WDC Investigates: Printing – Print Heads

There are two main types of inkjet print head. They both incorporate tiny nozzles, through which ink drops are fired in precise doses many hundreds or even thousands of times per second. The most widespread way of firing the ink out of the print head is thermally. The chamber behind the nozzle is filled with ink, a heater vaporises the ink and this causes it to expand and escape via the nozzle, where it cools and forms a droplet before it reaches the media surface.

Some printers, notably those made by Epson, use piezo technology. Pass a current over a piezo material and it changes shape. This forms the basis for a microscopic piezo pump behind each nozzle in the print head. The ink is not heated this time. Manufacturers using both types of print head variously claim that theirs is superior. But both types deliver very good results, though thermal heads do seem to be used in printers that are physically faster.

Incidentally, some printers incorporate their print heads in their disposable ink cartridges. Others have permanent print heads, which can only be replaced by the manufacturer’s service department and is often not economical. Kodak, some HP printers and some Canon printers have user-replaceable print heads that are independent of the ink cartridge, though in normal service you should not need to replace them.

Print Head Mechanisms

So how does the print head work?

Two main mechanisms lie behind the operation of inkjet print heads – thermal or ‘bubble’ jet and micro piezo. The former works by heating the ink, vaporising it, causing it to expand through the nozzle. Piezo electromechanically squeezes the ink through the nozzle without heating it.

In diagram form you can see how an Epson micro-piezo print head is constructed. Channels form into chambers behind each nozzle and when a current passes through the piezo material it deforms, squeezing the chamber and expels a droplet of ink.