EP1889674A1

EP1889674A1 - Method of manufacturing a necked container

Info

Publication number: EP1889674A1
Application number: EP06017136A
Authority: EP
Inventors: Henri Kwakkel; Hendrik Michel
Original assignee: Corus Staal BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2006-08-17
Filing date: 2006-08-17
Publication date: 2008-02-20
Also published as: US20080044256A1

Abstract

The invention relates to a method for manufacturing a necked container (1) starting with a metal body part (2) of which the overlapping edges (11) are welded together, after which at least part of the metal body is ironed in order to minimize or remove the difference in thickness between the welded part (11) and the other part of the wall of the metal body part (2). After the ironing operation the metal body part (2) is necked and provided with a top part (9). A bottom part (3) is connected to the metal body part (2) which can be done depending on the extent of the ironing process before or after ironing the metal body part (2) and before or after necking the metal body part (2). Furthermore, the invention provides a single seam method for bottom part (3) and metal body part (2), resulting in an inwardly directed seam (5,10).

Description

The present invention relates to a method of manufacturing a necked container.
Necked containers, wherein the diameter of the container is reduced from a first diameter to a second smaller diameter, are know per se. For instance beverage cans for beverages and aerosol cans for pressurized liquids. All these containers have in common that bottom and body part are made as a single piece, that is by deep drawing and wall ironing these parts from a single piece of metal, such as a steel or aluminium alloy. After forming bottom and body part the outer end of the body part is necked to a predetermined shape and final end diameter. Such necked containers are relatively costly because of the can making process and because of the prices of the metal alloys that are suitable for such a process.
If the costs for these cans are a real issue, it is an option to make the cans from three separate pieces, a body part, a bottom part and top part. The body part is formed from a rectangular piece of sheet metal, such as for example a tin plated steel, that is bend into a cylinder shape of circular cross-section, after which overlapping edges of the sheet are welded together. Next the bottom part and top part are connected to the body part. Such a container can be made for substantial less costs, however, the appearance of such a low cost container is also considerable less attractive in comparison to a deep drawn, wall ironed and necked container. A very important aspect of the appearance is the necked part of the container or rather the missing thereof with the low cost containers.
Therefore, it is an object of the invention to provide a low cost container with a necked body part.
It is another object of the invention to provide a welded body part for a container that is suitable for necking, in particular die necking.
It is still a further object of the invention to provide a container with a bottom part connected to the body part without the connection between bottom and body part being visible at the outside of the body part.
According to a first aspect of the invention at least one of these objects is achieved by providing a method of manufacturing a necked container comprising at least the steps of:

forming a metal body part of circular cross-section from a metal sheet, with two edges of the sheet overlapping,
connecting the overlapping edges of the metal body part to each other by means of welding,
ironing the wall of the metal body part over at least part of the length of the metal body part in order to reduce the thickness of the welded overlapping edges,
die necking the ironed part of the metal body part in a number of successive steps, and
providing the metal body part with at least a bottom part.

In order to be able to neck or more in particular to die neck the metal body part it is crucial that the wall of the metal body part is of equal thickness around the total circumference of the metal body part or that the variation in thickness is as minimal as possible. In die necking the metal body part is pushed inside a circular die, a knock-out within the die guiding the metal body part at the inside thereof. The gap between the die and the knock out should be close to the thickness of the wall of the metal body part in order to be able to guide the wall of the metal body part in the deformation from a first diameter to a smaller diameter and at the same time prevent wrinkling of the metal body part. With an overlapping welded metal body part the thickness at the weld is typically about 1.5 times the original thickness of the wall of the metal body part. With such a variation in thickness around the circumference of the wall of the metal body part the gap between the die and the knock-out has to be so large to be able to also receive the welded part that wrinkling of the metal of the metal body part will be inevitable. If the gap is made smaller in order to try to prevent wrinkling the top of the wall of the metal body part will be damaged at the place of the weld.
This problem is solved by the method according the invention by ironing the wall of the metal body part to reduce the thickness of in particular the welded overlapping edges of the metal body part. It is sufficient to iron the wall of the metal body part over the length needed for die necking one side of the metal body part to the chosen final shape. However, for a good appearance of the final container it might be preferable to iron the metal body part over the total length thereof. A further choice is to iron the metal body part only at the location of the weld or at the weld and the area directly adjacent to the weld or to iron the metal body part along the complete circumference thereof. With the last ironing method the final shape of the metal body part can be best controlled.
According to a further aspect of the invention it is provided that the thickness of the overlapping edges of the body part after welding is in the range of 1.1 - 1.5 times the thickness of the original material of the metal body part, more preferably in the range of 1.1 - 1.3 times said thickness. By starting with a difference as small as possible between the thickness of the welded overlapping part of the metal body part and the thickness of the wall of the metal body part it will be easier to iron out this difference in thickness. It is important to find a good compromise between the thickness of the weld and the thickness of the wall of the metal body part. The smaller the difference the less material has to be ironed away, however by minimizing the thickness of the weld the properties of the wall and/or the welded material may change considerably. This is important because the difference between properties of the welded and ironed overlapping part of the metal body party and the properties of the wall of the metal body part have to be kept as close as possible in order to be able to die neck the metal body part properly along the total circumference of the metal body part.
Although the container could be used as a container with an open top end, for most applications the metal container will be provided with a top part. Accordingly it is provided that the method further comprises the steps of:

