Deformation Threshold
Mechanical stress thresholds define the minimum shear stress required to initiate irreversible plastic flow in viscous textile chemical pastes, thickeners, and sizing polymers. Measuring yield stress determines the structural resistance of print pastes and coating formulations before liquid flow begins under applied force. The parameter governs printing paste stability, coating knife application, and pumping dynamics, ceasing to apply once liquid flow establishes and viscous shear rate dictates rheological behavior.
Rheological Limit
Formulated print pastes require a minimum yield stress to prevent dye dispersion settling during storage and to stop pattern spreading on fabric surfaces during screen printing. High yield stress values keep thickener pastes stationary inside screen meshes until squeegee blades exert sufficient mechanical force to induce flow. Once shear stress exceeds the threshold, paste viscosity drops rapidly to allow uniform fabric penetration.
Fluid Viscosity
Rheometers measure yield points using controlled stress sweeps or rotational shear vanes.
Mechanical Boundary
Insufficient yield stress causes dye paste migration outside printed pattern boundaries, destroying sharp print outlines on woven fabrics. Excessively high yield stress prevents smooth paste delivery through screen openings, producing thin, patchy color application. Formulators adjust thickener concentration and synthetic polymer crosslinking to tailor yield stress values to specific printing machinery.
