TechArchive

Original article date: June 1996

Although most of the low-cost colour printers currently on the market are referred to as inkjet printers, there are in fact two major technologies in use. Most 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, however, 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. Almost every other manufacturer of colour inkjet printers has chosen to go with the thermal method.

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, it must above all be resistant to heat since the firing process is heat-based. 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. 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 had been used quite 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 E L 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 more expensive printers). It used a piezo disk which was charged (pulled back) and then fired to release one droplet of ink at a time.

Conventional Piezo technology did, however, present some problems and overcoming these proved to be the main stumbling block and delaying factor. Despite its quality, reliability and speed, the piezo head was very large, making the printer very large in turn. It also required a high voltage drive, much larger than the thermal machines with their disposable heads.

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 20um thick.

The pressure that can be exerted by the multi-layer head is about 100 times that of earlier type heads. As a result, the charge is in fact bigger than for earlier heads, making the firing process more powerful and much faster. Simultaneously, the energy requirements for the new heads show a major improvement, as it needs only 20V rather than the 100 needed by the old type. This is significant also in that the price of circuitry is generally much lower at under 30V. At the same time, the multi-layer head has also kept the initial advantages of piezo in comparison to thermal technology, namely the fact 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 piezo-electric heads. Secondly, use of a permanent head produces much less waste.

Since the target of most inkjet printing is paper, 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 paper 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 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, creating an effect rather like a bow being strung, 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 are easily seen 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 which 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.

Since piezo technology uses electrical impulses to charge and fire, it also makes possible much better control of the size and positioning of the ink droplets. 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 which 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 up the way for using this printing technology for media other than paper.

The multi-layer actuator head also improves printing speed significantly. The frequency at which the head operates is ultimately regulated by the ink in the cavity, 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.

• Epson
• Tel: 01442 227291

June 1996