METHOD AND APPARATUS FOR PRODUCING A RECLOSABLE BAG FILM

Abstract: A method and apparatus is disclosed for producing a reclosable bag film. The bag film is produced by extruding a film web having a relatively thinner first portion and relatively thicker second portions which are formed unitarily with the first portion. Closure members preferably in the form of male and female fastener profiles, are formed separately from the film web and joined to the thicker second portions before the thicker second portions have become cooled to an ambient temperature.

Description: The present invention relates to reclosable plastic bags, and more particularly to a method and apparatus for joining closure members to a bag film web. and the reclosable bag film formed thereby. Such a method and apparatus is known from US-A- 4 295 919. The reclosable plastic bag film is shown from US-A- 3 338 284. Reclosable plastic bags typically include a bag film web portion and a closure member portion. The bag film web portion generally consists of a pair of opposed planar plastic sheets which are sealed on three sides to form a bag body. The closure member portion extends along the fourth side of the plastic sheets and includes a female (or groove) fastener profile and a male (or rib) fastener profile. The closure members can be interlocked by the consumer to form a sealed container, and disengaged to permit the consumer to gain access to the interior of the bag. An example of such a reclosable bag is the ZIPLOC® brand food storage bag manufactured by The Dow Chemical Company. One difficulty experienced by manufacturers of such reclosable bags involves the joining of the closure members to the bag film web portion. In order to produce a high quality reclosable bag, care must be taken to ensure that the fastener elements are positioned properly on the bag film web. Further, there should be no wrinkles or creases in the area of the bag in which the fastener profiles and bag film web are joined. Several approaches are described in the patent literature for forming reclosable bags. One approach has been to form the bag film web and the closure members in a single extrusion operation. See, e.g. Ausnit, U S -A- 3,338,284; Behr, U S -A-3,852,386 and Naito, U S -A- Re29,208. A second approach utilizes an adhesive, such as a strip of tape or glue, to join a closure member to a bag film web. For example, Ausnit, U.S. Patent No. 4,101,355 discloses a reclosable bag film forming process wherein a web and closure member are joined by a liquid adhesive which is supplied by an applicator having a nozzle. After the adhesive is applied, the web and closure members are pressed together by a presser roll and a backing roll. See also, Ausnit, U S -A- 3,226,787. A third approach involves the use of a preformed bag film web and a preformed closure member which are drawn from separate stock rolls. The bag film web and/or closure member are then heated and joined. For example, Howard, U S -A- 3,846,209 discloses a method for producing a closure member-containing bag film wherein a preformed plastic film is fed from a stock roll into a sealing station. A separate closure strip from a second stock roll is also fed into the sealing station. In the sealing station, the closure strip and plastic film are welded together by a pair of heating bars. In a fourth approach, a preformed bag film web is drawn from a supply roll and then joined to a relatively freshly extruded closure member. For example, Noguchi, U S -A- No. 3,904,468 discloses a method of making a reclosable bag film wherein a web of bag film material is unwound from a stock roll and then transferred to a heating station having a heating roll on which the film is heated to a desired temperature. The film is then transferred to a heated joining roll wherein the film is joined to the freshly extruded fastener profile strips. See also, Takahashi, U S -A- 4,279,677; and Ferrell, U S -A-4,582,549. Another variation on this fourth approach is disclosed in Yano, U S -A- 4,555,282. Yano discloses a method of bonding a closure member to a bag film web wherein a closure member having a base portion and a fastener profile portion is extruded shortly before being bonded to a bag film web. Intermediate the extrusion of the closure member and its joinder to the bag film web, the fastener profile portion of the closure member is cooled (and thereby solidified and stabilized) while the base portion is left heated (and thereby remains thermoplastic) when the closure member is joined to the film. Kamp, U S -A- 4,306,924 discloses another variation wherein the film web is extruded on to a casting roll to join an already formed closure member. A fifth approach involves the extrusion of a bag film web and a closure member in close proximity to each other, and the joinder of the bag film web and closure member to each other shortly after extrusion. For example, Kamp, U S -A- 4,428,788 discloses a method for forming a reclosable bag wherein a film, a tape, and a closure member are extruded by three separate dies. The film, tape and closure member dies are positioned so that the film, tape and closure members contact each other while still at a temperature of above 200°F. A chill roll