Acoustic-thermal-fluid-curing coupling effects of focused ultrasound printing: Mechanisms and process optimization

Focused ultrasound printing (FUP) is an emerging additive manufacturing process that enables non-contact and deep-penetration thermoset polymerization. However, the underlying acoustic-thermal-fluid-curing multiphysics coupling mechanism remains poorly understood, which severely restricts the printing precision and process controllability. Herein, we establish a fully coupled numerical model integrating the nonlinear acoustic field, fluid field, thermal field, and curing field to quantitatively characterize the multiphysics interactions of polydimethylsiloxane (PDMS) during FUP. Experimental validation using acoustic pressure measurement, thermal monitoring and visualizations confirms the accuracy of the multiphysics model. Our results reveal that acoustic streaming induces dominant upward flow and dual-vortex recirculation, which fundamentally decouples the spatial locations of the maximum acoustic pressure and peak temperature, leading to a dynamic upward shift of the hot spot. The curing kinetics exhibit a distinct sigmoidal evolution, transitioning from an induction phase to rapid cross-linking upon exceeding a critical temperature threshold, achieving full polymerization within 0.7 s under typical operating conditions. Parametric analyses demonstrate that increasing transducer surface pressure from 0.15 to 0.35 MPa reduces the time required for complete curing from 6.5 s to 0.25 s, while simultaneously narrowing the cured region. Within the frequency range investigated in this study, increasing the fundamental frequency of transducer from 1.0 to 3.0 MHz boosts acoustic intensity by 2.3-fold, sharply confines the thermal spot, and reduces the full curing time by 95%. This study also reveals an optimal processing window for ultrasonic parameters, which simultaneously improves the printing accuracy and enhances energy utilization efficiency of focused ultrasound printing.