Abstract: A device for forming a hollow profile formed as a multi-chamber profile (21) by means of an internal high pressure generated by a fluid pressure medium within the hollow profile containing a sealing die (1a) engaging in the profile chamber (20) with a carrier part (3a) and a collar (2a) arranged on the carrier part (3a) sealing the face side of the profile chamber (20). The die (1a) also has a passage opening (7a) as a pressure medium supply and/or extraction channel (9a). The collar (2a) is a resilient rubber die part arranged on the face of the carrier part (3a) facing the profile chamber (20) and on the free face (16a) of the collar (2a) is provided a preload plate (6a) which is anchored by way of the fixing means (8a) penetrating the collar (2a) in the carrier part (3a) lying on the collar (2a) so that the collar (2a) is squeeze-clamped between the preload plate (6a) and the carrier part (3a).