METHOD FOR PREPARING FLAT-BOTTOM THERMOPLASTIC SACK

Abstract: A method for preparing flat bottom thermoplastic sacks comprising process steps for forming a tube of thermoplastic film (14), collapsing the tube while forming two oppositely disposed gussets therein (18), forming two pairs of diagonal sealed seams in the gussets, forming two pairs of diagonal preweakened lines parallel to the diagonal seals, 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 (22) closely adjacent and parallel to the transverse sealed seam or forming a severing line along this line, removing the four double triangular regions bounded by the diagonal preweakened lines, the transverse preweakened lines and the side edges of the tube and collecting the resulting structures either while still interconnected or stacking the severed structures and then removing the triangular regions. The final structure can have handles or it can be handleless.

Description: The present invention relates to a method for preparing a thermostatic sack, and, more particularly, to a method for preparing such a sack having at least a substantially flat rectangular bottom. One type of bag contemplated for preparation by a process of the present invention is more particularly defined in US-A-4, 554, 192, granted on 19 November 1985. The bag structure of the above mentioned patent is of a thermostatic film material and it comprises front and rear bag walls connected by gussted side walls. The bag has an open mouth top portion, which is characterised by having handles located at opposite end regions thereof, the handles each being of two films as a result of being integral extension of the front, rear and gusseted side walls. The bag has a bottom wall planarly extensible so as to form a rectangle with at least no substantial excess film outside of the bulk volumetric capacity of the bottom region of the bag. An alternative manner of describing the bottom of such a bag is that the bottom is formed from integral extensions of the front, rear and side walls and the closure thereof is 4 two-film gusset-to-wall heat seals and 1 two-film front wall-to-back wall heat seals when the bottom is a rectangle. The present invention also contemplates forming the same type of bag without handles. In US-A-3748205 there is described a method and apparatus for preparing contoured bags from multiple layers of heat sealable flexible film. The bags are particularly useful to provide containers for articles having an irregular or contoured configuration, such as bananas. In EP-A-0021463 there is described a method of producing a web₂ of interconnected plastic bags, with longitudinal gusset folds. The bag include a central and two outer longitudinal gusset fold edges which bound a first and second gusset fold part. In EP-A-0092885 there is also described a plastic bag with gusset fold parts bounded by a central gusset fold edge and other outer gusset fold edges. Perforations are provided to provide ventilation to the interior of the bag. The present invention provides a method of continuously preparing an at least substantially flat, rectangular-bottom, thermoplastic film sack comprising: a) forming a tube of thermoplastic film; b) collapsing the tube while simultaneously forming therein two oppositely disposed, parallel gussets; characterised by: c) forming two pairs of diagonal sealed seams over the portions of the tube width corresponding to the gussets along lines diagonal to the length of the tube, the seal pairs of opposite sides being at an angle to one another and the seams of each pair being in registration with one another; d) simultaneously or spacially sequentially forming closely parallel to the line of the diagonal sealed seams a preweakened line in the film; e) forming a transverse sealed seam across the tube along a line which includes the inboard ends of the pairs of diagonal seams and simultaneously forming a preweakened transverse line losely adjacent and parallel to the transverse sealed seam or forming a severing line along the transverse sealed seam; f) individually separating these structures along the preweakened transverse line or the severing line and stacking them in registration; and g) removing from each bag of the stack the four double film triangular regions bounded by the preweakened lines parallel to the diagonal seam, the transverse seam and the side edges of the tube. Reference is now made to the accompanying drawings, in which: Figure 1 is a plan view, in schematic form illustrating a system and process sequence of one form of the process of the present invention. Figure 2 is a plan view of another form of the process of the present invention. As indicated in Figure 1, reference numeral 10 illustrates a system and process sequence for forming a handled, gusseted, flat bottomed thermoplastic sack. A tube of thermoplastic film 12 is in the process of being collapsed, by being passed through a gusset forming means 14. The tube collapsing and gusset-forming means 14 can be any device suitable for accomplishing these two steps substantially simultaneously. A typical gusset forming device is an open-ended box shaped arrangements of, for example, sheet metal material, which has two inwardly extending members which are brought to bear on the collapsing thermoplastic tube so as to form two oppositely disposed inwardly extending folds or gussets. The device gradually tapers to cause the gusseted tube to collapse completely. These gussets are illustrated in the drawing by dotted lines 16. The gusset forming means 14 can be prepared in a plurality of difference size gusset forming means so as to permit flexibility in the dimension of the resulting gusset in-fold. The gusset forming means can be such as to impress a gusset in a tube of anywhere from a fraction of an inch up to that providing gussets which extend all the way to the midline of a fully collapsed tube. The gusseted portion of the tube therafter traverses two Teflon coated plates 18, (or some other suitable non-stick separating surface) each of which extend into the full reach of the gusset 16. Thereafter, a sealed seam is impressed in each of the four