U.S. patent application number 10/007263 was filed with the patent office on 2002-11-07 for method of pressure-ram-forming metal containers and the like.
Invention is credited to Gong, Kevin, Hamstra, Peter, MacEwen, Stuart.
Application Number | 20020162371 10/007263 |
Document ID | / |
Family ID | 34632365 |
Filed Date | 2002-11-07 |
United States Patent
Application |
20020162371 |
Kind Code |
A1 |
Hamstra, Peter ; et
al. |
November 7, 2002 |
Method of pressure-ram-forming metal containers and the like
Abstract
A method of forming a bottle-shaped or other contoured metal
container by subjecting a hollow metal preform having a closed end
to internal fluid pressure to cause the preform to expand against
the wall of a die cavity defining the desired shape, and advancing
a punch into the die cavity to displace and deform the closed end
of the preform after expansion begins but before it is complete.
The pressure-subjecting step is performed by simultaneously
subjecting the preform in the die cavity to independently
controllable internal and external positive fluid pressures and
varying the difference between them to control strain rate.
Inventors: |
Hamstra, Peter; (Kingston,
CA) ; MacEwen, Stuart; (Inverary, CA) ; Gong,
Kevin; (Kingston, CA) |
Correspondence
Address: |
COOPER & DUNHAM LLP
1185 Ave. of the Americas
New York
NY
10036
US
|
Family ID: |
34632365 |
Appl. No.: |
10/007263 |
Filed: |
November 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10007263 |
Nov 8, 2001 |
|
|
|
09846546 |
May 1, 2001 |
|
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Current U.S.
Class: |
72/58 |
Current CPC
Class: |
B21D 51/16 20130101;
B21D 51/26 20130101; B21D 26/033 20130101; B21D 22/16 20130101;
B21D 26/047 20130101; B21D 26/049 20130101; B21D 26/041
20130101 |
Class at
Publication: |
72/58 |
International
Class: |
B21D 039/08 |
Claims
What is claimed is:
1. A method of forming a metal container of defined shape and
lateral dimensions, comprising (a) disposing a hollow metal preform
having a closed end in a die cavity laterally enclosed by a die
wall defining said shape and lateral dimensions, with a punch
located at one end of the cavity and translatable into the cavity,
the preform closed end being positioned in proximate facing
relation to the punch and at least a portion of the preform being
initially spaced inwardly from the die wall; (b) subjecting the
preform to internal fluid pressure to expand the preform outwardly
into substantially full contact with the die wall, thereby to
impart said defined shape and lateral dimensions to the preform,
said fluid pressure exerting force, on said closed end, directed
toward said one end of the cavity; and (c) after the preform begins
to expand but before expansion of the preform is complete in step
(b), translating the punch into the cavity to engage and displace
the closed end of the preform in a direction opposite to the
direction of force exerted by fluid pressure thereon, deforming the
closed end of the preform.
2. A method according to claim 1, wherein said punch has a
contoured surface, the closed end of the preform being deformed so
as to conform to said contoured surface.
3. A method according to claim 1, wherein said defined shape is a
bottle shape including a neck portion and a body portion larger in
lateral dimensions than the neck portion, said die cavity having a
long axis, said preform having a long axis and being disposed
substantially coaxially with said cavity in step (a), and said
punch being translatable along the long axis of the cavity.
4. A method according to claim 3, wherein said punch has a domed
contour, and wherein step (c) deforms said closed end of said
preform into said domed contour.
5. A method according to claim 3, wherein said die wall comprises a
split die separable for removal of the formed container following
step (c).
6. A method according to claim 5, wherein said defined shape is
asymmetric about said long axis of said cavity.
7. A method according to claim 3, wherein said punch is initially
positioned, at the start of step (b), to limit axial lengthening of
the preform by said fluid pressure.
8. A method according to claim 3, wherein step (c) is initiated at
substantially the same time that said portion of the preform begins
to come into contact with the die wall.
9. A method according to claim 3, wherein said preform is an
elongated and initially generally cylindrical workpiece having an
open end opposite said closed end and is substantially equal in
diameter to said neck portion of said bottle shape.
10. A method according to claim 9, wherein said workpiece has
sufficient formability to be expandable to said defined shape in a
single pressure forming operation.
