Necking die with redraw surface and method of die necking

Abstract

The invention provides a die set comprising an annular necking die and a knockout punch for necking-in a metal container, and a method of necking. In the method, in at least one necking-in step, the metal container is both reduced in diameter and the container wall is redrawn to minimize circumferential irregularities of wall thickness and/or circumferential rippling of the container wall caused by the reduction of diameter. The die of the die set is provided with a redraw surface to achieve this redrawing effect.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority right of copending U.S. provisional patent application Ser. No. 61/197,975 filed on Oct. 31, 2008 by applicants named herein. The disclosure of the aforesaid provisional application is specifically incorporated herein in its entirety by this reference.

BACKGROUND OF THE INVENTION

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 esthetic 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 or onto a succession of shaped annular dies of ever-decreasing (or increasing) diameter until the desired size reduction (or enlargement) 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.

While the die necking process is successful and is used on a large scale for the manufacture of beverage cans and the like, difficulties arise when a substantial reduction in diameter of a container body is required. The difficulties are of at least two kinds. Firstly, when the diameter of a container body is reduced by so-called “necking-in”, the walls of the body inevitably increase in thickness because the same amount of metal becomes distributed around a smaller circumference. However, because of small irregularities in the characteristics of the metal or metal wall anisotropy (differences in physical characteristics of a rolled metal sheet in the rolling and transverse directions due to metal grain elongation during rolling), this increase in thickness generally does not take place evenly around the circumference of the container wall. With every successive necking-in step, the radial unevenness may build up, and this can result in longitudinal creasing or pleating deformities after a number of necking steps. This tendency to crease or pleat is greater at positions where the rolling direction of the metal sheet is lined along the axis of the container pre-form than along the transverse direction of metal sheet due to the differences of hoop stress values as a function of metal lay.

Secondly, as necking-in proceeds, the extreme end of the container may exhibit circumferential lines or ripples that are visible in the finished article and detract from the desired smooth transition from one container diameter to another. These lines may result from the tendency of the metal to spring-back due to its elasticity, and this is made possible by the degree of free play between the die and associated knockout punch that allows the container wall to wrinkle slightly and prevents the wall from adopting an exact shape intended by the design of a necking-in die.

Such disadvantages, while encountered when shaping metal containers of many kinds, are especially acute when employing necking steps to produce so-called “metal bottles”. These are metal containers that mimic the shapes of glass bottles and may have contoured side walls and shoulder sections that greatly decrease in diameter or change in angle and merge smoothly with narrow cylindrical necks (sometimes closed with a screw-threaded cap or end closure). In such articles, curves can be sharp and changes of diameter may be significant over a short axial distance. This imposes particular demands on the die necking operation.

U.S. Pat. No. 5,497,900 issued Mar. 12, 1996 to Caleffi et al., assigned to American National Can Company, discloses a die necking method purporting to produce a smooth tapered container wall and a reduced diameter neck. However, there is still need for improvement in order to obtain smoother transitions during such shaping operations.

U.S. Pat. No. 4,403,493 issued Sep. 13, 1983 to Michael L. Atkinson, also assigned to Ball Corporation, discloses a procedure by which both a reduced diameter portion and a transition portion are reformed within controlled limits to provide a new reduced diameter portion and a new curvilinear transition portion.

U.S. Pat. No. 5,469,729 issued Nov. 28, 1995 to Milton S. Hager, assigned to Ball Corporation, discloses a procedure in which a plurality of venting ports are incorporated into a necking assembly which performs the necking operation. The container body may be centered with respect to the necking assembly which produces a double-neck container body configuration.

Despite these known procedures and equipment, there is still a need to overcome or reduce the problems mentioned above during shaping of containers.

SUMMARY OF THE EXEMPLARY EMBODIMENTS

One exemplary embodiment provides a die set for necking-in a metal container, in which the die set comprises a combination of a die and a knockout punch for the die having a generally cylindrical surface. The die has, in an axial direction from front to back of the die, an inwardly tapering necking surface having an inflection point at an innermost end thereof, preferably an undercut portion having a generally cylindrical surface extending axially rearwardly from the inflection point, a convex redraw surface extending inwardly of the die from a rear of the generally cylindrical surface of the undercut portion, and a generally cylindrical cutback surface having a diameter larger than the redraw surface at a peak thereof and extending axially from the redraw surface towards the back of the die, wherein the redraw surface at the peak defines an opening dimensioned to receive the knockout punch with a spacing effective to redraw a wall of a container necked in the die.

