Facilitating removal of deep drawn sheet metal can bodies from female die structure

In the deep drawing of unitary can bodies, method and apparatus which reduce frictional contact of the can body sidewall with the female die opening to permit removal of the drawn can body from the female die in the direction of removal of the male punch member without damage to the can body. Deep drawing of organically coated sheet metal stock and removal from the female die structure is also made possible while maintaining desired protection. Smooth, non-reentrant sidewall surfaces are maintained in the work product.

This invention relates to deep drawing of a unitary sheet metal can body 
having a smooth surface cylindrical sidewall and facilitating removal of 
such a deep drawn can body from the female die structure after the draw 
step. 
Unitary can bodies have been used for commercial manufacture of small 
containers requiring a relatively shallow draw, about 11/2" (3.8 cm) and 
having a diameter of at least twice the draw height. However, with the 
deep drawing operation required for unitary can bodies of greater sidewall 
heights, e.g. about 3" (7.62 cm) where the height to diameter ratio 
approaches one, difficulties are encountered which have inhibited wide 
commercial usage of unitary can bodies of extended sidewall height. 
During the drawing step required for such extended-height can bodies it has 
been found that the sidewall sheet metal bulges outwardly causing it to 
"hug" the internal surface of the female die opening. This creates high 
surface friction between the external sidewall surface of the drawn can 
body and the interior sidewall surface of the female die such that the can 
body cannot be readily removed without damage to the unitary endwall. 
The present invention reduces the sidewall frictional contact in deep 
drawing a longitudinally extended sidewall for a unitary sheet metal can 
body while maintaining the desired smooth, non-reentrant sidewall 
characteristics. Also, removal of can bodies drawn from organically coated 
sheet metal stocks is made possible while maintaining desired organic 
coating.

