch but only that it is softened sufficiently to enable its adhesion to the surface of a substrate when pressed against it in the transfer station. The tacky particles or regions of particles rendered tacky are believed to form individual films or contiguous films which following their transfer to a printing substrate may optionally yield thinner films, as a result of the pressure being applied upon contacting of the imaging surface (or a segment thereof) to the substrate (or a corresponding segment thereof) and/or of the optional further processing (e.g., fusing, drying, curing, etc.) of the transferred films. Such optional further processing may include heating of the already-transferred images and/or the receiving substrate by means which do not contact the transferred image or by means which contact the transferred images, both of which means are well known in the art. In the case of non-contact heating, such as hot air, radiant heating, radio frequency heating and the like, heating the transferred image may enhance its adhesion to the substrate, its abrasion resistance, its chemical resistance and the like. In the case of heating means which contact the image, such as silicone-coated fuser rolls or belts, in addition to the benefits of non-contact heating, the image film may also acquire higher gloss and scratch resistance.

The intended meaning of the term “monolayer” and different ways in which a monolayer can be achieved are disclosed in WO2016/189512 and WO2016/189513 which provide details of the particle size, polymer film thickness as well as the design and construction of a coating station for applying the particles.

Briefly, in order to facilitate replenishment of the particle coating on the imaging surface after each transfer, particles that adhere to the imaging surface more strongly than they do to one another are utilized. This results in an applied layer that is substantially a monolayer of individual particles, with little, if any, overlap, the thickness of the monolayer being therefore commensurate (e.g., 1-3-times) with the thickness of the particles. Stated differently, the layer is only one particle thick over a major proportion of the area of the imaging surface and most, if not all, of the particles have at least some direct contact with the imaging surface.

One advantage of having a monolayer is that it can provide for good thermal coupling between the particles and the imaging surface on which the particles are coated.

The thermoplastic particles may have a particle size of less than 40 μm, 20 μm, 10 μm, or less than 5 μm, or less than 1 μm, or within the range of 100 nm to 4 μm, or 300 nm to 1 μm, or 500 nm to 1.5 μm.

To permit the printing on the substrate of patterns corresponding to the selected regions exposed to heat applied by thermal conduction, the affinity of the heated tacky particles needs to be greater to the substrate than to the imaging surface. Moreover, this relatively higher affinity of the tacky particle to the substrate in the selected regions shall also be greater than the affinity of the bare substrate to the particles not rendered tacky. In the present context, a substrate is termed “bare” if lacking any desired image pattern to be printed by the present method or system. Though the bare substrate should for most purposes have substantially no affinity to the thermoplastic particles, to enable the selective affinity of the tacky ones, some residual affinity can be tolerated (e.g., if not visually detectable) or even desired for particular printing effects. Undesired transfer of particles to areas of the bare substrate is also termed parasite or parasitic transfer.

The term “thermoplastic particles” is used to refer to all particles (colored or not) comprising a thermoplastic polymer, whether coating the particle or forming substantially all of the particle, including any intermediate range of presence of the polymer allowing the thermoplastic particles to serve their intended purposes. In the latter cases, wherein the thermoplastic polymer(s) can be homogeneously present in the entire particle, not being particularly restricted to an external coating, the particles may also be said to be made of a thermoplastic polymer.

Such gradient of affinities between the particles (before and after heating), the fluid carrying the native particles for coating or replenishing of the transfer member, the imaging surface, the printing substrate, any such element of the method, can be modulated by selection of suitable materials or characteristics, such as hardness, smoothness, hydrophobicity, hydrophilicity, charge, polarity and any such properties known to affect interaction between any two elements.

For assisting in the transfer of the tacky film of particles from the imaging surface to the substrate, the imaging surface may be hydrophobic.

In some embodiments, the thermoplastic particles may themselves be hydrophobic. In such case, the relative affinity between the particles in their different states and the imaging surface can be based, at least partially, on hydrophobic-hydrophobic interactions. In some embodiments, attachment of the monolayer of particles to the imaging surface is assisted by the relative low hardness of the imaging surface as is further detailed below. A relatively soft imaging surface may assist in forming an intimate contact with each individual particle, such intimate contact manifesting itself in a relatively large contact area between the imaging surface and the particle, in contrast to the discrete contact formed between the particle and a relatively hard surface. Such intimate contact may thus further intensify effects of any short-range attraction forces between the imaging surface and the particles, such as, e.g., hydrophobic-hydrophobic interactions or Van der Waals forces.

In some embodiments, the thermoplastic particles and/or the imaging surface can alternatively or additionally achieve desired relative affinity one to another (and to any other fluid or surface suitable for a printing process according to present teachings) by way of charge-based interactions. For instance, positively charged particles may favor negatively charged surfaces. In such case, the relative affinity between the particles in their different states and the imaging surface can be based on charge-charge interactions.

