When several colors are printed on top of each other, there is a limit to the amount of ink or toner that can be put on paper. This maximum total dot percentage is referred to as either TIC (Total Ink Coverage) or TAC (Total Area Coverage).
When a designer ignores this technical limitation, the ink that gets laid down last won’t attach properly to the previous layers, leading to muddy browns in neutral areas. The ink also won’t dry properly on the press sheets. This can cause set-off where the ink of a still wet sheet rubs off on whatever is stacked on top of it.
Specifications for total ink coverage
Ink coverage is specified as a percentage: using 100% of each process color (cyan, magenta, yellow and black) equals 400% TIC. The actual maximum that is acceptable depends on a number of parameters:
- the printing process: digital, sheet offset, web offset (heatset or non-heatset),laser printer,…
- the paper stock (coated or uncoated,…)
- the speed at which the press runs
- how many colors are printed simultaneously (since intermediate drying time is important)
To know the TIC that the printer can handle, it is best to consult with them. The following percentages are industry averages:
- Sheetfed offset on coated paper (typical commercial printing jobs): 320 to 340%
- Heatset web offset (such as magazines): 300 to 320%
- SWOP: 300%
- Non-heatset web offset on uncoated paper (such as newspapers): 240 to 260% (In a 2011 analysis of thousands of advertisements that were sent to Dutch newspaper printers, 53% of all those ads had a TAC above 240%. The prepress systems at those sites fix this by ‘reseparating’ the files – which is not too difficult to do with today’s prepress systems but it is still a major hassle).
I haven’t found much information on inkjet devices or copiers but in general these systems seem to be able to handle values between 300 and 350%.
How to avoid exceeding total area coverage
One error that is frequently made is to use the ‘registration’ color for design objects. Since registration equals 100% of each separation, the total ink coverage is always exceeded when this color is used. Never use ‘Registration’ for design elements.
Incorrect separations settings in Photoshop are also a possible reason why images are ‘too heavy’. The default color settings of Photoshop are optimised for web coated paper.
Another easy way to cause TIC related issues is the inappropriate use of overprint.
How to check total ink coverage
InDesign has a Total Ink Coverage indicator in the Separations Preview window.
There are preflight tools that can check total ink coverage. Keep in mind that some of these applications only check vector objects and don’t check within images or take overprints into consideration.
Recent versions of Adobe Acrobat Professional have a ‘Total Area Coverage’ setting in the Output Preview settings. You specify a threshold value and any area within the PDF that exceeds this value is highlighted. Similar functions are offered by several prepress systems, which can check complete press sheets for any TIC related issues.
To discuss TIC, check this thread on the B4print forums.