Another exemplary embodiment provides an annular necking die which comprises, in a direction from front to back of the die, a tapering necking surface of decreasing diameter having an inflection point at an innermost end thereof, preferably an undercut portion having a generally axial surface, a convex redraw surface extending inwardly of the die from the generally axial surface of the undercut portion, and a cylindrical cutback surface having a diameter larger than the redraw surface and extending from the redraw surface towards the back of the die, wherein, during use of the die in a necking-in operation carried out on a cylindrical wall of a hollow metal container pre-form in conjunction with a cylindrical knockout punch dimensioned to fit snugly within the die, the redraw surface is positioned and dimensioned to contact a wall of the pre-form and to compress the wall against the knockout punch, thereby smoothing out circumferential irregularities of the wall caused by wall-thickening as the pre-form is necked in.

Yet another exemplary embodiment provides a method of necking-in a metal container pre-form having a cylindrical wall and an open end, the method comprising carrying out a plurality of necking-in steps by introducing the open end of the container into necking-in dies operated with cooperating knockout punches, wherein for at least one of the steps, the container wall is both reduced in diameter and the container wall is redrawn to minimize circumferential irregularities of wall thickness caused by the reduction in diameter.

Still another exemplary embodiment relates to a method of providing a necking-in die with capability of container wall redrawing, which comprises cutting an annular groove into a surface of a cylindrical land of a necking-in die and inserting in the groove an element having a convex redraw surface extending inwardly beyond the surface of the land.

The container wall generally has a range of angles through which it may be bent plastically, and wherein, during at least one step, the container wall is bent through an angle within the range.

The wall is preferably redrawn by forcing the wall through an annular gap within a necking-in die set comprising a die and a knockout punch, wherein the annular gap is formed between a redraw surface on the die and an external surface of the knockout punch positioned rearwardly of an inflection point on a necking-in surface of the die, and wherein the annular gap has a width that is the same as or less than an average thickness of the container wall at the inflection point. Preferably, a lubricant is fed to the redraw surface and container wall as the wall is passed through the gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a chart showing an outline of a typical shaped container body which the exemplary embodiments may produce (bottle profile diameter versus height), and FIG. 1B indicates the degrees by which the wall will bend and unbend at different positions along the axis of the container body (bottle profiles of gradients versus height);

FIG. 2 is a partial cross-section of an annular die and associated knockout punch showing a die necking operation and the problems associated therewith;

FIG. 3 is a partial cross-section similar to that of FIG. 2, but showing an exemplary embodiment of the present invention;

FIG. 4 is an enlargement of the part of FIG. 3 surrounded by the dotted circle marked IV;

FIGS. 5A and 5B are exaggerated schematic representations of transverse cross-sections of a container following necking-in; FIG. 5A shows radial variation of wall thickness, and FIG. 5B shows a smoothing of such variation obtainable by exemplary embodiments of the present invention;

FIG. 6 is a partial cross-section similar to FIG. 3 illustrating a profile of a die according to another exemplary embodiment of the invention; and

FIG. 7 is a view similar to that of FIG. 6 showing yet another profile according to yet another exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

As noted, FIG. 1A is a chart showing an arbitrary but typical shape of a metal to bottle of the kind of which manufacture is currently being attempted by die necking operations. The upper and lower curved lines A and B represent the walls of the container and it can be seen that it is intended to provide the container with a straight section C as well as a tapering shoulder D leading to a narrow cylindrical neck E. Such a shape is often made up of a number of curves comprising a mixture of intersecting circles of varying diameters. The slope of the shoulder D in the shape of FIG. 1A may be continuously smooth or may be a linearly increasing curve which reaches a maximum before it decreases. In FIG. 1B, which shows a range of relative angles of the container wall of curve B in FIG. 1A, line F represents the transitions in slope of the parts of the wall considered from the closed end to the open end of the container. At the points where line F changes direction, the metal undergoes a considerable degree of bending (possibly up to 55°, depending upon the bottle bottom base to neck diameters) and the difficulties mentioned in the introduction of this specification may be especially apparent. For example, in FIG. 1A, in typical die necking operations, the straight portion E on the neck approaches the die at zero degrees and encounters a die necking surface angled inwardly at say 18 degrees to create a bend. The wall will then bend back 18 degrees to form the shape shown in FIG. 1A at the last necking die. Other shapes may have much larger bend and un-bend angles. As can be seen, the most pronounced transition may take place where a shoulder region D merges into a neck region E (the curve at inch 6 on the chart of FIG. 1A). The illustrated curves are produced by a considerable number of die necking steps each requiring separate die sets. The exemplary embodiments relate to the necking-in steps of the shaping process, and particularly those involving a considerable change of angle.

