U.S. patent application number 12/819338 was filed with the patent office on 2010-12-23 for method for hydroforming hollow profile metal workpieces.
Invention is credited to Rafael Garcia Gomez, Jochem Grewe.
Application Number | 20100319428 12/819338 |
Document ID | / |
Family ID | 42932659 |
Filed Date | 2010-12-23 |
United States Patent
Application |
20100319428 |
Kind Code |
A1 |
Gomez; Rafael Garcia ; et
al. |
December 23, 2010 |
METHOD FOR HYDROFORMING HOLLOW PROFILE METAL WORKPIECES
Abstract
A method for hydroforming hollow metal workpieces includes
submerging the same in a reservoir of hydraulic fluid. A press is
provided having a hydroforming station located within the reservoir
and includes a forming cavity configured to receive the fluid
filled workpiece and create a fluid cushion spaced between the
outside of the workpiece and the inside of the forming cavity.
Sealing mandrels supply high-pressure hydraulic fluid to the
interior of the workpiece which hydroform the same to the shape of
the final part. The outward flow of fluid from the fluid cushion
space is controlled, along with the associated pressure, to
maintain the fluid cushion at least until such time as the mandrel
pushing step concludes, thereby alleviating friction and adhesion
between the formed part and the forming cavity.
Inventors: |
Gomez; Rafael Garcia;
(Paderborn, DE) ; Grewe; Jochem; (Paderborn,
DE) |
Correspondence
Address: |
PRICE HENEVELD COOPER DEWITT & LITTON, LLP
695 KENMOOR, S.E., P O BOX 2567
GRAND RAPIDS
MI
49501
US
|
Family ID: |
42932659 |
Appl. No.: |
12/819338 |
Filed: |
June 21, 2010 |
Current U.S.
Class: |
72/58 |
Current CPC
Class: |
B21D 26/043 20130101;
B21D 26/033 20130101; B21D 26/041 20130101 |
Class at
Publication: |
72/58 |
International
Class: |
B21D 26/02 20060101
B21D026/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2009 |
DE |
10 2009 030 089.2 |
Claims
1. A method for hydroforming hollow profile, metal workpieces using
internal high-pressure hydraulics, comprising: submerging a hollow
profile, metal workpiece in a reservoir of hydraulic fluid to
substantially fill the hollow interior of the workpiece with
hydraulic fluid; providing a press having a top die, a bottom die
and a hydroforming station disposed therebetween; forming a forming
cavity in the hydroforming station of the press at a location
wholly within the reservoir, configured to receive therein the
filled workpiece and create a fluid cushion space between the
outside surface of the filled workpiece and the inside surface of
the forming cavity; transporting the filled workpiece through the
reservoir into the forming cavity and creating a hydraulic fluid
cushion in the fluid cushion space between the outside surface of
the filled workpiece and the inside surface of the forming cavity;
pushing sealing mandrels against the ends of the filled workpiece
disposed in the forming cavity; hydroforming the filled workpiece
in the forming cavity into the shape of the forming cavity to
define a formed part by communicating high-pressure hydraulic fluid
through at least one of the sealing mandrels with the interior of
the filled workpiece in the forming cavity; and controlling the
outward flow of hydraulic fluid from the fluid cushion space and
the associated pressure of the fluid cushion during said
hydroforming step for maintaining the fluid cushion for a
predetermined time period as hydraulic fluid flow into the interior
of the filled workpiece decreases at least until such time as said
mandrel pushing step has concluded, thereby alleviating friction
and adhesion between the formed part and the forming cavity.
2. A method for hydroforming workpieces as set forth in claim 1,
wherein: said press providing step comprises providing a transfer
press with at least one additional processing station; and
including transporting the filled workpiece from the additional
processing station to the hydroforming station within the hydraulic
fluid in the reservoir.
3. A method for hydroforming workpieces as set forth in claim 1,
wherein: said hydroforming step comprises pumping a volume of
hydraulic fluid into the filled workpiece that is greater than can
be accommodated in the filled workpiece in addition to the
hydraulic fluid present in the filled workpiece.
