Advantageously the polyester of the polyester mixture of the sheath has a melt viscosity of 445 to 482 Pa???s, preferably of 455 to 475 Pa???s, in particular of 460 to 468 Pa???s, when measured at 245??????2??? C. and a shear gradient of 200???5 s-1 and of 245 to 282 Pa???s, preferably of 250 to 272, in particular of 255 to 270 Pa???s when measured at the same temperature with a shear gradient of