with the upper end of the stirrup 27 before the end of the descending stroke of the jack 29 the radial fingers 28 of the stirrup 27 are separated from the disc 23 of the feeler 21 since the lower end of the rod 22 thereof comes into contact with the thermoplastic band 1 in the zone 26 thereof and the feeler 21 then deforms this zone 26 under the effect of its own weight during a determined time less than the time gap separating two successive advance steps of the band 1 the deformation of the zone 26 of the band 1 brings about a vertical displacement of the feeler 21 this displacement being measured by sensor 25 provided in the guiding casing 24 it should be noted that at the end of descending stroke the connecting stirrup 27 rests on the guiding casing 24 and the length of the feeler 21 is sufficient in order that for this lower position of the stirrup 27 the disc 23 is still sufficiently apart from the fingers 28 of the stirrup 27 and still allows the feeler 21 to descend cf fig1 of course the feeler 21 is raised before each advance of the thermoplastic band 1 and the cooling plate 10 is lowered sufficiently to allow the deformed zone 26 of the said band 1 to pass whilst at the end of advance of the band 1 the feeler 21 is again lowered so as to rest freely on a zone of band surrounded by a cell 11 it will be readily understood that to raise the feeler 21.