Folding cartons have been printed by flexo for many years, but it has only been in the past 10 years that the industry has started using state-of-the-art central impression presses for this application. Pizza cartons illustrate the type of work that was originally relegated to the flexo process. The ability to print multi-colors in close register on wide-webs, however, has made the process more attractive to other point-of-purchase folding carton applications.
Normally the folding carton press is tied in to an in-line process that would include cutter-crease or platen die-cutting operations. In the past, most cutter-creasing operations were mated only with rotogravure or sheet fed offset presses. With the improved quality of flexographic printing, the use of water base or UV inks, and the lower cost of equipment, converters can no longer ignore the option of printing high quality folding cartons flexographically.
Folding carton equipment differs as a result of the substrate. Heavy board roll stock is used, which requires higher tension levels, larger idle rolls and more sophisticated splicing and control units. The unwind section must handle 72" diameter rolls and automatically splice them without an overlap (butt splicing). Idle rolls must be of a large enough diameter to eliminate the risk of weakening the board fiber bond or creasing the board.
From the unwind through the cooling drum and to the out-feed unit, the press operator is dealing with the usual tension zones and a constantly moving web. From this point on to the cutter creaser or platen die-cut section, the motion is intermittent (stop and go). Web control is critical to both print and die-cutting register, as the printing must be on a smooth flowing, tensioned web, and the die-cutting on a stationary, relaxed web. Electronic print sensors register the image on the web to the die cutter.