EP0215163B1

EP0215163B1 - Methods for preparing flat-bottom thermoplastic sack and systems therefor

Info

Publication number: EP0215163B1
Application number: EP85305690A
Authority: EP
Inventors: Gordon Leon Benoit
Original assignee: Mobil Oil AS; Mobil Oil Corp
Current assignee: Mobil Oil AS; ExxonMobil Oil Corp
Priority date: 1985-08-12
Filing date: 1985-08-12
Publication date: 1990-03-28
Anticipated expiration: 2005-08-12
Also published as: EP0215163A1; DE3576796D1; ATE51360T1

Abstract

A method and system for preparing flat bottom thermoplastic sacks comprising process steps and means for forming a tube of thermoplastic film, collapsing said tube while forming two oppositely disposed gussets therein, simultaneously or spatially sequentially opening a gusset fold on each side of said collapsed tube to their full, lay-flat extent; simultaneously or spatially sequentially forming in each of the four gusset folds, a diagonal sealed seam (20) over the portions of the tube width corresponding to the gussets along lines diagonal to the length of the tube so that the ends of the sealed seams of adjacent gusset folds are at least closely spaced or in contact so as to form an angle of at least approximately 90 DEG , returning said gusset folds to their original position so that their seal-pairs are superimposed, forming a transverse sealed seam (24) across the tube along a line which includes the inboard ends of the diagonal seams and forming preweakened transverse lines closely adjacent to said transverse sealed seam or forming a severing line along this line; removing the four double triangular regions (34) bounded by the diagonal seams, the transverse seam and the side edges of the tube; and collecting the resulting structures either while still interconnected or by stacking the severed sacks. The final structure can have handles (38) or it can be handleless.

Description

The present invention relate to methods and systems for preparing a thermoplastic sack, and, more particularly, to methods and systems for preparing such a sack having at least a substantially flat rectangular bottom.
European Patent Application 0 021 463 discloses a method of producing a web of a plurality of interconnected plastics bags with gusset folds, at least comprising a central and two outer longitudinal gusset fold edges, which bound a first and a second gusset fold part by providing a continuously supplied tubular plastics foil by heatsealing with a first transverse bottom seal which extends across the entire width of the tubular foil and moving the tubular foil over a predetermined distance to another sealing position characterised in that

a) a tubular foil is provided at both sides with at least bottom fold part seals each bottom fold part seal obtained by heatsealing a gusset fold part to the opposite outer foil layer, said bottom fold part seals extending from an outer gusset fold edge to the nearest central gusset fold edge;
b) subsequently the tubular foil as obtained under step a) is moved over a predetermined distance and immediately after having reached another sealing position a transverse bottom seal is formed by simultaneously heatsealing the opposite foil layers, extending between the central gusset fold edges and opposite bottom fold part seals to each other.

European Patent Application 0 064 321 discloses a web of a plurality of interconnected plastics bags with gusset foil formed from a tubular foil, at least comprising a central and two outer longitudinal gusset foil edges which bound a first and a second gusset fold part, each bag of said web comprising

a) a transverse bottom seal;
b) two first fold part seals at both sides of the web foil, which first fold part seals always connect an outer foil layer with an opposite gusset fold part, said first fold part seals extending between the central and outer longitudinal fold edges, in the region of said transverse bottom seal and diverging from said bottom seal to the outer fold edges;
c) second fold part seals at both sides of the web foil which second foil part seals always connect an outer foil layer with an opposite gusset fold part said second fold part seals extending between the central and outer longitudinal fold edges at the end of the bag remote from the transverse seal of this bag, said second fold part seals converging from the outer longitudinal fold edges to the transverse seal of a subsequent bag characterised in that at both sides of a bag an additional seal extends substantially parallel to the fold edges and in the region between inner and outer fold edges, said additional seals being connected with the second fold part seals and extending from said second fold part seals towards the transverse bottom seal of a subsequent bag.

