Shadows fill in, sheets mark or offset onto each other, and the job is slow to dry.
Filecheck fixes this automatically at upload
Total area coverage is the sum of the four CMYK percentages in the darkest part of your file. Solid black built as 100C 100M 100Y 100K is 400%, and no press will accept that.
Paper can only absorb so much ink before it stops behaving. Past the limit, the sheet does not dry in time, and the consequences appear after printing rather than during it: ink transfers onto the back of the sheet stacked above, the job marks when it goes through folding or cutting, and shadow detail closes up into a flat dark mass.
The number depends on press and paper, so treat these as orientation and use whatever your printer specifies:
Uncoated is the one people underestimate. The paper takes ink more readily, so it reaches its limit sooner, not later.
Rich black is the usual culprit. Designers build a deeper black by adding colour beneath it, which is right in principle, but 100K plus full C, M and Y is far past any sensible limit. A typical rich black is more like 60C 40M 40Y 100K, which totals 240%.
The other source is dark photography converted without a profile that enforces a limit. Converting through the correct ICC profile handles this automatically, because the limit is part of the profile’s definition.
Total coverage is measured at upload and compared against the limit for that product. Where a file exceeds it, the ink can be reduced automatically through the correct output profile, preserving the appearance of the shadows while bringing the coverage under the press limit.
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