flanging or curling the edge of the metal body part at the necked outer end of the metal body part, the flange or curl being directed outward,
connecting a top part to said flanged or curled edge of the metal body part.

Such a top part might be a top part provided with a tear off lip such as for cans holding beverages or it might be a top part with a valve cup as used for aerosol cans or the top part might only be a valve cap.
In principal the bottom part can be connected to the metal body part before or after ironing the metal body part and before or after die necking the metal body part. However, a practicable manner is to first iron the metal body part, then connect the bottom part to the ironed metal body part and next to die neck the ironed metal body part. In this sequence it is possible to use the standard die necking apparatus for containers in which a pusher is put against the bottom part of the container.
If only part of the weld is to be ironed it is also possible to first connect the bottom part to the metal body part, then iron the upper part of weld or the upper part of the weld and the corresponding upper part of the metal body part and finally neck the ironed part of the metal body part.
Still another possibility is to iron the lower and upper part of the weld or the lower and upper part of the weld with the corresponding parts of the metal body part and neck the metal body part at both ends. Necking the lower part of the container has the advantage that the seam of a bottom part seamed to the metal body part does not extend outside the maximal diameter of the body part. Instead of only the lower and upper part of the weld and corresponding parts of the metal body part also the whole weld and body may be ironed with this embodiment.
The bottom part is seamed to the metal body part at the outer end of the metal body part opposite of the necked part of the metal body part. This can be done in a standard operation, that is by connecting the bottom part to the metal body part with a double seam. Such a double seam results in a visible rim at the outside of the metal body part of the container. Although in comparison to the necked part of the container this will generally be less important to the total appearance of the container, it still will make a difference. Therefore, the method according to the invention also comprises the further steps of connecting a bottom part to the metal body part in a way that no rim will show at the outside of the metal body part.
Accordingly, a metal bottom part with a flanged edge is provided, the outside of the flanged edge corresponding to the inner side of the metal body part at the location where the bottom part is to be connected to the metal body part. The bottom part is placed within the metal body part at said location, and the outer end of the body part that extends beyond the flanged edge of the bottom part is folded inward against the inside of the flanged edge of the bottom part.
With this method the bottom part is bent to a final shape with a flanged edge in only one step, which is done prior to inserting the bottom part into the metal body part in the final position. Thereafter, the metal body part is folded in a simple operation without having to bend the bottom part at the same time as is done with the double seam method. With these simple bending and folding operations it is possible to use other sheet metals than is necessary with the double seam method, without the possible risk of flange cracks. Furthermore, it is possible to use different sheet metals for body and bottom and/or top part. For instance, a thinner sheet metal of greater hardness may be used for the bottom part. One of the advantages of this method is that no rim will be formed at least not at the outside of the metal body part as with the double seam method.
By folding the outer end of the metal body part against the inside of the flanged edge the bottom part is clamped securely against the inside of the metal body part. However, in order to also have a liquid and air tight connection between bottom part and metal body part it is further provided that at least the outer side of the flanged edge of the bottom part is provided with sealing means. Accordingly the bottom part is made out of a sheet metal provided at least at one side with a coating, the coating consisting of or comprising sealing means or provided with a coating that is suitable to be used as sealing means. Very good experimental results have been achieved by using a steel sheet that is commercially available from CORUS under the trademark "Protact". This steel sheet is provided at one or at both sides with a single or multi-layered polymer coating preferably comprising polyethylene terephthalate, polypropylene or a mixture of polypropylene and nylon.
In order to form a liquid and air tight connection a heating treatment is required to have the coating act as a sealing means. There are a number of possibilities to perform such a heating treatment, but preferably this is done by induction heating because this is an easy and fast manner to do so.
By using such a sheet metal with a coating suitable to be used as a sealing means, it is no longer necessary to use a separate sealing compound, that is normally used to this end. This not only means saving out costs for such a relatively expensive sealing compound but also the steps of applying the sealing compound to the bottom part and the following curing of the compound before the bottom part can be applied to the metal body part.
With this method the outer end of the metal body part that extends beyond the flanged edge of the bottom part may be folded such that the largest part of the inner side of said outer end of the metal body part butts against the inner surface of the flanged edge of the bottom part. With a bottom part that is provided with a coating at only the side that is facing the inside of the container and the inside wall of the metal body part a very good seal can be formed by induction heating as described. By providing a coating at both sides the seal is further improved, but this is not really necessary, if the length of the flanged edge of the bottom part is long enough.
However, a bottom part provided on both sides with a coating suitable to be used as sealing means gives the possibility for another advantageous way to fold the outer end of the metal body part. Accordingly, the outer end of the metal body part that extends beyond the flanged edge of the bottom part is curled inward such that the edge between outer and inner side of said outer end of the metal body part butt against the inner side of the flanged edge of the bottom part. Herewith, the edge of the metal body part, along which the sheet is cut and for that reason is not provided with a protective layer of for instance tin, chromium and/or organic coating, butts against the coating of the bottom part. With the heating treatment to form a seal between the bottom part and body part, also a seal between the inside of the flanged edge of the bottom plate and said edge of the body part is formed, protecting the edge and therewith preventing it from corrosion.
If the bottom part is used for aerosol cans, the bottom part is preferably formed into a dome shape, the dome pointing in a direction opposite to the direction in which the flanged edge of the bottom part points. With such a shape the transition from dome into the flanged edge can also be formed more easily than from a for instance flat bottom part.
The invention will be further described on hand of the examples given in the drawing, wherein:

