CA2145045A1 - Thermoplastic bag having gusset point stress relief - Google Patents
Thermoplastic bag having gusset point stress reliefInfo
- Publication number
- CA2145045A1 CA2145045A1 CA 2145045 CA2145045A CA2145045A1 CA 2145045 A1 CA2145045 A1 CA 2145045A1 CA 2145045 CA2145045 CA 2145045 CA 2145045 A CA2145045 A CA 2145045A CA 2145045 A1 CA2145045 A1 CA 2145045A1
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- Canada
- Prior art keywords
- bag
- zone
- gusset
- width
- walls
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Abstract
The present invention relates to a thermoplastic bag having stress relief zones located in the side wall internal gussets.
The stress relief zones are intentionally introduced structural defects that direct stress generated during loading away from the bag gusset points, thereby preventing catastrophic failure of the bottom heat seal. The stress relief zones can take the form of a slit, perforation or arcuate area.
The stress relief zones are intentionally introduced structural defects that direct stress generated during loading away from the bag gusset points, thereby preventing catastrophic failure of the bottom heat seal. The stress relief zones can take the form of a slit, perforation or arcuate area.
Description
F-~353-L 2 14 5 n 4 5 THERMOPLASTIC BAG HAVING GUSSET POINT STRESS RELIEF
-Field of the Invention The present invention relates to thermoplastic bags having 5 a stress relief area located in the internal gussets so as to direct stresses generated during loading and carrying away from the bag gusset points.
Background of the Invention Thermoplastic bags are steadily gaining acceptance where consumer goods of all types must be bundled up for transport away from the point of sale. These bags are used in many different types of retail settings to include grocery stores, department stores of all types, building supply stores and any 15 other setting where a lightweight, strong, easy to dispense bag, is required. The advantages of plastic bags over kraft paper bags are numerous. They are lighter in weight, take up less room when folded, resist water, and may be fabricated with integral handles that provide for easy transport of the loaded 20 bag. They are reusable as trash can liners or can be reused to carry other items. Thermoplastic bags are also recyclable.
Thermoplastic bags are commonly formed by collapsing a tube of plastic so as to have two in-folded gussets at opposite sides thereof; forming two spaced seals positioned transverse of the 25 collapsed tube; and removing from the end thereof a u-shaped segment which simultaneously forms two integral handles and a bag mouth opening. This configuration and the in-folded gussets create handles which have double film thicknesses and which provide greater carrying strength in the handles.
The evolution of the thermoplastic bags has included a gradual reduction of the thickness of the wall material of the bags. Advances in the field of polymer science have enabled manufacturers to maintain and even increase bag strength using ever decreasing wall thicknesses.
An important result of this evolution has been the occurrence of failures along the transverse heat strip seal at the bottom of the bag. In the region of the in-folded gusset, F-7353-L ~14 5 0 ~ 5 four layers of film are brought together in the outer regions of the lay-flat bag structure and, in-between, two layers of the front and back walls of the bag are brought together. Thus, the heat seal must simultaneously weld four layers together in the 5 out-board segments of the bag in its lay-flat condition and two layers in the central region of the bag. It has been found that the bags often fail by tearing open in the region of the heat seal portion of the bottom of the bag beginning at the seal transition from four layers to two layers.
Numerous attempts have been made to solve this problem.
U.S. Patent No. 4,613,988 to Maddock discloses the use of arcuate stress relief regions at the four-layer/two-layer junction. The arcuate regions result from the removal of the film area below a curve extending from a point at the bottom of i5 the four-layer region to a point at the bottom of the two-layer region.
A different approach is disclosed in U.S. Patent No.
4,812,055 to Prader and Benoit. That invention utilizes circular seals located slightly above the transverse seal line 20 and inside the in-folded gusset on both sides of the bag. The seal areas relieve strain at the weak regions at the bottom of the bag.
U.S. Patent No. 3,485,437 to Gruentzel et al. discloses the use in a bag of a weakened area positioned perpendicular to a 25 welded heat seal so as to interrupt a tear along the seam. The weakened area can take the form of an aperture, score line or slit for the purpose of redistributing stress forces over a wide area. In all embodiments of this invention, it is required that some interruption of the heat formed seam take place to provide 30 stress relief.
