Abstract: A process and apparatus for fluxing a mixable thermoplastic material is disclosed. In the process a blend of thermoplastic material is agitated in a mixer; at the same time vibrations of the mixer is monitored during this agitation step and when it reaches a predetermined level, a signal is generated so the mixer may be stopped or the material discharged. The apparatus includes a high intensity mixer, a means for monitoring vibrations of the mixer, together with a means for comparing these vibrations with a preset level to produce a signal to indicate the material in the mixer has reached the required degree of fluxing.