4. A method for hydroforming workpieces as set forth in claim 2,
including: pre-forming the workpiece at the additional processing
station to obtain a cross section configured for creating the fluid
cushion between the outside surface of the filled workpiece and the
inside surface of the forming cavity.
5. A method for hydroforming workpieces as set forth in claim 1,
wherein: when the forming cavity is in a closed condition, the top
die displaces less than one twelfth of the quantity of fluid in
which the bottom die is disposed.
6. A method for hydroforming workpieces as set forth in claim 1,
wherein: said hydroforming step includes selectively metering
hydraulic fluid from the fluid cushion through a gap that has a
defined width and is disposed in a separating gap between the top
die and the bottom die.
7. A method for hydroforming workpieces as set forth in claim 1,
wherein: said hydraulic fluid flow controlling step includes
selectively draining the hydraulic fluid cushion through grooves in
the forming cavity which extend toward at least one of the sealing
mandrels.
8. A method for hydroforming workpieces as set forth in claim 1,
including: heating the hydraulic fluid to a temperature greater
than room temperature.
9. A method for hydroforming workpieces, including: selecting the
hollow profile metal workpiece from a material comprising aluminum,
steel or magnesium alloy.
Description
CLAIM OF PRIORITY
[0001] Applicants hereby claim the priority benefits under the
provisions of 35 U.S.C. .sctn.119, basing said claim of priority on
German Patent Application Serial No. 10 2009 030 089.9, filed Jun.
22, 2009. In accordance with the provisions of 35 U.S.C. .sctn.119
and Rule 55(b), a certified copy of the above-listed German patent
application will be filed before grant of a patent.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method for hydroforming
hollow profile elements made of a metal material using internal
high-pressure hydraulics.
[0003] The state of the art is to produce metal components by means
of conventional hydroforming methods. Forming times are typically
on the order of approx. 1.5 to 3 seconds. These production times
are very long compared to so-called high-speed hydroforming (HSH).
Hydroforming times using HSH methods are normally well below 0.5
seconds. Cycle times are also very different for high-speed
hydroforming. While cycle times for conventional hydroforming are
on the order of for example 25 seconds, cycle times for HSH methods
are between 6 and 8 seconds.
[0004] It is a disadvantage when using internal high-pressure
hydroforming on softer materials such as for example aluminum
materials because these materials tend to stick or adhere to the
forming cavity. This results in aluminum accumulating on the tools.
This increases maintenance costs for the hydroforming tools.
Despite a theoretically faster forming speed, these advantages may
be outweighed by increased tool costs, and repair downtime.
SUMMARY OF THE INVENTION
[0005] An object of the present invention is therefore to provide a
method for internal high-speed hydraulic hydroforming of hollow
profile elements made of a metal material, wherein it is possible
to prevent method-related material accumulation on the tool
surfaces.
[0006] This object is attained in a method having the features of
patent claim 1.
[0007] Advantageous refinements of the present invention are the
subject matter of the subordinate claims.
[0008] In the present inventive method, a hollow profile element or
workpiece that is to be formed is submerged in a dip tank filled
with a hydraulic fluid. Thus, the hollow profile is filled not only
in the hydroforming station, but in other processing stations as
well. More specifically, the workpiece is filled with hydraulic
fluid prior to the hydroforming station. This is how the cavity of
the hollow profile element is completely filled with liquid prior
to the forming process. Thus, the workpiece is transported into the
actual hydroforming station through or in the hydraulic fluid. A
press that has a top die and a bottom die with a corresponding
forming cavity incorporates the hydroforming station. The hollow
profile element, filled with hydraulic fluid, is placed in the
forming cavity. After the top die has been lowered and the forming
cavity is closed, the ends of the hollow profile element are closed
using sealing mandrels. At the same time, internal pressure is
applied for internal high-pressure hydroforming of the
workpiece.
