The environmental conditions of your paper storage and pressroom can have an impact on what is often the largest, most expensive component of a printing project—paper. By nature paper is dimensionally unstable and susceptible to temperature and humidity. As the climate in the air changes, so does the climate within the paper. Establishing a policy and procedures to ensure paper is in an optimum condition prior to handling in the pressroom will help to avoid potential problems in production.
Air contains many gases, including oxygen, nitrogen, and water vapor. At a given temperature, air can only hold a specific, determinable, maximum amount of water vapor. Relative humidity (RH) is the percent ratio of the amount of water vapor present in the air to the greatest amount possible at the same temperature.
Paper readily absorbs and yields water vapor when it is exposed to different atmospheric conditions. Paper is subjected to various temperatures and humidities in the paper mill, during transport to the printing plant, in the plant’s storage area, and in the pressroom. If the temperature and moisture content of the paper and the surrounding air do not attain equilibrium in a controlled manner, it can lead to dimensional instability, runnability problems, misregister, or picking on press.
Paper Manufacture and Delivery
When paper fibers absorb or lose moisture, the diameter changes more drastically than the length of the fibers. This means that the paper dimensions change more across the grain than with the grain. The degree of dimensional change for any given change in moisture content depends upon the type of paper, its composition, and the extent of its refining. Printing papers are manufactured so that they have a minimal dimensional change consistent with other required properties.
Paper companies supply paper with a moisture content that, in their experience, satisfies most of their customers. It is impossible to produce paper that meets the RH requirements of the changing and adverse atmospheric conditions of many pressrooms, in transit, or in plant storage.
To ensure that the paper is received in proper condition, all deliveries should be checked on arrival. Wrappings or cartons having minor tears should be repaired. Skids or rolls having punctures, tears, or breaks in the protective wrapping should be examined very closely to determine if the material is defective or unusable.
Printers should photograph damaged skids, cartons, and rolls before they are removed from trucks or boxcars. The photographs act as proof of the condition of the paper when it arrived at the plant. In addition to protecting the printer against loss, photos can assist the mill in tracking 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 it is opened, and rewrapped immediately.
From Storage to Pressroom
When paper is removed from a cool area where it is in equilibrium with regard to the temperature and RH of the air around it and placed in, for example, a pressroom where the temperature is greater, the paper will gain heat from the air in the pressroom until it reaches equilibrium. Depending on the RHs of the paper and of the air in the pressroom, paper will gain or lose, moisture as well—even if it is properly wrapped.
This situation can be avoided by using a temperature-control system. However, many printers find it too costly to air condition both the paper storage area and the pressroom. If the storage area is not temperature-controlled, when paper is moved to the pressroom, it must be allowed to stand until it comes to pressroom temperature before being unwrapped. The length of time required depends on the size of the skid or roll and on the differences between the temperature of the paper and pressroom. If the paper storage area is maintained at the same temperature as the pressroom, stored paper can be unwrapped and used immediately.
Temperature and RH are Critical Factors
The greatest paper handling difficulties occur on cold days when the humidity inside the pressroom may become exceptionally low, causing tight edges that result in misregister, sheet wrinkling, and problems with static electricity. Even on a mild day in the northern United States and Canada, the pressroom RH may be only 15%. On the flip side, during a thunderstorm in the summer or in some areas in the south, a pressroom RH can reach as high as 90%. A high RH causes the paper to develop wavy edges (Figure 1).
Figure 1. Wavy-edged paper (left) and tight-edged paper (right).
Subjecting the paper to wide variations in temperature is a major cause of paper runnability issues. It is possible for cool paper in a warm pressroom to condense moisture in the same manner that moisture collects outside a glass of ice water. A press operator would not think of sprinkling water onto paper, yet when a skid of ice-cold paper is opened, moisture condenses on it the same way as if it had been sprinkled. The result is that the outside edges of the paper expand while the center of the sheet retains its original dimensions. When this occurs, wavy edges will develop.
A less common, but equally serious, problem will also occur when paper is exposed to extremely low RH in a warm pressroom. In this case, the edges of the paper lose water and shrink, while the inside center areas of the sheet remain unchanged in size. The result is a loose belly in the sheet or tight edges. Low RH also leads to static electricity and feeding and delivery problems.
Press Operators Can Determine Paper Temperature and RH
It is sometimes desirable for the press operator to determine the temperature and RH of paper. Temperature and RH can be measured using a sword hydrometer (Figure 2).
Figure 2. Measuring temperature and RH with sword hydrometer
Summary: Preventing Paper Problems
- Place orders far enough in advance to allow paper to reach pressroom temperature before removing protective wrapping.
- Photograph skids, cartons, and rolls of paper before removing them from trucks or boxcars. Report all damages to the carrier and paper mill immediately. Severely damaged paper should be rejected.
- Store paper at pressroom temperature, but not near a heater or cold wall.
- Do not unwrap paper until it is ready to be printed.
- If skids or rolls are unwrapped for sampling or testing purposes, they should be rewrapped immediately.
- All paper should be properly covered or wrapped with plastic between each pass through a press and after printing is completed
This article comes from Printing Industries of America’s Center for Technology and Research at http://www.printing.org/ctr.