EXPERTONLINE: Paper Misfeeding? Check Humidity

Few print production problems are easier to diagnose than those caused by dry air. Static electricity causes paper to double-sheet, slowing and jamming presses. Dry stock shrinks, creating problems from press infeed to finishing. Paper also can crack or break when scored or folded, causing waste and dust. Glue fails to cure consistently in dry air.

Such problems can affect press speeds, run times, waste, quality and profit. Frequently the culprit is a low level of relative humidity in the air.

Dry air problems are regional (in hot, arid climates) or seasonal, tied to heating plant air in colder weather. Heating air causes it to expand in volume. Unless moisture is added, relative humidity plummets.

In winter months plant humidity levels commonly drop to 10% or even less. For instance, when outside air that's 10° F with 60% relative humidity is heated to 70° F, its relative humidity drops to about 8%. Most types of problems begin to kick in somewhere between 30% and 40%. The ideal relative humidity for printing is 45% to 55% at around 70° F. Few printers can achieve these levels without adding large amounts of water to the plant air.

Paper and cardboard in the mill have about 6.5% to 9.0% water, but that begins to change the minute it leaves. The printer's goal is to keep moisture content as consistent as possible. This ensures paper does not get out of humidity balance and begin changing dimensions in transit or in storage.

Keeping paper tightly wrapped as long as possible, to reduce moisture loss, will go a long way to mitigate moisture loss. Once damage is done, it's usually impossible to undo.

Reducing or eliminating dry air problems is simple and inexpensive with modern engineered humidification systems. ROI is often less than two years—and the fix is usually permanent.

Static electricity frequently causes “haystacking” in the jogger. Plastics and synthetics are inherently sensitive to static charges and require higher relative humidity levels, typically around 50% to 55%.

In heatset printing, high temperatures and high speeds may mask evidence of moisture related problems, but challenges with static, web breakage, dust and inline glue issues will arise.

Today's humidification systems involve zones throughout an operation connected to a central programmable control. The control reads input from meters in each zone and activates the humidifiers when relative humidity falls below target levels.

High-pressure humidifiers create a fine, cool, fog-like mist. Because no heat is used, energy and maintenance costs are lower than other technologies: 10% to 15% that of compressed air systems, and less than 1% of electric steam systems.

For summaries of other types of humidification options, visit graphicartsonline.com.