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About Inkjet printers

An inkjet printer is a type of computer printer that creates a digital image by propelling variable-sized droplets of ink on to paper. Inkjet printers are the most commonly used type of printer[1] and range from small inexpensive consumer models to very large professional machines.[2]

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[citation needed].

The emerging ink jet material deposition market also uses inkjet 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.[3]

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 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.[4][5]Recent developments of the inkjet extend the operation from printing into manufacturing processes. The newest of these technologies is to deposit layers of plastic material as digital embossing ove the top of printed works[6]

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 compounds. 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 increased while ink cost has dropped significantly.[7]
UV-curable inks: these inks consist mainly of acrylic monomers with an initiator package. After printing, the ink is cured by exposure to strong UV-light. The advantage of UV-curable inks is that they "dry" as soon as they are cured, they can be applied to a wide range of uncoated substrates, and they produce a very robust image. Disadvantages are that they are expensive, require expensive curing modules in the printer, and the cured ink has a significant volume and so gives a slight relief on the surface. Though improvements are being made in the technology, UV-curable inks, because of their volume, are somewhat susceptible to cracking if applied to a flexible substrate. As such, they are often used in large "flatbed" printers, which print directly to rigid substrates such as plastic, wood or aluminum where flexibility is not a concern.

UV Curable Ink Properties and Functions:
• Photoinitiators: Absorb the UV energy from the light source on the print head. Chemical reaction occurs that converts the liquid ink into a solid film.
• Monomers: Used as solvents because of their ability to reduce viscosity (thickness) and combine with other ink components. 100% percent solids and do not release VOCs (volatile organic compounds). Monomers also add improved film hardness and resistance properties.
• Oligomers: Determine the final properties of the cured ink film, including its elasticity, outdoor performance characteristics and chemical resistance.
• Colorants: Can be dye-based or pigment-based. Usually, pigment-based because of the greater light fastness and durability of pigments compared with dyes. Pigments used in outdoor advertising and display applications have similar requirements to those used in automotive paints. Consequently, there is some crossover of use. While a pigment is selected on the basis of the required application, size control and reduction along with dispersion technique are major components of ink formulation.

UV Ink Printing Process:

Ink is exposed to UV radiation where a chemical reaction takes place where the photo-initiators cause the ink components to cross-link into a solid.
Typically a shuttered mercury-vapor lamp is on either side of the print head, and produces a great amount of heat to complete the curing process (this lamp is used for free radical UV ink, which is what the majority of flatbed inkjet systems use).
UV inks do not evaporate, but rather cure or set as a result from this chemical reaction.
No material is evaporated or removed, which means about 100% of the delivered volume is used to provide coloration.
This reaction happens very quickly, which leads to instant drying that results in a completely cured graphic in a matter of seconds. This also allows for a very fast print process.
As a result of this instant chemical reaction no solvents penetrate the substrate once it comes off the printer, which allows for high quality prints.
[8] [9]

Dye sublimation inks: these inks contain special sublimation dyes and are used to print directly or indirectly on to fabrics which consist of a high percentage of polyester fibres. A heating step causes the dyes to sublimate into the fibers and create an image with strong color and good durability.
[edit] Inkjet head design

Inkjet heads:
Disposable head (left) and
Fixed head (right) with ink cartridge (middle)There are two main design philosophies in inkjet head design: fixed-head and disposable head. Each has its own strengths and weaknesses. Most inkjets are used for photo printing.

Fixed head
The fixed-head philosophy provides an inbuilt print head (often referred to as a Gaither Head) that is designed to last for the life of the printer. The idea is that because the head need not be replaced every time the ink runs out, consumable costs can be made lower and the head itself can be more precise than a cheap disposable one, typically requiring no calibration. On the other hand, if a fixed head is damaged, obtaining a replacement head can become expensive if removing and replacing the head is even possible. If the printer's head cannot be removed, the printer itself will then need to be replaced.

Fixed head designs are available in consumer products but are more likely to be found on industrial high-end printers and large format plotters. In the consumer space, fixed-head printers are manufactured primarily by Epson and Canon. Hewlett-Packard also offers a few fixed-head models, such as the HP Photosmart 3310. Industrial fixed-head print heads are manufactured by these companies: Kodak Versamark, Trident, Xaar, Spectra (Dimatix), Hitachi / Ricoh, HP Scitex, Brother, Konica Minolta, Seiko Epson, and ToshibaTec (a licensee of Xaar)[citation needed].