11. A method according to claim 9, including a preliminary steps of
placing the workpiece in a die cavity smaller than the
first-mentioned die cavity and subjecting the workpiece therein to
internal fluid pressure to expand the workpiece to an intermediate
size and shape smaller than said defined shape and lateral
dimensions, before performing steps (a), (b) and (c).
12. A method according to claim 3, wherein said preform is an
elongated and initially generally cylindrical workpiece having an
open end opposite said closed end and is larger in diameter than
said neck portion of said bottle shape; and including a further
step of subjecting the workpiece, adjacent said open end, to a spin
forming operation to form a neck portion of reduced diameter, after
performance of steps (a), (b) and (c).
13. A method according to claim 1, wherein said preform is an
aluminum preform.
14. A method according to claim 13, including the step of making
the preform from aluminum sheet having a recrystallized or
recovered microstructure with a gauge in a range of about 0.25 to
about 1.5 mm, prior to performance of step (a).
15. A method according to claim 14, wherein said preform is
produced as a closed end cylinder by subjecting said sheet to a
draw-redraw operation or back extrusion.
16. A method according to claim 1, wherein, during step (b), fluid
pressure within the preform occurs in successive stages of (i)
rising to a first peak before expansion of the preform begins, (ii)
dropping to a minimum value as expansion commences, (iii) rising
gradually to an intermediate value as expansion proceeds until the
preform is in extended though not complete contact with the die
wall, and (iv) rising from the intermediate value during completion
of preform expansion; and wherein initiation of translation of the
punch in step (c) to displace and deform the closed end of the
preform occurs substantially at the end of stage (iii).
17. A method according to claim 1, wherein, during step (b), the
closed end of the preform assumes an enlarged and generally
hemispherical configuration as said portion of the preform comes
into initial contact with the die wall in step (b); and wherein
initiation of translation of the punch in step (c) to displace and
deform the closed end of the preform occurs substantially at the
time that the preform closed end assumes said configuration.
18. A method according to claim 1 wherein step (b) comprises
simultaneously applying internal positive fluid pressure and
external positive fluid pressure to the preform in the cavity, said
internal positive fluid pressure being higher than said external
positive fluid pressure.
19. A method according to claim 18, including controlling strain
rate in the preform by independently controlling the internal and
external positive fluid pressures to which the preform is
simultaneously subjected for varying the differential between said
internal positive fluid pressure and said external positive fluid
pressure.
20. A method of forming a metal container of defined shape and
lateral dimensions, comprising (a) disposing a hollow metal preform
having a closed end in a die cavity laterally enclosed by a die
wall defining said shape and lateral dimensions, with a punch
located at one end of the cavity and translatable into the cavity,
the preform closed end being positioned in proximate facing
relation to the punch and at least a portion of the preform being
initially spaced inwardly from the die wall; (b) simultaneously
subjecting the preform within the cavity to internal positive fluid
pressure and to external positive fluid pressure less than said
internal pressure such that there is a positive internal-external
fluid pressure differential, while independently controlling said
internal pressure and said external pressure, to expand the preform
outwardly into substantially full contact with the die wall,
thereby to impart said defined shape and lateral dimensions to the
preform, said fluid pressure differential exerting force, on said
closed end, directed toward said one end of the cavity; and (c)
after the preform begins to expand but before expansion of the
preform is complete in step (b), translating the punch into the
cavity to engage and displace the closed end of the preform in a
direction opposite to the direction of force exerted by said fluid
pressure differential thereon, deforming the closed end of the
preform.
21. A method according to claim 20, wherein the internal and
external positive fluid pressures are independently controlled for
varying the differential between said internal positive fluid
pressure and said external positive fluid pressure to control
strain rate in the preform.
22. A method according to claim 21, wherein the metal is
aluminum.
23. A method according to claim 20, wherein the external positive
fluid pressure is applied by controllably supplying fluid under
positive pressure to the die cavity between the die wall and the
preform.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of applicants'
copending U.S. patent application Ser. No. 09/846,546, filed May 1,
2001, the entire disclosure of which is incorporated herein by this
reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to methods of forming metal
containers or the like, utilizing internal fluid pressure to expand
a hollow metal preform or workpiece against a die cavity. In an
important specific aspect, the invention is directed to methods of
forming aluminum or other metal containers having a contoured
shape, e.g. such as a bottle shape with asymmetrical features.
