January 2006
In This Issue...
Resources
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Infrared Application of the Month #1:
Glueing of Direct Mail Pieces
A specialist direct mail printer uses Carbon IR technology to apply glue to printed pieces inline as they leave the litho printer.
Two IR modules are arranged in two individually controlled zones allowing specific control of the area to be glued and dried. The glueing and drying section of the process has a small footprint: less than one yard on the production line. As installed, the system has the capacity to glue and dry at a rate of more than 800 feet per minute.
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Infrared Application of the Month #2:
Welding of Glass-filled Polypropylene
High powered shortwave IR heaters from Heraeus help a manufacturer of cylindrical pressure tanks for water softening equipment.
Glass-filled polypropylene (PP) is used for the tanks because of its strength and durability. The cylindrical housing consists of two pieces joined with a plastic weld. The complete assembly must withstand high pressure. Non-contact IR welding was chosen since it could do the job without compromising the structure of the plastics.
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Tech Center Spotlight:
Medium Wave Heaters
Stable and Efficient
Plastics, water and other solvents absorb medium wave radiation especially well. The use of medium wave infrared heaters helps in the effective drying of paints and lacquers and in the economical processing of plastic foils and sheet.
Because of their long life, these heaters are best suited for continuous process. Surface films and very thin materials are heated up extremely efficiently. Medium wave infrared heaters are manufactured as twin tubes in three different tube formats and in any required length up to 20 feet.
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Special Designs:
Ring Heater
Designed for horizontal use, this shortwave ring heater, internally focused for intense rapid heat-up of rods, stakes etc. is made of quartz glass, has a diameter of 8 mm, a two-sided connection, and is available with or without the Heraeus gold reflector.
A wide assortment of ring heaters are in stock at Heraeus. Click HERE to for details.
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Engineering Aspects of Radiation Theory
continued from last month's issue
Laws of Radiaiton and Their Practical Significance
Turning now to the oscillatory nature, the radiation passes through successive identical states at precise time intervals measured in seconds. The rate at which the states recur, or frequency, is measured in cycles per second so that frequency is equal to the reciprocal of time. The velocity of propagation (in a vacuum ) for all radiation is 3 x 10 8 meters per second: the speed of light. From this we can deduce that the distance between successive identical states -- the wavelength -- is the product of velocity and time. See figure below.
Expressing these statements mathematically ,
t = time interval in seconds which separates the passage of radiation through two successive identical states
f = frequency in cycles per second
λ = wavelength in meters per second
V = speed of light in meters per second
Infrared and visible wavelengths are normally expressed in microns (or micro -meters), this unit being one millionth of a meter. Radiaiton visible to the human eye occurs over a very narrow band, from 0.4 to 0.76 microns. The broad region occupied by infrared extends from 0.76 microns (that is just beyond the red end of the visible end of the spectrum) to 400 microns. However, the radiation used for process heating occurs between wavelengths of 1 and 5 microns in order to obtain adequate source temperatures. This represents a temperature range of 2200 °C to 300°C.
Continuing well beyond the infrared or thermal region to much longer wavelengths of the order of centimeters and meters, the spectrum is occupied by microwave, radar, television and radio communications equipment.
The radiation emitted by a body can be determined if the temperature and nature of its surface (emissivity) are known.
These are the key parameters required to calculate the radiation emitted by a surface at a particular wavelength or over a band of wavelengths.
the starting point in the discussion on the laws of thermal radiation is the concept of the "black body" or Planckian radiator. This is an ideal body which totally absorbs all incident radiation at all wavelengths. The reflectivity is therefore zero (Note that the term "black body" does not have any color connotation in the visual sense). In addition to being a perfect absorber, it is also a perfect radiator: it will radiate the maximum amount of energy at any given temperature. This concept is very convenient in the mathematical and graphical treatment of infrared theory and the development of relationships.
A near approximation to the black body is provided by an isothermal enclosure (see graphic at right), which represents a hollow metal sphere with a small radial hole through its wall. Any radiation entering this hole undergoes multiple internal reflections and absorptions until total absorption is achieved. Conversely, if the sphere is heated, the hole will radiate as if it were a black body. This applies even if the sphere is heated to an incandescent temperature.
From this theoretical phenomenon, it is possible, for example, to visualize the interior of an enclosed furnace with all the walls at a constant temperature behaving almost as a black body. However, in practice all bodies are less than perfect radiators or absorbers, and are therefore referred to as "grey bodies," or more strictly "nonblack bodies." The maximum radiation intensity W produced by a black body unit interval of wavelength is obtained from Planck's Law...
This article will be continued in our next issue.
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That's it for this month's issue of Application Notes for IR Heating. Feel free to encourage your colleagues to subscribe. Just click HERE to send them an invitation to subscribe. It's quick, easy, FREE, and no-obligation.
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