The present invention relates to apparatus and a method for holding a dye donor element against a dye receiver element in dye transfer printing.

In dye diffusion thermal transfer printing, dye donor and receiver elements, such as sheets, are held in intimate contact with one another, and selected regions of the donor sheet are heated to cause dye to diffuse from the heated regions into the receiver sheet to form a corresponding print image therein. The heat may be supplied by a thermal print head comprising a number of independently heatable elements, or by a modulated scanning laser beam. Transfer may also be initiated in other ways, such as by ultrasound, and, instead of diffusion, the dye may transfer by melting or sublimation. In the latter case, the dye is vaporised and crosses an air gap between the two sheets (the gap may be provided by beads embedded in the donor sheet) .

Typically, the receiver sheet is mounted on the surface of a platen roller, and a dye donor ribbon is pressed against the receiver sheet along a contact line adjacent a print station where a laser scans' across the roller. Printing is carried out line by line as the receiver sheet is rotated past the print station.

In such systems, material handling is not particularly straightforward, and the present invention aims to provide an alternative and improved apparatus and method for mounting dye donor and receiver elements together during dye transfer printing.

Viewed from a first aspect, therefore, the present invention provides printing apparatus comprising a (preferably substantially flat) support bed on which may be mounted dye donor and receiver elements one above the other, and a resilient pressure member having an arcuate surface convex to the bed, the member and bed being

relatively movable towards and away from one another to sandwich and release the donor and receiver elements therebetween, the arcuate surface of the pressure member deforming as it contacts the donor and receiver elements such that it lies against the donor and receiver elements and presses them together over a region across which printing is to occur.

Such apparatus is inexpensive and simple to use. The deformed arcuate surface firmly holds the donor and receiver elements in intimate contact with one another, and squeezes out air bubbles and wrinkles from between the two elements as it deforms and gradually presses the elements together over more and more of the print region. As a flat bed is preferably used, the material handling may be made much easier, and the invention is particularly useful in printing credit and security cards, glass slides and other flat objects.

The pressure member may be solid, for example a shallow arc section pad, and made of, for example, a sponge-like material.

Preferably, however, the pressure member comprises a portion of a sprung hoop.

When using a pad, it can be difficult to ensure that an even pressure is applied across the whole print region, as most of the pressure provided by the deformation of a solid member would tend to be applied to the middle of the print region, and the donor and receiver elements would tend not be held in close enough contact at the peripheries of the print region. By using a hoop, the pressure is able to be applied more evenly over the whole print region.

The hoop is preferably mounted so that it naturally reverts to a substantially semi-circular or a near semi- circular cross-section when not pressed against the bed. This ensures a more even pressure when the hoop is depressed.

The hoop may be made of any suitable materials, for

example nylon, but is preferably of metal, such as a sheet of sprung steel. It is preferred for the surface of the pressure element to be smooth, so that an even pressure is applied across the print region, without pockets appearing in which the donor and receiver elements may not be in intimate contact. The surface should also not damage the receiver or donor elements, and the pressure member may be coated with a suitable material, such as soft rubber, to provide these properties.

Preferably, means are provided for exerting further pressure on the centre region of the hoop. This could, for example, take the form of a resilient pad or a smaller, second, inner hoop provided within the main hoop. The second hoop may have a higher curvature than the main outer hoop. Such means are advantageous in situations where the print regions are large and where the outer hoop may have a tendency to rise at its centre. Such rising may be undesirable, as the donor and receiver elements may not remain in intimate contact across the raised area. The force applied by, for example, an inner hoop is able to prevent this rise from occurring.

Dye transfer may be carried out in any suitable manner and may be by thermal transfer. Heating is preferably provided by a scanning laser beam or an array of laser beams. The beams could be applied through the pressure element, if transparent to the laser radiation, but, preferably, the support bed or a portion thereof is transparent, and the laser light is imaged therethrough, as this provides for a more simple arrangement. Any suitable scanning system could be used, and the support bed and pressure member may be movable together relative to the beam to provide a scanning action. The invention is particularly suited to colour printing when using a receiver sheet transparent to laser light, e.g. 35 mm slides or other transparencies. The receiver may then be mounted on the transparent

support bed, and colour printing may be carried out using dye sheets of different colours placed sequentially over the receiver, or by using a dye ribbon wound on sequentially from one colour region to another. As the receiver does not move, there is no need to re¬ register it after each individual colour print has been made.

Instead of using a hoop, which applies pressure firstly to the centre o ^' f the donor and receiver elements and then outwardly towards their edges, the pressure member could comprise a coiled spring fixed at one end above the support bed, and including means for uncoiling the spring over the print area. This may be achieved by moving its centre, e.g. its free end, across the surface of the print area. As the spring uncoils, it will automatically press against the donor and receiver elements, and will squeeze out any air bubbles and wrinkles by exerting pressure gradually along the elements and over the whole print region from one end to the other. The spring may for example be a coiled sheet of steel.

