The ultimate guide for printhead technologies

Simon Eccles finds out more about inkjet printheads and takes a look at the next generation making waves in the print industry. 

Drop-on-demand, continuous inkjet, piezo-electric, thermal, solid, binary, greyscale. These are all terms glibly bandied about when describing inkjet printers, and specifically their print head types.

If you know what they mean, then these terms let you predict pretty well what the printer is for and how it will operate. If you don’t, it’s rare for anyone to stop and explain them.

So here’s where we stop and explain them. Some of the terms describe the fundamental design of print heads, others describe what they do or how they operate. Some can double up for a more precise explanation, such as a piezo greyscale head, others are mutually exclusive - you can’t have a binary greyscale head.

This then is FESPA’s jargon-busting guide to inkjet print heads. Starting with what is a print head anyway?

The component of an inkjet printer that projects drops of ink at the media. This is a very high precision unit and manufacturing it involves a lot of intellectual property (know-how) and heavy investment in clean-room factories. Modern printheads often use manufacturing techniques (such as thin film silicon MEMS) that have a lot in common with microchip fabrication.

Inside a typical printhead there are driver electronics, ink feed attachments, and at least one and usually hundreds of ink chambers leading to nozzles, which are holes in the nozzle plate.

The ink input channels are only a few tens of microns across and the nozzles are typically 20-50 microns. A human hair is approximately 80 microns across.

Most printheads used in signage and other graphics application will have hundreds of nozzles that are individually controlled to generate and project drops (see also “Drop on Demand”). Generating what can be millions of drops in a pass of the head and making sure they hit the media in the right place takes very advanced electronics.

Some inkjets have a single nozzle and project a continuous stream of drops, which are deflected towards or away from the media be either electrostatic plates or air blasts. These tend to be used in coding and marking systems rather than graphics. See Continuous Inkjet.

Printhead makers

An exploded view of a printhead showing its components, in this case a Xaar 1001 piezo type. 

 

Although there are hundreds of printer makers worldwide, they all obtain their printheads from a relatively small number of specialist manufacturers, and then integrate them into the printers themselves with a combination of mounts, electronics, ink feeds, firmware and driver software.

Only a handful of large format printer makers have their own printhead factories, including Canon, Epson/Seiko-Epson, Fujifilm (though its subsidiary Fujifilm Dimatix), HP and Xerox.

All the others buy in heads or operate joint ventures with printer makers. Most of the manufacturers mentioned above will supply heads to other manufacturers on an OEM basis (though sometimes they keep the latest models for themselves). Other head makers include Konica Minolta, Kyocera, Panasonic, Ricoh, Toshiba TEC and Xaar.

Drop-on-demand (DoD)

This is a general term for the type of print head most normally found in modern inkjets used for high quality graphics, including all the wide format printers you’ll see at FESPA shows and on this website.

Drop-on-demand means that inkjet nozzles generate and project ink drops when and where they are needed to produce a mark on the media. The term was mainly coined to contrast with the earlier continuous flow type heads (see continuous flow below).

Drop-on-demand heads are further subdivided into thermal or piezo-electric types – see below.

Continuous inkjet

Principle of continuous inkjet, showing stream deflection. Source: Xaar. 

An inkjet printhead that projects a continuous stream of droplets all the time the printer is running. Normally there will only be one nozzle per head, but an array of heads may be used to build up a wider printing swathe.

The stream is deflected towards or away from the media by either charged metal plates with an electrostatic field, or (in the case of Kodak) by precisely timed blasts of air. Unwanted ink is collected in a catch gutter and may be filtered and returned to the storage tank.

Today these heads are usually to be found in coding and marking systems rather than sophisticated graphics printers.

The exception is the Kodak Prosper family of printheads which use a highly developed continuous inkjet technology called Stream, giving very high image quality. At present Prosper and Stream are not used in any dedicated sign and display type printers.

Thermal printheads

Caption: Principle of a thermal inkjet. Source: Xaar. 

