I. Field of the Invention
This invention relates to the shaping of metal containers by means of a succession of necking steps using dies that gradually modify the container walls into a desired finished shape. More particularly, the invention relates to the design of dies to improve die necking operations and to methods of die necking.
II. BACKGROUND ART
Thin walled metal foodstuff containers, beverage cans, aerosol canisters, and other such containers for consumer or industrial products are often provided with inwardly- or outwardly-flared walls for aesthetic reasons or for reasons of practicality or economy. For example, beverage can bodies are often provided with an inward flare adjacent to their upper ends primarily to reduce the size of the required metal end closure walls. Such end closure walls are necessarily made of a metal of a much thicker gauge than that required for the walls of the container bodies, so any reduction in their size results in a considerable saving of metal. Containers of this kind are often made from rolled metal sheet that is cut into blanks, cupped, drawn and ironed to elongate the side walls, and then finally trimmed to produce a straight-walled open-ended container body pre-form. Such container body pre-forms are then provided with flared ends or other shapes of the above-mentioned kind by a process known as die necking whereby the open end of a tubular pre-form is forced into a succession of shaped annular dies of ever-decreasing diameter until the desired size reduction of the tubular wall at the open end is achieved. A large number of small changes of diameter are carried out in order to avoid metal buckling, ripping or tearing that generally occurs if abrupt size changes are attempted in a single step. More details of a typical necking operation may be obtained from U.S. Pat. No. 5,497,900 issued to Caleffi et al. on Mar. 12, 1996 and from PCT publication WO 2007/136608 A2, published on Nov. 29, 2007 (the disclosures of which documents are specifically incorporated herein by reference).
The necking dies work in conjunction with correspondingly-sized knockout punches that fit within the central openings of the dies and help to support and shape the container pre-forms to be necked. The purpose of the die is to reduce the diameter of the opening and impart a shape that is aesthetically pleasing at the top portion of the container pre-form. The purpose of the knockout punches is to control the metal by diverting it back towards the die so that the size reduction of a particular die necking stage is not larger than intended, to prevent failures of buckling in the neck, and to knock the container pre-form out of the die after shaping has been accomplished in that stage.
As the container pre-form is forced into the die, considerable friction is generated even though a lubricant is usually present on the die and metal surfaces. The friction thus generated hinders the smooth shaping operation and increases the risk of metal buckling. It also limits the degree of necking (the extent by which the open end may be necked-in at each necking step) because the container pre-form can sustain only a certain maximum axial load without deformation, and a greater degree of necking-in requires a greater degree of axial load regardless of the generated friction. Therefore, increased friction detracts from the axial load that can be applied to necking-in.
U.S. Pat. No. 5,711,178 which issued to Hogendoorn et al. on Jan. 27, 1998 discloses a die for use in a die-necking process of a metal body. The die is designed to reduce axial force needed in the die necking steps.
U.S. Pat. No. 4,881,394 which issued to Jansen on Nov. 21, 1989 and U.S. Pat. No. 5,168,742 which issued to Heyes et al. on Dec. 8, 1992 may also relate to the minimization of axial force, but these patents relate to ironing dies which are quite different from necking dies.
It would therefore be advantageous to provide alternative ways of reducing the amount of friction generated between the metal pre-form and a necking die without compromising a desired shaping operation.
One exemplary embodiment of the invention provides a necking die set for necking-in a metal container preform. The die set comprises a knockout punch having a generally cylindrical surface and a die having, in an axial direction from front to back of the die, an inwardly tapering in-feed surface, a forming radius, a generally cylindrical land defining a die bore diameter, a discharge surface following the land, and a relief surface having a diameter larger than the die bore diameter. The land has an axial length of less than 0.1 inch. This dimension limits the number of metal contacts with the land to one or two as a metal container preform is necked in the die in co-operation with the knockout punch.
The axial length of the land is preferably from 0.010 to 0.0950 inches, and still more preferably from 0.0127 to 0.0827 inches.
The knockout punch preferably has an outer diameter and the die preferably has a bore diameter effective to leave a gap therebetween, the gap being greater than the top-wall thickness of a container preform necked in the die set. Alternatively, the gap may be the same as the top-wall thickness or less than the top-wall thickness, to effect re-drawing of the container preform during the necking step. When the gap is less than the top-wall thickness, the gap is preferably up to 10% smaller than the thickness of the top-wall, and more preferably up to 5% smaller.
The container preform may preferably have a top-wall thickness of 0.0058 to 0.010 inch, and the die may preferably have a forming radius of 0.2 and 0.5 inches.
