Get Your Dry Right

A primer on what you need to know to get the right dryer and use it correctly.
March 1, 2009

The Adelco JetForce Split Belt gas textile dryer has two belts, each with its own motor and speed controls so you can cure two different types of jobs at once. Set one side with a fast belt speed and shorter retention for curing plastisol on light garments and set the other side with a slow belt speed and longer retention time for water-based ink on dark garments. The Adelco JetForce Split Belt, distributed by Hirsch Intl., provides production flexibility to high-volume shops.

By James Ortolani, Contributing Writer

When screen printers began using plastisol inks in the late '50s, they found they needed a heat source that would cure screen printed prints on finished garments. Don Petrey was an early innovator with Flexible Products Co. and he helped pioneer the process by developing a screen printable plastisol ink for Russell Athletics. Don printed this new plastisol formulation and cured the printed athletic jerseys in an industrial gas oven.

We have come a long way since Don did that first plastisol print one half century ago! I have had the unique opportunity to learn about various curing methods for textile inks from many industry pioneers like Don who have contributed to the development of the screen printing process and our industry. I hope you enjoy the following information that I have compiled through the years.

Beginnings
The introduction of the conveyor textile dryer has been one of the most important developments in our industry, as it helped stimulate the growth of garment screen printing by providing a way to expedite the printing and curing process. Before commercial conveyor dryers were introduced in the early '60s, most garments were printed as piece goods or direct printed with water-based inks and air dried.

It doesn't matter how fast a garment can be printed if you can't cure the inks fast enough to keep up with production. I have come to realize that the single most important piece of equipment in a screen print shop is the textile dryer. Think about it — a wide variety of apparatus can be used to hold the screen in place while the ink is printed on the garment. In fact, door hinges will work to hold a screen for a one-color print! But without a reliable belt dryer, your shop will suffer slow production times and you could ruin entire print runs with under-cured inks and wash out problems. A good dryer is worth the investment.

Dryer or Oven?
I am often asked about the correct terminology to use when referring to a textile conveyor curing system? Is it a dryer or is it an oven? And my reply is that it depends on the application. If you are printing with water-based inks, you are using the curing unit as a dryer because these inks cure when evaporating water from the ink — the heat source merely speeds the process (water-based prints also can be air dried).

On the other hand, when curing plastisol ink prints, the unit is being used more like an oven due to the "melting" process that cures this type of ink. Each terms is correct depending on the application, and I will alternate between the two terms throughout this article.

Terminology
The three main heat sources associated with curing inks on textiles include radiant, convection and conduction.

Radiant heat is the energy emitted from electric infrared (IR) heat panels or quartz tubes. Convection heat is heated air produced by a gas dryer or electric forced air dryer. And lastly, conduction occurs when a heating element comes in direct contact with the substrate being cured.

A good example of conduction heat curing is the use of a heat press to cure digital direct-to-garment inks. The printed garment is removed from the inkjet printer and cured by locking down the heat element of the heat press in direct contact with the freshly printed design. A Teflon sheet should be used as a barrier between the heat element and the garment to keep the heating element clean.

Fit Your Needs
There are two main categories of conveyor dryers found in a textile screen print shop. These types include; electric and gas dryers.

Electric dryers fall into the category of radiant heat, since heat radiates from the electric infrared panels. Small to midsize screen print shops are more likely to utilize small electric conveyor ovens for curing needs rather than gas dryers.

Why? Well, for starters, our industry has many manufacturers of electric dryers that are perfect for the small shop. These units typically feature a 24-inch-wide conveyor belt and oven sections from 4 to 6 feet long. Gas dryers simply are not built that small. Most gas ovens are built with 36- to 48-inch-wide conveyor belts and oven sections in lengths of 8 to 12 feet. These parameters make most gas dryers overkill for smaller production runs and too big of an investment for a small print shop.

Since electric dryers rely on infrared panels to radiate heat, it's important to understand the basics of this heat source.

