Guest post by John M Viavattine, Manager, Media Technology Center Laboratory, and Principal Engineer, Digital Printing Substrates and Applications.
Experienced print professionals understand the impact of paper moisture on jobs, and less seasoned printers should certainly get up to speed as well. “Bad” moisture affects printing uptime and output quality. So how to know what’s bad and what’s good moisture content? Measurement.
Moisture content gets measured as relative humidity (RH), and most papers are designed to be stable at 45-55% RH in a 72°F environment. Typically, manufacturers produce paper to an absolute moisture content between 4.0 and 6.0%, meaning water makes up 4 to 6% of the paper’s total weight. But once unpacked, paper can either absorb or release moisture.
Paper Wants Equilibrium
“Paper is hygroscopic,” explains Howie Fenton. “This means it picks up moisture from its environment, and it will release moisture into the environment to try to reach equilibrium with the moisture in the air.” Paper exposed to a 50% RH environment accumulates moisture content of up to 7% in as little as four hours.
When dealing with papers for digital printing, take precautions to ensure optimum press performance:
- Locate the press in a controlled temperature and humidity environment of 65-75°F and 40-50% RH.
- Because paper loves moisture, store paper in the original wrapper. The ream wrappers are designed as moisture barriers.
- Acclimate paper in its wrapper at print room conditions for 24 hours before printing. This eliminates temperature differences between the paper and the environment.
Standard Methods to Measure Paper Moisture
The paper industry uses three methods to measure moisture content:
Oven Dry Method – A paper sample is weighed and then exposed to a 221°F oven. When the samples are re-weighed after cooling, the gravimetric difference is calculated as “percent moisture.” This is the most accurate method and serves as the reference for any instrumental measurements.
Moistrex Moisture Meter – This simple, accurate instrumental measurement uses microwaves to measure moisture content as a percent. However, accuracy depends on calibration using the Oven Dry Method.
Moisture Saber – This is a common method in offset printing. The instrument looks like a sword, and when inserted into a stack of paper, it reads the paper’s RH to calculate moisture content. The degree of accuracy is acceptable for approximate measurements.
Paper mills retain accurate moisture data for the lots they produce, and you can get the information by request. Xerox also retains proof of specification compliance on all Xerox-branded paper.
Paper Moisture and Printing Methods
Moisture content in paper relates to the printing method, and different printing methods work best with certain amounts of moisture. Papers intended for digital marking aim for a target of 4.5% to ensure minimal curl, excellent image quality and optimum toner adhesion. Papers for offset are made to moisture levels of 5.5% – 6%, which aren’t compatible with digital’s hot roll fusing.
Understand the importance of purchasing and using paper with the right moisture content for your purpose, whether digital or offset.
Key Takeaways for Managing Paper Moisture Content
Here are two key takeaways for working with paper. First, understand the importance of purchasing and using paper with the right moisture content for your purpose, whether digital or offset. The second point is to maintain stable moisture content through good paper handling within recommended guidelines. Learn more about good paper handling practices in this short video.