[0003] Metal cans are well known and widely used for beverages.
Present day beverage can bodies, whether one-piece "drawn and
ironed" bodies, or bodies open at both ends (with a separate
closure member at the bottom as well as at the top), generally have
simple upright cylindrical side walls. It is sometimes desired, for
reasons of aesthetics, consumer appeal and/or product
identification, to impart a different and more complex shape to the
side wall of a metal beverage container, and in particular, to
provide a metal container with the shape of a bottle rather than an
ordinary cylindrical can shape. Conventional can-producing
operations, however, do not achieve such configurations.
[0004] For these and other purposes, it would be advantageous to
provide convenient and effective methods of forming workpieces into
bottle shapes or other complex shapes. Moreover, it would be useful
to provide such procedures capable of forming contoured container
shapes that are not radially symmetrical, to enhance the variety of
designs obtainable.
SUMMARY OF THE INVENTION
[0005] The present invention broadly contemplates the provision of
a method of forming a metal container of defined shape and lateral
dimensions, comprising disposing a hollow metal preform having a
closed end in a die cavity laterally enclosed by a die wall
defining the shape and lateral dimensions, with a punch located at
one end of the cavity and translatable into the cavity, the preform
closed end being positioned in proximate facing relation to the
punch and at least a portion of the preform being initially spaced
inwardly from the die wall; subjecting the preform to internal
fluid pressure to expand the preform outwardly into substantially
full contact with the die wall, thereby to impart the defined shape
and lateral dimensions to the preform, the fluid pressure exerting
force, on the preform closed end, directed toward the aforesaid one
end of the cavity; and, after the preform begins to expand but
before expansion of the preform is complete, translating the punch
into the cavity to engage and displace the closed end of the
preform in a direction opposite to the direction of force exerted
by fluid pressure thereon, deforming the closed end of the preform.
Translation of the punch is effected by a ram which is capable of
applying sufficient force to the punch to displace and deform the
preform. This method will sometimes be referred to herein as a
pressure-ram-forming (PRF) procedure, because the container is
formed both by applied internal fluid pressure and by the
translation of the punch by the ram.
[0006] As a further feature of the invention, the punch has a
contoured surface, and the closed end of the preform is deformed so
as to conform to the contoured surface. For instance, the punch may
have a domed contour, the closed end of the preform being deformed
into the domed contour.
[0007] The defined shape, in which the container is formed, may be
a bottle shape including a neck portion and a body portion larger
in lateral dimensions than the neck portion, the die cavity having
a long axis, the preform having a long axis and being disposed
substantially coaxially within the cavity, and the punch being
translatable along the long axis of the cavity.
[0008] Advantageously and preferably, the die wall comprises a
split die separable for removal of the formed container. With a
split die, the defined shape may be asymmetric about the long axis
of the cavity.
[0009] The punch is preferably initially positioned close to or in
contact with the preform closed end, before the application of
fluid pressure, in order to limit axial lengthening of the preform
by the fluid pressure. Translation of the punch may be initiated
after the expanding lower portion of the preform has come into
contact with the die wall.
[0010] The preform, for forming a bottle-shaped container or the
like, is preferably an elongated and initially generally
cylindrical workpiece having an open end opposite its closed end.
In particular embodiments of the invention, it may be substantially
equal in diameter to the neck portion of the bottle shape, and may
have sufficient formability to be expandable to the defined shape
in a single pressure forming operation. If it lacks such
formability, preliminary steps of placing the workpiece in a die
cavity smaller than the first-mentioned die cavity, and subjecting
the workpiece therein to internal fluid pressure to expand the
workpiece to an intermediate size and shape smaller than the
defined shape and lateral dimensions, are performed prior to the
PRF method described above.
[0011] Alternatively, if the elongated and initially generally
cylindrical workpiece is larger in initial diameter than the neck
portion of the bottle shape, the method of forming a bottle-shaped
container may include a further step of subjecting the workpiece,
adjacent its open end, to a spin forming operation to form a neck
portion of reduced diameter, after performance of the PRF
procedure.
[0012] The preform may be an aluminum preform (the term "aluminum"
herein being used to refer to aluminum-based alloys as well as pure
aluminum metal) and may be made from aluminum sheet having a
recrystallized or recovered microstructure with a gauge in a range
of about 0.25 to about 1.5 mm. It may be produced as a closed end
cylinder by subjecting the sheet to a draw-redraw operation or back
extrusion.
