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.

Drop formation

Various drop formation technologies exist, and can be classified into two main types: continuous inkjet and drop-on-demand.
While CIJ has a straightforward drop creation and sophisticated drop trajectory manipulation, DOD has sophisticated drop creation and no trajectory manipulation.

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. Commercial printheads can have tens to thousands of nozzles.
The two leading technologies for forcing ink out of a nozzle on demand are thermal DOD and piezoelectric DOD. Additional technologies include electrospray, acoustic discharge, electrostatic membrane and thermal bimorph.

Piezoelectric DOD

Piezoelectric DOD was invented in the 1970s. 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 for subtractive color and suggestions from fellow employees about investment casting encouraged this patent.

Thermal inkjet (TIJ) DOD

Thermal DOD was introduced in the 1980s by Canon and Hewlett-Packard. 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. 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 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 or multiple jets.
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.
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.
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

The printhead must have heating capability to print metallic alloys such as lead, tin, indium, zinc and aluminium. The process of printing of low-melting point metals is called "direct melt printing".

Photo inkjet printhead

Tandem printheads

Multiple printheads

Fabrication approaches

The printed material is sometimes only one step in the process, which may include deposition of a precursor followed by a catalyst, sintering, photonic curing, electroless plating etc., to give the final result.

Subtractive inkjet fabrication

Inkjettable 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.
Drop formation is governed by two 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.
Generally, lower viscosity allows better droplet formation and in practice only liquids with viscosities of 2-50 mPa s can be printed. More precisely, liquids whose Ohnesorge number is larger than 0.1 and smaller than 1 are jettable.