The present invention is concerned with the method of continuously preparing, at least substantially flat, rectangular bottom, thermoplastic film sack comprising:

a) forming a tube of a thermoplastic film;
b) collapsing said tube while simultaneously forming therein two oppositely disposed, parallel gussets;
c) opening, simultaneously or spatially sequentially, a gusset fold on each side of said collapsed tube, to their full lay-flat extent;
d) simultaneously or spatially sequentially forming in each of the four gusset folds, a diagonal sealed seam over the portions of the tube width corresponding to the gussets along lines diagonal to the length of the tube so that the ends of the sealed seams of adjacent gusset folds are at least closely spaced or in contact so as to form an angle therebetween of at least approximately 90%;
e) returning said gusset folds to their original position so that their seal-pairs are superimposed;
f) forming a transverse sealed seam across said tube along a line which includes the inboard ends of said pairs of diagonal seams and simultaneously forming a pre-weakened transverse line closely adjacent and parallel to said transverse sealing seams; and
g) removing the four double film triangular regions bounded by said diagonal seams, said transverse seam and the side edges of said tube.

This invention also provides a system for preparing an at least substantially flat, rectangular-bottom, thermoplastic film sack comprising in combination:

a) means for forming a tube of thermoplastic film;
b) means for collapsing said tube including means for simultaneously forming therein two oppositely disposed, parallel, gussets;
c) means for opening, simultaneously or spatially sequentially, a gusset fold on each side of said collapsed tube to their full, lay-flat extent;
d) means for simultaneously or spatially sequentially forming in each of the four gusset folds a diagonal seam over the portion of the tube width corresponding to the gussets along lines diagonal to the length of the tube so that the ends of the sealed seams of adjacent gusset folds are at least closely spaced or in contact so as to form an angle of at least approximately 90_°;
e) means for returning said gusset folds to their original position so that their seal-pairs are superimposed;
f) means for forming a transverse sealed seam across said tube along a line which includes the inboard ends of said pairs of diagonal seams and means for simultaneously forming a pre-weakened transverse line closely adjacent and parallel to said transverse sealed seams; and
g) means for removing four double film triangular regions bounded by said diagonal seams said transverse seam and the side edges of said tube and while still interconnected at said pre-weakened transverse lines.
- Figure 1 is a plan view, in schematic form, illustrating a system and process sequence of one form of the present invention.
- Figure 2 is a plan view of another form of the process and system of the present invention.

