WO2002053380A1

WO2002053380A1 - Thermal transfer printing on articles other than sheets or webs

Info

Publication number: WO2002053380A1
Application number: PCT/GB2002/000037
Authority: WO
Inventors: Jeffrey Michael Howell; Andrew Alec Clifton
Original assignee: Imperial Chemical Industries Plc
Priority date: 2001-01-06
Filing date: 2002-01-07
Publication date: 2002-07-11
Also published as: GB0100330D0

Abstract

Apparatus for printing an image onto an image-receiving surface (22) of an article (10) by a thermal transfer printing process comprises a heat-conductive member (18) comprising polymeric elastomer material and having a surface (20) shaped to be substantially complementary to the image-receiving surface of the article; and means (16) for heating the member. In use, a carrier sheet (24) bearing an image to be printed is held in contact with the image-receiving surface of the article by the member, via the complementary surface of the member. The member is heated by the heating means, with the sheet being held in contact with the article for a suitable time and with the member at a suitable temperature to print the image onto the surface of the article. Because heat is applied via the member, rather than being applied via the article, the article is exposed to less heat so that printing can be performed on articles of a wider range of materials than has been possible hitherto, including thermoplastics materials, provided the article is suitably supported when heated so the shape of the article is retained. The invention also covers a method of printing an image on an image-receiving surface of an article, and a printed article.

Description

THERMAL TRANSFER PRINTING ON ARTICLES OTHER THAN SHEETS OR WEBS

Field of the Invention

This invention relates to thermal transfer printing, and particularly concerns apparatus for printing an image on an article, a method of printing and an article bearing a printed image.

Background to the Invention

Thermal transfer printing, in the context of the present specification, is used to mean a process in which an image on a carrier sheet is printed onto an image-receiving surface of an article by placing the image in contact with said surface of the article, and heating the image to transfer the image onto the article surface. Usually force is applied to exert pressure on the sheet and maintain the sheet in contact with the surface during printing.

The image is typically formed on the carrier sheet by a first printing step. In this case, the overall two stage process can be considered as a retransfer printing process. Retransfer printing techniques are commonly used for printing on articles other than flexible sheet material.

One example of a retransfer printing process is dye diffusion thermal retransfer printing. In a first stage, an image is formed by dye diffusion thermal transfer printing on a retransfer intermediate carrier sheet. Dye diffusion thermal transfer printing is a process in which one or more thermally transferable dyes are caused to transfer from selected areas of a dye-donor sheet to a receiver by thermal stimuli, thereby to form an image. This is generally carried out in a printer having a thermal head or laser energy source, depending on the kind of dye- donor sheet used. Using a dye-donor sheet comprising a thin substrate supporting a dyecoat containing one or more uniformly spread dyes, printing is effected by heating selected discrete areas of the dye-donor sheet while the dyecoat is pressed against a dye-receptive surface of a receiver sheet, thereby causing dye to transfer to corresponding areas of the receiver. The shape of the image transferred is determined by the number and locations of the discrete areas which are subjected to heating. Full colour images can be produced by printing with different coloured dyecoats sequentially in like manner, and the different coloured dyecoats are usually provided as discrete uniform panels arranged in a repeated sequence along a ribbon-shaped dye-donor sheet or ink ribbon. In retransfer printing, the receiver sheet is in the form of a retransfer intermediate sheet which comprises a supporting substrate having a dye-receptive imageable layer on one side, usually with a backcoat on the other side to promote good transport through the initial printer. Suitable retransfer intermediate sheets are disclosed, e.g, in WO 98/02315. The image-carrying intermediate sheet formed in the first stage of the process is separated from the dye-donor sheet, and in the second stage of the process, is pressed against the article, with its image-containing layer contacting a dye-receptive surface of the article. Heat is then applied to effect transfer of the image, usually over the whole area of the image simultaneously. This is commonly carried out in a press shaped to accommodate the article. See, for example, the mug press disclosed in US 5643387, which is designed for retransfer printing a heat-resistant article that is cylindrical in shape.

Thermal retransfer techniques are of particular applicability to the customisation of three dimensional articles such as mugs and tiles which can withstand the temperatures involved without distortion. The carrier sheet can be printed by a desktop printer, and the press can retransfer the image in a total time of less than 5 minutes. Thus the method has advantages over direct printing by silk-screen, offset or gravure processes because it can add personalisation at point-of-sale.

In general, prior art thermal retransfer techniques can only be used with articles made of materials able to withstand the elevated temperatures (and possibly also elevated pressures) involved without distorting or breaking, typically being restricted to articles of metal or refractory materials such as ceramics. This applies, eg, to the mug press disclosed in US 5643387, which employs high pressures to ensure compliance of the press with the surface of the mug to be printed, restricting the approach to refractory objects of certain forms (having only simple curves, in only one plane) which do not distort or break at the temperatures and pressures involved.

The need for precise conformance of the printing tool with the article is avoided in DE 29704538 by using a tool comprising silicone resin foam. However, the foam has a poor thermal conductivity so the heat must be applied from a radiation source such as a quartz lamp through the body of the article. This severely limits the form of the press, and restricts it to use with refractory articles.

A printing method for articles of arbitrary shape disclosed by EP 606189 Al uses a thin, flexible membrane which is externally pressurised to conform to the article. The image- bearing sheet is placed between the membrane and the article, and the whole is immersed in a heating oil. Although the article is not subject to any external distorting forces, because it experiences a uniform hydrostatic pressure, it is still prone to distortion from internal stress. The hot oil is also hazardous and this factor, together with the extra preparation needed for each article, makes this approach unsuitable for point-of-sale use.

JP 06-247058 discloses sublimation transfer printing onto a moulded article of heat-resistant material, using a supporting jig.