While each of the inventions described above addresses the problem of bag failure, they each have certain disadvantages.
The arcuate regions described in the Maddock patent can cause consumers to doubt the strength of the bag since those regions 35 will appear as holes in the bottom of the bag. Moreover, any liquid that may leak from the contents of that bag will then leak from the bottom of the bag. The heat seal regions of the 21450~
Prader and Benoit patent requires a costly extra manufacturing step in order to achieve the stress relief feature. Finally, the use of the weakened area of the Gruentzel patent could result in a new bag having an unsightly tear after the product 5 is loaded therein. Again the consumer will be displeased with the appearance of the bag and may doubt its strength. There is a need in the art for a gusset point stress relief feature in a gusseted plastic bag that is hidden from sight and that effectively relieves the stresses generated along the bottom 10 heat seal of the bag.
Summar~ of the Invention In accordance with the present invention, there is provided a thermoplastic film bag having front and rear walls;
15 gusseted side walls connecting said front and rear walls, said side walls having an internal gusset in the lay-flat condition;
an open mouth defined by said walls; handles extending upwardly from either side of said open mouth, said handles being integral extensions of said walls; a transverse seal connecting the 20 bottoms of said walls; stress relief zones in said gusseted side walls, each zone located on the internal gusset of said side walls, said zone spaced above said transverse seal so as to relieve stress generated at the junction of said internal gusset and said transverse seal when said bag is loaded.
This invention is also concerned with a thermoplastic film bag having side walls each with an internal gusset in a lay flat condition, and a transverse seal connecting the bottoms of said walls. The bag has the improvement comprising stress relief zones in said side walls, each zone located on the internal 30 gusset of said side walls, said zone positioned above the junction of said internal gusset and said bottom seal so as to relieve the stress generated when said bag is loaded.
The bag of the present invention has the advantage of providing a stress relief feature which dramatically reduces the 35 occurrence of gusset point failure as defined herein below. The bag achieves this stress relief without any exterior evidence of its use. Thus, consumer confidence in the strength of the F-7353-L 21~504~
bag is increased.
Therefore it is an object of this invention to provide stress relief zones in a thermoplastic bag which direct bag failures away from the bag bottom seal.
It is another object of this invention to provide a stress relief feature that is not visible from the outside of a loaded bag.
It is still another object of this invention to prevent catastrophic failure of a thermoplastic bag by providing a 10 stress relief zone in a less stressed area of the bag.
Yet another object of this invention is to enable the use of thin gauge films for the manufacture of thermoplastic bags without encountering failure at the heat sealed bottom strip.
Brief Description of the Drawings Fig. 1 is an elevation view of a prior art bag.
Fig. 2 is an end view of a thermoplastic tube having oppositely disposed gussets in partially collapsed form.
Fig. 3 is an elevation view of the bag of the present 20 invention.
Fig. 4 is a detail view of the stress relief structure of the present invention.
Detailed Description of the Invention The present invention is directed to a recurring failure observed in the bag structure of the prior art as shown in Fig.
1. This structure is formed from a collapsed tube of thermoplastic film as shown in Fig. 2. In a lay-flat condition the bag has a front wall A, a rear wall D and gusseted side 30 walls represented by B and C. Each side wall is folded during manufacture to form an internal gusset E in Fig. 2 and shown as the vertical dotted lines 2 in Figure 1. As a consequence of the commonly employed method of manufacture of the prior art bags, there are four layers of film in the gusseted regions 3 35 of the bag and two layers of film in the central region 4 of the bag. This construction is fully described in U.S. Patent No.
4,812,055 to Prader and Benoit, the contents of which are - 2145~45 incorporated herein in their entirety.
The walls of the bag in Fig. 1 are connected at the bottom by a transverse heat seal 5. The intersection of the internal gussets 2 and the heat seal 5 forms two gusset points 6.
5 Between these gusset points and the outside edge of the bag there are four layers of film. Between the gusset points themselves there are two layers of film. The gusset points therefore represent the junction of four layer bag material and two layer bag material. It has been observed that as bags of 10 the prior art are loaded the gusset points act as concentrators of stress. The stress is developed as the walls of the bag expand and the internal gussets 2 unfold to accept the load.
