Paper conditioning and pressroom humidity: why stock is acclimatised before a run and what goes wrong when it is not
Paper reacts to the air around it
Paper is made from plant fibres, and those fibres take up and release moisture all the time. When the air is damp, sheets absorb water and the fibres swell, mostly across their width. When the air is dry, they give it back and shrink. A single sheet changes size by only a fraction of a millimetre, but on a press running thousands of sheets at high speed, and on a job where four colours have to line up to within a tenth of a millimetre, that fraction matters.
That is why careful print shops control pressroom humidity and why paper often sits wrapped in the pressroom for a day or more before it is used. This article explains what paper conditioning is, what goes wrong without it, and what print buyers can do to avoid problems on large or demanding runs.
Relative humidity, moisture content and equilibrium
Paper leaves the mill at a moisture content chosen to be close to balance with typical pressroom air. The wrapping (usually a moisture-resistant ream or pallet wrap) is meant to keep it that way until it is used. Once the wrap is opened, the paper starts moving towards the moisture level of the surrounding air.
The key ideas:
- Relative humidity (RH) is how much water the air holds compared with the most it could hold at that temperature. Many pressrooms aim for a stable value in roughly the 45 to 55 percent range, and paper mills often publish recommended conditions for their stocks.
- Equilibrium is the point where the paper stops gaining or losing moisture. If the paper and the room match, the sheet stays flat and stable.
- Temperature matters as much as humidity. Cold paper brought into a warm room can collect condensation on the outer sheets, and its edges react before its centre does.
Why paper is acclimatised before a run
Paper delivered from a cold warehouse or a lorry in winter may be several degrees colder than the pressroom. If the wrapping is opened immediately, the outer sheets and the edges of the stack take on moisture faster than the middle. The result is a pile of sheets that are no longer flat.
Acclimatising, or conditioning, means leaving the paper in its unopened wrapping in the pressroom until it reaches room temperature. The wrapping stops moisture exchange while the temperature evens out. The time needed depends on the size of the pallet and the temperature difference; large pallets can need a day or longer. Mills and paper merchants publish guidance tables, and experienced press managers follow them closely in winter and in humid summers.
What goes wrong when it is not conditioned
Wavy edges and tight edges
If the edges of a stack absorb moisture while the centre does not, the edges grow longer and form waves, a defect usually called wavy edges. If the edges dry out instead, they shrink and the centre bulges, known as tight edges. Both make sheets feed badly and print unevenly, because the sheet does not lie flat against the blanket.
Curl
When one side of a sheet gains or loses more moisture than the other, the sheet curls. Coated papers with different coatings on each side, and sheets printed heavily on one side, are particularly prone. Curl causes feeding problems on the press and makes finishing harder later.
Misregistration
Because paper grows and shrinks mostly across the grain, a sheet that changes size between colour units, or between the first and second pass on a two-pass job, will not line up. Fine detail and small text show colour fringes. We describe how operators monitor this in registration, colour bars and the press sheet.
Static and double feeding
Very dry air lets static build up. Sheets cling together, two go through the press at once, and the delivery pile does not jog square. Dry paper also cracks more easily when it is folded, even with scoring.
Drying and set-off
Humidity affects how inks dry. In damp conditions some inks dry more slowly, increasing the risk of set-off in the delivery pile. For more on that, see ink drying and set-off.
Conditioning after printing matters too
The press itself changes the moisture in a sheet. Offset printing applies water as well as ink, and heat-set or UV drying removes moisture. A freshly printed pile is often slightly out of balance with the room. Printers usually let printed sheets settle before cutting, folding or binding, because a sheet that is still moving will not trim accurately or fold cleanly. The relationship between moisture and grain direction is covered in our article on paper grain direction, and the effect on cutting in guillotine cutting and trim accuracy.
| Condition | Typical defect | What the printer does |
|---|---|---|
| Cold paper, warm room | Wavy edges, condensation on outer sheets | Leave wrapped until it reaches room temperature |
| Very dry air | Tight edges, static, cracking on folds | Humidify the pressroom, condition stock longer |
| Very damp air | Curl, slow drying, misregistration | Dehumidify, keep reams wrapped until needed |
| Partly used reams left open | Edges different from centre | Rewrap unused paper immediately |
Digital presses are sensitive too
Humidity is often thought of as an offset problem, but toner and inkjet presses react to it as well. Toner transfer depends on the electrical properties of the paper, which change with moisture. Paper that is too damp can give patchy solids and poor toner fusing, so the image rubs off on folds. Paper that is too dry builds static and jams. Production inkjet adds water to the sheet as it prints, so heavily covered pages can cockle unless the paper and dryer settings suit each other.
Digital presses usually work from smaller paper stocks loaded into trays, which makes it easy to forget about conditioning. A carton opened on a cold morning and loaded straight into the press can behave differently from the same paper an hour later.
What print buyers can do
Most of this is the printer's responsibility, but buyers influence it more than they might think.
- Allow time for supplied paper. If you are supplying your own stock, or the printer is ordering a special paper, allow time for it to arrive and condition before the press date.
- Store finished work properly. Printed jobs kept in a damp storeroom or a hot car can curl or cockle after delivery. Keep cartons off the floor and away from walls and heaters.
- Mention two-pass and laser jobs. Jobs printed on one press and then overprinted later, or digitally personalised after offset printing, are the most sensitive. Tell the printer so they can plan the gap between passes.
- Be wary of rushing heavy stocks. Thick boards and duplex stocks take longer to settle. A very tight schedule on a heavy-coverage job is when conditioning gets skipped.
How we handle paper at our print shop
Our pressroom runs with controlled humidity and temperature, and incoming paper stays wrapped on the floor until it has reached room conditions. For long runs and jobs with fine registration, we check sheets at the start of the run and between passes, and printed work is left to settle before it moves to cutting and binding. If you are supplying your own paper or need a two-pass job such as offset shells overprinted later, tell us when you book so we can plan the timing properly.
You can read more about our general printing services, or contact us with the specification of your next large run and we will advise on stock and schedule.
