An inkjet printer is a type of computer printer that reproduces a digital image by propelling variably-sized droplets of liquid material (ink) onto a page. Inkjet printers are the most common type of printer and range from small inexpensive consumer models to very large and expensive professional machines.
The concept of inkjet printing dates back to the 19th century and the technology was first developed in the early 1950s. Starting in the late 1970s inkjet printers that could reproduce digital images generated by computers were developed, mainly by Epson, Hewlett-Packard and Canon. In the worldwide consumer market, four manufacturers account for the majority of inkjet printer sales: Canon, Hewlett-Packard, Epson, and Lexmark.
The emerging ink jet material deposition market also uses ink jet technologies, typically piezoelectric crystals, to deposit materials directly on substrates.
Technologies
There are two main technologies in use in contemporary inkjet printers: continuous (CIJ) and Drop-on-Demand (DOD). Drop-on-demand is further divided into thermal DOD and piezoelectric DOD.
Continuous inkjet
The continuous inkjet method is used commercially for marking and coding of products and packages. The idea was first patented in 1867, by Lord Kelvin and the first commercial devices (medical strip chart recorders) were introduced in 1951 by Siemens.
In continuous inkjet technology, a high-pressure pump directs liquid ink from a reservoir through a gunbody and a microscopic nozzle, creating a continuous stream of ink droplets via the Plateau-Rayleigh instability. A piezoelectric crystal creates an acoustic wave as it vibrates within the gunbody and causes the stream of liquid to break into droplets at regular intervals – 64,000 to 165,000 droplets per second may be achieved. The ink droplets are subjected to an electrostatic field created by a charging electrode as they form; the field varies according to the degree of drop deflection desired. This results in a controlled, variable electrostatic charge on each droplet. Charged droplets are separated by one or more uncharged “guard droplets” to minimize electrostatic repulsion between neighbouring droplets.
The charged droplets pass through an electrostatic field and are directed (deflected) by electrostatic deflection plates to print on the receptor material (substrate), or allowed to continue on undeflected to a collection gutter for re-use. The more highly charged droplets are deflected to a greater degree. Only a small fraction of the droplets is used to print, the majority being recycled.
Continuous ink jet is one of the oldest ink jet technologies in use and is fairly mature. The major advantages are the very high velocity (~50 m/s) of the ink droplets, which allows for a relatively long distance between print head and substrate, and the very high drop ejection frequency, allowing for very high speed printing. Another advantage is freedom from nozzle clogging as the jet is always in use, therefore allowing volatile solvents such as ketones and alcohols to be employed, giving the ink the ability to "bite" into the substrate and dry quickly.
The ink system requires active solvent regulation to counter solvent evaporation during the time of flight (time between nozzle ejection and gutter recycling) and from the venting process whereby air that is drawn into the gutter along with the unused drops is vented from the reservoir. Viscosity is monitored and a solvent (or solvent blend) is added in order to counteract the solvent loss.
Thermal/thermal DOD inkjet
Most consumer inkjet printers, from companies including Canon, Hewlett-Packard, and Lexmark (but not Epson), use print cartridges with a series of tiny chambers each containing a heater, all of which are constructed by photolithography. To eject a droplet from each chamber, a pulse of current is passed through the heating element causing a rapid vaporisation of the ink in the chamber to form a bubble, which causes a large pressure increase, propelling a droplet of ink onto the paper (hence Canon's tradename of Bubble Jet for its technology). The ink's surface tension as well as the condensation and thus contraction of the vapor bubble, pulls a further charge of ink into the chamber through a narrow channel attached to an ink reservoir.
The inks used are usually water-based ( aqueous ) and use either pigments or dyes as the colourant. The inks used must have a volatile component to form the vapour bubble, otherwise droplet ejection cannot occur. As no special materials are required, the print head is generally cheaper to produce than in other other inkjet technologies. The thermal inkjet principle was discovered by Canon engineer Ichiro Endo in August 1977.
Thermal inkjet printers are not the same as thermal printers, which produce images by heating thermal paper, as seen on older fax machines, cash registers, ATM receipt printers, and lottery ticket printers.
Piezoelectric/piezoelectric DOD inkjet
Most commercial and industrial inkjet printers and some consumer printers (those produced by Epson) use a piezoelectric material in an ink-filled chamber behind each nozzle instead of a heating element. When a voltage is applied, the piezoelectric material changes shape, which generates a pressure pulse in the fluid forcing a droplet of ink from the nozzle. Piezoelectric (also called Piezo) inkjet allows a wider variety of inks than thermal inkjet as there is no requirement for a volatile component, and no issue with kogation, but the print heads are more expensive to manufacture due to the use of the specialist piezoelectric material (usually PZT, lead zirconium titanate). Piezo inkjet technology is often used on production lines to mark products - for instance the use-before date is often applied to products with this technique; in this application the head is stationary and the product moves past. Requirements of this application are a long service life, a relatively large gap between the print head and the substrate, and low operating costs. There is a drop-on-demand process, with software that directs the heads to apply between zero to eight droplets of ink per dot and only where needed. As of June 2009, the fastest cut-sheet inkjet printer on the market is the RISO ComColor 9050, which prints 146 USLetter and 150 A4 full-color pages per minute in both one-sided and two-sided printing modes.
Inkjet Inks
The basic problem with inkjet inks are the conflicting requirements for a coloring agent that will stay on the surface and rapid dispersement of the carrier fluid.
Desktop inkjet printers, as used in offices or at home, tend to use aqueous inks based on a mixture of water, glycol and dyes or pigments. These inks are inexpensive to manufacture, but are difficult to control on the surface of media, often requiring specially coated media. HP inks contain sulfonated polyazo black dye (commonly used for dying leather), nitrates and other compunds. Aqueous inks are mainly used in printers with thermal inkjet heads, as these heads require water in order to perform.
While aqueous inks often provide the broadest color gamut and most vivid color, most are not waterproof without specialized coating or lamination after printing. Most Dye-based inks, while usually the least expensive, are subject to rapid fading when exposed to light. Pigment-based aqueous inks are typically more costly but provide much better long-term durability and ultraviolet resistance. Inks marketed as “Archival Quality” are usually pigment-based.
Some professional wide format printers use aqueous inks, but the majority in professional use today employ a much wider range of inks, most of which require piezo inkjet heads and extensive maintenance:
- Solvent inks: the main ingredient of these inks are volatile organic compounds (VOCs), organic chemical compounds that have high vapor pressures. Color is achieved using pigments rather than dyes for excellent fade-resistance. The chief advantage of solvent inks is that they are comparatively inexpensive and enable printing on flexible, uncoated vinyl substrates, which are used to produce vehicle graphics, billboards, banners and adhesive decals. Disadvantages include the vapour produced by the solvent and the need to dispose of used solvent. Unlike most aqueous inks, prints made using solvent-based inks are generally waterproof and ultraviolet-resistant (for outdoor use) without special over-coatings. The high print speed of many solvent printers demands special drying equipment, usually a combination of heaters and blowers. The substrate is usually heated immediately before and after the print heads apply ink. Solvent inks are divided into two sub-categories:
- Hard solvent ink offers the greatest durability without specialized over-coatings but requires specialized ventilation of the printing area to avoid exposure to hazardous fumes.
- Mild or "Eco" solvent inks, while still not as safe as aqueous inks, are intended for use in enclosed spaces without specialized ventilation of the printing area. Mild solvent inks have rapidly gained popularity in recent years as their color quality and durability have inc
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