Influence of dielectric properties of polybutylene terephthalate and respective foam beads on process behavior in radio‐frequency welding

Abstract

Radio‐frequency welding shows great potential for a more sustainable manufacturing of lightweight bead foam products based on local dielectric heating of the part. This study provides the first insight into the material‐process interactions required to enable interdiffusion between foam particles and thus adhesion. The complex relative permittivity is analyzed by impedance spectroscopy of the bulk polymer and foam beads using specially designed measurement cells. Due to the increased flexibility of the polymer chains at elevated temperatures, the relaxation mechanisms are promoted, resulting in a shift of the curves to higher frequencies. At a fixed frequency, a peak in the imaginary part is observed above temperature, revealing the superposition of these two parameters. The foamed beads show similar trends with an attenuated signal due to the high volumetric fraction of air within the multiphase system. The linear dependence between and volumetric power as direct process feedback due to dielectric heating was observed during welding. Compared to the delayed signals from the shielded temperature sensors, the analysis of power consumption allows for higher quality real‐time monitoring throughout the process cycle. The correlation also allows prediction of power levels at defined temperatures.