Disposable head
The disposable head philosophy uses a print head which is supplied as a part of a replaceable ink cartridge. Every time a cartridge is exhausted, the entire cartridge and print head are replaced with a new one. This adds to the cost of consumables and makes it more difficult to manufacture a high-precision head at a reasonable cost, but also means that a damaged print head is only a minor problem: the user can simply buy a new cartridge. Hewlett-Packard has traditionally favoured the disposable print head, as did Canon in its early models. This type of construction can also be seen as an effort by printer manufacturers to stem third party ink cartridge assembly replacements, as these would-be suppliers don't have the ability to manufacture specialized print heads.

An intermediate method does exist: a disposable ink tank connected to a disposable head, which is replaced infrequently (perhaps every tenth ink tank or so). Most high-volume Hewlett-Packard inkjet printers use this setup, with the disposable print heads used on lower volume models.

Canon now uses (in most models) replaceable print heads which are designed to last the life of the printer, but can be replaced by the user if they should become clogged. For models with "Think Tank" technology, the ink tanks are separate for each ink color.

Cleaning mechanisms
The primary cause of inkjet printing problems is due to ink drying on the printhead's nozzles, causing the pigments and dyes to dry out and form a solid block of hardened mass that plugs the microscopic ink passageways. Most printers attempt to prevent this drying from occurring by covering the printhead nozzles with a rubber cap when the printer is not in use. Abrupt power losses, or unplugging the printer before it has capped the printhead, can cause the printhead to be left in an uncapped state. Further even when capped this seal is not perfect, and over a period of several weeks the moisture can still seep out, causing the ink to dry and harden. Once ink begins to collect and harden drop volume can be affected, drop trajectory can change, or the nozzle can fail to jet ink completely.

To combat this drying, nearly all inkjet printers include a mechanism to reapply moisture to the printhead. Typically there is no separate supply of pure ink-free solvent available to do this job, and so instead the ink itself is used to remoisten the printhead. The printer attempts to fire all nozzles at once, and as the ink sprays out, some of it wicks across the printhead to the dry channels and partially softens the hardened ink. After spraying, a rubber wiper blade is swept across the printhead to spread the moisture evenly across the printhead, and the jets are again all fired to dislodge any ink clumps blocking the channels.

Some use a supplemental air-suction pump, utilizing the rubber capping station to suck ink through a severely clogged cartridge. The suction pump mechanism is frequently driven by the page feed stepper motor – it is connected to the end of the shaft. The pump only engages when the shaft turns backwards, hence the rollers reversing while head cleaning. Due to the built-in head design, the suction pump is also needed to prime the ink channels inside a new printer, and to reprime the channels between ink tank changes.

Professional solvent- and UV-curable ink wide-format inkjet printers generally include a "manual clean" mode that allows the operator to manually clean the print heads and capping mechanism and to replace the wiper blades and other parts used in the automated cleaning processes. The volume of ink used in these printers often leads to "overspray" and therefore buildup of dried ink in many places that automated processes are not capable of cleaning.

The ink consumed in the cleaning process needs to be collected somewhere to prevent ink from leaking all over the surface under the printer. The collection area is known as the spittoon, and in Hewlett Packard printers this is an open plastic tray underneath the cartridge storage and cleaning/wiping station. In Epson printers, there is typically a large fibrous absorption pad in a pan underneath the paper feed platen. For printers several years old, it is common for the dried ink in the spittoon to form a pile that can stack up and touch the printheads, jamming the printer with sticky slime. Some larger professional printers using solvent inks may employ a replaceable plastic receptacle to contain waste ink and solvent which needs to be emptied and/or replaced when full.

The type of ink used in the printer can also affect how quickly the printhead nozzles become clogged. While the official brand of ink is highly engineered to match the printer mechanism, generic inks cannot exactly match the composition of the official brand since the actual ink composition is a trade secret. Generic ink brands may alternately be too volatile to keep the printhead moist during storage, or may be too thick and jellied leading to frequent printhead channel clogging.