In drawing operations to form a cup-shaped can body, a sheet metal blank is 
held about its periphery between the clamping surfaces of the female die 
means and a clamping ring on the male side of the structure. This clamping 
action is shown in FIG. 1 which depicts the general relationship of the 
die structure parts and the sheet metal during the drawing operation to 
form a cup-shaped can body. 
In FIG. 1, female die structure 10 includes an internal cylindrically 
shaped surface 12 defining a die opening which is concentric with central 
longitudinal axis 16. The sheet metal is clamped around its periphery 18 
between a female die clamping surface 20 and clamping ring 22 on the male 
side. Male die plunger 24 is moved in a direction coincident with the 
central longitudinal axis during its work stroke into the female die 
opening and the sheet metal takes on the cup-shaped configuration 26. 
Changing the sheet metal from a substantially flat configuration to a 
cylindrical configuration requires large forces to be applied against the 
internal surface bottom wall. As a result, there may be a "spring-back" or 
"memory" action in the sheet metal tending to return to its original shape 
after the work stroke stops, which causes the sidewall sheet metal to 
bulge against the female die opening. The male plunger, on the other hand, 
can generally be withdrawn with little or no resistance. 
As indicated by FIG. 1, a tight frictional contact of sidewall 28 with 
working surface 30 of the female die opening results when deep drawing a 
can body while a loose relationship exists between the external sidewall 
surface of the male die plunger 24 and the sheet metal. Such 
tight-adherence, high-friction contact between the internal surface of a 
prior art female die opening and the external surface of the can body 
holds the can body in such female die structure while the male plunger is 
removed. In drawing steel can bodies when the height of the sidewall 
exceeds half the diameter of the cup-shape and especially in drawing sheet 
metal of thickness gages commonly used for can bodies of 3" (7.62 cm) and 
greater sidewall heights, the forces required for removal of such can 
bodies in the direction of withdrawal of the male punch are such that 
damage results in an unacceptable percentage of product. Removal in the 
direction of withdrawal of the male die plunger is required for most food 
pack can bodies in order to provide flanging metal for double-seam joinder 
of a closure to the open end of the can body. 
In FIG. 1 the direction of the work stroke is indicated by arrow 29. 
Because of the loose relationship with the male die plunger 24, its 
withdrawal opposite to the direction of arrow 29 is not of significant 
help in removing the drawn product. Removal must take place by force being 
applied to the external surface 34 of the bottom wall of can body 26. This 
not only resulted in damage to an unacceptable percentage of uncoated 
sheet metal can bodies but ruled out precoating with organic coatings 
because of damage to such coatings. 
The invention teaches a reduction in this sidewall frictional contact along 
the internal sidewall of the female die opening while maintaining 
sufficient surface contact for drawing the sidewall to produce the desired 
smooth, non-reentrant sidewall surface characteristics. The male die 
member and clamping structures used in the invention are not significantly 
changed from those shown schematically in FIG. 1; nor are the metal 
chemistry and physical properties of the tool used for manufacturing the 
female die structure of the invention changed significantly from those 
generally known and accepted in the art. 
FIG. 2 shows a portion of the female die structure of the invention, on one 
side of the central longitudinal axis 40, in enlarged cross section. The 
internal surface of the opening of this female die structure is formed 
with a series of raised, ring-shaped, cylindrical surface portions 42, 44, 
46 and 48 distributed longitudinally along the working surface of the die 
opening; these provide a female drawing surface of substantially reduced 
area. Such raised portions are separated by a series of recessed portions 
50, 52, 54 and 56 of ring-shaped cylindrical configuration, which support 
the raised portions 42,44,46, and 48 without frictional contact with the 
work product. 
The female die structure includes clamping surface 60, entry radius 62, and 
a cylindrical entry portion 64 having substantially the desired diameter 
for the external surface of the can body being drawn. The diameter of 
entry portion 64 is the same as that of raised portion 42,44,46 and 48. 
The longitudinal dimension of the raised portions 42,44,46 and 48 is 
selected to provide the necessary support to the can body sidewall during 
drawing in order to produce a smooth non-reentrant sidewall. The 
longitudinal dimension of a recessed portion is a multiple of the 
longitudinal dimension of a raised portion; such multiple extending from 
about five (5) to about thirty (30) times the longitudinal dimension of 
the surface contact of a raised portion. In a typical example, the 
recessed ring shaped portions would have a longitudinal dimension of 1/2" 
(1.25 cm), the cylindrical portion 64 at the entry end of the die would 
have a longitudinal dimension of about 1/2" (1.25 cm), and the raised 
ring shaped portions would have a longitudinal dimension between 1/64" 
and 1/8" (0.39 mm and 3.18 mm). For a 5" (11.7 cm) diameter can body, the 
entry portion and raised ring shape portions of the die would have a 
diameter of about 5.010" (11.725 cm), and the recessed portions a diameter 
of 5.015" (11.738 cm). 
Considering typical dimensions for a smaller diameter can body: in the 
female die structure 66 of FIG. 3 (entry from above) the recessed ring 
shaped diameter 68 has a dimension of 2.877" (7.307 cm) and the entry 
portion and raised ring shaped portions diameter 70 has a value of 2.875" 
(7.302 cm). Die structure 66 includes clamping surface 74 and recess means 
76 for holding the die structure in the drawing machinery (not shown). 
While the features of the invention have application where the depth of 
draw exceeds half the diameter, its contributions are more pronounced in 
drawing a cup shape where ratio of the sidewall height to the diameter 
equals or exceeds one, especially where the sidewall height of a can body 
is 4" (10.16 cm) or longer. 
Removal of can bodies drawn from sheet metal precoated with conventional 
organic coatings, e.g. vinyls, epoxies, phenolics and acrylics, is made 
possible by the present invention. Organic coating of flat rolled stock 
can be carried out in continuous strip lines. After cutting into blanks, 
the coated blanks are drawn in the described die structure. The reduction 
of surface friction provided enables removal of the coated drawn cup 
shape, in the direction of withdrawal of the male punch, while maintaining 
the protective and/or decorative coating on both surfaces of the can body. 
The organic coating is maintained on the external surface of the can body 
sidewall without damage as a result of removal from the female die 
opening. 
The invention in addition to being useful in the drawing of steel thickness 
gages for can bodies between about 0.0065" and about 0.011" (about 0.15 mm 
to about 0.28 mm) is also applicable in the drawing of aluminum can bodies 
typically having gages between 0.007" (0.178 mm) and 0.015" (0.374 mm). 
Other configurations and interrelations of dimensions are available than 
those specifically set forth; therefore the scope of the invention is to 
be determined from the appended claims.