Thermal V.S. Infrared Radiation The term Thermal Radiation simply describes - heat transferred by electromagnetic radiation. Infrared radiation is a type of electromagnetic waves, which could also transfer heat. Thus it is commonly known as “heat radiation”. However, only part of the infrared radiation could carry heat (highlighted in dash). There is non-thermal infrared radiation used in telecommunications applications. For example, infrared remote control and Wifi, they all employ infrared radiation. Figure 1 depicts distinction between thermal radiation and infrared radiation.

Figure 1

Thermal radiation could directly transfer heat to an object over a long distance. For example, our earth constantly receives Sun’s thermal radiation through space. Despite the obvious advantage of direct heating, radiation heating could be complex. Due to the wave nature of radiation, some energy will be reflected, some will be transmitted and the rest are absorbed. Only the absorbed energy is used to heat up the material. The proportion of reflected, absorbed and transmitted energies depend on type of materials, their surface smoothness and incoming radiation wavelength.

100 % incoming

Glass

100 % incoming

5 % reflected

Metal

5% absorbed

40% reflected

60% absorbed

90 % transmitted

Figure 2. For instance, glass is a good transmitter. Most of the thermal radiation will penetrate through glass. On the contrary, all thermal radiation will be either absorb on metal surface or reflected. Figure 2 shows a typical thermal radiation on a glass and a metal. A highly polished metal surface will reflect most of the thermal radiation. Figure 3 shows polished parabolic metal surface reflects and focus solar radiation.

Figure 3.

Castool Superoven

The efficiency of a radiant oven depends on type of materials heated and radiation wavelength generated. A properly designed die oven considers the properties of die and radiation wavelength all together. Die is usually made of H13 hot work tool steel with polished surface. Figure 4 shows the absorption behavior of H13 steel. It has peak absorption for thermal radiation at narrow range 2 – 2.5 um for almost 95%. The two grey bands represent air absorption wavelength. Heat will be absorbed by the air inside the die oven – the efficiency will be low [1]. Wavelength absorbed by air

Absorption ratio

Figure 4

AISI H13 1.5

2.0

2.5

3.0

3.5

4.0

4.5

Wavelength( um) Best wavelength for H13 heating

Superoven Radiation Profile 100

Nominal Distribution

90 80 70 60 50 40 30 20 10 0 0.0

1.0

2.0

3.0 4.0 5.0 Wavelength, micrometers

6.0

7.0

The thermal radiation in Castool’s Superoven has the peak radiation tuned to maximum absorption for H13 to provide better radiation heat transfer (See appendix 1 for more information)

A closer look at the absorbed thermal radiation energy on metal surface reveals the energy is absorbed at a very thin layer near the surface, around 0.1 μm(See Appendix 2 for more information). This phenomenon dictates what is the best way to increase the Die temperature without over heating the surface. The heat transfer within the H13 die is controlled by thermal conduction of the metal itself. The best way to heat the die is using all the available surface area. Superoven uses 4 sides heating to take advantage of all surface area on the die. The die is encased in high temperature surface layer. Then the heat is conducted to the core of the die by conduction. After all larger die has more surface area, the time it takes to heat up a larger dies is essentially the same compare to smaller dies.

Heating time ~ 75 mins

Larger Die, More Surface area Heating time ~ 75 mins

Reference 1. Chang-Da Wen, “Investigation of Steel Emissivity Behaviour : Examination of Multispectral Radiation Thermomtry Emissivity Models”, International Journal of Heat and Mass Transfer 53( 2010) 2035-2043.

Appendix 1. Planck’s Law describes the electromagnetic radiation emitted by a materials at any given temperature.

B: Spectral Radiance T : Absolute temperature kB: the Boltzmann constant h: the planck constant c: the speed of light 2. In a conductor, steel, the electromagnetic energy is dissipated near the surface skin. It can be described by the following

ρ: the resistivity of the conductor ω: the angular frequency of the electromagnetic wave μ: the absolute magnetic permeability of the conductor