Sealing of thermoplastic by the application of radio frequency

Abstract

Radio and electrical engineers have long understood the physical phenomenon by which metallic conductors as well as semi-conductors become heated when electric currents are forced through the materials in question. When an alternating electric current is passed through a mass of material, the manner in which the current flows and consequently the manner in which heat is generated, is greatly dependent on the conductivity of the material and on the frequency of the alternations. It both the conductivity and the frequency are high, the current tends to crowd to the outside of the mass and surface heating results. On the other hand, if the conductivity is low, then the current distribution tends to be uniform and heat is generated throughout the mass. It is thus possible to heat the material internally without the difficulty of conducting the heat through the outer layers. Once the heat is generated, however, thermal conduction takes place, so that the temperature distribution through the mass differs greatly from the law which describes the distribution of power generation. The object of this project is two fold. Firstly, to establish why and how radio-frequency is applied for sealing thermoplastic materials and secondly, to establish the theory involved due to the heat transfer which takes place. This project is also backed by anaylitic and graphical results brought about by the mathematical· anaylisis used. Finally, the theory established gives rise to a practical experimental model which the author designed, but due to reasons beyond the author's capability, this experimental model could not be tested.