An envelope blank is conveyed by a transport system (12) in an envelope making machine to a folding mechanism (10) that folds one or more flaps of the blank. In folding a side flap (72) the leading edge (88) of the blank (70) adjacent to the score line (76) is conveyed along a convex face (112) of a plowshare (48). The plowshare face (112) applies at the leading edge (88) of the blank (70) adjacent to the score line (76) a force which bends the side flap (72) through an angle of 180 DEG to a position where the flap (72) is folded to overlap the blank (70). A crease is formed on the score line (76) to permanently set the flap (72) on the blank (70). The folding of the flap (72) is initiated by the convex face (112) of the plowshare (48). Extending from the upper edge of the plowshare face (112) is a grid-like network (130) of flexible rods (146) mounted to follow the helical path of the plowshare face (112). The free edge portion (92) of the flap (72) is supported by the grid (130) to follow the flap leading edge (88) being folded by the plowshare (48). The flap folding forces are applied at the region of the flap (72) adjacent to the score line (76) where the flap (72) has the least resistance to bending. The folding forces are removed from the flap free edge (92) which is supported by the grid (130) to follow the movement of the flap leading edge (88). The trailing edge (92) of the flap (72) is stabilized by a guide plate (150) positioned oppositely of the plowshare (48) in the feed path (20). The guide plate (150) prevents the flap trailing edge (90) from falling downwardly ahead of the flap leading edge (88) to assure that the fold is maintained in alignment with the score line (76).
