Inkjet technology

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.

Applications

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

  • optical devices.[11]

Personalized medications

Drop formation

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)

In this method, drops of ink are released individually, on demand, by a voltage signal. Released drops fall vertically without any trajectory manipulation or require special fire timing when drops are projected horizontally from a rotary printhead spinning at 121 RPM to form perfect shaped characters (Howtek color printer 1986). Commercial printheads can have a single nozzle (Solidscape) or thousands of nozzles (HP) and many other variations in between. DOD is a very complex technology that few people can really appreciate. For example: Arrayed Inkjet Apparatus (John G Martner patent 4468680, 1984 Exxon Research and Engineering Co) was invented after testing a Piezo DOD epoxied on the end of a piano wire 30 inches long and inserted into a ink fluid chamber leading to a nozzle. The tiny piezo either was pulling the wire in and out of the fluid chamber or transmitting a sound wave through the wire to impart acoustic energy into the fluid to fire a drop. The object of the invention was to build a printhead to reduce crosstalk (sound or any energy into closely placed nozzles for text printing).

The two leading technologies for forcing ink out of a nozzle on demand are thermal DOD and piezoelectric DOD. Notice the DOD may use a "Fill before firing a drop" or "Fire before fill" and Thermal DOD just "fires before fill". Drops must be precisely controlled with Piezo DOD or Thermal DOD. A standard Piezo DOD can fire drops at 9 feet per second drop velocity. Piezo 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
The piezoelectric voltage pulse determines the jetted volume.

Piezoelectric Drop-On-Demand (DOD) was invented in the 1970s.[19][20] One disadvantage of the piezoelectric-DOD method is that jettable inks must have viscosity and surface tension within a relatively strict range to expel smaller drops without spray or satellite drops. 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) with suggestions from a fellow inventor/employee, Alan Hock, about investment casting encouraged this patent. The patent is focused on printing complex solid 3D objects printed with a clean burning material when placed in an investment casting process primarily in the jewelry industry but also favored by electronics, automotive and medical industries in the early 1990's. Howtek style inkjets and Thermoplastic materials were created to print documents and images and later Braille characters.

There are many patents and methods to expel drops with the Piezoelectric devices. A piezo changes shape when voltage is applied. The amount of dimensional change is extremely small. Piezos also be made in many different sizes. The smaller the piezo the smaller the shape displacement. The use of a DOD piezo to print a text character (the size of these letters) requires the piezos to be placed side by side in a housing. Drops must be smaller than .005 inches and be placed precisely in lines to form letters. Piezo's placed side by side at frequencies high enough to print a full sheet of

paper vibrate loudly and effect the drops in nearby. Drop-On-Demand (DOD) printheads have manufacturing limits with single nozzles. Multijet DOD

printing is most common with inkjet printers for this reason.

Thermal inkjet (TIJ) DOD
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)

In this method, a stream of ink is released continuously from the nozzle. A garden hose jet stream is a good example of a continuous flow from a nozzle except CIJ nozzles are tiny (less than .005 inch or about 1/10 millimeter). The ink stream naturally breaks into separate drops due to the Plateau–Rayleigh flow instability. Fluid streams can be broken into different size drops with vibration from a piezoelectric device. The use of a piezoelectric device should not be confused with Drop-On-Demand Inkjet which used the piezo to generate sound waves in nozzles or expand the fluid chamber size to push single drops from a nozzle. The CIJ 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). CIJ is popular in industry and publishing and not usually seen in any common retail printers for home use.

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 printed stock market quote symbols on a moving metal substrate belt and dropped on the table to be used as signage or remelted in the recorder to print other symbols. This was possibly the earliest example of printing "fabricated objects" with an inkjet.

The printhead

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. Waterbased inks may not added need heat. 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

Tandem printheads

Multiple printheads

Fabrication approaches

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
  • 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
  • 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.

Inkjet fluid materials

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. The liquid is positioned in the nozzle aperture by gravity (fluid storage tank must be slightly lower in height to the nozzle). The fluid surface tension also holds the fluid at the edge of the nozzle orifice (hole). The action of expelling a drop alters this natural steady fluid position condition. This condition is commonly called the meniscus of the fluid. The meniscus acts like a barrier and most be overcome to allow drop ejection. The meniscus also exerts strong forces when stretched. The lower the storage tank height the higher the force to expel a drop. The meniscus spring action timing alters the drop size, drop velocity and drive voltage in drops formation. Firing drops more frequently means the characteristics of the drop change constantly because of meniscus position. Each jettable material has different physical properties and requires different printer parameters and tank height settings. Materials can not just be switched. The temperature of the inkjet must be close controlled to maintain surface tension and viscosity more in a DOD system than in a Continuous (CIJ) jet system.

Generally, lower viscosity allows better droplet formation[25] and in practice only liquids with viscosity 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
  • Hutchings, Ian M.; Martin, Graham D., eds. (December 2012). Inkjet Technology for Digital Fabrication. Cambridge: Wiley. ISBN 978-0-470-68198-5.
  • de Gennes, Pierre-Gilles; Brochard-Wyart, Françoise; Quéré, David (2004). Capillarity and Wetting Phenomena. Springer New York. doi:10.1007/978-0-387-21656-0. ISBN 978-1-4419-1833-8.
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