A gusseted bag of a synthetic material for holding foodstuffs

Abstract

 A length of gusseted tube of a film of a thermoplastic material is sealed at one end with two or three parallel sealing lines (22, 24) that are connected by transverse bracing seals (26) extending at intervals across them. The bracing seals are preferably arranged at uniform intervals and may extend at right angles to the parallel sealing lines or with alternately opposed inclinations. The film may comprise a single layer or two co-extruded layers, and preferably one of the layers is made of polyethylene.

Description

[0001] This invention is concerned with a gusseted bag of a synthetic material for holding foodstuffs, particularly intended for home freezing of foodstuff.

[0002] Bags are available on the market for freezing food such as vegetable and fruit produce in the home. The bags are manufactured from an extruded tube of polyethylene film that is flattened, cut into pieces of a desired length, and simultaneously heat-sealed transversely near one end. In order to give the bag a larger capacity, and also in order to avoid the formation of air pockets within the bag, the bag is usually provided with lateral gussets that are obtained by forming two indented folds in the film tube during flattening.

[0003] However, in consequence of the formation of the gussets, when the bag bottom is sealed by heat sealing, a critical central area develops between the gussets, where there are only two overlapping film layers that are to be bonded together, in contrast to the side areas where the overlapping layers that are to be bonded are in number of four.

[0004] Due to the difference in the amounts of heat required for sealing in the two-layer area and in the four-layer area (gussets), the sealing conditions necessarily are a compromise between the optimum values that would be appropriate for two layers and for four layers, respectively. What usually happens is that the seal in the central area is weaker, and therefore liable to failures in case of excessive filling, squeezing, or inflating.

[0005] Moreover, in the transition from the two-layer area to the four-layer area there is necessarily a step or discontinuity, where the outer films diverge obliquely before overlapping on the gusset. In such two discontinuities the seal is wanting anyway.

[0006] In an attempt to improve the bottom sealing in the above-mentioned gusseted bags, IT-U-236009 discloses a seal comprising two parallel sealing lines, a few millimeters apart from each other. This solution highly enhances the strength of the bottom sealing, even in case of considerable loads such as due to water filling or bag squeezing. However, the inventor has found that, because of the high deformability of the bag material, usually polyethylene, the sealing line that is more stressed from the inside will stretch until it is quite close to the other sealing line, the reinforcement caused by the two sealing lines being thereby reduced.

[0007] Accordingly, the object of the invention is to improve the effectiveness of the bottom seal with respect to gusseted bags of the prior art, particularly bags as disclosed in IT-U-236009.

[0008] Another object is to provide the above high-strength bag at a manufacturing cost that is substantially identical as for a conventional double-seal bag as disclosed in IT-U-236009.

[0009] The invention attains the above objects, as well as other objects and advantages such as will appear from the disclosure, by a gusseted bag of a polyethylene film having the features recited in claim 1.

[0010] A few preferred embodiments of the invention are described below, with reference to the attached drawings, wherein:

Fig. 1 is a perspective view of a bag of polyethylene film according to the invention;

Fig. 2 is an enlarged and broken-away front view of the bag of Fig. 1;

Fig. 3 is a further enlarged front view of the sealing pattern of the bag according to Fig. 2;

Fig. 4 is a view similar to Fig. 3, for a sealing variant according to the invention;

Fig. 5 is a view similar to Fig. 3, for another sealing variant according to the invention.

[0011] On Fig. 1, a tube of polyethylene film 10 is flattened so that it includes lateral indented folds 12,14, by means known to persons skilled in the art. At one of its ends, the tube is sealed with a transverse heat seal 15 (described below in more detail) so that a gusseted bag is obtained. Accordingly, seal 15 extends both along a central area 16 comprising two overlapping film layers, and along two lateral areas 18, 20, comprising four overlapping film layers.

[0012] The bag is shown in an enlarged, broken away front view on Fig. 2. seal 15 is seen to comprise two parallel sealing lines 22, 24, with bracing seals 26 extending at intervals across lines 22, 24. The seal so patterned can be defined as a "truss seal", similar to so-called cross-linked trusses, where an enhanced bending strength is obtained with respect to a plain beam of similar weight, due to the interwoven action of the struts and tierods acting between the two main beams.

