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How Ink Dries

There are several different ways for ink to dry. Litho and letterpress sheet-fed inks are the fastest drying inks. They dry primarily by one of the following two methods:

*Absorption - The ink soaks into the paper stock.
*Oxidation - The ink dries on the surface of the paper stock when exposed to air.

The way ink is formulated to dry is determined by the ink's intended suitability for a particular press, paper stock (coated vs. uncoated), or printing application.

Three factors that affect ink drying are:

*Fountain Acidity
If the pH of the fountain solution is too acidic (below 4.0), the ink will dry slower.

*Temperature
Ink dries faster on warm paper stock than on cold paper stock.

*Humidity
Ink dries slower on paper stock that is damp and limp.

Ink Emulsification

Offset lithography is a demanding science where many printing factors and variables interact. Some of these variables have little effect on the printing process, while others determine the quality of the printed job. One of the major factors the printer must understand and control is the amount of ink emulsification that occurs on the press.

In order for an ink to print well it must absorb or pick up a sufficient quantity of water. When the ink picks up too much water it emulsifies. Emulsified ink loses its tack and transfers poorly from the ink rollers to the plate. The result is poor ink density and washed out copy.

The four main causes of ink emulsification are:

* Poor quality ink

* Fountain solution mixed too strong

* Fountain solution pH below 4.0

* Press water setting too high

To minimize ink emulsification:

* Use an oil or soy based ink that exhibits wide water balance and eventually reaches an equilibrium between ink and water.

* Avoid mixing the fountain too strong.

* Maintain a pH of 4.5 to 6.5.

* Adjust for and maintain a minimum water setting.

* Avoid over-wetting the plate prior to engaging the ink form rollers and printing.

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