is a type ofcomputer printingthat recreates adigital imageby propelling droplets of ink onto paper, plastic, or other substrates.Inkjet printers are the most commonly used type of printer,and range from small inexpensive consumer models to expensive professional machines.
The concept of inkjet printing originated in the 20th century, and the technology was first extensively developed in the early 1950s. Starting in the late 1970s, inkjet printers that couldreproduce digital imagesgenerated by computers were developed, mainly byEpsonHewlett-Packard(HP), andCanon. In the worldwide consumer market, four manufacturers account for the majority of inkjet printer sales: Canon, HP, Epson, and Brother.
The emergingink jet material depositionmarket also uses inkjet technologies, typically printheads usingpiezoelectriccrystals, to deposit materials directly on substrates.
The technology has been extended and the ink can now also comprise living cells,for creatingbiosensorsand fortissue engineering.
There are two main technologies in use in contemporary inkjet printers: continuous (CIJ) and Drop-on-demand (DOD).
The continuous inkjet (CIJ) method is used commercially for marking and coding of products and packages. In 1867,Lord Kelvinpatented thesyphon recorder, which recorded telegraph signals as a continuous trace on paper using an ink jet nozzle deflected by a magnetic coil. The first commercial devices (medical stripchart recorders) were introduced in 1951 bySiemens.
In CIJ 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 thePlateau-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 another 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.
CIJ is one of the oldest ink jet technologies in use and is fairly mature. The major advantages are the very high velocity (20m/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 allowingvolatilesolvents such asketonesand 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 to counteract solvent loss.
Drop-on-demand (DOD) is divided into thermal DOD and piezoelectric DOD.
Most consumer inkjet printers, including those fromCanon(FINE),Hewlett-Packard, andLexmark, use thethermalinkjet process. The idea of using thermal excitation to move tiny drops of ink was developed independently by two groups at roughly the same time: John Vaught and a team at Hewlett-Packards Corvallis Division, and Canon engineer Ichiro Endo. Initially, in 1977, Endos team was trying to use thepiezoelectriceffect to move ink out of the nozzle but noticed that ink shot out of a syringe when it was accidentally heated with a soldering iron. Vaughts work started in late 1978 with a project to develop fast, low-cost printing. The team at HP found that thin-film resistors could produce enough heat to fire an ink droplet. Two years later the HP and Canon teams found out about each others work.
In the thermal inkjet process, the print cartridges consist of a series of tiny chambers, each containing a heater, all of which are constructed byphotolithography. To eject a droplet from each chamber, a pulse of current is passed through the heating element causing a rapid vaporization of the ink in the chamber and forming a bubble, which causes a large pressure increase, propelling a droplet of ink onto the paper (hence Canonstrade nameofBubble Jet). The inkssurface tension, as well as the condensation and resultant 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 involved are usually water-based and use eitherpigmentsordyesas the colorant. The inks must have a volatile component to form the vapor 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.
Most commercial and industrial inkjet printers and some consumer printers (those produced byEpsonandBrother Industries) use apiezoelectric materialin an ink-filled chamber behind each nozzle instead of a heating element. When a voltage is applied, the piezoelectric material changes shape, generating a pressure pulse in the fluid, which forces 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 (buildup of ink residue), but the print heads are more expensive to manufacture due to the use of piezoelectric material (usually PZT,lead zirconium titanate).
A DOD process uses software that directs the heads to apply between zero and eight droplets of ink per dot, only where needed.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 high speed, a long service life, a relatively large gap between the print head and the substrate, and lowoperating cost.
The basic problem with inkjet inks is the conflicting requirements for a coloring agent that will stay on the surface vs. rapid dispersement of the carrier fluid.
Desktop inkjet printers, as used in offices or at home, tend to useaqueous inksbased on a mixture of water,glycolanddyesorpigments. 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 dyeingleather), nitrates and other compounds.Aqueous inks are mainly used in printers with thermal inkjet heads, as these heads require water to perform.
While aqueous inks often provide the broadestcolor gamutand most vivid color, most are not waterproof without specialized coating orlaminationafter printing. MostDye-based inks, while usually the least expensive, are subject to rapid fading when exposed to light or ozone.Pigment-based aqueous inks are typically more costly but provide much better long-term durability andultravioletresistance. Inks marketed asArchival Qualityare 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:
The main ingredient of these inks arevolatile organic compounds (VOCs), organic chemical compounds that have highvapor pressures. Color is achieved with 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, uncoatedvinylsubstrates, 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 andultraviolet-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:
ink offers the greatest durability without specialized over-coatings but requires specialized ventilation of the printing area to avoid exposure to hazardous fumes, while
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.
