Inkjet technology

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Inkjet technology is a method for accurately depositing liquid droplets on a substrate in very thin layers. It was originally developed for the publishing industry with text and images, but has become a popular method in digital fabrication of electronic and mechanical devices and especially jewelry. Although terms, "jetting", "inkjet technology" and "inkjet printing", are commonly used interchangeably, inkjet printing usually refers to the publishing industry, used for printing graphical content, while Jetting usually refers to the general purpose fabrication via particle deposition.


Inks must have high conductivity, high oxidation resistance and low sintering temperature.

  • optical devices.[11]

Personalized medications[edit]

Drop formation[edit]

Various drop formation technologies exist, and can be classified into two main types: continuous inkjet (CIJ) and drop-on-demand (DOD).[12]

While CIJ has a straightforward drop creation and sophisticated drop trajectory manipulation, DOD has sophisticated drop creation and no trajectory manipulation.

A Howtek Inkjet nozzle uses a tubular thin wall piezo that produces a sound wave in the fluid chamber reflecting of both ends of the nozzle at the fluid speed of sound. The leading edge of a square wave signal triggers it and the lagging edge of the square wave signal in coincidence with the pressure wave expels the drop.This DOD single jet is acoustic. The 120C Tefzel nozzle is not rigid and does not squeeze.

Representation of Howtek Inkjet Nozzle
Classification of inkjet technologies[13]
Drop-on-demand (DOD) Continuous (CIJ) Electrospray
Thermal Piezoelectric Single jet Multiple jet
Face shooter Side shooter Shear Extension Unimorph/bimorph Squeeze Acoustic modulation Thermal modulation

Drop-on-demand (DOD)[edit]

In this method, drops of ink are released individually, on demand, by a voltage signal. Released drops fall vertically without any trajectory manipulation. Commercial printheads can have a single nozzle (Solidscape) or thousands of nozzles (HP) and many other variations in between.

The two leading technologies for forcing ink out of a nozzle on demand are thermal DOD and piezoelectric DOD. Notice the DOD uses a "Fill before firing a drop" and Thermal DOD just "fires a drop". Drops are precisely controlled with DOD. Standard DOD fires drops at 9 feet per second drop velocity. DOD drop target positioning is very accurate with every drop fired horizontally or vertically.

Additional technologies include electrospray,[14][15] acoustic discharge,[16] electrostatic membrane[17] and thermal bimorph.[18]

Piezoelectric DOD[edit]

The piezoelectric voltage pulse determines the jetted volume.

Piezoelectric DOD was invented in the 1970s.[19][20] One disadvantage of the piezo-DOD method is that jettable inks must have viscosity and surface tension within a relatively strict range. One big advantage is DOD piezoelectric jets can be designed to work with high temperature Thermoplastics and other hot-melt inks in the temperature range of 100-130C. This allowed for three-dimensional droplets to be printed on substrates and opening this industry up to investment casting and 3D modelling. The Richard Helinski 3D patent US5136515A started a new era in inkjet printing. Helinksi's experience at Howtek, Inc from 1984 -1989 and his many other patents including subtractive color (layering colored drops) and suggestions from fellow employees about investment casting encouraged this patent.

Thermal inkjet (TIJ) DOD[edit]

Comparison between piezoelectric jet (left) and thermal jet (right)

Thermal DOD was introduced in the 1980s by Canon[21] and Hewlett-Packard.[22] Thermal printing does not use high temperature inks.

One disadvantage of this method is that the variety of inks compatible with TIJ is essentially limited, because this method is compatible with inks that have high vapour pressure, low boiling point and high kogation stability.[23][24] Water being such a solvent, limited the popularity of this method for non-industrial photo printing only, where water-based inks are used.

Continuous inkjet (CIJ)[edit]

In this method, a column of ink is released continuously from the nozzle. The ink column spontaneously breaks into separate drops due to the Plateau–Rayleigh flow instability. The formed ink drops are either deflected by an electric field towards the desired location on the substrate, or collected for reuse. CIJ printheads can be either have a single jet (nozzle) or multiple jets (nozzles).

One disadvantage of the CIJ method is the need for solvent monitoring. Since only a small fraction of the ink is being used for actual printing, solvent must be continually added to the recycled ink to compensate the evaporation that takes place during flight of the recycled drops.[23]

Another disadvantage is the need for ink additives. Since this method is based on electrostatic deflection, ink additives, such as potassium thiocyanate, may deteriorate the performance of the printed devices.[23]

CIJ can directed through a magnetic field using low temperature metal alloy ink as described in Johannes F Gottwald Liquid Metal Recorder patent US3596285A which issued on 7/27/71. The .003 inch aperture glass nozzle formed stock market quote symbols on a moving metal substrate belt and dropped on the table to be handled or reused in the recorder for other symbols. This was possibly the earliest example of fabricated objects with inkjet.

