A process for molding a beading in particular on the periphery of a window or for molding a plastic window, in which a constituent plastic of said beading or of said plastic window respectively is introduced into a molding cavity in which at least one ferromagnetic insert, such as a trim, has been placed beforehand, wherein said insert is held in position in said molding cavity during the introduction of said plastic by at least one magnet which can move between two positions, an active position in which it exerts an attractive force on said insert across the internal surface of the molding cavity and an inactive position I in which it exerts no attractive force on said insert.