Editing Inkjet printing (section)

== Methods ==

Fluid surface tension naturally pulls a stream into droplets. Optimal drop sizes of {{Convert|0.004|in|mm}} require an inkjet nozzle size of about {{Convert|0.003|in|mm}}. Fluids with surface tension may be water based, wax or oil based and even melted metal alloys. Most drops can be electrically charged. There are two main technologies in use in contemporary inkjet printers: continuous (CIJ) and drop-on-demand (DOD). Continuous inkjet means the flow is pressurized and in a continuous stream. Drop-on-demand means the fluid is expelled from the jet nozzle one drop at a time. This can be done with a mechanical means with a push or some electrical method. A large electrical charge can pull drops out of a nozzle, sound waves can push fluid from a nozzle or a chamber volume expansion can expel a drop. Continuous streaming was investigated first many years ago. Drop-on-demand was only discovered in the 1920s.{{Citation needed|date=October 2022}}

=== Continuous inkjet ===
[[File:INKJET-PRINTER-INDUSTRI.gif|thumb|Schematic diagram of a continuous inkjet printing process]]

The '''continuous inkjet''' (CIJ) method is used commercially for marking and coding of products and packages. In 1867, [[William Thomson, 1st Baron Kelvin|Lord Kelvin]] patented the [[syphon 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 strip [[chart recorder]]s) were introduced in 1951 by [[Siemens]].<ref name="CRC" /> using the patent US2566443 invented by [[Rune Elmqvist]] dated September 4, 1951.

In CIJ technology, a high-pressure pump directs liquid ink from a reservoir through a gunbody and a microscopic nozzle (usually .003 inch diameter), creating a continuous stream of ink droplets via the [[Plateau-Rayleigh instability]]. A piezoelectric crystal  may be used to create 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 irregular-sized ink droplets per second may be achieved.<ref name=":2">{{Cite book|last=Kenyon|first=R.W.|title=Chemistry and technology of Printing and Imaging Systems|publisher=Blackie Academic & Professional|year=1996|isbn=978-94-010-4265-9|location=Glasgow UK|pages=114–115, 119–120, 128, 131, 133}}</ref> The ink droplets are subjected to an electrostatic field created by a charging electrode or by a magnetic flux field as they form; the field varies according to the degree of drop deflection desired. This results in a controlled deflection by electrostatic charge on each droplet. Charged droplets may be separated by one or more uncharged "guard droplets" to minimize electrostatic repulsion between neighboring droplets.

The droplets pass through another electrostatic  or magnetic field and are directed (deflected) by electrostatic deflection plates or flux field to print on the receptor material (substrate), or allowed to continue on deflected 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 (1951) ink jet technologies in use and is fairly mature.{{Citation needed|date=October 2022}} Drop-on-demand was not invented until later.{{Citation needed|date=October 2022}} The major advantages of CIJ are the very high velocity (≈20&nbsp;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 [[Volatility (chemistry)|volatile]] solvents such as [[ketones]] and alcohols to be employed, giving the ink the ability to "bite" into the substrate and dry quickly.<ref name=":2" /> 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.

In the later 1950s, heated wax inks became popular with CIJ technologies. In 1971, Johannes F. Gottwald patent US3596285A, Liquid Metal Recorder used molten metal ink with a magnetic flux field to fabricate formed symbols for signage. This may have been the first 3D metal object printed using magnetic core memory as data to produce each symbol.

=== Drop-on-demand ===
[[file:Micro Piezo Comparison.gif|thumb|Piezoelectric (left) and thermal (right) drop generation schematic. A print head will contain several such nozzles, and will be moved across the page as paper advances through the printer.]]
[[file:Canon S520 ink jet printer - opened.jpg|thumb|A Canon inkjet with [[CMYK]] cartridges]]
[[file:EPSON Piezoelectric InkJet Print Nozzle.jpg|thumb|Piezoelectric printing nozzle of an EPSON C20 printer]]
[[file:JetFluid.jpg|thumb|Howtek style inkjet nozzle (tubular piezo not shown)]]

There are many ways to produce a drop-on-demand (DOD) inkjet. Common methods include thermal DOD and piezoelectric DOD to speed up the frequency of drops.<ref>{{Cite book |last1=Salehi |first1=Mojtaba |url=https://books.google.com/books?id=ppRLDwAAQBAJ&dq=drop-on-demand+common+methods+thermal+piezoelectric&pg=PA89 |title=Inkjet Based 3D Additive Manufacturing of Metals |last2=Gupta |first2=Manoj |last3=Maleksaeedi |first3=Saeed |last4=Sharon |first4=Nai Mui Ling |date=2018-01-02 |publisher=Materials Research Forum LLC |isbn=978-1-945291-45-6 |language=en}}</ref> DOD may use a single nozzle or thousands of nozzles.<ref>{{Cite book |last=Cost |first=Frank |url=https://books.google.com/books?id=LkX6QtHrEt4C&dq=drop-on-demand+may+use+a+single+nozzle+or+thousands+of+nozzles&pg=PA62 |title=The New Medium of Print: Material Communication in the Internet Age |date=2005 |publisher=RIT Cary Graphic Arts Press |isbn=978-1-933360-03-4 |language=en}}</ref> One DOD process uses software that directs the heads to apply between zero and eight droplets of ink per dot, only where needed.{{citation needed|date=June 2015}} Inkjet fluid materials have expanded to include pastes, epoxies, hot-melt inks, biological fluids, etc. DOD is very popular and has an interesting history. Mechanical DOD came first, followed by electrical methods including piezoelectric devices and then thermal or heat expansion methods.

