Fluid Behavior
Flow properties determine how printing pastes and dyeing liquors deform under mechanical forces. The rheology of a print paste controls its behavior during application through the screen and onto the fabric. If the paste has the wrong flow properties, it can block the screen or spread too far on the fabric.
This parameter is monitored in the laboratory using a viscometer.
Shear Stress
Printing processes subject the paste to high shear stress as it is forced through the screen by the squeegee. Under these conditions, the viscosity of the paste must drop to allow it to pass through the mesh. This is the core of rheology in textile printing where shearthinning behavior is required.
Once the squeegee passes, the paste must recover its viscosity to prevent spreading and ensure sharp outlines on the printed surface.
Paste Stability
Storage and mixing alter the flow behavior of finishing chemicals over time. A stable print paste maintains its rheology under continuous agitation in the supply tanks. If the viscosity changes during the run, the color intensity and print definition will vary.
This requires continuous monitoring and the addition of thickeners.
Process Optimization
Mill operators adjust the flow properties by changing the concentration of thickeners. This optimization of paste rheology reduces print defects like pinholes. The viscosity must be tailored to the specific screen.
