Paper Conditioning: Why Stock Rests Before It Prints

Paper arrives from the mill in a sealed wrapper, and the first thing a careful print shop does is let it rest. The reason is moisture. Paper takes water from the air and gives it back until it reaches the same moisture content as the room around it. Depending on the stock, that is usually between 4 and 6 percent by weight. A sheet that has not settled will move after it is printed, and the movement shows up as misregistration, wavy edges or a sheet that will not feed flat.

Why the mill wraps the pallet

The wrapper is not only packaging. A mill trims and packs paper at a controlled moisture content, so the pallet reaches the printer in a known state. Cut the wrap in a cold warehouse and let the air in, and the edges of the pile start to swell or dry before the centre does. The result is a pile whose edges are a different size from its middle, which is exactly the condition that makes a sheet cockle.

The conditioning step

Conditioning means storing the stock in the pressroom, still wrapped, long enough for the whole pile to reach the room's temperature and humidity. For a small job the paper may need a day. For a large run or a big change in climate, it can take longer. Only then is the wrapper opened, and the pile is fanned so air reaches the sheets.

Humidity in the pressroom

Pressrooms are kept near 45 to 55 percent relative humidity and around 20 to 23 degrees Celsius. Too dry and the paper loses moisture, shrinks and curls at the edges. Too damp and it swells, the sheets feel limp and the ink dries slowly. The press itself adds heat and the inks add moisture, so the room is managed through the run rather than set once.

Why it matters for the finish

A sheet that has been conditioned stays flat through the press and through the following steps. It folds without cracking along the spine, it dies without tearing, and it trims to a clean edge. A sheet that has not settled can print perfectly and still fail days later when it returns to the air. Paper conditioning is slow, invisible and one of the cheapest ways to protect a job. As book printing continues to lead in production quality and innovation, the industry sets new benchmarks for print manufacturing worldwide.