Fig.1: shows schematically a perspective and partially exploded view of a necked container,
Fig.2: shows schematically the ironing of the overlapping welded part of the metal body part,
Fig.3A, B, C: show schematically different stages and a detail of the die necking of the metal body part,
Fig.4: shows a detail of an embodiment of a container with the outer end of the body part folded flat against the flanged edge of the bottom part, and
Fig.5: shows a detail of an embodiment with the outer end of the body part curled against the inner side of the flanged edge of the bottom part.

Fig.1 shows a perspective and a partially exploded view of a necked container 1 with a body part 2 with a cylindrical circumferential wall 6 and a necked part 9 with a curled edge 10 at the top thereof. Connected to the body part is a bottom part 3 with a dome 4 pointing upward. The body part 2 is cut out of a sheet of metal, folded into a cylindrical sleeve of which the overlapping portions are welded together, which weld is indicated with broken line 11. The curled edge 10 is curled in outward direction and is sized to receive a valve cap, not further indicated in the drawing.
The thickness of the metal body 2 at the overlapping parts after welding is reduced by ironing the weld 11, as is schematically shown in Fig.2. The weld is ironed to such a thickness that the metal body part 2 will fit into a standard necking device, such as a die necking device. The overlapping parts of wall 6 of body part 2 after being welded together are ironed with an ironing device 12 as schematically shown. Such an ironing device 12 may extend over some distance at both sides of the weld or may be annular shaped so as to engage the total circumference of the metal body part 2.
In Fig.3A-C an initial step, a final step as well as a detail of the die necking process is shown. The container 1 is pushed with the rim of the metal body part 2 into the die 13, thereby guided by the so-called "knock-out" 14 preventing the wall 6 from wrinkling, see also fig.3B. The container 1 is pushed at the bottom by means of a pusher device 15. To reduce the diameter of a container in this manner a great number of successive dies of successive smaller diameter are needed to arrive at the final stage according to fig.3C. It is essential for this die necking process that the gap between die 13 and knock-out 14 is only very little larger than the thickness of the circumferential wall 6 of metal body part 2 and consequently this die necking process can only be applied for welded containers if the weld can be ironed out to come sufficiently close to the thickness of wall 6 of metal body part 2.
After die necking metal body part 2 a flange or curl 10 is formed at the top end of wall 6 of metal body part 2. A flange is formed if a top part is to be connected to the metal body part 2 and a curl 10 is formed if the diameter of the metal body part can be reduced sufficiently to connect a valve cap directly to the metal body part 2. A standard valve cap has a diameter of 1 inch.
In Fig.4 a cross-section of the bottom side of container 1 is shown with body part 2 and bottom part 3. The body part 2 has a closed circumferential wall 6. In the most simple shape such a wall 6 is of circular cross-section and with a constant diameter along the length of the body. However, other shapes, such as tapered shapes and/or shapes of non-circular cross-section are also possible. The bottom part 3 is formed with a dome shape 4 pointing in upward direction and is further provided with a flanged edge 5. The outer size and shape of flanged edge 5 of bottom part 3 corresponds to the size and shape of body part 2 at the inside at the location where the bottom part 3 is to be connected to the body part 2.
After placing the bottom part 3 at the right location in the body part 2, that is at a certain distance above the lower edge of body part 2, the outer end 7 of body part 2 that extends below the flanged edge 5 of bottom part 3 is folded such that the largest part of the inner side of said outer end 7 of body part 2 butts against the inner side of the flanged edge 5 of bottom part 3. By folding the outer end 7 in this way bottom part 3 is clamped securely against body part 2.
The bottom part 3 is provided at one or both sides with a coating that comprises a sealing means or that may be used as a sealing means. The bottom part that was experimented with is made from a steel sheet commercially available under the trade name "Protact", which is provided at one or both sides with a single or multi-layered polymer coating preferably comprising polyethylene terephthalate, polypropylene or a mixture of polypropylene and nylon. To activate the sealing means a heating treatment is necessary, which heating treatment is preferably an induction heating treatment. After this heating treatment the sealing means not only provide a liquid tight and air tight seal but it also improves the mechanical connection between the bottom and body parts 2, 3.
In the embodiment shown in Fig.5 the outer end of the body part 2 is curled inward in such a way that the edge 8 of body part 2 butts against the inner side of the flanged edge 5 of bottom part 3 and therewith against the coating comprising a sealing means. After heating the edge 8 will be securely embedded in the sealing means therewith preventing any corrosion of edge 8 of body part 2.
With this embodiment sealing of the container 1 is mainly achieved by the sealing means acting on the outer side of the flanged edge 5 of bottom part 3 and the inner side of the circumferential wall 6 of body part 2, which however is sufficient. For this embodiment it is essential that bottom part 3 is provided on both sides with the coating containing sealing means.