These inks consist mainly of acrylicmonomerswith an initiator package. After printing, the ink iscuredby exposure to strong UV-light. 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 or UV LED is used for the curing process. Curing processes with high power for short periods of times (microseconds) allow curing inks on thermally sensitive substrates. 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.
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 aluminium where flexibility is not a concern.
These inks contain specialsublimation dyesand are used to print directly or indirectly on to fabrics which consist of a high percentage ofpolyesterfibres. A heating step causes the dyes to sublimate into the fibers and create an image with strong color and good durability.
Inkjet heads: disposable head (left) and fixed head (right) with ink cartridge (middle)
There are two main design philosophies in inkjet head design:fixed-headanddisposable head. Each has its own strengths and weaknesses.
Thefixed-headphilosophy provides an inbuilt print head (often referred to as agaiter- 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 printers 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 OfficeJet Pro X576dw.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).
Thedisposable headphilosophy uses a print head which is supplied as a part of a replaceableink cartridge. Every time a cartridge is exhausted, the entire cartridge and print head are replaced with a new one. This adds to the cost ofconsumablesand makes it more difficult to manufacture a high-precision head at a reasonable cost, but also means that a damaged or clogged print head is only a minor problem: the user can simply buy a new cartridge.Hewlett-Packardhas 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 dont 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. A similar approach is used byKodak, where the printhead intended for permanent use is nevertheless inexpensive and can be replaced by the user. 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 should they become clogged.
The primary cause of inkjet printing problems is ink drying on the printheads 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. Even when the head is capped, this seal is not perfect, and over a period of several weeks the moisture (or other solvent) can still seep out, causing the ink to dry and harden. Once ink begins to collect and harden, the drop volume can be affected, drop trajectory can change, or the nozzle can completely fail to jet ink.
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 printers 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 feedstepper 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 to prevent ink from leaking in the printer. The collection area is called thespittoon, and in Hewlett Packard printers this is an open plastic tray underneath the cleaning/wiping station. In Epson printers, there is typically a large 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. Some larger professional printers using solvent inks may employ a replaceable plastic receptacle to contain waste ink and solvent which must be emptied or replaced when full.
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. For ink to spray from the cartridge, air must enter to displace the removed ink. The air enters via an extremely long, thin labyrinth tube, up to 10cm long, wrapping back and forth across the ink tank. The channel is long and narrow to reduce moisture evaporation 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 an airtight, collapsible bag that requires no vent. The bag merely shrinks until the cartridge is empty.
The frequent cleaning conducted by some 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 standard test pattern on the page. Some software workaround methods are known for re-routing printing information from a clogged nozzle to a working nozzle.
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Compared to earlier consumer-oriented color printers, inkjet printers have a number of advantages. They are quieter in operation than impactdot matrixordaisywheel printers. They can print finer, smoother details through higher resolution. Consumer inkjet printers with photographic-quality printing are widely available.
In comparison to technologies likethermal waxdye sublimation, andlaser printing, inkjets have the advantage of practically no warm up time, and often lower cost per page. However, low-cost laser printers can have lower per-page costs, at least for black-and-white printing, and possibly for color.
For some inkjet printers, monochrome ink sets are available either from the printer manufacturer or from 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 acleaning cartridge. Special software or at least a modifieddevice driverare usually required, to deal with the differentcolor mapping.
Some types of industrial inkjet printers are now capable of printing at very high speeds, in wide formats, or for a variety of industrial applications ranging from signage, textiles, ceramics and 3-D printing into biomedical applications and conductive circuitry. Leading companies and innovators in hardware include HP, Epson, Canon, Konica Minolta, FujiFilm, EFi, Durst, Brother, Roland, Mimaki, Mutoh and many others worldwide.
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Many intelligent ink cartridges contain amicrochipthat 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, to prevent consumers from refilling cartridges. For example,Epsonembeds a chip which prevents printing when the chip claims the cartridge is empty, although a researcher whoover-rodethe 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.Third-party ink suppliers sell ink cartridges at significant discounts (at least 10−30% off OEM cartridge prices, sometimes up to 95%, typically averaging around 50%), and also bulk ink and cartridgeself-refill kitsat even lower prices. Many vendors intelligent ink cartridges have beenreverse-engineered. It is now possible to buy inexpensive devices to reliably reset such cartridges to report themselves as full, so that they may be refilled many times.
Long-term durability of early inkjet prints was quite poor, though improved ink formulations have greatly improved this attribute. See the section ondurabilityfor more information.
The very narrow inkjet nozzles are prone to clogging. The ink consumed cleaning themeither during cleaning invoked by the user, or in many cases, performed automatically by the printer on a routine schedulecan account for a significant proportion of the ink used in the machine. Inkjet printing head nozzles can be cleaned using specialized solvents; or by soaking in warm distilled water for short periods of time, for water-soluble inks.
