A flow molding process and associated molding apparatus for making plastic parts or pieces utilizing a radio frequency heating fluid. The molding apparatus includes a pair of spaced electrodes, at least one of which supports a molding material which may be in the form of an imprinted diaphragm (mold) for forming an imprint on at least one side of the plastic piece being fabricated. The apparatus is, in particular, employed for forming pieces having different thicknesses throughout, the purpose being to provide uniform heating throughout the piece being fabricated regardless of the varying thicknesses thereof. This is accomplished by maintaining the capacitance constant throughout all different thickness sections of the piece. This may be accomplished by equalizing the dielectric constants between the thermoplastic or the like material and the diaphragm (mold) material. In another version of the invention the capacitance is equalized by altering the spacing between the main electrodes in the different thickness sections.