Bulk piezo.

Bulk piezo is the older and still dominant printhead construction, in which the actuator is sintered PZT ceramic machined into channel walls rather than a film deposited on a wafer (see thin-film piezo for how the two are made). What separates one bulk head from another is how the wall is driven. In shear mode the field runs across the poling direction and a single piece of poled ceramic deforms sideways; Xaar's chevron mode fuses two oppositely poled pieces so the wall bends at the join, and its shared-wall design lets one wall fire either of the two channels it sits between, which is how channel density rises without thinning the ceramic. In that mode the walls flex at roughly 150 kHz and it is the acoustic wave running along the channel, not the wall movement by itself, that ejects the drop. The practical consequence is uniformity: drop volume and velocity vary from nozzle to nozzle across a bulk array, which is why vendors ship per-nozzle waveform and voltage trimming, and an array that has drifted out of trim prints density steps at the head joins that no color profile will correct.