Since the early days of radio communication, experts in the field have known that many kinds of materials in a high-frequency field exhibit a heating effect. There are two methods of heating, depending on the material to be heated – Dielectric heating and Induction heating.
Stanelco RF Technologies is proud to have been developing and supporting a full range of radio frequency applications since the 1940s. Dielectric heating is particularly relevant to welding plastic sheet materials. RF Induction heating is applicable to precise heating of electrically conductive materials.
Dielectric heating is a process for heating electrical insulators. Without forgetting the achievements of Nikola Tesla (1856-1943), Dielectric heating is historically independently attributed to Jacques Arsène d'Arsonval (1851-1940), together with the medical application of high frequency heating, Diathermy.
Most electrically insulating materials are made of molecules which have separate regions of positive and negative electric charge. If the material is placed in an electric field the molecules will rotate to align with the field, in much the same way that a compass needle aligns with the Earth's magnetic field.
Our Dielectric heating equipment uses an electric field which alternates at radio frequencies of typically 13.56, 27.12 or 40.68 million times per second, dependent on the application. The rapidly alternating electric field causes the molecules to vibrate, resulting in a heating effect, like food heating up in a microwave oven.
Dielectric heating is particularly appropriate for welding plastic sheet materials. Traditional heat sealing uses hot tooling to conduct heat into the plastic. To do this the tool must be hotter than the desired fusion temperature. When the tool comes into contact with the plastic, the surface is melted before the weld interface. Surface melting often produces a weak weld with a poor surface appearance.
Dielectric welding uses cold or warm tooling at temperatures significantly below the temperature at which plastic melts. Radio frequency power is applied, causing a uniform heating effect in the weld area. Because the tooling is significantly below the melting point of the plastic it cools the surface of the material, allowing the weld interface to melt before the surface. The end product is a stronger, more reliable weld.
Induction heating is a process for heating electrically conductive materials. In 1831 Michael Faraday invented the solenoid and independently discovered the principle of Induction, leading to the understanding and exploitation of the related heating effects. In contrast to Dielectric heating which generates heat uniformly throughout the receiving material, Induction heating generates heat near the surface of the material, penetrating to a depth that is proportional to the frequency of the alternating current in the solenoid.
It works like this.
A metal component placed within or adjacent to an induction coil is heated by passing an electrical current through that coil, which in turn induces another current within the component. Heat is produced by resistance to that induced current, according to the I2R law (I=Current and R=Resistance) and also by hysteresis loss in magnetic materials. Hysteresis can be simply explained as; the friction between molecules as they are magnetised first in one direction and then in the other. It is this friction that is turned into heat. In magnetic materials the hysteresis effect disappears at the Curie temperature (approximately 1400°F, 760°C).
At frequencies used for induction heating, the current tends to flow in the surface of the conductor, to a depth dependant on the resistivity of the conductor, the frequency of the alternating current and the effective permeability of the conductor. The effective depth of current penetration, in metric form, is given by the formulae:
p = depth of current penetration
r = resistivity in microhm centimetres
F = frequency in cycles per second
µ = effective permeability
(µ = 1 for non magnetic materials)
By choosing the correct frequency, we can control how much of the material is heated. If we use a high frequency, the effective penetration would be very small. As the frequency decreases, so the depth of penetration increases.