Bend Mode

Bend Mode-Piezoelectric Method

PicoJet printheads use the bend mode design. In this method the piezoceramic plates are bonded to a diaphragm forming an array of bilaminar electromechanical transducers used to eject ink droplets. As an electrical field is applied to the piezoceramic, the piezoceramic expands and contracts. This action forces the diaphragm to which the piezoceramic is bonded to buckle in and out generating pressure similar to the operation of an oil can.

View the Piezoelectric method in action!

Bend Mode Advantages

The piezoelectric bend mode method has many advantages over the piezoelectric shear mode method. Both shear mode and bend mode require the use of electrodes to operate the piezoceramic. Shear mode printheads incorporate the electrodes on the piezoceramic inside the ink chamber. The bend mode design separates the electrode and piezoceramic from the ink chamber using a diaphragm. The shear mode design allows for interaction between ink and the electrode on the piezoceramic. Ink interaction with the electrode on piezoceramic can result in corrosion that can release particles into the ink channel causing eventual nozzle clogging.

To prevent the interaction with the electrode, shear mode printhead manufacturers apply elaborate passivation coatings and electrical insulation inside the ink channel. However, this additional effort adds to the complexity and cost of the shear mode technology.

Since the piezoelectric material in a shared wall shear mode printhead serves as an ink chamber wall the ink chambers on either side of the active ink chamber are inoperable. This prevents the operation of all jets at one time which limits the high drop frequency operation of the printhead. .

In addition, in the shear mode design, the shearing or twisting motion of the piezoceramic material makes it very difficult to control the pressure profile in the ink chamber. The sudden pressure fluctuation at high frequency in the ink chamber can cause air bubbles to grow out of the ink solution. The trapped air bubbles can then absorb the energy from the pressure wave generated by the piezoceramic. This ultimately results in the failure of the jet function. To minimize this effect, systems sometimes called “lungs” are incorporated with shear mode printheads to circulate, heat and degas the ink before it reaches the ink chamber. This additional circulating, heating and degassing system adds complexity and cost to the printhead system.

In the bend mode design the piezoceramic expansion and contraction can be controlled by the slope and amplitude of the printhead drive wave form. This allows bend mode design printheads to control the pressure profile inside the ink chamber preventing the release of air bubbles from the ink solution. As a result, circulation, heating and degassing systems are normally not required by Bend Mode printheads for jetting inks at high frequency.