This stress can typically build up to the point that a failure develops at the junction of the gusset point and the heat seal 15 5. The failure once developed can propagate quickly along the seal via a zippering effect causing sudden, catastrophic failure of the bag. The failure can also manifest itself as a vertical rip in either bag wall A or D. The goal of the present invention is to provide a sacrificial stress relief zone close 20 to the gusset point so as to direct the destructive forces just described away from the gusset point and into a non-critical area of the bag.
Fig. 3 shows an elevation of the bag of the present invention. In this embodiment the bag has front and rear 25 walls, gusseted side walls and handles 11 which are integral extensions of the bag walls. The walls define an open mouth 12 disposed between the walls. In a lay flat condition the depth of the gussets will vary with the size and intended use of the bag. The present invention incorporates stress relief zones 7 30 positioned near the gusset points 8. The zones are located in and occupy a portion of the internal gussets 9. The zones are positioned between the front walls A and D of the bag just above and spaced away from the heat seal 10 and are therefore not visible from the exterior of the bag. The positioning of the 35 zones is detailed in Fig. 4 which is a close-up view of one corner of the bag of the present invention. The size and location of each zone is defined by dimensions a and b which are F-7353-L 214 5 0 ~ 5 ;
discussed in more detail below. The stress relief zones 7 operate as the forces generated upon loading concentrate at the gusset points 8. Those forces are directed away from the gusset points 8 and are absorbed in the stress relief zones 7. These 5 zones tear inside the bag leaving the heat seal intact and thus maintaining the integrity of the heat seal 10. In this invention a structural defect is introduced into the bag in a nonvisible, non-critical location in such a manner that the defect grows and absorbs the potentially destructive forces that 10 could destroy the bag. The present invention achieves a stress relief function without any interruption of the heat seal 10 at the bottom of the bag. Thus the customer's confidence in the strength of the bag is increased because there is no visible defect from the outside of the bag.
In order to define the optimum qualities for dimensions a and b, a store simulation test was conducted on a number of bag structures. This standard procedure consisted of loading the bags with simulated and real grocery items, transporting the bags to simulate actual store conditions and then unloading the 20 bags. The bags were then checked for the occurrence of gusset point holes. Any resultant holes were then classified as small or medium. Small holes were defined as those less than 2 inches in size. Medium holes were defined as those 2 inches to six inches in size.
STORE SIMULATION TEST RESULTS
The store simulation test described above was conducted on five sample runs of 50 bags each. Because there are two gusset points in each bag, each sample run could have a maximum of gusset point 100 holes. The bags used were constructed of high 30 molecular weight, high density polyethylene with a wall thickness of about 13.97 microns (0.55 mil). The first sample run was conducted using a prior art bag without the stress relief zones. The remaining four samples were conducted using a variety of combinations of dimensions a and b to determine the 35 most effective size and location of the stress relief zones.
Dimension a is the width of the stress relief zone 7. Dimension b is the parallel distance from the heat seal 10 to the stress relief zone 7. In each of sample runs 2 through 5 the stress relief zone was a straight slit, parallel to the heat seal. In sample runs 1 through 4 the average bag load was 18.5 pounds.
The bag load in sample run 5 was 12.0 pounds.
The following test- results were observed:
Small Medium Total Holes Holes Holes Sample #1 80 7 87 10 Prior Art Bag Without Stress Relief Sample Run #2 24 10 34 Dim. a: 6.35mm (1/4") 15 Dim. b: 3.12mm (1/8") Sample Run #3 1 0 Dim. a: 12.7mm (1/2") 20 Dim. b: 3.12mm (1/8") Sample Run #4 0 4 4 Dim. a: 12.7mm (1/2") Dim. b: 9.53mm (3/8") Sample Run #5 1 2 3 Dim. a: 12.7mm (1/2") Dim. b: 9.53mm (3/8") The test results clearly demonstrate that gusset point failures are dramatically reduced using any embodiment of the stress relief zones of the present invention. Optimum performance was obtained using a width of 12.7mm (1/2 inch) and 35 a parallel spacing from the heat seal of 3.18mm (1/8 inch).
Further testing has indicated that the present invention will function using a parallel spacing of up to about 7.6 cm (3 in).
