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Lithographic inks are paste inks, The press works the ink , thereby heating it and
reducing its viscosity or body , making it flow readily to provide a uniform ink film to
the image areas of the plate . Litho inks must be heavily pigmented
Formulation of the litho inks
Sheeted lithographic ink contains the following materials .
• Pigment: carbon black, phthalo blue rubine or others
• Varnish: long oil alkyd , phenolic or urethane litho varnish
• Drier: cobalt and manganese salts
• Solvent: heat-set oil
• Modifier: wax compound for rub resistance
Additive and subtractive colour
White is defined as the presence of all colours and black as their absence . When red ,
green and blue are blended white is the resulting color . When red, green and blue are
masked out the resulting color is black . Ink subtracts color and acts as a color filter .
Black filters out all colors, yellow filters out blue , magenta filters out green and cyan
filters out red.
Pigments
Among the most important properties of printing ink are its color, its flow , and its drying
;that is , the color and behaviour of the ink on the press and on the print . Color is
determined principally by the pigment , flow by both the pigment and the varnish, and
drying principally by the varnish or vehicle .
Pigment characteristics of importance in ink formulation include tinctorial strength,
opacity , shade , gloss , durability, particle size, specific gravity , refractive index,
hardness or texture , wettability, dispersibility ,light-fastness and chemical resistance .
Flow
The study of the flow of fluids is known as rheology . The printer is interested mostly in
the flow of liquids and the control of viscosity , body and tack . The word viscosity refers
to the resistance to flow; the greater the resistance to flow , with a given applied force ,
the greater the viscosity .
In shear independent or Newtonian liquids the velocity of flow is directly related to the
force applied. Lithographic inks are shear dependent or Non Newtonian . |
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