A typical multi-color dye donor web that is used in a dye transfer or thermal printer is substantially thin and has a repeating series of three different rectangular-shaped color sections or patches such as a yellow color section, a magenta color section and a cyan color section. In addition, there may be a transparent colorless laminating section immediately after the cyan color section.
Each color section of the dye donor web consists of a dye transfer area which is used for dye transfer printing and a pair of opposite longitudinal edge areas alongside the dye transfer area which often are not used for printing. The dye transfer area may be about 152 mm wide and the two longitudinal edge areas may each be about 5.5 mm wide, so that the total web width is approximately 163 mm.
To make a multi-color image print using a thermal printer, a motorized donor web take-up spool draws a longitudinal portion of the dye donor web off a donor web supply spool in order to successively move an unused single series of yellow, magenta and cyan color sections over a stationary liner array (bead) of selectively heated resistive elements on a thermal print head between the supply and take-up spools. Respective color dyes within the yellow, magenta and cyan color sections are successively heat-transferred line-by-line, via the selectively heated resistive elements, onto a dye receiver medium such as a paper or transparency sheet or roll, to form the color image print. The selectively heated resistive elements often extend across the entire width of a color section, i.e. across the dye transfer area and the two longitudinal edge areas comprising that color section. However, only those resistive elements that contact the dye transfer area are selectively heated. Those resistive elements that contact the two longitudinal edge areas are not heated. Consequently, the dye transfer occurs from the dye transfer area to the dye receiver medium, but not from the two longitudinal edge areas to the dye receiver medium.
As each color section is drawn over the selectively heated resistive elements, it is subjected to a longitudinal tension particularly by the forward pulling force of the motorized donor web take-up spool. Since the dye transfer area in the color section is heated by the resistive elements, but the two longitudinal edge areas alongside the dye transfer area are not, the dye transfer area is significantly weakened and therefore vulnerable to stretching as compared to the two longitudinal edge areas. Consequently, the longitudinal tension will stretch the dye transfer area relative to the two longitudinal edge areas. This stretching causes the dye transfer area to become thinner than the non-stretched edge areas, which in turn causes some creases or wrinkles to develop in the dye transfer area, most acutely in those regions of the dye transfer area that are close to the non-stretched longitudinal edge areas. The creases or wrinkles occur most acutely in the regions of the dye transfer area that are close to the non-stretched edge areas because of the sharp, i.e. abrupt, transition between the stretched (thinner) transfer area and the non-stretched (thicker) edge areas.
As the dye donor web is pulled by the motorized donor web take-up spool over the selectively heated resistive elements, the creases or wrinkles tend to spread from a trailing (rear) end portion of a used dye transfer area at least to a leading (front) end portion of the next dye transfer area to be used. A known problem that can result is that the creases in the leading (front) end portion of the next dye transfer area to be used will cause undesirable line artifacts to be printed on a leading (front) end portion of the dye receiver medium. The line artifacts printed on the dye receiver medium, although they may be relatively short, are quite visible.
The question presented therefore is how to solve the problem of the creases or wrinkles being created in an unused transfer area so that no line artifacts are printed on the dye receiver medium during the dye transfer.
The Cross-Referenced Applications and Patent
The cross-referenced applications each disclose a thermal printer capable of preventing crease formation in a dye transfer area of a dye donor web that can cause line artifacts to be printed on a dye receiver during the dye transfer from the dye transfer area to the dye receiver. To prevent crease formation, there is provided a crease-preventing platen roller that is movable to hold a dye transfer area and the two longitudinal edge areas alongside the dye transfer area against a print head. The crease-preventing roller has a pair of roller end portions that apply a constant pressure against the two longitudinal edge areas, and a roller main portion between the roller end portions that applies a lesser pressure against the dye transfer area. Since the pressure applied against the two longitudinal edge areas is greater than the pressure applied against the dye transfer area, the two longitudinal edge areas will be stretched the same as the dye transfer area, so that creases will not be formed in the dye transfer area. This is so notwithstanding that the dye transfer area is heated by the print head, but the two longitudinal edge areas are not.
In contrast to the cross-referenced applications, the referenced incorporated (prior art) patent discloses a thermal printer that is adapted to optimize print image quality by preventing undesired pressure variations along the line of contact between the dye donor web and the linear array of selectively heated resistive elements in the thermal print head (the patent does not discuss the problem of crease formation). To optimize print image quality, there is provided a plurality of pressure applying rods that bear down on the thermal print head to urge the selected heated resistive elements into pressure contact with the dye donor web. The amount of pressure applied by each rod and the location of each rod along the print head is individually adjusted in response to sensed changes in different operating parameters that negatively affect print image quality, such as print head temperature when printing dark vs. light image portions, and thickness and/or stiffness of the dye donor web.