Polystyrene foam sheet manufacture

A foam sheet material made in accordance with the method which comprises

• introducing atmospheric gas as the blowing agent into a molten thermoplastic resin in an amount of less than 2% by weight,
• coextrudng a plastic film layer and a foam layer such that the foam layer and plastic film layer form a frustoconical tubular web comprising a foam layer and a plastic non-foam film at an angle to the axis of the extruder,
• passing said frustoconical web over an internal cooling mandrel to cool the interior of the web,
• simultaneously applying cooling air to the outside of the frustoconical web,
• said step of applying cooling air to the outside of the frustoconical web comprising applying cooling air axially to the outside of said frustoconical web at an acute angle tangentially to the web whereby the cooling air moves in a direction parallel to the frustoconical web from adjacent the extruder to adjacent the internal cooling mandrel such that the external surface of the frustoconical web is cooled by cooling air and by the ambient air drawn by the cooling air against the external surface of the frustoconical web,
• such that the resultant coextruded sheet material comprises a sheet material which
• (a) has substantially no residual blowing agent,
• (b) is dimensionally stable at ambient temperature and having substantially no corrugations,
• (c) is sufficiently resilient that it can be compressed to remove gauge bands if they had occurred,
• (d) has shrinkage properties at elevated temperatures which do not change substantially when the foam material is stored over extended periods of time,
• (e) has cell size, opacity, stiffness, smoothness, caliper, density that do not change over extended time of storage as contrasted to foam thermoplastic material formed using fluorocarbons or hydrocarbon blowing agents.

A coextruded thermoplastic heat shrinkable foam product comprising

• a foam thermoplastic material which has been formed by introducing an atmospheric gas into a molten thermoplastic resin, continuously extruding said thermoplastic resin through an extruder to form a foam layer and coextruding a non-foam layer of thermoplastic resin on said foam layer to form a frustoconical coextruded tubular web at an angle to the axis of the extruder, continuously pulling and passing said frustoconical coextruded web over an internal cooling mandrel to cool the interior of the web, simultaneously applying external cooling air to the outside of the frustoconical tubular web which when extruded from said extruder, subjected to pulling and cooling internally and externally comprises a tubular foam product which is characterized by
• (a) said product having substantially no residual blowing agent,
• (b) said product being dimensionally stable at ambient temperature and having substantially no corrugations,
• (c) said product being sufficiently resilient that it can be compressed to remove gauge bands if they had occurred,
• (d) said product having shrinkage properties at elevated temperatures which do not change substantially when the foam material is stored over extended periods of time,
• (e) said product having cell size, opacity, stiffness, smoothness, caliper, density that do not change over extended time of storage as contrasted to foam thermoplastic material formed using fluorocarbons or hydrocarbon blowing agents.

The method of making foam sheet material which comprises

• introducing atmospheric gas as the blowing agent into a molten thermoplastic resin in an amount less than 2% by weight,
• coextruding a plastic film layer and a foam layer such that the foam layer and plastic film layer form a frustoconical tubular web comprising a foam layer and a plastic film at an angle to the axis of the extruder,
• passing said frustoconical web over an internal cooling mandrel to cool the interior of the web,
• simultaneously applying cooling air to the outside of the frustoconical web,
• said step of applying cooling air to the outside of the frustoconical web comprising applying cooling air axially to the outside of said frustoconical web at an acute angle tangentially to the web whereby the cooling air moves in a direction parallel to the frustoconical web from adjacent the extruder to adjacent the internal cooling mandrel such that the external surface of the frustoconical web is cooled by cooling air and by the ambient air drawn by the cooling air against the external surface of the frustoconical web,
• such that the resultant coextruded sheet material comprises a foam layer and a plastic film has substantially no residual blowing agent, and is substantially dimensionally stable at ambient temperatures, and shrinkage properties which do not change substantially when the foam material is stored.