Method of manufacturing hollow elements from thermoplastic materials

Claim 1

A method for the manufacture by moulding with thermoplastic resins in powdered form of hollow elements of substantially cylindrical shape comprising a cylindrical body and two ends and having an internal volume of at least one thousand liters, and the method comprising the steps of introducing a particulate thermoplastic resin material into a cylindrical mould, said thermoplastic resin being selected from the group consisting of polyethylene, polypropylene, thermoplastic polyolefin copolymers and a self-condensate of 11-aminoundecanoic acid, a first stage of heating during which a rotational movement about a central longitudinal axis of said cylindrical mould is imparted to the mould while at the same time heating both the body and the ends of the mould, said longitudinal axis being maintained horizontal; a second stage of moulding the ends during which rotational movement in alternate directions in sequence is imparted to the mould about said axis of rotation while simultaneously heating said body and ends of said mould, said second stage further including the step of alternately tilting said mould in opposite directions about a central axis transverse to said axis of rotation at predetermined intervals, said longitudinal and transverse axis both passing through the gravity center point of the mold; which intervals at least exceed the time required for several revolutions of said mould, with said mould being retained in tilted position during each entire interval, while said alternate rotation about said axis of rotation continues, the mould thereby being tilted and thus inclined first to one side, then to the other, while heating both said body and ends of the mould; a third stage of progressively cooling the ends of the mould by first reducing and then completely terminating the heating of the ends of the mould, with the alternating tilting and rotation of the mould continuing as in said second stage; a fourth stage of moulding the body in which the same alternating rotational movement as in the second stage is imparted to the mould, but where the axis of rotation remains substantially horizontal and heat is applied only to said body of the mould; a fifth stage of cooling the body in which the method of the fourth stage is continued and the heating of the body is terminated, and a sixth stage of removing the mould, said heating being sufficient to melt said powder at least partially and to cause said powder to agglomerate.

Claim 2

A method for the manufacture by moulding with thermoplastic resins in powdered form of hollow elements of substantially cylindrical shape comprising a cylindrical body and two ends and having an internal volume of at least one thousand liters, and the method comprising the steps of introducing a particulate thermoplastic resin material into a cylindrical mould, said thermoplastic resin being selected from the group consisting of polyethylene, polypropylene, thermoplastic polyolefin copolymers and a self-condensate of 11-aminoundecanoic acid, a first stage of heating during which a rotational movement about a central longitudinal axis of said cylindrical mould is imparted to the mould while at the same time heating both the body and the ends of the mould, said longitudinal axis being maintained horizontal; a second stage of moulding the ends during which rotational movement in alternate directions in sequence is imparted to the mould about said axis of rotation while simultaneously heating said body and ends of said mould, said second stage further including the step of alternately tilting said mould in opposite directions about a central axis transverse to said axis of rotation at predetermined intervals, said longitudinal and transverse axis both passing through the gravity center point of the mold; which intervals at least exceed the time required for several revolutions of said mould, with said mould being retained in tilted position during each entire interval, while said alternate rotation about said axis of rotation continues, the mould thereby being tilted and thus inclined first to one side, then to the other, while heating both said body and ends of the mould; a third stage of progressively cooling the ends of the mould by first reducing and then completely terminating the heating of the ends of the mould, with the alternating tilting and rotation of the mould continuing as in said second stage; a fourth stage of moulding the body in which the same alternating rotational movement as in the second stage is imparted to the mould, but where the axis of rotation remains substantially horizontal and heat is applied only to said body of the mould; a fifth stage of cooling the body in which the method of the fourth stage is continued and the heating of the body is terminated, and a sixth stage of removing the mould, said heating being sufficient to melt said powder at least partially and to cause said powder to agglomerate.