apparatus including an extruder , a die member and a vacuum device connected in series . the vacuum device includes a chamber and a device at at least the outlet thereof to air - tightly seal the chamber around the extruded mass of foamed thermoplastic resin . the air - shielding or sealing device is an annular tube which is inflatable radially inwardly having a flexible inner face which seals around the extruded mass . a durable , abrasion - resistant , flexible protective sheet can be interposed between the mass and the tube .