PURPOSE: To form tubular packaging material of substantially circular cross-sectional configuration into package containers of quadrilateral cross-sectional configuration without inviting deformation by pinching the material between flaps.

CONSTITUTION: This device consists of pairwise cooperating forming flaps 11 which are of U-shaped cross-section. In order to prevent packaging material from being pinched between shank surfaces of the flaps 11 when the flaps 11 are caused to surround a packaging material tube 7, the shank surfaces are designed to be undulatory, i.e., alternatingly with projections and recesses which fit in one another and guide the packaging material tube into the U-shaped recesses of the flaps 11 without pinching the packaging material tube.