Abstract: The present invention provides a radio frequency welding apparatus for joining multiple layers of thermoplastic materials. The apparatus includes an electrical voltage source that is electrically connected to a radio frequency energy generator. The apparatus also includes a first die and a second die that are configured to form a tooling capacitor when thermoplastic sheets to be welded are positioned between them. The tooling capacitor is electrically connected to the radio frequency energy generator and the dielectric includes a thermoplastic sheet. A processor is electrically connected to the voltage source and is configured to monitor an electrical property applied to the electric circuit and vary voltage supplied to the radio frequency generator such that the electrical property approaches a predetermined value. The radio frequency energy generator is configured to provide energy to the tooling capacitor such the energy can be controlled as the voltage is controlled.