Infrared energy is a form of light — it can be focused, directed and reflected. The infrared light wave can be found on the electromagnetic spectrum between radio waves and visible light. With sunlight, the infrared light waves are the waves that make you feel warm. Infrared heat travels at the speed of light and heats up any substrate that it contacts — in our case the substrate being a printed T-shirt.

Infrared heating panels are used in a variety of industrial applications, including powder coating batch ovens, glass curing ovens and more. In a textile oven, the IR heaters are generally hung 7 to 8 inches above the conveyor belt and temperature is controlled by either a thermocouple or a percentage timer.

Electric infrared/air dryers combine infrared and convection heat, and this combination is an excellent choice for drying printed plastisol or water-based inks on finished garments. The magic behind the forced hot air is that the hot air actually dries the moisture out of the garment allowing the infrared heat waves to bring the ink up to the proper curing temperature at a much faster rate than a dryer that does not have airflow. Straight IR energy alone works to cure plastisol, but at a slower rate. Heat doesn't transfer as fast due to the moisture trapped in the garment.

Not all combination electric infrared/air dryers are capable of curing water-based inks because airflow designs vary from one dryer brand to the next. If you print water-based inks, look for an electric dryer with a very high cfm rating. Cfm stands for cubic feet per minute and is a measure of the volume of heated air that the fan in a dryer can move. You want something in the range of 1,500 to 1,800 cfm, and I recommend you consult the dryer manufacturer for a recommendation on curing water-based inks.

Gas dryers are convection type dryers capable of maintaining very even temperatures across the entire belt width. The air is heated with a gas burner and forced through a series of nozzles or air knives. Thus, the heat is forced into the printed fabric, accelerating the drying process. And because of the volume of the forced hot air, the fabric heats up very quickly — evaporating the moisture out of the garment. The cfm airflow rating for a gas dryer is almost twice that of the electric cfm rating, due to the large blower motors. An average gas dryer produces up to 3,000 cfm of forced air, making the gas dryer an excellent choice for curing discharge and water based-ink systems.

Things to Consider
Oven section length varies, and is an important factor in which model is suited for your business. It is important to have a realistic projection of how many garments per hour your shop can produce before you purchase a textile dryer. Bigger is not always better, if you end up buying more oven than you actually need. At the same time, you don't want to purchase a dryer with an oven section that is too short to handle your production needs.

Keeping it simple, just remember that a garment needs to be in the drying chamber for approximately one minute to achieve a full cure. (Always follow your ink manufacturer's ink curing guidelines.) Speeding up the belt to handle faster printing times can get you in trouble unless you have a long enough oven tunnel to allow the garment to stay under the heat elements for the full minute. A small manual print shop can usually get by with a short oven length — 4-foot to 6-foot — depending on shop conditions. A larger shop running automatic presses will need a long oven in its production lines — usually 8- to 12-feet long.

It is important to note that any conveyor dryer is only as good as its operator in terms of getting drying times correct and properly curing the ink deposit. Always follow the ink manufacturer's curing guidelines and monitor your dryer temperature on a consistent basis with tools that work with your equipment. If you use a "ray gun"-type thermometer, be sure to hold the "barrel" of the ray gun well into the exit end of the drying tunnel and aim the sensor at a large area of ink.

Remember that the actual ink temperature will reach its highest temperature about three-fourths of the way through the oven, and the garment will begin to cool down rapidly in the last one to two feet of travel as it exits the oven and begins to mix with cooler, ambient air.

Also, temperature tapes are a great tool to use to check the ink temperature, as are digital temperature probe devices.

When checking your dryer temperature be sure to use a reliable temperature testing tools and remember the best safeguard is to always do periodic wash tests to ensure your dryer settings are correct. Investing in a good reliable textile dryer will be the best move your business can make.

James Ortolani has more than 25 years experience in the decorated apparel industry, specializing in hands-on direct screen printing and heat transfer production. He has worked for many industry suppliers, and currently serves as national sales manager for Hix Corp. For more information or to comment on this article, email him at jortolani@hixcorp.com.

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