[0013] During the step of subjecting the preform to internal fluid
pressure, the fluid pressure within the preform occurs in
successive stages of (i) rising to a first peak before expansion of
the preform begins, (ii) dropping to a minimum value as expansion
commences, (iii) rising gradually to an intermediate value as
expansion proceeds until the preform is in extended though not
complete contact with the die wall, and (iv) rising from the
intermediate pressure during completion of preform expansion.
Stated with reference to this sequence of pressure stages, the
initiation of translation of the punch to displace and deform the
closed end of the preform in preferred embodiments of the invention
occurs substantially at the end of stage (iii).
[0014] Typically, when the internal fluid pressure is applied, the
closed end of the preform assumes an enlarged and generally
hemispherical configuration as the preform comes into contact with
the die wall; and initiation of translation of the punch occurs
substantially at the time that the preform closed end assumes this
configuration.
[0015] Also in accordance with the invention, the step of
subjecting the preform to internal fluid pressure comprises
simultaneously applying internal positive fluid pressure and
external positive fluid pressure to the preform in the cavity, the
internal positive fluid pressure being higher than the external
positive fluid pressure. The internal and external pressure are
respectively provided by two independently controllable pressure
systems. Strain rate in the preform is controlled by independently
controlling the internal and external positive fluid pressures to
which the preform is simultaneously subjected for varying the
differential between the internal positive fluid pressure and the
external positive fluid pressure. In this way, problems associated
with excessive strain rates are avoided and additional beneficial
results, such as reduction in the hydrostatic stress that may cause
microstructural damage to the container wall, are achieved.
[0016] Further features and advantages of the invention will be
apparent from the detailed description hereinafter set forth,
together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a simplified and somewhat schematic perspective
view of tooling for performing the method of the present invention,
in illustrative embodiments;
[0018] FIGS. 2A and 2B are views similar to FIG. 1 of sequential
stages in the performance of a first embodiment of the method of
the invention;
[0019] FIG. 3 is a graph of internal pressure and ram displacement
as functions of time, using air as the fluid medium, illustrating
the time relationship between the steps of subjecting the preform
to internal fluid pressure and translating the punch in the method
of the invention;
[0020] FIGS. 4A, 4B, 4C and 4D are views similar to FIG. 1 of
sequential stages in the performance of a second embodiment of the
method of the invention;
[0021] FIGS. 5A and 5B are, respectively, a view similar to FIG. 1
and a simplified, schematic perspective view of a spin-forming
step, illustrating sequential stages in the performance of a third
embodiment of the invention;
[0022] FIGS. 6A, 6B, 6C and 6D are computer-generated schematic
elevational views of successive stages in the method of the
invention;
[0023] FIG. 7 is a graph of pressure variation over time (using
arbitrary time units) illustrating the feature of simultaneously
applying independently controllable internal and external positive
fluid pressures to the preform in the die cavity and comparing
therewith internal pressure variation (as in FIG. 3) in the absence
of external positive pressure;
[0024] FIG. 8 is a graph of strain variation over time, derived
from finite element analysis, showing strain for one particular
position (element) under the two different pressure conditions
compared in FIG. 7; and
[0025] FIG. 9 is a graph similar to FIG. 7 illustrating a
particular control mechanism that can be used in the forming
process when internal and external positive fluid pressures are
simultaneously applied to the preform in the die cavity.
DETAILED DESCRIPTION
[0026] The invention will be described as embodied in methods of
forming aluminum containers having a contoured shape that need not
be axisymmetric (radially symmetrical about a geometric axis of the
container) using a combination of hydro (internal fluid pressure)
and punch forming, i.e., a PRF procedure.
[0027] The PRF manufacturing process has two distinct stages, the
making of a preform and the subsequent forming of the preform into
the final container. There are several options for the complete
forming path and the appropriate choice is determined by the
formability of the aluminum sheet being used.
[0028] The preform is made from aluminum sheet having a
recrystallized or recovered microstructure and with a gauge in the
range of 0.25 mm to 1.5 mm. The preform is a closed-end cylinder
that can be made by, for example, a draw-redraw (-redraw) process
or by back-extrusion. The diameter of the preform lies somewhere
between the minimum and maximum diameters of the desired container
product. Threads may be formed on the preform prior to the
subsequent forming operations. The profile of the closed end of the
preform may be designed to assist with the forming of the bottom
profile of the final product.
