Method of manufacturing a tough board of thermoplastic resin foam having integral skins and a dense intermediate layer

Claim 1

A method of manufacturing a tough board of thermoplastic resin foam of low density having solid skin layers formed on the upper and lower sides thereof, a solid intermediate high density layer lying substantially halfway between the upper and lower surfaces of the board and generally coextensive with said skin layers and large cells elongated in the direction of the thickness of the board arranged in two rows separated by said intermediate layer which comprises: (a) introducing into an extruder thermoplastic resin, 2 to 20 parts by weight of a volatile foaming agent based on 100 parts by weight of said resin and soluble in said resin at extrusion temperature and less than 3 parts by weight of a nucleating agent based on 100 parts by weight of said resin; (b) causing said foaming agent to become dissolved in said resin; (c) extruding a sheet of the resulting mixture from said extruder through a die orifice at a temperature ranging between the softening or melting point of said mixture and a temperature 30° C. higher than the softening or melting point of the thermoplastic resin itself; (d) cooling the upper and lower surfaces of said sheet rapidly to form solid skin layers thereon; (e) thereafter cooling the sheet slowly over a period of between about 2 to 25 minutes so that foam nuclei formed therein grow slowly into cells during said cooling period causing said sheet to expand transversely, and (f) pressing the expanding sheet throughout said cooling operation with expansion restricting means on the upper and lower surfaces thereof to restrict the thickness thereof, said pressing being conducted in first and second phases, (g) the pressing in said first phase causing the skin layers of the expanding sheet to diverge from one another as the sheet travels downstream from said extruder, (h) the pressing in said second phase causing the skin layers of the board issuing from said first phase to remain parallel while the cooling thereof for said period is substantially completed providing the tough board aforesaid having a thickness between about 50 to 95% of that thickness which would be realized by the extruded board if foamed under atmospheric pressure in the absence of said restrictive pressing.

Claim 2

A method of manufacturing a tough board of thermoplastic resin foam of low density having solid skin layers formed on the upper and lower sides thereof, a solid intermediate high density layer lying substantially halfway between the upper and lower surfaces of the board and generally coextensive with said skin layers and large cells elongated in the direction of the thickness of the board arranged in two rows separated by said intermediate layer which comprises: (a) introducing into an extruder thermoplastic resin, 2 to 20 parts by weight of a volatile foaming agent based on 100 parts by weight of said resin and soluble in said resin at extrusion temperature and less than 3 parts by weight of a nucleating agent based on 100 parts by weight of said resin; (b) causing said foaming agent to become dissolved in said resin; (c) extruding a sheet of the resulting mixture from said extruder through a die orifice at a temperature ranging between the softening or melting point of said mixture and a temperature 30° C. higher than the softening or melting point of the thermoplastic resin itself; (d) cooling the upper and lower surfaces of said sheet rapidly to form solid skin layers thereon; (e) thereafter cooling the sheet slowly over a period of between about 2 to 25 minutes so that foam nuclei formed therein grow slowly into cells during said cooling period causing said sheet to expand transversely, and (f) pressing the expanding sheet throughout said cooling operation with expansion restricting means on the upper and lower surfaces thereof to restrict the thickness thereof, said pressing being conducted in first and second phases, (g) the pressing in said first phase causing the skin layers of the expanding sheet to diverge from one another as the sheet travels downstream from said extruder, (h) the pressing in said second phase causing the skin layers of the board issuing from said first phase to remain parallel while the cooling thereof for said period is substantially completed providing the tough board aforesaid having a thickness between about 50 to 95% of that thickness which would be realized by the extruded board if foamed under atmospheric pressure in the absence of said restrictive pressing.