SOP

Dot Gain, Color And Other Common Questions

To help our clients address and avoid common issues that can potentially impact the quality of a job, we've gathered some useful information and answers to questions we often receive. Please feel free to contact us if you have any additional questions or concerns.

Dot Gain

What is it?

Dot gain, a common culprit of color variation during a press run is caused by a variation in ink feed or printing pressure — ink trap is often a main contributor to dot gain. On longer press runs, changes in pressroom humidity can change ink/water balance thus affecting color. Ink absorption by the paper can also vary (and with it the color) if the paper was sheeted from multiple rolls. Absorption differences are also seen from the felt to the wire side of the sheet which produces variation on backup.

Dot Gain on our coldset open web press varies between 15-20%. On some types of paper, dots often appear larger than they should, causing loss of detail and darker tones than desired.

How can I control it?

Dot gain can be compensated for in the film stage by reducing the size of the film dot. Dot compensation may be varied in different areas of the image, with less compensation in highlight areas and greater compensation in shadow areas. Often a dot no longer then a pinpoint is called for in highlight areas. Be sure to adequately compensate for dot gain: if dot size is not reduced sufficiently, dots run together and a condition known as plugging occurs. This results in overly dark, muddy shadow areas. Overcompensation for dot gain (reducing the dot size too much) must also be avoided. Overcompensation cause gray-out — a condition in which the image appears weak and washed out.

Color

Why does our magenta ink change color after heatset drying?

Some pigments used in magenta ink will darken when heated. This could explain the change in color. The loss of gloss could also be caused by excessive penetration of the ink vehicle, resulting in loss of gloss.

How can I avoid this?

We recommend using a more heat-resistant pigment, and can minimize the hue variation by reducing drying temperature to just enough to dry the ink (without overdrying).

Why does the color vary from one run to another?

There could be a variety of causes for this, including a change in density and dot gain from the first run to the second.

How can I correct this?

Check for dot grain and density changes with a densitometer used to measure the solid ink density of the color bar. If the density is the same as the first run, check the got grain at the mid-tone dot. The dot grain should be the same as on the previous run and also within 3% over the cyan, magenta and yellow to ensure gray balance. Any increase of decrease in dot grain range over 3% will cause a color shift visible to the human eye.

Halftones

How do they work?

Halftones can be printed with any ink color dense enough to hold the dots in all areas of the photograph. Varying the formuation of black ink can change the feeling of a halftone. Add blue ink for a cooler feeling. Add red ink for a warmer feeling.

How do I achieve the right halftone?

Ink colors are usually best selected by evaluating tonal values of ink samples screened to 50% to 70% . This is because every area of a halftone consists of a screen value. To most easily achieve a high contrast halftone, use an original photograph with simple composition. High contrast film will automatically eliminate the lighter grays and register darker grays as black.

Size Changes

Why are my book bodies and covers changing sizes one week after assembly?

In some instances, when you print the body of book using heatset web and the cover using sheetfed, you may notice either the body grows bigger or the covers become smaller a week or so after assembly. This occurs when the dyer on the heatset web reduces the moisture content to almost zero percent. This causes the paper to shrink to its minimum dimensions. As it reconditions, the paper will grow perpendicular to the grain (cellulose fiber grows in width not length).

How can I avoid this?

Allow the paper to recondition before binding to avoid having to perform additional trimming later.