The prior art techniques are not generally applicable to printing on articles of thermoplastic materials that would soften and distort under the printing conditions employed.

Summary of the Invention

In a first aspect the invention provides apparatus for printing an image onto an image- receiving surface of an article by a thermal transfer printing process, the apparatus comprising a heat-conductive member comprising foamed polymeric elastomer material and having a surface shaped to be substantially complementary to the image-receiving surface of the article; and means for heating the member. In use, a carrier sheet of material bearing an image to be printed is held in contact with the image-receiving surface of the article by the member, via the complementary surface of the member. Typically, a sheet is located on the article in an appropriate position and the member is brought into contact with the exposed face of the sheet and force applied to exert pressure on the sheet sufficient to promote wetting contact between the sheet and the member. The member is heated by the heating means, possibly being pre-heated to a desired temperature prior to contact with the sheet and/or prior to contact of the sheet with the article. The sheet is held in contact with the article for a suitable time with the member at a suitable temperature (typically up to about 200°C) to print the image on the surface of the article. Suitable printing conditions can be readily determined by experiment. The member is removed and the article allowed or caused to cool to ambient temperature. The sheet is usually removed from the article surface.

Because heat is applied via the member, rather than being applied via the article as in DE 29704538, the article is exposed to less heat. The apparatus therefore enables printing to be performed on articles of a wider range of materials than has been possible hitherto. In particular, the invention makes it possible to print on articles of heat-distortable materials that would normally distort under printing conditions, particularly thermoplastic materials such as many widely used polymers such as acetal resin, acrylic resins, polycarbonates etc, provided the article is suitably supported when heated so the shape of the article is retained.

The apparatus accordingly desirably includes a support for the article, shaped to support or receive the article while leaving exposed the image-receiving surface thereof. In embodiments intended for use in printing articles of thermoplastic material, the support should be shaped to be complementary to a part of the article so as fully to support the article with a snug fit. The support will be discussed further below.

In a second aspect the invention provides apparatus for printing an image onto an image- receiving surface of an article by a thermal transfer printing process, the apparatus comprising a heat-conductive member comprising polymeric elastomer material and having a surface shaped to be substantially complementary to the image-receiving surface of the article; means for heating the member; and a support for an article to be printed, shaped to support or receive the article while leaving exposed the image-receiving surface thereof, the support having a lower thermal conductivity than that of the member.

A third aspect of the invention resides in apparatus for printing an image onto an image- receiving surface of an article by a thermal transfer printing process, the apparatus comprising a heat-conductive member comprising polymeric elastomer material and having a surface shaped to be substantially complementary to the image-receiving surface of the article; means for heating the member; and a support for an article to be printed, shaped to support or receive the article while leaving exposed the image-receiving surface thereof, the support having a greater hardness than that of the member.

Apparatus in accordance with the second and third aspects is used as described above in relation to the first aspect of the invention.

During printing, the sheet should be held firmly in contact with the article to produce a print of good quality. The apparatus therefore preferably includes suitable urging means, eg for applying force between the member and support, e.g. in the form of a spring loading arrangement or a hydraulic system. Where the apparatus is intended for printing onto curved surfaces, particularly those of high compound curvature, the urging means preferably ensure intimate contact between the sheet and article in curved regions. In such cases it may sometimes also be beneficial to apply an additional force in the horizontal direction, and this is exemplified by the use of a collar. Other means might also be used, for example the use of a vacuum pump to withdraw the air from between the transfer sheet and the article to be printed. This will tend to force the sheet against the article.

The member preferably comprises foamed material, ie. material including a plurality of small openings, pockets or voids within the body of the material, unlike the apparatus disclosed in JP 06-247058. The voids are typically filled with gas, commonly air. The voids should be small (preferably having a maximum dimension less than 1mm) and are ideally substantially uniform in the size and are preferably reasonably uniformly dispersed through the material, for uniformity of properties, such that any variation in conductivity is not such as to produce prints with a variation in optical density. The presence of the voids, coupled with the elastomeric nature of the member material, mean that the member is relatively soft and has a degree of elasticity and so can, if necessary, deform to an extent (at least when heated) to enable the surface thereof to conform to the image-receiving surface of an article, thus providing uniform contact between the member and article. Provided the member is sufficiently compliant to allow good wetting contact with the sheet and article under compression, variation between the shape of the member and article surfaces is tolerable. Thus, while said surface of the member should be shaped to be substantially complementary to the image-receiving surface of the article, exact complementarity is not required as a degree of variation can be accommodated by the deformability of the member. The degree of variation tolerable will depend on factors including the degree of deformability of the member, the pressure applied in use and the complexity of shape of the image-receiving surface of the article. The extent to which the transferred image can wrap around curves in the printed article will in part depend on the curvature of the member. So, for example, a flat member will deform sufficiently to allow transfer of an image around a gentle curve. A more difficult situation is presented where a printable surface has a compound curved surface (concave or convex), such as a part spherical surface having curvature in two planes, especially where smaller radii of curvature and greater angles (up to, say 90°) are present. In this case, a high degree of complementarity is required to enable the sheet to conform to the surface without creasing. References to "substantially complementary" in this context should be construed accordingly.

The void volume of the material should be selected to give desired properties, and should be sufficiently high to provide the necessary elasticity without being so high as to adversely affect thermal conductivity. Generally, the void volume is in the range of 1 to 35%, preferably 5 to 25% by volume, typically being about 10% by volume.

Suitable foamed polymeric elastomer materials are well known to those skilled in the art, and are commercially available or can be produced by known techniques. A foamed material can be created with a controlled void level by several means, including but not limited to mechanical entrainment of gas, adding volatile liquids, dissolving gases in one or more components, using a chemical reaction which evolves gas, adding a material which decomposes to evolve a gas during curing. For example, air-filled voids can be created by vigorously mixing material with air to generate an aerated paste which sets, eg on addition of a curing agent, to produce a solid foamed material.

