A differential pressure machine for differentially pressure forming an article in a sheet of thermoplastic material including a frame, upper and lower platens movable on the frame between spread and less spread positions. Mechanism is provided for moving the platens between the spread and less spread positions for releasably locking the platens in the less spread positions. Upper and lower, opposed differential pressure molds mounted on the upper and lower platens, respectively, for movement therewith between open positions and closed positions for engaging opposite sides of a thermoplastic sheet of material and differentially pressure forming an article therein. Fluid pressure operated mechanism mounts one of the upper and lower molds on one of the upper and lower platens for movement therewith and for movement relative thereto when the platens are locked in the less spread positions to incrementally move one of the molds relative to one of the platens toward the other of the molds to tightly sandwich the sheet between the upper and lower molds including a variable volume, fluid pressure chamber between confronting portions of one of the platens and one of the molds. The invention is characterized in this that a separator plate is disposed in the chamber between the confronting portions of one platen and one mold and opposed annular seals are disposed between, and in sealing engagement with, the separator plate and the one platen and the one mold.