The device includes a supporting surface, with an initial curved surface portion, and a substantially flat surface portion. Above the supporting surface, extends at least part of a first path and of a second path of a first ply and of a second ply. The two paths are disposed adjacent to the supporting surface and the initial curved surface portion has a convexity facing the first path. Also provided is a system to supply air between the first path of the first ply and the supporting surface, to help support the web material with respect to the supporting surface.