a clamping frame for use in plastic forming apparatus which includes a plurality of tubular elements joined together to define an opening over which a sheet of plastic material is mounted with the edges of the plastic sheet being held against supporting surfaces of the tubular elements by clamps . each tubular element is formed of a thermally conductive material and includes an interiorly disposed heating element located adjacent the supporting surfaces of the tubular element . the tubular elements include a plurality of interconnections to permit selective positioning of the tubular elements with respect to one another for varying the size of the opening defined thereby , and these interconnections also electrically connect the heating elements within the tubular elements at all adjusted positions thereof to form a closed electrical circuit which may be connected to an electrical source whereby the edges of the plastic sheet between the clamps and the tubular element supporting surfaces may be heated .