method and apparatus for joining together edge surfaces of polytetrafluoroethylene material by a butt joint in which abutting surfaces of the material are placed in a chamber bounded by clamps along the abutting surfaces which press together the abutting surfaces , and heating elements in the space between the clamps to heat both sides of the material between the clamps , the boundaries of the chamber inhibiting the volume expansion of the material resulting from heating . pressure generated in chamber may be controlled by moving a heating element . additional pressure during cooling - down is exerted on the butt joint by moving one clamping in the direction of the butt joint . a thin foil of a fluorine - containing thermoplastic inserted between the abutting surfaces facilitates joining of the surfaces .