Food-packaging tray and method of making same

Abstract

A tray package for food has a stiff tray having a rim and formed inward of the rim with at least one throughgoing hole and a cover film fixed to the rim. A lining made of a flexible film is fixed to the rim between the tray and the cover film and forms with the cover film a closed compartment adapted to hold the food and with the tray an outer compartment that is open to the exterior through the hole.

We claim: 
     
         1 . A tray package for food, the package comprising:
 a stiff tray having a rim and formed inward of the rim with at least one throughgoing hole;   a cover film fixed to the rim;   a lining made of a flexible film, fixed to the rim between the tray and the cover film, and forming with the cover film a closed compartment adapted to hold the food and with the tray an outer compartment that is open to the exterior through the hole.   
     
     
         2 . The tray package defined in  claim 1 , wherein the tray includes a substrate composed of polyester, polypropylene, polystyrene, or polycarbonate, and a cover layer of weldable polymer on an inside face of the tray. 
     
     
         3 . The tray package defined in  claim 2 , wherein the tray is between 150 μm and 2000 μm thick. 
     
     
         4 . The tray package defined in  claim 1 , wherein the film of the lining is composed of a multilayer barrier film that includes at least one barrier layer for oxygen. 
     
     
         5 . The tray package defined in  claim 4 , wherein the barrier film includes polyolefin outer layers and at least one core layer composed of EVOH as an oxygen barrier layer. 
     
     
         6 . The tray package defined in  claim 5 , wherein the EVOH layer is embedded between two polyamide layers. 
     
     
         7 . The tray package defined in  claim 6 , wherein the barrier film is between 50 μm and 350 μm thick. 
     
     
         8 . The tray package defined in  claim 1 , wherein the cover is a multilayer film that includes at least one oxygen barrier layer and a sealing layer composed of a weldable polymer on an inner surface of the package. 
     
     
         9 . A method of making a package comprising the steps of sequentially:
 juxtaposing a thermoshapable flexible film with a perforated tray film having a hole;   bonding the film to the tray around a forming region;   thermoshaping the bonded together film and tray with subatmospheric pressure to form a two-layer tray with an upwardly open cavity;   charging food to be heat treated into the cavity; and   welding to the rim over the flexible film and above the food a cover film so as to enclose the food and a gas in the cavity in a hermetically sealed inner compartment between the forming region of the flexible film and the cover film, the flexible film and the tray forming an outer compartment vented to the exterior through the hole.   
     
     
         10 . The package-making method defined in  claim 9 , wherein the flexible film is bonded to the tray film in an annular region that subsequently forms a rim when the tray film is thermoshaped into the stiff tray. 
     
     
         11 . The package-making method defined in  claim 9 , wherein the flexible film is bonded to the rim by welding. 
     
     
         12 . The package-making method defined in  claim 9  wherein the tray film and flexible film form a two-layer laminate from which the tray is thermoshaped and the tray film and flexible film are bonded together in an annular region within which they are not bonded together such that they can separate and form the outer compartment after thermoshaping of the tray. 
     
     
         13 . The package-making method defined in  claim 12  wherein the tray film and flexible film are bonded together only at the annular region by an adhesive, or one of the tray and flexible films is coated over its entire surface with the adhesive and the other of the films is coated within the annular region with a release agent. 
     
     
         14 . The package-making method defined in  claim 13 , wherein the adhesive is applied to the flexible film or to the tray film in a cellular pattern comprising an adhesive area and a plurality of adhesive-free areas each surround by adhesive forming an annular region. 
     
     
         15 . The package-making method defined in  claim 9 , further comprising the steps of:
 subjecting the entire package to a heat treatment with a high-pressure fluid such that the fluid flows into the outer compartment through the vent hole and the gas in the inner is compressed to reduce the volume of the inner compartment with deformation of the flexible film and both inner and outer faces of the tray are exposed to the high pressure such that the tray does not deform; and thereafter   reducing the pressure to ambient such that the fluid is forced back out of the outer compartment by expansion of the gas in the inner compartment without substantial deformation of the tray.