Claims

Method of manufacturing a necked container comprising at least the steps of:
- forming a metal body part of circular cross-section from a metal sheet, with two edges of the sheet overlapping,

- connecting the overlapping edges of the metal body part to each other by means of welding,

- ironing the wall of the metal body part over at least part of the length of the metal body part in order to reduce the thickness of the welded overlapping edges,

- die necking the ironed part of the metal body part in a number of successive steps, and

- providing the metal body part with at least a bottom part.
Method according to claim 1, wherein the wall of the metal body part is ironed over at least the length needed for die necking one side of the metal body part to the chosen final shape.
Method according to claim 1 or 2, wherein the method further comprises the steps of:
- flanging or curling the edge of the metal body part at the necked outer end of the metal body part, the flange or curl being directed outward,

- connecting a top part to said flanged or curled edge of the metal body part.
Method according to claim 3, wherein the top part that is connected to the curled edge of the metal body part is a valve cap.
Method according to any one of claims 1-4, wherein the thickness of the overlapping edges of the body part after welding is in the range of 1.1 - 1.5 times the thickness of the original material of the metal body part, more preferably in the range of 1.1 - 1.3 times said thickness.
Method according to any one of claims 1-5, wherein the bottom part is seamed to the metal body part at the outer end of the metal body part opposite of the necked part of the metal body part.
Method according to claim 6, wherein the bottom part is connected to the metal body part by means of a double seam.
Method according to claim 6, wherein the method comprises the steps of:
- providing a metal bottom part with a flanged edge, the outside of the flanged edge corresponding to the inner side of the body part at the location where the bottom part is to be connected to the body part,

- placing the bottom part within the body part at said location, and

- folding the outer end of the body part that extends beyond the flanged edge of the bottom part inward against the inside of the flanged edge of the bottom part.
Method according to claim 8, wherein the outer end of the body part that extends beyond the flanged edge of the bottom part is folded such that the largest part of the inner side of said outer end of the body part butts against the inner surface of the flanged edge of the bottom part.
Method according to claim 8, wherein the outer end of the body part that extends beyond the flanged edge of the bottom part is folded inward such that the edge between outer and inner side of said outer end of the body part butts against the inner side of the flanged edge of the bottom part.
Method according to any one of claims 8-10, wherein at least the outer side of the flanged edge of the bottom part is provided with sealing means.
Method according to any one of claims 8-11, wherein the bottom part is made out of a sheet provided at least at one side with a coating, the coating consisting of or comprising sealing means or provided with a coating that is suitable to be used as sealing means.
Method according to any one of claims 1-12, wherein at least the metal body part is made of tin plated steel.
Method according to any one of claims 1-13, wherein the metal body part is die necked and the die necked outer end is curled outward such that a standard sized valve cup can be connected to the curled outer end.