It is known that it is highly favorable to use a molding method applying heating by electromagnetic induction, notably for rapidly and efficiently heating plastic or composite materials which have to be shaped, but also for heating parts in metals or metal alloys before stamping them and quenching them in the tool.
According to such an induction-heating method, the molding device comprises inductors through which flows a medium frequency current igen delivered by a power generator and generating a variable electromagnetic field over time. This variable field is at the basis of the well known phenomenon of electromagnetic induction: when applied to a current conducting material, it creates a variable magnetic flux over time and an induced voltage in the conducting material, which in turn generates induced currents at the surface of the conducting metal, over a depth called the skin thickness δ and given by the relationship:
  δ  =            2              μ        ·        σ        ·        ω            
wherein μ is the magnetic permeability of the material with μ=μ0.μr, σ is the electric conductivity of the material (reciprocal of the electric resistivity Rel), ω is the angular frequency and is equal to 2.π.f wherein f is the frequency of the excitation current and of the generated magnetic field.
When the conducting material is not ferromagnetic, the value of μr is close to 1 and the skin thickness is given by the relationship:
      δ    amagn    =      500    ⁢                  Rel        f            
For this purpose, the material induction-heating method is known from FR 2 867 939 which describes a mold with which the precursor material which will assume its definitive properties after heat treatment, may be received in a cavity. Supplying the inductor with a medium frequency electrical current generates induced currents in the skin thickness of an intermediate part in contact with the material to be heated, which limits the volume of the part of the mold, which has to be heated.
Moreover, inside this intermediate element, blocks of inserts made from materials having distinct electric resistivities or magnetic permeabilities may be positioned in order to obtain different surface temperatures.