Acoustic Quality Control in Rubber Compounding

Abstract

When rubber is compounded in an internal mixer, distinct noises and vibrations occur over the mixing time. Operators of internal mixers often report these characteristic noises, which vary during the mixing process and are directly related to the properties of the compound. Therefore a process monitoring system using vibration measurement has been developed by the Institute of Plastics Processing (IKV) and the company CT Datentechnik GmbH in Nienburg to determine whether the rubber compounding process in an internal mixer can be characterised and controlled with an acoustic fingerprint. An acceleration sensor is mounted to the outside of the mixing chamber to measure and record the vibration. Then the vibration signals are converted from time domain to frequency domain with a Fast Fourier Transform to establish frequency bands that are relevant to the evaluation of the acceleration level. The compounds are tested for their mechanical and rheological properties and the results of the tests are correlated with the acoustic signals. The correlation of acoustic signals and material properties of rubber compounds allows the mixing state during the compounding process to be described. With this knowledge the mixing process can be optimised in order to reduce waste and to shorten the mixing time.