Inkjet: head to head

By Simon Eccles, Monday 24 April 2017

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If you’re new to the industry or are looking to take on a new process, inkjet technology can look complicated. To address this we are publishing a quick, back-to-basics primer on printhead technology.

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Inkjets dominate the professional wide-format sector of print, used for everything from signage and vehicle graphics to fine art and photography. Inkjet ‘presses’ are increasingly important, though by no means the only technology, used for digital printing of work such as documents, brochures, books, transactional work, direct mail personalisation and so on. Other inkjets are also increasingly used for products such as wallcoverings, instrument panels and direct to object items such as phone covers and promotional giftware.

The beating heart of any inkjet is the printhead and so here we look at how the various main types work. 

What’s the choice?

Most people will choose a printer based on its overall capability, such as size, speed, image quality, and the ink type and characteristics, which in turn determine what substrate it can be used with, and the subsequent characteristics of the printed substrate, such as gamut, durability, scratch resistance, fade resistance, flexibility, stretchability and so on. 

The head technology is a vital part of any inkjet printer, but it probably is not a major factor in a buying decision, largely because you don’t get a choice. Think of a car: if you buy a BMW, it will have a BMW-supplied engine and you can’t order it with one from Mercedes. Likewise if you buy a Mimaki large-format printer, you are buying it for its particular price/performance and ink type, but probably not because it uses Ricoh heads. 

While there appears to be a bewildering variety of inkjet heads on the market, in reality almost all inkjets are variations of three main technology families: thermal, piezo-electric and continuous. There are also valve jets, a technology with roots in the 19th century, but only used today in coding and marking systems. 

All head technologies work by generating small ink drops that are then projected onto the substrate, where they attach and dry/cure by various means depending on the combination of ink chemistry, substrate structure and energy applied.

Thermal and piezo-electric are classed as drop-on-demand (DOD) types, meaning they only generate ink drops when needed and those are all destined to go onto the final substrate surface. Nearly all of today’s commercial inkjet printers, whether wide-format or narrow, single-pass or multi-pass, use DOD heads. 

They are called that to distinguish them from continuous inkjet heads (CIJ), which have nozzles and ink feeds that generate a continuous stream of droplets all the time they are running. 

Piezo-electric DOD

By far the majority of commercial press-type inkjets, and many desktop and wide-format printers, use piezo-electric heads, usually with hundreds of nozzles per head. They have advantages such as long lifetime, the ability to handle a great variety of fluid chemistries, viscosities and pigment particle sizes, and often the facility to generate variable-sized drops from the same nozzle. This is called ‘greyscale’ though it refers to colours too. 

Piezo-electric heads come in many different configurations, but they all rely on the piezo-electric effect. This involves putting an electric current into a particular type of crystal. By pulsing the current, the crystal vibrates rapidly. 

When built into a small chamber that is filled with ink, with a feed coming in and a nozzle facing out, the piezo crystal acts as a pump, forcing ink out of the nozzle and drawing fresh ink in through the feed. 

The two main configurations work by either ‘bend’ or ‘shear’. Bend uses simple expansion to force a drop out of the head. Shear sets up an acoustic pressure wave in the ink that also generates drops but with less energy. 

Thermal DOD

Canon and HP invented the underlying technologies of thermal heads simultaneously in the 1970s. As with piezo, there can be hundreds of nozzles in a head.

Thermal printheads incorporate a thin film resistor element inside each ink-filled chamber. This is rapidly heated by electrical current. A bubble of superheated vapour forms and forces a drop of ink out of the nozzle (hence Canon’s trade name of BubbleJet). Then the heat is switched off, the bubble condenses rapidly and the loss of pressure draws replacement ink into the chamber. The process is very rapid, so thermal heads can operate very fast. 

These heads tend to be low-cost and easily replaceable, which is just as well because bubble-collapse cavitation and thermal stresses mean they have relatively short lifetimes. 

Their biggest limitation is that they only really work well with aqueous inks, that have a water-based carrier liquid for the pigments and other constituents. In many cases this means they can only print on paper, or other media that has a special (and expensive) ink-receptive coating that stops spread and aids drying. 

More volatile solvents flash off too quickly in the heads, while UV and other high-viscosity inks are too gloopy for the bubble effect to work. 

