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Types of inkjet printing

The term ‘inkjet printing’ is used to describe the process involved in directing individual ink droplets onto a substrate (media). There are two basic processes available in wide format printers today:

Drop on demand printers (DOD) work on a simple principle – when the control mechanism wants to fire a droplet of ink onto a substrate it sends a signal to the inkjet head to tell it to do just that (one signal=one droplet). While both thermal and piezo rely on strategically placing drops of ink on to the media, how the drops are dispelled from the inkjet cartridge nozzles is what differentiates the two.

Drop On Demand Thermal

Thermal DOD inkjet printing works by heating the ink inside the print cartridge to about 400°F. As the ink travels from a cartridges reservoir, it passes through a micro-filter (which removes particles which could clog the jets) to the ink channel. When printing starts, the thermal heaters heat the ink in the channel, causing it to expand and shoot through the jets. The thermal heater cools and the vacant ink channel are replenished with filtered ink. This cycle can repeat up to 6,900 times per second.

The big difference between thermal and piezo inkjet printers is that thermal printers use disposable print cartridges that contain both the ink supply and the print head. Typically, these cartridge-driven units produce excellent print quality at 300-600dpi but can tend to produce slower print times.drop on demand thermal printing sequence

Printing sequence:

Drop On Demand Piezo

Piezo DOD inkjet printing, in contrast to thermal, uses a permanent print head. The technology is very reliable and has the benefit of allowing greater freedom in ink development.

Piezo inkjet printing relies on different principles for the expulsion of ink from the cartridge nozzles. With this technology, an electrical charge is applied to the cartridge nozzles and excites a small piezo crystal that is inside. When the crystal is stimulated, the crystals change shape and squeeze the ink chamber. The ink is then forcefully expelled from the nozzle onto the media.

Printing sequence:drop on demand piezo printing sequence

Since the piezo electric process does not utilise heat, the cartridge life of these printers is greatly expanded – cartridges should typically last a minimum of one year under heavy usage.