gusset folds as shown at 20. Simultaneously a line of perforations 21 is formed parallel to each seal seam 20. This will subsequently permit the removal of all corners from a stack of bags without there being any film-loss or bag destruction due to misregistration during stacking. The Teflon plate prevents one seam from fusing to the other on each side of the tube. These seams are best formed by heat sealing members which are 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 but not severed. These diagonal sealed seams extend over the portions of the tube width corresponding to the gussets along lines diagonal to the length of the tube and the seal pairs of opposite sides are at an angle to one another, preferably at an angle of at least approximately 90°. The seams of each pair of seams on each side of the gusseted tube are in registration with one another. In other words, beneath seam 20, and on the other side of Teflon separator plate 18, there is an identical seal 20. The diagonal line of perforations parallel to the diagonal seams may also be lines of reduced tickness. The perforations or reduced thickness can be formed by any piercing or punching means or by any scoring means. As the tube, seals 20 and perforations 21 progress through the system there is subsequently formed a bottom seal 22 which simultaneously seals and severs the bottom of the gusseted structure. This is accomplished by means of bottom sealing and severing device 24. Seal line 22 is a transverse sealing and severing seam which extends across and through the collapsed tube along a line which includes the inboard ends 26 of each pair of diagonal seams. Transverse sealing and severing device 24 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 28 from the collapsed and gusseted tube. Sack blank 28 will have a seal 30 across the top thereof and 4 triangular regions 32 will be bounded by perforation lines 21, 22 and the side edge of the gusseted structure. The sack blank structures 28 may be separately or collectively converted into bag structures by removal of the triangular regions 32 and by the formation of bag mouth opening 34 which simulataneously forms handle loops 36, 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 gussets fully, thereby forming a flat bottom, rectangular in nature, having a length larger than its width. In a preferred embodiment of the process illustrated in the drawing, a linear low density polyethylene tubular film 12, of about 1 mil in thickness, is collapsed and simultaneously gusseted by passing through gusset forming means 14. This action infolds the collapsed tube to yield gussets 16. As the gusseted tube progresses through the system, seals 20 and 22 and perforation lines 21 are simultaneously formed. This action produces bottom seal 26 and top seal 30 at the same time. This also isolates sack blank 28. Sack blank 28 can then be removed to a station which can separately remove triangular sections 32 along perforation lines 21 and remove film to form bag mouth opening 34 and form handles 36 on individual blanks. Alternatively, the blanks 28 can be stacked in vertical registration and, through the means of a suitable removing mechanism, e.g., a clamp, shear, etc., the triangular regions are removed and the handle and bag mouth opening can be formed. A typical bag can have a bag front and rear wall width of about 30.5 cm (12 inches), 10.2 cm (4 inch) gusset folds, yielding 20.3 cm (8 inch) side walls and a bag length of about 50.8 to 63.5 cm (20 to 25 inches) with or without handles. For the formation of bags which do not contain a handle, the process and the system are as shown in Figure 2. The process 10a 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 38 is shown impressing seal seam 40 in the flattened gusseted tube and, simultaneous therewith, perforation line 42 is formed closely adjacent and parallel to seal line 40. Seal line 40 does not sever the tube, merely seals it. Seal line 40 and perforation line 42 are impressed in the gusseted tube at the same time diagonal seals 20 and diagonal perforation line 21 are impressed into the gussets. Diagonal perforation line 21 and transverse perforation line 42 touch or intersect for easy removal of the four triangular regions 44. After such removal, this will yield sack 46 attached by perforation line 42 to a like structure. Thereafter, these sacks may be rolled upon themselves to form a cylindrical package, collected in a zig-zag folded stack arrangement or individually separated and folded or stacked. 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. The sack resulting from the process of Figure 2 will have a rectangular bottom with a length larger than its width. In a modification of the process shown in Figure 2, open mouth handleless structures can be formed by severing each structure along transverse perforation line 42, stacking the structures with the triangular corners still attached and thereafter removal of all corners at once along diagonal perforation lines 21. Handled bags of the structure described can be formed with a pair of registered tabs as integral extensions of the bag mouth 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. The term "polyolefins" is employed generically to include all forms of the polymer species including, for example, low density polyethylene, linear low density copolymers of ethylene and another alpha 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 0.0076 to 0.13 mm (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 the 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, e.g. 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 are also contemplated. As disclosed, the bag structure formed by the 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. It is to be understood that the preweakened lines described herein can be lines of perforations, lines of reduced thickness, such as score lines, etc. It is also to be understood that the diagonal preweakened lines can be formed prior to, simultaneously with or after the formation of the diagonal seals. This will permit use of the methods described in US-A-4 571 235 and US-A-4 816 104.