Aqueous ink is generally not durable enough for outdoor signage applications. However, since 2008 HP has been offering Latex ink, an emulsion of heat-activated pigment-carrying resins in a water carrier, that runs in its thermal heads. It is suited to a wide variety of media including vinyls and other plastic-based outdoor media. Mimaki and Ricoh also offer an ink they call Latex for use  with Ricoh’s piezo heads. 

As a general rule thermal heads only generate one drop size. However HP gets around that in its latest PageWide HDNA head by pairing nozzles with larger and smaller diameters, giving two drop sizes. By pairing these pairs in turn, up to five grey levels can be printed, or alternatively very high speeds (up to 305m/min) or very high resolutions (2,400dpi) can be achieved. 

HP uses PageWide thermal heads as single-pass arrays in its very-high-speed web presses and a few wide-format printers intended to compete with Memjet-based models. These all use aqueous inks. 

Memjet, a US-owned printhead manufacturer, also uses thermal technology in its high resolution, very-high-speed single-pass arrays, which are used by third parties including Canon and Xerox in A1 plan/poster printers, by Delphax in its élan A2+ digital press and by numerous small document and label printer OEMs. Memjet nozzles are particularly fine, which so far limits them to using aqueous pigment inks rather than the longer-lived types. 

Continuous Inkjet

CIJ generates a stream of ink drops from its nozzles all the time it is running, with some form of deflector system that directs the stream either onto the media where an image is needed or into a catch gutter and recirculation system. 

Most CIJs put an electrical surface charge on the drops, then switch a charge on or off in a metal plate that deflects the stream. However Kodak’s Stream technology uses puffs of air. 

Early photographic, fine art and proofers (from the late 1980s to mid-1990s) used CIJ, but these were wiped out by Epson with its first low-cost micro piezo printers. CIJ is still widely used for coding and marking heads by companies such as Domino or Videojet. These mostly use volatile solvent inks for a variety of surfaces such as paper, glass, metal and plastics, with pigment or dye colourants. 

Only Kodak has persevered with CIJ for commercial quality printing, originally with its VersaMark web-fed transactional printers, then the much higher quality Stream family of heads, used in its Prosper S-series bolt-on overprint units and in its Prosper Press family. 

Stream allows high-quality single-pass printing at high speeds that are competitive with HP’s thermal-head web presses and seem to beat any piezo based high-speed inkjet to date. All Stream applications so far have used aqueous inks. 

At Drupa 2016 Kodak revealed a next-generation CIJ technology it calls UltraStream, claiming it will give “printing of 600x1,800dpi at speeds of up to 150m/min on the widest array of paper and plastic substrates”. Intended mainly for OEM licensees, it can be configured in single-pass arrays from 203mm to 2,490mm wide. 

Electrical gubbins

It is fairly easy to understand how the different printhead technologies work and how this affects inks, image quality and so on. What you don’t often hear about is the vital importance of print head controllers. These take instructions from the digital front-end RIPs and turn them into electrical signals that actually drive the heads. Developing these is often handled by third-party specialists that are commissioned by printer developers once they have decided which heads and inks to use. Examples in the UK are Global Inkjet Systems and TTP Meteor (recently bought by Global Graphics), both based near Cambridge. 

This has been a quick introductory scamper through heads and how they work. To do it in more detail would fill a book. I know, I co-authored one in 2013. It’s called Talking Heads, but sadly it’s out of print. Whatever happened to print-on-demand?

Heads and inks

Here’s a small breakdown of what head technologies can be used with particular ink and other fluid types.

Piezo-electric

Aqueous (dye or pigment), eco-solvent, mild-solvent, strong-solvent, UV-cure with colourants, UV-cure clear varnish, bonder/primer fluids, Solvent-UV hybrid, water-UV hybrid, Latex (Mimaki or Ricoh) other resin-emulsion inks, opaque heavy pigment fluids (white, metallic, fluorescent, speciality), phase change (solid wax), Canon UVgel, some 3D printer fluids (mainly UV-resin binders), electro-conductive inks, food-safe edible aqueous inks. 

Thermal

Aqueous (dye or pigment), food-safe edible aqueous, Latex (HP), bonder primer inks (mainly saline solutions), water-UV hybrid (in principle, though nothing is available). 

Continuous inkjet 

Various volatile solvents (pigment or dye), aqueous.

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