The high cost of OEM ink cartridges and the intentional obstacles to refilling them have been addressed by the growth of third-party ink suppliers. Many printer manufacturers discourage customers from using third-party inks, stating that they can damage the print heads due to not being the same formulation as the OEM inks, cause leaks, and produce inferior-quality output (e.g. of incorrect color gamut).Consumer Reportshas noted that some third-party cartridges may contain less ink than OEM cartridges, and thus yield no cost savings,whileWilhelm Imaging Researchclaims that with third-party inks the lifetime of prints may be considerably reduced.However, an April 2007 review showed that, in adouble-blindtest, reviewers generallypreferredthe output produced using third-party ink over OEM ink.In general, OEM inks have undergone significant system reliability testing with the cartridge and print-head materials, whereas R&D efforts on third-party ink material compatibility are likely to be significantly less. Some inkjet manufacturers have tried to prevent cartridges being refilled using various schemes including fitting chips to the cartridges that log how much the cartridge has printed and prevent the operation of a refilled cartridge.
The warranty on a printer may not apply if the printer is damaged by the use of non-approved supplies. In the US theMagnusonMoss Warranty Actis a federal law which states that warrantors cannot require that only brand name parts and supplies be used with their products, as some printer manufacturers imply. However, this would not apply if non-approved items cause damage. In the UK, a printer manufacturer cannot lawfully impose such conditions as part of its warranty (Regina Vs Ford Motor Company refers) although many attempt to do so illegally. As long as the product used was sold as being for the printer it was used in, then the sale of goods act applies, and anything so sold must be of merchandisable quality and fit for purpose. Moreover, under UK law, it is the retailer and not the manufacturer that is legally liable, for 2 years on electrically operated items specifically, and as such the retailer is where one would seek redress.
Inkjet documents can have poor to excellentarchivaldurability, depending on the quality of the inks and paper used. If low-quality paper is used, it can yellow and degrade due to residualacidin the untreated pulp; in the worst case, old prints can literally crumble into small particles when handled. High-quality inkjet prints onacid-free papercan last as long as typewritten or handwritten documents on the same paper.
Because the ink used in many low-cost consumer inkjets is water-soluble, care must be taken with inkjet-printed documents to avoid even the smallest drop of moisture, which can cause severe blurring or running. In extreme cases, even sweaty fingertips during hot humid weather could cause low-quality inks to smear. Similarly, water-basedhighlightermarkers can blur inkjet-printed documents and discolor the highlighters tip. The lifetime of inkjet prints produced using aqueous inks is generally shorter (although UV-resistant inks are available) than those produced with solvent-based inkjets; however, so-called archival inks have been produced for use in aqueous-based machines which offer extended life.
In addition to smearing, gradual fading of many inks can be a problem over time. Print lifetime is highly dependent on the quality and formulation of the ink. The earliest inkjet printers, intended for home and small office applications, used dye-based inks. Even the best dye-based inks are not as durable as pigment-based inks, which are now available for many inkjet printers. Many inkjet printers now utilize pigment based inks which are highly water resistant: at least the black ink is often pigment-based. Resin or silicone protected photopaper is widely available at low cost, introducing complete water and mechanical rub resistance for dye and pigment inks. The photopaper itself must be designed for pigment or for dye inks, as pigment particles are too large to be able to penetrate through dye-only photopaper protection layer.
The highest-quality inkjet prints are often calledgicleprints, to distinguish them from less-durable and lower-cost prints. However, the use of the term is no guarantee of quality, and the inks andpaperused must be carefully investigated before an archivist can rely on their long-term durability.
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Inkjets use solvent-based inks which have much shorter expiration dates compared to laser toner, which has an indefinite shelf life.Inkjet printers tend to clog if not used regularly, whereas laser printers are much more tolerant of intermittent use.Inkjet printers require periodical head cleaning, which consumes a considerable amount of ink, and will drive printing costs higher especially if the printer is unused for long periods.
If an inkjet head becomes clogged, third-party ink solvents/head cleaners and replacement heads are available in some cases. The cost of such items may be less expensive compared to a transfer unit for a laser printer, but the laser printer unit has a much longer lifetime between required maintenance. Many inkjet printer models now have permanently installed heads, which cannot be economically replaced if they become irreversibly clogged, resulting in scrapping of the entire printer. On the other hand, inkjet printer designs which use a disposable printhead usually cost significantly more per page than printers using permanent heads.By contrast, laser printers do not have printheads to clog or replace frequently, and usually can produce many more pages between maintenance intervals.
Inkjet printers have traditionally produced better quality output than color laser printers when printing photographic material. Both technologies have improved dramatically over time, although the best qualitygicleeprints favored by artists use what is essentially a high-quality specialized type of inkjet printer.
A common business model for inkjet printers involves selling the actual printer at or below production cos.