is provided for cooling the film, tape and closure members. Sutrina et al., U S -A- 4,295.919, discloses an apparatus for forming a reclosable bag film wherein a fastener profile and its base strip are extruded integrally, and then joined to a film web. The die for extruding the bag film web is disposed adjacent to the fastener die block so that the fastener profile and bag film are joined shortly after both are extruded. See also Goto, U S -A- 3,462,332 and Zieke. U S -A-4,522,678. Although the above discussed patents all disclose various methods for forming reclosable bags, room for improvement exists in the fabrication of a reclosable bag which can be manufactured to consistently high quality standards. Accordingly, it is an object of the present invention to provide a method and apparatus which can produce high quality reclosable bag film webs consistently. In accordance with the present invention, a plastic bag film is provided for producing a reclosable bag comprising: a film web including a relatively thinner first portion having relatively lesser heat retention properties, and relatively thicker second portions formed unitarily with said first portion and having relatively greater heat retention properties and a closure member, characterized in that the relative thickness ratio of said second portions to said first portion is from 1.3:1 to 8:1, and the closure members are formed separately from said film web and joined to said thicker second portions before said thicker second portions are cooled to an ambient temperature. Additionally, a method is disclosed for forming plastic bag film for reclosable plastic bags by extruding a film web and closure members separately, joining the closure members to said film web by heat fusion and cooling said closure members containing bag film to fuse said closure members and said film web together, characterized in that the film web is extruded having a relatively thinner first portion with relatively lesser heat retention properties and relatively thicker second portions with relatively greater heat retention properties, and that the film web is precooled before the extruded closured members are joined to said thicker second portions, whereby the thicker second portions have considerable residual heat and have not yet cooled to an ambient temperature and wherein any additional heat to cause the fusion between the closure member and the thicker second portion is provided by the heat of the freshly extruded closure member. Further an apparatus is provided for producing an integrated bag film having closure members comprising a first extrusion means for extruding a film web, second extrusion means including an extrusion slot for extruding closure members and a chilled roll for cooling said closure members and said film web to fuse said closure members and said film web together, characterized in that the first extrusion means includes a generally continuous extrusion slot having a relatively thinner first portion and relatively thicker second portions for extruding a film web having a relatively thinner first portion and relatively thicker second portions, said film web is passed around a first casting roll before said film web encounters a joining means positioned for joining said closure members to said second portions,of said film web before said second portions have become cooled to an ambient temperature, and in that said joining means comprises a roller means positioned for receiving said film web and said closure members in a properly aligned relation. One feature of the present invention is that the thickened portion of the bag film web retains considerable residual heat at the time when the fastener profile is applied. This retained residual heat has the advantage of facilitating the adhesion of the freshly extruded fastener profile to the bag film by reducing the energy required to soften the thickened portion sufficiently to achieve a fusion between the fastener profile and the thickened portion. Another feature of the present invention is that as the fastener profile is extruded separately from the bag film web, the material used for the fastener profile can be different than the material used for the bag film web. This feature has the advantage of providing a manufacturer with greater flexibility in his choice of materials. Thus, the choice of material for use with a bag film web is not constrained by material requirements imposed by the closure member. These and other features of this invention and their inherent advantages will become apparent to those skilled in the art from the following description of preferred embodiments and the accompanying drawings illustrating the best mode of carrying out the invention. Figure 1 is a schematic view of the reclosable bag forming apparatus of the present invention; Figure 2 is an end view of an extrusion die block for extruding a bag film web of the present invention; Figure 3 is a sectional view of the bag film web of the present invention; Figure 4 is a perspective view of the closure member extrusion and joining station of the present invention; Figure 5 is a sectional view taken along lines 5-5 of Figure 4, and Figure 6 is a sectional view taken along lines 6-6 