Last week we started a conversation about how temperature and humidity affect digital printing. This week we will delve a little deeper. The effect of moisture on paper is nothing new. What is new are the newer printing technologies and the effect of moisture content on the paper transport, toner adherence and color quality due to the absorption of inkjet inks and inkjet papers.
The first thing to know is that paper is hygroscopic, which 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. Over the years, I have been in shops filled with high-speed laser type printers to address complaints about paper jamming. Believe it or not, jamming issues are often the result of moisture content and improper paper handling, which affect static electricity and curl. Paper curl can be caused by several factors but one of them is due to the wavy edges and tight edges of paper that is open to the humidity in the air.
As paper fibers lose moisture, they shrink in diameter – when they pick up moisture, they swell. The problem with open reams of paper in humid areas is that only the edges that are exposed to the moist air will swell (or shrink in lower humidity areas) or only about the outer .5” will be affected, while the inner part of the paper stays the same size. As the outer edges swell (or shrink), the paper starts to distort in shape, creating wavy or tight edges – most consumers just call it curl. When sheets of wavy paper are fed through a copier, printer or digital press, jamming can occur.
Static electricity is an electrical charge that builds up on anything moving including paper. Typically, in normal humidity, the charge is easily released (discharged) and does not have an opportunity to build up enough to cause issues; however, in low humidity conditions, the dry air acts as an insulator and the charges build-up causing the paper to stick together.
Want to avoid production issues? Focus on these six basic paper handling tips:
- Store the paper in a room that is temperature and humidity controlled. If that is not possible, then leave the paper in the room that it will be printed in for 24 hours.
- Keep the paper shrink wrapped and away from outside walls.
- Before you remove the wrap, make sure that the paper is the same temperature as the surrounding air.
- Keep your production space between 45 – 55% RH, temperatures constant and near comfortable conditions.
- Watch outside doors for moisture changes, especially from loading docks, and consider installing a second barrier such as plastic or forced air between the docks and production areas.
- Acclimate the paper after printing to allow moisture return (offset, laser) or “outgas” (inkjet).
Have you discovered any other tricks to help avoid production problems due to paper moisture or humidity?