set off

Set off occurs when wet ink from a freshly printed sheet transfers onto the back of another sheet or onto the roller, guide, or surface that contacts it before the ink has dried. In offset lithography, the problem is most acute because sheets must move quickly through the press: if the printed side of one sheet touches another sheet, the ink acts as a temporary adhesive and smudges across the unprinted surface. The result is unwanted marks, loss of image clarity, and wasted paper.

The severity of set off depends on ink formulation, paper type, press speed, and drying time. Fast-drying inks and coated papers that absorb less ink are less prone to set off than slow-drying inks and uncoated stock. Reducing press speed gives ink more time to set before the next sheet arrives, but this cuts productivity. Anti-set-off spray powder, a fine starch or talc applied between sheets as they stack, creates a micro-separation that prevents the wet ink surface from contacting the next sheet directly.

Set off can occur during the printing run itself if the delivery pile is compressed or if sheets are not properly staggered. It also happens during post-press handling if printed sheets are stacked while still damp. In web printing and flexography, set off is less common because sheets do not touch until fully dry, but it remains a hazard with high-speed offset presses running heavy ink coverage on uncoated papers.

Press operators monitor set off by inspecting the underside and face of printed sheets at the delivery end. Visible ghosting, smudging, or ink marks on the back indicate that ink is setting too slowly relative to press speed. Adjustments include switching to faster-drying ink, increasing anti-set-off powder application, reducing sheet stacking height, or slowing the press. The term derives simply from ink that has set off from one surface onto another.