[0009] Another aspect of the present inventive method is that a
fluid cushion that is provided between the forming cavity and the
hollow profile elements, and is maintained in a controlled manner
for a period of time as the hydroforming fluid flow decreases. This
control provides good lubrication when the sealing mandrels push
against the hollow profile element. Therefore, hydraulic fluid is
used to form a fluid cushion that must not under any circumstances
be removed too rapidly. In contrast to prior high-speed
hydroforming, which forms the component as rapidly as possible and
brings the component to its final contour as rapidly as possible,
in the present inventive method, the final contour of the workpiece
is not attained at the beginning of the pushing using particularly
high internal pressure, especially in those areas that the sealing
mandrel is to push against. Rather, the final contour is not
attained until as late a point in time as possible, more
specifically, not until the insertion of the sealing mandrel has
been concluded. At this moment, the fluid cushion is no longer
needed. The fluid cushion should be maintained for as long as the
hydroforming process is taking place, especially in those areas
where the sealing mandrel pushes against the workpiece. Preferably,
the thickness of the fluid cushion should decrease
continuously.
[0010] It would be best if the workpiece material did not come into
direct contact with the tool at all while it is being pushed
against. When the method is designed ideally, there is no temporal
delay compared to high-speed hydroforming, which does not use such
a fluid cushion. This is because the present forming process, or
the pushing by the sealing mandrel, does not occur slower when the
fluid cushion of the present invention is employed.
[0011] It is possible to significantly reduce the friction forces
between the workpiece and the forming cavity using the present
invention. This has a positive effect on the force that is to be
transferred via the sealing mandrel. Accumulations of adhered
workpiece material are avoided. The standing time for the tools is
increased and overall efficiency is improved.
[0012] The inventive method exhibits its advantages in particular
with materials that are softer than steel, such as for example
aluminum. However, the method is also just as suitable for other
metal materials, such as for example steel or even magnesium.
[0013] The press used is preferably a transfer press that has
automatic transport systems. The press may be either a
hydraulically driven press or a mechanically driven press. It is
also possible to use presses driven by servo-motors.
[0014] A so-called transfer bar transports the hollow profile
elements or workpieces from station to station. With the present
invention, this transfer occurs entirely inside or within a
hydraulic fluid bath, i.e., somewhat below the fluid level.
[0015] Another process station can be used for a pre-forming
station or the like in which the hollow profile element obtains a
cross section that is suitable for creating a fluid cushion between
the hollow profile element and the forming cavity. For instance,
the hollow profile element may be given a wavy cross section, at
least in those areas that are to be pushed against. This is so that
there are as few points of contact as possible between the hollow
profile element and the forming cavity. The goal is to create a
defined fluid cushion. Therefore, the cross-sectional contour of
the blank can be very different from the contour of the completed
part sought through internal high-pressure hydroforming. Thus, the
goal of pre-forming the hollow profile element is not to create a
contour that is as close as possible to that of the finished
product, but rather to create deliberate differences that
facilitate forming the fluid cushion.
[0016] As a rule, the hollow profile elements or workpieces
prepared for internal high-pressure hydroforming are bent, or even
just deformed, so that the ends are not completely even against the
sealing mandrels. This necessarily results in leaks. In prior
hydroforming processes, these leaks would have to be eliminated in
a separate production step, wherein the pre-formed components are
either compressed or trimmed. However, with the present invention,
the increase in pressure during hydroforming is so high, and the
corresponding flow is so great, that such leaks at the ends of the
hollow profile element can be ignored, and the workpiece considered
operably sealed. Therefore, it is not necessary for the end of a
hollow profile element to be placed completely flat against the
sealing mandrel. Because of the great excess of hydraulic fluid
flow, the relatively small quantity of hydraulic fluid that escapes
through leaks is negligible. High-speed hydroforming using the
present invention can be performed with no problem.
[0017] Thus, the intent is to pump a greater volume of hydraulic
liquid into the hollow profile element during internal
high-pressure hydroforming than can be accommodated therein, in
addition to the hydraulic fluid already present in the filled
hollow profile element. This is a function of the final contour of
the completed part at the end of the internal high-pressure
hydroforming process.
