the invention relates to a device which is used for the heat - sealing of a thermoplastic synthetic film to a thermoplastic synthetic container . the inventive device has at least one thermal electrode which is made from a material with high thermal conductivity . the electrode is equipped with a metal section having electrical connection terminals at its ends . a heat flux sensor comprising two electrical connections is also provided , and the lower face is fixed mechanically to the upper part of the above - mentioned section . in addition , the upper face of the heat flux sensor is fixed to the lower face of a thermal capacitor which is made from a material with high thermal diffusivity and conductivity . furthermore , a thermocouple is mounted in a cavity in the metal section .