Claims: 1. A method of continuously preparing an at least substantially flat, rectangular-bottom, thermoplastic film sack comprising: (a) forming a tube of thermoplastic film (12); (b) collapsing the tube (12) while simultaneously forming therein two oppositely disposed, parallel gussets (14); (c) characterised by: forming two pairs of diagonal sealed seams (20), over the portions of the tube width corresponding to the gussets (14) along lines diagonal to the length of the tube, the seal pairs of opposite sides being at an angle to one another and the seams (20) of each pair being in registration with one another; (d) simultaneously or spacially sequentially forming closely parallel to the line of the diagonal sealed seams a preweakened line (21) in the film; (e) forming a transverse sealed seams (22, 40) across the tube along a line which includes the inboard ends (26) of the pairs of diagonal seams (20, 40) and simultaneously forming a preweakened transverse line ( 42) closely adjacent and parallel to the transverse sealed seam, or forming a severing line along the transverse sealed seam (22); (f) individually separating these structures along the preweakened transverse line (42) or the severing line and stacking them in registration; and (g) removing from each bag of the stack the four double film triangular regions (32, 44) bounded by the preweakened lines (21) parallel to the diagonal seam (20), the transverse seam (22, 40) and the side edges of the tube. 2. A method according to claim 1, characterised in that the diagonal preweakened lines (21) are formed prior to the diagonal sealed seams (20). 3. A method according to claim 1, characterised in that the diagonal preweakened lines (21) are formed after the diagonal sealed seams (20). 4. A method according to Claim 1, characterised in that the preweakened lines (21) are lines of perforations. 5. A method according to claim 1, characterised in that the preweakened lines (21) are lines of reduced thickness. 6. A method according to claim 1, characterised in that the thermoplastic film is a polyolefin. 7. A method according to claim 1, characterised in that each gusset (16) is equal and the centerfold of each gusset (16) contacts the other and the angle between the diagonal seams (20) is at least about 90 degrees so that the resulting sack has an at least substantially flat square bottom when planarly extended. 8. A method according to claim 1, characterised in that each gusset (16) is equal in width and the centerfold of each gusset (16) is spaced one from the other and the angle between projected diagonal seams lines (20) is at least 90 degrees so that the resulting sack has an at least substantially flat rectangular bottom of unequal dimensions when planarly extended. 9. A method according to claim 1, characterised in that simultaneously with forming the preweakened transverse line (42), a second transverse sealed seam is formed across the tube parallel to the preweakened transverse line (42) so that the preweakened line (42) is between the closely spaced transverse sealed seams. 10. A method according to claim 9, characterised in that simultaneously with or subsequent to the removal of the four folded triangles (32), forming a handle (36) and sack mouth opening (34) at the opposite end of the stacked structure and bonding the stack together.

Inventors: BENOIT, GORDON LEON
Applicants: MOBIL OIL CORPORATION
Application year: 1986
Main classification: B31B37
IPC codes: B31B35, B31B19, B31B23, B31B37, B31B1