As will be readily appreciated by a person of ordinary skill in the art, many other combinations of those width and spacing 40 dimensions are possible and may be desirable depending on the size and the planned use of the bag. The stress relief zone of this invention will preferably not extend along the entire width of the internal gusset.-A stress relief zone in the form of a straight slit was 45 used for the testing described above. However, it is alsocontemplated that other structures can also perform adequately.
In a nonlimiting example, a perforation can be used in the stress relief zone. The advantage of a perforation is the ability to achieve a controlled failure of the stress relief zone. The rate of the sacrificial failure of the stress relief 5 zone may be controlled by changing the width of the bag material between perforations. As that width increases, the rate of failure rate increases providing a more gradual and controlled associated stress relief. An additional advantage of the perforation is that the amount of tear of the perforation would 10 match the load placed in the bag. Again, the optimum combination of zone width and distance from the heal seal may vary with bag size and intended use.
Another nonlimiting example of a stress relief zone configuration is an arcuate area in the internal gusset. Such 15 an arcuate area would start on the internal gusset, extend away from the internal gusset towards the outside edge of the bag and return to the internal gusset. Size and positioning of the arcuate area may vary depending on the size and intended use of the bag.
In one embodiment the bags of the present invention are fabricated using a blown-film extrusion process. In this process a molten polymeric material of any type described below is extruded through the annulus of a concentric die using extrusion equipment well-known in the art. Air pressure 25 introduced inside the resulting tube expands the tube to its ungusseted diameter and cools the tube as it is drawn from the extruder. The expanded molten material, referred to as a bubble, is further cooled to a solid state by air rings positioned around the exterior circumference of the bubble. The bubble is 30 then collapsed and collected for further processing. The collapsed tube then passes through a cutting or perforating mechanism which creates the stress relief zones bag lengths apart along the tube. The novel tube is then gusseted and processed into bags using technology well-known in the art.
While bags incorporating the present invention can be made of any thermoplastic material, polyethylene and polyethylene blends are preferred. The term polyethylene is employed herein - F-~353-L ~145~5 g in its generic sense to include low density polyethylene (LDPE) having a density of from about 0.910-0.939, linear low density polyethylene (LLDPE), which is a copolymer of ethylene and an alpha olefin, having density ranging from about 0.910-0.939, 5 high molecular weight, high density polyethylene (HDPE) having a density ranging from about 0.940-0.970 and any blends thereof.
A preferred material for handled grocery sacks is a blend of LLDPE and LDPE with the latter being present in about 0-20% by weight. When employing this material, the film gauge can range 10 from 0.3 to 1 mil or more in thickness. Another preferred polyethylene resin is high density polyethylene (HDPE) alone or in combination with from 0 to 50 weight % of LLDPE. A preferred combination is a blend of the two which would yield a density of from 0.935-0.955. When film of this density is employed, 15 unusually strong film having a gauge thickness of from 0.3 to 1 mil can be employed in forming the thermoplastic sacks contemplated by the present invention.
Although the present invention has been described with preferred embodiments, it is to be understood that modifications 20 and variations may be ut-ilized without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims.
-Field of the Invention The present invention relates to thermoplastic bags having 5 a stress relief area located in the internal gussets so as to direct stresses generated during loading and carrying away from the bag gusset points.
Background of the Invention Thermoplastic bags are steadily gaining acceptance where consumer goods of all types must be bundled up for transport away from the point of sale. These bags are used in many different types of retail settings to include grocery stores, department stores of all types, building supply stores and any 15 other setting where a lightweight, strong, easy to dispense bag, is required. The advantages of plastic bags over kraft paper bags are numerous. They are lighter in weight, take up less room when folded, resist water, and may be fabricated with integral handles that provide for easy transport of the loaded 20 bag. They are reusable as trash can liners or can be reused to carry other items. Thermoplastic bags are also recyclable.
Thermoplastic bags are commonly formed by collapsing a tube of plastic so as to have two in-folded gussets at opposite sides thereof; forming two spaced seals positioned transverse of the 25 collapsed tube; and removing from the end thereof a u-shaped segment which simultaneously forms two integral handles and a bag mouth opening. This configuration and the in-folded gussets create handles which have double film thicknesses and which provide greater carrying strength in the handles.
The evolution of the thermoplastic bags has included a gradual reduction of the thickness of the wall material of the bags. Advances in the field of polymer science have enabled manufacturers to maintain and even increase bag strength using ever decreasing wall thicknesses.
