Process and apparatus for the production of a can body by wall ironing

Abstract

A process for the production of a can body including a base and a tubular body from sheet metal which is coated on at least one side with a polymer layer, in which process, firstly, a round disc is produced from the sheet metal, which disc is then deep-drawn into a cup which has a polymer layer at least on the outside, after which this cup is formed into a can body by wall ironing, the wall ironing taking place in a single stroke by moving the cup successively through a redraw die and one or more wall-ironing rings.

Claims

1. A process for producing a can body comprising a base and a tubular body from sheet metal which is coated on one or both sides with a polymer film, the process comprising: producing a round disc from the coated metal sheet, then deep-drawing the disc into a cup, followed by redrawing the cup and subsequently forming the redrawn cup into a can body by wall ironing, the wall ironing taking place in a single stroke by moving the redrawn cup successively through one or more wall-ironing rings by means of a punch, wherein the punch has a front end wall and sidewalls, the front end wall for contacting the base, the sidewalls defining a cylindrical front end portion with a diameter D0, a transition portion, and a rear end portion toward a rear end of the punch with a diameter D1, wherein D1<D0 and wherein the front end portion is separated from the rear end portion by the transition portion, wherein the diameter of the punch sidewalls gradually decreases over the transition portion and wherein the shape of the transition portion of the front end portion of the punch to the rear end portion is a continuous curve wherein the tapering (a) angle between the tangent of the curve and the centreline of the punch is not constant over the transition portion and wherein the first derivative of the curve has at least one inflection point in the transition portion, wherein the sheet metal is a steel sheet, wherein the tangent of the continuous curve at the connecting point between the curve and the front end portion and/or at the connecting point between the curve and the rear end portion is equal to the tangent of the front end portion and/or the rear end portion respectively to provide smooth transfer from curve to punch.

2. The process according to claim 1, wherein the tangent of the continuous curve at the connecting point between the curve and the front end portion and at the connecting point between the curve and the rear end portion is equal to the tangent of the front end portion and the rear end portion respectively to provide smooth transfer from curve to punch.

3. The process according to claim 1, wherein the coated metal sheet is obtained by means of film lamination or direct extrusion coating at least one surface of a metal sheet with an organic resin, wherein the organic resin is a polyester resin and the resin film has a thickness of 5 to 100 μm in case of a single-layer film or a total thickness of 5 to 100 μm in case of a multi-layer film.

4. The process according to claim 3, wherein the sheet metal is coated on both sides with a polymer film.

5. The process according to claim 1, wherein an entry angle (α) for the first wall-ironing ring is between 3.5 and 4.5° and an exit angle (β) for the first wall-ironing ring is between 2.5 and 3.5°.

6. The process according to claim 1, wherein no external coolant is directly applied to the can body during the wall-ironing operation.

7. The process according to claim 1, wherein the sheet metal is selected from a group of sheet metals consisting of uncoated steel sheet (blackplate), tin coated steel sheet (tinplate), chromium-chromium oxide coated steel sheet (ECCS), tinplate which was diffusion annealed to form an iron-tin alloy consisting of at least 80% of FeSn (50 at. % iron and 50 at. % tin) thereupon, chromium-chromium oxide coated steel sheet produced by electroplating from a trivalent chromium electrolyte (TCCT).

8. A wall ironing apparatus having a punch and one or more wall-ironing rings for reducing the wall thickness of a redrawn cup by forcing the redrawn cup through the one or more wall-ironing rings by the punch wherein the punch has a front end wall and sidewalls, the front end wall for contacting a base of the cup, the sidewalls defining a cylindrical front end portion with a diameter D0, a transition portion, and a rear end portion toward a rear end of the punch with a diameter D1, wherein D1<D0 and wherein the front end portion is separated from the rear end portion by the transition portion, wherein the diameter of the punch sidewalls gradually decreases over the transition portion and wherein the shape of the transition portion of the front end portion of the punch to the rear end portion is a continuous curve wherein the angle between the tangent of the curve and the centreline of the punch is not constant over the transition portion and wherein the first derivative of the curve has at least one inflection point in the transition portion, wherein the tangent of the continuous curve at the connecting point between the curve and the front end portion and/or at the connecting point between the curve and the rear end portion both its ends is equal to the tangent of the front end portion and/or the rear end portion respectively to provide smooth transfer from curve to punch.

9. The apparatus according to claim 8, wherein the tangent of the continuous curve at the connecting point between the curve and the front end portion and at the connecting point between the curve and the rear end portion is equal to the tangent of the front end portion and the rear end portion respectively to provide smooth transfer from curve to punch.

