where DP is penetration depth εr is relative dielectric loss factor and is microwave speed. The penetration depth of the microwave
power is calculated according to Eq. (2), which shows how it
depends on the dielectric properties of the material. It is noted
that products with huge dimensions and high loss factors, may
occasionally overheat a considerably thick layer on the outer layer.
To prevent such phenomenon, the power density must be chosen
so that enough time is provided for the essential heat exchange
between boundary and core. If the thickness of the material is less
than the penetration depth, only a fraction of the supplied energy
will become absorbed. Furthermore, the dielectric properties of