BACKGROUND OF THE INVENTION

Technical Field

This invention relates generally to resistive thermal printers, and more particularly to such printers having a reusable dye donor member.

Background Art

Color dye transfer thermal printers use a dye donor member which may be a sheet, but usually is in the form of a web advanced from a supply roll to a take-up roll. The dye donor member passes between a printhead and a dye receiver member. The thermal printhead comprises a linear array of resistive heat elements. In operation, the resistive heat elements of the printhead are selectively energized in accordance with data from a printhead control circuit. As a result, the image defined by the data from the printhead control circuit is placed on the receiver member.

A significant problem in this technology is that the dye donor members used to make the thermal prints are generally intended for single (one time) use. Thus, although the member has at least three times the area of the final print and contains enough dye to make a solid black image, only a small fraction of this dye is ever used.

After printing an image, the dye donor member cannot be easily reused, although this has been the subject of several patents. The primary reason that inhibits reuse of the dye donor members is that the dye transfer process is very sensitive to the concentration of dye in the donor layer. During the first printing operation, dye is selectively removed from the layer thus altering its concentration. In subsequent printings, regions of the donor member which had been previously imaged have a lower transfer efficiency than regions which were not imaged. This results in a ghost image appearing in subsequent prints.

The cost associated with having a single use donor ribbon is large because of the large area of ribbon required, as well as the large excess of dye coated on the donor member. While this technology is able to produce high quality continuous tone color prints, it is desired to provide an approach which has all of the good attributes of thermal dye transfer imaging but without the limitations associated with single use donor members.

Some work has been done by others to accomplish similar goals. U.S. Patent No. 5,334,574 describes a reusable dye donor ribbon for thermal dye transfer printing. This reusable ribbon has multiple layers containing dye which limit the diffusion of dye out of the donor sheet. This enables the ribbon to be used to make multiple prints. In addition, the ribbon may be run at a slower speed than the dye receiver sheet, enabling additional utilization. Although reusable thermal dye transfer ribbons are known, these ribbons attempt to control the diffusion of dye out of the ribbon so that they could printed multiple times, rather than enable the re-diffusion of dye back into the ribbon as in the present invention concept.

DISCLOSURE OF THE INVENTION

The invention in its broad form resides in apparatus for re-applying dye to a dye donor element of a dye transfer thermal printer including a thermal dye donor element, a printing station at which dye is image-wise transferred from the dye donor element to a receiver medium, at least partially depleting the dye donor element of dye, said apparatus characterized by:

• a reservoir containing a supply of dye; and
• means for transferring dye from the reservoir to the dye donor element by diffusion of dye into the dye donor element.

The invention in its broad form also resides in a process for reapplying dye to a dye donor element of a dye transfer thermal printer including providing a thermal dye donor element, providing a printing station at which dye is image-wise transferred from the dye donor element to a receiver medium, at least partially depleting the dye donor element of dye, said process characterized by the steps of:

• providing a reservoir containing a supply of dye; and
• transferring dye from the reservoir to the dye donor element by diffusion of dye into the dye donor element.

It is a feature of the present invention to provide apparatus for reapplying dye to a dye donor element of a dye transfer thermal printer. A reservoir containing a supply of dye that is transferred from the reservoir to the dye donor element by diffusion of dye into the dye donor element.

It is another feature of the present invention to provide apparatus for re-applying dye to a dye donor element of a dye transfer thermal printer. A reservoir containing a supply of dye has a diffusion controlled permeation membrane through which dye is delivered to the dye donor element.

It is still another feature of the present invention to provide apparatus for re-applying dye to a dye donor element of a dye transfer thermal printer. A reservoir containing a supply of dye and carrier has a diffusion controlled permeation membrane. The diffusion controlled permeation membrane inhibits diffusion of the carrier, whereby the dye partitions between the reservoir and the dye donor element but the carrier does not. The reservoir may also include a porous sub-layer covered by the diffusion controlled permeation membrane through which dye is delivered from the sub-layer to the dye donor element. Further, the reservoir may be a roller with the membrane forming a cylindrical cover for the sub-layer.

In this method, dye is thermally transferred from a reservoir to the depleted donor patch. The dye and a carrier are contained in the reservoir. The reservoir is covered with a diffusion controlled permeation membrane. With the addition of heat, dye diffuses through the membrane and is delivered to the donor patch. The dye partitions between the reservoir and the donor patch reestablishing the original dye concentration.

The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawing, which is a schematic side view of a dye donor ribbon thermal printer according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.

