the present invention concerns an apparatus and method for thermoforming a plastic article from a sheet of plastic material heated to a predetermined forming temperature . the apparatus includes a mold having forming surfaces defining a forming cavity generally conforming to the shape of the plastic article to be formed . the heated plastic sheet is urged into contact with the forming surfaces of the mold to form the plastic article . the mold includes at least one undercut forming element moveable between an advanced position wherein the element projects into the forming cavity to form an undercut portion in the plastic article and a retracted position wherein the element is withdrawn from the forming cavity to enable the formed plastic article to be removed from the forming cavity . after the plastic article has been formed , the undercut forming element is moved to the retracted position and a stripper plate is utilized to remove the formed plastic article from the forming cavity . in the preferred embodiment of the invention , the undercut forming element is retracted simultaneously with the stripping movement of the stripper plate to minimize the cycle time associated with producing the plastic article .