A device for shrinking flat thermoplastic packaging materials onto articles or sets of articles, comprising at least - a tunnel housing with two openings, through which the articles or sets of articles with their respective thermoplastic packaging material can enter and exit the tunnel housing, - a transport device, which provides a horizontal transport plane for moving the articles or sets of articles with their respective thermoplastic packaging material through the tunnel housing, - a means or a plurality of means for introducing thermal energy into the tunnel housing, and - at least one unit through which an opening cross-section of at least one of the two openings can be enlarged or reduced - wherein the at least one unit features at least one flap, which is positioned in the region of the respective opening and is arranged, at least in sections, in the tunnel housing, - with the at least one flap for enlarging and reducing the opening cross-section of the respective opening being pivotable in an inwards direction with respect to the tunnel housing and/or in an outwards direction with respect to the tunnel housing about an axis oriented at an angle and preferably perpendicularly to the horizontal transport plane, characterised in that the one means or the plurality of means comprise at least two shaft walls positioned in the tunnel housing and facing each other, which shaft walls each have several openings for a temperature-controlled fluid flow in order to introduce thermal energy into the tunnel housing, wherein the at least one flap positioned in the region of the respective opening is arranged in such a manner that the at least one flap positioned in the region of the respective opening adjoins in a defined pivot position one of the shaft walls facing each other at least approximately. - a tunnel housing with two openings, through which the articles or sets of articles with their respective thermoplastic packaging material can enter and exit the tunnel housing, - a transport device, which provides a horizontal transport plane for moving the articles or sets of articles with their respective thermoplastic packaging material through the tunnel housing, - a means or a plurality of means for introducing thermal energy into the tunnel housing, and - at least one unit through which an opening cross-section of at least one of the two openings can be enlarged or reduced - wherein the at least one unit features at least one flap, which is positioned in the region of the respective opening and is arranged, at least in sections, in the tunnel housing, - with the at least one flap for enlarging and reducing the opening cross-section of the respective opening being pivotable in an inwards direction with respect to the tunnel housing and/or in an outwards direction with respect to the tunnel housing about an axis oriented at an angle and preferably perpendicularly to the horizontal transport plane, characterised in that the one means or the plurality of means comprise at least two shaft walls positioned in the tunnel housing and facing each other, which shaft walls each have several openings for a temperature-controlled fluid flow in order to introduce thermal energy into the tunnel housing, wherein the at least one flap positioned in the region of the respective opening is arranged in such a manner that the at least one flap positioned in the region of the respective opening adjoins in a defined pivot position one of the shaft walls facing each other at least approximately.