The magic behind inkjet printing

In this first of a series of articles, we would like to look at the heart of all inkjet printers – the print head. With a better understanding of inkjet printers and how they work, you will be able to capitalize on this technology to propel your business to the next level in an ever-changing graphics world where inkjet is quickly becoming a predominate technology in many areas.

Print head technologies

So first and foremost, the “engine” of an inkjet printer, the print head, determines image quality and print speed of an inkjet printer. There are different types of print heads – thermal, piezoelectric and continuous inkjet. These, in turn, may use aqueous, UV, solvent, latex, solid, oil and eco-solvent inks, which in turn may use dye- or pigment-based colorants.

An inkjet printer reproduces a digital image by firing variably-sized droplets of liquid or molten material (ink) onto a page. The history of inkjet printing dates back to the 19th century and the technology was first developed in the early 1950s. Starting in the late 1970s, inkjet printers that could reproduce digital images were developed, mainly by Canon, Epson and Hewlett-Packard. Inkjet printing technologies are used in a wide range of applications including home, office, industrial, three-dimensional, medical and for textile printing.

There are three main technologies in use in today’s inkjet printers: thermal, piezoelectric and the continuous inkjet method. As we will see over the next few series of articles, certain print head technologies lend themselves better to different applications.

One of the most dramatic developments over the last few years has been the increase in the number of nozzles per print head and how print heads are being combined to create wider arrays. It wasn’t long ago that the largest print heads were about 1-inch wide and contained 128 nozzles, but now Canon makes a high-density print head with a total of 15,360 nozzles. HP also has a new scalable printing technology (SPT) that combines five heads in a single 4.25-wide print head array with a total of 10,560 nozzles.

Thermal print heads

Thermal printheadMost of the consumer inkjet printers (Canon, Lexmark and Hewlett-Packard) use print cartridges with a series of tiny electrically-heated chambers constructed by a photolithography process. To produce an image, the printer runs a pulse of current through the heating elements causing a steam explosion in the chamber to form a bubble, which propels a droplet of ink onto the paper (hence Canon’s trademark name of Bubblejet for its inkjet printers). The ink’s surface tension as well as the condensation and contraction of the vapour bubble, pulls a further charge of ink into the chamber through a narrow channel attached to an ink reservoir.

The ink used is known as aqueous or water-based ink, usually consisting of pigment or dye ink droplets. The principle was discovered by Canon engineer Ichiro Endo in August 1977.

Piezoelectric print heads

Piezoelectric printheadMost commercial and industrial type of inkjet printers as well as some consumer printers (Epson) uses a piezo-electric material (piezo-ceramic) in an ink-filled chamber behind each nozzle instead of a heating element.

The word “piezo” is derived from the Greek “piezein,” meaning to squeeze or push. When a voltage is applied, the piezoelectric material changes shape or size, which generates a pressure pulse in the fluid forcing a droplet of ink from the nozzle. This is essentially the same mechanism as the thermal inkjet but generates the pressure pulse using a different physical principle. Piezoelectric print heads (also called piezo print heads) allow a wider variety of inks to be used because there is no heat involved in the jetting process, but the print heads are also more expensive. Piezo print head technology is often used on production lines to mark products – for instance the use-before date is often applied to products with this technique; in this application, the head is stationary and the product moves past.

Continuous inkjet

Continuous ink jetContinuous inkjet is one of the oldest inkjet technologies in use and is fairly mature. The idea was first patented in 1867, by Lord Kelvin, and the first commercial devices (medical strip chart recorders) were introduced in 1951 by Siemens. One of its advantages is the very high velocity at which the ink droplets are fired, and this allows for a relatively long distance between print head and substrate. A piezoelectric crystal creates an acoustic wave as it vibrates within the gunbody and that, in turn, causes the stream of liquid to break into droplets at regular intervals – 64,000 to 165,000 drops per second may be achieved.

Another advantage of this continuous inkjet technology is that the nozzles don’t clog as easily because the inkjets are always firing, which allows volatile solvents such as ketones and alcohols to be used, giving the ink the ability to “bite” into the substrate and dry quickly. Therefore, continuous inkjet printing is ideal for marking and coding packages.