A process for the production of at least two-layer thermoplastic sheets via thermal welding of at least one first thinner thermoplastic sheet with density and of at least one second thinner thermoplastic sheet with density, where the density of the first thinner thermoplastic sheet is smaller than the density of the second thinner thermoplastic sheet, comprising the following steps a) to e): a) the first thinner thermoplastic sheet and the second thinner thermoplastic sheet are oriented parallel to one another at a distance (a) from one another, thus forming an intervening space, b) at least one first heating element and at least one second are introduced along mutually parallel offset planes into the intervening space in a manner that is parallel to the two thinner thermoplastic sheets, where the surfaces of the thinner thermoplastic sheets and the surfaces of the heating elements do not touch one another and with a distance between the first heating element and the first thinner thermoplastic sheet and a distance between the second heating element and the second thinner thermoplastic sheet, c) the heating elements are introduced between the two thinner thermoplastic sheets to an extent such that, in relation to every location on the surface of the first thinner thermoplastic sheet, the first heating element has been present at least temporarily between the two thinner thermoplastic sheets, and to such an extent that, in relation to every location on the surface of the second thinner thermoplastic sheet, the second heating element has been present at least temporarily between the two thinner thermoplastic sheets, where the first heating element transfers a quantity of energy to the surface of the first thinner thermoplastic sheet and the second heating element transfers a quantity of energy to the surface of the second thinner thermoplastic sheet, where the quantity of energy that the first heating element transfers to the surface of the first thinner thermoplastic sheet is smaller than the quantity of energy that the second heating element transfers to the surface of the second thinner thermoplastic sheet, d) the heating elements are removed entirely from the intervening space, e) at least one of the two thinner thermoplastic sheets is pressed against the surface of the respective other thinner thermoplastic sheet. a) the first thinner thermoplastic sheet and the second thinner thermoplastic sheet are oriented parallel to one another at a distance (a) from one another, thus forming an intervening space, b) at least one first heating element and at least one second are introduced along mutually parallel offset planes into the intervening space in a manner that is parallel to the two thinner thermoplastic sheets, where the surfaces of the thinner thermoplastic sheets and the surfaces of the heating elements do not touch one another and with a distance between the first heating element and the first thinner thermoplastic sheet and a distance between the second heating element and the second thinner thermoplastic sheet, c) the heating elements are introduced between the two thinner thermoplastic sheets to an extent such that, in relation to every location on the surface of the first thinner thermoplastic sheet, the first heating element has been present at least temporarily between the two thinner thermoplastic sheets, and to such an extent that, in relation to every location on the surface of the second thinner thermoplastic sheet, the second heating element has been present at least temporarily between the two thinner thermoplastic sheets, where the first heating element transfers a quantity of energy to the surface of the first thinner thermoplastic sheet and the second heating element transfers a quantity of energy to the surface of the second thinner thermoplastic sheet, where the quantity of energy that the first heating element transfers to the surface of the first thinner thermoplastic sheet is smaller than the quantity of energy that the second heating element transfers to the surface of the second thinner thermoplastic sheet, d) the heating elements are removed entirely from the intervening space, e) at least one of the two thinner thermoplastic sheets is pressed against the surface of the respective other thinner thermoplastic sheet.