US 3372857 A
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SIDE SEAL BAG CONSTRUCTION WITH STRESS RELIEF NOTCH Filed April 7, 1967 FIG 2 PRIOR ART T R A R l R P ,Is I
INVENTOR. RALPH A. BRAYLA 6 V 1 J m A \\\Mu W m m 5 y My FIG, 3
United States Patent 3,372,857 SIDE SEAL BAG CONSTRUCTION WITH STRESS RELIEF NOTCH Ralph A. Brayla, Canandaigua, N.Y., assignor to Mobil Oil Corporation, a corporation of New York Filed Apr. 7, 1967, Ser. No. 629,164 6 Claims. (Cl. 229-53) ABSTRACT OF THE DISCLOSURE Thermoplastic bag constructions adapted for employment on automatic loading equipment which comprise a front wall and a rear Wall said walls being heat seamed together along the opposite longitudinal edges thereof and said bag being opened at one end for the introduction of product therein; rupture of said heat seams during loading operations is inhibited by a stress relief notch located adjacent the upper ends of said heat seams.
BACKGROUND OF THE INVENTION 1. Field of the invention This invention relates to novel side seal bag constructions fabricated of thin thermoplastic film wherein the bag sides are heat sealed to one another and more specilically relates to a novel bag structure which eliminates bag failure by zippering of the side seals by provision, in the longitudinal heat seal seams of the bag, of a stress relief notch.
II. Description of the prior art In the manufacture of bags of thin thermoplastic film such as films of polyethylene or polypropylene, for example, a bag is often formed from a continuous sheet of plastic film Which is folded over longitudinally of said sheet as an initial step in the bag-making operation. Sub-- sequently the folded-over sheet is passed through a standard bag-forming device wherein a heated 'knife member severes the folded-over web transversely and simultaneously seals the severed film edges together. As the foldedover sheet continually advances, this heat sealing and severing operation is repeated at a position in the foldedover Web downstream of the first heat sealing and severing operation whereupon a side seal bag is produced. The bottom of the bag is coincident with the folded-over portion of the plastic sheet; the edges of the bag are constituted by the heat welds formed by the severing and sealing operations of the hot knife and the mouth of the bag is open. The bag may be fabricated so that the back Wall of each individual bag is longer than the front wall thereof whereby a lip is provided which may be employed with fastening means to maintain a number of such bags in stacked relationship. Such a superimposed stack of bags may subsequently be employed in semiautomatic and/ or automatic packaging operations. In such loading devices the uppemost bag in the stack is opened by a draft of air directed at the bag opening between the front and back wall of said bag. The bag is retained on the stack by the aforesaid fastening means in the lip of the bag. A pair of loading horns enters the mouth of the open bag and applies opening stresses thereto which are usually concentrated along the longitudinal heat sealed seams and particularly at the upper portions of said seams, i.e. at the mouth of the bag. Such loading stresses when applied to the bag heat seams oftentimes result in heat seam failure usually initiating at the open end of the bag. Upon the initiation of such heat seam failure, the heat seam will zipper open very rapidly down towards the bottom of the bag resulting in, not only a loss of the individual bag, but also the product contained therein, when such 3,37Z"857 Patented Mar. 12, 1968 failure does not become evident until the product has been thus loaded.
In order to ascertain the course of such side seam failure, the side seal seam areas of such bags have been examined, for example, by photomicrographs thereof, and it has been found that the upper end of the seal, i.e. that portion of the seal adjacent the bag opening, is generally non-uniform and weaker than the remainder of the seal extending towards the bag bottom. Since during the bag loading operation, as aforedescribed, opening and loading stresses tend to concentrate at these points of weakness, there is a tendency for the bag seals to fail with the attendant zippering apart of the bag Walls along the seam areas. Attempts made in the past to remedy this problem include reinforcement of the weakened areas of the side seam seals by providing an enlarged seal area at the terminal end or corners of the seal portions at the open mouth of the bag construction. Such a construction is disclosed in US. Patent No. 3,142,437. However, this attempt to solve the problem is not completely satisfac tory as it, at best, only seams to temporarily arrest the tearing or splitting of the seals at the corners when they are subjected to opening stresses. Other attempts to remedy this problem, as disclosed in US. Patent No. 3,142,- 437, comprise forming cutaway portions or cutout notches along the open upper edges of the bag walls in order to distribute opening forces away from the weak areas along the longitudinal heat sealed seams of the bag. However, it has been found that such cutout portions in the bag lip, in addition to necessitating extra process steps during bag fabrication, are not completely satisfactory in eliminating side seal seam failure during bag opening and loading operations.
SUMMARY OF THE INVENTION heated knife edge, results in the area of the bag walls surrounding the notch, i.e. the notch periphery, being heat welded together. It has been found that with such a construction, the bags, when loading streses are applied to the mouth thereof during opening and filling operations, resist the tendency exhibited by similar unnotched side seal bags to tear or zipper along the longitudinal heat sealed edges.
