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Electrostatic printers use electrostatic processes found in normal office copiers. First, the surface of a light-sensitive drum (or sometimes a belt) is given an electrical charge. Next, a light shines on portions of the surface, causing the drum to lose its charge where struck by light. The drum surface then passes through a toner which is attracted to the drum areas not struck by the light. The toner is then deposited and bonded to the media by a fuser. Some electrostatic copiers provide four-color toner (C, M, Y, K) cartridges that require four exposure passes to produce color output. Such copiers are very expensive.
There are two types of ink jet printers—continuous and drop on demand. Continuous ink jet printers produce a continuous stream of electrically charged ink drops which may be of varying size depending on the particular printer. Unwanted ink drops are collected into a print head ink gutter using electrical force and returned to the ink well while the remaining drops impact the media. Continuous ink jet printers are classified as either vibrated (drops produced by vibrations) or non-vibrated.
Drop on demand printers produce individual drops under printer control and are either piezoelectric or thermal. Piezoelectric printers use either liquid colorant or solid colorant (called hot melt) consisting of solid wax blocks, pellets, or powder. The colorant is melted by a heating element and fed to a print element. Supplying electrical pulses to the one or more actuators connected to a print element pumps (similar to a piston) the actuator, forcing colorant out a nozzle and onto the media. The colorant instantly solidifies when it strikes the paper, avoiding ink wicking (seepage or bleeding). Applying heat to the media fuses the colorant to the media which can be any type of paper. Piezoelectric printers produce poor transparency output.
Thermal ink jet printers operate the same way and use either disposable print cartridges (Hewlett-Packard) or permanent print heads (Canon). HP invented thermal ink jet technology in 1979 and developed the first disposable print head in 1982. HP’s first ink jet product was the 96 dpi ThinkJet printer announced in 1984. In 1988, HP announced its first 300 dpi DeskJet printer.
HP ink jet printer cartridge print heads use thin-film semiconductor technology suitable for high volume manufacturing. Ink jet cartridges may have one or more color ink wells, each with their own set of print elements. A print element produces ink spots under printer control. It resides on a monocrystalline silicon substrate and consists of a microscopic chamber with an electrically resistive heating element connected to an electrical conductor. One side of the chamber has a small hole, or nozzle, that faces the media.
To emit an ink drop, 5 to 20 microjoules of electrical power are applied to the heating element which heats at 100º C per microsecond and heats to over 500º C before power is removed. After 15 microseconds, an ink drop begins emerging from the hole, propelled by vapor bubble pressure caused by vaporized ink. After 25 microseconds, the ink drop escapes the nozzle, the remaining ink subsequently regroups, and the nozzle meniscus helps keep the chamber filled with ink by sucking ink from the ink well.
Because HP ink jet printers use multiple, fast-response nozzles, it achieves 300 dpi resolutions. However, because ink drops arrive propelled by expanding vapor, small aerosol drops may impact around the primary drop impact area, producing satellites that reduce effective resolution. When ink drops strike normal paper media, paper fibers absorb the ink vertically and horizontally, causing ink bleed as the ink diffuses (spreads over a wider area). It follows that using specially treated media that restricts ink bleed substantially improves actual output resolution by minimizing dot gain. It also eliminates the surface warping caused by soaking paper with ink.
Thermal transfer printers are also known as thermal wax and dot thermal transfer printers. Fujicopian Co. LTD (Japan) developed the technology and holds the fundamental thermal transfer technology patent. Thermal transfer printers use a continuous roll ribbon, sometimes called a donor, with page-sized, alternating CMY sheets (bands). The ribbon cannot be reused and the colorants usually require special smooth paper to avoid spotty colorant adhesion. The paper is usually clay-coated to improve colorant adhesion though Tektronix has a novel approach that uses a special ribbon that applies a special foundation to normal paper.
Thermal transfer printers require one complete pass per color plane, also called a pane, to produce color output. During each pass, the colorant is spot heated, typically at 300 dpi, and pressed into the media (paper or transparencies). Because there is little colorant bleed, thermal printers produce excellent detail and rich, saturated colors. However, the colorant can scratch or crack if the media is folded.
Thermal transfer printers initially used a 5.7 mil ribbon substrate and produced fixed size dots. In 1994, Fujicopian Co. LTD introduced a thinner, 3.5 mil donor ribbon. This allows variable sized dot transfers since printing operations require less heat and provide better control. Combined with a new film-like paper with micropores (that absorbs colorant into paper rather than allowing it to bleed) and a new rectangular pixel print head, thermal printers can now produce 16 intensities per color per dot (4 bits per pixel). One study indicates that under proper conditions, the human eye may perceive this as continuous color output.
Dye sublimation is also known as thermal dye diffusion, dye diffusion, and dye sub. Dye sublimation requires carefully matched colorants and media consisting of special paper or polyester-coated transparencies. Like thermal transfer, the media makes multiple passes. However, with dye sublimation, discrete thermal elements vaporize a sublimable dye(s) on ribbon. The gas vapor diffuses onto the special receiver media and is absorbed by a polyester coating. Controlling the discrete heating elements controls the colorant vapor amount, producing near-photographic quality using 24 bits per color. Dye sublimation is excellent for full page color since diffusion blends the colorants smoothly, though the diffusion reduces detail. Dye sublimation printers and print page costs are very expensive.
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