2 Types of drop on demand print heads

Since its creation in the 1980s drops on demand inkjet printer system has been used predominately in industrial applications.

The DOD process generates ink in the ink reservoir. This causes valves open and close to form ink drops on demand drops are projected through the nozzle when and where they are needed to produce a mark on the substrate according to the printing requirement.

DOD is a common term for print heads found in inkjets used for high-quality graphics and a broad classification of inkjet printing technology. It can be sub-categorized into:

  • thermal print heads
  • piezo print heads

Thermal print heads:

This was the first type of DOD print head. It is quite efficient and can produce high-quality images. Its production speed is competitive with the piezo print heads. However, this type of print head can only work with water-based inks and as a result, is restricted to indoor applications.

The process:

The process involves providing heat to an ink chamber inside the print head. The heat causes the ink to vaporize because it is liquid. As a result of a gas bubble forms that then expands and deposits drops of ink through the nozzle which is located at the end of the chamber.

When the heating element is switched off it causes the gas to cool, condense and contract. A disadvantage is that thermal stresses wear the heads out quickly so heads are designed to be replaced.

Piezo print heads process

Piezo heads are currently a very popular choice of technology for most industrial applications. Piezo crystal is distorted by expanding or contracting when an electric current is passed through it and switched off again. This mechanically creates a pressure pulse and is used as the basis of a pump in the ink chamber. Which will cause a drop of ink to be ejected from the nozzle.

The current can be switched on and off very rapidly and the expansion/contraction can happen almost instantaneous.

Piezo heads are greyscale and can drop variable sizes of ink from the same chamber and nozzle giving different ink densities on the substrate.

Piezo print head advantages:

These print heads will work well with any fluid so it can utilize solvent-based inks, water-based inks and UV cured inks.

One of the biggest advantages Piezo heads have over thermal heads is that they have a longer head life. This is because there is less thermal stress on the Piezo head. These heads are designed to last for the printing machines operational lifetime, as long as there is no external damage. As a result, they cost more to make and buy than thermal print heads.

Article submitted by Needham Ink