DESCRIPTION OF THE DRAWINGS DESCRIPTION OF SPECIFIC EMBODIMENTS In accordance with the present invention, thermoplastic bags, and particularly bags commonly referred to as side seal bags are constructed with a front wall and a rear wall, the walls being heat seamed together along the opposite longitudinal edges thereof and being open at one end for the introduction of product therein. Such bags may be employed for use on automatic packaging equipment wherein loading stresses are applied to the mouth of the bag during opening and filling thereof. These stresses are transmitted to the longitudinal heat seams and particularly the upper portions thereof near the bag opening. In the improved structure of the present invention, liability to rupture of said longitudinal heat seams is inhibited by locating a stress relief notch just below the upper ends of said heat seams, the notch extending through the heat seam and terminating in the bag walls immediately adjacent to the seams. It is preferred that such a notch be formed with a hot knife whereby the area of the bag Walls which form the periphcry of the notch are heat welded together.
As illustrated in the drawing shown in FIGURE 1, the prior art, side seal bag structures comprise a front wall 12 and a back wall 13, the bag being fabricated from a single sheet of thermoplastic material folded over at 14. The front Wall 12 is sealed to back wall 13 along longitudinal heat seam lines 15 (and 15). in the illustration shown in FIGURE 1, back wall 13 is slightly longer than front wall 12 whereby a single layer lip 16 is provided. Lip 16 may contain one or more apertures 17 whereby a group of such bags may be stacked together and secured in stacked alignment by fastening means (not shown) which pass through apertures or perforations 17. As can be seen more clearly in FIGURE 2, when it is desired to open such a bag structure, especially on automatic packaging equipment wherein loading horns are forced between front wall 12 and back wall 13 in the vicinity of the bag mouth opening, stresses tend to concentrate at the opposite corners of the bag mouth near the upper end of longitudinal side seal seams 15 (and 15). Since, as hereinbefore indicated, the top portions of the side seal identified in FIGURE 2 as x are generally quite weak and not uniform, there is a tendency of the side seal to rupture at these points and, once seal rupture occurs, little resistance is offered to a complete zippering or tearing apart of the bag walls along heat sealed seams 15 (and 15). In the bag construction of the present invention as illustrated in FIGURES 3 and 4, a notch is provided in that area of the side seams 1.5 (and 15') immediately below the upper edge 18 of front wall 12. The notch 20 is formed with a heated knife and extends completely through front wall 12 and back wall 13. Those portions of the back walls 12 and 13 which surround notch 20 are heat sealed together at the periphery of the notch. Thus, as shown more clearly in FIGURE 4, When opening stresses are now applied to the mouth of the bag, and if a rupture of the seal area at x does occur, rather than the seal failure being propagated throughout a substantial portion of the length of the seam 15 (or 15) such a zippering action will be inhibited by notch 23. When the seal failure reaches this point, it terminates at apex A of notch 20. As loading and opening forces continue to cause stress in the side seal area after the tear or rip has reached point A, these stresses are now distributed and effectively absorbed without further seal failure along area y of the bag side seal seams 15 (and 15').
Ithas been found that side seal bags of a structure similar to those embodied herein but in which the stress relief notch does not have the periphery thereof sealed,
4; often result in, upon loading and subsequent side seal seam failure, a tearing of the bag wall immediately adjacent the notch. On the other hand, by forming the stress relief notch so that the periphery thereof is sealed as embodied by the novel bag products described herein, a substantial improvement has been found to result in that, upon loading, the opening stresses appear to be uniformly and continuously resisted by the side seal, upon failure thereof, up to the apex (A) of the notch, whereupon the seal failure terminates.
It has been found that in a particularly preferred embodiment, the angle which the notch subtends with respect to the longitudinal edge of heat seal seams 15 (and 15) of the bag is preferably less than and more desirably 60 or less. Although a notch which is disposed at any angle to this longitudinal bag edge will retard seam failure, an angle on the order of 60 or less has been found to be particularly effective in inhibiting the tendency of the seal area below such a notch to zipper during bag opening, loading and handling operations.
Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be resorted to, without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such variations and modifications are considered to be within the purview and scope of the appended claims.
What is claimed is:
1. A thermoplastic bag comprising a front wall and a rear wall, said walls being heat seamed together along the opposite longitudinal edges thereof and being open at one end for the introduction of product therein; said bag being adapted for use on automatic filling equipment whereby loading stresses are applied to the mouth of the bag during opening and filling thereof and transmitted to said longitudinal heat seams; the improved structure in which liability to rupture of said heat seams is inhibited which comprises a stress relief notch located adjacent the upper end of each of said heat seams, said notch extending through said heat seams and terminating in said bag walls adjacent said seams, the area of said bag walls surrounding said notch being heat welded together.
2. A thermoplastic bag as defined in claim 1 wherein the front and rear walls of said bag comprise a polyolefin.
3. A thermoplastic bag as defined in claim 2 wherein the polyolefin is polyethylene.
4. A thermoplastic bag as defined in claim 1 wherein said rear wall is longer than said front wall thereby providing a lip at said bag mouth.
5. A thermoplastic bag as defined in claim 1 wherein said stress relief notch forms an angle of less than 90 with said longitudinal heat seam.
d. A thermoplastic bag as defined in claim 1 wherein said stress relief notch forms an angle of from about 25 to about 60 with said longitudinal heat seam.
References Cited DAViD M. BOCKENEK, Primary Examiner.