The printhead[edit]

The printhead must have heating capability to print any material influenced by viscosity changes. Oil based inks are sensitive to temperature. Waxes and hot-melt materials are solids at room temperature. It is even possible to print with metallic alloys such as lead, tin, indium, zinc and aluminium. The process of printing of low-melting point metals is called "direct melt printing" and was introduced in 1971 by Johannes F Gottwald patent, US3596285, "Liquid Metal Recording" with a Continuous inkjet (CIJ) long before any form of 3D Printing was ever considered. Thermoplastic DOD inkjets print at or above the piezoelectric Curie temperature and must be continuously poled to work. Piezo D33 displacement had to be optimized to lower drive voltages. See Piezoresponse force microscopy (PFM) for relevant theory. Prior research in 1980 by James McMahon about the six piezo physical poling states and tests to maximize piezo resonant and anti-resonant frequencies sped up the development time. Howtek, Inc., manufactured these state of the art inkjets in 1985 before 3D printing with inkjets was invented on 8/4/1992. Original DOD inkjet printheads were made of glass in 1972 by Steve Zoltan. These early single nozzle inkjet printheads printed with water based inks. Later a housing was needed to surround the inkjet with a stable thermal mass. Glass inkjet nozzles were hard to duplicate and the molded nozzles were introduced by Howtek, Inc. Glass nozzles had to be made with drawn glass tubes, cut to size and polished to produce a flat nozzle orifice surface. Howtek introduced single nozzle Tefzel molded nozzles sliced with a razor and produced a full color thermoplastic material sheet Pixelmaster printer in 1986 with 32 single nozzles (8 for each primary color). The Tefzel nozzle material operating at 125C allowed only the voltage pulse energy to trigger an acoustical pressure wave in the fluid without coupling the high frequency vibrations from the piezo that cause spray and fluid vibration as the drops are ejected. Earlier inkjet designs with glass nozzles were also resonance sources and when packed with vibration dampening material could never eliminate spray. The object of the design was to have clean spray-free drops ejected over the frequency range of the nozzle length. The Howtek jets run nicely from 1-16,000 Hertz. No other company has produced printheads with this design to this day. One 3D printer in use in 2020 has a Howtek style nozzle that was manufactured in 1986. It has the hex-shaped metal structure with an offset nozzle orifice that allowed the jet drops to be directed toward a target to align properly for print quality when it was previously installed in the Howtek Pixelmaster. Over 1500 Howtek style inkjets were acquired by early Solidscape when production of the Modelmaker 6 Pro was first started in 1994. The Modelmaker 6 pro uses 2 inkjets per machine. The inkjets are installed in a special printhead directing the drops straight downward for 3D printing. The Pixelmaster projected the drops horizontally from a 121 rpm rotating printhead to print 2D characters or images on paper. A Braille character printer was introduced and sold in 19901991 with raised font printed on plain paper. This required layers of drops to stack up for each Braille feature. This was an early example of the three-dimensional material printing but Additive Manufacturing (AM) does not reference historical references to material properties used in 3D printing. Each of these printheads or inkjets is a single nozzle design.

Photo inkjet printhead[edit]

Tandem printheads[edit]

Multiple printheads[edit]

Fabrication approaches[edit]

The printed material is rarely only one step in the process, which may include direct material deposition followed by a mechanical roller or a controlled surface milling step. It may be a deposition of a precursor followed by a catalyst, sintering, photonic curing, electroless plating etc., to give the final result. See Ballistic particle manufacturing which uses a Solid ink single nozzle, heated to 125C and a 5 axis printing technique that required no other process for fabrication.

  • Direct deposition is jetted material directly applied to a substrate or surface
  • Mask printing
  • Etching
  • Inverse printing
  • Powder bed

Additive inkjet fabrication[edit]

  • The application of any jetted material having sufficient three-dimensional properties to achieve a Z axis dimension when printed over itself many times. It can include other fabrication steps as listed above under Fabrication approaches.

Subtractive inkjet fabrication[edit]

  • The use of a milling step after deposition. Solidscape 3D inkjet printers use this technique in their model forming process. Layer thickness of 0.0005 inches require the 4 mil drops to be printed and the material spreading out but a milling step reduces the Z dimension to 0.0005 before the next layer is deposited. More than 50% of the material is removed on these thin layers but part excellent part quality is achieved with little stair stepping on the model sloped surfaces.

Inkjettable materials[edit]

The ink must be liquid, but may also contain small solids if they do not cause clogging. The solid particles should be smaller than 1/10 of the nozzle diameter to avoid clogging and be smaller than 2 mircon to reduce satellite drop spray. Fine detail inkjet printing has material filtered by 1 micron filters to prevent spray and fluid lines protected by 15 micron filters to prevent clogging.

Drop formation is governed by two main physical properties: surface tension and viscosity. The surface tension forms ejected drops into spheres, in accordance with Plateau–Rayleigh instability. The viscosity can be optimized at jet time by using an appropriate printhead temperature. Drop volume is controlled by drive pulse timing width and drive voltage amplitude. Each inkjet assembly will have a slight variation in drop size and maintaining all material and jet parameters is necessary for optimum performance. Drop formation and volume varies with drop frequency and jet orifice meniscus position.

Generally, lower viscosity allows better droplet formation[25] and in practice only liquids with viscosities of 2-50 mPa s can be printed.[13] More precisely, liquids whose Ohnesorge number is larger than 0.1 and smaller than 1 are jettable.[26][27][28]


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Further reading[edit]