Necking die with redraw surface and method of die necking
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.
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 “r redraw ”, 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.