Wood pulp fibres ‘the building blocks of paper making’ can be affected by moisture. Paper is made to certain actual moisture specifications depending on its end us, i.e. web offset papers have lower moisture content (approximately 4%) than sheet fed offset (approximately 7%).
In a controlled press room of 20-23C and 45-55% Relative Humidity, most grades of paper will retain their original dimensions and moisture content values. This is on the proviso that the paper has been allowed time to reach the same temperature as the press room before being unwrapped.
However, it appears that many press rooms in South Africa are not climate controlled. Therefore, papers cannot possibly produce papers to match all of these varying conditions. It is important to understand the possible consequences.
When fibres absorb moisture, they swell,
when they lose moisture, they shrink
Absorbing moisture will result in wavy edges which, if great enough, will cause creasing and register ‘fit’ problems. The opposite (loss of moisture) will cause tight edges which again, distort the dimensions of the sheet and cause creasing and mis-register.
The reason for the stock being affected by moisture can be caused by:
The difference of temperature of the pressroom and the temperature of the paper or,
The difference in the relative humidity of the paper and the relative humidity of the press room.
Firstly, the ‘temperature’, let’s say the paper has come from a cold paper store in the basement and the actual temperature of the stack of wrapped paper is 3C. The print room is heated to around 20C. This will cause wavy edges very quickly when the cold paper is unwrapped in a warm environment. The air surrounding the stack condenses and this excess condensation is absorbed by the paper. To minimize this, it would be advisable to have the paper in the press room for an adequate period of time for equilibrium in temperature to be reached.
As a simple guide, we will list approximate recovery times for a pallet of paper e.g. 750 kgs.
|
Temperature Recovery Times |
|
|
Temperature difference between Material and Press Room |
|
|
Difference in C |
Time Needed |
|
10 °C |
24 hours |
|
15 °C |
40 hours |
|
20 °C |
75 hours |
|
25 °C |
90 hours |
The second issue is ‘Relative Humidity’. Two scenarios follow:
If the paper is ream wrapped in moisture proof paper (at the mill) at 50% RH and is opened when the relative humidity in the press room is 80%, excess moisture will be absorbed and the fibres will expand in the cross grain direction. The result will be wavy stock.
However, if the paper is packed at 50% in its packets and the press room is now 20% RH, the paper will lose moisture to the atmosphere and contract the fibres, primarily in the cross grain direction. This shrinkage is commonly referred to as ‘tight edges’. This is quite common during the winter months as the factory temperature increases to combat the cold conditions outside, the relative humidity drops. To minimize or avoid this, introduce humidification, or if in a small room, increase relative humidity locally by using buckets of water or plenty of wet rags.
Do’s and Don’ts
- If possible, take your paper to the press room 24 hours before printing. If this is not possible, try and limit the temperature differential between the paper storage and press room.
- Leave the mill waterproof wrappers on until paper is to be press loaded and printed
- Protect your paper with polythene covers between passes and back-ups
- Protect printed paper with covers between printing and guillotine/trimming and folding
- Protect printed work with polythene covers or stretch film in transit to the binding or finishing house. This applies to web offset folded sections or sheets, particularly.
- Don’t immediately print paper that has just been unloaded from the delivery vehicle
- Don’t unwrap your paper until just before it is going to print
- Don’t leave paper unprotected for any length of time
- Don’t leave partially printed paper unprotected for any length of time
- Don’t leave paper unprotected in the press feeder overnight.
- Don’t guillotine trim paper in readiness for printing and leave it unprotected. Cover stacks with polythene.