Airing Out the Options

There is a large selection of air-drying equipment available; which to choose depends greatly on the application.

by Kate Tomlinson, Associate Editor

NO MATTER WHAT you are printing, be it tags and labels, flexible packaging, folding cartons, or corrugated, a dry web is a basic essential. Drying technologies are plentiful, but choosing the right method for your application plays a crucial part in the end product’s success. Unfortunately, air-drying equipment comes with a bit of fine print. And as long as you can answer a couple quick questions (types of inks/coatings/adhesives and typical substrates), suppliers are more than willing to spell out the pros and cons of each type of machinery.

“Simply put, the web-coating process involves applying a diluted solution to the paper, film, or metal substrate and drying the solution,” says Matt Litzler, marketing manager at C.A. Litzler. “Continuous web dryers are used to evaporate the coating from the web’s surface. The evaporation of the coating depends on heat transfer, vapor transfer, and removal of the vaporized diluent.”

Flotation dryers

Converters producing coated web products are turning to air flotation dryers because of their efficiency, final product quality, and relatively low-cost operation versus roll support dryers.

Litzler explains that the moving coated web in a flotation dryer is supported on a cushion of heated air emitted from a series of airbars above and below the web. Stable flotation is achieved because the positive pressure pad of the nozzles reacts against the vertical components of web tension and weight. “Staggered air bars form a sine wave in the web, which eliminates edge curl while providing web transport without contact,” he concludes.

Bob Narang, presient of Radiant Energy Systems, adds that flotation dryers are good when the web is coated on both sides or with very delicate substrates. He says they are more expensive than other drying technologies but offer advantages including shorter equipment lengths and heights, while boasting the same line speed capabilities as impingement dryers.

“The flotation dryers are also good when you do not want film to touch the rolls, such as when you need to prevent scratching or contamination on decorative film or medical applications.” Narang also touched upon the largest challenge associated with this machinery: “You cannot run a large variety of substrates or coating weights in a flotation dryer, because the web is supported on a bed of air and these requirements will change with the weight.”

Hot-air impingement dryers

According to Narang, hot-air impingement dryers are the most commonly used dryers in the converting industry. The reason is that they provide a very cost-effective and efficient way to dry water- and solvent-based inks, coatings, and adhesives.

Generally these dryers have idlers arranged in an arch for proper web tension control. The idlers may be driven for handling delicate webs, which can be specially designed to avoid stretching or deformation. Most impingement dryers also contain features to keep the rolls cool, which maintains web stability while it travels through the long, hot oven.

“Many older lines have ovens that are merely ‘hot boxes,’ which worked well for solvent-based inks and coatings, but are totally inadequate for water-based coatings and the high speeds converters are running today,” continues Narang. “For example, last year we replaced a seven-year-old, 150´-long dryer, that struggled to reach 125 fpm line speed. Today this company enjoys drying and curing solvent-based adhesives on polyethylene film using their 140´-long, four-zone impingement system that easily runs at 300 fpm.”

Combination air/infrared dryers

Seen most often in applications running coatings, the most common combination dryers pair infrared energy with air. These dryers are very efficient, relatively short in length, and are also cost effective. The right combination of infrared and air is essential to achieving a completely dry web without running into problems such as web damage on line, stops, or skinning of the coating because of the infrared’s high intensities.

One combination is high-velocity, hot-air impingement with infrared heaters between the air nozzles. “The typical solution to problems with drying waterborne coatings is to slow down the line, giving the product more dwell time in the dryer, or use longer ovens,” says Jim Alimena, Glenro’s vice president of marketing. “Unfortunately, any gains in product quality achieved in these ways may be offset by losses in productivity and plant floor space. Glenro’s hot air/infrared systems allow you to run coating lines at maximum speed without sacrificing product quality, and without wasting precious floor space.”

“The combination of hot-air impingement and infrared energy also solves aggravating coating blistering problems, because water doesn’t become trapped beneath the coating’s surface,” Alimena concludes.

New air-drying solutions

Recently, several new air drying and combination air/IR drying units have hit the marketplace. Fuchs Devries introduced a Hot Airknife Drying System, offering the heat and control needed to maximize press output. It was designed to dry water-based coatings by incorporating high-volume hot air with velocity and temperature controls. This PLC-controlled system maintains temperature to within 1°F accuracy.

C.A. Litzler introduced the new Compact Split-Hood PERFECT FLOAT® flotation dryer at Labelexpo. The dryer is designed with the fans and burners built into the dryer box to eliminate external ductwork. The compact split-hood allows access to the dryer for web-up, maintenance, and cleaning while using minimal floor space.

Dri-Tec’s latest design consists of two sections, top and bottom, with split parallel to web pass line. The top section is hinged at the backside, allowing it to be swung open, exposing the internal workings of the drying chamber.

The ROLLTRACKER roll support dryer, by MEGTEC Systems, offers high-efficiency ductwork, heat source, and air-handling components enclosed entirely within the dryer box. This self-contained design requires less floor space than conventional dryers with remote air-handling systems and it reduces requirements for structural support and platforms.