[0018] Since from the beginning of the present process, the hollow
profile element is to be surrounded as completely as possible by a
fluid cushion, it is useful for the end of the sealing mandrel to
be blunt. In the context of the present invention, a blunt seal
shall be construed to be a sealing mandrel having an end face that
is disposed perpendicular to the longitudinal direction of the
workpiece without projections or depressions that are specially
adapted to the inner contour of the hollow profile element. This
end face, that is disposed perpendicular to the direction of
advancement of the mandrel, extends across a significantly larger
area than just the wall thickness of the hollow profile element to
be formed. This is specifically because the shape of the hollow
profile elements is not close to the final contour of the completed
part, but rather is deliberately formed wavy for creating the fluid
cushion, and in particular, extends at a distance from the walls of
the forming cavity. Some circumferential areas of the hollow
profile element are therefore displaced much farther radially
outward than other areas during the internal high-pressure
hydroforming. The sealing mandrel has a flat, i.e., blunt,
positioning surface of a corresponding size, so that no
obstructions occur in the area of the sealing mandrel. There are no
special sealing mechanisms for reducing the leaks in the transition
area between the sealing mandrel and the hollow profile element.
This type of leak-tolerant seal has the advantage that the ends of
the hollow profile element do not have to be prepared in a
particular manner in order to be able to perform high-speed
hydroforming using the present method. In addition, the ends of the
fully hydroformed hollow profile elements do not have to be cut
off. This results in material savings.
[0019] It is considered advantageous when the top die displaces
less than one twentieth of the quantity of fluid in which the
bottom die is disposed when the forming cavity is closed. The ratio
of displaced volume to bath or reservoir volume must be selected to
be high enough. Very high pressures are used in the present
inventive method, and fluid from leaks flows back into the fluid
reservoir or bath. The flow from leaks can be damped by a
corresponding quantity of hydraulic fluid, so that the hydraulic
fluid does not spray out in an uncontrolled manner. To this end,
the leak locations are preferably disposed deep under the fluid
level of the reservoir. Additional shielding measures are also
useful.
[0020] Hydraulic fluid is selectively drained or metered from the
fluid cushion in a controlled manner from a defined gap disposed
between the top die and the bottom die. This gap borders on the
forming cavity. In other words, a gap is created in the separating
gap between the top die and the bottom die, and it permits exactly
enough hydraulic fluid to drain off, so that only when the
insertion process for the sealing mandrel has concluded, the fluid
cushion has been completely removed. Alternatively, or in addition,
grooves can be provided in the forming cavity, and the hydraulic
fluid can be selectively drained or metered using these grooves as
well. This functionally occurs towards the sealing mandrel where
even larger flows from leaks may occur.
[0021] The temperature of the hydraulic fluid can be controlled so
that the hollow profile elements, comprised of a metal material,
can be somewhat hot-formed during the present high-speed
hydroforming method. Semi-hot-forming and hot-forming of metal
increases formability. As the temperature of the fluid bath
increases, the hollow profile element heats faster, so that the
subsequent internal high-pressure hydroforming operation can also
be performed at an accelerated pace. Heating the hollow profile
elements in the fluid bath has the advantage that the hollow
profile elements can be heated conductively with a medium that is
in direct contact with the hollow profile element. This method is
more effective than furnace heating because liquids conduct heat so
well.
[0022] These and other advantages of the invention will be further
understood and appreciated by those skilled in the art by reference
to the following written specification, claims and appended
drawings.
[0023] The invention is explained in greater detail in the
following using the exemplary embodiments depicted in the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic representation of an apparatus for
performing the method embodying the present invention.
[0025] FIG. 2 is a section through the forming cavity of a
hydroforming station.
[0026] FIGS. 3a through 3c depict a segment of a longitudinal
section through a forming cavity of a hydroforming station at three
different times during the processing.
[0027] FIG. 4 depicts a variant of the depiction in FIG. 3c, having
leak areas on the sealing mandrel.
