A rectangular tray-type carton formed from a blank of plastics-coated board has a peripheral flange for attachment of a closure lid. The flange is formed after the erection of the carton side wall in a punch and die arrangement, by folding down elongate panels carried along the free edges of the side wall and by subsequently joining the panels together at their ends by means of overlapping tabs which are heat-sealed together. In order to avoid damage to the tabs during formation of the side wall the uppermost tabs are folded out of the plane of the blank before it enters the punch and die arrangement.