Until this invention, the top was severed from the container portion by routing the joining or waste material between the top and the container.  This practice left a rough and uneven edge on both parts.  It also caused the entire joining strip to be ground up into non-recyclable material.  The parts when cut with the router or other similar rotating tools required further trimming by hand.  Furthermore, because the rotating tool is prone to producing an uneven cut, scrap articles were frequently produced. 
Cutting open blow-molded containers using a knife blade rather than a rotating tool is shown in U.S. Pat.  No.  3,662,633.  The closed container is rotated under a knife blade to remove the expansion head.  However, there are drawbacks to the use of this apparatus: square or other non-circular objects would be difficult if not impossible to cut; the knife blade is inherently under stress when cutting and therefore plows and gouges the plastic or is likely to be broken; the workpiece must have an integral groove and head for guiding and controlling the knife; and many revolutions of the article are necessary for severing.