Tips on Controlling Dot Gain
by Dennis Cheeseman
Dot gain is an unpleasant fact of life. We can't wish it away, but we can control its effects and predictability.
Dot gain is defined as the increase in the diameter of a halftone dot during the pre-press and printing process. Total dot gain is the difference between the dot size on a film negative and the corresponding dot size on the printed page. For example, a dot pattern that covers 30 percent of the image area on film, but covers 50 percent when printed, is said to show a total dot gain of 20 percent.
Every stage of the process-from film, through platemaking, to the several stages of printing the final image-contributes to dot gain.
Screen Value (Percent) |
Not all dots grow the same amount. The area of greatest dot gain is in the midtones (40-to-60 percent). Above this range, as the dots progressively touch each other, the perimeter available for the growth of each dot diminishes, resulting in a tapering-off effect (see chart, top of page). Dot gain becomes more pronounced as the screen ruling becomes finer and is often one of the factors limiting the choice of screen.
Dot gain in pre-press can occur during platemaking. Exposure times and contact between the negative and plate should be carefully monitored. The dot gain in this area should be less than 2 percent.
Dot gain on the press stems from several effects. Each time the ink is split (plate to blanket, and blanket to paper) a degree of "squash" increases the physical diameter of the printed dot. Also, when the dot is transferred to newsprint, it is absorbed. This occurs both vertically (into the paper) and sideways, contributing to an increased dot diameter.
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An optical effect is also at work (see diagram at right). Light striking a dot throws a shadow onto the paper. This shadow can be perceived by both the eye and a reflection densitometer, and has the same effect as printing a larger dot. Optical dot gain can be larger if the shadow is more pronounced and depends on the opacity and surface of the newsprint.
If the press and its settings are under control, dot gain should be consistent, and it should be possible to compensate for it at the pre-press stage. It is necessary, therefore, to benchmark the dot gain of a press under normal printing conditions. For each color, prepare a plate with a dot-gain target (a series of tone squares usually ranging from 5 percent to 95 percent).
Measure the plates to determine the gain produced in platemaking. After a run, when the press is warm and the ink and water are in balance, mount the plates and run them at normal operating speeds. Using a densitometer, measure the dot gain or dot area on the paper for each tone target.
The densitometer may use the Murray-Davies equation, which relates the integrated density of the tone square to the dot size. Some densitometer manufacturers use other equations and give different results, so it is important to know which equation the densitometer uses.
Listed in the green box are typical total dot-gain ranges for printing on newsprint (calculated from the midtone area).
| Black | Cyan | Magenta | Yellow | |
| GATF | 25-35% | 25-35% | 25-35 % | 25-35% |
| SNAP | 28-34% | 28-34% | 28-34% | 28-34% |
| X-Rite | 34% | 33% | 30% | 28% |
After benchmarking a press, a printer may be able to isolate and concentrate on an area where the gain is excessive. The following press variables have been found to affect dot gain:
Ink: tack and viscosity, ink/water balance, pigment strength, temperature, ink-film thickness (print density)
Paper: brightness, whiteness, opacity, porosity/holdout, smoothness, web tension
Fountain Solution: pH/conductivity, hardness of incoming water, type of dampening system, formulation
Blanket: compressibility, age, tension, surface characteristics
Plate: exposure time and vacuum, processing, reactions (age, light, chemicals), tension
Rollers: durometer, settings, glazing
Speed: low/high
Packing: over/under
Coverage: light/heavy.
Dot gain should be benchmarked immediately after any of these variables is changed. If you wait too long, bringing the process back under control may be difficult.
Also, it is important to note that ink-film thickness will have a dramatic effect on dot gain. It is therefore critical to monitor and control print density, so dot gain will remain consistent throughout the run.
Dennis Cheeseman is director of customer services for US Ink Corp. Phone is (201) 935-8666; fax, (201) 933-2291.