[0028] FIG. 5 is a graph in which the movement curve for the
sealing mandrel, the pressure applied for internal high-pressure
hydroforming, and the thickness of the fluid cushion are plotted
over time.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] For purposes of description herein, the terms "upper",
"lower", "right", "left", "rear", "front", "vertical", "horizontal"
and derivatives thereof shall relate to the invention as oriented
in FIG. 1. However, it is to be understood that the invention may
assume various alternative orientations and step sequences, except
where expressly specified to the contrary. It is also to be
understood that the specific devices and processes illustrated in
the attached drawings, and described in the following
specification, are simply exemplary embodiments of the inventive
concepts defined in the appended claims. Hence, specific dimensions
and other physical characteristics relating to the embodiments
disclosed herein are not to be considered as limiting, unless the
claims expressly state otherwise.
[0030] FIG. 1 depicts a press 1 embodied as a transfer press, and
having a hydroforming station 6. The press 1 has a press table 2. A
reservoir or dip tank 3 is disposed on table 2, and is filled with
a hydraulic fluid. There are four processing stations inside dip
tank 3. The first station is a fill station 4, which is followed by
a pre-forming station 5. Thereafter is a hydroforming station 6,
and finally a final station 7. Raw materials 8 are transported to
the fill station 4 by means of a robot 9 in the processing sequence
from left to right. At the fill station 4, the blank or raw
material 8, which constitutes a hollow profile element or workpiece
10, is filled with hydraulic fluid, or the hydraulic fluid fills
the hollow profile element 10. Then, the hollow profile element 10
with hydraulic fluid therein is transported to the next station 5
by means of a transfer bar 11, as shown in the FIG. 1 drawing.
Transport from one station to the next station occurs below the
fluid level of tank 3 until such time as the hollow profile element
10 that has been hydroformed with internal high pressure is finally
removed from tank 3 by another robot 12 at the final station 7. The
robot 12 puts the finished parts 13 away.
[0031] The hollow profile element 10 is actually processed in the
pre-forming station 5, the hydroforming station 6, and the final
station 7. To this end, the press 1 has a press ram 14. Appropriate
top dies 15 for each of the stations 5, 6 and 7 are arranged on the
press ram 14. A piston/cylinder unit 16 is arranged on the top die
15 of the hydroforming station 6, and presses hydraulic fluid into
the interior of the hollow profile element 10 during the internal
high-pressure hydroforming. The press 1 is connected (not shown in
greater detail) to a pressure control system and pressure
regulator, such as that described in DE 10 2005 057 863 B3. A
bottom die 17 is associated with the top die 15 in a known
manner.
[0032] FIG. 2 is an enlarged depiction of a cross section through
the closed hydroforming station 6. It can be seen that a hollow
profile element 10, that is to be formed, is arranged within a
forming cavity 18, which has an essentially rectangular
cross-sectional shape disposed between the top die 15 and the
bottom die 17. The exterior surface of the hollow profile element
10 is in contact with the interior surface of the forming cavity 18
as little as possible, i.e., only at selected points. This is
because the hollow profile element 10 has been pre-formed into a
shape that creates a fluid-filled free space between the inside
surface of the forming cavity 18 and the outside surface of the
hollow profile element 10. A fluid cushion 19 forms in this free
space. In addition, the forming cavity 18 has grooves 20, which in
this exemplary embodiment, are disposed adjacent the center of the
top die 15 and bottom die 17, and extend generally in the
longitudinal direction of the forming cavity 18. These grooves 20
are not provided to create a contour in the hollow profile element
during the internal high-pressure hydroforming process. Rather,
they are provided to channel the hydraulic fluid from the fluid
cushion 19 when the internal pressure "p" in the interior of the
hollow profile element 10 increases during the internal
high-pressure hydroforming, and the hollow profile element 10 is
thereby caused to expand. In the lateral area of forming cavity 18,
the hydraulic fluid can be selectively drained or metered from the
fluid cushion 19 via a gap 21 that is disposed between top die 15
and bottom die 17. The gap 21 is so narrow that no material from
the hollow profile element 10 penetrates into the gap 21 during the
internal high-pressure hydroforming. The same is true of the
grooves 20. Naturally, it is also possible to provide a plurality
of additional grooves 20 in the top die 15 and/or bottom die 17.
