Dot gain, more recently referred to as tonal value increase is a fact of life in printing. What the pressman must do is learn to control it. But controlling it is not so easy. So many factors come into play that the average pressman can be lost trying to keep it in check.
For example, many physical press factors come into play:
Web tension
Impression squeeze
Roller settings
Press temperature
These are just a few mechanical factors affecting dot gain. In addition, chemical factors need to be considered. Here are just three.
Condition of the ink
Fountain solution chemistry
Ink and water balance
Let’s take a look more closely at just a few factors and see how they affect dot gain. Remember, you cannot take dot gain away, but you can control it. Educating yourself about what causes it will help you to keep it in check.
1. Density
By far the greatest contributor to dot gain is density. By simply applying ink to an image, you incur dot gain. By continuing to add ink, you cause the ink to be squeezed outside the boundary of the image area.
2. Blanket Squeeze
Whether you work on an offset press that is blanket-to-blanket or one that is blanket to impression cylinder, there must be a certain amount of pressure to properly transfer the ink. The key is to have as little pressure as possible to do the best transfer. Finding that balance is the key. Consult your press manual to find the optimum blanket height and you will achieve a dot gain that is consistent and controllable.
3. Roller Settings
Another “squeeze” that takes place is the setting of the ink and water forms to the plate. The principle is the same: ink must transfer to the plate with minimal pressure and maximum transfer. Be sure your roller settings are not heavier than recommended. Generally the larger the diameter of the roller, the heavier it should be.
4. Ink Viscosity
Generally considered the measurement of the degree that an ink resists flow. Generally a high-viscosity ink (one that does not flow easily) will incur less dot gain. This is because when the ink is transferred by squeezing one medium to the other, the ink has a tendency to spread. High viscosity inks resist that spread. How can you know what your viscosity measurement is? Consult your ink manufacturer.
Of course one factor on press that affects ink viscosity and thus dot gain is heat. If your ink fountain, roller train, or plates are hot, viscosity will drop and thus dot gain will go up.
5. Slur
If the paper slips in any way while printing, slur will occur. Slur is like a phantom form of dot gain. By making a double image, it gives the appearance of dot gain. If printing on a web press, web tension is critical to control. Side to side movement is also. In addition, backlash of gears from non servo driven presses will cause dot gain.
Once the dot gain is controlled on the press, there is much software that can help control dot gain by focusing on just the 40% and 80% screens. Adjustments can be made to the dot gain curve on those screens or anywhere throughout the tonal dot gain curve to match press conditions.
