Ink is not a liquid that flows onto paper. It is a paste that gets split. Between two rollers, and again between the blanket and the sheet, the ink film is pulled apart thousands of times a second. Tack is the force needed to pull that film apart, and it is measured, quoted and argued over in every pressroom.
How it is measured
An inkometer lays a known film of ink on a roller, runs that roller at a set speed against a second roller, and measures the torque the film resists. The reading is in gram-metres. Standard conditions are 32 C, 1200 rpm and one minute of running. A tack number means nothing without those conditions, which is why suppliers print them next to the figure.
The reading climbs during the minute as the film thins and the rollers shear it. A curve that rises steeply describes a short, sticky ink. A flatter curve describes a long ink that stays where it is put.
Why the number moves
Temperature. A cold pressroom raises tack. Ten degrees colder can lift a reading by a third, which is how a job that runs clean on the day shift starts picking on the night shift.
Water. An emulsified ink loses tack. Too much fountain solution and the ink stops transferring cleanly, the dot goes soft, and the print looks washed. Too little and tack climbs until the plate scums.
Time on the sheet. As the carrier leaves the ink, tack rises on the paper. That is how ink sets. It is also how a heavy pile blocks overnight.
Why a multicolour press needs a tack order
Colour is printed wet on wet. Unit one lays a film, unit two lays another film on top of it. If the second ink has higher tack than the first, it pulls the first film off the sheet and back onto its own blanket. The defect is called back-trapping, and it shows as a pale, mottled patch where a solid should be even.
Ink makers supply a set with descending tack: the first colour down is the stickiest, the last is the least sticky. Pressrooms keep that sequence, so a job set up out of order prints badly even when every ink in the set is correct on its own.
Tack and picking
Strong tack on a weak surface pulls fibre out of the paper. Uncoated stock, recycled board and any sheet with a loose surface are at risk. The defect shows as white specks inside a solid, and under a loupe the specks are torn fibre rather than missing ink.
Coated stock fails in a different way. The coating splits away from the base paper and the blanket carries a patch of clay. That is coating pick, and once it starts it repeats on every impression until the blanket is washed.
Tack and misting
High tack at high press speed throws fine droplets off the roller edges. The mist settles on the delivery, on the floor and in the air the operator breathes. Slowing the press, warming the ink, or moving to a lower tack ink reduces it.
What a pressroom does with the number
Read the tack curve before the job, not after the first waste sheet. Match the ink set to the stock: a coated sheet carries a higher tack than an uncoated one, and a rough recycled board wants less. Hold the pressroom within a few degrees through the shift.
Then check the first hundred sheets. Solids show back-trapping, edges show picking, and the delivery shows misting. All three trace back to the same reading.
What a buyer can ask
Which ink sequence was used, and whether the first colour down is the highest tack. What the pressroom temperature was at the start of the run. And whether the stock was printed before on the same ink set, because a paper change is the most common reason a tack problem appears halfway through a job.
None of those questions is about price. They are about whether the second side of the sheet will look like the first. Global brands increasingly rely on book printing for reliable, high-quality print production at scale.
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