The cross section of the grooves and gaps is adapted in a
particular manner, specifically such that the hydraulic fluid can
only flow out of the fluid cushions 19 at a reduced, controlled
flow speed. The goal is to maintain the fluid cushion 19 with a
continuously decreasing quantity of fluid for a certain period of
time, specifically at least until the sealing mandrel has been
inserted.
[0033] The cross-sectional view of FIG. 2 depicts that stage of the
present process wherein the hollow profile element 10 is only
slightly expanded by the internal high-pressure hydroforming.
Specifically, workpiece 10 is shown expanded only to the extent
that portions of the same come to be positioned against the forming
cavity 18, without being elongated and/or experiencing a reduction
in the wall thickness. The actual expansion using internal high
pressure occurs in other areas, and the depicted cross-sectional
contour merely indicates those portions of the aforesaid areas that
are to undergo more significant expansion are being pushed against.
The depicted cross section is thus especially showing deformation
in those areas positioned adjacent to the sealing mandrel. The
corresponding grooves 20 and gaps 21 for the fluid cushion 19 are
also located there.
[0034] FIGS. 3a-c illustrate how the hydroforming method proceeds.
A longitudinal section through the forming cavity 18, similar to
FIG. 2, is shown. It can be seen in FIG. 3a that the sealing
mandrel 22 is inserted into the forming cavity 18. Hydraulic fluid
is pumped into the interior of the hollow profile element 10 via a
channel 23 in mandrel 22, and a pressure "p" builds up. In FIG. 3b,
the sealing mandrel 22 has been urged in the direction of the arrow
P1, in order to push against the end of hollow profile element 10.
A recess or convexity is provided in an area (not depicted in
greater detail) of the forming cavity 18. The hollow profile
element 10 is to be pressed into this convexity by internal
high-pressure hydroforming. Workpiece material is pushed against
the end in order to prevent a reduction in the material wall
thickness of the workpiece. The fluid cushion 19 is maintained in
these areas of the hollow profile element 10 that are pushed
against. Small quantities of the hydraulic fluid can escape from
the fluid cushion 19 out of the forming cavity 18 towards the
sealing mandrel 22 via grooves 20 in the top die 15 and bottom die
17. It can also selectively escape (not shown in greater detail)
from the gap 21 between the top die 15 and the bottom die 17. The
hollow profile element 10 generally floats during this phase of the
hydroforming, i.e., while the sealing mandrel 22 is displaced by
the path "W", to some extent in the hydraulic fluid, and is carried
by the fluid cushion 19. The hollow profile element 10 is not
forced against the inside surface of the forming cavity 18, as
depicted in FIG. 3c, until the pushing process is in the process of
concluding or has fully concluded. At this point, the inner
pressure "p" has fully expanded the hollow profile element 10, such
that the fluid cushion 19 has been removed. It can be seen that the
hollow profile element 10 has not penetrated into the grooves 20.
Only the pushing process or the pushing of the sealing mandrel 22
has concluded in the depicted position. Meanwhile, the expansion of
areas (not depicted in greater detail) of the hollow profile
element 10 can be continued, because the inner pressure "p" is
still being applied after the fluid cushion 19 has been
removed.
[0035] FIG. 4 depicts a hollow profile element 10, the end of which
is not positioned flat against the sealing mandrel 22. The circled
area "L" illustrates that the end face of the hollow profile
element 10 is disposed a spaced apart distance from the sealing
mandrel 22 in the vicinity of the top die 15. Leaks therefore occur
there. The pressure "p" for internal high-pressure hydroforming can
still be effectively applied, because a very large quantity of
fluid is being supplied via the channel 23 in the sealing mandrel
22. It is to be understood that the size of the area "L" shown in
FIG. 4 is exaggerated for illustration purposes. In practice, the
amount of hydraulic fluid that escapes in the area "L" is not so
great that the desired hydroforming pressure "p" cannot be
attained. Thus, the present inventive method can also be performed
when there are leaks in the area of the sealing mandrel 22.