The foamed polymeric elastomer material preferably comprises a foamed silicone resin, but other foamed polymers that exhibit sufficient flexibility and stability under printing conditions may alternatively be employed. Suitable materials are commercially available.

The member comprising foamed polymeric elastomer material is heat conductive so that in use heat can be conducted through the member to an image-bearing sheet in contact with the article. It is desirable for the member to have relatively high thermal conductivity properties so that in use the sheet reaches the transfer temperature rapidly, thus avoiding the need for prolonged exposure of the article to elevated temperatures and so reducing the likelihood of thermal degradation of the article. The member preferably has a thermal conductivity of at least 0.2 W/m/K, and in general desirably has as high a thermal conductivity as possible provided other properties are not compromised.

The presence of voids in the polymeric elastomer material reduces the thermal conductivity of the material. In order to raise the thermal conductivity, preferably to a value greater than that of the polymeric elastomer material in unvoided, unfoamed condition (eg unfoamed silicone resin), it is convenient to include in the material particles of high thermal conductivity material, having a thermal conductivity greater than that of the polymeric elastomer material and typically having a thermal conductivity of at least 1 W/m K. Suitable high thermal conductivity materials include metals and metal oxides. The particles should be reasonably uniformly dispersed through the material, for uniformity of properties. The particles are preferably spherical or near spherical in form and relatively uniform in size in order not to affect adversely the flexibility and deformability of the member. For the same reason, the particles are preferably relatively small, eg having a maximum particle size in the range 3 to 300 microns, preferably less than 40 microns. The particles are desirably present in sufficient quantity to ensure that the image reaches and stays at the transfer temperature for printing for the required period of time, without being present at such high levels that flexibility of the material is compromised. Suitable particle loadings can be readily determined by experiment. The particles are preferably present at a level of at least about 10% of the volume, more preferably at least about 15%, yet more preferably at least about 20% of the volume of the material.

Addition of the thermally conductive particles allows a desirable level of thermal conductivity to be achieved, but an undesirable side-effect is an increase in the stiffiiess of the elastomer mixture. This can be partially offset by the presence of voids, but in order to obtain the best mechanical compliance from the member (as discussed below), it is often desirable to plasticise the resin mixture. Many formulations of elastomer already contain plasticiser, but additional quantities can give further benefit. For the plasticisation of silicone resins, polydimethylsiloxane) (PDMS) is frequently used.

Such further additions of plasticiser increase the compliance of the member to the extent that it is sometimes possible to obtain good transfer without the presence of voids.

The member is preferably made of material that is substantially isotropic, with voids and particles, if present, reasonably uniformly distributed therethrough.

The means for heating the member conveniently comprises a heatable plate or jacket in contact with the member other than said shaped surface thereof. The plate or jacket may be heatable in known manner, eg incorporating or being linked to electrically operated heating elements.

Alternatively or additionally, the member be rendered electrically conductive by the incorporation in the foamed material of particles of electrically conductive material, preferably carbon, so that the member may be heated by the passage of an electrical current therethrough.

The apparatus preferably includes a support, as noted above.

The support preferably has a lower thermal conductivity than that of the member, eg being of the same base material as the member but not in foamed condition and without added particles of high thermal conductivity material. For instance, the support conveniently comprises unfoamed silicone resin. This is because, in use, heat flow through an article tends towards a steady-state value during printing. Unless the thermal conductivity of the support is less than that of the foam, the support will suck heat away from the article, making it harder for the article to attain a suitable temperature for printing and necessitating the use of higher printing temperatures, which is generally undesirable. It is nevertheless possible to produce satisfactory prints using apparatus in which the support has higher thermal conductivity than that of the member. At least in embodiments intended for use in printing on articles of thermoplastic materials, the support should be comparatively hard, rigid and non-deformable.

The voids in the foamed material serve to make the material more compliant and deformable. Hence the voids are used in the heat-conductive member on the printing side in order to allow good conformation between the member and the combination of carrier sheet and article to be printed. The voids have the undesirable side effect of reducing the thermal conductivity of the material. This effect is more than compensated by adding thermally conductive filler material to the composition. In the case of the support, the material should not be too soft and deformable, or it will not be capable of providing the desired mechanical support. It is thus preferred that the support has a greater hardness (eg as represented by Shore A hardness value), i.e. a lower mechanical compliance, than the member. For this reason, unless the support is constructed of a rigid material, it is prefened that there should be no voids present. It is also desired that the thermal conductivity of the support should be lower than that of the other member. The absence of voids is a disadvantage from this point of view. However, the desired result can be achieved by not incorporating the thermally conductive filler in the support.

The support is preferably not heated during printing, to reduce exposure of the article to heat, and also for more rapid cooling of the article after printing.

The apparatus may include cooling means for cooling the article after printing and after removal of the member. For instance, means such as a fan may be provided for directing a flow of cold air over the printed surface of the article. Additionally or alternatively, the support may have an associated coolable member. The support is typically vertically below the member in use of the apparatus, but this is not essential.

Apparatus in accordance with the invention is typically designed for use with a particular article, with the member, and support if present, being tailored to the shape of that particular article. If the article surface to be printed has a high degree of curvature, as discussed above, then the member needs to be tailored to provide a close fit to the surface. It may even be desirable to build up the surface of the member in critical areas, so that extra pressure is applied during printing in regions of high curvature of the article surface.

In a further aspect, the invention provides a method of printing an image on an image- receiving surface of an article, comprising holding a carrier sheet bearing an image to be printed in contact with said image-receiving surface of the article by means of a heat- conductive member comprising foamed polymeric elastomer material and having a surface shaped to be substantially complementary to said image-receiving surface of the article; and heating the member to print the image on said surface.

