Relative humidity and the temperature of the pressroom contribute significantly to the paper’s “runnability” on press. To ensure dimensional stability of the paper, it is important to measure the relative humidity of the paper and pressroom on a regular basis.
What is relative humidity?
Relative humidity can be defined as the amount of moisture, in vapour form, in relation to the amount of moisture the air can contain at a given temperature. It is important to consider the relative humidity of the pressroom environment as it has a direct impact on the paper. Paper is hygroscopic, meaning it absorbs and loses water readily. When the moisture in the paper is in balance with the air’s relative humidity, the paper is in equilibrium with its surrounding atmosphere. At equilibrium, the paper can neither gain nor lose moisture. This is a characteristic that the printer must ensure that the paper adapts to. To achieve this, the printer should conduct a test that measures the relative humidity and the temperature of the immediate surroundings, while eliminating any influences in the external atmosphere. The ideal relative humidity of a paper’s dimensional stability is between 35-50 percent. With that said, the 40-45 percent range has been found optimum for many sheetfed-offset plants with a pressroom temperature of 21-29 degrees celsius. Pressrooms that are kept at this temperature along with the relative humidity helps to facilitate faster drying time; the environment is more conducive to the oxidation and polymerization because of the higher temperature and lower relative humidity. As a result of obtaining a balance between the RH of the paper and pressroom, there will be fewer mis-registers and paper-edge problems.
Impact of relative humidity
One of the most significant issues that occurs in the pressroom is the waving of paper, which is a result of humidity. This is why it is important that printers have a thorough understanding of the effects that relative humidity and temperature have on paper through its life cycle, as a lack of knowledge can lead to unnecessary expenses and frustrations. The printer must take into consideration how the paper is stored and the piling techniques, as improperly managed stock can lead to damages and thus make the paper unprintable. Furthermore, creating tall stacks of paper can cause wavy edges due to the uneven pressure on the centre and the edges of the stack, especially in cases where the paper must be stacked higher than usual. Another consideration printers should take into account is how the paper is packaged during shipping. Tightly packaged stacks of paper can cause delays for the printers and hold up the printing job because the moisture content in the stack of paper will be reduced due to the pressure from the tight packaging. This causes the paper to become hard and brittle, decreasing resiliency and smoothness under printing impressions. Therefore, the printer must wait for the paper to reach its equilibrium in order to print. Furthermore, when the relative humidity of the surrounding air is lowered, the paper will lose moisture. In dry conditions, moisture is drawn from the exposed surfaces of the paper stack and paper will shrink, but the centre of the paper still maintains its original moisture content, which causes tight edges. Taking into account the moisture content of the paper is essential for printers to be familiar with as it leads to dimensional changes and can cause problems such as curl, paper distortion and major register problems.
The temperature and humidity in the pressroom and stockroom should be kept as stable as possible at all times. When moving paper to the pressroom, a sufficient amount of time must be allowed for the paper to properly adjust to the humidity change (attempting to keep the pressroom and stock room at the same humidity will decrease the amount of time and lower the risk of paper damage). Before running a job on press ensure that the operator checks the humidity of the pressroom and paper to avoid any possible problems that may occur. Lastly, be aware of the weather changes, as this can affect your pressroom temperature and humidity levels of the paper stacks.