Thermal Printing Technologies
Printing technologies that employ the controlled application of thermal energy via a contacting printhead to activate either physical or chemical image formation processes come under this general classification. There are four thermal technologies in current use: direct thermal, direct thermal transfer, dye diffusion thermal transfer, and resistive ribbon thermal transfer.
Direct Thermal
This is the oldest and most prolifically applied thermal technology. The imaging process relies on the application of heat to a thermochromic layer of approximately 10 µm in thickness coated onto a paper substrate. The thermally active layer contains a leuco dye dispersed along with an acid substance in a binder. Upon heating fusion melting occurs resulting in a chemical reaction and conversion of the leuco dye into a visible deeply colored mark. Key to this process is the design of the printhead, which can be either a page-wide array or a vertical array or a scanning printhead. Two technologies are in use, thick film and thin film. Thick-film printheads have resistor material between 10 and 70 µm. The resistive material is covered with a glass layer approximately 10 µm thick for wear resistance. The thin-film printheads bear a strong resemblance to those found in thermal ink jet printheads. They employ resistive material, such as tantalum nitride, at 1 µm thickness and a 7-µm-thick silicon dioxide wear layer. Thin-film heads are manufactured in resolutions up to 400 dpi. In each case the resistors are cycled via electrical heating pulses through temperature ranges from ambient (25◦C) up to 400◦C. Overall, the thin-film printheads excel in energy efficiency conversion, print quality, response time, and resolution. For these reasons the thin-film printheads are used when high resolution is required, whereas the thick-film printhead excels in commercial applications such as bar coding, airline tickets, fax, etc.
FIGURE 23.11 Schematic of wax transfer process: (a) Intimate contact between printhead, ribbon, and paper is required for successful transfer, (b) design elements of thin-film thermal printhead. Thermal barrier insulates heater for the duration of the heat pulse but allows relation of heater temperature between pulses.
Direct Thermal Transfer
These printers transfer melted wax directly to the paper (Fig. 23.11(a) and Fig. 23.11(b)). The wax that contains the colorant is typically coated at 4 µm thickness onto a polyester film, which, in common implementations, is approximately 6 µm thick. A thermal printhead of the kind described previously presses this ribbon, wax side down, onto the paper. As the individual heating elements are pulsed, the wax melts and transfers by adhesion to the paper. The process is binary in nature; but, by the use of shaped resistors, which produce current crowding via an hourglass shape, for example, the area of wax transferred can be modulated. Common implementations employ page width arrays at 300 dpi with some providing vertical addressability of 600 dpi. The thermal ribbons are also packaged in cassettes for scanning printhead designs in desktop and portable printers. Power consumption is an issue for all thermal printers, and efforts to reduce this for direct thermal transfer have focused on reducing the thickness of the ribbon.
Dye Diffusion Thermal Transfer
This technology involves the transfer of dye from a coated donor ribbon to a receiver sheet via sublimation and diffusion, separately or in combination. The amount of dye transferred is proportional to the amount of heat energy supplied; therefore, this is a continuous tone technology. It has found application as an alternative to silver halide photography, graphics, and prepress proofing. As with all thermal printers the energy is transferred via a transient heating process. This is governed by a diffusion equation and depending on the length of the heating pulse will produce either large temperature gradients over very short distances or lesser gradients extending well outside the perimeter of the resistor. Much of the design, therefore, focuses on the thicknesses of the various layers through which the heat is to be conducted. In the case of thermal dye sublimation transfer a soft-edged dot results, which is suitable for images but not for text. Shorter heating pulses will lead to sharper dots.
Resistive Ribbon Thermal Transfer
This technology is similar to direct thermal transfer in that a thermoplastic ink is imaged via thermal energy onto the substrate. The ribbon is composed of three layers: An electrically conductive substrate of polycarbonate and carbon black (16 µm thick), an aluminum layer 1000–2000 A˚ , and an ink layer which is typically 5 µm. The aluminum layer serves as a ground return plane. Heat is generated by passing current from an electrode in the printhead in contact with the ribbon substrate through the polycarbonate/carbon layer to the aluminum layer. The high pressure applied through the printhead ensures intimate contact with the paper, which does not have to be especially smooth for successful transfer. Printed characters can be removed by turning on all electrodes at a reduced energy level and heating the ink to the point that it
bonds to the character to be corrected but not to the paper. The unwanted character is removed as the printhead passes over it. This technology does not adapt to color printing in a straightforward way.