The invention also covers a method of printing an image on an image-receiving surface of an article, comprising placing the article on a support so as to leave exposed the image- receiving surface of the article; holding a carrier sheet bearing an image to be printed in contact with the exposed image-receiving surface of the article by means of a heat- conductive member comprising polymeric elastomer material and having a surface shaped to be substantially complementary to said image-receiving surface of the article, wherein the support has a lower thermal conductivity than that of the member; and heating the member to print the image on said surface.

Another aspect of the invention resides in a method of printing an image on an image- receiving surface of an article, comprising placing the article on a support so as to leave exposed the image-receiving surface of the article; holding a carrier sheet bearing an image to be printed in contact with the exposed image-receiving surface of the article by means of a heat-conductive member comprising polymeric elastomer material and having a surface shaped to be substantially complementary to said image-receiving surface of the article, wherein the support has a greater hardness than that of the member; and heating the member to print the image on said surface.

The image on the carrier sheet may be produced by a variety of different printing techniques, including electrophotography, mass transfer printing, ink jet printing, melt or wax transfer printing and dye diffusion thermal transfer printing, using appropriate carrier sheet material in each case.

Particularly when printing onto a curved surface of an article, it is desirable to use a carrier sheet that is extensible (preferably in both dimensions of the plane of the sheet) at the printing temperature (typically up to about 200°C) so that the sheet conforms to the shape of the surface. Extensibility in two dimensions is difficult to measure, so it is preferred isntead to measure a linear extensibility by placing a strip of the material under load at the printing temperature (typically 200°C) and measuring the ratio of the extended length to the initial length (without tearing) when a force of ION is applied to a 10mm wide strip of the material. It is preferred to use carrier sheet having linear extensibility measured in this way of at least 1.5, more preferably at least 2.5. When printing onto a surface of high compound curvature, it is desirable to use a sheet having linear extensibility of at least 3 in order to obtain full wrap around of the surface.

When printing by a dye diffusion thermal retransfer printing process using a retransfer intermediate sheet as discussed above may be adopted. A retransfer intermediate sheet typically comprises a supporting substrate having a dye-receptive coating, or receiver layer, on one surface thereof, e.g. as described in EP 409514, with the other surface optionally carrying a coating to improve the friction, release or static properties of the sheet during printing and preferably also carrying the heat-resistant backcoat, as described in EP 912349 (WO98/02315). The substrate may comprise paper, possibly polyolefin-coated paper, with the preferred substrate comprising a laminated material prepared by laminating an opaque, voided polypropylene film to at least one side of a cellulose paper base material, e.g. as disclosed in JP 06-84119-B. The voids in the polypropylene film layer improve the compliance of the sheet. Substrates of high extensibility, as discussed above, include polyolefm synthetic paper (eg Yupo from New Oji Paper (Yupo is a Trade Mark) or Teslin from PPG Inc. (Teslin is a Trade Mark)), or other extensible materials, such as PNC (polyvinylchloride). In general, it is preferred not to have a cellulosic core when a nigh degree of wrap is desired. It is also desirable that the receiver layer itself should have high extensibility under the retransfer conditions. This may be achieved, for example, by limiting the degree of crosslinking in the receiver coating.

In electrophotography, an image is initially produced by transferring particles of fusible toner onto an image-receiving carrier sheet. A toner film (made up of toner particles that have fused together during the fusing stage of the initial electrophotographic printing process) can then be thermally transferred onto an article in accordance with the invention. It is preferred to use a carrier sheet having a substrate comprising plastics material e.g. polyester such as Melinex (Melinex is a Trade Mark) or sealed paper so that the toner particles do not penetrate into and adhere within the body of the material (as they would tend to with unsealed paper substrate, sticking to paper fibres), hindering release. The carrier sheet may comprise a receiver layer on the substrate, which receiver layer may be transferred to the article along with the toner in the second stage printing, so providing a protective layer over the transferred image on the article. In order to obtain full wrap, as discussed above, it is desirable to use a carrier film with high extensibility. In order to obtain reliable printing in an electrophotographic printer, it may be desirable to use a laminated structure, with an inextensible backing an extensible top layer, that can subsequently be removed for retransfer.

Ink jet printing may be used to print an image on a carrier sheet comprising a thermally fusible receiver layer, into which the ink is printed. Suitable carrier sheets include those normally used for iron-on T-shirt printing. The receiver layer and incorporated ink may be thermally transferred together to an article in the second stage printing. In order to obtain full wrap, it is again desirable to use an extensible substance.

The thermal transfer printing method of the invention may result in transfer to the image- receiving surface of the article of only the image, or the image together with one or more layers of thermally transferable material from the carrier sheet, to provide a protective coating over the transferred image. These possibilities are discussed above in relation to electrophotography and ink jet printing. Similarly, when using a carrier sheet bearing an image produced by a melt or wax transfer process, the material transferred could be just the imaging material or a transferable receiver layer in addition. This also applies to an image produced by dye diffusion thermal transfer printing, using a carrier sheet comprising a substrate bearing a removable dye-receiver layer such that the thermal transfer printing method of the invention results in transfer to the image-receiving surface of the article of the receiver layer and dye together. The receiver layer coating on the article constitutes a protective coating over the image. In order to facilitate the mass transfer of the receiver layer it is possible optionally to use an additional coating when printing of the original image, e.g. in the form of an additional panel of material in each sequence of an ink ribbon. The additional panel comprises a suitable polymer that can act as a thermally transferable adhesive.

