A mold for uniform heating and molding of work material to form a workpiece of variable thickness using a microwave oven includes upper and lower mold members and a sidewall mold member. Metal inserts, with irregular inner surfaces defining a mold cavity, are positioned adjacent opposed planar surfaces of the upper and lower mold members. The mold members are formed from material selected to have an effective thermosensitivity equal to that of the work material. The effective thermosensitivity of work material is its dissipation factor divided by the product of its dielectric constant, density and specific heat multiplied by the fraction of the mass of work material divided by the total mass of work material and metal inserts. The effective thermosensitivity of the material of inner layer of mold member is its dissipation factor divided by the product of its dielectric constant, density, and specific heat and multiplied by the fraction of the mass of its inner mold layer divided by the total mass of this mold member.