How Rheology Effects the Printed Image
Rheology falls within the branch of physics that deals with the deformation and flow of matter, especially the non-Newtonian flow of liquids and the plastic flow of solids.
Rheology, in a broad sense, is the study of physical behavior of materials under stress. All matter possesses four rheological properties: viscosity, elasticity, plasticity and rigidity. Not all materials possess all characteristics at the same time. A liquid may be mostly viscous, plastic is mostly elastic, clay is plastic, and rocks are rigid. However, the characteristic of a material can be changed under stress (like coal becoming a diamond) and go from a perhaps elastic to plastic or viscous to elastic.
The point of yield or change requires a fifth concept; strength, the property by which the material resists deformation.
Many of the screenprinting substances – inks, pastes, resists and other chemicals – can be classified as viscous. However, we can also see an elasticity in some screenprinting substances. When a screenprinting substance is both viscous and elastic it becomes a viscoelastic substance, such as Plastisol (PVC), most adhesives and even the acrylic-based products we use. The four most important characteristics of screenprinting substances are plastic, pseudoplastic, dilatant and thixotropic.
Plastic (also called Bingham plastic), pseudoplastic and dilatant are non-time dependent in that changes occurring due to stress mostly instantaneous. Thixotropic substances are time dependent in that changes in viscosity depend on events prior to change, and may be decreasing or increasing over time.
Bingham also showed that for many real fluids, such as in our screenprinting substances, a critical level of stress must be attained in order to initiate flow. Below this critical stress, τy, the material behaves as a solid, absorbing the stress energy without flowing. Once the threshold of critical stress has been reached, the material yields to flow, hence the term, yield stress.
The differences in yield stress can be observed using two common household substances honey (a Newtonian substance) and ketchup (a plastic fluid). During pouring these from the container, the honey will flow well enough, but the ketchup will only flow when we hit the bottom of the bottle to force the change. Although we perceive ketchup to be thicker (more viscous) than the honey to now learn that the opposite is true and the ketchup flows more freely than the honey.
Effect When Printing
When screenprinting with a thixotropic substance, the rheology of the ink or coating may effect the wet deposit. The viscosity of a thixotropic substance decreases under constant shear rate – given a steady speed as it is moved across the screen. The viscosity at the end of the stroke is lower than at the beginning and produces more deposit. The larger the image the more pronounced the effect.
This is combated to some degree by the fill stroke moving from back to front and the squeegee stroke from front to back or vice versa.