An important result of this evolution has been the occurrence of failures along the transverse heat strip seal at the bottom of the bag. In the region of the in-folded gusset, F-7353-L ~14 5 0 ~ 5 four layers of film are brought together in the outer regions of the lay-flat bag structure and, in-between, two layers of the front and back walls of the bag are brought together. Thus, the heat seal must simultaneously weld four layers together in the 5 out-board segments of the bag in its lay-flat condition and two layers in the central region of the bag. It has been found that the bags often fail by tearing open in the region of the heat seal portion of the bottom of the bag beginning at the seal transition from four layers to two layers.
Numerous attempts have been made to solve this problem.
U.S. Patent No. 4,613,988 to Maddock discloses the use of arcuate stress relief regions at the four-layer/two-layer junction. The arcuate regions result from the removal of the film area below a curve extending from a point at the bottom of i5 the four-layer region to a point at the bottom of the two-layer region.
A different approach is disclosed in U.S. Patent No.
4,812,055 to Prader and Benoit. That invention utilizes circular seals located slightly above the transverse seal line 20 and inside the in-folded gusset on both sides of the bag. The seal areas relieve strain at the weak regions at the bottom of the bag.
U.S. Patent No. 3,485,437 to Gruentzel et al. discloses the use in a bag of a weakened area positioned perpendicular to a 25 welded heat seal so as to interrupt a tear along the seam. The weakened area can take the form of an aperture, score line or slit for the purpose of redistributing stress forces over a wide area. In all embodiments of this invention, it is required that some interruption of the heat formed seam take place to provide 30 stress relief.
While each of the inventions described above addresses the problem of bag failure, they each have certain disadvantages.
The arcuate regions described in the Maddock patent can cause consumers to doubt the strength of the bag since those regions 35 will appear as holes in the bottom of the bag. Moreover, any liquid that may leak from the contents of that bag will then leak from the bottom of the bag. The heat seal regions of the 21450~
Prader and Benoit patent requires a costly extra manufacturing step in order to achieve the stress relief feature. Finally, the use of the weakened area of the Gruentzel patent could result in a new bag having an unsightly tear after the product 5 is loaded therein. Again the consumer will be displeased with the appearance of the bag and may doubt its strength. There is a need in the art for a gusset point stress relief feature in a gusseted plastic bag that is hidden from sight and that effectively relieves the stresses generated along the bottom 10 heat seal of the bag.
Summar~ of the Invention In accordance with the present invention, there is provided a thermoplastic film bag having front and rear walls;
15 gusseted side walls connecting said front and rear walls, said side walls having an internal gusset in the lay-flat condition;
an open mouth defined by said walls; handles extending upwardly from either side of said open mouth, said handles being integral extensions of said walls; a transverse seal connecting the 20 bottoms of said walls; stress relief zones in said gusseted side walls, each zone located on the internal gusset of said side walls, said zone spaced above said transverse seal so as to relieve stress generated at the junction of said internal gusset and said transverse seal when said bag is loaded.
This invention is also concerned with a thermoplastic film bag having side walls each with an internal gusset in a lay flat condition, and a transverse seal connecting the bottoms of said walls. The bag has the improvement comprising stress relief zones in said side walls, each zone located on the internal 30 gusset of said side walls, said zone positioned above the junction of said internal gusset and said bottom seal so as to relieve the stress generated when said bag is loaded.
The bag of the present invention has the advantage of providing a stress relief feature which dramatically reduces the 35 occurrence of gusset point failure as defined herein below. The bag achieves this stress relief without any exterior evidence of its use. Thus, consumer confidence in the strength of the F-7353-L 21~504~
bag is increased.
Therefore it is an object of this invention to provide stress relief zones in a thermoplastic bag which direct bag failures away from the bag bottom seal.
It is another object of this invention to provide a stress relief feature that is not visible from the outside of a loaded bag.
It is still another object of this invention to prevent catastrophic failure of a thermoplastic bag by providing a 10 stress relief zone in a less stressed area of the bag.
Yet another object of this invention is to enable the use of thin gauge films for the manufacture of thermoplastic bags without encountering failure at the heat sealed bottom strip.
Brief Description of the Drawings Fig. 1 is an elevation view of a prior art bag.
