Transfer printing.

0001This invention relates to transfer printing on to a painted substrate.

0002From one aspect the present invention provides a method of transfer printing in which a continuous length of strip is coated with a thermo-setting material and brought into contact immediately after curing whilst at a temperature of between 180°C and 280°C with a continuous strip of printed material, the contact being effected during the passage of both strips between resiliently surfaced rollers at a pressure of at least 50 p.s.i., the ink print being transferred to the strip by sublimation .

0003The ink may be printed on a paper substrate and the strip to which this ink is transfer printed may be a steel strip on to which e.g. an alkyd, polyester, polyurethane or epoxy paint has been applied. This paint may in turn be surfaced with a thermo-setting lacquer.

0004In accordance with this invention then transfer printing on to a metal substrate is effected in a continuous line which has not been achieved before and very high speeds may be achieved, e.g. 10-100 metres per minute, utilising the residual heat in the strip following the paint curing. Since this is the sole source of heat, the paper roll is "cold", a significant saving in energy is additionally achieved compared with single sheet batch processes adopted hitherto or "web" transfer, where a continuous paper web is held under pressure over a_heated roll, around which is wrapped the material to which the printing is to be applied.

0005In order that the invention may be fully understood, one embodiment thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawing which shows part of a continuous strip coating line on which transfer printing is effected in accordance with this invention.

0006Referring now to the drawing, a steel strip la which has been prepared, primed and coated on its upper surface with a thermo-setting paint by e.g. a roller coater, including (optionally) a surface coating of thermo-setting lacquer, issues from a curing oven 2 at a temperature of between 190°C and 250°C, preferably around 230°C, at a speed of say 25 to 40 metres per minute. Twin idler rollers 3,4 are sited upstream of the oven. These rollers have a siliconised rubber coating on their outer surface and are water-cooled internally by spray units 5,6. More particularly, the surface of the upper roller (which is fixed) has a typical Shore hardness of 70 whilst the surface of the lower (hydraulically movable) roller has a Shore hardness of 90. A pay-off reel 7 from which printed paper 8 is dispensed and a take-up reel 9 for collecting this (exhausted) paper flank the roller 3, and the paper together with the coated strip pass between the nip of rollers 3,4, via a bowed anti- creasing 'Mount Hope' roller 10 and a diverter roll 12. Each of the reels 7 and 9 is provided with a clutch mechanism and a brake in order to maintain the correct tension, and the whole roller assemblies may be steered to maintain alighment.

0007The paper is such that it is non-absorbent to the ink, and any convenient printing process may be employed to impart the pattern, e.g. photogravure flexography, screen printing, letterpress or photolithography.

0008The print on the paper is in contact with the painted surface of the steel strip under pressure, typically around 1000 p.s.i., at the area of roller contact and at the elevated temperature of the strip the dye in the ink sublimes, that is, it transposes directly from the solid to the gaseous phase without melting; the resulting chemical change in the contacting painted steel strip yields a very definitive and accurately reproduced copy of the original print in this painted surface. The period of contact - which is almost 'line' contact save for the yielding displacement of the resilient surfaces of the rollers 3,4 - is very short, the patterned painted strip lb then passing through a quenching station 13 before being waxed, if required, and coiled for dispatch.

0009The paint may or may not have a colouring pigment and as mentioned it may be surfaced with a clear lacquer.

0010The ink employed may contain dissolved or finely dispersed dyes, which of course sublime under the conditions stated, a solvent mixture - advantageously anhydrous - and a binder or thickener which is stable to heat.

0011The continuous coating line on which this process may be adopted may be quite conventional embodying the usual treatment stations, tension levellers, accumulators and stitching stations (for joining coils).

0012Thus the method may readily be adopted in existing plant consistent with siting the equipment immediately 'downstream' from the final curing oven so that the residual temperature of the strip may be utilised for the sublimation phenomenon.

0013Although the invention has been described with reference to the particular embodiment illustrated, it is to be understood that various modifications may readily be made without departing from the scope of this invention. For example, steel is only one substrate medium, other metals, or nonmetals provided they retain sufficient heat following curing, may readily be coated. Further the inked pattern may be deposited on a medium other than paper, the only essential prerequisite being that the dye/dyes be transferable by sublimation. Again, the pressures adopted may vary dependent on the various material characteristics and other operating parameters; 1000 p.s.i. has been disclosed as being typical but other pressures in excess of 50 p.s.i. and up to say 1300/1400 p.s.i. may readily be utilised.