Humidity Control for Printing Paper
I always knew that in successful printing of any kind, paper, ink, and water had to always be in a fine balance, or that any number of printing problems could occur. I saw that when I went to commercial printers to check press sheets for the jobs I had designed, and I even began to understand it back in my own office as I saw how heat affected the paper in my laser printer or how inkjet ink would soak into the printing paper and make it ripple. Back then I couldn’t quite articulate what it all meant, but I knew that balance was vital in order to avoid disaster.
So I was pleased this week to find an article explaining optimal humidity for various kinds of commercial printing. It is entitled "Why Proper Humidity Control Is Vital to Paper and Print Quality Results." It was written by Caine Ruckstuhl. I found the article on www.piworld.com, on 05/16/18, and noted its attribution to PrintingImpressions. I’m sure you can find it. I think you will find it helpful.
These are the characteristics of paper Ruckstuhl’s article describes:
1. Paper is "hygroscopic." It absorbs water from the surrounding air (that is, in the form of humidity). Paper can both absorb moisture and also release moisture in order to maintain balance, or equilibrium, with the surrounding environment. Because the fibers in the paper absorb or release moisture, the physical character of the paper changes, and this will affect the printing processes in which the paper is used.
2. When printing paper is manufactured, it is initially mostly water. The pulp mixture used is then reduced until the final paper has four to six percent of its weight in water. When the mill packages the paper, its water weight is not static. Rather it varies depending on the moisture content in the surrounding air. This is why rolls or sheets of printing paper are brought from storage into the pressroom prior to the print run, so they can become conditioned to the ambient relative humidity. This way the paper fibers can expand or contract and reach equilibrium before being fed into the offset presses.
3. Too much or too little water in the fibers will distort the paper. This might include making it less dimensionally stable (i.e., making it stretch under the pressure of the press rollers), or making it curl inappropriately or become wavy. Sheets of paper can also stick together, and there can be problems with static electricity. Overall, such problems will contribute to equipment downtime and paper waste. It will be a struggle to produce usable printed press sheets. Moreover, lack of proper humidity (too much or too little) will even shorten the life of press components such as cylinders and plates.
Ruckstuhl’s article then goes on to explain exactly how the ideal ambient conditions for different printing technologies include different levels of relative humidity. Ruckstuhl addresses the humidity requirements for digital printing (laser printing), offset lithography, and inkjet printing.
Digital Laser Printing
Laser printing, also known as xerography or electrophotography, works by fusing toner particles to the surface of the printing paper using intense heat and pressure. (This is the mechanical process, apart from the laser-produced electrostatic charges used to position the toner particles.) Obviously, high heat draws water out of the air and out of the paper traveling through the laser printer. Therefore, to keep the paper moving smoothly, the relative humidity of the paper (RH) should be between 50 and 55 percent.
Too much or too little relative humidity can make it hard for the toner to properly adhere to the paper substrate. It can also cause changes in the temperature of the printer rollers, leading to paper jams. If the RH falls below 40 percent, the paper can change shape, and electrostatic charges can cause the paper to stick together and jam in the printer. Or, the paper can attract dust, which can cause uneven application of toner particles to the paper surface.
The offset printing process (which includes the additional complexity of an ink/water balance in order to work properly) requires higher relative humidity than laser printing. Ruckstuhl’s article, "Why Proper Humidity Control Is Vital to Paper and Print Quality Results," notes that offset printing works best at 55 percent RH. Not having the proper humidity can cause problems in the "interaction between the ink [and] the press, the paper [and] the press, and the ink [and] the paper" ("Why Proper Humidity Control Is Vital to Paper and Print Quality Results," Caine Ruckstuhl).
According to Ruckstuhl, if the RH is lower than 55 percent, "the dry air will cause issues, such as curling, creasing, and dot doubling." If the humidity is too low, "electrostaic buildup is also common and can also cause misfeed, as well as problems with stacking, trimming, and folding when the paper starts sticking together." ("Why Proper Humidity Control Is Vital to Paper and Print Quality Results," Caine Ruckstuhl). When offset printing paper is fed back into the press to print the opposite side of the sheets, "it can change shape again and cause cracking along the folds" in the post-press finishing department. Ruckstuhl doesn’t mention this directly, but if the paper changes shape when the press is printing the reverse side of the sheet, the images on the front and back of the paper will not be properly aligned once the press signature has been folded and trimmed. All of this can lead to massive amounts of downtime, slow printing, and paper waste: a real nightmare.
The inkjet printing process prefers 45 to 55 percent relative humidity. This is the standard range for most inkjet printing paper, and to vary from this range can cause ink spreading (in which inkjet dots expand rather than staying crisp and precisely positioned).
(Although Ruckstuhl does not specifically mention this, I would be concerned that spreading inkjet dots could also change the printed color, just as spreading halftone dots in conventional offset printing can cause color shifts, depending on the colors involved and the extent of the dot gain.)
Show-through of ink and problems with the drying of the inkjet ink on the paper can also come from not maintaining an ambient RH of 45 to 55 percent.
Fortunately, Ruckstuhl notes that humidifiers can be used in the press environment to properly regulate humidity levels, so each of these printing processes can function at optimal efficiency.
[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]