of Figure 4. An apparatus 10 is shown in Figure 1 for producing a reclosable bag film by the method of the present invention. The apparatus 10 includes an extruder 12 for extruding a bag film web. The extruder 12 includes a hopper 14 for receiving plastic material, a body 16 which contains a heating means (not shown) for heating the plastic material into a molten state, and a screw auger (not shown) for advancing the molten material to and through die block 18. The die block 18 and the film extruded from the die block 18 are best shown in Figures 2 and 3 respectively. The die block 18 includes a generally continuous die slot 22 having a relatively thinner first portion 24 and a pair of relatively thicker second portions 28 and 30 disposed at either end of the relatively thinner portion 24. The first portion 24 and second portions 28 and 30 each have a generally contoured cross section. Preferably, the first portion 24 generally has a thickness A of from 0.5 to 1 mm and a width B generally equal to slightly more than twice the height of the body portion of a finished bag assembly (not shown). Preferably, the thickness D of the second portions 28, 30 is from 0.75 to 2 mm and preferably 1.1 mm. The ratio of thickness of the second portions 28, 30 to the first portion 24 (D:A) may be in the range of from 1.5:1 to 2:1 and preferably about 1.75:1. The width E of the second portions is preferably in the range of from 1.8 to 3.2 cm each and preferably about 2.5 cm. The adjacent surfaces 38 of each of the second portions 28, 30 may contain a plurality of grooves 40, the purpose of which will be explained below. The bag film web 42 extruded from the die block 18 is shown in Figure 3 as having a cross-sectional shape similar to the cross-sectional shape of the die slot 22 from which the bag film web 42 is extruded. The bag film web 42 includes a relatively thinner first portion 44 which is extruded from the relatively thinner first portion 24 of the die slot 22, and a pair of relatively thicker second portions 46 and 48 which are extruded respectively from the relatively thicker second portions 28 and 30 of the die slot 22. Typically, the thickness H of the first portion 44 is from 0.025 to 0.075 mm and the thickness J of the second portions 46, 48 is from 0.075 to 0.2 mm, depending on the type of bag being made from the bag film web 42. The ratio of the thickness J of the second portions 46, 48 to the thickness H of the first portion 44 is in the range of from 1.3:1 to 8:1, and preferably is about 3:1. The width F of the first portion 44 and the width G of the second portions 46, 48 are slightly less than the widths B, E of the respective first and second portions 24 and 28, 30, of the die slot 22. The thickness H of the first portion 44 and the thickness J of the second portions 46 and 48 of the bag film web 42 are considerably less than the respective thicknesses A and D of the die slot 22. The thicknesses H and J are typically much less than the thickness of the respective first and second portions of the die slot 22. This reduction in thickness is due to the fact that a 10:1 to 30:1 draw down ratio is typically utilized in the manufacture of reclosable plastic bags. It will be appreciated that the exact draw down ratio utilized for the production of any particular reclosable bag is often a matter of choice, with the choice being dictated by the characteristics desired of the bag so formed. For example, a higher draw down ratio (e.g. 30:1) might be used for the production of a bag film web for sandwich bags having a thickness of 0.025 mm from a die slot having a thickness of 0.75 mm, whereas a lower draw down ratio (e.g. 10:1) could be used for the production of freezer bags having a thickness of 0.075 mm from the same die slot having a thickness of 0.75 mm. The adjacent surface 51 of the second portions 46, 48 of bag film web 42 contains a plurality of parallel, raised ridges 52, which are formed by the grooves 40 in die slot 22. In the finished bag assembly, these ridges 52 serve as engaging means for aiding the user in disengaging the fastener profiles. Referring now to Figure 1, the film web 42 emerges from die block 18 and passes around a driven rotatable casting roll 54. The casting roll 54 drives the web 42 along its path and aids in the cooling of the bag film web 42. The casting roll 54 can cool the first portion 44 and second portions 46, 48 below their crystallization points. Notwithstanding this crystallization, the relatively thicker second portions 46, 48, will still retain substantial residual heat after emerging from the casting roll 54 due to their relatively lower surface to volume ratio as compared to the thinner first portion 44. The amount of cooling imparted by the casting roll 54 to the bag film web 42 is related to the temperature of the perimetrical surface of the casting roll 54 and the amount of time the bag film web 42 is in contact with the casting roll 54. The size chosen for the casting roll 54 used should be large enough to foster the solidification of the first portion 44 of the bag film web, yet small enough so as not to cool the second portions 46, 48 to an ambient temperature. The bag film web 42 then passes around idler rollers 62 and 