Not only must paper be handled carefully during shipment but it must able be handled carefully in the plant.
Paper must be brought to pressroom temperature before opening so that it does not curl due to the gain or loss of moisture. Paper should remain tightly wrapped until it is ready for the press, and it should be wrapped tightly after printing to keep the press sheets from developing tight wavy edges.
An instrument called a hygroscope is used to measure the moisture content of a pile of paper relative the humidity of the pressroom. Readings from a hygroscope help the press crew to determine if paper conditioning is necessary.
Figure 5-7 shows the guidelines for decision on printing sheetfed paper, assuming paper at 45% R.H.
Temperature conditioning. Temperature conditioning means that the paper is not unwrapped until it reaches the same temperature as the pressroom where it will be used.
In some printing plants, paper is stored in the pressroom. Consequently, it is pressroom temperature when it goes to press. Because the paper is stored in the pressroom, temperature conditioning is unnecessary. In other plants, however, paper is kept in a warehouse or separate storage area. Paper is brought to the pressroom only as needed. Temperature conditioning is necessary because the temperature of the storage room is usually different from that of the pressroom (Figure 5-8).
Temperature changes, which occur when paper a cold warehouse or truck is delivered to a warm pressroom, cause trouble even if the paper is wrapped and sealed to protect it from humidity changes. When cold paper is unwrapped in the pressroom, moisture condenses on the edges of the paper. As a result, the exposed to as wavy-edged paper.
If the temperature of the paper is higher than the temperature of the pressroom when unwrapped, the surrounding air becomes warmer and its relative humidity is lowered. The paper's exposed edges will lose moisture to the atmosphere and shrink; the paper is often referred to as tight-edged paper.
Chilling the air with an air cooling unit dries the air and creates tight edges in the paper. Air conditioning requires control of both temperature and humidity.
Air conditioning paper is manufactured so that it stays flat and does not develop conditioned pressroom greatly reduces moisture problems. If the pressroom R. H. (and temperature) is kept at a constant level, the printer can order paper that is in reasonable balance with a specified relative humidity.
The R.H. of most paper manufactured in the United States and Canada is in the range of 35%~50%. The R.H. of an air-conditioned pressroom should be maintained at a selected value within this range.
Paper with an R.H. 5%~8% higher than the pressroom R.H. is desirable for highquality, close-register printing that requires more than one pass through the press. Paper with this R.H. loses moisture to the pressroom atmosphere at about the same rate that it picks up moisture from the press, assuming that the pressroom R.H. is controlled. Since moisture change is minimal between successive printings, the printing of tight-register work is more easily accomplished.
Paper cannot be produced to meet the R.H. requirements of pressrooms in which the R.H. and temperature are not controlled. Papermakers produce papers that meet the R.H. conditions of the average pressroom. If the pressroom is not temperature-and humidity-controlled, the paper should still be temperature-conditioned.
Storage. To ensure that paper is received in proper condition, all deliveries should be checked upon arrival. Wrappings or cartons having minor tears should be repaired. Skids with punctures, tears, or breaks in the protective wrapping should be rejected. The delivery should note the damage on the way bill and initial it. Printers should photograph damaged skids and cartons before they are removed from trucks or railroad box cars. The photograph acts as proof of the paper condition upon arrival. In addition top protecting the printer, the photograph helps the papermill track down the causes of damaged paper. If paper wrappings must be removed for sampling or testing purposes, the wrapped paper should be brought into temperature balance before opening and then rewrapped immediately. Careful handling of paper minimizes unloading damage.
Ideal warehouse and storage conditions minimize the movement and rehandling of paper from the time it is received by the printer until it arrives at the press. Each time paper is moved, either to gain access to other skids of paper or to be transported to the press, the likelihood of handling damage is increased.
Paper must not contact concrete or damp basement floors. The moisture damages and distorts the paper. Consequently, platforms or racks that elevate the paper above the floor are recommended. Papers should be stored away from any object (such as a radiator) that heats the paper, but warehouses should be heated in winter.
After paper is printed, protective, protective plastic covers should be placed tightly over the pile to minimize changes in R.H. Figure 5-9.