device for extruding hollow strands from thermoplastic material , with an extruder head , having a mandrel , and a calibrating device , for making a dimensional change while production is in progress , and with a radially adjustable inlet , at least one ranque vortex chamber being formed in the mandrel , the cooling air outlet of which chamber leads into a cooling tube , which extends as an axial extension of the mandrel through the inlet of the calibrating device and has a cooling air outlet opening out into the calibrating device . this device achieves the object of providing a device with which effective interior cooling is achieved in calibrating devices designed for making a dimensional change while operation is in progress .