Apparatus for forming flaps at the top of an open-top carton after it has been packed, with the flap formation correlated to the level of the top of the contents of the carton, having means for internally scoring the walls of the carton generally at said level, means for slitting the corners of the carton generally down to said level, and means for trimming off the tops of the side walls of the carton so that they may be folded on top of the carton without overlap. In forming the flaps, the apparatus is adapted to compress the contents of the carton. The scoring means includes a set of scoring blades which fit in a carton on top of the contents and a set of external presser members for pressing the carton walls against the blade edges. The scoring blades are replaceable with blades of different sizes and provision is made for adjustment of the presser members and trimming means to operate on cartons of different sizes.
