Profit through Innovation - Developments in inkjet print head technology

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Developments in inkjet print head technology

Written by Pira International

One of the key areas in the overall progress of inkjet printing is that of print heads. Development is ongoing, working towards improved print speeds, quality and resolution, versatility in handling different ink bases and viscosity, robustness of the print heads for industrial applications, and width of printing swathe.

For packaging and converting lines there needs to be a balance between the image resolution and the speed at which the line travels. This is accomplished in many different ways, including variable dot size, dithering, greyscale enhanced resolution, higher frequency ejection of ink droplets and formation of wide arrays. The challenge is to achieve the best balance between all these items, and drop-on-demand (DoD – see below) heads are currently best placed to deliver high-resolution printing for full-colour images, flexibility in ink and substrate selection for many commercial packaging applications, including card, carton, board, corrugated, plastics, foil, film and so forth.

Current leading manufacturers include Aprion, Domino, Imaje, Kodak, Screen/Epson, Sony LD Shot, Spectra, Toshiba Tec and Xaar.

In March 2004, Xaar announced that it was working with Agfa to develop second-generation OmniDot greyscale inkjet print heads. Agfa had acquired Dotrix’s the.factory about the same time. Other collaborations emerged at May 2004’s drupa exhibition, including Domino’s agreement with Heidelberg, Kodak unveiling its Graphic Communications Group, and Xaar’s collaboration with MAN Roland to develop inkjet printing systems for in-line coating applications. Konica Minolta has also created a new division, Konica Minolta IJ Technologies, to expand its print head component business to target the commercial and textile inkjet markets. These associations reflect the increasing interest in the attractions of inkjet printing: faster speeds, improved quality and reliability.

In the wide format sector, inkjet printing is now widely used for building wraps and fleet advertising on trucks. The substrates used (PVC and mesh in numerous variants) and the use of solvent inks is causing environmental concerns in certain sectors. Manufacturers are being pressed to move from solvent inks to UV ink solutions, which are going to affect large-format manufacturers such as Vutek and NUR.

Source: Pira International

Types of inkjet printers

The most important aspect of the digital inkjet technologies is that they are (in the main) non-contact forms of printing. Because of this, they can be used to print onto almost any surface, whether rigid, flexible, porous or non-porous, even or uneven.

The second aspect is that they are digital forms of printing and capable of using a variety of ink types. They can manage variable information on demand, using print heads that have large numbers of nozzles, printing with relatively small drops of ink to give fast coverage of the substrate with high quality.

These attributes mean that this relatively recent form of printing can be applied to many areas of the existing printing trade to advantage.

Figure 1. Principle of Xaar head. Source: DPS

Continuous inkjet

With continuous inkjet technology, the main feature is that pumping ink through a nozzle on which a piezo crystal is constantly vibrating generates a continuous stream of ink droplets. An electric charge is applied to the droplets to enable their deflection onto the substrate: some bubbles do not hit the substrate but are collected and returned to the ink reservoir.

The main types are binary deflection, multiple deflection and Hertz. The continuous inkjet principle is used mainly for very high-speed coding and marking applications, and has a place in the wide format and large format inkjet sectors.

Binary deflection

In this process, pumping ink through a nozzle on which a piezo crystal continuously vibrates generates a continuous stream of ink droplets. By applying a voltage to a charge electrode, each droplet can be individually given a charge. Conversely, removal of the voltage removes the charge. Charged drops may be passed through an electrostatic field and be deflected by that field to hit predetermined targets on the substrate.

Multi-level deflection

This is a modification of binary deflection – with drop generation as well as print head configuration being the same for both technologies. With multi-level deflection, the ink droplets are electrically charged and deflected to different levels on the substrate. By using this technique, it is possible to print complete characters (i.e. more than one row of pixels), for instance, in a single movement of the print head relative to the substrate.

Drop-on-demand (DoD) inkjet processes

With the DoD process, electric signals are applied to a piezo crystal in the print head and because of this, the crystal deforms immediately to mechanically eject the ink onto the substrate – there is no ink left over to be recirculated to an ink reservoir.

Figure 2. Principle of bitjet head. Source: DPS

Bubble jet/thermal inkjet

In this technology, the ink is heated in the ink channel until a steam bubble is created. This bubble creates a local high pressure that ejects the ink out of the print head onto the substrate. Bubble jet is mainly used in desktop printing applications, due to its relatively short lifetime and limitations on the inks that it can handle.