[0029] As illustrated in FIG. 1, the tooling assembly for the
method of the invention includes a split die 10 with a profiled
cavity 11 defining an axially vertical bottle shape, a punch 12
that has the contour desired for the bottom of the container (for
example, in the illustrated embodiments, a convexly domed contour
for imparting a domed shape to the bottom of the formed container)
and a ram 14 that is attached to the punch. In FIG. 1, only one of
the two halves of the split die is shown, the other being a mirror
image of the illustrated die half; as will be apparent, the two
halves meet in a plane containing the geometric axis of the bottle
shape defined by the wall of the die cavity 11.
[0030] The minimum diameter of the die cavity 11, at the upper open
end 11a thereof (which corresponds to the neck of the bottle shape
of the cavity) is equal to the outside diameter of the preform (see
FIG. 2A) to be placed in the cavity, with allowance for clearance.
The preform is initially positioned slightly above the punch 12 and
has a schematically represented pressure fitting 16 at the open end
11a to allow for internal pressurization. Pressurization can be
achieved, for example, by a coupling to threads formed in the upper
open end of the preform, or by inserting a tube into the open end
of the preform and making a seal by means of the split die or by
some other pressure fitting.
[0031] The pressurizing step involves introducing, to the interior
of the hollow preform, a fluid such as water or air under pressure
sufficient to cause the preform to expand within the cavity until
the wall of the preform is pressed substantially fully against the
cavity-defining die wall, thereby imparting the shape and lateral
dimensions of the cavity to the expanded preform. Stated generally,
the fluid employed may be compressible or noncompressible, with any
of mass, flux, volume or pressure controlled to control the
pressure to which the preform walls are thereby subjected. In
selecting the fluid, it is necessary to take into account the
temperature conditions to be employed in the forming operation; if
water is the fluid, for example, the temperature must be less than
100.degree. C., and if a higher temperature is required, the fluid
should be a gas such as air, or a liquid that does not boil at the
temperature of the forming operation.
[0032] As a result of the pressurizing step, detailed relief
features formed in the die wall are reproduced in inverse
mirror-image form on the surface of the resultant container. Even
if such features, or the overall shape, of the produced container
are not axisymmetric, the container is removed from the tooling
without difficulty owing to the use of a split die.
[0033] In the specific embodiment of the invention illustrated in
FIGS. 2A and 2B, the preform 18 is a hollow cylindrical aluminum
workpiece with a closed lower end 20 and an open upper end 22,
having an outside diameter equal to the outside diameter of the
neck of the bottle shape to be formed, and the forming strains of
the PRF operation are within the bounds set by the formability of
the preform (which depends on temperature and deformation rate).
With a preform having this property of formability, the shape of
the die cavity 11 is made exactly as required for the final product
and the product can be made in a single PRF operation. The motion
of the ram 14 and the rate of internal pressurization are such as
to minimize the strains of the forming operation and to produce the
desired shape of the container. Neck and side-wall features result
primarily from the expansion of the preform due to internal
pressure, while the shape of the bottom is defined primarily by the
motion of the ram and punch 12, and the contour of the punch
surface facing the preform closed end 20.
[0034] Proper synchronization of the application of internal fluid
pressure and operation (translation into the die cavity) of the ram
and punch are important in the practice of the invention. FIG. 3
shows a plot of computer-generated simulated data (sequence of
finite element analysis outputs) representing the forming operation
of FIGS. 2A and 2B with air pressure, controlled by flux.
Specifically, the graph illustrates the pressure and ram time
histories involved. As will be apparent from FIG. 3, the fluid
pressure within the preform occurs in successive stages of (i)
rising to a first peak 24 before expansion of the preform begins,
(ii) dropping to a minimum value 26 as expansion commences, (iii)
rising gradually to an intermediate value 28 as expansion proceeds
until the preform is in extended though not complete contact with
the die wall, and (iv) rising more rapidly (at 30) from the
intermediate value during completion of preform expansion. Stated
with reference to this sequence of pressure stages, the initiation
of translation of the punch to displace and deform the closed end
of the preform in preferred embodiments of the invention occurs (at
32) substantially at the end of stage (iii). Time, pressure and ram
displacement units are indicated on the graph. The effect of the
operations represented in FIG. 3 on the preform (in a computer
generated simulation) is shown in FIGS. 6A, 6B, 6C and 6D for times
0.0, 0.096, 0.134 and 0.21 seconds as represented on the x-axis of
FIG. 3.
