Measurement of mass of single inkjet drops with a quartz crystal microbalance QCM

Monitoring inkjet performance requires control of parameters such as drop velocity, direction and drop volume. Present methods to determine drop volume utilize optical vision systems or calculation of an average drop mass from large numbers of drops on a precision balance. An alternative technique based on QCM (Quartz Crystal Microbalance) was assessed to measure the mass of single drops. Low-cost plano-convex 6 MHz AT-cut quartz resonators were used to measure single inkjet drops. Since the footprint of these ink drops is of the order 100 μm the QCM detector was used in a 'localized spot' measurement mode in contrast to the typical large area detection mode. The sensitivity of an inner 0.5 mm circle was determined to be 5.46 x 10-10 g/Hz for solid silver films. Single drops of an oil-based ink of 50 pL nominal volume were jetted using a Xaar126 piezo inkjet printhead onto the QCM target area and produced signals with a SNR better than 70:1. This paper presents the technical challenges relating to liquid droplet volume measurements using higher frequency oscillators.