As discussed above, the substrate is preferably not too stiff or inelastic, at least at printing temperatures and preferably also at ambient temperature, at least for use in printing on a compound curved surface (concave or convex) of an article, such as a part spherical surface having curvature in two planes, to enable the sheet to conform to the surface without creasing. It may be desirable to pre-heat the sheet prior to holding the sheet in contact with the article so that the sheet is in softened condition and more readily able to conform to the shape of the surface. Such pre-heating can be achieved by radiant heat from the member in heated condition, being held in close proximity to the sheet located on the image-receiving surface of the article (but not held in contact therewith), prior to contact of the sheet by the member and application of force to press the sheet into engagement with the article.

The image-receiving surface of the article may optionally be coated with suitable material to enhance susceptibility of the surface to printing, in known manner. Examples of suitable coating materials are given in US 5643387 at column 2 lines 23 to 52.

The invention is applicable to printing on articles of a wide range of materials, including thermoplastic materials as well as metals and ceramic materials, and in a wide range of shapes, including printing on flat surfaces as well as on more complex, curved shapes (concave or convex) including compound curves. The invention can thus be used for printing on articles such as casings of mobile telephones (the entire casing or part or parts thereof) made of polymers such as polycarbonate that soften and would otherwise distort under printing conditions of temperature and pressure. With such articles even slight distortion is unacceptable because the article must meet tight tolerances to fit correctly with other components. The invention also finds use in the personalisation of other thermoplastic objects, for example computer mice, hand-held computers, gearstick knobs, etc.

The invention also includes within its scope an article bearing a printed image produced by use of apparatus or a method in accordance with the invention.

The invention will be further described, by way of illustration, with reference to the accompanying drawings, in which: -

Figure 1 is a schematic sectional view of one embodiment of apparatus in accordance with the invention; and

Figure 2 is a view similar to Figure 1, showing the apparatus in use.

Detailed Description of the Drawings

Referring to the drawings, the illustrated apparatus comprises a printing press designed to print an image onto a surface of an article in the form of part of a polycarbonate casing 10 of a mobile telephone by a dye diffusion thermal retransfer printing process.

The apparatus comprising a support 12 of silicone resin, which is a precisely moulded part shaped to fit closely within the casing 10. The support is mounted on a coolable backing plate 14.

The apparatus further comprises a heatable aluminium element 16, having the general form of an inverted open box. Element 16 has associated electric heaters (not shown) either inserted therein or attached to the major outer face thereof. Element 16 carries a rubber-like foam member 18, the inner surface 20 of which is shaped to be complementary to the outer face 22 of the casing 10. Member 18 is formed of foamed silicone resin containing both gas- filled voids and thermally conductive particles.

In use, the casing 10 is fitted on support 12. A retransfer intermediate carrier sheet 24, bearing an image to be printed on one face 26, produced by dye diffusion thermal transfer printing as discussed above, is located on the outer face 22 of the casing, with the image positioned over the region of the casing on which it is desired to print. Element 16 is heated, thus heating member 18 by conduction. The member 18 is pre-heated in this way until it is at a temperature above the retransfer printing temperature, typically being up to about 200°C. Heat radiating from the surface 20 of member 18 will heat up, and may cause softening of, sheet 24. The member 18 (and associated heater 16) are moved into contact with the sheet 24 and exposed regions of the casing outer face 22, to the position shown in Figure 2, and suitable force applied by a spring loaded arrangement (not shown) to press the sheet 24 into engagement with the surface 22 of the casing. In this position, the casing 10 is fully enclosed between support 12 and member 18. This condition is maintained for a suitable time, typically about 1 or 2 minutes. During this time the sheet and image and also the casing are rapidly heated to a temperature at which dye diffuses to the casing surface, creating a durable image. The entire apparatus may optionally be located in a suitable enclosure and air evacuated to ensure good contact between the sheet and article during printing.

Any air trapped between the article and the sheet will inhibit the transfer of the dye on printing and give rise to unprinted spots on the article. This can be avoided by extracting the air from the press prior to closing it or by arranging the shape of the foam to squeeze the air out progressively from the point of contact to the edges as the press is cooled. This can be achieved, e.g., by having a slight dome (not shown) in the centre of member surface 20 so that as pressure is progressively applied an outwardly enlarging area of contact between the surface 20 and the sheet 24 and article face 22 is progressively formed, moving from the centre to the edges and forcing out air.

The foam of member 18 is soft enough to conform closely to the surface 22 of the casing, deforming to accommodate any differences in shape, and so ensuring uniform contact. The voids provide sufficient elasticity to maintain an even pressure despite dimensional variations between components. If the article to be printed has a high degree of curvature and especially compound curves, it may be necessary to take special steps to apply pressure in critical regions. For example, it the main face of the article is horizontal, then additional sideways pressure may have to be applied. The particles give sufficient heat capacity and thermal conductivity to ensure that the image reaches and stays at the retransfer temperature for the required period of time. The support is shaped to prevent the casing deforming even though the retransfer may take place at a temperature above the softening temperature of the casing material.

After printing, the press is opened by raising the member 18 and element 16, leaving the casing 10 and sheet 24 on the support 12. The thermal conductivity of the support is such as to allow the casing to cool quickly to below its softening temperature. Cooling may be assisted by directing cold air over the casing. After cooling, the casing 10 can be removed from the support without risk of distortion of the casing. The sheet 24 can be peeled off the casing, leaving a printed image (and possibly also a protective coating as discussed above) on the surface 22.

Example 1

A sample of a silicone resin foam containing 40% w/w of aluminium powder was prepared as follows: 20 grams of Silastic 'S' silicone base (Dow Corning) (Silastic 'S' is a Trade Mark) were mixed vigorously with 13.3 grams of fine aluminium powder of particle size 3- 30 microns (Fisher Chemicals A/1605/53) to form a finely aerated paste. Then 2 grams of Silastic 'S' curing agent were mixed in. The resulting paste was cast into a slab and cured at 100°C for 3 minutes. The volume fraction of aluminium powder is limited by the thickening of the paste and the need to retain flexibility in the cured resin.