64 and travels to a closure member extrusion and joining station 68. The extrusion and joining station 68 is best shown in Figure 4 as including a first extruder 70 for extruding a female fastener profile 72, and a second extruder 74 for extruding a male fastener profile 76. As the fastener profiles 72, 76 are extruded separately from the bag film web 42, it will be appreciated that the fastener profiles 72 and 76 need not be formed of the same material as the bag film web 42. For example, the fastener profiles 72 and 76 need not be formed of the same material as the bag film web 42. For example, the fastener profiles 72, 76 can be extruded from a first polyethylene material, and the bag film web 42 from a second different polyethylene material. As the desired characteristics of the material chosen for the bag film web 42 may be different than the characteristics desired of the material and for the fastener profiles 72, 76, the separate extrusion provided by the instant invention permits the manufacturer to optimize his choice of materials for each of the bag film web 42 and fastener profiles 72, 76 without being forced to choose a "compromise" material having some characteristics desirable for a bag film web and some characteristics desirable for a closure member. As shown in Figure 4, the female and male fastener profiles 72, 76 are also extruded separately from each other. Thus, if so desired, the manufacturer can also choose to make the two fastener profiles 72, 76 from different materials. Alternately, the fastener profiles 72, 76 can be produced from a single extruder (not shown) having a pair of spaced die slots for extruding the fastener profiles 72, 76. The shape of the extruded fastener profiles 72, 76 is best shown in Figure 5. After emerging from their extruders 70, 74 the fastener profiles 72, 76 are joined to the upper surfaces 79, 80 of the thicker second portions 46, 48 of the bag film web 42 as the bag film web 42 passes around a idler roller 77. The positioning of the path of travel of the bag film web 42, the extruder and the idler roller 77 ensure that the fastener profiles 72, 76 are properly aligned on the respective thicker second portions 46, 48 of the bag film web 44. Due to the relatively short distance between the die slot of the extruders 70, 74 and the point at which the fastener profiles 72, 76 engage the second portions 46, 48 of the bag film web 42, the fastener profiles 72, 76 will generally maintain their proper rotational alignment to ensure that the lower surface 81 of female fastener profile 72 will be engaged with the upper surface 79 of the thickened second portion 46, and the open upper end 82 of the female fastener profile 72 will face away from upper surface 79. Similarly, the lower surface 83 of the tail of the male fastener profile 76 will be engaged with the upper surface 80 of thickened second portion 48, and the apex 84 of the male fastener profile 76 will point away from upper surface 80. The heat of the freshly extruded fastener profiles 72, 76 helps to fuse the fastener profiles to the second portions 46, 48 causes them to cool much more slowly than the thinner first portion 44, which by the time it reaches the closure and joining station 68 may be cooled to an ambient temperature. Thus, when the fastener profiles 72, 76 are joined to the thicker second portions 46, 48, the thicker second portions 46, 48 still have considerably residual heat and have not yet cooled to an ambient temperature. This residual heat facilitates the fusion of the fastener profiles 72. 76 to the thickened second portions 46, 48. Any additional heat which may be necessary to cause the fusion between the fastener profiles 72, 76 and the thickened second portions 46, 48 is provided by the heat of the freshly extruded closure members 72, 76. Without this residual heat, it is likely that a poor bond would be formed between the fastener profiles 72, 76 and the thickened second portions 46, 48. A chilled casting roll 78 is positioned in the path of travel of the bag film web 42 to engage the bag film web 42 shortly after the fastener profiles 72, 76 are joined to the bag film web 42. The coolness of the chilled casting roll 78 causes the thickened second portions 46, 48 and fastener profiles 72, 76 to become substantially solidified, thus cementing the bond between the second portions 46, 48 and the fastener profiles 72, 76, and maintaining the fastener profiles 72, 76 and the second portions 46, 48 in their proper conformation. A cooling fluid can be passed through the casting roll 78 to maintain the outer surface of the casting roll at a desired temperature. After passing around the chilled casting roll 78, the film web 42 can pass around a series of idler and drive rollers, such as roller 86, to transport the web 42 to its desired location. Thus, an apparatus and method for forming a bag film web is provided wherein the bag film web 42 and fastener profiles 72, 76 are separately formed to permit the manufacturer to optimize his choice of materials. Further, the method and apparatus of the present invention provides for a reclosable bag film web 42 having fastener profiles 72, 76 which are well bonded and properly aligned on the bag film web 42.

