The present invention relates to printing and in particular printing onto three dimensional surfaces.

Dye diffusion or dye sublimation thermal or heat transfer printing is a process by which an ink is applied to an intermediate substrate and diffused on application of heat onto an ultimate substrate. It is used for instance in printing graphic and/or digital images, applied by a computer controlled ink jet printer or the like to the intermediate substrate and thence to the ultimate surface. The distinction between “dye diffusion” and“dye sublimation” is loose. Strictly in the former the ink diffuses as a liquid and in the latter it sublimes. However, as the terms have come into more common usage, they have lost their scientific origin and are often used

interchangeably, in particular with the latter being used where sublimation as such does not occur.

To avoid ambiguity, we use the term dye transfer via substrate to encompass both.

In the nature of the process, it lends itself to printing on two dimensional surfaces, such as textiles.

It has been proposed for printing onto 3D surfaces. For instance, the abstract of GB 2442824A is as follows:

The apparatus preferably comprises an infinitely variable speed fan 11 providing hot air flow at a low speed or Reynolds number to soften a film, and at a high speed or Reynolds number for dye diffusion. In the preferred embodiment, heaters (8, electric or 9, gas) are modulated to maintain air in an enclosure (acting as a hot air reservoir) between 160 and 260 degrees centigrade. Film 6, which may carry a dye, is mounted onto frame 5 on movable carriage la above workpiece (if dying) or former (if vacuum forming) 3. The carriage is then moved under chamber 12a, shutter 7 withdrawn and fan speed reduced to heat, soften and dry the film. The vacuum platen is then raised to seal around the film, and apply vacuum to form the film around the workpiece or former. The fan speed is then increased while dye is being transferred to the workpiece. The shutter then returns and the dyed, moulded component is removed when the carriage is returned to its starting point. The component may be removed following a cooling period. Compressed air may be used to remove the film.

Other proposals have been made for 3D dye diffusion printing, particularly where the surfaces to be printed are largely flat, such as a smart phone case.

The object of the present invention is to provide an improvement in dye transfer via substrate printing.

According to a first aspect of the invention there is provided a method of dye transfer via substrate printing, consisting in the steps of:

• providing a sheet-like substrate for a sheet to be coated;

• providing a substantially impervious layer on the substrate;

• providing an ink receptive coating on the impervious layer;

• the coating including at least one thermoplastic component enhancing integrity of the coating on conforming of the substrate to a 3D surface of an object to be printed;

• applying an image in ink to the receptive coating of the coated sheet;

• providing a chamber in which the object is placed with the inked coating of the sheet against the 3D surface of the object;

• applying vacuum and heat in the chamber, to conform the substrate to the 3D surface with elastomeric resilience of the coating, whereby the ink sublimes and/or diffuses from the coating onto the surface for absorption there as a printed image; and

• restoring ambient conditions and removing the substrate from the printed surface of the object.

According to a second aspect of the invention there is provided a coated sheet for dye transfer printing comprising:

• a sheet-like substrate,

• a substantially impervious layer on the substrate and • an ink receptive coating on the impervious layer, the coating including:

• at least one thermoplastic component,

whereby the coated sheet is adapted to conform to a 3D surface of an object to be printed onto with stretching of the coating, so that with an ink image applied to the coating can be caused to sublime and/or diffuse from the coating onto the 3D surface for absorption there as a printed image.

The substrate is preferably of polyester, PET or APET.

Whilst the impervious layer could be a crosslinked polymeric layer, it is preferably a metallic layer. Normally it will be of deposited aluminium either vacuum deposited or sputtered.

The thermoplastic component can be provided as a thermoplastic elastomer. It can be for instance a styrene copolymer, preferably styrene-acrylic copolymer.

Whilst an elastomeric, thermoplastic component can provided good results with relatively smooth 3D objects; those with more surface roughness, such as woven fabrics in shoes to be printed onto, may be gripped by the elastomeric component to the extent of inhibiting removal of the substrate.

To improve on this situation, we have found that thermoplastic components experiencing plastic yield at or approaching ink sublimation/diffusion temperatures grip the surface to be printed to a lesser extent, with eased removal of the substrate.

A suitably plastically deformable thermoplastic component is a

vinylpyrrolidone vinylacetate copolymer.

Preferably, the ink receptive coating includes an aqueous absorption component and at least one binding component. Further another preferred component is a dispersant.

Normally the coating will be applied to the impervious layer as a slurry of 65% to 90%, preferably 75% to 80%, water. Two binders can be used as in the instance of the thermoplastic elastomer being a styrene copolymer and the dry weight percentages (%) of the components of the coating are preferably as follows:

In this instance the one binder can be polyvinyl alcohol and the other binder can be polyacrylate.

A single binder can be used as in the instance of the thermoplastic elastomer being a vinylpyrrolidone vinylacetate copolymer and the dry weight percentages (%) of the components of the coating are preferably as follows:

It should be noted that we avoid the inclusion of any cationic binders.

Preferably, the object to be printed will be of plastics material or will have a coating thereof.

In this instance the one binder can be polyvinyl alcohol. To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic view of a reverse image bearing sheet of the invention loaded into a chamber for printing onto a 3D object in accordance with the invention;

Figure 2 is a view similar to Figure 1 of sheet drawn down onto the object to transfer the image;

Figure 3 is a plan view of the sheet of Figure 1 showing the reverse printed image;

Figure 4 is a perspective view of the printed object and

Figure 5 is a scrap cross-sectional view of the sheet showing its coating having received reverse printing.

Referring to the drawings, printing of an image on a 3D curved object 1 by dye transfer printing will be described.

