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Drop on demand inkjet printing (DOD)

What is drop on demand inkjet printing?

Drop on demand (DOD) is a broad classification of inkjet printing technology where drops are ejected from the printhead only when required. In general, the drops are formed by the creation of a pressure pulse within the printhead. The particular method used to generate this pressure pulse creates the primary subcategories within DOD, namely thermal, piezo and electrostatic.

Thermal inkjet - diagram Thermal inkjet (TIJ) is the technology most used in consumer desktop printers but is also making some inroads into industrial inkjet. In this technology, drops are formed by rapidly heating a resistive element in a small chamber containing the ink. The temperature of the resistive element rises to 350-400慢, causing a thin film of ink above the heater to vaporise into a rapidly expanding bubble, causing a pressure pulse that forces a drop of ink through the nozzle. Ejection of the drop leaves a void in the chamber, which is then filled by replacement fluid in preparation for creation of the next drop.

The advantages of thermal inkjet include the potential for very small drop sizes and high nozzle density. High nozzle density leads to compact devices, lower printhead costs and the potential for high native print resolution. The disadvantages of the technology are primarily related to limitations of the fluids which can be used. Not only does the fluid have to contain a material that can be vapourised (usually meaning an aqueous or part-aqueous solution) but must withstand the effects of ultra high temperatures. With a poorly designed fluid, these high temperatures can cause a hard coating to form on the resistive element (kogation) which then reduces its efficiency and ultimately the life of the printhead. Also, the high temperature can damage the functionality of the fluid due to the high temperatures reached (as is the case with certain biological fluids and polymers).

Piezoelectric Inkjet - diagram Piezoelectric (piezo) drop on demand inkjet is currently the technology of choice for most existing and emerging industrial inkjet applications. In this technology, a piezoelectric crystal (usually lead zirconium titanate) undergoes distortion when an electric field is applied. This distortion is used to create a pressure pulse in the ink chamber, which causes a drop to be ejected from the nozzle. There are many variations of piezo inkjet architectures including tube, edge, face, moving wall and piston, which use different configurations of the piezo crystal and the nozzle.

The advantages of piezo inkjet technology include the ability to jet a very wide variety of fluids in a highly controllable manner and the good reliability and long life of the printheads. Perhaps the only disadvantage is the relatively high cost for the printheads, which limits the applicability of this technology in low cost applications.