Piezo and thermal print heads
Through the years inkjet has grown to be a reliable solution for digital printing. Inkjet printing has already been implemented in multiple designs and can be used for various applications. The market of inkjet printers is dominated by only two different technologies. Although there are many manufacturers, their printers are equipped with either thermal print heads or piezo print heads.
A thermal inkjet head is equipped with a resistor. This resistor heats the ink by means of electrical power. The vapor inside the firing chamber expands and pushes the ink out of the nozzle. The remaining vapor bubble collapses after cooling and sucks new ink into the firing chamber. By changing the heat energy, the dropsize can be tuned.
A piezo inkjet head also uses an electric pulse to jet. When the piezoelectric crystal is electrically charged, it pressurizes the firing chamber, which pushes the ink out. The piezoelectric effect is used for inkjet print heads in many different architectures and with different piezoelectric operating principles, e.g. push mode, bend mode, shear mode.
Pros and cons
Thermal inkjet heads are cheaper, but have a shorter lifetime. The ink is heated and the vapor bubble creates a rapid thermal shock. The ink must evaporate with little thermal energy and early drying of the ink in the nozzle is a problem. The ink formulation should be adapted to minimize kogation and to stand the thermal shock.
Although the manufacturing cost of piezo is higher than for thermal inkjet print heads, the piezo print heads are very reliable and have a longer lifetime. The main advantage of piezoelectric inkjet heads is the flexibility in using and developing inks. Vapor bubbles do not have to be generated and other ink types than water based and solvent based inks can be used.
The piezo print head is also more resistant to aggressive chemicals. Moreover, since a mechanical process enables jetting, the ink is not subject to thermal shock, as it is with thermal heads. The viscosity can be higher and so it becomes simpler to develop special inks for substrates that have special requirements.