Disclosed is an extrusion mandrel fastener (2) that is embodied as a two-piece unit comprising a supporting beam (4) and a holding beam (5) which can be joined together and are provided with semi-cylindrical recesses (7, 8) complementing each other so as to form cylindrical recesses (9). An extrusion mandrel (3) is inserted into each cylindrical recess (9) and is retained therein. The extrusion mandrel fastener (2) and the extrusion mandrels (3) are provided with ducts (13, 14, 15, 16, 24, 25) which allow fluid circulation so as to temper the extrusion mandrels (3) and communicate with each other as well as with openings (11, 26) that are disposed on the extrusion mandrel fastener (2) via cylindrical connecting sleeves (30). Said connecting sleeves (30) are preferably sealed by means of sealing rings and fix the extrusion mandrel (3) to the extrusion mandrel fastener (2). Preferably, the holding beam (4) is configured as a two-piece element comprising an extrusion mandrel connection beam (17) and a fluid connection beam (18) which can be joined together and are provided with ducts (13, 14, 15, 24, 25). Said ducts (13, 14, 15, 24, 25) communicate with each other so as to allow fluid circulation between the openings (11, 26) and through the extrusion mandrels (3) or the ducts (16) that are disposed therein. Preferably, the fluid connection beam (18) is embodied as a two-piece member comprising a body (19) and a lid (20) which can be joined together, the ducts (24, 25) being configured as grooves that are mounted on the body (19) of the fluid connection beam (18) and are covered by the lid (20).