The invention describes a method for generating a shell mold for castings, in which the shell mold is described from a multiplicity of finite volume elements andor surface elements which are connected to one another via node points and which together form a mesh points for describing the surface of the shell mold. The reinforcement material for rubber has a flat coil shape where, when the material is in a single free state, loop portions are partly superposed on each other in sequence, and is embedded in a rubber product such as a pneumatic tire. The oscillation frequency of the magnetron is locked to the frequency of the reference signal by injection of the reference signal.