[0035] At the outset of introduction of internal fluid pressure to
the hollow preform, the punch 12 is disposed beneath the closed end
of the preform (assuming an axially vertical orientation of the
tooling, as shown) in closely proximate (e.g. touching) relation
thereto, so as to limit axial stretching of the preform under the
influence of the supplied internal pressure. When expansion of the
preform attains a substantial though not fully complete degree, the
ram 14 is actuated to forcibly translate the punch upwardly,
displacing the metal of the closed end of the preform upwardly and
deforming the closed end into the contour of the punch surface, as
the lateral expansion of the preform by the internal pressure is
completed. The upward displacement of the closed preform end cannot
move the preform upwardly relative to the die or cause the side
wall of the preform to buckle (as might occur by premature upward
operation of the ram) owing to the extent of preform expansion that
has already occurred when the ram begins to drive the punch
upward.
[0036] A second embodiment of the method of the invention is
illustrated in FIGS. 4A-4D. In this embodiment, as in that of FIGS.
2A and 2B, the cylindrical preform 38 has an initial outside
diameter equal to the minimum diameter (neck) of the final product.
However, in this embodiment it is assumed that the forming strains
of the PRF operation exceed the formability limits of the preform.
In this case, two sequential pressure forming operations are
required. The first (FIGS. 4A and 4B) does not require a ram and
simply expands the preform within a simple split die 40 to a larger
diameter workpiece 38a by internal pressurization. The second is a
PRF procedure (FIGS. 4C and 4D), starts with the workpiece as
initially expanded in the die 40 and, employing a split die 42 with
a bottle-shaped cavity 44 and a punch 46 driven by a ram 48, i.e.,
using both internal pressure and the motion of the ram, produces
the final desired bottle shape, including all features of the
side-wall profile and the contours of the bottom, which are
produced primarily by the action of the punch 46.
[0037] A third embodiment is shown in FIGS. 5A and 5B. In this
embodiment, the preform 50 is made with an initial outside diameter
that is greater than the desired minimum outside diameter (usually
the neck diameter) of the final bottle-shaped container. This
choice of preform may result from considerations of the forming
limits of the pre-forming operation or may be chosen to reduce the
strains in the PRF operation. In consequence, manufacture of the
final product must include both diametrical expansion and
compression of the preform and thus can not be accomplished with
the PRF apparatus alone. A single PRF operation (FIG. 5A, employing
split die 52 and ram-driven punch 54) is used to form the wall and
bottom profiles (as in the embodiment of FIGS. 2A and 2B) and a
spin forming or other necking operation is required to shape the
neck of the container. As illustrated in FIG. 5B, it is preferred
to employ a spin forming procedure of the type set forth in
copending U.S. patent application Ser. No. 09/846,169 (filed May 1,
2001, by two of the present applicants, the entire disclosure of
which is incorporated herein by this reference), utilizing plural
tandem sets of spin forming discs 56 and a tapered mandrel 58 to
shape the bottle neck 60.
[0038] In the practice of the PRF procedure described above, PRF
strains may be large. Alloy composition is accordingly selected or
adjusted to provide a combination of desired product properties and
enhanced formability. If still better formability is required, the
forming temperature may be adjusted, since an increase in
temperature affords better formability; hence, the PRF operation(s)
may need to be conducted at elevated temperatures and/or the
preform may require a recovery anneal, in order to increase its
formability.
[0039] The present invention differs from known pressure-forming
operations such as blow-forming of PET containers, in particular,
in adding an external punch-forming component. An internal punch,
as sometimes used for PET bottle-forming, is not required. At
present, there is no way known to applicants to produce an aluminum
container with a shaped profile with the range of diameters that
can be achieved with the present invention. Furthermore, there is
no way currently known to applicants to produce an asymmetric
profile (for example, feet on the bottom or spiral ribs on the side
of the container).
[0040] The method of the invention could also be used to shape 15;
containers from other materials, such as steel.
