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Printheads

Martin Johns, market development manager for Pro Graphics at Epson UK, tracks the path of printheads over the last few years, and takes a look at the next generation making waves in the industry

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Epson printheads in a linehead configuration

The progression of printheads

Inkjet printhead technology has come a long way in the past 20 years, and it is safe to predict that there are major advances to come over the next 20, with some commentators pitching them at the heart of a new manufacturing revolution. 

SignLink readers will be well aware of inkjets in graphical applications, such as forming printed images. How-ever, related technologies are also revolutionising sectors on the fringe of printing, such as ceramic tile decoration and printed micro-electronics, and starting to address entirely different applications, such as 3D materials deposition for manufacturing, and even medical applications. 

The microelectronics and computer technology needed to drive practical inkjets arrived in the 1970s, leading to early document printers. Since then the major changes have come in the development of printheads and the fluids that they jet. 

There are essentially two inkjet printhead technologies, thermal and piezo, with piezo being the most flexible in terms of the variety of fluids it can handle

There are essentially two inkjet printhead technologies, thermal and piezo, with piezo being the most flexible in terms of the variety of fluids it can handle. Large-format inkjets for signage started appearing in the early 1990s, with the definitive piezo printhead types able to handle solvent and UV-curing outdoor-durable inks appearing later in that decade. 

Although today there is what may seem a bewildering choice of printhead types, in reality there is just a handful of distinct technologies and only a few printhead manufacturers worldwide. Some printer manufacturers own their own printhead factories (including Epson and its sister company Seiko Infotech), but the majority buy their printheads elsewhere and then integrate them into their own hardware solutions. 

Demand the best 

Graphics detailing the components of Epson’s PrecisionCore printhead technology

Nearly all inkjet printheads used in today’s printing industry are drop-on-demand (DoD). These can be precisely controlled to produce ink drops when required. There are two major types of DoD printhead: thermal and piezo-electric. Thermal works by using a heating element in each ink-filled chamber to form a bubble of super-heated vapour, that expands to force a drop of ink out of the nozzle. Piezo uses a piezo-electric material in the ink chamber that responds to an electric current by expanding, causing a pressure wave that ejects a drop of ink out of the nozzle. Thermal heads are low-cost to make and replace, but suffer fairly short lives due to thermal stress. They are widely used for office, photo, and poster work, but can only operate with aqueous (water-based) inks. 

Nearly all inkjet printheads used in today’s printing industry are drop-on-demand (DoD). These can be precisely controlled to produce ink drops when required

Piezo printheads are permanent and will last the lifetime of the printer. They can handle a much wider range of fluids than thermal heads, including all of the ink types used in commercial print, such as aqueous, solvent, UV-cured, solvent-UV hybrids, dye sublimation, various textile inks and phase-change solid wax inks. The ability to handle viscous fluids and large pigment particles has led to the development of white and metallic inks. They can also handle non-print fluids for electronics, medical, and additive 3D manufacturing work.

Some piezo heads feature variable sized droplet technology, which means they can generate different drop sizes on the fly. This can create a visual effect of more detail than an equivalent ‘binary’ head with drops all the same size. All of Epson’s current large-format and industrial printheads are DoD piezo-electric variable droplet types. During the 2000s Epson introduced its advanced Micro Piezo Thin Film Piezo (TFP) technology that allowed the generation of very precisely controlled, very small drops by printheads with a high nozzle density. Micro Piezo TFP has been the basis of the Epson Stylus Pro and the current SureColor printer ranges. Similar technology is also used to print semiconductor electronics as well as colour filters for LCD TVs. 

The new kid on the block

Graphics detailing the components of Epson’s PrecisionCore printhead technology

This year Epson launched a significant new technology, Micro TFP, in a new generation, printhead technology called PrecisionCore. It is the result of one of the largest research and development (R and D) investments in the company’s history. Micro TFP doubles the piezo element’s flexing power, allowing further miniaturisation and scalability opening up new possibilities for fluid handling and substrate compatibility. 

Today nearly all large-format inkjets depend on a microweave technique to build up an image by multiple passes of the printhead carriage over the substrate. This minimises the number of printheads needed, but limits the throughput in terms of sheets per minute or metres per hour. An alternative that is increasingly being used for digital label printing and fast commercial inkjet web presses, is to arrange a fixed array of printheads over the full width of the head, so the image is printed at high speed in a single pass of the substrate. 

Today nearly all large-format inkjets depend on a microweave technique to build up an image by multiple passes of the printhead carriage over the substrate

What will the future hold? Printhead design and development continues apace and we are already seeing their ability to handle a range of challenging fluids—such as granular metallic inks and heavy pigmented white and viscous varnish inks—for signage and label applications. We are also seeing printheads that can jet a wider range of drop sizes to give users the choice of high quality or high coverage, or the optimum combination of both. 

The increasing ability of printheads coming to market to jet a range of fluids onto a range of materials reliably, quickly, and with consistent quality within industrial environments, is open-ing up exciting new markets and commercial opportunities for inkjet. And the printhead is the core technology that will drive innovation in many sectors.  

Inkjets for graphics will continue to advance, but for many people the promise of advanced manufacturing and additive manufacturing is particularly interesting. Desktop inkjets have already been modified to print edible inks for cake decorations, and maybe one day we’ll even see the 3D printable pizza.

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