Color transfer thermal printers use a color 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 color 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 color 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 colorant to make a solid black image, only a small fraction of the color is ever used.
After printing an image, the color donor cannot be easily reused, although this has been the subject of several patents. The primary reason that inhibits reuse of the color donor is that the color transfer process is very sensitive to the concentration of the colorant in the donor layer. During the first printing operation, color is selectively removed from the layer thus altering its concentration. In subsequent printings, regions of the donor 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 colorant coated on the donor member. While this technology is able to produce high quality continuous tone prints, it is desired to provide an approach which has all of the good attributes of thermal color transfer imaging but without the limitations associated with single use donor members.
Some work has been done by others to accomplish similar goals. For example, U.S. Pat. No. 5,286,521 discusses a reusable wax transfer ink donor ribbon. This process is intended to provide a dye donor ribbon that may be used to print more than one page before the ribbon is completely consumed. U.S. Pat. No. 4,661,393 describes a reusable ink ribbon, again for wax transfer printing. U.S. Pat. No. 5,137,382 discloses a printer device capable of re-inking a thermal transfer ribbon. However, again the technology is wax transfer rather than dye transfer. In the device, solid wax is melted and transferred using a roller onto the reusable transfer ribbon.
U.S. Pat. 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. U.S. Pat. No. 5,118,657 describes a multiple use thermal dye transfer ink ribbon. This ribbon has a high concentration dye layer on the bottom and low concentration dye layer on the top. The low concentration dye layer meters or controls dye transfer out of the ribbon. This enables the ribbon to be used multiple times. U.S. Pat. No. 5,043,318 is another example of a thermal dye transfer ribbon which can be used multiple times.
Another problem with the resistive head thermal printers described above is high the large amount of energy used by thermal resistive printing. This is because the heat generated by resistive printing head must first heat the support layer before the heat reaches the color donor layer. While the color donor layer can be quite thin and therefore have a low heat capacity, the support layer must be relatively thick, particularly if the support is to be used for multiple printing events without tearing or distorting from the insult of the printing operation.
Yet another problem of the printers described above is that of low resolution images. As the heat from the printhead moves through the support toward the color donor layer, the heat spreads in all directions, so that the spot of color printed is necessarily larger than when it is first generated at the print head. This means the final image is of lower resolution then might be possible with a thinner color donor web.