ALLENTOWN, PENNSYLVANIA — One of the most critical steps in the screen printing process is drying, or curing, of ink after it has been applied to the screen printed item. This is accomplished by heating the ink to the temperature at which it cures, as a printed garment or other item passes through a dryer on a conveyor belt. Popular plastisol inks require cure temperatures of approximately 320° to 340°F (160° to 171°C), while other types of ink cure/bond at slightly different temperatures.
Choosing Between Gas and Electric
Gas and electric dryers offer different advantages depending on budget, required capacity, available floor space and the cost of gas versus electricity.
Gas dryers have an open flame that heats air within a chamber and transfers the heated air to the area through which garments or other printed items are conveyed. The heat is then transferred to the ink by air convection, equivalent to heating food slowly in a gas oven.
An electric dryer, by contrast, uses infrared radiation that causes ink molecules to vibrate, generating heat that raises the temperature of the ink rapidly without heating the air around it, akin to heating food in a microwave.
The primary advantage of a gas dryer is its ability to maintain a constant temperature and avoid overheating of the ink. Primary disadvantages are dwell times of two to three minutes to heat the ink, requiring long dryer lengths that increase capital cost and consume floor space. Additionally, thick insulation is needed to prevent the outer walls from attaining dangerous temperatures. As a result, gas dryers are generally recommended for large print shops, providing that natural gas or propane is available and less expensive than electric is locally.
Gas dryers are typically larger and more expensive than infrared conveyor dryers for several reasons: 1) Gas dryers handle significant volumes of high temperature air, requiring costly air handling systems with motors positioned away from the hot air stream, 2) Complex safety controls are required to protect operators from combustible pressurized gas and 3) thick insulation is needed to prevent the outer walls of gas dryers from attaining dangerous temperatures.
By comparison, infrared conveyor dryers are smaller and less expensive because they do not require insulation, and are shorter in length due to reduced dwell times, since infrared radiation attains ink curing temperatures more rapidly and efficiently than heated air, particularly when curing plastisol inks. However, if items are conveyed through an infrared dryer too slowly, the ink can overheat and break down chemically.
Unlike installed gas dryers, infrared conveyor dryers are also expandable. For example a relatively large infrared dryer capable of curing approximately 1000 screen printed items per hour, can cure 2000, 3000 or 4000 per hour by adding one, two or three heating chambers, in addition to lengthening the conveyor and increasing its belt speed.
In terms of size, the infrared electric dryer shown in Figure 1 measures 7 to 8 ft (2.1 to 2.4 m) in length and cures approximately 300 items per hour, versus approximately 10 to 12 ft (3.1 to 3.7 m) for a gas dryer of equivalent capacity.
Remove Moisture from Water based Inks Rapidly
A common misconception is that forced hot air is needed to evaporate moisture from water based inks and to expel it from the dryer. In reality, it is infrared radiation that heats the ink and causes evaporation, which is then removed from the heating chamber by a high velocity exhaust system with high efficiency — on par with the efficiency of an infrared dryer curing plastisol inks.
For example, a Vastex Big Red infrared conveyor-dryer (FIG.2) exhausts 400 cu ft/min (11 cu m/min) of air from the drying chamber and supplies roughly 500 cfm of air, warming but not superheating the air to pull moisture from the ink and exhaust it, while preventing outside air from entering through the dryer exit.
Compare Quartz Tubes with Infrared Panels for Generating Infrared Heat
If your application calls for an infrared electric dryer, you can maximize results by understanding different ways in which infrared radiation is generated.
Some dryers use quartz tubes while others use infrared panels. Because quartz tubes radiate 360°, they require a reflector to direct energy radiated from the top portion of the tube, back toward the garment, losing efficiency in the process.
Infrared panels are made of a ceramic quartz fabric and are flat, directing virtually all energy toward the garment, offering higher efficiency (less power usage) per unit of radiation.
Quartz tubes are also relatively fragile and typically have a shorter life, whereas infrared panels are more durable and have a useful life of up to 20 years, making them less costly in the long run.
Infrared radiation is measured in terms of wavelength (not temperature), which is expressed in microns (one millionth of a meter [0.000039 in.]). For most screen printing inks, the optimum frequency is 3.2 microns (0.000126 in.), which is at the top end of the mid-micron range. At this frequency, the infrared radiation is readily absorbed by the ink, causing its molecules to vibrate and generate heat.
However, most quartz tubes in dryers for screen printed items generate infrared radiation in the mid-micron range making them less efficient for curing of screen printing inks.
Eliminate Cold Spots with Proper Core Density
An infrared panel is powered by a long electric wire routed back and forth in a parallel pattern across the panel, with the distance between the wires determining its "core density." Because the temperature between wires spaced widely (2 in/51 cm) can drop by approximately 100°F (30°C), you should specify a heater having wires spaced at approximately .75 in. (1.9 cm), providing sufficient core density to prevent the ink from cooling as it passes between wires.
Keep Your Options Open with a Wider Dryer
It is advisable to specify a wide dryer if budget permits, for drying larger garments or two smaller garments side-by-side, as well as blankets, signage, binders, metal parts, advertising specialties, and other screen printed items. A dryer with a 30 in. (76.2 cm) wide belt, for example, cannot cure an overall printed shirt that measures 40 in. (102 cm) sleeve-to-sleeve.
If your budget is limited, the higher cost of a wider dryer can be offset by purchasing a shorter (less costly) dryer initially, and then adding heating chambers (depending on brand) at a future date to increase capacity as you grow.
Anticipate applications beyond printing and drying of textiles
It is typically less expensive to purchase a dryer that offers greater capabilities than you need today, than it is to purchase a larger dryer when you require greater capacity or the flexibility to expand beyond textile printing.
Consider that the same screen printing presses and dryers used daily by thousands of textile printers worldwide, are also used to print and dry a diversity of signage, and hard goods as previously mentioned.
One example is screen printing on medical delivery trays (FIGS. 3 and 4) that contain and identify surgical instruments. Because the trays are utilized several times per day, they are sterilized in autoclaves where they are subjected to pressurized steam after each use, placing rigorous demands on the screen printed graphics.
Screen printing has long been the industry standard for printing on instrument delivery systems because of the need to use highly specialized inks that stand up to repeated autoclaving, and due to the varying shapes of the trays.
A growing portion of the manufacturer's trays are printed on two, Vastex one-color tabletop 2000HD Heavy Duty Screen Printing Presses and cured in a Vastex EconoRed 30 Infrared Dryer.
The industrial-grade presses are constructed with tube steel legs, square steel rotor arms, heavy-gauge steel rotor assemblies and non-warp steel pallets, with all critical moving parts riding on ball bearings. The dryer is equipped with a 4300 watt heater and a 66 in. (168 mm) long, heavy duty conveyor capable of printing 190 pieces per hour.
This application involves specialized inks and uncompromising quality standards, while others involve irregularly shaped items, extra large (heavy) screens, extremely short runs requiring fast set up and micro registration, extremely tight multi-color registration and other future requirements that may hinge on the capabilities of your screen printing equipment. It is therefore prudent to select equipment that will expand, rather than limit, opportunities that may arise during your screen printing career, even if you cannot utilize some of the equipment's capabilities at the onset.