They are generally made of two spaced thin steel sheets, welded to each other.For the production of the bed the patent in suit proposes the pillow plate technology, in which two thin steel sheets are first laser welded together along their outline as well as on a series of spots distributed over the surface, then the space between the plates is pressurized to deform one or both plates in order to provide a space in which a heating (or a cooling) medium can circulate.