As shown in Figure 1, there is illustrated a system and process sequence 50 for forming a handled, gusseted flat-bottomed thermoplastic sack. A hollow cylinder or tube of thermoplastic film 12 is in the process of being collapsed by being passed through a combined gusset forming and gusset opening means 14. This device can be of any construction which accomplishes the steps of collapsing the tube, forming oppositely disposed gussets therein and opening one fold of each gusset to the maximum so that they are spread open in a lay-flat position. By this is meant, that the gusset, which normally is foled upon itself at an acute angle approaching 0_°, is opened by flipping or turning one of the folds over so that the gusset lays flat at an angle approaching 180_°. Depending upon the size of the gussets, this step can be accomplished simultaneously. If both gussets are small enough, they can be opened completely side-by-side in a lay-flat position. As the gusset depth increases only one gusset fold can be opened at a time. As shown in Figure 1, and as suggested by the mid-fold line 16, both gussets were opened completely by the gusset forming and opening means 14. In a situation where the gussets are too large to be able to be opened side-by-side, they can be opened spatially sequentially. By this is meant, first one gusset will be opened as the tube progresses means 14 and at a point downstream in the system, after the first gusset returns to its closed position, the opposite gusset will be opened by an equivalent means to the lay-flat position. As indicated above, device 14 can be a sheet- metal device which has an entrance side for receiving a flexible tube of cylindrical film. Sheet metal plates therein cause an infolding or gusset to be fashioned in the structure as the tube passes therethrough. The exit dimension progressively diminishes to present a fully collapsed gusseted tube. This device contains turning plates or rolls so that after the gusset is formed, the upper fold is progressively opened until it assumes a lay-flat, 180_° position.
The gusset or gussets are opened so that a pair of diagonal sealed seams can be impressed in each of the four gusset folds as shown at 20. This is accomplished by means of a heat-sealing member 18 which includes Teflon-coated resistance wires heated to a temperature which will permit the two layers of each of the four gusset folds to be heat sealed together. The diagonal sealed seams extend over the portions of the tube width corresponding to the gussets along lines diagonal to the length of the tube. The seal-pairs of each gusset in the lay-flat position form an angle of approximately 90_° to one another. Thereafter, a gusset closing device 22 flips or turns the open gusset and folds it back to the closed position so that the angle therebetween again approaches 0°. In this position the sealed seams 20 of each gusset are superimposed and in registration with one another.
As the collapsed tube with its seals 20 progress through the system, there subsequently is formed a bottom seal 24 which simultaneously seals and severs the bottom of the gusseted structure. This is accomplished by means of bottom sealing and severing device 26. Seal line 24 is a transverse sealing and severing seam which extends across and through the collapsed tube along a line which in- dudes the inboard ends 28 of each pair of diagonal seams. Transverse sealing and severing device 26 can be any suitable means which will bond and sever the collapsed tube along the defined line. This sealing and severing action will isolate sack blank 30 from the collapsed and gusseted tube. Sack blank 30 will have a seal 32 across the top thereof and the four triangular regions 34 will be bounded by seal lines 20, 24 and the side edge of the gusseted structure.
The sack blank structures 30 may be separately or collectively converted into bag structures by removal of the triangular regions 34 and by the formation of bag mouth opening 36 which simultaneously forms handle loops 38, each of which are formed of two layers of film by virtue of the gusseted structure. This will complete the formation of a handled sack, the bottom of which can be planarly extended so as to have a flat bottom. By "planarly extended" is meant that the bag is extended in internal volume to its utmost by expanding the gusssets fully, thereby forming a flat-bottom, rectangular in nature, having a length larger than its width.
In a preferred embodiment of the process and system illustrated in the drawing, a linear low density polyethylene tubular film 12, of about one mil in thickness, is collapsed and simultaneously gusseted with the gussets then opened fully to approximately 180° as the tube passes through the gusset forming and opening means 14. As the gusseted tube progresses through the system, seals 20 and 24 are simultaneously formed. This action produces bottom seal 24 and top seal 30 at the same time. This also isolates sack blank 30. Sack blank 30 can then be removed to a station which can separately or simultaneously remove triangular sections 34, form bag mouth opening 36 and form handles 38. This can be accomplished on individual blanks or alternatively, blank 30 can be stacked in vertical registration and, through the means of a suitable cutting die mechanism, the triangular regions removed and the handle and bag mouth opening can be formed.
