The present invention relates to the art of folding paperboard cartons in general and to windowed folding cartons in particular. The windows extend into two or more contiguous walls of the carton and they are covered by transparent thermoplastic sheets. Cartons of this general type are well known to the art and are disclosed, for example, in U.S. Pat. Nos. 4,826,004; 4,846,775; and 4,733,916. It is to improvements in this type of windowed folding carton having an "around-the-corner" window that the present invention is directed.
When the flat paperboard blanks from which folding cartons are die cut are formed with large apertures which serve as viewing windows in the finished cartons, those windows extend over to contiguous panels of the blank (eventually walls of an erected carton) and the windows are typically closed off and sealed completely about their peripheries with sheets of transparent plastic sheet. The bonding of the clear plastic sheet to the paperboard about the edges of the window aperture is effected by means of an adhesive which is typically applied in a circumscribing pattern about the window aperture. While cartons erected from this type of windowed blank are very attractive and desirable for the purposes of providing large display areas of packaged goods, the folding and bending of the comparatively stiff plastic sheet along with the underlying paperboard panels often creates tension as well as compression stresses in the joined materials which often results in the tearing of the paperboard along the fold lines at the overlap of the plastic sheet with the paperboard or forms wrinkles in the overlying plastic sheet near the edges of the aperture forming the borders of the window at the area of the fold line or actually destroys the adhesive bonding of the overlying window material to the paperboard itself. Any of these failures, or worse still, several of them simultaneously ruins the appearance and the acceptability of the windowed carton.
Accordingly, many efforts have been made in composite folding cartons, (those in which window panels are adhered to underlying paperboard blanks in areas which extend over two or more contiguous panels) to eliminate the deleterious aesthetic effects of crinkling, tearing, separation or combinations thereof in the window area. These efforts have concentrated on eliminating or reducing the stressing of the composite blank in the sensitive areas, the zones of the window periphery where the plastic sheet is adhered to the paperboard blank along fold lines between contiguous panels. The present invention provides a solution to this problem which permits the production of folding cartons having a comparatively heavy plastic sheet material closing off display windows formed by apertures extending over two or three contiguous main carton panels in a manner which avoids stress wrinkling of the adhered plastic sheet material, which avoids cracking and/or bursting of the paperboard blank at the score lines where the contiguous panel members are adhered to the plastic sheet, and which avoids delamination, separation or destruction of the sealing adhesive which secures the carton blank to the overlying plastic to the paperboard blank.