3. Types of Dot Gain
There are two primary components making up the sum of the total dot gain levels inherent in the printing process.
This can be caused by a number of the factors and
materials used during the printing process and include ;
- Paper - An uncoated, more absorbent paper surface usually causes higher dot-gain
- Inks - Higher ink film thickness and lower ink tack values usually result in higher dot-gain
- Blankets - Level of compressibility, age and surface condition affect dot reproduction
- Ink rollers - Condition, accuracy of settings and durometer readings of rollers also tend to increase dot-gain.
- Dampening - Usually, alcohol dampening systems produce sharper dots
- Line Screens - Finer screen rulings tend to produce higher levels of dot gain
- Press - Poor mechanical condition, improperly set pressures over-packed blankets etc. All contribute to higher dot gains.
However, the level of dot-gain is not uniform across the whole tonal range.
The highest level of gain usually occurs in the mid tone areas between the 40% to 60% screens and gradually tapers off towards the highlight and shadow areas.
A typical dot-gain profile on a web press would be as shown Fig #5. This is for illustration only as different colours and substrates exhibit slightly different curves.
A point to note is that the finer the screen ruling used, the higher the level of dot-gain with substrates and ink densities remaining constant.
This is illustrated in Fig #6
This is one reason why unnecessarily high screen rulings which are not compatible with the substrate and print process used should be discouraged.
A
normal person reading at a distance of about 300mm under standard lighting
conditions of between 4000k to 5000k will not be able to discern
dot patterns more than 150 l.p.i.
Anything above this is really over kill.