For the formation of bags which do not contain a handle, the process and system is as shown in Figure 2. The process and system 50a is different from that of Figure 1, beginning at the point where the bottom of the bag is sealed. Thus, a bottom seal and perforating mechanism 40 is shown impressing seal- seam 42 in the flattened gusseted tube and, simultaneously therewith, perforation line 44 is formed closely adjacent and parallel to seal line 42. Seal line 42 does not sever the tube, merely seals it. Seal line 42 and perforation line 44 are impressed in the gusseted tube at the same time diagonal seals 20 are impressed into the opened gussets in the next succeeding gusseted tube structure. Thereafter, by any suitable die cutting mechanism, the four triangular regions 46 are removed from the structure. This will yield sack 48 attached by perforation line 44 to a like structure. Thereafter, these sacks may be rolled upon themselves to form a cylindrical package or collected in a zig-zag folded stack arrangement. Either structure will permit a sack to be torn free along the perforation lines to yield an open mouth bag which when extended fully to the total width of the gusset, will yield a flat-bottomed sack. Instead of rolling or folding this structure, each bag may be automatically separated at the perforation line and folded or stacked. The sack resulting from the process and system of Figure 2 will have a rectangular bottom with a length larger than its width.
In a modification of the process and system shown in Figure 2, open mouth handleless structures can be formed by forming a severance transversely across the tube at the same time bottom seal 42 is formed. Thereafter, the comers of the sack blank corresponding to regions 46 can be severed to complete the bag structure.
As disclosed, the bag structure formed by the system and process illustrated in Figures 1 and 2 will yield a sack having a rectangular bottom when planarly extended, which bottom will have a length longer than its width. In other words, the sack bottom will not be a square. In order to form a square bottom sack either with or without handles the process and means merely need be modified so that the gusset folds are both infolded to the maximum. That is, the collapsed tube will be gusseted to the longitudinal mid-line of the tube. In this case the center- fold of each gusset will contact the other and the angle between the thereafter formed diagonal seams will be at least about 90_°. The resulting sack will then have a substantially flat square bottom when planarly extended.
Bags of the structures described can be formed with a pair of registered tabs as integral extensions of the bag mouth opening as shown at 39 in Figure 1. The bags can be stacked in registration to contain 50, 100, 150, etc, bags and the tabs 39 fastened together in some suitable manner. For example, the tabs can be ultrasonically welded together. A perforation line across the tab will form a convenient tear-off site for individual bags to be removed from the bag pack.
Bags of the structures described can be formed by employing any suitable thermoplastic material, for example, polyolefins. In employing the term "polyolefins" it is used generically to include all forms of the polymer species, including, for example, low density polyethylene, linear low density copolymers of ethylene and another olefin, high density polyethylene, mixtures and blends of the same, and other monomer counterparts, etc. Thermoplastic coated paper stock is also contemplated. The bag film can be of any gauge, for example, from about 0.3 to about 5 mils. in thickness.
In forming the flat-bottomed bag, it is preferred that the angle between the diagonal seal line and the bottom seal be at least approximately 135_° when said bag is in its lay-flat condition. It is also preferred that the open mouth portion of the handled sacks of the present invention have stress relief curves or arcs extending from the base of the handle to a raised portion of the bag mouth. This will militate against splitting or tears occurring in the bag mouth during handle extension or when the bag is being filled with bulky material. The handles and bag mouth opening can have any configuration.
In forming the bag structure, the pertinent angles need not be precise. For example, they need not be exactly 90_°, 45_°, or 135_°. Reasonable processing latitude should permit some deviation from these guidelines. Likewise in forming seal seams the ends thereof can be curved or angled in order to accommodate or insure good closure at all film interfaces and joints. Double or plural line seams and seals also can be employed.