These were the first type of drop-on-demand print heads and were used in the first desktop inkjets in the early 1980s. Thermal printheads are efficient and can give very high image quality and speeds that compete with piezo-electric heads, but unlike piezo they only work with water-based inks so are normally confined to indoor applications.

HP’s Latex inks are an exception: they work with HP thermal heads. The reason is that they have a heat-activated polymer in a water suspension that’s fine for outdoor use.

Thermal technology was invented independently and simultaneously in the 1970s by printhead technolo in Japan and Hewlett-Packard in the USA, which decided to pool their patents rather than fight each other.

The principle is that an element inside an ink chamber in the print head is rapidly heated to the point that the liquid ink vaporises and forms a bubble of gas, which expands and forces a drop of ink out of a hole (the nozzle) at one end of the chamber.

The heat element is then switched off, so the gas bubble cools, condenses and contracts. Surface tension at the nozzle stops air being drawn in backwards, so instead more liquid ink is drawn into the chamber from the feed pipes. Canon, joint inventor of thermal heads, coined the term Bubble jet because of the way they work.

So far there are no true greyscale thermal heads, so they are all binary, meaning the drops are always the same size. However HP has developed paired nozzles of different sizes that go some way towards a greyscale effect.

The thermal stresses wear the heads out quickly, so the heads are designed to be consumables, so they can be easily and cheaply replaced after a few tens or hundreds of operating hours.

Piezo-electric printheads

Principle of a bend mode piezo-electric inkjet. Source: Xaar

Often just called piezo heads. These drop-on-demand heads started appearing in early large format printers in the 1990s and revolutionised the sector. For the first time it meant that solvent and UV-cured inks originally associated with screen process printing could now be printed digitally.

Piezo heads are all based on the principle that a particular type of crystal (often lead zirconate titanate in inkjets, written as PZT) expands or contracts when an electric current is passed though it and switched off again. This expansion/contraction is used as the basis of a pump in the ink chamber.

Depending on the configuration of the crystals (called in “bend” or “shear” modes), a two-way expansion either draws in ink and then forces it ink out of the chamber via the nozzle (Epson uses this), or it sets up acoustic pressure waves that have the same effect but with less energy (Xaar uses this).

The electrical current can be switched on and off very rapidly and the expansion/contraction of the crystal is likewise almost instantaneous, so there is far more scope for controlling dot formation than with thermal heads.

Among other things this means that some piezo heads can generate drops of variable size from the same chamber and nozzle, giving different ink densities on the media. These are called greyscale heads (see below).

The piezo-electric effect works with pretty well any fluid, so piezo print heads can be built to handle solvent based inks, UV cured inks (including some used for 3D printing), and aqueous inks. They can also be used for challenging fluids such as electro-conductive inks, large-particle opaque white and metallic inks, 3D printing inks and phase-change inks that are a liquid when they reach the ink chamber.

Piezo printheads last much longer than thermal heads because there is less thermal stress and the piezo crystals can expand/contract millions of times. A piezo head is normally intended to last for the lifetime of the machine, as long as there is no fatal blockage or external damage. However they also cost considerably more to make and buy than thermal heads, so users need to put more effort into maintaining them.

Binary or greyscale?

This Epson Micro piezo PrecisonCore TFT printhead has a native resolution and generates variable drop sizes from 1.5 to 23 picolitres. 

These terms indicate whether the printhead fires drops all of the same size or if they can be varied in some way so the density of the ink reaching the media can be controlled with lighter shades. Combined with halftoning techniques, greyscale can considerably extend the tonal range of an inkjet while allowing relatively modest nozzle pitches or fewer passes to be used.

Piezo printheads were originally always binary, meaning they would only generate ink drops of all the same size. You can get a good range of tones from a binary head by using halftone techniques, but highlight tones can look a bit grainy unless you use ultra-fine nozzle pitches (and/or add extra, lighter coloured inks).

