knock up

In printing, to knock up means to tap and square a stack of paper so that all sheets align flush on at least two edges, typically the head and left side. The operator holds the stack firmly and strikes it sharply against a flat surface or against a metal or wooden edge, using repeated quick blows to settle and compress the pile. This ensures that when the stack feeds into a press, cutter, or collating machine, sheets travel straight without binding or jamming.

The knock-up operation is essential after sheets arrive from a paper supplier, after a guillotine cut, or after folding. Paper undergoes compression and slight curling during manufacture, storage, and transport; moisture changes also cause sheets to shift against each other. A well-knocked stack reduces false counts and missed sheets during finishing. The operation takes seconds but prevents costly misfeeds and spoilage.

Operators use several methods. Hand-knocking against a bench or metal angle-iron works for small quantities. Powered stack knockers, fitted to many finishing machines, use pneumatic or mechanical rams to strike the pile at the head and side simultaneously, holding the stack rigid. Some equipment includes vibration tables that settle paper without impact. The force needed depends on sheet size, weight, and how badly the stack has settled; 80 gsm bond requires less aggressive knocking than 200 gsm cover stock.

The verb "knock up" comes from the literal action of knocking or tapping, common in English craft trades for decades. In North America, the same operation is sometimes called "jogging" a stack, particularly when done on a powered jogging head. Regardless of terminology, the goal is identical: to present a true, parallel stack to the next machine or hand.