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 ( 40 ) inside the cup, moving an annular wall-ironing die ( 24 ) 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 ( 24 ) back in an opposite direction to remove the can body from the die ( 24 ). 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;
moving an annular wall-ironing die axially over a closed end of the cup towards an 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;
forming a profile in a base of the cup at the closed end to form the can body, wherein said die is shaped such that when the profile is formed in the base of the cup the die passes across a region beyond said axially spaced position toward the open end without ironing that region; and
moving the wall-ironing die back in an opposite direction to remove the can body from the die.
2. A method according to claim 1 , wherein said position is between 1 mm and 20 mm from the open end of the cup.
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 1 , further comprising removing the punch from within the cup prior to moving the wall-ironing die back in the opposite direction.
6. A method according to claim 5 , wherein the forming the profile in the base of the cup further comprises pressing a tool against an outer surface of the base of the cup at the closed end.
7. A method as claimed in claim 1 , wherein the metal cup comprises a flange surrounding the open end, and the method further comprises applying a force to the flange to remove the can body from the punch.
8. A method as claimed in claim 7 , 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 and wherein the applying force to the flange occurs when the burnishing is complete.
9. A method as claimed in claim 1 , wherein the punch comprises an internal longitudinal channel, and the method further comprises introducing compressed air into the can body through the channel to remove the can body from the punch.
10. A method as claimed in claim 1 , wherein the cup is made from sheet metal pre-coated with a polymer such as polyester or polypropylene.