Thermal printing technology eliminates the need for impact altogether by using heat to form characters and graphic images on paper. Instead of fomting dots through a matrix of electromechanical solenoids, thermal print heads employ an array of resistive semiconductor heating elements (called dot heaters). Electrical pulses from the printer's driver circuits transfer energy to each desired dot heater causing them to heat up rapidly. This in turn will discolor the appropriate points on temperature-sensitive paper or a thermal transfer ribbon. Thermal dot-matrix (TOM) print heads are available in serial (moving head) and line-head versions, which can be chosen depending on the particular application. Before you learn about the operation of thermal printing in detail, however, you must understand the way in which TDM heads are constructed.
Thermal Head Fabrication
TDM heads are not built as an assembly of other parts the way other heads are. They contain no wires, solenoids, linkages, or other individual parts. Instead, they are fabricated much like an integrated circuit; one layer of material at a time is deposited onto previous layers in masked patterns that form resistive elements, their interconnections, electrical insulators, and protective coatings. Three popular fabrication methods are discussed as follows. Thermal head thick-fllm fabrication is shown in 3-11. A semiconductor resistance element is deposited between two conductors and covered with a protective glass film. Everything is fabricated onto a ceramic substrate (or support structure) insulated by a glass insulating layer. Thick-film technology can achieve resolutions of 12 dots per millimeter. Each dot heater exhibits a resistance that can range from 160 to 3,000 n depending on the number of dots and the print head's intended application. Each dot is roughly round and faintly defined. The working temperature of each dot can easily exceed 350°C at the resistive element itself, yet typical thick-film heater elements will last for more than 30 million pulses, and they are highly resilient to power overload conditions. Line-print head versions are often manufactured using thick-film technology. Another issue to consider with TDM print heads is wear; after all, the print head must be in physical contact with the page surface (or thermal transfer ribbon). Over time, the friction encountered at the page surface will wear away the TDM head. Today, protective film coverings can endure more than 30 kilometers of wear against the page surface before failure. typical thin-film fabrication technologies. Both s are variants of the same approach. A ceramic substrate and glass insulating layer form the foundation of a thin-film heater. Unlike the thick-film technique, a thin-film resistor is much thinner, and is fabricated underneath its electrical conductors. Finally, two separate protective layers are added. Thin-film technology offers several performance differences versus thick-film devices. Thin-film devices can achieve resolutions as high as 16 clots per millimeter. Each dot heater can range from 1.5 to 50 n depending on the number of dots and intended application of the head. Dots appear square and are sharply defined. Thin-film dot heaters can easily survive more than 50 million pulses, but they are not very tolerant of driver overloads. The hard tantalum covering can undergo more than 70 kilometers of surface wear, more than t\vice that of a thick-film covering. Thin-film technology can also be used to manufacture either serial or line-print heads.
Serial Head Operation
A thermal heater can be fired much the same way as an impact solenoid as shown in 3-14. Data for such a dot driver circuit is applied to a tri-state buffer. Note the extra input marked "print." This is an enable input that controls the tri-state buffer. While "print" is brought logic high, the buffer's output state will equal its input state (i.e., a logic 1 input will yield a logic 1 output, and vice versa). This enable line is very handy because it allows data to be set up in advance on any number of dot heaters. The "print" signal can then be strobed on all drivers simultaneously. For the simple driver circuit of 3-14, a logic 1 will fire a dot heater, and a logic 0 will not. Each dot heater will have its own driver circuit.
Line Head Operation
TDM line-print heads take a bit more complicated approach. Instead of assembling images as a series of vertical dots scanned from side to side, a line-print head uses a single horizontal row of dot heaters to assemble complete images as paper advances.a ROHM TDM line-print head. One dot heater is available for every possible point in a horizontal line. Depending on the width of a head (which will probably be as wide as the paper it is printing), there might be as few as 160 or as many as 7,000 dot heaters.
Heat generated by each dot must be transferred to a page surface as required in order to form permanent images. This is accomplished in either of two ways: direct contact or thermal transfer. As the name implies, direct contact requires that a print head be in direct contact with the paper surface. Heat is used to activate heat-sensitive chemicals in the paper, which cause those points to discolor and leave permanent marks. Most heat-sensitive papers generate either blue or black marks. This offers a simple, reliable printing approach, but it is not without its disadvantages. Perhaps the most notable disadvantage is that standard paper (such as 20-lb bond xerography-grade paper) cannot be used. Temperatures developed by a thermal print head are insufficient to cause discoloration on normal paper without burning it. Heat-sensitive chemicals cause thermal paper to discolor at much lower temperatures. Such sensitized paper is usually delicate and sensitive material. It only has a limited shelf life (typically a few years). Age, sunlight, humidity, and a variety of chemical vapors will ruin the paper. Finally, thermal paper is manufactured on long, continuous rolls. Its tendency to curl makes it difficult to handle.
Advantages and Disadvantages of TDM Printing
Thermal dot-matrix technology enjoys several advantages over other printing methods. First, TDM is very quiet. Even though contact is required, there are no moving parts to generate noise. Power consumption is also very low in TDM heads. Thermal dot heaters in serial heads often require less than 2 W per dot. Lineprint heads need even less (in the range of 0.2 W per dot). Compare this to a DMI head that usually takes more than 12 W per solenoid. Low power consumption makes TDM heads ideal for mobile and battery-powered applications. TDM heads are simple and reliable devices. They are totally self-contained; there are no moving parts or apertures that can jam or clog. About the only preventive maintenance required for TDM heads is an occasional wipe-down to remove accumulations of dust and dirt. Line-print heads simplify the printer by eliminating the need for a carriage mechanism. They also incorporate on-board circuitry for data handling and dot driving. This too can greatly simplify the printer's corresponding ECU. Print is clear and crisp, with resolutions that can rival that of an electrophotographic printer. TDM print heads have a few disadvantages. Print time is slightly slower than other printing methods because of cooling time added to each printing cycle. This is just a slight difference. Thermal head life is shorter than most DMI heads, largely due to the surface wear caused by paper friction. TDM heads are usually good for 10 million characters or less, where DMI heads can support 50 million characters or more. Finally, TDM heads cannot be serviced; if a dot heater burns out or its internal circuitry fails, the entire head assembly must be replaced.