A sheet 2 of 200 micron (pm) APET has an aluminium layer 3 deposited on it in a known manner. The layer is a few tens of nm thick and its surface needs no further treatment. It provides an impervious barrier for a coating 4 deposited on it.

The coating as such is described below. When thoroughly dried - and in practice it will be prepared well in advance of its use - it is able to receive ink jet printing, with the moisture from the ink being drawn into the coating and then able to evaporate, without wicking sideways of dyes in the ink. The result is a reverse image 7 on the coating 4 of the image to be printed.

With the sheet gripped at its margin 8, it and the object 1 are placed in a chamber 9 with the image aligned with its intended position on the object. Vacuum is applied to the object side of the sheet, with the grip 10 maintaining the vacuum at the margin 8. Thus the sheet is drawn onto the object. This being a three dimensional object, the sheet 2 stretches as it conforms to the shape of the object. The image also stretches. The image as printed may take account of this whereby the final image does not appear stretched. Once the image is conformed or during the conforming, the sheet is heated, by circulation of hot air on the non-image side, where the pressure remains ambient, or by radiant heating 11. This drives the dyes of the ink to diffuse and transfer to the object. After a period of time to allow diffusion to occur, the heating is discontinued, the vacuum is released and the object is removed from the chamber. The sheet can then be removed from it, leaving the image 12 printed onto it.

The sheet 2 and in particular the aluminium layer and the coating 4 will now be described in more detail.

Without an impervious layer, ink printed onto the coating could migrate into the APET sheet 2. The aluminium layer is provided to avoid this. Its surface is untreated from deposit to application of the coating, which adheres to it.

The coating is prepared as a slurry of between 75% to 80% water of the following dry components:

It should be noted that the +/- variations shown are the amount by which the individual components can be varied from their nominal proportion in the coating, with the other components all being varied by an equivalent reduction or increase to keep the sum of their percentages to 100.

It is applied as a coating of substantially 18 to 26 pm to the aluminium and set aside to dry. It can be actively dried by fan and/or heating. The PVOH making up substantially half the coating - not including the water - is a water soluble polymer and acts as a binder for the coating. It does this in conjunction with polyaciylate, the two acting as co-binders. In use it also acts to absorb the dyes of the printed ink.

The co-polymer of styrene-acrylic is a thermoplastic, elastomeric polymer and makes up substantially one fifth of the coating. Its function in the coating is to give resilience and counteract tendency to crack. It also acts with the polyacrylate as an adhesive of the coating to the aluminium layer. With the coating adhered to the aluminium, the elastomer reinforces the coating as the sheet is stretched in

conforming to the 3D object to be printed.

The polyacrylate also makes up substantially one fifth of the coating and acts as a binder in binding together the components of the coating. It does this as a cobinder with the PVOH. Further it acts as an adhesive to the aluminium layer. It also acts as a temporary dye fixative discouraging sideways migration of dye ink in the coating.

The silica makes up the balance of substantially one tenth of the coating and is an absorbent. It is the main absorbent of the ink dye on its printing onto the coating.

The small quantity of alcohol ethoxylate acts as a dispersant for even dispersion of the components of the coating prior to its coating and likewise dispersion of the components on the aluminium layer after actual coating of the latter.

It will be noted that the components of the coating are not unique in their function, i.e. it is not the case of one being a dye absorbent, one a binder and one an adhesive. Rather they complement each other.

Where the coated sheet is used with an object having a tight radius of curvature, there is a risk of the aluminium layer reaching its fracture point and cracking. If the coating has too high a stress imposed on it, it also will fracture.

However, our experiments have shown that it normally fractures only after the aluminium. The coating slurry is prepared by adding the components in comminuted form to a small amount of water. Conveniently it can be rolled onto the APET sheet. Once the coating has dried for use and been printed onto, moisture in the ink is drawn away from the dye in the ink. However the dye does not migrate appreciably beyond its printed extent. The printing can be done with a conventional inkjet printer.

When the sheet is drawn against the object to be printed, the combined effect of the vacuum and the heating causes the dye to migrate from the coating to the object. The levelling of heating need not be high, typically to 130 to 160°C.

The heating may be initiated prior to conforming the sheet to the 3D object, to assist the sheet in stretching around the object, but it should not be to the final level, to avoid premature drawing of the ink dye from the coating.

Heating of the object can assist in making its surface more receptive to the printing. The invention is not intended to be restricted to the details of the above described embodiment. For instance the co-polymer of styrene-acrylic can be replaced with styrene butadiene copolymer, although it is believed that the former adheres to the aluminium layer better than the latter. Further we have now, since the original invention, developed another coating, comprising the following dry components, again applied as a slurry comprising 75% to 80% water:

In this coating, the styrene-acrylic copolymer as the thermoplastic elastomer is replaced by the vinylpyrrolidone vinylacetate copolymer. The polyacrylate as the other binder is not replaced, its function also being taken by the vinylpyrrolidone vinylacetate copolymer. The polyvinylalcohol, the silicon dioxide and the alcohol ethoxylate perform the same functions as in the above described coating. This coating is applied in the same way, to the same thickness and dried in the same way before being printed onto.

The vinylpyrrolidone vinylacetate copolymer is markedly less elastomeric than the co-polymer of styrene-acrylic, stretching to reach plastic limit at tens of percentage strain as opposed to hundreds of percent. This results in thinning of the coating as it conforms to the 3D object. It is better able to do this if the conforming is with application of heat. The final temperature can be somewhat lower than with the co-polymer of styrene-acrylic, namely 110°C to 130°C.

The thinning of the coating is accompanied by plastic deformation of it. Thus it does not fracture with thinning. However after dye transfer, the 3D surface is not gripped in the same elastomeric manner as with the co-polymer of styrene-acrylic.

We therefore prefer this coating.