stack

In offset printing, stacking occurs when ink accumulates on the blanket cylinder or printing plate faster than it transfers to the substrate. Instead of a clean, controlled film of ink depositing onto paper, cloth, or other material, excess ink pools and creates heavy, uneven deposits. This results in blurred halftone dots, filled-in fine lines, and muddy color. The printed surface becomes sticky and may show what pressmen call "set-off," where wet ink transfers backward onto the back of the sheet above it in the stack.

Stacking typically happens when ink viscosity is too high for the press speed and substrate being run. A thick, sticky ink designed for slower speeds will build up on the cylinder if you run faster than the ink film can transfer. It also occurs with dry or absorbent paper stocks that don't accept ink quickly enough, or when fountain solution concentration drops and the water-ink balance fails. Nip pressure set too high compounds the problem by forcing excess ink into the gap between blanket and substrate.

You diagnose stacking by watching the first few impressions off the press and checking the blanket and impression cylinder surfaces. A blanket that feels tacky or looks shiny and wet is a sign. The printed sheets show heavy color, lost detail, and slow drying. To correct it, reduce ink flow at the fountain keys, lower nip pressure if the press allows it, or thin the ink with appropriate reducer. Adding fountain solution helps restore the water-ink balance. On some presses, slight speed reduction allows the ink film to transfer cleanly.

Stacking is distinct from other ink transfer problems. Picking, where fibers tear from the paper surface, happens at the point of pull. Setoff is the consequence of stacking and occurs in the pile. Chalking is the opposite: ink dries too fast and rubs off. A pressman must distinguish between these to apply the right fix; adding water won't help a picking problem, and thinning ink won't help a fountain solution mix that is too weak.