For comparison, a slab of the same silicone resin was prepared, but in non- voided form and without added aluminium powder.

The two materials were tested, and details of test results are as follows:-

In the priority document, the values for thermal conductivity of normal silicone and aerated silicone were given incorrectly as 0.101 and 0.244, respectively, due to an error in calculation. This also resulted in the thermal diffusivity values being incorrectly specified as 80 and 190. These errors have now been corrected.

The volume composition of the novel aerated silicone is calculated from its density as 71% resin, 18% metal, 11% voids.

A printing press generally as illustrated in Figures 1 and 2 was constructed, with member 18 formed of the novel aerated silicone as described above and support 12 formed of normal silicone as described above. The press was designed for printing on an outer surface of polycarbonate shell of shallow dish shape with a diameter of 30mm and a mean thickness of 1mm. This was the back part of the casing of a mobile telephone. Member 18 was formed by casting the aerated silicone at a thickness of 4mm around the outer surface of the shell, with support 12 being cast at a thickness of 12mm inside the shell.

The outer surface of the shell was coated with a clear epoxy material, and a print was formed on the coated outer surface of the shell by using the press as described above, with an image on a retransfer intermediate sheet of NP retransfer paper from ICI Imagedata. The retransfer paper comprises a 128 gsm paper core laminated on both sides with a 35 microns thick commercial pearl film such as Toyopearl SS (Toyopearl SS is a Trade Mark). The upper layer of the substrate is coated with a filled whitening layer upon which the receiver layer is coated. The member 18 was heated to a temperature of 160°C. When the press was closed, the shell was subjected to a pressure of 500 Νewtons for 2 minutes. The press was then opened and the shell left on the support to cool, with a flow of air being directed over the support and shell for 30 seconds for additional cooling effect.

A full colour (black, cyan, magenta, yellow) image was transferred into the clear epoxy coating on the shell in this way. The neutral optical density measured with a Macbeth TR- 1224 densitometer in reflection mode was 1.63. The shell was measured for distortion which was found to be less than 1% anywhere in the shell. The shell could be replaced onto the remainder of the mobile telephone casing without excessive force, and remained firmly attached as originally intended.

When the member of normal silicone was used in place of the aerated silicone, the neutral optical density was 0.87.

Example 2

The above printing process was repeated using a transfer sheet on a Teslin (Teslin is a Trade mark) base prepared as described in EP 690793-A. It was found that the image transferred almost up to the edges of the shell.

Example 3

The above process was repeated using a similar transfer sheet based on Yupo (Yupo is a Trade Mark). Good wrap was again achieved, although there were occasional creases in areas of compound curvature.

Example 4

The shell used in the previous Examples was replaced by one with greater curvature, in which the four sides were vertical when the main body of the shell was horizontal. The transfer sheet used was the same as in Example 2. Using a substantially flat member, it was found that there was insufficient pressure around the edges of the shell, and no transfer occurred there. When the member was moulded so as to be closely complementary to the shell, then transfer occurred nearly to the edges.

Example 5

Using the shell and transfer sheet of Example 4, the apparatus was modified by applying a collar round the member in order to apply a horizontal force around the sides of the shell. This was found to give full wrap printing right to the edges.

Example 6

The following samples were prepared in order to determine the mechanical properties of silicone resin containing different amounts of plasticiser and aluminium. Air was deliberately removed from the resin mixture in order to avoid any variability in the air entrapment and allow measurement of the intrinsic properties of the mixture. The materials used were as follows:

Silastic S and curing agent as in Example 1

Fine aluminium powder as in Example 1

Poly(dimethyl siloxane) of viscosity nominally 5000 centistokes at 25 deg C (PDMS) - product code PS044 from Fluorochem Ltd, U.K.

It is believed that Silastic S contains about 5% of PDMS in order to act as a plasticiser.

The samples were made by kneading the tabulated proportions of the components, and were then cast into moulds of diameter 56 mm to a thickness of 6mm. They were de- aerated by standing in an evacuated chamber for 15 minutes in order to remove entrapped air, before being heated to 90°C for 10 minutes in order to effect cure.

Each of the cured samples was subjected to compression using an Instron model 6021 stress-strain measuring device. The sample was compressed using a 15 mm diameter aluminium disc being urged by the activator of the Instron device via a 12.7 mm diameter spherical-tip probe. The recorded curves were then translated to an equivalent Shore A hardness value by an empirically fitted curve.

Measurements were carried out at a sample temperature of 20°C and at 200°C.

The following hardness figures were determined at 20°C:

PDMS 0% 6.5% 12%

(w/w) Aluminium (w/w)

0% 36 24 19

20% 43 30 21

40% 55 43 29

The same samples gave the following figures at 200°C

PDMS 0% 6.5% 12%

(w/w)

Aluminium

(w/w)

0% 30 22 17

20% 41 25 20

40% 47 32 26

It can be seen that addition of aluminium increases the hardness of the test samples, but that increasing the level of plasticiser allows the hardness to be reduced.

Example 7

A member was prepared with the ingredients as described in Example 6 so as to contain 3.2% added PDMS plasticiser and 40% aluminium powder (w/w). The polymer was carefully de-aerated as described so it was effectively free of voids. The surface of this member was flat, and its thickness was 6 mm, so that conformance with the curved surface of the substrate could only occur through deformation of the member. A support was prepared from epoxy resin and a mobile telephone back measuring 50 by 73 mm was printed as described in Example 1. The total press force was kept constant for the retransfer duration of 80 seconds. Mean penetration into the pad was measured by examination of the printed area. The hardness of the member and support at 20°C was measured with a Durometer gauge made by Coats Machine Tool Co. Ltd. Results are given below.