Fig. 2 is an end view of a thermoplastic tube having oppositely disposed gussets in partially collapsed form.
Fig. 3 is an elevation view of the bag of the present 20 invention.
Fig. 4 is a detail view of the stress relief structure of the present invention.
Detailed Description of the Invention The present invention is directed to a recurring failure observed in the bag structure of the prior art as shown in Fig.
1. This structure is formed from a collapsed tube of thermoplastic film as shown in Fig. 2. In a lay-flat condition the bag has a front wall A, a rear wall D and gusseted side 30 walls represented by B and C. Each side wall is folded during manufacture to form an internal gusset E in Fig. 2 and shown as the vertical dotted lines 2 in Figure 1. As a consequence of the commonly employed method of manufacture of the prior art bags, there are four layers of film in the gusseted regions 3 35 of the bag and two layers of film in the central region 4 of the bag. This construction is fully described in U.S. Patent No.
4,812,055 to Prader and Benoit, the contents of which are - 2145~45 incorporated herein in their entirety.
The walls of the bag in Fig. 1 are connected at the bottom by a transverse heat seal 5. The intersection of the internal gussets 2 and the heat seal 5 forms two gusset points 6.
5 Between these gusset points and the outside edge of the bag there are four layers of film. Between the gusset points themselves there are two layers of film. The gusset points therefore represent the junction of four layer bag material and two layer bag material. It has been observed that as bags of 10 the prior art are loaded the gusset points act as concentrators of stress. The stress is developed as the walls of the bag expand and the internal gussets 2 unfold to accept the load.
This stress can typically build up to the point that a failure develops at the junction of the gusset point and the heat seal 15 5. The failure once developed can propagate quickly along the seal via a zippering effect causing sudden, catastrophic failure of the bag. The failure can also manifest itself as a vertical rip in either bag wall A or D. The goal of the present invention is to provide a sacrificial stress relief zone close 20 to the gusset point so as to direct the destructive forces just described away from the gusset point and into a non-critical area of the bag.
Fig. 3 shows an elevation of the bag of the present invention. In this embodiment the bag has front and rear 25 walls, gusseted side walls and handles 11 which are integral extensions of the bag walls. The walls define an open mouth 12 disposed between the walls. In a lay flat condition the depth of the gussets will vary with the size and intended use of the bag. The present invention incorporates stress relief zones 7 30 positioned near the gusset points 8. The zones are located in and occupy a portion of the internal gussets 9. The zones are positioned between the front walls A and D of the bag just above and spaced away from the heat seal 10 and are therefore not visible from the exterior of the bag. The positioning of the 35 zones is detailed in Fig. 4 which is a close-up view of one corner of the bag of the present invention. The size and location of each zone is defined by dimensions a and b which are F-7353-L 214 5 0 ~ 5 ;
discussed in more detail below. The stress relief zones 7 operate as the forces generated upon loading concentrate at the gusset points 8. Those forces are directed away from the gusset points 8 and are absorbed in the stress relief zones 7. These 5 zones tear inside the bag leaving the heat seal intact and thus maintaining the integrity of the heat seal 10. In this invention a structural defect is introduced into the bag in a nonvisible, non-critical location in such a manner that the defect grows and absorbs the potentially destructive forces that 10 could destroy the bag. The present invention achieves a stress relief function without any interruption of the heat seal 10 at the bottom of the bag. Thus the customer's confidence in the strength of the bag is increased because there is no visible defect from the outside of the bag.
In order to define the optimum qualities for dimensions a and b, a store simulation test was conducted on a number of bag structures. This standard procedure consisted of loading the bags with simulated and real grocery items, transporting the bags to simulate actual store conditions and then unloading the 20 bags. The bags were then checked for the occurrence of gusset point holes. Any resultant holes were then classified as small or medium. Small holes were defined as those less than 2 inches in size. Medium holes were defined as those 2 inches to six inches in size.
STORE SIMULATION TEST RESULTS
The store simulation test described above was conducted on five sample runs of 50 bags each. Because there are two gusset points in each bag, each sample run could have a maximum of gusset point 100 holes. The bags used were constructed of high 30 molecular weight, high density polyethylene with a wall thickness of about 13.97 microns (0.55 mil). The first sample run was conducted using a prior art bag without the stress relief zones. The remaining four samples were conducted using a variety of combinations of dimensions a and b to determine the 35 most effective size and location of the stress relief zones.