FIG. 2 is a schematic representation of a necking-in die and knockout punch combination 100 illustrating a necking-in step that may typically be attempted to produce a metal bottle of the kind described above. In this view, for simplicity, a cross-section of the end part of only one half of a metal container pre-form 10 (referred to simply as the “container” in the following) is shown, and similarly, only a cross-section of half of a necking-in die 11 and a knockout punch 12, but the illustrated profiles are present all around the wall of the container, the die and the punch and are symmetrical around a central longitudinal axis of the die. The wall 13 of the metal container is necked-in upon being pushed into the inwardly-tapering surface 18 of the annular necking die 11 while surrounding the knockout punch 12. The latter is generally cylindrical except for an enlarged step 19 used for the knock-out procedure at the end of the necking-in step. More details of a typical necking operation may be obtained from U.S. Pat. No. 5,497,900 mentioned above (the disclosure of which is incorporated herein by reference), and typical die shapes are shown in PCT publication WO 2007/136608 A2 published on Nov. 29, 2007 (the disclosure of which is also incorporated herein by reference).

In the container 10 shown in FIG. 2, curve 14 is a neck-shape formed on the container at a previous necking station (this embodiment being different from that shown in FIG. 1A). Letters P, Q, R, S and T are used to identify different points on the die 11. Point T represents the front of the die at its maximum opening diameter, and points R and S indicate the rear wall of the die. The container 10 is moved relative to the die in the direction X during a necking operation and the knockout punch 12 is also moved in direction X at approximately the same speed during this stage of necking. The surface 18 between points T and P causes the container pre-form to neck-in (i.e. to bend in) as it is pushed into the die. The length of this surface is typically 0.02-0.5 inch, depending on the dimensions of the container pre-form and the necking step involved. The thickness of the container wall is caused to increase as it passes along this surface as a consequence of the reduction in diameter of the necked-in part of the container. Point P on the die is referred to as an inflection point and it may contain a small forming radius. This is the point where the metal loses contact with the surface 18 and the slope of the container wall is caused to bend dramatically upon contact with an outer surface 22 of punch 12. A radial component of frictional force generated upon contact with the surface 18, and acting in the in the direction PT, also aids in forcing the container wall to bend back towards the die. Hence, it is desirable to provide the die surface 18 and/or the metal in contact with surface 18 with a degree of roughness in order to increase the frictional force. From point P, in a direction moving towards the rear of the die to point Q, the profile of the die forms a cylindrical land 15 having a diameter that is intended to define the diameter of the neck of the container at this stage of the necking operation and a wall that is parallel to the axis of the die. However, because the metal of the container wall is not fully plastic and has a retained elasticity, and because there may be insufficient radial force from the friction generated with the contact with surface 18, and further because there is normally a gap or degree of play between the land 15 of the die 11 and the outer surface 22 of the punch 12 that is greater than the thickness of the wall of the container (to prevent the metal jamming in the die), the wall follows a curve 16 (which in actuality may rebound between the land 15 and the surface 22 several times) that extends above the land 15 by an amount that may be as large as 0.002 inches. The metal then reverse bends (relaxes) towards the end of the land 15 (at point Q).

The curve 16 thus formed may have a radius “r₁” but this radius tends to be smaller than that which would allow the metal to bend plastically. For many materials, the elastic modulus (E) is about 1000 times the yield strength (S) such that the bend ratio at the elastic limit, for metal thickness “t” and bend radius of R, is given by (R/t)=[E/(2×S)] which gives (R/t) a value of 500. This means that the outer fibers (layers) of the bend reach the yield point when the radius of curvature is about 500 times the metal thickness. Typical curvatures are 2 to 20 times the thickness and bending is generally fully plastic. If the surface is bent over a large radius, then bending is plastic because, as the curvature decreases, the bending becomes plastic. At a low radius of curvature the bend is fully plastic. As described in WO 2007/136608 A2 (mentioned earlier), a large radius at point P is desired to decrease the angle of contact made between the metal at the inflection point, at first contact with the knockout punch, such that the normal component of the axial stress from friction and ram force remains high to avoid the arc above the land.