Example 8

As described in Example 7 above, but the mixture was not de-aerated. In order to incorporate as many voids as possible, it was subjected to additional vigorous mixing for a period of 5 minutes at room temperature with a mechanical stirrer. The void fraction was calculated from the member density as 13%. Printing was carried out using exactly the same technique as described in the previous example.

The penetration of the phone back into the member (in mm) for these Examples 7 and 8 was as follows:

It can be seen that the voided pad allows better conformance to the curved surface of the telephone back, even though the measured hardness and thermal conductivity are comparable.

Claims

1. Apparatus for printing an image onto an image-receiving surface of an article by a thermal transfer printing process, the apparatus comprising a heat-conductive member comprising foamed polymeric elastomer material and having a surface shaped to be substantially complementary to the image-receiving surface of the article; and means for heating the member.

2. Apparatus according to claim 1, wherein the member has a thermal conductivity of at least 0.2 W/m/K.

3. Apparatus according to claim 1 or 2, wherein the member comprises foamed silicone resin.

4. Apparatus according to claim 1, 2 or 3, wherein the conductive member comprises foamed polymeric elastomer material including particles of high thermal conductivity material.

5. Apparatus according to claim 4, wherein the particles comprise metal or metal oxide.

6. Apparatus according to claim 4 or 5, wherein the particles comprise at least about 10% by volume of the material.

7. Apparatus according to any one of the preceding claims, wherein the means for heating the member comprise a heatable plate or jacket in contact with the member other than said shaped surface thereof.

8. Apparatus according to any one of the preceding claims, further comprising a support for an article to be printed, shaped to support or receive the article while leaving exposed the image-receiving surface thereof.

9. Apparatus according to claim 9, wherein the support has a greater hardness than the member.

10. Apparatus according to claim 8 or 9, wherein the support has a lower thermal conductivity than that of the member.

11. Apparatus according to claim 8, 9 or 10, wherein the support comprises silicone resin.

12. Apparatus according to any one of the preceding claims, further comprising means for cooling an article after printing.

13. Apparatus according to any one of the preceding claims, including urging means for holding a carrier sheet bearing an image to be printed in contact with the article surface.

14. Apparatus according to claim 13, wherein the urging means are adapted to ensure intimate contact between the carrier sheet and regions of the surface of compound curvature.

15. Apparatus according to any one of the preceding claims, for printing onto a surface of article of compound curvature.

16. Apparatus according to any one of the preceding claims, adapted for printing on part or parts of the casing of a mobile telephone.

17. A method of printing an image on an image-receiving surface of an article, comprising holding a carrier sheet bearing an image to be printed in contact with said image- receiving surface of the article by means of a heat-conductive member comprising foamed polymeric elastomer material and having a surface shaped to be substantially complementary to said image-receiving surface of the article; and heating the member to print the image on said surface.

18. A method according to claim 17, wherein the carrier sheet comprises a retransfer intermediate sheet bearing an image formed by dye diffusion thermal transfer printing.

19. A method according to claim 17, wherein the carrier sheet comprises a retransfer intermediate sheet bearing an image formed by electrophotography.

20. A method according to claim 17, wherein the carrier sheet comprises a retransfer intermediate sheet bearing an image formed by ink jet printing.

21. A method according to any one of claims 17 to 20, wherein the carrier sheet has a linear extensibility at the printing temperature of at least 1.5.

22. A method according to claim 21, wherein the carrier sheet has a linear extensibility at the printing temperature of at least 2.5.

23. A method according to any one of claims 17 to 22, wherein the member is pre-heated prior to contact with the sheet and article.

24. A method according to any one of claims 17 to 23, wherein the sheet is pre-heated prior to printing.

25. A method according to any one of claims 17 to 24, wherein the image-receiving surface has compound curvature.

26. A method according to any one of claims 17 to 25, wherein the article comprises thermoplastic material and is supported on a support during printing.

27. An article bearing a printed image produced by use of apparatus in accordance with any one of claims 1 to 16 or by a method in accordance with any one of claims 17 to 26.

28. An article according to claim 27, wherein the article is a mobile telephone casing or part thereof.

29. Apparatus for printing an image onto an image-receiving surface of an article by a thermal transfer printing process, the apparatus comprising a heat-conductive member comprising polymeric elastomer material and having a surface shaped to be substantially complementary to the image-receiving surface of the article; means for heating the member; and a support for an article to be printed, shaped to support or receive the article while leaving exposed the image-receiving surface thereof, the support having a lower thermal conductivity than that of the member.

30. Apparatus according to claim 29, wherein the support has a greater hardness than the member.

31. Apparatus according to claim 29 or 30, wherein the support comprises silicone resin.

32. Apparatus according to claim 29, 30 or 31, wherein the member comprises foamed material.

33. Apparatus according to any one of claims 29 to 32, wherein the member has a thermal conductivity of at least 0.2 W/m/K.

34. Apparatus according to any one of claims 29 to 33, wherein the member comprises foamed silicone resin.

35. Apparatus according to any one of claims 29 to 34, wherein the conductive member comprises foamed polymeric elastomer material including particles of high thermal conductivity material.

36. Apparatus according to claim 35, wherein the particles comprise metal or metal oxide.

37. Apparatus according to claim 35 or 36, wherein the particles comprise at least about 10%) by volume of the material.

38. Apparatus according to any one of claims 29 to 37, wherein the means for heating the member comprise a heatable plate or jacket in contact with the member other than said shaped surface thereof.

39. Apparatus according to any one of claims 29 to 38, further comprising means for cooling an article after printing.

40. Apparatus according to any one of claims 29 to 39, including urging means for holding a carrier sheet bearing an image to be printed in contact with the article surface.

