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Storage handling and conditioning |
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All paper contains a small amount of moisture, and is highly sensitive to changes in temperature and humidity in the surrounding air. In humid environments, the paper’s fibres expand and make the sheet swell. If on the other hand the environment is dry, the opposite occurs and the paper releases moisture. These phenomena can cause dimensional changes or waviness, which in turn can lead to printing problems. In order to reduce this external influence, and to protect the paper against dirt, damage and other wear before it reaches the customer, the paper is wrapped.
However, the wrapping should be kept on even after the paper reaches the customer, as it will begin to acclimatise to the surrounding humidity level as soon as the packing is opened. If the premises do not maintain the right temperature and humidity, this can lead to the problems mentioned above.
To reduce the risk of dimensional change, the paper should also be stored at the same premises that it will be used at, to make sure it adapts to the right temperature. This is of course especially important if the paper has been transported a long distance or has previously been stored in a cooled ware-house or over the winter.
There are suggested conditioning times of original wrapped paper depending on temperature differences between paper and environment, see the below table.
Short memo table for one normal pallet
Note that this is more approximate but give a hint of time needed.
Temperature difference 5oC 10oC 20oC 25oC
Time in hours 12h 24h 48h 64h
The table shows the volume of paper and different in temperatures and the time needed to reach equilibrium between paper in a pallet and temperature in the surrounding environment. A “normal” pallet weight and size gives that the 0,6m3 often is give a good estimation on full normal pallet. To calculate the volume in pallet more exact, following calculation can be done; w x l x h for example a 64x90 pallet at a height of 110cm will be 0,64x0,90x1,10 = 0,63m3
Humidity, after temperature, is the second major factor which has influence on behavior of paper. Suggested difference between relative humidity of paper and printing hall, shouldn’t exceed 10%. Otherwise paper processing problems might occur. In the case of standard offset paper, suggested humidity of the printing house environment should be in the 40-60% range at temperature 22-23o C.
Also bear in mind that once the dimensional changes, caused by excessive humidity difference, have occurred the paper will never return to its original shape, even if the correct air humidity level is restored.
There are several different kinds of wrapping, the most common being paper and plastic. Paper wrapping usually consists of a special type of kraft paper with a thin embedded plastic film, and this is wrapped around the reel of paper. This maintains the existing moisture level while keeping water out. Corrugated board end-discs are used to cover the ends of the reels.
There are also several types of plastic packaging. One of these is stretch film, whereby thin plastic wrap is wound several times around a pallet or reel to ensure a stable parcel. The film also provides protection.
Another plastic wrapping is shrink film. The pallet is wrapped in plastic – polythene – which then shrinks after passing through an oven. Finally, any air is removed by compression. Following this process, the pallet is practically vacuum-packed and very steadily supported and well protected.
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As you are bound to already know, paper has different life spans. You can probably picture what an old newspaper looks like without actually seeing one. Yellow and slightly brittle. As you may expect, this phenomenon is called yellowing.
The quality of a paper and its ingredients determine how resistant it is to ageing. Generally speaking, wood-containing paper grades, i.e. paper made from mechanical pulp, have a shorter life span as they contain lignin, which makes the paper turn yellow after prolonged exposure to sunlight (UV light). If the product is intended to last a really long time, woodfree paper is to be recommended, i.e. paper made from chemical pulp. However, there is no lifetime guarantee – after enough time in sunlight, any paper will yellow or change shade in some way. Either way, the information in the printed product will have long ceased to be relevant before the paper is no longer any good – the paper can be left on a book-shelf for several years before it starts turning yellow.
The general rule is to avoid exposing wood-containing paper to direct sunlight. |
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