Referring to the drawing, a reusable dye donor member is provided, such as in the form of a belt 10 that is trained about a pair of rollers 12 and 14. At least one of the two rollers is driven to advance belt 10 past a plurality of dye reservoir rollers 16, 18, and 20; one or more re-ink heads 22; and a printhead 24 at a printing station.

Donor member belt 10 comprises a support 26 and a dye donor element such as a plurality of dye donor patches 28, 30, and 32. Any material can be used as the support for the dye-donor element of the invention provided it is dimensionally stable and can withstand the heat of the laser or thermal head. Such materials include aluminum or other metals; polymers loaded with carbon black; metal/polymer composites such as polymers metalized with 500-1000 Å of metal; polyesters such as polyethylene terephthalate, polyethylene naphthalate, etc.; polyamides (such as nomex); polycarbonates; cellulose esters such as cellulose acetate; fluorine polymers such as poly(vinylidene fluoride) or poly(tetrafluoroethylene-co-hexafluoropropylene); polyethers such as polyoxymethylene; polyacetals; polyolefins such as polystyrene, polyethylene, polypropylene or methylpentene polymers; and polyimides such as polyimide-amides and polyether-imides. The support generally has a thickness of from about 5 m to about 200 m and may also be coated with a subbing layer, if desired, such as those materials described in U. S. Patents 4,695,288 or 4,737,486.

In the illustrated embodiment, the dye donor element is form of a distinct dye donor patch on the support for each color. However, a continuous dye donor element over the entire support surface may be used, with machine logic subdividing the single element into dedicated color regions. Likewise, more than three patches may be used. The dye donor is dispersed in a polymeric binder such as cellulose and derivatives of cellulose to include cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, and cellulose triacetate, poly(vinyl acetal), poly(vinyl alcohol-co-butyral) and any of the polymers described in U.S. Patent No. 4,700,207; polyurethanes, polyesters, polyamides, polyacrylamides, acrylates, poly(vinyl alcohol), polyimides, polyethers, polystyrene, poly(siloxanes), polysulfone, polycarbonate, acrylics, gelatin, polyolefin, poly(nitrile), poly(dienes), polyacetal, polybutural and their coplymers.

A conventional dye receiver medium 34 is drawn through a nip formed between printhead 24 and a platen roller 36 by a capstan drive roller pair 38 and 40. Dye receiver medium 34 is conventional, and includes a support 42 and a receiving layer 44. Image-wise activation of linear printhead 24 causes dye to be transferred from the dye donor element of belt 10 into the dye receiving layer of medium 34; at least partially image-wise depleting portions of the patches of dye.

Dye reservoir rollers 16, 18, and 20 include a permeation membrane. Examples of membrane material include cellulose and derivatized cellulose used alone or blended with other components, polyesters, polyamides, polysufone, crosslinked polystyrene, phenol/formaldehyde resin and fluorinated polymers to include polytetrafluoroethylene and polyvinylidene fluoride, polycarbonate, poly(vinyl alcohol) and silicon containing polymers. Membranes can be constructed from a dense layer of polymer supported on a porous sub-layer. These polymeric membranes can be crosslinked to further reduce permeability.

Dye reservoir rollers 16, 18, and 20 may be replaced by wicks formed of similar materials, but not mounted for rotation.

Each dye reservoir roller is opposed by a re-ink head 22 (only one head is illustrated in the drawing), and the rollers are selectively raised and lowered into contact with belt 10 as necessary. When a dye reservoir roller is lowered to the belt, and the associated re-ink head activated, heat and/or pressure between the dye reservoir roller and belt 10 effects re-inking of the dye donor element, and the depleted dye donor layer of the patch is re-saturated with dye from the dye reservoir roller.

In this method, dye is thermally transferred from a reservoir to the depleted donor patch. The dye and a carrier are contained in the reservoir. The reservoir is covered with a diffusion controlled permeation membrane. With the addition of heat dye diffuses through the membrane and is delivered to the donor patch. The dye partitions between the reservoir and the donor patch reestablishes the original dye concentration.

It is a feature of one aspect of the present invention that, during the re-diffusion, dye separates from the solvent. A semi-permeable membrane allows only the dye to diffuse out of the dye supply and into the donor member. Solvent is retained within the supply. Other methods of replenishment require that solvent is removed either prior to the replenishment step (intermediate transfer) or after transfer of dye to the donor ribbon. Solvents must be volatile in these alternative approaches. In addition, the removal of solvent results in more complex hardware as well as the potential health and safety problems associated with this process.

Dye transfer from the reservoir through the semi-permeable membrane may not require any carrier solvent. In a solid dye transfer mechanism, dye would melt and diffuse through the membrane to re-ink the donor patch.