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What could be causing my paper to misfeed?
October 9, 2007

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Question: My paper stock recently began misfeeding and seeks to be curling. What could be the problem?

Answered by Pierre Husson of Husson, Inc.

The problem could be dry air in your plant—especially this time of year. 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.

The RH Factor

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.

Built-in Water

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.

That Static Cling Thing

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 RH 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 system.

Types of Humidification Systems

 

Steam systems produce efficiently absorbed fine droplets, but usually have high energy and maintenance costs and add unwelcome heat to the environment. Maintenance to prevent build-up of corrosive residues on heating elements is also expensive.

 

Centrifugal systems use centrifugal force to atomize water. Units have low installed and operating/maintenance costs, but larger droplets are slowly absorbed, often leading to condensation problems. Air quality concerns have been raised over bacterial growth in open-water baths.

 

Ultrasonic humidifiers are effective and energy-efficient, but air quality and health concerns have been raised over open-water baths here, too.

 

Compressed-air systems drive water through a nozzle creating fine, easily absorbable droplets. Energy and maintenance costs are low, but nozzle noise can be annoying, so they’re best used some distance from workstations.

 

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

Posted by Mark Vruno on October 9, 2007 | Comments (0)