The land 15 normally extends axially in the region PQ by a distance of 0.1 to 0.2 inch or more and is then followed between points Q and R by a cylindrical cut-back region 17 of greater diameter than the land 15 and that consequently does not exert any significant force on the adjacent wall of the container during the necking operation (as there is no contact). The cut-back region 17 prevents the pre-form from becoming jammed in the die and allows for the accumulation of debris and dirt without harmful effects on the necking operation. However, the large amount of space between the die and the punch in the cut-back region 17 may allow the container wall to ripple or wrinkle into small waves (as represented in FIG. 2) to take up the available space. Such rippling may occur in a space that is larger than required and is the result of the compressive forces created as the pre-form is pushed into the die. However, this rippling does not always occur as the contact between the container wall and the knockout punch may cause the container wall to remain in close contact with the surface 22 of the punch.

Additionally, as previously mentioned, as the wall of the pre-form thickens during necking-in, the thickening may not take place evenly around the circumference of the wall of the container. If the punch does not move concentrically with the die axis, it will allow wall thickness to grow unevenly. Grossly thick and thin wall sections will result, which will also lead to axial wrinkling and pleating of the formed neck, especially after a number of necking-in steps.

FIG. 3 shows a die profile similar to that of FIG. 2, but illustrating one exemplary embodiment of the invention. The part of FIG. 3 within the broken-line circle IV is shown in greater magnification in FIG. 4. In this exemplary embodiment, the land 15 of FIG. 2 (shown in dotted lines in FIG. 4) has been fully or partially replaced by an inserted die element 20 that stands proud of the former surface of the land. The die element 20 is annular and sits in an annular recess 28 in what was formerly the land 15. Referring in particular to FIG. 4, it can be seen that the size, shape and positioning of this element is such that, after moving past the inflection point P, the container wall 13 starts to bend in the usual way, but is briefly contacted on its opposite sides by the surface 21 of the element 20 and the adjacent surface 22 of the knockout punch 12. The brief pinching or compression created by this contact causes the metal to be slightly to “redrawn” (i.e. reshaped) by the element. That is to say, the metal is plastically re-shaped by the contact. Ideally, the contact is such that only parts of the metal wall thickened more than average during the necking-in step are flattened and redrawn to a desired average thickness of the container wall, but the contact may be made such that a slight reduction in the average wall thickness is also achieved. This thickness reduction is generally less than 10% of the average thickness of the contacted container wall, more preferably less than 5%, and may (as noted) be essentially zero (the intended average wall thickness at this position in the die). FIGS. 5A and 5B show in a very simplified schematic way the “redrawing” that takes place at this position. FIG. 5A represents a cross-section of the container immediately in advance of the contact with the redraw element 20 and shows how the thickness may vary around the circumference of the metal wall (radial variations in thickness). A form of wrinkling or pleating may occur particularly around the 0° and, to a lesser extent, the 90° axes to the rolling direction of the sheet from which the container is made (due to metal anisotropy). As shown in FIG. 5B, after contact with the redraw element 20 backed by the knockout punch 12, the profile of the wall is smoothed out to a constant thickness “t” (which may be slightly less than the wall thickness of the container shown in FIG. 5A) and is smoothed to an average surface evenness.

If the distance between the redraw element 20 and the knockout punch 12 is smaller than necessary for this minor amount of redrawing, or if the redrawing contact of the container wall with the redraw element 20 takes place over too great a distance, the container may jam in the die or undergo buckling and result in a failure of the necking step. The optimum is therefore to make the contact brief (extending over a minimal axial distance) and with minimum friction, but significant enough to create the desired redrawing effect. This is made possible by the convex nature of surface 21 which is such that the surface extends away from the container wall immediately beyond the point of contact (actually a circumferential line of contact around the die).

It should also be mentioned that the positioning of the element 20 with respect to the shaping surface 18 of the die may be important in some cases. The element 20 should preferably be shaped and positioned such that the wall of the container may contact the element without being lifted away from the surface 18 or bent inwardly towards the center of the die. More preferably, the die is positioned such that the metal may bend slightly outwardly, following point P, before contacting the element 20. This is illustrated in FIG. 4 by arrow K that is slightly bent towards the right at the head end.

