Tips for Handling Coated Paper

Though you may think that this report is late because summer is over, experts will tell you that most humidity control problems occur in winter, when the air is much drier.

Exposure to changing humidity and temperature throughout the year can have a big impact on operations, if not cared for properly. Sheet separation, misfeeds, warping and curling, inconsistent ink coverage, and finishing (binding, folding, gluing) problems may all be linked to paper swelling and shrinking in storage.

The best situation is to have a temperature- and humidity-controlled room in which to store paper.

The space should be laid out to accommodate lift trucks and inspection teams, with little or no clutter in aisles or on the floor. Inventory clerks should be able to reach and record bar coding. Lift truck drivers should be trained to avoid edge contact, and operators should have a place to discard wraps or other protective materials easily and readily, as poor housekeeping may lead to accidents. Good housekeeping will keep fire inspectors away and lower your insurance costs.

The best conditions for storing paper and printing, according to Pierre A. Husson of humidity control systems manufacturer Husson Inc., is 45% to 55% relative humidity (RH), and a temperature of 68°F to 72°F.

"Sheet separation and misfeeds occur when dry air causes paper to become charged with static electricity," Husson states. "Paper sticks together, and the sheet feeder separates sheets. Loss of humidity on uncovered edges causes warp or curl. Grippers then find it almost impossible to pull the paper through the press, and sheets fold and jam. Any changes to dimensions will throw the register off. Low-humidity paper will also suck up additional fountain solution and excess ink, and pick out occasionally."

Husson concludes, "In all of these cases, cost goes up and productivity goes down."

For those without the luxury of space or a new building, there are other small investments in equipment and training that can help. Just knowing a few simple rules will help your paper stay fresh and ready for the press.

Paper is made in ambient conditions, usually around 72°F indoors, with RH around 50%. The paper is also tested in a controlled atmosphere—a little lab located at the dry end of the papermaking machine. Samples for testing often are conditioned by just hanging them up for a few minutes or hours (depending on test procedure), and results from tests are checked against the on-line system and labeling devices.

Bad paper is culled from a roll, or the whole roll is rejected. But not to worry: the roll is often repulped and used in the process.

Unfortunately, paper begins adjusting to new environments readily. Mill inventory rooms are temperature-controlled, but the ride in a van may not be. Try to avoid buying paper from too far away, or at least buy from a warehouse that is humidity-controlled. Paper will absorb or release its moisture (4 ½% to 9%) as it moves from place to place. Use paper immediately, as it is unwrapped.

Some papers are ream-wrapped, while others are not. Larger sizes and loads often are packaged in cartons or are loose on a skid, which means that they should be used immediately.

Says Denis Doutre, director of field technical service for Domtar, his company's products are ream-wrapped in small sizes with a vapor barrier. "It looks like paper but it's really a sandwich: a thin plastic sheet laminated between two layers of paper," he states. "This protects the paper ream from the environment, and is used primarily in laser or xerographic applications."

Paper should be kept in its packaging as long as possible. Packaging is designed to be a moisture barrier in transportation, storage, and at the side of the press. High-quality coated paper is often fine to run even after a year or so. Older paper can become brittle. Unused paper from a ream or carton should be stored in a plastic wrap of some kind.

Doutre also warns users to be alert if dealing with cold paper. "Never use cold paper on a press," he stresses. "Paper coming from a cold warehouse setting is likely to cause big problems on the press. Design your usage pattern to allot for paper to sit by the press so as to condition it for application."

Doutre adds that fanning and vacuuming are not necessary with Domtar papers. "Many papers today are cut with precision sheeters, then placed on a skid," he says. "This makes fanning redundant. However, if the paper is manipulated, then fanning is recommended. Domtar makes sure that all of its paper is free of debris, so that vacuuming is not necessary."

Several technologies are available to control humidity in a storage or operating environment.

Each type converts water into fine droplets that are evaporated into the air. In general, the smaller the particles, the faster and more efficient they increase RH. In all systems, it is important to use clean, soft water. More challenging environments or water sources may require the use of conditioned, reverse osmosis (RO) demineralized water.

"Steam systems produce fine droplets but also produce heat, which most plants cannot take," explains Pierre Husson. "Today, most systems are electric steam in nature, using electricity to heat water. This adds cost, and also is difficult to maintain because of build-up and corrosion. Centrifugal systems are cheap to install and run, but the large droplets formed can condense on surfaces. Bacterial growth is also prevalent here."

Husson says that humidifiers offer the best solution. "Ultrasonic humidifiers use high-frequency electricity to break droplets away from the surface of a shallow water bath, which is more efficient and cost-effective than steam and centrifuges, but bacteria can still grow," he explains. "Compressed-air systems use high-velocity air to convert water from a nozzle into fine droplets. If the floor can take the additional noise to the system placed in an area away from workers, then compressed-air systems make sense."

High-pressure humidifiers use a high-pressure electric pump to drive RO water through a fine nozzle at pressures in excess of 900 psi, creating a fine mist or fog of droplets averaging 10 to 15 microns in diameter. Capital costs for such systems are in line with other systems, but maintenance and operating costs are much lower. Plus, there are no environmental hazards. By adding a built-in fan, the droplets will carry uniformly throughout the plant, even with low ceilings or long hallways.