Another exemplary embodiment provides a necking die set for necking-in a metal container preform. The die set comprises a knockout punch having a generally cylindrical surface and a die having, in an axial direction from front to back of the die, an inwardly tapering in-feed surface, a forming radius, a generally cylindrical land defining a die bore diameter, a discharge surface and a relief surface having a diameter larger than the die bore diameter. The land has an axial length that limits the number of metal contacts to one.
The exemplary embodiments also extend to dies designed for use in the aforesaid die sets.
Another exemplary embodiment provides a method of necking-in a metal container preform having a top-wall thickness. The method includes the steps of directing an open end of a metal container preform surrounding a cylindrical knockout punch into an annular necking die to effect necking-in of the container preform, and then using the knockout punch and optionally pressurized gas to knock out the container body preform from the die. The method also includes providing the die with, or selecting the die to have, a land having an axial length effective to limit a number of contacts between the container preform with the land during the necking step to one or two. The distance between the land and the knockout punch is also preferably reduced to cause a resizing or redistribution of metal of the necked-in part of the container wall.
In the following, for the sake of simplicity, reference is made to a “container” rather than a “container body pre-form”, although the latter is generally intended.
During neck forming, as represented in
As previously mentioned, and as shown more clearly in the still further enlarged view of
An exemplary embodiment of the present invention minimizes the friction generated in this way by reducing the axial length of the land below the minimum length conventionally employed (about 0.1 inch). This decreases the number of times the metal contacts the surface of the land 22 and/or decreases the area of contact. Ideally, the land is made so short that there is only one contact of the metal with the land, but as many as two metal contacts may be accepted. This is illustrated schematically in
It may be possible to determine the number of contacts made with the land by microscopic examination of the land surface of a well-used die since the contacts change the surface appearance or physical wear on the land surface, which appear as surface bands. Moreover, a test die made of a tough transparent material, e.g. polycarbonate or other strong polymer, may be used to allow visual observation of movements of the container wall during the necking-in process.
The axial length of the land 22 required to produce the desired reduction in friction is a function of the forming radius 21, the metal properties of the top wall of the container, the top wall thickness of the un-necked container body (generally 0.0058 to 0.010 inch), and the clearance between the knockout punch and the land. For most applications using aluminum can body stock (e.g. container bodies made of alloys AA3004, AA3014, X319, X343, etc.) the forming radius 21 will fall between 0.2 and 0.5 inches, and the knockout punch and die clearance over metal (the metal being at the gauge to which it thickens in that stage of the necking operation) will fall between 0.0005 and 0.001 inch per side. In such circumstances, the preferred land lengths will be within the range of 0.027 and 0.060 inch in axial length. It should be noted that the length of the land is the length of the portion that is parallel to the central axis 14 and does not include any part of the discharge surface 23 or the forming radius 21. The preferred working range for the length of the land is 0.010 to 0.0950 inch, and generally an amount less than 0.1 inch. These dimensions are normally suitable for all conventional necking speeds and stroke lengths.
Table 1 below shows the land lengths that are optimum for achieving a single contact with the die land.
Thus, it can be seen from the above table that, for forming radii below 1.2 inch, the land preferably has a minimum length above about 0.1 inch and a maximum length below about 0.095 inch, and preferably below 0.09 inch.
The inventor has also surprisingly found that, when the length of the land is shortened in the indicated manner, the free play or spacing conventionally provided between the container wall and the confronting surfaces of the land 22 and knockout punch 12 may be eliminated without increasing friction unacceptably and without increasing any tendency of the container to jam in the die. This was evident by the fact that containers necked in this way did not collapse during the necking step, even when provided with a weakened mid section (e.g. a narrow waist). It is also noted that, when the free play or spacing is eliminated, or the spacing between land and knockout punch is made slightly less than the thickness of the adjacent container wall (top-wall thickness), the metal in the container wall may be redistributed or “re-sized” to eliminate or reduce circumferential irregularities of thickness that may build up as the container is necked-in. Indeed, the wall thickness after the neck reduction may be reduced in this process by 10% or less, and preferably 5% or less, when compared to an equivalently necked container where conventional free play or spacing is provided. This wall thickness reduction produces an even wall-smoothing effect. This advantage can be achieved without further modifying the land, i.e. while maintaining the flat cross-sectional profile of the surface of the land 22, as shown. It is theorized that, although the re-sizing of the container wall can accomplished in this way, this can be done without significant increases in friction because the re-sizing of the metal takes place to only over a short axial distance due to the reduced length of the land.
This embodiment is illustrated in
This application claims the priority right of co-pending U.S. provisional patent application Ser. No. 61/197,976 filed on Oct. 31, 2008 by applicants named herein. The disclosure of the aforesaid provisional patent application is specifically incorporated herein in its entirety by this reference.
Number | Date | Country | |
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61197976 | Oct 2008 | US |