Sheet metal container making – Apparatus for can body making
Patent
1997-12-04
1999-11-16
Coan, James F.
Sheet metal container making
Apparatus for can body making
72348, 220906, B21D 2230, B21D 4508
Patent
active
059846046
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of drawing hollow articles from a blank. In particular, it relates to a method of drawing a cup-shaped blank into a drawn and wall-ironed (DWI) one-piece can body.
In known methods of drawing cans, the blank is held on a punch and carried through a succession of dies for drawing the shallow cup and ultimately strikes a bottom former to produce the desired base profile. For beverage cans, this base profile is typically a dome, whereas for food cans the base profile typically has a plurality of concentric annular panels surrounding a central panel. Alternatively, the base profile may be formed in a separate process which combines pressing the inner annular beads and then roll forming a deeper outer "anti-peaking" bead.
The material used for can manufacture is costly and so efforts have been made over recent years to reduce the thickness of the material required so as to reduce material costs accordingly. However, limitations in the thickness reduction are imposed by the forming process and by the particular base profile which is required in order to cope with thermal processing and pasteurisation and with conditions imposed by the product itself, such as carbonated beverages.
Food cans are often formed from a ferrous material, for example single reduced (SR) or double reduced (DR) steel. The steel is typically in the form of tinplate such as T57 tinplate. This tinplate has a yield strength of 200 to 300 Nmm.sup.-2 and UTS of 330 to 410 Nmm.sup.-2. Minimum elongation to fracture is 23% and proof/UTS is 80 to 90%. Usually the tinplate finish used for food cans is matt although flow brightened tinplate is used for some applications such as partially lacquered cans. The tin coating is usually selected according to the product for which the can is to be used, for example T57 tinplate cans used for human food have a tin coating of 2.8/2.8 gm.sup.-2.
The profile used for the base of one-piece can bodies formed in a single process exhibits thinning around the tight bead radii due to the tensile forces arising during base formation. Base forming loads are particularly high where the can is wall-ironed. Thinning is a particular problem at the innermost bead and if the material is too thin will lead to splitting of the base at this point. Consequently, the minimum thickness which it is possible to use for formation of a one-piece 73 mm diameter DWI can body in a single process from T57 tinplate is 0.275 mm SR, or 0.270 mm SR for a 65 mm DWI food can. Conventional bases may be formed from 0.270 mm SR material without splitting but these are not strong enough to withstand some processing pressures.
SUMMARY OF THE INVENTION
According to the present invention there is provided a can body formed by the steps of: the cup side wall; dies; and profile; wall including at least one annular bead surrounding a central panel, the or one of the bead(s) having an internal radius of between 0.8 mm and 1.4 mm.
Typically the internal radius may be 1.4 mm for a 73 mm diameter can body but may be reduced to as low as 0.8 mm for the same can body by the introduction of fluid in accordance with the invention. These radii are much tighter than has been found possible using conventional base forming methods and the resultant base profile is much stronger. This radius may usually be what is known as the "countersink radius". The radii are not related to specific can diameters but typical can diameters for which these profiles would be used are 65 and 73 mm.
The can body may be formed from tinplate having a UTS value of up to 650 Nmm.sup.-2, preferably 500 Nmm.sup.-2 or less. The tinplate may be double reduced steel and may have a thickness of at least 0.15 mm.
This can body is preferably drawn and wall ironed as it passes through the series of dies.
According to a second aspect of the present invention, there is provided a can body formed by the steps of: the cup side wall; dies; and profile; wall including a peripheral channel portion having a depth of between 4% and 8% of the can body diameter.
Typically
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patent: 3812696 (1974-05-01), Kneusel et al.
patent: 4065951 (1978-01-01), Lyu
patent: 4151927 (1979-05-01), Cvacho et al.
patent: 4373368 (1983-02-01), Heijting
patent: 4930330 (1990-06-01), Weishalla
patent: 5056350 (1991-10-01), Moen et al.
Carnaudemtal Box Inc.
Coan James F.
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