DIGTAL PRINT TECHNOLOGY: PIEZOELECTRIC

Piezoelectric printheads can be grouped into one of the following types based on the way electrical current deforms the piezoceramic activator plate: shear mode, bend mode, push-piston mode, and squeeze tube. In addition, one device is a hybrid combining two of these modes in the same printhead on separate piezoceramic plates.

Piezoelectric driver plates are almost universally Lead-Zirconium

-Titanate (PZT).

 

PIEZOELECTRIC SHEAR MODE

Shear mode print heads use an electric field perpendicular to the polarization of the piezoelectric PZT driver. Electric charge causes a shearing action in the distortion of the PZT piezoplates against the ink causing the ink to eject from the nozzle opening in drops. Shear mode piezoelectric printheads include those from Spectra, and those based on Xaar’s patents, such as Xaar, Nu-Kote MIT, Brother, and Olympus.

 

LaserMaster, now ColourSpan, employs Spectra sheer mode heads for its hot melt DisplayMaker Express. Polaroid uses them for its DryJet Color Proofing System. Luscher and Kiwo employ them for their computer to screen printers. 3D Systems manufactures prototype building devices which print dimensional prototype models using Spectra shear mode printheads. These print systems melt thermoplastic resins which Spectra heads shoot layer upon layer to form models.

 

Spectra heads have the advantage of high reliability, proven performance, robust capability, wide ink choice, and can process inks in the 20 to 25 cp. (centipoise) range, which is relatively high for

piezoelectric inkjet. Spectra manufactures a number of head versions made if materials varying from sintered graphite to stainless steel. Spectra Inc. has advanced shear mode with the use of CNC machined sintered polycrystalline graohitic carbon as the structural print head base, the placement of a filter between the piezo pumping chamber and the nozzle, and the edge shooting placement of the piezoelectric transducer. The shear mode action makes it possible to achieve tightly packed assembly of many jets in a printhead with just one piece of piezoelectric plate. Although these heads have the disadvantage of high sticker price, printheads developed with shear mode technology

can deliver lower cost per jet at higher speeds with superior jet uniformity for jetting various inks on a wide variety of substrates.

 

The Xaar type of this technology is also known as shared-wall shear mode. Its electrodes are exposed in the ink channel in their native unpassivated mode. Water-based inks corrode these electrodes unless they are passivated, that is coated to prevent corrosion. Without passivation, these heads must use non-corroding solvent-based inks. Xaar, Brother and others have successfully passivated the printhead electrodes to permit the printing of water-based inks. The Xaar piezoelectric print technology is termed shared wall because each of the piezoelectric activated membrane walls is shared with its neighboring ink channel. This means that only every other chamber can fire simultaneously. In actuality, one can only fire every third chamber due to the possibility of accidental droplet generation from the chambers immediately adjacent. By adjusting the angular orientation of the printheads to the direction of substrate movement, one can achieve a workable pattern of ink deposition to compensate for this head firing limitation. The Nu-kote/MIT version of the Xaar print

heads can print either 200 or 360 dpi depending on the angle of orientation.

 

A number of OEMs employ Xaar shear mode printhead technology. Using the MIT version are Raster Graphics fir its PiezoPrint 5000 and 6000 printers, and Daniel Instruments Limited of Winterthur, Switzerland for its Polijet Digital Inkjet Print Systems (DIPS). These systems use three rows of precision positioned printheads to enable

continuous line printing. Xerox employs the Olympus version of Xaar technology in a two row array producing up to 720 dpi resolution. Mechatron is also using an array of Xaar piezo printheads for use with its XY flatbed plotter systems tool head. Xaar heads typically have the advantage of relatively low cost. They have improved in performance,

but still suffer from limited reliability and the restrictions of in channel electrodes and shared-wall activation.

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