Accordingly, from a further aspect, the invention provides printing apparatus comprising a (preferably substantially flat) support bed on which may be mounted dye donor and receiver elements one above the other, and coiled spring means mounted above the support bed so that on being uncoiled the spring means is brought into contact with the dye and receiver elements to press the elements together against the bed over a region across which printing is to occur.

As with the hoop embodiments, the use of a spring, i.e. an arcuate strip-like resilient pressure means, as opposed to a solid member, allows for an even application of pressure to the receiver and donor elements. In a preferred form, therefore, the invention provides a printing station comprising a laser source, a support bed, and a sprung arcuate sheet or strip-like pressure means relatively movable towards and away from

the bed, with receiver and donor elements being mountable, one above the other, between the bed and the pressure means, wherein the pressure means deforms to lie against the donor and receiver elements, thereby pressing the donor and receiver elements into contact with one another against the bed.

The donor and receiver elements may take any suitable forms, such as ribbons or individual sheets. Where the dye donor element is a ribbon, it may be wound on to a cassette, and the hoop may press against the ribbon through a window in the cassette.

As printing is preferably conducted on a flat bed, the receiver could comprise coated glass, and in 35 mm slide applications, glass thereby printed to could be mounted directly in a slide holder.

The invention also extends to a method of dye transfer printing, in which donor and receiver elements are held together by placing the elements between a (preferably substantially flat) support bed and a resilient pressure means having an arcuate surface convex to the bed, and by moving the pressure means and the bed towards one another such that the arcuate surface of the pressure means deforms to lie against the elements and thereby presses them into contact with one another against the support bed.

The invention further extends to apparatus and methods as above, but wherein the support bed is not flat, but may, for example, be arcuate, as the resilient means, such as the sprung steel hoop, may still conform to the support bed and sandwich the donor and receiver elements therebetween to provide an advantageous hold- down system.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows, in schematic form, a double hoop embodiment of the present invention, in a non-printing position;

Figure 2 shows the Figure 1 embodiment in its printing position;

Figure 3 shows a steel spring embodiment of the invention, in a non-printing position; and Figure 4 shows the Figure 3 embodiment in its printing position.

Referring to Figures 1 and 2, the print station comprises a flat support bed 1 having a window 2 therein which is transparent to a scanning laser beam 3. A pressure means 4 is mounted above the support bed 1, and comprises a sheet of sprung steel as an outer hoop 5 and a sheet of sprung steel as an inner hoop 6 both mounted on a fixing plate 7 movable towards and away from the bed 1. When printing, a receiver sheet 8 and a dye sheet 9 are mounted one above the other on the bed 1 over the window 2. Plate 7 is then lowered, so that hoops 5 and 6 press the receiver sheet 8 and dye sheet 9 into intimate contact with one another against the support bed 1.

During movement of the pressure means 4 towards the bed 1, hoop 5 firstly presses the receiver sheet 8 and dye sheet 9 together along their centres, and then gradually applies pressure outwardly as it deforms to lie flat. Inner hoop 6 is deformed over a smaller area, and applies pressure mainly to an inner centre region of the outer hoop 5, so as to prevent it from bending upwardly at its centre away from the sheets 8 and 9. With the receiver sheet 8 and dye sheet 9 thus firmly held together in intimate contact, the laser beam 3 may be scanned across the window 2, and modulated to heat selected regions of the dye sheet 9, and thus transfer dye to the receiver sheet 8 to produce a print image therein. In this particular arrangement, the receiver sheet 8 must be transparent to the laser light (for example, it may be an overhead projector transparency or a 35 mm slide) . Of course, the receiver sheet 8 and dye sheet 9 may be arranged in reverse, so

that it is the dye sheet 9 which rests on the support bed 1. The receiver sheet 8 need not then be transparent, and may, for example, comprise a credit or security card. To provide scanning over the whole of the print region defined by the window 2, the laser beam may be scanned in one direction, and the support bed 1 and pressure means 4 may be moved together perpendicular to this scanning direction. In a specific embodiment put into practice, the outer and inner hoops 5 and 6 were of sprung steel 0.1 mm thick and 50 mm wide, with respective lengths of 135 mm and 103 mm. The outer hoop 5 was coated with soft rubber to prevent damage to, and to ensure good contact with, the dye sheet 9. The support bed window 2 was of 2 mm thick glass', and from first contact with the dye sheet 9, the plate 7 was movable a further 13 mm towards the bed 1. The system was used with a 150 mW diode laser (SDL 5422) of 819 nm to produce a print using standard ICI dye and receiver sheets.

Figures 3 and 4 show a further embodiment of the invention, in which the pressure means 4 comprises a coiled steel spring 10 mounted at one end to the plate 7, the plate 7 being fixed in position relative to the bed 1. A rod 11 is mounted to the other end' of the spring 10, i.e. at its centre, and, by moving the rod 11 in the direction of the arrow across the window 2 (using, for example, a crank - not shown) , the spring 11 gradually uncoils and presses down against the receiver sheet 8 and dye sheet 9, from one end to the other to hold them firmly together over the window 2.