Claims: A plastic bag film for producing a reclosable bag comprising a film web (42) including a relatively thinner first portion (44) having relatively lesser heat retention properties, and a relatively thicker second portions (46, 48) formed unitarily with said first portion (44) and having relatively greater heat retention properties and a closure member, characterized in that the relative thickness ratio of said second portions (46, 48) to said first portion (44) is from 1.3:1 to 8:1, and the closure members (72, 76) are formed separately from said film web (42) and joined to said thicker second portions (46, 48) before said thicker second portions (46, 48) are cooled to an ambient temperature. The bag film of Claim 1, wherein said relative thickness ratio is about 3:1. The bag film of Claim 1, wherein the film web (42) is comprised of a synthetic resinous material different than the synthetic resinous material from which the closure members (72, 76) are comprised. The bag film of Claim 1, wherein said first portion (44) has a thickness of from 0.025 mm to 0.075 mm and said second portions (46, 48) have a thickness of from 0.075 mm to 0.2 mm. The plastic bag film of Claim 1, wherein said first portion (44) is disposed between a pair of said second portions (46, 48), and said closure members (72, 76) comprise a female fastener profile (72) disposed on one of said second portions (46) and a male fastener profile (76) disposed on the other of said second portions (48), and wherein said first portion (44) has a greater surface to volume ratio than said second portions (46, 48). A method for forming plastic bag film for reclosable plastic bags by extruding a film web (42) and closure members (72, 76) separately, joining the closure members (72, 76) to said film web (42) by heat fusion and cooling said closure members (72, 76) containing bag film to fuse said closure members (72, 76) and said film web (42) together, characterized in that the film web (42) is extruded having a relatively thinner first portion (44) with relatively lesser heat retention properties and relatively thicker second portions (46, 48) with relatively greater heat retention properties, and that the film web (42) is precooled before the extruded closured members (72, 76) are joined to said thicker second portions (46, 48), whereby the thicker second portions (46, 48) have considerable residual heat and have not yet cooled to an ambient temperature and wherein any additional heat to cause the fusion between the closure member (72, 76) and the thicker second portion (46, 48) is provided by the heat of the freshly extruded closure member (72, 76). The method of Claim 6, wherein said step of extruding a film web (42) comprises the step of extruding said film web (42) from a slotted extrusion die (18) having a relatively thinner first portion (24) through which said first portion (44) of said web (42) is extruded and relatively thicker second portions (28, 30) through which said second portions (46, 48) of said film web (42) are extruded, and wherein said relatively thinner first portion (24) of said extrusion die (22) has a width of from 0.5 to 1 mm and said relatively thicker second portions (28, 30) have a width of from 0.75 to 2 mm. The method of claims 6 and 7, whereby the thicker second portions (46, 48) and the thinner first portion (44) of the film web (42) is formed in a relative thickness ratio between 1.3:1 and 8:1. An apparatus (10) for producing an integrated bag film having closure members (72, 76) comprising a first extrusion means (16) for extruding a film web (42), second extrusion means (70, 74) including an extrusion slot for extruding closure members (72, 76) and a chilled roll (78) for cooling said closure members (72, 76) and said film web (42) to fuse said closure members (72, 76) and said film web (42) together, characterized in that the first extrusion means (16) includes a generally continuous extrusion slot (22) having a relatively thinner first portion (24) and relatively thicker second portions (28, 30) for extruding a film web (42) having a relatively thinner first portion (44) and relatively thicker second portions (46, 48), said film web (42) is passed around a first casting roll (54) before said film web (42) encounters a joining means (68) positioned for joining said closure members (72, 76) to said second portions (46, 48) of said film web (42) before said second portions (46, 48) have become cooled to an ambient temperature, and in that said joining means (68) comprises a roller means (77) positioned for receiving said film web (42) and said closure members (72, 76) in a properly aligned relation. The apparatus of Claim 9, wherein said first extrusion means (16) comprises a slotted die (18), said first portion (24) of said die slot (22) being generally rectangular in cross section and having a width of from 0.5 to 1 mm, and said second portions (28, 30) of said die slot (22) being generally contoured in cross section and having a width of from 0.75 to 2 mm and a length of from 1.8 to 3.2 cm. The apparatus of Claim 9 or 10, wherein the ratio of the width of said second portions (28, 30) and said first portion (24) of said die slot (22) is from 1.5:1 to 2:1.

Inventors: GEIGER, HERBERT B., KAIN, DONALD L., JR.
Applicants: DOWBRANDS INC.
Application year: 1987
Main classification: B29C47
IPC codes: B29C65, B29D5, B29C47, B65H20, B65D33, B29L7