Claims

1. A method of continuously preparing an at least substantially flat, rectangular-bottom thermoplastic film sack comprising:

(a) forming a tube (12) of thermoplastic film;

(b) collapsing said tube while simultaneously forming therein two oppositely disposed, parallel gussets;

(c) opening, simultaneously or spatially sequentially, a gusset fold, on each side of said collapsed tube to their full lay-flat extent;

(d) simultaneously or spatially sequentially forming in each of the four gusset folds, a diagonal sealed seam (20) over the portions of the tube width corresponding to the gussets along lines diagonal to the length of the tube so that the ends of the sealed seams of adjacent gusset folds are at least closely spaced or in contact so as to form an angle of at least approximately 90°;

(e) returning said gusset folds to their original position so that their seal-pairs are superimposed;

(f) forming a transverse sealed seam (24/42) across said tube along a line which includes the inboard ends (28) of said pairs of diagonal seams and simultaneously forming a pre-weakened transverse line closely adjacent and parallel to said transverse sealing seams; and

(g) removing the four double film triangular regions (34) bounded by said diagonal seams, said transverse seam and the side edges of said tube.

2. The method of claim 1 further comprising the additional step of (h) collecting the resulting structures while still interconnected at the pre-weakened transverse lines.

3. The method of claim 2 further comprising the additional step of (i) forming a handle (38) and sack mouth (36) opening at the opposite end of the sack bottom.

4. The method of claim 2 or 3 wherein steps (g) and (h) are reversed in order.

5. The method of any one of the preceding claims wherein said pre-weakened line is a line of perforations.

6. The method of any one of the preceding claims wherejn said pre-weakened line is a line of reduced thickness.

7. The method of any one of the preceding claims wherein said resulting structures are collected in a roll.

8. The method of any one of the preceding claims wherein said resulting structures are collected in a zig-zag stack.

9. The method of any one of the preceding claims wherein the resulting interconnected structures are individually separated and folded or stacked.

10. The method of any one of the preceding claims wherein said thermoplastic film is of a polyolefin.

11. The method of any one of the preceding claims wherein each gusset is equal and the center-fold (16) of each gusset contacts the other and the angle between said seal-pairs is at least about 90_° so that the resulting sack has an at least substantially flat, square bottom when planarly extended.

12. The method of any one of the preceding claims wherein each gusset is equal and the centerfold of each gusset is spaced one from the other and the angle between projected diagonal seam lines is at least about 90° so that the resulting sack has an at least substantially flat rectangular bottom of unequal dimensions when planarly extended.

13. The method of any one of the preceding claims wherein said two pairs of diagonal sealed seams are formed simultaneously.

14. The method of any one of the preceding claims wherein said two pairs of diagonal sealed seams and said transverse sealing and severing seams are formed simultaneously on adjacent sack structures.

15. The method of any one of the preceding claims wherein the sealed and severed structures are collected in a stack and then said four double film triangular regions are removed from said stack simultaneously.

16. The method of any one of the preceding claims wherein said rectangular-bottom is a square.

17. The method of any one of the preceding claims wherein said rectangular-bottom has an unequal length and width dimension.

18. A system for preparing an at least substantially flat, rectangular-bottom, thermoplastic film sack comprising in combination:

(a) means for forming a tube of thermoplastic film (12);

(b) means (14) for collapsing said tube including means for simultaneously forming therein two oppositely disposed, parallel, gussets;

(c) means for opening, simultaneously or spatially sequentially, a gusset fold on each side of said collapsed tube to their full, lay-flat extent;

(d) means (18) for simultaneously or spatially sequentially forming in each of the four gusset folds a diagonal seam (20) over the portion of the tube width corresponding to the gussets along lines diagonal to the length of the tube so that the ends of the sealed seams of adjacent gusset folds are at least closely spaced or in contact so as to form an angle of at least approximately 90_°;

(e) means (22) for returning said gusset folds to their original position so that their seal-pairs are superimposed;

(f) means (26) for forming a transverse sealed seam (24) across said tube along a line which in- dudes the inboard ends of said pairs of diagonal seams and means for simultaneously forming a pre-weakened transverse line closely adjacent and parallel to said transverse sealed seams; and

(g) means for removing four double film triangular regions (34/46) bounded by said diagonal seams said transverse seam and the side edges of said tube and while still interconnected at said pre-weakened transverse lines.

19. The system of claim 18 further comprising (h) means for collecting the resulting structures while still interconnected at said pre-weakened transverse lines (44).

20. The system of claim 18 or 19 further comprising (i) means for forming a pair of handles (38) and sack mouth opening (36) at the opposite end of the sack bottom.

21. The system of any one of claims 18, 19, or 20 wherein said means for forming said diagonal seams include non-stick means.

22. The system of any one of claims 18-21 wherein said means for forming said pre-weakened transverse line is a perforating means.

23. The system of any one of claims 18-22 wherein said means for removing said triangular regions is a cutting means.

24. The system of any one of claims 18-23 wherein said means for collecting said interconnecting structure is a means for forming said structures into a roll.

25. The system of any one of claims 18-24 wherein said collecting means is a means for collecting said structures in a zig-zag sack.