In the erection of rectangular tray-type cartons from blanks of foldable sheet material, corner tabs which are to be uppermost in the erected carton are first folded up from the plane of the blank, which is then passed through a die by a punch to form the side wall. On completion of the stroke of the punch, the pair of opposed elongate panels which carry the uppermost tabs are folded outwardly by folding bars carried by the punch. The partially erected carton is then transferred to a continuously moving conveyor and the other pair of elongate panels are folded outwardly by oscillating fingers and plough bars. The upstanding tabs are then heated by travelling hot air jets and pressed down by pressure rollers to seal them to the other tabs and complete the horizontal peripheral flange which extends continuously around the carton.