The most significant problem with thermal inkjets is power consumption needed for the rapid boiling of water to produce a vapour bubble in a fraction of a second to expel a droplet of ink. Typically, this happens about 12,000 times a second. Water has a very high heat capacity and must be superheated in thermal inkjet applications. This leads to an efficiency of around 0.02 per cent from electricity input to drop momentum (and increased surface area) output.

Piezoelectric

In piezoelectric inkjets, a crystal is placed on the wall of the ink chamber in the print head. When an electrical pulse is applied to the crystal, the nozzle head deforms to create a pressure wave that causes the ink droplet to be ejected. In some systems this happens 20,000, or more, times a second.

Xaar’s piezo drop-on-demand binary inkjet

This printing process uses lead zirconate titanate (PZT) material that deforms when an electric field is applied to it. The deformation is harnessed in order to eject ink from the print head in a highly controlled and repeatable manner. Each pixel on the substrate is either covered with ink, or not – a binary choice.

Xaar is now working in partnership with Toshiba TEC to produce a print head that has a six pico-litre drop. This technology allows up to seven shots at the same pixel point to produce images with a far higher (apparent) resolution. The technology achieves a greyscale effect by firing micro-droplets so fast that in flight they merge to become one. This has the effect of reducing the inherent dot gain in the process. In this way, the size of the droplet reaching the substrate is varied, and the final quality is almost photographic in impact.

Figure 3. Principle of thermal inkjet. Source: DPS

This technology is used in a range of industrial printing applications, including coding and marking, and is a versatile technology able to print on a wide range of surfaces with virtually any type of ink. Shared channel walls divide the ink chambers in the print head, with an electrode attached to each. When an electric current is passed through the electrodes, the ink channel walls deform to eject ink directly onto the substrate. This deformation is in the manner of the movement of a pack of cards when sheared to one side – the height and width of the chambers does not alter. It’s the shift to the side that ejects the ink.

Inkjet machines that take advantage of Xaar’s greyscale technology work in a similar way to binary inkjet systems, but have the ability to fire a range of ink drop sizes – currently up to eight levels of greyscale – that makes these heads different to others on the market. This technology achieves extremely high apparent print resolutions.

PicoJet print head

The robust design of the PicoJet print head is based on piezoelectric bend-mode technology. Bend-mode technology is claimed to greatly enhance print-head reliability over other modes of piezo inkjet technology. First, by bonding the piezo ceramic to a stainless-steel diaphragm, the ceramic is separated from the ink, preventing blockage. Second, since the ink has no contact with the piezo structure, electrical field interference with ink is eliminated. Finally, only one-tenth of a micron of motion is required to eject a drop using the robust print head design. With so little motion required in the device, reliability is greatly improved

PicoJet’s PJ-N32 series of print heads uses bend-mode piezoelectric drop-on-demand inkjet technology. This operates by providing an electrical charge to the piezoelectric ceramic material. The ceramic material is attached to a stainless steel diaphragm that deforms as the ceramic material expands and contracts. The deformation of the diaphragm causes the ink volume to change in the pressure chamber to generate a pressure wave directed to the print head orifice.

Thermal versus piezo

Figure 4. Principle of multi-deflection inkjet. Source DPS

Some inkjet printers, including the popular bubble-jet machines, use what is known as thermal technology, a method whereby heat is used to squirt (or fire) ink onto the paper through tiny nozzles. The actual squirting is initiated by heating the ink to create a bubble until it bursts, driven by the pressure thus created. The technology, used by Epson in its Stylus series, for example, achieves the same end of squirting ink onto the paper but uses a different method. Instead of using heat to fire the ink, it uses a piezoelectric cell that exerts mechanical pressure on the ink to force it out of the chamber.

Epson was also drawn to the greater flexibility that piezo technology offered in terms of the type of ink that could be used. Compatibility between ink and paper is an important element in ultimate print quality. Thermal technology imposes a severe limitation in that

whatever type of ink is used must be resistant to heat, since the firing process is heat-based. The use of heat in thermal printers creates a need for a cooling process as well, which requires more time and consumes more energy.

