A method for infusing gas into a thermoplastic material includes positioning a sheet of a thermoplastic material into a vertical or substantially vertical position; positioning a sheet of a gas-permeable material into a vertical or substantially vertical position; and winding together the positioned sheet of thermoplastic material sheet and the positioned sheet of gas-permeable material to form a roll of the thermoplastic material interleaved with the gas-permeable material, the interleaved roll having a longitudinal axis oriented in a vertical or substantially vertical position. The method also includes exposing the interleaved roll to an atmosphere of a gas pressurized to infuse the thermoplastic material with the gas, while the longitudinal axis of the interleaved roll is oriented in a vertical or substantially vertical position; and then unwinding the gas-infused interleaved roll, while the longitudinal axis of the interleaved roll remains in a vertical or substantially vertical position.