Important Aspect of Viscosity in Gravure Printing Process

IMPORTANT ASPECTS OF VISCOSITY IN GRAVURE PRINTING PROCESS

MAKARAND DOLE

T.E. PRINTING

FEATURING…

• Understanding Liquids…

• Understanding Viscosity…

• Comparing Viscosities…

• Automation in Viscosity measurement

• Effect in Gravure Printing

• Factors influencing Viscosity Control…

• Conclusion

UNDERSTANDING LIQUIDS…Newtonian Liquids:

• The liquid will flow even the small force will be applied

UNDERSTANDING VISCOSITY…• Father of Viscosity: Sir Isaac Newton

• In simple words it is ‘Resistance to flow’

• Plays a major part in determining how ink, varnishes, adhesives and other fluids behave in printing and subsequent processes

• Flow of liquid affects its distribution on machine

• As well as rate of penetration and evaporation

EXPERIMENT OF SIR ISAAC NEWTON

The speed and shear rate were obtained from his test apparatus.

Viscosity formula

This constant is known as the Viscosity of the liquid and its units are Ns/m2 (formerly

Poise, p, where 1N/m2 = 10 p)

VISCOSITY OF GRAVURE INKS:

• Through the physical necessity, the viscosity of the gravure ink is low, e.g. 15-25 secs Zahn 2(18-56 centipoises) under normal conditions

• This enables the ink to flow quickly and uniformly into the cells as well as out of the cells during contact with substrate

• For the critical works the narrow range of the viscosities will give the desired result

THE CORRECT VISCOSITY OF THE GRAVURE INK IS GOVERNED BY :

• The cell

• The range of the depth of cells

• The speed of the press

• The evaporation rate of the solvents

• The distance between the Doctor blade and the point of the impression

• And to the affinity and the absorbency of the substrate for the ink

COMPARING VISCOSITIES• Capillary Viscometers

• Falling Sphere Viscometers

• Rotational Viscometers

• Cone and Plate Viscometer

• Flow cups

CAPILARY VISCOMETERS• Used for low Viscosity fluids

FALLING SPHERE VISCOMETERS

• Used for unpigmented vehicles

• Also for Adhesives

• Can be used in Newtonian as well as non Newtonian liquids

• Particularly useful for letterpress and litho inks

• May be used for non Newtonian liquids

ROTATIONAL VISCOMETERS• Two cylinders- Outer and Inner

• Outer exerts a drag on inner

• Deflection of the scale is proportional to the drag, which is depend upon the Viscosity

CONE AND PLATE VISCOMETERS

• The plate is stationary and the cone rotates

• The torque required for rotate the cone is measured

FLOW CUPS• Simple and basic procedure to measure

the viscosity

• Used for low viscosity fluids

AUTOMATION IN VISCOSITY MEASUREMENTViscosity Sensor Types:

• Rotation Deep Sensor

• Inline Sensor

• Inline Falling Body Sensor

EFFECT IN GRAVURE PRINTING• With increasing Viscosity, Drying speed

also increases

• At higher viscosity, ink do not transfer completely out of the cell because drying is too fast

• Reduced Viscosity will allow more ink to transfer resulting higher ink density

• The ink density reduces after reaching a saturation point

FACTORS INFLUENCING VISCOSITY CONTROL…

• Ink Consistency

• Temperature

• Color Consistency

CONCLUSION• The handling of viscosity has a direct

influence on final print results and quality

• Quality production is the driving factor in selecting any viscosity or temperature control device

• Viscosity increases with increasing temperature

• Continuous monitoring by electronic viscosity controllers control solvent additions