Multi-layered foamed polymeric objects and related methods

A method for making a multi-layer foamed thermoplastic material sheet from a solid monolithic thermoplastic material sheet, the solid thermoplastic material sheet having a first bulk crystallinity level, the method comprising:

• absorbing an effective amount of gas into the solid thermoplastic material sheet to yield a reversibly plasticized thermoplastic material sheet having:
• a second bulk crystallinity level that is greater than the first bulk crystallinity level;
• and
• a crystallinity gradient across a thickness of the sheet such that the crystallinity level is lower in the middle of the sheet's thickness than in the sheet's surface,
• wherein the absorbing occurs at a pressure in a range of 0.345 MPa to 9.65 MPa at a temperature in a range of −20° F. to 150° F.;
• desorbing at least some of the gas from the plasticized thermoplastic material sheet at a reduced pressure, as compared to the absorbing pressure, and a temperature in a range of −40° F. to 150° F., wherein the desorbing comprises a first desorbing stage in which the plasticized thermoplastic material sheet is placed in a freezer at a first temperature for a first predetermined time period to allow at least some of the gas to desorb, removing the plasticized thermoplastic material sheet from the freezer after the first predetermined time period, and, subsequent to removing the plasticized thermoplastic material sheet from the freezer, allowing at least some of the gas to desorb at a second temperature, greater than the first temperature, for a second predetermined time period in a second desorbing stage;
• and
• heating the plasticized thermoplastic material sheet to yield the multi-layer foamed thermoplastic material sheet having:
• a third bulk crystallinity level that is greater than or equal to the second bulk crystallinity level;
• and
• a plurality of discrete inner foamed microcellular layers and discrete unfoamed layers comprising:
• first and second discrete unfoamed skin outer layers;
• two substantially identical second inner foamed layers positioned immediately adjacent to the first and second discrete unfoamed skin outer layers;
• two substantially identical third inner foamed layers positioned immediately adjacent to the second inner foamed layers;
• and
• a fourth inner foamed layer positioned immediately adjacent to and interposed between the third inner foamed layers,
• wherein closed cells within the plurality of discrete inner foamed microcellular layers have a diameter ranging from about 0.1 to about 100 microns, and
• wherein the fourth inner foamed layer comprises cells having a size greater than cells of the second inner foamed layers, and the third inner foamed layers comprise cells having a size greater than cells of the fourth inner foamed layer.