Labyrinth air vent tubes on the top of an Epson Stylus Photo 5-color ink tank. The long air channels are molded into the top of the tank and the blue label seals the channels into long tubes. The yellow label is removed prior to installation, and opens the tube ends to the atmosphere so that ink can be sprayed onto the paper. Removing the blue label would destroy the tubes and cause the moisture to quickly evaporate.There is a second type of ink drying that most printers are unable to prevent. In order for ink to spray out of the cartridge, air needs to enter somewhere to displace the removed ink. The air enters via an extremely long, thin labyrinth tube, up to 10 cm long, wrapping back and forth across the ink tank. The channel is long and narrow to slow down moisture from evaporating out through the vent tube, but some evaporation still occurs and eventually the ink cartridge dries up from the inside out. To combat this problem, which is especially acute with professional fast-drying solvent inks, many wide-format printer cartridge designs contain the ink in a special airtight, collapsible bag that does not require a vent as the ink level drops. The bag merely shrinks until the cartridge is empty.

The frequent cleaning conducted by printers can consume quite a bit of ink and has a great impact on cost per page determinations.

Clogged nozzles can be detected by printing a pattern on the page. Methods are known for re-routing printing information from a clogged nozzle to a working nozzle.[citation needed]

Print quality
A method to quantify the spatial resolution of printed inkjet images is described in Tunable Laser Applications.[10] This method consists in printing transmission gratings with the inkjet printer to be assessed and measuring the modulation of laser light in an N-Slit interferometer. For gratings with spatial frequencies in the 0.25-5.0 lines/mm range, modulations in the 0.71-0.87 range are reported for various printers.[10] The closer the measured modulation is to unity (~ 1.0) the higher the spatial resolution of the print.

Inkjet advantages

Compared to earlier consumer-oriented color printers, inkjets have a number of advantages. They are quieter in operation than impact dot matrix or daisywheel printers. They can print finer, smoother details through higher printhead resolution, and many consumer inkjets with photographic-quality printing are widely available.

In comparison to more expensive technologies like thermal wax, dye sublimations, and laser printers, inkjets have the advantage of practically no warm up time and lower cost per page (except when compared to laser printers).

For some inkjet printers, monochrome ink sets are available either from the printer manufacturer or third-party suppliers. These allow the inkjet printer to compete with the silver-based photographic papers traditionally used in black-and-white photography, and provide the same range of tones – neutral, "warm" or "cold". When switching between full-color and monochrome ink sets, it is necessary to flush out the old ink from the print head with a cleaning cartridge.

Inkjet disadvantages
Inkjet printers may have a number of disadvantages:

The ink is often very expensive. (For a typical OEM cartridge priced at $15, containing 5 mL of ink, the ink effectively costs $3000 per liter—or $8000 per gallon.) According to the BBC (2003), "The cost of ink has been the subject of an Office of Fair Trading investigation. Which? magazine has accused manufacturers of a lack of transparency about the price of ink and called for an industry standard for measuring ink cartridge performance".[11]
Many "intelligent" ink cartridges contain a microchip that communicates the estimated ink level to the printer; this may cause the printer to display an error message, or incorrectly inform the user that the ink cartridge is empty. In some cases, these messages can be ignored, but some inkjet printers will refuse to print with a cartridge that declares itself empty, in order to prevent consumers from refilling cartridges. Thus, Epson embeds a chip which prevents from printing when the chip claims the cartridge is empty, although a researcher who over-rode the system found that in one case he could print up to 38% more good quality pages, even though the chip stated that the cartridge was empty.[11]
The lifetime of inkjet prints produced by inkjets using aqueous inks is limited; they will eventually fade and the color balance may change. On the other hand, prints produced from solvent-based inkjets may last several years before fading, even in direct sunlight, and so-called "archival inks" have been produced for use in aqueous-based machines which offer extended life.
Because the ink used in most consumer inkjets is water-soluble, care must be taken with inkjet-printed documents to avoid even the smallest drop of water, which can cause severe "blurring" or "running." Similarly, water-based highlighter markers can blur inkjet-printed documents.
The very narrow inkjet nozzles are prone to clogging with dried ink. The ink consumed cleaning them - either during cleaning invoked by the user, or in many cases, performed automatically by the printer on a routine schedule - can account for a significant proportion of the total ink installed in the machine.
These disadvantages have been addressed in a variety of ways: