Metal containers and methods of manufacture
A method of reducing a thickness and increasing a height of a cylindrical wall of a metal cup to form a can body comprises positioning a wall-ironing punch inside the cup, moving an annular wall-ironing die axially over the closed end of the cup towards the open end of the cup, but not beyond the open end of the cup, in order to iron the cylindrical wall from the closed end up to a position axially spaced from the open end and moving the wall-ironing die back in an opposite direction to remove the can body from the die. Methods of altering a diameter of one or more regions of a can body are also disclosed.
1. A method of reducing a thickness and increasing a height of a cylindrical wall of a metal cup so as to form a can body, the method comprising:
positioning a wall-ironing punch inside the cup, the punch including:
a first punch portion having a first diameter,
a second punch portion having a second diameter,
a third punch portion having a third diameter;
a first transition between the first punch portion and the second punch portion such that the second punch portion diameter is greater than the first punch portion diameter;
wherein the first punch portion corresponds to a closed end of the cup; and
a second transition between the second punch portion and the third punch portion such that the third punch portion diameter is greater than the second punch portion diameter;
moving an annular wall-ironing die axially over the closed end of the cup towards an open end of the cup, such that the wall-ironing die does not move beyond the open end of the cup, in order to iron the cylindrical wall from the closed end up to a position axially spaced from the open end, thereby forming the can body;
moving the wall-ironing die back in an opposite direction to remove the can body from the die; and
forming a curl at an uppermost end of the can body;
thereby forming a body-wall of the can body having a wall thickness proximate a base of the can body that is greater than a wall thickness of a second body-wall portion.
2. A method as claimed in claim 1 , where said step of moving the annular wall ironing die stops at a position that is between 1 mm and 20 mm from the open end of the formed can body.
3. A method as claimed in claim 1 , wherein the step of moving the wall-ironing die back in the opposite direction is carried out with the punch positioned inside the can body in order to burnish a portion of the cylindrical wall.
4. A method as claimed in claim 3 , wherein burnishing comprises reducing a thickness of the portion of the cylindrical wall by around 1%.
5. A method as claimed in claim 3 , the method further comprising pressing a tool against an outer surface of the base of the can body as the wall-ironing die is moved back in the opposite direction in order to form a profile in the base.
6. A method as claimed in claim 1 , the method further comprising removing the punch from within the can body prior to moving the wall-ironing die back in the opposite direction.
7. A method as claimed in claim 6 , the method further comprising pressing a tool against the outer surface of a base of the can body as the wall-ironing die is moved towards the open end of the cup in order to form a profile in the base.
8. A method as claimed claim 1 , wherein the punch comprises an internal longitudinal channel and the method comprises introducing compressed air into the can body through the channel to remove the can body from the punch.