[0013] Fig. 3 shows a seal fragment, and also shows the shape that a hot bar should have in order to produce the seal so patterned. Parallel lines 22, 24 correspond to two respective metal ribbons, in which respective heating electric currents flow. The bracing cross-seals 26 are made by ties (not shown) integral with the ribbons, and heated by them by conduction.

[0014] In a preferred example, concerning a bag of about 160 mm width, the two parallel sealing lines 22, 24 have each a width of about 1 mm, are about 4 mm apart from each other, and transverse bracing seals 26 also have a width of about 1 mm and are placed at uniform intervals of about 6 mm.

[0015] Fig. 4 shows a modification of the sealing, which still comprises two parallel sealing lines, between which extend bracing seals inclined at angles of 45-60° in alternately opposite directions.

[0016] Fig. 5 shows a further modification of the seal, similar to the seal of Fig. 3, though it further comprises a third sealing line 28 extending parallelly in mid position between lines 22, 24. The third sealing line corresponds to an additional current conductor in mid-position in the hot bar, causing the cross elements to be heated more effectively, and therefore giving rise to a more secure seal.

[0017] For a better sealing of the bag, as it appears from the above prior document, it is further preferable to use a double-layer co-extruded film, rather than single-layer film. As a consequence of this step, the micro-holes that (as known to persons skilled in the art) are often formed in thin films are mutually shut between the two co-extruded layers.

[0018] The manufacture of bags with truss seals as disclosed above can be carried out by conventional procedures and equipment, by feeding a flattened tube of a thermoplastic film having indented folds beneath a hot bar. However, the hot bar, rather than having one or two hot sealing ribbons, has a hot surface comprising two hot ribbons interconnected at intervals by cross elements. A cutting blade is sunk onto the film tube synchronously with the hot bar, as known to persons skilled in the art, in order to sever a length of tube.

[0019] Tests were made by inflating bags having double seals according to the above mentioned IT-U-236009, on the one hand, and bags having truss seals according to the present invention, on the other hand. The latter have shown a definitely better strength, in that they tolerated a higher internal pressure. This can also be seen visually in that the two sealing lines in the bag of the invention remain apart from each other when the bag is subjected to an internal pressure which is sufficient to cause the sealing lines to abut against each other in the case of bags sealed with a double line but without any bracing seals.

[0020] Obviously, the distances between the transverse seals, rather than being uniformly spaced as shown and described above, could be different from each other, e.g. they might widen from the central axis of the bag toward its sides. Moreover, although the disclosure only mentions polyethylene film, the invention encompasses use of any film made from a sealable synthetic material. Also, although the examples shown always refer to heat sealing, it should be understood that the same arrangements can be used when the bags are manufactured by electronic microwave sealing.

Claims

1. A bag comprising a length of a gusseted tube of a film of a thermoplastic material, sealed at one end with two parallel sealing lines, characterized in that transverse bracing seals (26) extend at intervals between said parallel sealing lines (22, 24).

2. The bag of claim 1, further characterized in that between said two parallel sealing lines (22, 24) extends a third intermediate parallel sealing line (28) which intersects said transverse seals (26).

3. The bag of claim 1 or 2, further characterized in that said bracing seals (26) are arranged at uniform intervals along said parallel sealing lines (22, 24, 28).

4. The bag of claim 1, 2 or 3, further characterized in that said bracing seals (26) are substantially at right angles to said parallel sealing lines.

5. The bag of claim 1 or 2, further characterized in that said bracing seals (26) are at an angle to said parallel sealing lines (22, 24) with alternately opposed incline.

6. The bag of any of claims 1 to 5, further characterized in that said said seal is a heat seal.

7. The bag of any of claims 1 to 6, further characterized in that said thermoplastic material is polyethylene.

9. The bag of any claims 1 to 8, further characterized in that said film is formed of a plurality of co-extruded layers.

Drawing