which extend outwardly from the seal bands and which are of reduced thickness to provide space for pressurized cooling air or a pressurized air water mixture which is supplied via a duct 17 and discharged through the bores 16 for intense cooling of the seal bands fig5 shows an embodiment for the side sea structure wherein the calibrating seal bands 12 12 39 moving along the sides of the compression bands 3 3 39 do not have guide beads 37 but are held in position by the rollers 23 of a roller chain 24 which moves at half the speed of the compression bands wherein the rollers firmly engage the sides of the seal bands for retaining and at the same time cooling them the chain rollers being supported and cooled by a guide track 27 provided with passages 25 through which a coolant 26 is circulated the guide track 27 is supported so as to be pressed onto the rollers 23 of the roller chain 24 in a resilient manner by support means which are not shown in the drawing fig6 shows the arrangement of fig5 in the cross sectional area b b 39 in which the material between the compression bands is fully densified and the side seals have moved fully together as described in connection with.