Typical binary drop sizes are from 30 to 100 picolitres. It’s possible to achieve smaller droplets for finer results, but this means that more passes are needed to build up the density of solid areas in the print, so printing is slower.

Greyscale heads can vary the density of individually printed dots, so a drop might be able to show anything from 30% or 50% to 100% colour. The advantage is that lower resolutions and fewer head passes can achieve the same “effective resolution” as binary heads with much higher native resolutions.

For example a resolution of 360 dpi with a greyscale head is said to give the same effect as 1,000 dpi binary, which is as good as you’ll normally ever need for photographs and blends even for close-up viewing.

Piezo heads vary the dots sizes by several different methods, usually depending on the individual manufacturer and what patents it holds or want sot avoid infringing. Depending on the precise methods there may be between three and size drop sizes available.

The smallest size on the finest printheads (often used for photography) is less than 2 picolitres). For signage printers, sizes of 10 to 20 picolitres are more common for the smallest drops, as speed and coverage matters more than close-in viewing quality.

Thermal greyscale

True variable drop sizes are so far only possible with piezo heads. However HP has developed a form of greyscale for its thermal PageWide heads, called High Definition Nozzle Architecture. So far this is only used on its huge T-series inkjet web presses for commercial print, and not on the wide format PageWide XL single-pass models that are so far mainly used for CAD and plan work.

Although the drops are always the same size from each nozzle, it pairs a large and a small nozzle very closely together in the printhead and treats them as one imaging element. It then takes two pairs of nozzles and controls them as a single imaging element for greyscale purposes.

By firing different combinations of two small and two large nozzles, five grey levels can be achieved (actually it is white plus four levels). The HDNA nozzle pitch is 2,400 dpi, so the nozzle pairs have a native resolution of 1,200 dpi and the greyscale sets are 600 dpi.

Further density control is possible by using different ink colours in the large and small nozzles (eg cyan and light cyan). The nozzle sets can also be controlled separately for higher speeds or resolutions, with fewer grey levels.

Native resolution

This Memjet Waterfall printhead is 222.8 mm wide and is designed for single-pass printing. It has 70,400 nozzles in two rows, giving a native resolution of 1,600 dpi. 

This is a description of the nozzle pitch, meaning the actual number of ink drops a print head can produce over a given area. The industry normally states these as dots per inch, rather than a metric measure. So if a printhead is 1.5 inches (38 mm) wide and has 540 nozzles across its width, then the native resolution is 360 dpi.

Many wide format inkjets build up images in a series of overlapping passes, so there may be many more drops per inch on the media than the native resolution alone can give. The higher the dpi, the more the final print can look like a continuous tone photograph.

Greyscale heads allow a range of different dot densities to be created, giving more tonal range compared to a binary head of the same nozzle pitch. which in turn gives a better simulation of continuous tone.

It’s therefore common for greyscale printer makers to talk about “equivalent” resolutions, meaning for example that a 360 dpi greyscale head might give the perceived quality equivalent of a 1,000 dpi binary head.

There are printheads with very high native resolutions too, such as Epson’s Micro Piezo PrecisionCore TFT heads (used on its SureColor printers) have a native 600 dpi resolution and five drop sizes ranging from 1.5 to 23 picolitres.

HP’s PageWide HDNA, mentioned above, has a nozzle pitch of 2,400 dpi by alternating large and small nozzles, but as they are controlled as pairs then the native resolution can be considered as 1,200 dpi. 

Members of the industry keen to learn more about HP and Epson kits and the benefits they can offer to their businesses can speak with experts from the companies at FESPA 2017, which runs from May 8-12 at the Hamburg Messe in Germany. 

HP and Epson will be two of more than 700 brands that will have a presence at the event, which is expected to attract a record number of visitors.

To find out more about FESPA 2017, visit: http://www.fespa2017.com.  Visitors can get free entry to the exhibition by registering online, quoting reference code: FESG702.