Dimension a is the width of the stress relief zone 7. Dimension b is the parallel distance from the heat seal 10 to the stress relief zone 7. In each of sample runs 2 through 5 the stress relief zone was a straight slit, parallel to the heat seal. In sample runs 1 through 4 the average bag load was 18.5 pounds.
The bag load in sample run 5 was 12.0 pounds.
The following test- results were observed:
Small Medium Total Holes Holes Holes Sample #1 80 7 87 10 Prior Art Bag Without Stress Relief Sample Run #2 24 10 34 Dim. a: 6.35mm (1/4") 15 Dim. b: 3.12mm (1/8") Sample Run #3 1 0 Dim. a: 12.7mm (1/2") 20 Dim. b: 3.12mm (1/8") Sample Run #4 0 4 4 Dim. a: 12.7mm (1/2") Dim. b: 9.53mm (3/8") Sample Run #5 1 2 3 Dim. a: 12.7mm (1/2") Dim. b: 9.53mm (3/8") The test results clearly demonstrate that gusset point failures are dramatically reduced using any embodiment of the stress relief zones of the present invention. Optimum performance was obtained using a width of 12.7mm (1/2 inch) and 35 a parallel spacing from the heat seal of 3.18mm (1/8 inch).
Further testing has indicated that the present invention will function using a parallel spacing of up to about 7.6 cm (3 in).
As will be readily appreciated by a person of ordinary skill in the art, many other combinations of those width and spacing 40 dimensions are possible and may be desirable depending on the size and the planned use of the bag. The stress relief zone of this invention will preferably not extend along the entire width of the internal gusset.-A stress relief zone in the form of a straight slit was 45 used for the testing described above. However, it is alsocontemplated that other structures can also perform adequately.
In a nonlimiting example, a perforation can be used in the stress relief zone. The advantage of a perforation is the ability to achieve a controlled failure of the stress relief zone. The rate of the sacrificial failure of the stress relief 5 zone may be controlled by changing the width of the bag material between perforations. As that width increases, the rate of failure rate increases providing a more gradual and controlled associated stress relief. An additional advantage of the perforation is that the amount of tear of the perforation would 10 match the load placed in the bag. Again, the optimum combination of zone width and distance from the heal seal may vary with bag size and intended use.
Another nonlimiting example of a stress relief zone configuration is an arcuate area in the internal gusset. Such 15 an arcuate area would start on the internal gusset, extend away from the internal gusset towards the outside edge of the bag and return to the internal gusset. Size and positioning of the arcuate area may vary depending on the size and intended use of the bag.
In one embodiment the bags of the present invention are fabricated using a blown-film extrusion process. In this process a molten polymeric material of any type described below is extruded through the annulus of a concentric die using extrusion equipment well-known in the art. Air pressure 25 introduced inside the resulting tube expands the tube to its ungusseted diameter and cools the tube as it is drawn from the extruder. The expanded molten material, referred to as a bubble, is further cooled to a solid state by air rings positioned around the exterior circumference of the bubble. The bubble is 30 then collapsed and collected for further processing. The collapsed tube then passes through a cutting or perforating mechanism which creates the stress relief zones bag lengths apart along the tube. The novel tube is then gusseted and processed into bags using technology well-known in the art.
While bags incorporating the present invention can be made of any thermoplastic material, polyethylene and polyethylene blends are preferred. The term polyethylene is employed herein - F-~353-L ~145~5 g in its generic sense to include low density polyethylene (LDPE) having a density of from about 0.910-0.939, linear low density polyethylene (LLDPE), which is a copolymer of ethylene and an alpha olefin, having density ranging from about 0.910-0.939, 5 high molecular weight, high density polyethylene (HDPE) having a density ranging from about 0.940-0.970 and any blends thereof.
A preferred material for handled grocery sacks is a blend of LLDPE and LDPE with the latter being present in about 0-20% by weight. When employing this material, the film gauge can range 10 from 0.3 to 1 mil or more in thickness. Another preferred polyethylene resin is high density polyethylene (HDPE) alone or in combination with from 0 to 50 weight % of LLDPE. A preferred combination is a blend of the two which would yield a density of from 0.935-0.955. When film of this density is employed, 15 unusually strong film having a gauge thickness of from 0.3 to 1 mil can be employed in forming the thermoplastic sacks contemplated by the present invention.
