Simply Defined, Viscosity is a measure of flow. Practically defined , Viscosity is a measureable property of how thick or thin the color paste is.
The thickness or thinness is measured with a viscometer and given a number. As a relative example a color paste with a viscosity number of 5000 cps is thin (weak resistance to flow), Whereas a color paste viscosity of 25000 cps is thick (strong resistance of flow )
Keeping this in mind , most screen printers like to work with print paste viscosities in the broad range of 10,000 – 20,000 for pigments . But more specifically they find 12000-15000 quite acceptable .
Why do screen printers prefer this range ? Through many years of experience , this viscosity range has proven to be the best for color paste “behaviour “ in the screen printing equipment . By behaviour we mean the physical way the color paste arts inside the screen as printing take place.
For example, when the print machine stops temporarily, The color paste remains in place . The resistance to flow is strong enough to prevent it from pooling and leaking out the sides of the screens. But most importantly , the color paste will flow effectively inside the screen during printing to allow a continuous , even transfer through the screen mesh to the fabric surface.
There are some situation where a lower than normal or higher than normal viscosity will be preferred .
Low Viscosity – Less than 10000 , Provides better performace for printing on fabrics with uneven surfaces . eg towel, rugs, blankets
High Viscosity – Greater than 20000 – Provides better control for fine line printing