Factors That Affect Dot Gain

Our printing group has done some detailed testing regarding dot gain recently.  It is driven by the ever present goal of reaching the ISO standard.  But that's another story I'll get into one day.  Today though, I would like to explain the chart above.  It is a guideline we go by to chase dot gain issues on our presses and it helps us to troubleshoot possible causes of dot gain, or sometimes called tonal value increase (TVI).  It shows the factors that affect dot gain in relation to how much of an influence they have.

Dot Gain Causes in the Offset Printing Process


1.  Ink.  The ink itself plays the single most influential role in controlling dot gain.  Properties such as viscosity and tack are a good start.  These properties we look at first when chasing dot gain problems.
2. Temperature.  The best place to start with this is the temperature of your oscillating rollers.  Use a infrared sensor to measure the temperature across each side of the transfer rollers, and in particular the oscillating rollers if you have a heat set press.  The optimum temperature?  27 degrees Celsius.  How do we know that?  I'll cover that in a future article.  But I'll say in short that this is the temperature with which you will get the least piling.  
3.  Pressure.  Blanket to blanket, plate to blanket and form roller to plate all play a role.  To find out more, I've written another more detailed article about how this happens.
4.  Blankets.  We've found that when the blankets get old, dot gains will go a little sketchy.  Sometimes all you have to do is change the blanket for the problem to get better.  Also, the more piling on the blanket, the sharper your dots become.
5.  Paper. Glossy or matte, you will get different dot gain settings based on how the print lays down. 
6.  Machine.  In this instance, it's not only the condition of the bearers that make a difference.  which could be wearing or causing slur.  The conditions of all the bearings, gears and rollers come into play.  
7.  Ink and water balance.  Some put this factor much higher, but it shouldn't play such a large factor if you are in the proper ink and water balance window.
8.  Plates.  Again, raw materials will make a difference in this aspect.  Additionally, plate wear can cause dot gain.
9.  Plate making.  If the plate is scrubbed properly can make a difference.  Additionally, the type of dots used can make a difference.  For example, we recently went to an elliptical dot.  This meant about a two percent increase in dot gain.

Optical Dot Gain

Outside of the offset press spectrum we have optical dot gain.  This varies according to the color printed and the sharpness of the printed dot.  Light will actually go through the ink and get diffused inside the substrate.  This causes the illusion that the dot is larger than it really is.

When light hits the printed surface, it becomes slightly diffused around the dots. The human eye (as well as measuring devices) perceive this as a darkening. Dots appear to be larger than they really are.

It's always a work in progress for us troubleshoot the cause of dot gain.  It is something inherent to the process that will unfortunately never go away.  However I hope our little table above can give you a little help in case you're hunting for ways to reduce it.


Related Media
How to Troubleshoot Offset Printing Ink
The Web Press Troubleshooting Guide