Referring again to FIG. 4, it will be seen that, in this embodiment, the inflection point P is immediately followed (in the axial direction front to back of the die) by an undercut portion 25 similar to the start of the land 15 of a die of the kind of FIG. 2 but provided at a greater distance from the center of the die. This undercut portion 25 is preferred (but not essential) to form a reservoir for lubricant for the metal in advance of the redrawing process. The lubricant in the reservoir may be replenished during the knockout process when the container is removed from the die (lubricant is generally liberally applied to the die and knockout punch during the necking operation and is thus available for replenishment of the reservoir). The presence of the undercut portion 25 also avoids support for the container wall beyond the inflection point P. The undercut portion 25 preferably has a generally axial (cylindrical) surface at its maximum diameter (i.e. it may be exactly axial as shown or slightly sloping up or down relative to the die axis). If the undercut portion 25 is absent, the die surface in this region may follow the contour of the original land 15 as shown in broken lines.

The redraw surface 21 rises beyond the undercut portion 25 (and the surface of the original land 15) as shown and engages the outer (lower) surface of the container wall 13 as the metal curves under the bending force imposed by the knockout punch 12 and the frictional force. The redraw surface is convex and has a peak 23 at which it is closest to the central axis of the die. The maximum height “h₂” of the redraw surface 21 above the surface of the original land 15 (the distance from the surface of land 15 to the peak 23) causes a reduction in the diameter of the container wall at this point by an amount of 2×h₂ (as the container is necked in from all sides). This height is chosen to cause the neck 26 of the container to become reduced in wall thickness by the indicated amount of less than 10%, and consequently to elongate the metal slightly as it is extruded. Height “h₂” is preferably 1/20^th (5%) of the wall thickness at a maximum and 1/200^th (0.5%) of the wall thickness at a minimum so that, as explained previously, the metal is deformed plastically in the curve. In terms of actual height, the distance “h₂” is generally from 0.0001 to 0.012 inch, preferably 0.0001 to 0.006 inch, depending on wall thickness and the stage of the necking operation. Height “h₁” is the height of the peak 23 above the surface of the undercut portion 25, and height “h₃” is the height of the step 19 above the cut back surface 17 (see FIG. 3).

The redraw surface 21 is preferably in the form of a convex curve or arc that has a radius “r₂” (see FIG. 3). The radius is chosen according to the desired reduction of wall thickness. Generally, the radius varies from 0.02 to 4 inches. As the number of necking steps increases, the metal may become harder and it may be preferable to reduce the radius “r₂”, e.g. from a range of 2 inches to 0.02 inch with a base “x” (see FIG. 4) ranging from 0.4 to 0.03 inch.

If the redraw surface 21 extends for a distance “x” in the axial direction front to back of the die, and if the redraw radius is “r₂”, then the radius “r₂” is preferably determined by the formula:

$r_2=\frac{{\left[\frac{x}{2}\right]}^2}{\left(2\times h_2\right)}$

In embodiments having an undercut portion 25, “h₁” (the radial height above the cylindrical surface 25 or point P) may be used in the above formula instead of “h₂”.

The center of the redraw surface is preferably positioned so that the flow of metal makes greater contact with the redraw surface on the entry side (closer to the front of the die) than the exit side. When using a redraw element 20 as shown in FIGS. 3 and 4 as an insert into the land 15 of a die of the kind shown in FIG. 2, then rather than inserting the die centrally within the land, it is preferable to offset it towards the inflection point P by a short distance, e.g. 0-0.2 inch, to produce the desired greater contact with the entry side. The curve on the redraw side is longer and thus the pinch starts just before the minimum gap between the knockout punch and the peak 23 of the redraw radius. As noted, it is most preferable to provide the surface 21 with a regular convex curved profile as shown. This is to ensure that, once the redrawing contact has been made with container wall 13, such contact is eased or withdrawn after the metal moves a very short distance towards the rear of the die. The redrawing contact thus occurs only over a very short annular part of the surface 21 and a corresponding part of the surface 22 of the knockout punch 12. Moving the element 20 away from point P (towards the rear of the die) reduces the arc of contact on the element 20 without reducing the gap at which the pinching of the metal occurs. Conversely, moving the element towards point P increases the arc of contact. Increasing the wrap angle on the die side reduces the stress on the die side. As in asymmetric rolling, the metal flow can be preferentially controlled over the redraw radius. The dimensions and positioning of the redraw element 20 or surface 21 depends to some extent on the thickness and nature of the metal of the wall of the container. The metal is usually steel or an alloy of aluminum. In the latter case, the following metal thickness ranges are usual: 0.002-0.080 inch (0.051-2.03 mm), and more preferably 0.005-0.025 inch (0.127-0.635 mm).

