The invention relates to a device and to a method for sealing two partially overlapping packaging parts (11, 12) in a liquid-tight manner, wherein one packaging part is a tubular main body (11), in particular formed from a blank made of cardboard/plastic composite material, and a further packaging part (12) forms the packaging head or packaging bottom, said device comprising at least a mandrel (D) and a die (M), wherein the mandrel (D) has at least two spreading elements (1A, 1B, 1C, 1D), which are moved toward each other from a working position of the spreading elements to an idle position in order to reduce the cross-section such that the tubular main body (11) can be easily slid on, wherein the spreading elements (1A, 1B, 1C, 1D) have spreading jaws (2A, 2B, 2C, 2D), which form a closed peripheral outside contour (AK) in a working position of the spreading jaws, which outside contour corresponds to an inside contour (IK) formed by an opening (8) in the die (M), wherein a narrow ring gap for the application of a pressing force to the overlap region (13) of the packaging parts (11, 12) from the inside remains between the inside contour (IK) and the outside contour (AK), and wherein the spreading elements (1A, 1B, 1C, 1D) are each pivotably arranged on a common base element (5). In order to optimize the kinematics and synchronization of the spreading elements and to achieve a simple, low-wear, reliable, and easy-to-clean design, by means of which aspectically filled packages also can be produced, each spreading element (1A, 1B, 1C, 1D) is fastened to at least one torsionally stiff leaf-spring element (4A, 4B, 4C, 4D), which enables exact pivoting of the particular spreading element (1A, 1B, 1C, 1D) between the idle position and the working position.