Although the present invention has been described with preferred embodiments, it is to be understood that modifications 20 and variations may be ut-ilized without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims.
Claims (16)
1. A thermoplastic film bag comprising:
(a) continuous front and rear walls;
(b) continuous gusseted side walls connecting said front and rear walls, said side walls having internal gussets in a lay-flat condition;
(c) an open mouth defined by said walls;
(d) handles extending upwardly from both sides of said open mouth, said handles being integral extensions of said walls;
(e) a transverse seal connecting the bottoms of said walls;
(f) gusset points formed at the intersection of said internal gussets and said transverse seal;
(g) stress relief zones in said gusseted side walls, said zones occupying a portion of the transverse width of said internal gusset, said zones spaced above said transverse seal so as to sacrificially deform and thereby direct stresses generated at said gusset points away from said seal when said bag is loaded.
(a) continuous front and rear walls;
(b) continuous gusseted side walls connecting said front and rear walls, said side walls having internal gussets in a lay-flat condition;
(c) an open mouth defined by said walls;
(d) handles extending upwardly from both sides of said open mouth, said handles being integral extensions of said walls;
(e) a transverse seal connecting the bottoms of said walls;
(f) gusset points formed at the intersection of said internal gussets and said transverse seal;
(g) stress relief zones in said gusseted side walls, said zones occupying a portion of the transverse width of said internal gusset, said zones spaced above said transverse seal so as to sacrificially deform and thereby direct stresses generated at said gusset points away from said seal when said bag is loaded.
2. The bag of claim 1 wherein said zone is a film slit.
3. The bag of claim 2 wherein said zone is spaced from 3.1mm to 9.5mm above said heat seal.
4. The bag of claim 2 wherein said zone is from 6.3mm to 12.7mm in width.
5. The bag of claim 1 wherein said zone is a perforated area, said perforated area having a width of bag film material located between perforations.
6. The bag of claim 5 wherein said zone is spaced from 3.1mm to 9.5mm above said heat seal.
7. The bag of claim 5 wherein said zone is from 6.35mm to 12.7mm in width.
8. The bag of claim 5 wherein said deformation is controlled by varying the width of bag material between said perforations.
9. In a thermoplastic film bag having a front wall, a rear wall and side walls, said side walls having an internal gusset in a lay flat condition and a transverse heat seal connecting the bottoms of said walls, said internal gusset forming a gusset point at the junction of said internal gusset and said heat seal, the improvement comprising:stress relief zones located in the internal gusset of said side walls, said zone positioned adjacent to the junction of said internal gusset and said bottom seal so as to sacrificially deform and relieve the stress generated when said bag is loaded.
10. The bag of claim 9 wherein said zone is a film slit.
11. The bag of claim 10 wherein said zone is spaced from 3.12mm to 9.53mm above said heat seal.
12. The bag of claim 10 wherein said zone is from 6.35mm to 12.7mm in width.
13. The bag of claim 9 wherein said zone is a perforated area, said perforated area having a width of bag film material located between perforations.
14. The bag of claim 13 wherein said zone is spaced from about 3.12mm to about 9.53mm above said heat seal.
15. The bag of claim 13 wherein said zone is from 6.35mm to 12.7mm in width.
16. The bag of claim 13 wherein said deformation is controlled by varying the width of bag material between said perforations.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US24191794A | 1994-05-12 | 1994-05-12 | |
| US08/241,917 | 1994-05-12 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2145045A1 true CA2145045A1 (en) | 1995-11-13 |
Family
ID=22912702
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2145045 Abandoned CA2145045A1 (en) | 1994-05-12 | 1995-03-20 | Thermoplastic bag having gusset point stress relief |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2145045A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US11267618B1 (en) | 2016-09-16 | 2022-03-08 | Robert DeMatteis | Algorithmic construction of a plastic bag |
-
1995
- 1995-03-20 CA CA 2145045 patent/CA2145045A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US11267618B1 (en) | 2016-09-16 | 2022-03-08 | Robert DeMatteis | Algorithmic construction of a plastic bag |
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