The average ink savings by switching from, for example, a 175 lpi AM/XM halftone to a 20 micron FM halftone, is about 10-15% (depending on the range of tones in the original art). Since the average sheetfed printer will spend about 3-4% of their gross earnings on ink, as an example, for a $10,000,000 dollar a year printer the ink savings using FM screening will range from about $35,000 to $52,500 a year.
The reduction in ink usage for the same final presswork appearance, comes primarily from the use of a cut-back curve applied to the FM screened plate, the thinner ink film characteristic of FM screening, and the higher percentage of the tonality being created optically (optical dot gain) .
How higher dot gain saves ink
The smaller the halftone dot is, the greater its perimeter to area ratio is.
Since dot gain occurs at the perimeter of the dot this results in smaller dots having more dot gain. And since FM halftone screens are made up of very small dots they will initially have a higher dot gain compared to a conventional AM/XM halftone. That higher dot gain compared with, for example, a 175 lpi screen, must be “normalized” by using a tone reproduction curve applied to the plate to achieve the same final presswork appearance. As a result, to get the same final tone on press that a 50% dot at 175 lpi gives - you might only need a 40% dot with an FM screen. Because a 40% dot area carries less ink than a 50% dot area - the result is a reduction in ink usage.
For example, here is a 4/C image rendered at 175 lpi:Left bottom is a 50% tone, center bottom is the 50% tone on plate, right bottom is the final result: a 65% tone on the press sheet. The look of this presswork will be the target for the FM screen presswork.
Next is the same image but this time rendered with a 20 micron FM screen:Left bottom is a 50% tone, center bottom is the 50% tone on plate, right bottom is the final result: an 80% tone on the press sheet. The presswork is now too dark compared to the 175 lpi AM/XM target.
However, by applying a tone compensation curve to the plate, the extra dot gain can be factored out:Left bottom is a 50% tone, center bottom is the 50% tone mapped to 40% on the plate, right bottom is the final result: a 65% tone on the press sheet. Now the FM presswork is tonally aligned to the AM/XM target presswork. And since lighter tones on plate carry less ink area, the result is a reduction in ink consumption.
FM's thinner ink film also helps save ink
Small FM dots cannot carry as thick a film of ink as larger AM/XM dots can because there is not as much dot area to carry the ink. This characteristic also contributes to ink savings.
The below image shows the thickness of ink on a 175 lpi AM/XM halftone. Ink density has been mapped to height to show the thickness (depth exaggerated for illustrative purposes).
Here is the same 3D projection - this time with a 20 micron FM screen:
In North America, approximately 80% of telephone directory printers and 60% of newspaper flyers (as well as 90% of WalMart flyers) are printed using FM screening in order to take advantage of ink savings for cost and well as environmental impact reductions.