; Thermal DOD printing: Most consumer inkjet printers, including those from [[Canon (company)|Canon]] (FINE Cartridge system, see photo), [[Hewlett-Packard]], and [[Lexmark]], use the thermal inkjet process.<ref name=":1">{{Cite book|last=Webster, Edward.|url=https://www.worldcat.org/oclc/46611664|title=Print unchained : fifty years of digital printing, 1950-2000 and beyond : a saga of invention and enterprise|date=2000|publisher=DRA of Vermont, Inc|isbn=0-9702617-0-5|location=West Dover, VT|pages=53–54|oclc=46611664}}</ref> 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-Packard's Corvallis Division, and Canon engineer Ichiro Endo. Initially, in 1977, Endo's team was trying to use the [[piezoelectric]] effect 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. Vaught's 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 other's work.<ref>{{cite news | title=Spitting image | date=19 September 2002 | url=http://socrates.berkeley.edu/~scotch/innovation/inventing_injet.htm | newspaper=The Economist}}</ref><ref>{{cite news | title=History of ThinkJet Printhead Development | date=May 1985 | author=Niels J. Nielsen | url=http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1985-05.pdf | work=Hewlett-Packard Journal | access-date=31 January 2015 | archive-date=24 September 2015 | archive-url=https://web.archive.org/web/20150924031511/http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1985-05.pdf | url-status=dead }}</ref>
; Thermal inkjet: In the thermal inkjet process, the print cartridges consist of 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 vaporization of the ink in the chamber and forming a bubble,<ref>{{Cite AV media |url=https://www.youtube.com/watch?v=9yeZSaigBj4 |title=How inkjet printer work |date=2017-02-19 |last=Webmundo |access-date=2025-05-06 |archive-url=https://ghostarchive.org/varchive/youtube/20211211/9yeZSaigBj4 |archive-date=2021-12-11 |via=YouTube}}</ref> which causes a large pressure increase, propelling a droplet of ink onto the paper (hence Canon's [[trade name]] of ''Bubble Jet''). Early thermal heads ran at just 600–700 dpi<ref name=":1" /> but improvements by HP increased the firing range of 8–12&nbsp;kHz per chamber and as high as 18&nbsp;kHz with 5-picoliter drop volume by the year 2000.  Thermal printheads do not have the power of piezo DOD or continuous inkjet, so the gap between the face of the head  and paper is critical. The ink's [[surface 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 either [[pigment]]s or [[dye]]s as 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.
; [[Piezoelectricity|Piezoelectric]] DOD printing: Piezos are electrically polarized ceramic devices, just as a magnet is polarized. Most commercial and industrial inkjet printers and some consumer printers (those produced by [[Micro Piezo|Epson]] (see photo) and [[Brother Industries]]) use a [[piezoelectricity|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, generating a pressure pulse in the fluid, which pushes a droplet of ink from the nozzle. Single nozzle tubular inkjets actually are fluid resonator chambers and the drops are expelled by sound waves in the ink chamber. The 1972 patent called them squeeze tube inkjets but later it was discovered to be acoustical inkjets.  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]]). However, the ink cartridges can be separate from the head itself and individually be replaced as needed. Piezo, then has the potential for lower running costs. Piezo heads are said to achieve firing rates that are faster than thermal heads at comparable drop volumes.<ref name=":1" />
; Piezo inkjet: 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. This application requires a relatively large gap between the print head and the substrate, but also yields a high speed, a long service life, and low [[operating cost]].
; [[Thermoplastic]]/[[3D printing]]: In the 1970s, the first DOD inks were water-based and higher-temperature use was not recommended. In the late 1970s, wax- and oil-based inks were used in the Silonics in 1975, Siemens PT-80i in 1977 and Epson and Exxon in 1980s DOD inkjets.<ref name=":1" /> In 1984, a small company, Howtek, Inc.,<ref name=":0" />  found that [[solid ink]]<ref name=":1" /> materials (thermoplastics) could be jetted at {{Convert|125|C|F}} by maintaining the piezo poling charge while printing. In 1986, Howtek launched the Pixelmaster solid ink-jetting printer, which opened the door to printing three-dimensional plastic inks and led to a 1992 3D patent, US5136515A. This patent was licensed by the first three major [[3D printing|3D printer]] companies (Sanders Prototype, Inc, Stratasys and 3D Systems).
; Braillemaster: In the late 1980s, Howtek introduced the Braillemaster, a printer that used four layers of solid ink per character to create documents in Braille that could be read by people who were blind.
; Howtek: Solidscape, Inc., currently uses the Howtek-style thermoplastic materials and Howtek-style single nozzle inkjets (see illustration) very successfully. Ballistic Particle Manufacturing also used the Howtek style materials and inkjets.<ref>{{Cite book|last=Cooper|first=Kenneth G.|url=https://www.worldcat.org/oclc/45873626|title=Rapid prototyping technology : selection and application|date=2001|publisher=Marcel Dekker|isbn=0-8247-0261-1|location=New York|pages=26–43|oclc=45873626}}</ref> These inkjets can produce up to 16,000 drops per second and shoot drops at 9 feet per second. Originally designed to only print on standard letter-sized paper sheets they now can print 3D models requiring hundreds of layers.
; Thermojet: The thermoplastic inks in piezoelectric inkjets (called Thermojet technology by Howtek) are sometimes confused with the thermal (heat expansion) bubble-jet technology but they are completely different. Bubble jet inks are not solid at room temp and are not heated. Thermojet inks require 125&nbsp;°C to reduce fluid viscosity in jetting range. Howtek was the first to introduce an inkjet color printer using thermoplastic inks<ref name=":1" /> in 1984 at Comdex, Las Vegas.