10. The apparatus according to claim 8, wherein D0 is constant or wherein both D0 and D1 are constant.

11. The apparatus according to claim 8, wherein an entry angle (α) for the first wall-ironing ring is between 3.5 and 4.5° and an exit angle (β) for the first wall-ironing ring is between 2.5 and 3.5°.

12. The apparatus according to claim 8, wherein additional wall-ironing rings, positioned behind the first wall-ironing ring, are used wherein the entry angle of each successive wall-ironing ring is smaller than that of the preceding ring.

13. The apparatus according to claim 8, wherein the second wall-ironing ring is present and the entry angle (α) for the second wall-ironing ring is at least 1.75 and at most 2.25°.

14. A can produced according to the process of claim 1.

15. The process according to claim 2, wherein the coated metal sheet is obtained by means of film lamination or direct extrusion coating at least one surface of a metal sheet with an organic resin, wherein the organic resin is a polyester resin and the resin film has a thickness of 5 to 100 μm in case of a single-layer film or a total thickness of 5 to 100 μm in case of a multi-layer film.

16. The process according to claim 15, wherein the sheet metal is coated on both sides with the polymer film.

17. The apparatus according to claim 9, wherein D0 is constant or wherein both D0 and D1 are constant.

18. The apparatus according to claim 17, wherein an entry angle (α) for the first wall-ironing ring is between 3.5 and 4.5° and an exit angle (β) for the first wall-ironing ring is between 2.5 and 3.5°.

Description

EXAMPLES

(1) A three layer polymer coating system with a total thickness of 30 μm is applied to one side of a steel strip (the side becoming the outside of the can) with a thickness of between 0.10 and 0.50 mm by means of film lamination. In this example the coated strip obtained is used to produce, in two steps, a cup with a diameter of 73 mm, the polymer-coated side forming the outside of the cup. In the first step, a cup with a diameter of 100 mm is deep drawn from a round disc with a diameter of 150 mm. In the second step, this cup is formed into a cup having the final diameter of 73 mm by a further deep-drawing operation. See FIG. 1 for the schematic representation. This cup is fed to a wall ironing machine in which the wall thickness of the cup is reduced by wall ironing at a speed of between 180 and 600 strokes per minute and using a redraw ring followed by a first wall-ironing ring with an entry angle α.sub.1 and an exit angle β.sub.1, which reduces the wall thickness of the cup by a value (RED1) between 10 and 60% and a second wall-ironing ring an entry angle α.sub.2 and an exit angle β.sub.2, which reduces the wall thickness of the cup by a value RED2 between 2 and 25%.

(2) TABLE-US-00001 Application Adhesion layer Main layer Surface layer Food Optimised for Optimised for Optimised for sterilisation non-blushing forming performance or performance or coating colour content release Aerosol Optimised for Optimised for Optimised for print heat barrier resistance performance or coating colour General Line Optimised for Optimised for (Paint) chemical barrier resistance properties or coating colour Beer and Optimised for Optimised for Optimised for beverage adhesion barrier forming performance, performance and deformation stresses print and coating colour

(3) These experiments showed that an entry angle of 4° and an exit angle of 3° without external coolant provided excellent results with the majority of polymer coated strips. No scuffing was observed. Comparative experiments showed that the angle is critical in obtaining a good result. The method according to the invention is particularly suitable for polymer coatings which contain no or only insignificant amounts of titanium dioxide. However, the inventors found that white layers are somewhat more prone to scuffing because of the loading of the film with hard particles, such as TiO.sub.2 which have an abrasive effect even though these films could also be processed with the settings as claimed. The inventors found that when using a white coating on the intended outside of the can which is pigmented with titanium dioxide that the entry angle α for the first wall-ironing rings is preferably between about 1.5 and 2.5°. This is believed to be caused by the hard titanium dioxide particles have a scouring effect that increases the risk of damage to the film when being processed with an entry angle for the first wall-ironing ring of between 3.5 and 4.5°. A white titanium dioxide pigmented coating can be further processed with the exit angle (β) of the first ring between 2.5 and 3.5° and an entry angle (α) for the second wall-ironing ring, if present, of between 1.5 and 2.5° and the exit angle (β) is between 2.75 and 3.25°, similar to the method according to the invention.

