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Color deviations can sometimes occur within a print run. They are usually recognisable only when random copies are viewed side by side.
There are numerous causes:
Evaporation of ink solvent
Printing inks contain a fairly high proportion of solvents. Some solvent evaporates during the print run, increasing the pigment concentration - i.e. higher density, accompanied by a darker or more saturated color.
Variations of the printing speed can occur color deviations too.
One other reason for colour deviations can be the squeegee:
Differing in blade hardness, changing the squeegee angle or a defective or inadequate blade edge can cause deviations in color.
Adding ink as steadily as possible helps to prevent variations in density and viscosity of the ink
Printing parameters established at the art of printing must be maintained throughout the run
The finished result must be continually compared with the original, both visually and by measurment with densitometer