Predicting Stress Development and Incomplete Conversion in Thick Thermoset Tooling Runs
Predicting cure exotherms and volumetric shrinkage gradients prevents matrix cracking, low core conversion, and dimensional distortion in thick thermoset tools.
Ultrasonic Shear Velocity
Meaning
Propagation speed of high-frequency transverse waves through a material provides a direct measurement of the shear modulus of a curing polymer. Monitoring ultrasonic shear velocity enables real-time tracking of the transitions from liquid to gel and finally to a solid state. This acoustic technique provides nondestructive data without interrupting the curing cycle.
Wave Propagation
Shear waves require a solid or highly viscous medium to propagate effectively, unlike longitudinal waves. As the polymer crosslinks, the rising shear stiffness causes the shear wave to travel faster. Recording this change in speed allows for the calculation of the shear storage modulus.
Transition Tracking
The gel point is identified by the rapid increase in velocity as the resin begins to support shear stress. Vitrification occurs later when the velocity reaches a stable plateau, indicating that the polymer has entered a glassy state. These milestones indicate that the reaction has reached the target conversion level.
Process Monitoring
Utilizing this acoustic method on the manufacturing floor replaces scheduled cure durations with actual cure states. If the resin is not fully cured, the wave speed will be lower than the demonstrated baseline value. This mismatch warns operators that the part is not ready for demolding, protecting the manufacturing run from curing defects and laminate distortion.