As well as smoothing out radial irregularities in the manner indicated above, it is also found that the action of the redraw element 20 helps to avoid the previously-described wrinkle or ripple in the neck region 26 positioned beyond the redraw element 20. This is assisted further if the cut-back region 17 is of smaller diameter than in the die of FIG. 2 so that there is less distance between the cut-back portion 17 and the adjacent surface 22 of the knockout punch 12. Ideally, this distance should be a minimum so that the knock-out punch 12 does not wobble. The diameter of the cut-back region may therefore be less than the diameter of the surface of the undercut portion 25 ahead of the redraw element 20, as shown in FIGS. 3 and 4.

The step 19 at the end of the knock-out punch is a bearing surface that may ride on the cut-back surface 17 and may always be in contact with this surface. The small gap or clearance “h₃” allows the step 19 to slide on the surface 17 with minimum friction as the knockout punch usually moves slightly faster than the metal during the necking operation. The gap “h₃” is preferably in the range of 0.001 inch to 0.000050 inch, e.g. about 0.0007 inch. The lower end of the range may be achieved by setting this gap effectively to zero and then roughening the surface 17 to a roughness value (R_a) of 5 to 10 micro-inches and polishing off the peaks to create a run-in surface which acts as lubrication bearing surface for the knockout punch. As indicated earlier, on the return stroke, the lubrication is transferred to the entry side of the element 20 into the undercut portion 25 above P in FIG. 4 (a pumping action of the larger diameter step 19 of the knockout punch), which acts as a reservoir for lubricant for reducing the redraw ironing force for the next stroke.

While the above embodiment employs a redraw element 20 positioned within a slot 28 within the region of the land 15 of a die of the type shown in FIG. 2, this is not essential, although it may be a preferred way of modifying existing dies of this kind or manufacturing new dies. Instead, however, the die itself may be provided with an internal shape provided with a redraw surface 21 and (if desired) an undercut portion 25. This is illustrated in FIGS. 6 and 7 which show alternative profiles. FIG. 6 shows the profile of a die 11 having a necking-in zone TP, and inflection point P, an undercut portion 25, a convex curved redraw surface 21, and a cut-back region 17. The position of a land 15 of the type shown in FIG. 2 is shown by a dotted line to illustrate the extra height of the redraw surface 21 above this level. In the case of FIG. 7, the surface of the undercut portion 25 is not quite axial (it slopes or tapers slightly radially inwardly towards the central axis of the die) and the redraw surface 21 is asymmetrically shaped to minimize contact at the entry side. The provision of a smooth arc for surface 21 is normally considered preferable, but the shape of FIG. 7 may be useful if a sharper radius is required to fit into the area previously forming the land. The preferred radius at P is one that is as long as possible for a smooth transition to be obtained on the necked-in portion. The profile of FIG. 7 allows a large radius at P while providing the required undercut portion 25.

The drawings of the present application show a single die and knockout punch set designed for one of many necking-in steps of a particular shaping operation. Each such operation will have a specially designed die and knockout punch set as the profile of the container is gradually shaped to a final profile. Each of these dies may be provided with a redraw surface according to the exemplary embodiments, but it is more usual to provide the surface in only those dies used for the last few steps. This is because the deformities in the container shapes start out small and build up with each successive necking step, so it is only at the end stages where the deformities are significant and need to be corrected. The number of dies needing a redraw surface can easily be determined by trial and error, or with the benefit of experience.

Claims

1. A die set for necking-in a metal container, which die set comprises a die and a knockout punch having a generally cylindrical surface, the die having, in an axial direction from front to back of the die, an inwardly tapering necking surface having an inflection point at an innermost end thereof, a convex redraw surface extending inwardly of the die, and a generally cylindrical cutback surface having a diameter larger than said redraw surface at a peak thereof and extending axially from the redraw surface towards the back of the die, wherein said redraw surface at said peak defines an opening dimensioned to receive said knockout punch with a spacing effective to redraw a wall of a container necked by said die.

2. The die set of claim 1, wherein said redraw surface is positioned such that a container necked by said die is caused to bend plastically when extending from said inflection point to said peak.

3. The die set of claim 1, wherein said die has an undercut portion with a generally cylindrical surface immediately following said inflection point forming a reservoir for lubricant supplied to said die during use.