Piezo technology also offers greater potential for controlling the shape and speed of ink droplets since the process of charging the chamber and firing the ink is a simple electrical one.

Piezo technology was used extensively in early generations of printers, but there was a general perception that it was well past its prime. The original concept had been developed in Germany by EL Kyser in the 1970s, and is a form of on-demand inkjet technology (which releases droplets as needed, rather than the alternative continuous method, which is a feature of some of the larger, more expensive printers). It uses a piezo disk that is charged (pulled back), and then fired to release one droplet of inkat a time.

The development in 1989 of the multi-layer actuator head solved the problems of earlier heads. Instead of using a single piezo element that is large and bulky, the multi-layer actuator head uses a multi-layer ceramic construction (including thin piezo layer elements). Each piezo element is only about 20µm thick. The pressure that can be exerted by the multi-layer head is about 100 times that of earlier type heads, and the droplet is expelled faster. At the same time, the multi-layer head has also kept the initial advantages of piezo in comparison with thermal technology, namely that it is a permanent attachment rather than a disposable element.

Using a permanent head offers several advantages over the disposable heads used in thermal inkjet printers. First of all, the need to replace the head on a regular basis increases the overall cost of ownership of the printer, but is inevitable for thermal inkjet printers since the heat element puts high levels of stress on the heads. This problem does not exist with piezoelectric heads. Secondly, use of a permanent head produces much less waste.

The way ink droplets are fired is one of the most crucial elements in establishing print quality. Getting each droplet cleanly from the nozzle to the substrate (paper, more often than not) without creating misting is the ultimate target for all inkjet printers. To achieve this, the multi-layer actuator head controls the meniscus on the surface of the nozzle when firing the droplet. The make-up of the multi-layer actuator head developed by Epson for its Stylus printers, for example, consists of multi-layered piezo elements sitting behind a vibrating plate that lies against the ink chamber, with a tiny nozzle at the top of the chamber.

When a very short electrical pulse is applied to the piezo element, it expands and then contracts. This motion is linked with the motion of the meniscus through the vibrating plate. The meniscus on the surface of the nozzle expands in firing and then contracts. This happens at very great speed and at very high pressure, making the droplet fly out without misting. In contrast, thermal heads use heat to create pressure, thus firing the bubbles. This technology only makes it possible to expand the meniscus, not to contract it. Both technologies succeed in the basic task of getting the ink from the head to the page, but the piezo head allows for much greater finesse in doing this.

The results of this high degree of control can be seen clearly in slow-motion photographs of ink droplets being fired from a piezo head and a thermal head. The piezo droplets are almost perfectly round and fly out evenly. On the other hand, the thermal droplets are elongated and carry a long tail that disintegrates into smaller droplets at its end. The result of this is that the ink is liable to mist, be uneven and create satellites as it hits the page.

To achieve high-quality printing, the size of the droplet has to be accurate and even, and the positioning has to be on target. With piezo heads, both the charging and firing process can be controlled with a high degree of precision in terms of pressure, timing and speed, so as to determine both the even volume of each droplet and exact positioning of each droplet on target.

The ability to calibrate precisely the way the ink droplets are fired onto the paper makes a big difference to print quality. In the case of text, the letters are clearly outlined, lacking the jagged edges that are often associated with inkjet printing. This is particularly significant when it comes to full-colour printing, since misaligned dots can create unpleasant bleed effects resulting in non-true colour reproduction.

Since with the piezo head there is no need to worry about heat resistance, inks can be chosen exclusively for the brilliance of their colour and their ability to dry more or less quickly. In the longer-term, this will also open the way for using this printing technology for media other than paper. The multi-layer actuator head also improves printing speed significantly. The ink in the cavity ultimately regulates the frequency at which the head operates, but the piezo head also has a latent power of 100kHz at its disposal. Thermal heads need cooling-down time after each firing, which limits the degree to which speed can be increased. Another approach to increasing printing speed is to increase the number of ink nozzles, but that too has its limits for thermal inkjet printers, since the same problem of energy generation and cooling remain.

Inkjet printers have become the most popular choice at the low end of the market due to their undoubted quality at a low price. The two technologies, continuous and drop-on-demand, continue to develop, with many manufacturers still evolving solutions. Although there are many manufacturers, their printers are all equipped with either thermal or piezo print heads.

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