[0041] The importance of moving the ram-driven punch 12 into the
die cavity 11 to displace and deform the closed end 20 of the
preform 18 (as in FIGS. 2A and 2B) may be further explained by
reference to FIG. 3 (mentioned above) as considered together with
FIGS. 6A-6D, in which the white line represents the vertical
profile of the die cavity 11, and the displacement (in inches) of
the dome-contoured punch 12 at various times after the initiation
of internal pressure is represented by the scale on the right-hand
side of that white line.
[0042] The ram serves two essential functions in the forming of the
aluminum bottle. It limits the axial tensile strains and forms the
shape of the bottom of the container. Initially the ram-driven
punch 12 is held in close proximity to, or just touching, the
bottom of the preform 18 (FIG. 6A). This serves to minimize the
axial stretching of the preform side wall that would otherwise
occur as a result of internal pressurization. Thus, as the internal
pressure is increased, the side wall of the preform will expand to
contact the inside of the die without significant lengthening.
Typically, the central region of the preform will expand first, and
this region of expansion will grow along the length of the preform,
both upward and downward. At some point in time the bottom of the
preform becomes nearly hemispherical in shape, with the radius of
the hemisphere approximately equal to that of the die cavity (FIG.
6B). It is at or just before this point in time that the ram must
be actuated to drive the punch 12 upwards (FIG. 6C). The profile of
the nose of the ram (i.e. the punch surface contour) defines
completely the profile of the bottom of the container. As the
internal fluid pressure completes the molding of the preform
against the die cavity wall (compare the bottle shoulder and neck
in FIGS. 6B, 6C and 6D), the motion of the ram, combined with the
internal pressure, forces the bottom of the preform into the
contours of the punch surface in a manner that produces the desired
contour (FIG. 6D) without excessive tensile strains that could,
conceivably, lead to failure. The upward motion of the ram applies
compressive forces to the hemispherical region of the preform,
reduces general strain caused by the pressurizing operation, and
assists in feeding material radially outwards to fill the contours
of the punch nose.
[0043] If the ram motion is applied too early, relative to the rate
of internal pressurization, the preform is likely to buckle and
fold due to the compressive axial forces. If applied too late, the
material will undergo excessive strain in the axial direction
causing it to fail. Thus, coordination of the rate of internal
pressurization and motion of the ram and punch nose is required for
a successful forming operation. The necessary timing is best
accomplished by finite element analysis (FEA) of the process. FIG.
3 is based on results of FEA.
[0044] The invention has been thus far described, and exemplified
in FIG. 3, as if no positive (i.e., superatmospheric) fluid
pressure were applied to the outside of the preform within the die
cavity. In such a case, the external pressure on the preform in the
cavity would be substantially ambient atmospheric pressure. As the
preform expands, air in the cavity would be driven out (by the
progressive diminution of volume between the outside of the preform
and the die wall) through a suitable exhaust opening or passage
provided for that purpose and communicating between the die cavity
and the exterior of the die.
[0045] Stated with specific reference to aluminum containers, by
way of illustration, it has been shown by FEA that in the absence
of any applied positive external pressure, once the preform starts
to deform (flow) plastically, the strain rate in the preform
becomes very high and is essentially uncontrollable, owing to the
low or zero work hardening rate of aluminum alloys at the process
temperature (e.g. about 300.degree. C.) of the pressure-ram-forming
operation.
[0046] That is to say, at such temperatures the work hardening rate
of aluminum alloys is essentially zero and ductility (i.e., forming
limit) decreases with increasing strain rate. Thus, the ability to
make the desired final shaped container product is lessened as the
strain rate of the forming operation increases and the ductility of
aluminum decreases.
[0047] In accordance with a further important feature of the
invention, positive fluid pressure is applied to the outside of the
preform in the die cavity, simultaneously with the application of
positive fluid pressure to the inside of the preform. These
external and internal positive fluid pressures are respectively
provided by two independently controlled pressure systems. The
external positive fluid pressure can be conveniently supplied by
connecting an independently controllable source of positive fluid
pressure to the aforementioned exhaust opening or passage, so as to
maintain a positive pressure in the volume between the die and the
expanding preform.
