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Paper Storage Temperature and Humidity Control

CFW has many years of experience in designing systems for climate control of paper storage environments. A wide range of desiccant and refrigerative dryers and fan equipment is available. Complete, bespoke solutions of any scale can also be designed and installed.

paper rolls

Paper is a material that is subject to various environmental stresses. Whether unopened reams or valuable documents are to be preserved, both short and long-term storage conditions need to be seen to. In sensitive applications such as printing presses, temperature and humidity levels must not be allowed to fluctuate much. Heat can cause paper to dry out and curl, and lead to problems with mould and discolouration. Curl can also occur if the air is too dry, or after rapid heating in a printer and subsequent cooling.

Causes of Paper Deterioration

Acids in the paper, which may be present in the wood pulp used to make it or in chemicals added to improve the paper's characteristics, tend to cause yellowing or browning in the long run. This process is accelerated by higher temperatures, which increase the rate at which the fibres break down. Some paper can be subject to delamination or "picking" (the pulling out of ink-covered fibres from the paper) when cold.

Since paper is hygroscopic, it tends to absorb or lose moisture until its moisture levels reach equilibrium with its environment. Different grades of paper have different hygroscopic characteristics, but all kinds of paper must be acclimatised properly and stored in a stable environment, in which relative humidity does not fluctuate too much. Changes in humidity tend to result in non-uniformity as the fibres expand or contract unevenly across the surface of the paper, and also lead to changes in stiffness. This decreases the printer runnability of the paper.

Results of Paper Deterioration

Low relative humidity (RH) can encourage print distortion and the build-up of static (which in turn can cause sticking, improper stacking or misfeeds at the press or office printer). High humidity can result in slow ink drying. Sudden increases in humidity are the most common cause of wavy edges, but cold paper which is unwrapped in a warm room can also lead to waviness. This is because the paper absorbs moisture unevenly and expands in those areas where more moisture is absorbed, either because of high air moisture or condensation on cold paper. Tight edges occur under the opposite conditions: low ambient humidity or relatively cool ambient temperatures when the paper is warm. This is a problem for large newspaper printers, where the edges of reels tend to dry, causing the paper to be subject to tearing when run through the press. The result is downtime and production losses.

Any of these kinds of paper deformation can cause a sheet to stick in printers, causing a paper jam. Misfeeds can also cause creasing. Distortion of short-grain paper also results in altered paper dimensions. If this is not controlled, fan-out (i.e. increasing size in printed images between printing stages) is harder to manage and can negatively affect press functioning. Wrinkles, colour misregister (where the colours successively printed on the paper do not line up) or slur, including blanket pre-slap, can result from dimensional instability. Misregister can result from even small dimensional changes.

Guidelines for Appropriate Storage Conditions

Ideal ambient temperatures are between 18 and 22 ° C. Significant deviations in temperature can result in irreversible deterioration. Relative humidity should be kept between 45 and 60 per cent. Dehumidifiers can be used to regulate RH in conjunction with hygrometers to monitor air moisture levels. Proper monitoring of ambient humidity will also help determine how long paper will need to acclimate to an environment such as a pressroom.