a guide contour 134 that merges smoothly with the expanded lower end of outlet opening 126 shaft 130 of the guide blades is pretensioned upward by a spring 136 and acted upon by a lever 138 of an adjusting mechanism said lever being located on the cooling ring in a housing 140 radially outside outward opening 126 guide blade 128 is shaped so that it closes outlet opening 126 in its upper end position so that the cooling air can flow unimpeded to die gap 118 on the other hand if guide blade 128 is pressed downward by lever 138 guide contour 134 of the guide blade causes a portion of the cooling air to be deflected into outlet opening 126 so that a partial stream b is branched off main cooling air stream a hence only the remaining partial stream c of cooling air reaches die gap 118 so that a reduced cooling air throughput is achieved at the die gap outlet openings 126 and guide blades 128 located circumferentially are separated from one another by partitions 142 distributed radially in annular chamber 120 when two adjacent guide blades are set to different positions so that the cooling air throughputs downstream of the guide blades differ from one another partition 142 prevents premature combination of partial streams c and equalization of the throughputs in.