IR heat transfer occurs when a temperature difference exists between two bodies (i.e. the lamp, and the substrate/ink film). Unlike convection (dryer) or conduction (hot air gun) heat transfer, IR does not require physical contact between the source (lamp) and the target for heat to flow. (Conversely, with convection, the heat source heats the air, which in turn heats the substrate/ink film).

Convection/conduction is based upon the first power relationship of the source temperature. For example, if the element heats the air to 275 Fahrenheit, the air, in turn, heats the substrate/ink film to 275 Fahrenheit. IR is basically "invisible" to gases (like air), so it doesn't heat the atmosphere inside the curing unit. If the system is focused, the lamp(s) will heat the reflector, which is usually water-cooled.

Infrared heat transfer is proportional to the fourth power. If the IR has a source temperature twice that of a convection heat element, the radiant heat transfer is 16 times higher with IR than with convection. Obviously, the result is a significantly lower curing time. Cooling requirements for the product are also reduced, since IR doesn't need to get the substrate hot in order to cure; only the ink film.

Other benefits of IR compared to convection are "instant on-off", whereas convection ovens need to be pre-heated. Conduction (hot guns), like the Leister hot air dryers used by Innovative Marking Systems, maintain a constant internal element temperature and use forced air to transfer that heat to the substrate/ink film. Essentially, conduction in the case of the Leister units offers "instant on-off" as well, although the heat transfer is still based on the first power, resulting in a longer dwell-being necessary than with IR.

IR does have some limitations, particularly with regard to the effective removal of VOC's from the ink film. If you have a specification related to out-gassing (i.e. automotive), IR is probably not going to be the way to go. IR also yields a less consistent cure than conduction or convection, and is more expensive to purchase (by a factor of 2X on average) and maintain (lamps have an average life of 1500 to 5000 hours and are expensive to replace). Power usage for IR, versus an efficient convection or conduction system, is basically a wash.

Finally, ink manufacturers typically don't provide IR curing parameters for their products, so it becomes the responsibility of the ink/IR user to determine processing parameters.

In conclusion, the main advantage of using IR is a faster curing cycle and a reduced footprint. The main disadvantages are a narrow processing window, entrapment of VOC's, a higher capital investment, and higher maintenance costs.

Sources: Infrared Curing Article Missing Some Facts, Mario Bassi, Bassi Trading Company, Italy. Posted 4/18/2002 @ www.circuitree.com and High Speed [adhesive] Curing with Focused Infrared, Norm Cox, MSME. Research, Inc. 1998 Adhesives Conference Presentation.
 

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Periodic Table of Pad Printing Elements

Decorating Process Comparison Chart

Dealing with Heat and Humidity

Infrared vs. Convection or Conduction in Curing Pad Printing Inks