41. Apparatus according to claim 40, wherein the urging means are adapted to ensure intimate contact between the carrier sheet and regions of the surface of compound curvature.

42. Apparatus according to any one of claims 29 to 41, for printing onto a surface of article of compound curvature.

43. Apparatus according to any one of claims 29 to 42, adapted for printing on part or parts of the casing of a mobile telephone.

44. A method of printing an image on an image-receiving surface of an article, comprising placing the article on a support so as to leave exposed the image-receiving surface of the article; holding a carrier sheet bearing an image to be printed in contact with the exposed image-receiving surface of the article by means of a heat-conductive member comprising polymeric elastomer material and having a surface shaped to be substantially complementary to said image-receiving surface of the article, wherein the support has a lower thermal conductivity than that of the member; and heating the member to print the image on said surface.

45. A method according to claim 44, wherein the carrier sheet comprises a retransfer intermediate sheet bearing an image formed by dye diffusion thermal transfer printing.

46. A method according to claim 44, wherein the carrier sheet comprises a retransfer intermediate sheet bearing an image formed by electrophotography.

47. A method according to claim 44, wherein the carrier sheet comprises a retransfer intermediate sheet bearing an image formed by ink jet printing.

48. A method according to any one of claims 44 to 47, wherein the carrier sheet has a linear extensibility at the printing temperature of at least 1.5.

49. A method according to claim 48, wherein the carrier sheet has a linear extensibility at the printing temperature of at least 2.5.

50. A method according to any one of claims 44 to 49, wherein the member is pre-heated prior to contact with the sheet and article.

51. A method according to any one of claims 49 to 50, wherein the sheet is pre-heated prior to printing.

52. A method according to any one of claims 44 to 51, wherein the image-receiving surface has compound curvature.

53. A method according to any one of claims 49 to 52, wherein the article comprises thermoplastic material and is supported on a support during printing.

54. An article bearing a printed image produced by use of apparatus in accordance with any one of claims 29 to 43 or by a method in accordance with any one of claims 44 to 53.

55. An article according to claim 54, wherein the article is a mobile telephone casing or part thereof.

56. Apparatus for printing an image onto an image-receiving surface of an article by a thermal transfer printing process, the apparatus comprising a heat-conductive member comprising polymeric elastomer material and having a surface shaped to be substantially complementary to the image-receiving surface of the article; means for heating the member; and a support for an article to be printed, shaped to support or receive the article while leaving exposed the image-receiving surface thereof, the support having a greater hardness than that of the member.

57. Apparatus according to claim 56, wherein the support has a lower thermal conductivity than that of the member.

58. Apparatus according to claim 56 or 57, wherein the support comprises silicone resin.

59. Apparatus according to claim 56, 57 or 58, wherein the member comprises foamed material.

60. Apparatus according to any one of claims 56 to 59, wherein the member has a thermal conductivity of at least 0.2 W/m K.

61. Apparatus according to any one of claims 56 to 60, wherein the member comprises foamed silicone resin.

62. Apparatus according to any one of claims 56 to 61, wherein the conductive member comprises foamed polymeric elastomer material including particles of high thermal conductivity material.

63. Apparatus according to claim 62, wherein the particles comprise metal or metal oxide.

64. Apparatus according to claim 62 or 63, wherein the particles comprise at least about 10% by volume of the material.

65. Apparatus according to any one of claims 56 to 64, wherein the means for heating the member comprise a heatable plate or jacket in contact with the member other than said shaped surface thereof.

66. Apparatus according to any one of claims 56 to 65, further comprising means for cooling an article after printing.

67. Apparatus according to any one of claims 56 to 66, including urging means for holding a carrier sheet bearing an image to be printed in contact with the article surface.

68. Apparatus according to claim 67, wherein the urging means are adapted to ensure intimate contact between the carrier sheet and regions of the surface of compound curvature.

69. Apparatus according to any one of claims 56 to 68, for printing onto a surface of article of compound curvature.

70. Apparatus according to any one of claims 56 to 69, adapted for printing on part or parts of the casing of a mobile telephone.

71. A method of printing an image on an image-receiving surface of an article, comprising placing the article on a support so as to leave exposed the image-receiving surface of the article; holding a carrier sheet bearing an image to be printed in contact with the exposed image-receiving surface of the article by means of a heat-conductive member comprising polymeric elastomer material and having a surface shaped to be substantially complementary to said image-receiving surface of the article, wherein the support has a greater hardness than that of the member; and heating the member to print the image on said surface.

72. A method according to claim 71, wherein the carrier sheet comprises a retransfer intermediate sheet bearing an image formed by dye diffusion thermal transfer printing.

73. A method according to claim 71, wherein the carrier sheet comprises a retransfer intermediate sheet bearing an image formed by electrophotography.

74. A method according to claim 71, wherein the carrier sheet comprises a retransfer intermediate sheet bearing an image formed by ink jet printing.

75. A method according to any one of claims 71 to 74, wherein the carrier sheet has a linear extensibility at the printing temperature of at least 1.5.

76. A method according to claim 75, wherein the carrier sheet has a linear extensibility at the printing temperature of at least 2.5.

77. A method according to any one of claims 71 to 76, wherein the member is pre-heated prior to contact with the sheet and article.

78. A method according to any one of claims 71 to 77, wherein the sheet is pre-heated prior to printing.

79. A method according to any one of claims 71 to 78, wherein the image-receiving surface has compound curvature.

80. A method according to any one of claims 71 to 79, wherein the article comprises thermoplastic material and is supported on a support during printing.

81. An article bearing a printed image produced by use of apparatus in accordance with any one of claims 56 to 70 or by a method in accordance with any one of claims 71 to 80.

82. An article according to claim 81, wherein the article is a mobile telephone casing or part thereof.