Grocery sacks of high density polyethylene film (HDPE) are now in the position of an idea whose time has come. Paper grocery sacks are used by the billions, but plastic has significant advantages over paper including greater toughness, ultimate wet strength and moisture resistance, lighter weight, good appearance and excellent reuse potential.
Grocery sacks of HDPE are known and one such sack is shown in U.S. Pat. No. 3,669,347 assigned to the assignee of this application. Another commonly owned U.S. Pat. No. 3,606,822, discloses a method and apparatus for making such a sack. The problem is in rapidly, continuously, and automatically making such sacks as the prior art is substantially a sack-by-sack hand operation which by its nature is slow and consequently uneconomical.
The prior art of making paper bags is ancient and quite well worked, see e.g. U.S. Pat. No. 634,081, but the solutions to problems in making paper sacks including suitable means for folding, sealing, cutting and the like, are not applicable to a similarly constructed bag of plastic such as HDPE due primarily to the differences in the characteristics of the materials. In other words, because HDPE plastic does not seal, cut or fold like paper, machines for making paper bags are presently useless for making sacks or bags of HDPE.
Certain of the advantages offered by thermoplastic materials pose difficulties in bag forming operations, and particularly folding, in that electrostatic effects may tend to maintain a gusseted web in a flattened condition, and flexural properties increase resistance to fold and crease formation, especially where multiple and complex folds are required to be performed within a small region and at high speed.