Seaming plastic film using solvent-based adhesive bead
View Patent ↗A solvent-based adhesive is used to form a continuous, adhesively bonded and welded seam at the longitudinal edge portions of plastic film which have been overlapped. First, a bead of the adhesive is disposed on the edge portion of a web of film passing over an applicator roll. Next, as the film moves between the applicator roll and nip rolls, the adhesive dissolves a portion of the film to form a longitudinal channel having a generally semicircular cross-section. Then the edge portion is overlapped onto the other edge portion with the bead of adhesive between them. Finally the overlapped edge portions are squeezed between the nip rolls, so that the adhesive exudes laterally outward from the groove and is distributed smoothly between the overlapped edge portions, until the adhesive and dissolved film form a uniform band having a rounded hat-shaped cross-sectional profile.
1. A method of forming a seam at overlapped first and second longitudinal edge portions of plastic film comprising the steps of
providing an applicator roll, a dispensing valve with a dispensing tip spaced closely to the circumferential surface of the applicator roll, two nip rolls downstream of the applicator roll, and at least one web of plastic film;
moving the film longitudinally over the rotating applicator roll and then between the rotating nip rolls;
dispensing, through the valve and tip, a round, continuous bead of a solvent-based adhesive, and laying that bead onto the first edge portion of the film while the film is supported by the applicator roll;
dissolving into the adhesive, while the film is moving between the applicator roll and the nip rolls, part of the first edge portion of the film so as to form therein a longitudinal channel having a generally semicircular cross-section;
overlapping the second edge portion onto the first edge portion, with the bead of adhesive between the two overlapped edge portions;
squeezing the overlapped edge portions between the nip rolls, whereby the adhesive exudes laterally outward from the channel and is distributed smoothly between the overlapped edge portions, until the adhesive and dissolved film form a uniform band having a rounded hat-shaped cross-sectional profile.
2. A method according to claim 1 wherein the first and second edge portions are the opposite edge portions of a single web of film which has been folded into a tube.
3. A method according to claim 1 wherein the first and second edge portions are the edge portions of two webs of film.
4. A method according to claim 1 wherein the film is shrinkable.
5. A method according to claim 1 wherein the film is heat-shrinkable.
6. A method according to claim 1 wherein the dispensing tip is spaced from the film by a distance of from 0 inch to about 0.060 inch.
7. A method according to claim 1 wherein the dispensing tip is spaced from the film by a distance of from about 0.001 inch to about 0.010 inch.
8. A method according to claim 1 wherein the dispensing tip is spaced from the film by a distance of about 0.005 inch.
9. A method according to claim 1 wherein the inner diameter or width of the dispensing tip is greater than 0.003 inch.
10. A method according to claim 1 wherein the dispensing tip is circular or elliptical.
11. A method according to claim 1 wherein the dispensing tip is circular and the inner diameter of the dispensing tip is from about 0.003 inch to about 0.010 inch.
12. A method according to claim 1 wherein the inner diameter of the dispensing tip is from about 0.003 inch to about 0.005 inch.
13. A method according to claim 1 wherein the dispensing tip is elliptical and the width of the tip is from about 0.003 to about 0.08 inch.
14. A method according to claim 1 wherein the film is moving at a speed of at least 50 feet per minute.
15. A method according to claim 1 wherein the time period during which the solvent dissolves the film between the applicator roll and the nip rolls is at least 0.2 seconds.
16. A method according to claim 1 wherein the time period during which the solvent dissolves the film between the applicator roll and the nip rolls is at least 0.3 seconds.
17. A method according to claim 1 wherein the width of the brim is greater than about 0.3 mm.
18. A method according to claim 1 wherein the width of the brim is from about 0.3 mm to about 3.0 mm.
19. A method according to claim 1 wherein the width of the brim is from about 0.3 mm to about 8.0 mm.
20. A method according to claim 1 which precisely controls the width of the bead and comprises the further steps of
moving the plastic film at a first speed,
sensing that first speed,
selecting a first predetermined value of fluid pressure in the delivery line which corresponds to that first speed and causes the dispensing tip to dispense a bead having a desired width and cross-sectional profile,
establishing that first predetermined fluid pressure in the delivery line,
moving the plastic film at a second, different speed while continuing to practice said method,
sensing that second speed,
selecting a second, different predetermined value of fluid pressure in the delivery line which corresponds to that second speed and causes the dispensing tip to dispense a bead having precisely the same width and cross-sectional profile, and
establishing that second predetermined fluid pressure in the delivery line.
21. A method according to claim 11 wherein said sensing, selecting, and establishing steps are performed automatically and without operator intervention.
22. A method according to claim 11 wherein the change in fluid pressure in the delivery line from the first value to the second value is sufficient to cause the bead dispensed at the second value to retain precisely the same width and profile, without any additional control of the volume of adhesive flow.
23. A method according to claim 11 wherein the first speed is less than 100 feet per minute and the second speed is greater than 500 feet per minute.