What is Set-Off and how does it happen?
April 17, 2009 – 11:42 amIn printing, lithographically only, ink is tacky as it is transferred onto the paper. If there is a lot of coverage, i.e. a 4-colour black (CMYK - Mentioned in this article) of e.g. 100% of all colours, this would make that area of the print particularly tacky to touch and as such, when it is delivered onto a pallet at the end of the press it starts to stick to the sheet below and above it.
Then, when the print process is complete and they are ready to be guillotined, you hear a cracking and tearing noise when picking up the piles, this is because they have been stuck together, not usually enough to tear the sheets, but enough to damage the print. This is often caused simply by the amount of ink and weight of paper bearing down ontop of each other, for instance you may find the bottom sheets “set-off” worse than the top few.
To avoid this, printers use less colour combinations to achieve the same colour, i.e. 30,30,30,100 (CMYK) looks the same as 100,100,100,100 (CMYK) except uses a 210% less ink. This makes the black look the same colour but be less thick and “rich” and allow it to be printed quicker and trimmed quicker.
Set off can also occur if print is trimmed to early after printing and hasn’t been given sufficient time to dry out, the clamp pressure which ranges from 1-2 tons of the guillotine can then cause the sheets to stick together causing the same problem.