RF Theory
The terms “RF welding” or “RF heat sealing” are often used interchangeably with High Frequency (“HF”) or dielectric heat sealing or welding. When a dielectric material comes into contact with an electromagnetic field, some portion of the electromagnetic energy will go through a change of state and be dissipated as heat with the dielectric. The degree to which this conversion of energy will occur is dependent on the atomic and molecular structure of the material, the frequency of the electromagnetic field and the field strength.
The term dielectric heating correctly describes this phenomenon at any frequency while RF or HF heating describes this process over the limited frequency range (1 to 200 megahertz). In the case of RF or HF welding of thermoplastics, the effective mechanisms producing heat in the dielectric are Dipolar and Interfacial polarization.
- Dipolar Polarization: Molecules that are permanently polarized by chemical forces that are realigned when in contact with an electromagnetic field.
- Interfacial Polarization: The accumulation of a charge at a discontinuity within the material due to the migration of electrons under the influence of an electromagnetic field (in effect, current flow in a capacitor).
An alternating electromagnetic field causes oscillatory displacement of the polarized components as re-alignment with the positive and negative oscillations occurs. This phenomenon, often analogized as friction on an atomic or molecular level, is the source of the heat generated in the dielectric. The amount of energy dissipated or heat generated in a dielectric can be described by the following formula.
P = .555 f E2 e' (tan δ x 10-6)
Where:
P = Heat generated in watts/cm3
f = Frequency of the electromagnetic field MHz/sec
E = Field strength in Volts/cm
e' = Dielectric constant of the material
tan δ= Loss tangent, Ie/Ic
From the above formula, it can be readily seen that materials with higher "loss tangents" and higher "dielectric constants" will heat more readily than those with lower valued loss tangents and dielectric constants. Hence, if we multiply the (loss tangent) by the (dielectric constant) we obtain by definition the "loss factor", a very useful number and accurate predictor of how readily any given material will heat in an alternating RF field. The higher the loss factor of any specific material, the more efficiently it will heat in an alternating RF field. Materials with loss factors of .3 or greater are considered excellent candidates for RF welding, between .2 and .3 good candidates, below .2 but greater than .1 fair to poor.