4. The die set of claim 3, wherein said undercut portion extends rearwardly from said inflection point by a distance in a range of 0 to 0.2 inches.

5. The die set of claim 3, wherein said peak of said redraw surface extends inwardly from said generally cylindrical surface of said undercut portion by an amount in a range of 0.0001 to 0.012 inch.

6. The die set of claim 3, wherein said peak of said redraw surface extends inwardly from said generally cylindrical surface of said undercut portion by an amount in a range of 0.0001 to 0.006 inch.

7. The die set of claim 1, wherein said redraw surface is in the form of an arc having a radius of 0.02 to 4 inches.

8. The die set of claim 3, wherein said redraw surface is in the form of an arc that extends axially of said die by a distance “x”, extends diametrically inwardly beyond said generally cylindrical surface of said undercut portion by a distance “h” and has a radius “rredraw”, and wherein r redraw = [ x 2 ] 2 ( 2 × h ).

9. The die set of claim 1, wherein said knockout punch has a step at a rear end thereof which extends diametrically outwardly towards said cutback surface, and wherein said cutback surface is spaced diametrically from said step by a distance of 0.001 to 0.00005 inch.

10. The die set of claim 1, wherein said knockout punch has a step at a rear end thereof which extends diametrically outwardly towards said cutback surface, and wherein said cutback surface is spaced diametrically from said step by a distance of about 0.0007 inch.

11. A die set according to claim 9, wherein said step acts as a pump to transfer lubricant to said redraw surface during a knockout step.

12. The die set of claim 3, wherein said cutback surface has a smaller diameter than said generally cylindrical surface of said undercut portion.

13. A die set according to claim 1, wherein said redraw surface is formed on a redraw element inserted into an annular groove in said die immediately rear of said inflection point.

14. The die set of claim 1, wherein said redraw surface is formed on a redraw element that is integral with a remainder of the die.

15. The die set of claim 1, wherein redraw surface is asymmetrical in axial profile.

16. An annular necking die which comprises, in a direction from front to back of the die, a tapering necking surface of decreasing diameter having an inflection point at an innermost end thereof, a convex redraw surface extending inwardly of the die, and a cylindrical cutback surface having a diameter larger than said redraw surface and extending from the redraw surface towards the back of the die, wherein, during use of the die in a necking-in operation carried out on a cylindrical wall of a hollow metal container pre-form in conjunction with a cylindrical knockout punch dimensioned to fit snugly within said die, said redraw surface is positioned and dimensioned to contact a wall of said pre-form and to compress said wall against said knockout punch, thereby smoothing out circumferential irregularities of said wall caused by wall-thickening as said pre-form is necked in.

17. The necking die of claim 16, wherein said die has an undercut portion with a generally cylindrical surface immediately following said inflection point forming a reservoir for lubricant supplied to said die during use.

18. A method of necking-in a metal container pre-form having a cylindrical wall and an open end, said method comprising carrying out a plurality of necking-in steps by introducing the open end of the container into necking-in dies operated with cooperating knockout punches, wherein for at least one of said steps, said container wall is both reduced in diameter and said container wall is redrawn to minimize circumferential irregularities of wall thickness caused by said reduction in diameter.

19. The method of claim 18, wherein said container wall has a range of angles through which it may be bent plastically, and wherein, during said at least one step, said container wall is bent through an angle within said range.

20. The method of claim 18, wherein said container wall is reduced in thickness by an amount of 10% or less as said wall is redrawn.

21. The method of claim 18, wherein said container wall is reduced in thickness by an amount of 5% or less as said wall is redrawn.

22. The method of claim 18, wherein said container wall is made of even thickness at positions around a periphery thereof as said wall is redrawn.

23. The method of claim 18, wherein said wall is redrawn by forcing said wall through an annular gap within a necking-in die set comprising a die and a knockout punch, wherein said annular gap is formed between a redraw surface on said die and an external surface of said knockout punch positioned rearwardly of an inflection point on a necking-in surface of said die, and wherein said annular gap has a width that is the same as or less than an average thickness of said container wall at said inflection point.

24. The method of claim 18, wherein a lubricant is fed to said container wall as said wall is passed through said gap.

25. A method of providing a necking-in die with capability of container wall redrawing, which comprises cutting an annular groove into a surface of a cylindrical land of a necking-in die and inserting in said groove an element having a convex redraw surface extending inwardly beyond said surface of the land.