|Duke Water Pickup Tester|
Water pickup value should tell us the size of the ink and water balance window - or so it would seem to a pressman. It is however often misunderstood as the determining factor in the size of the lithographic window. For example, an ink producer may put a number like 50 on the certificate of analysis from a certain batch of ink. What this really means is that the ink retained about 30% of the water in the test performed, perhaps a Duke Water Pickup Tester. A rating of 10 would mean 9% whereas a reading of 100 would mean 50%.
The reading in fact relates less to the lithographic process as it does to the ink’s rheological properties that are affected by the rating. For example, viscosity and flow are two factors that can be greatly affected by this value. So really it is the inks behavior as opposed to an ink and water balance window measurement by a pressman.
What further complicates the matter is that different vehicles and pigments will behave differently when introduced to water. Therefore the number that is given for a cyan may not affect a magenta ink the same way. Furthermore ink companies that use different vehicles will have different results with the same water pickup number.
|Water Pickup Values Vary Based on Ink Type|
Another factor to consider is that the fountain solution formulation has a large bearing on water pickup. An ink manufacturer may get one result with pure water, but a different one when fountain solution is introduced into the picture. Furthermore, the inks properties will change whether or not fountain solution is introduced and what type is used. Therefore it is very important for an ink manufacturer to know what type of fountain solution is being used by the pressman.
In the real world of offset printing, some manufacturers will try to increase their tack values to get a sharper dot and less dot gain. As in anything, this increase in quality will come at a price as the ink cannot absorb as much water. The general rule is: the higher the tack, the less water can be absorbed – hence a lower water pickup value.
So in dispelling the myth that water pickup value determines the water window, it pays to reflect on the real reason behind this test. Simply to simulate how an ink will behave when on the press in the real world.
by Michael Brent