Thin sheets of paper, card, or tape pasted underneath a printing plate, woodcut, or type block to bring its surface exactly up to type-high (0.918 inches). This adjustment ensures the rollers ink the block evenly and the platen or cylinder applies uniform pressure during the impression. Unlike an overlay, which adjusts pressure on the tympan side, an underlay physically shims the printing element from the bed.
Heard on the job
We need another sheet of tissue underlay beneath that halftone block to get it printing heavy enough.
Insider tip
Always start by checking the block's height with a type gauge before adding underlay, as over-packing can damage the plate or the rollers.
Do not mix it up with
- Overlay: An underlay goes under the plate to raise it, while an overlay goes on the impression cylinder or tympan to adjust pressure.