Rheology
Rheology - The study of the flow and deformation of matter. The printer is primarily interested in the flow of liquids (inks and varnishes). Rheology enables us to quantify such terms as consistency, tack, length of flow, stiffness and body. For inks we typically talk about properties such as viscosity, yield value and thixotropy. Viscosity is the ratio of shearing stress (force per unit area) to shearing rate (strain rate or velocity gradient of flow).
Mathematically this is
where 
is the shear stress,
is the viscosity and 
= dvy/dx is the shear component of the velocity
gradient or the shear rate.
Fluids for which h is a constant are called Newtonian fluids. Water and liquid hydrocarbons are Newtonian liquids. Fluids for which h is a function of shear rate are called non-Newtonian fluids.
Printing inks are non-Newtonian. Litho inks are highly non-Newtonian. An ideal plastic material is one where flow commences only when the shear stress reaches a specific yield stress t0.
Once flow starts, the slope of the stress versus shear rate curve is linear.
Thus,
is a constant, but
depends on shear rate.
An ideal plastic material hence is non-Newtonian, but in a simple way. A pseudoplastic fluid is one in which the viscosity decreases with increasing shear rate. Such fluids are also called shear-thinning. Many polymeric liquids and suspensions such as printing inks are shear-thinning. A dilitant fluid is one for which the viscosity increases with increasing shear rate. Such fluids are also called shear-thickening. Shear-thickening may occur for inks where the pigment particles tend to pack to form rigid structures, i.e. for large amounts of pigment or extender.
Most inks are thixotropic, i.e. apparent viscosity decreases with time. This is generally associated with some sort of gel structure, which is broken down by stirring. Generally shear thinning and thixotropy go together, but not always. Thixotropy is essential to the performance of litho in particular, allowing them to flow more easily when “worked” by the rollers in the ink train. Both pseudo plastic and dilitant fluids may also have a finite yield stress, but the stress versus shear rate plot is not linear.
Litho inks generally display a non-zero yield value in addition to being thixotropic and shear-thinning. Other examples of viscous liquids not having and having a yield value are honey and ketchup respectively. Ink flow behavior is strongly dependent on temperature. Thus, during measurement it is important to carefully control the temperature. The viscosity of a liquid generally decreases with increasing temperature.
Conventional methods for measuring viscosity are with a capillary viscometer, a falling sphere viscometer, a rotational viscometer, such a a Brookfield, a cone and plate viscometer and a falling bar viscometer. The effective viscosity for liquid printing inks is measured using a cup viscometer. These are cylindrical cups with a capillary (Shell cup) or a machined hole (Zahn cup) which controls the rate of liquid from from the cup. These may be calibrated with an oil of a known viscosity.
Ink “length” is often used as a measure of rheological properties. A “long” ink can be stretched somewhat with an ink knife and is generally more viscous than a “short” ink. An ink that is too long will tend to fly or mist. Tack - A measure of the forces necessary to split a single film of ink. Characterized by a tack number which is measured with an inkometer. An inkometer consists of rotating rollers connected to transducers which measure the torque on the rollers.
The torque in units of gram-meters is the tack. To fully specify the tack number the revolution rate (in rpm) must be specified. For sheetfed offset 800 rpm is generally used, while 1200 rpm is generally used for web offset. If the tack is too high, it may cause picking of paper or pulling of coating from coated papers. Tack is important for multicolor printing. The highest tack ink should go down first. We have an inkometer, which is in the proofing press lab.