(4) TABLE-US-00002 Examples of successful combinations (OK = no scuffing, Not OK = scuffing or damaged, Just OK = acceptable), exit angle = 3°. *White film, TiO.sub.2 pigment. Entry Entry angle Reduction angle Reduction # Rings (°) RED1 (%) (°) RED2 (%) Result 1 4 43 — — OK 1 4 32 OK 1 4 26 OK 1 4 45 OK 1 4 41 OK 1 4 31 OK 1 4 46 OK 1 4 45 OK 1 4 48 OK 1 4 50 OK 1 4 48 OK 1 4 48 OK 1 4 25 OK 1 (Comparison) 5 48 Not OK 2 4 48 2 6.9 OK 2 2 14.6 OK 2 2 21.5 OK 2 2 27 OK 2 2 13 OK 2 2 19.2 OK 2 2 10 OK 2 (Comparison) 4 48 3 13 Not OK 2 (Comparison) 4 48 4 13 Not OK  1* 2 46 — — OK  1* 2 50 OK  1* 2 48 OK  1* 2 25 OK  2* 2 48 2 31 OK  2* 2 50 OK  2* 2 48 OK  2* 2 25 OK 2*(Comparison) 2 48 4 32 Not OK  2* 4 45 — — Just OK

(5) The polymer coated steel substrates that can be processed by the process according to the invention are preferably based on polycondensates, such as polyesters, co-polyesters (including PET, PBT, polyethylene furanoate (PEF), poly(lactic acid) (PLA)) or polyamides, polyolefins, elastomers, crystallisable polyaddition polymers, or any other polymer that can be formed in a film by extrusion. The polymer coating may consist of one or more layers. Preferably the polymer coating layer comprises or consists of polyethylene terephthalate, IPA-modified polyethylene terephthalate, CHDM-modified polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene furanoate, poly (lactic acid) or copolymers or blends thereof.

(6) The procedure of the novel process and apparatus is illustrated in more detail in the appended figures, in which FIG. 1 shows various processing systems in various processing phases, FIG. 2 shows a schematic cross section of a polymer coated metal sheet, in this case provided with a polymer film on both sides. FIG. 3 shows a schematic and exaggerated portion of the punch with the rear end portion 1a, the transition portion 1b and the front end portion 1c, as well as the connecting points 14 and 15 and a cut out to illustrate the meaning of the tapering angle (ϕ) in the transition. FIG. 4 shows a detail of a wall ironing operation and FIG. 5 shows a schematic detail of the workface of an ironing ring with a land zone between the (frusto-conical) entry and exit plane.

(7) FIG. 1 illustrates how a preformed deep-drawn cup 3 is formed into a finished wall-ironed can body 9. The cup 3 is placed between a redraw sleeve 2 and a redraw die 4. When punch 1 moves to the right, the cup 3 is brought to an internal diameter of the final finished can 9 by the redrawing step.

(8) Then, the punch 1 successively forces the product through (in this example) two wall-ironing rings 6 and 7. Ring 8 is an optional stripper ring. Wall ironing provides the can body 9 to be formed with its ultimate wall thickness and wall length. Finally, the base of can body 9 is formed by moving punch 1 towards an optional base tool 10.

(9) Retracting punch 1 allows to detach can 9 from the punch 1 so that it can be discharged in the transverse direction. The optional stripper ring may assist in this. The can 9 is then subsequently trimmed, optionally necked and provided with a lid after filling.

(10) FIG. 2 provides a detailed illustration of the passage of a part of the can wall to be formed through, for example, wall-ironing ring 5. Punch 1 is indicated diagrammatically.

(11) The entry plane for wall-ironing ring 5 runs at an entry angle α to the direction of the axis of the wall-ironing ring. The thickness of the material of the wall to be formed is reduced between punch 1 and wall-ironing ring 5. This material comprises the actual metal can body wall 11 with layers of polymer 12 and 13 on either side. The layer of polymer 12 becomes the outside of the can body, and the layer of polymer 13 becomes the inside of the can body, eventually coming into contact with the contents of the can. The figure illustrates how the thickness of all three layers 11, 12 and 13 is reduced.

(12) FIG. 5 shows a schematic detail of the workface of an ironing ring with a land zone between the (frusto-conical) entry and exit plane. The radii of the transfer between the land zone and the entry plane and the radius of the transfer between the land zone and the exit plane are between 0.1 and 10 mm, preferably between 0.2 and 5 mm.

(13) The wall ironing rings are preferably provided with a land zone of length L which is located between connects the frusto-conical entry surface and the frusto-conical exit surface of the ring. The land zone is a cylindrical ring and has a length of at most 0.6 mm, preferably of at most 0.5 mm, and even more preferably of at most 0.3 mm.

(14) FIG. 6 shows an example of the prior art process with a straight punch where there is significant hair formation (A) and in the bottom half (B) the punch according to the invention shows no hair formation.