Prediction of Printable Area for Needle-Valve Type Piezoelectric Dispensing System Using Non-Newtonian Fluid

Research is currently underway to mitigate satellite droplets and minimize droplet tails to achieve stable, high-speed droplet jetting in drop-on-demand material dispensing systems. The operational region where stable droplet jetting occurs is defined as the printable area or printability. This is characterized using dimensionless numbers, specifically the Z number, which is the reciprocal of the Ohnesorge number (Oh), and the Weber number (We). Predicting the printable area for non-Newtonian fluids, which exhibit shear-thinning behavior, poses challenges because dimensionless numbers typically apply to Newtonian fluids with constant viscosity. In a needle-valve type piezoelectric dispensing system capable of handling non-Newtonian fluids, the viscosity and velocity of the fluid, which vary with needle movement, were determined using transient response Computational Fluid Dynamics (CFD) simulation. Subsequently, droplet shapes were categorized into four groups, and the distribution of dimensionless numbers for each group was compared and analyzed. The dimensionless numbers Z and We were computed by applying trapezoidal driving signals that adjusted to changing viscosity. Experimental verification confirmed their influence on the printable area. It was observed that stable droplet jetting is achievable when the rise time comprises more than 50% and the fall time less than 40% of the parameters of the trapezoidal signal at 120 Hz.