[0048] FIGS. 7 and 8 compare the pressure vs. time and strain vs.
time histories for pressure-ram-forming a container with and
without positive external pressure control (the term "strain"
herein refers to elongation per unit length produced in a body by
an outside force). Line 101 of FIG. 7 corresponds to the line
designated "Pressure" in FIG. 3, for the case where there is no
external positive fluid pressure acting on the preform; line 103 of
FIG. 8 represents the resulting strain for one particular position
(element) as determined by FEA. Clearly the strain is almost
instantaneous in this case, implying very high strain rates and
very short times to expand the preform into contact with the die
wall. In contrast, lines 105, 107 and 109 of FIG. 7 respectively
represent internal positive fluid pressure, external positive fluid
pressure, and the differential between the two, when both internal
and external pressures are controlled, i.e., when external and
internal positive fluid pressures, independently controlled, are
simultaneously applied to the preform in the die cavity; the
internal pressure is higher than the external pressure so that
there is a net positive internal-external pressure differential as
needed to effect expansion of the preform. Line 111 in FIG. 8
represents the hoop strain (strain produced in the horizontal plane
around the circumference of the preform as it is expanding) for the
independently controlled internal-external pressure condition
represented by lines 105, 107 and 109; it will be seen that the
hoop strain shown by line 111 reaches the same final value as that
of line 103 but over a much longer time and thus at a much lower
strain rate. Line 115 in FIG. 8 represents axial strain (strain
produced in the vertical direction as the preform lengthens).
[0049] By simultaneously providing independently controllable
internal and external positive fluid pressures acting on the
preform in the die cavity, and varying the difference between these
internal and external pressures, the forming operation remains
completely in control, avoiding very high and uncontrollable strain
rates. The ductility of the preform, and thus the forming limit of
the operation, is increased for two reasons. First, decreasing the
strain rate of the forming operation increases the inherent
ductility of the aluminum alloy. Second, the addition of external
positive pressure decreases (and potentially could make negative)
the hydrostatic stress in the wall of the expanding preform. This
could reduce the detrimental effect of damage associated with
microvoids and intermetallic particles in the metal. The term
"hydrostatic stress" herein refers to the arithmetic average of
three normal stresses in the x, y and z directions.
[0050] The feature of the invention thus described enhances the
ability of the pressure-ram-forming operation to successfully make
aluminum containers in bottle shapes and the like, by enabling
control of the strain rate of the forming operation and by
decreasing the hydrostatic stress in the metal during forming.
[0051] The selection of pressure differential is based on the
material properties of the metal from which the preform is made.
Specifically, the yield stress and the work-hardening rate of the
metal must be considered. In order for the preform to flow
plastically (i.e., inelastically), the pressure differential must
be such that the effective (Mises) stress in the preform exceeds
the yield stress. If there is a positive work-hardening rate, a
fixed applied effective stress (from the pressure) in excess of the
yield stress would cause the metal to deform to a stress level
equal to that applied effective stress. At that point the
deformation rate would approach zero. In the case of a very low or
zero work-hardening rate, the metal would deform at a high strain
rate until it either came into contact with the wall of the mold
(die) or fracture occurred. At the elevated temperatures
anticipated for the PRF process, the work-hardening rate of
aluminum alloys is low to zero.
[0052] Examples of gases suitable for use to supply both the
internal and external pressures include, without limitation,
nitrogen, air and argon, and any combinations of these gases.
[0053] The plastic strain rate at any point in the wall of the
preform, at any point in time, depends only on the instantaneous
effective stress, which in turn depends only on the pressure
differential. The choice of external pressure is dependent on the
internal pressure, with the overall principle to achieve and
control the effective stress, and thus the strain rate, in the wall
of the preform.
[0054] FIG. 9 shows a different control mechanism that can be used
in the forming process. Finite element simulations have been used
to optimize the process. In FIG. 9, line 120 represents internal
pressure (Pin) acting on the preform, line 122 represents external
pressure (Pout) acting on the preform, and line 124 represents the
pressure differential (Pdiff=Pin-Pout). This figure shows the
pressure history from one control method. In this case, the fluid
mass in the internal cavity is kept constant and the pressure in
the external cavity (outside the preform) is decreasing linearly.
Strain rate-dependent material properties are also included in the
simulation. This latter control mechanism is currently preferred
because it results in a simpler process.
[0055] It is to be understood that the invention is not limited to
the procedures and embodiments hereinabove specifically set forth
but may be carried out in other ways without departure from its
spirit.
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