Abstract: The invention provides a method and apparatus for the high pressure injection of polyurethane foam into a hollow envelope using means of the type including a chamber for mixing the constituents of the polyurethane foam, a cylinder communicating with said chamber and open at both its ends, one of its ends opening into the orifice of the hollow envelope and the other end serving as inlet for a piston adapted for sliding in said cylinder and driving the polyurethane foam therefrom into said envelope, wherein said envelope is substantially vertical, its upper end opens into the orifice of the hollow envelope and is of a volume close to half that defined by the envelope and the movement of the piston in said cylinder is adjustable in synchronism with the cycle of the mixing head.