Process for skin foam

Claim 1

In a method of making a foamed plastic product having a relatively low density cellular core structure and a substantially non-cellular outer layer or skin wherein a first or foam stream of foamable normally solid molten polymer containing blowing agent is fed to a co-extrusion die under pressure and expanded on exiting from said die to form said cellular core and a second or skin stream of said normally solid molten polymer is supplied to said co-extrusion die to form said skin, the improvement which comprises: incorporating a volatile organic blowing agent and a nucleating agent in said first or foam stream of polymer, incorporating a volatile organic blowing agent in said second or skin stream of polymer in an amount effective to match the viscosity of said second stream to that of said first stream and maintain stable flow of said second stream relative to said first stream, supplying said second stream to said co-extrusion die at a temperature sufficient to produce foam formation on expansion of the polymer leaving the die, and supplying said second stream to said co-extrusion die at a temperature lower than that required to producing foaming in said second stream of polymer on release of pressure in said co-extrusion die.

Claim 4

A method of making a foamed plastic product having a non-cellular outer surface or skin of relatively high density and a foamed core having a relatively low density cellular structure comprising: feeding a first or foam stream of a foamable normally solid molten polymer containing a blowing agent and a nucleating agent under pressure to one outlet of a co-extrusion die where it is expanded to substantially atmospheric pressure with foaming a formation of said inner core structure; feeding a second or skin stream of a foamable normally solid molten polymer containing a volatile organic liquid blowing agent in an amount effective to match the viscosity of said second stream to that of said first stream and maintain stable flow of said second stream relative to said first stream and free from any added nucleating agent to said co-extrusion die under pressure wherein it is extruded with said first stream of polymer; supplying said first or foam stream to said co-extrusion die at a temperature sufficient to cause foaming of said polymer on release of pressure; and supplying said second or skin stream to said co-extrusion die at a temperature below that at which foaming of said polymer occurs spontaneously on release of pressure thereby forming a skin on said foamed core.

Claim 5

A method of making a foamed plastic product having a non-cellular outer surface or skin of relatively high density and a foamed core having a relatively low density cellular structure comprising: feeding a first or foam stream of a foamable normally solid molten polymer containing a blowing agent and a nucleating agent under pressure to one outlet of a co-extrusion die where it is expanded to substantially atmospheric pressure with foaming a formation of said inner core structure; feeding a second or skin stream of a foamable normally solid molten polymer containing a volatile organic liquid blowing agent in an amount effective to match the viscosity of said second stream to that of said first stream and maintain stable flow of said second stream relative to said first stream and free from any added nucleating agent to said co-extrusion die under pressure wherein it is extruded with said first stream of polymer; supplying said first or foam stream to said co-extrusion die at a temperature sufficient to cause foaming of said polymer on release of pressure; and supplying said second or skin stream to said co-extrusion die at a temperature below that at which foaming of said polymer occurs spontaneously on release of pressure thereby forming a skin on said foamed core.