Therefore, the sealing mandrel 22 can have an end face that runs
perpendicular to the advancing direction, without additional
sealing means that would be inserted into the hollow profile
element 10 to be hydroformed. Thus, even when there are larger
fluid cushions, or when the spaces between the pre-formed hollow
profile element 10 and the wall of the forming cavity are larger,
it is possible to ensure that the hollow profile element can move
uninhibited transversely to the sealing mandrel 22, i.e., in the
direction of expansion.
[0036] The speed with which the fluid cushion is dissipated or
removed is essential in the present inventive method, and shall be
explained using the graph in FIG. 5. The curve K1 represents the
progression of pressure for a press 1 over time with an
electronically or hydraulically controlled pressure system for
hydroforming according to the prior art. The pressure begins to
build at zero and climbs above the working point "A" for the
internal high-pressure hydroforming method to the top dead point B1
on the curve K1. Then the pressure drops again through the pressure
drop point C1 to point D1.
[0037] Curve K2 depicts the path, i.e., the stroke, of a mechanical
press. In the press that is used here, the top die is embodied with
an additional piston/cylinder unit 16 for producing pressure. After
it has reached its bottom-most dead point B1, the press stroke
moves back in the direction of the top dead point (cannot be shown
in the graph) through pressure drop points C1 and D1. The press is
still held down between the lower dead point B1 of the press
stroke, the curve K1, and the pressure drop point C1. The press
moves up starting at pressure drop point C1, the pressure is
removed, and the press 1 opens the hydroforming tool at point D1.
The top dead point OT (not shown) is passed through without a
temporal delay. The curve K2 depicts the pressure progression for a
press as is described in DE 10 2005 057 863 B3. In that press, a
pressure control system and a pressure regulator made of at least
one piston/cylinder/spring unit are provided. The press is provided
with another apparatus for additional production operations.
Additional production operations are performed in the time window
for the pressure plateau B1-B2.
[0038] The curve K3 is a movement curve for the sealing mandrel 22.
In the first movement phase, in the zero to "R" range, the hollow
profile element that is to be formed provides relatively little
resistance. There is an adequate fluid cushion between the hollow
profile element and the forming cavity during this period of time.
The R-S segment is normally the critical segment for the entire
movement curve because, in this range, the hydraulic fluid drains
out of the fluid cushion rapidly since the hollow profile element
10 is positioned against the forming cavity in this phase. In the
S-T phase, the sealing mandrel 22 holds its position until it is
finally withdrawn (T-D2).
[0039] The curve K4 illustrates the thickness of a fluid cushion.
It can be seen that the thickness decreases relatively quickly
between the point "G" and the point "H", and in particular
approaches zero, before the sealing mandrel has completely passed
through the R-S range in the movement curve. This means that the
fluid flows out rapidly. There is increased friction between the
workpiece and the tool, which can lead to material adhesion and the
disadvantages discussed above. In accordance with the present
invention, it is provided that the thickness of the fluid cushion
decreases at a significantly slower rate, as is illustrated by the
curve K5. It can be seen that the sealing mandrel has already
passed through the R-S range, while the thickness of the fluid
cushion has not even decreased 50 percent. It is only at point "J",
which is temporally after the end of the sealing mandrel insertion
process, that the thickness of the fluid cushion approaches zero.
However, at this point in time, there is no more friction between
the workpiece and the tool, so that the fluid cushion is no longer
needed. Thus, what is critical is that the point "J" for curve K5
on the time axis be located to the right of the point "S", wherein
point "S" denotes the end point for the sealing mandrel insertion
process.
[0040] The curves depicted in FIG. 5 are purely schematic. In
practice, curves may result instead of the straight lines shown.
What is essential is that the speed with which the hydraulic fluid
selectively escapes the fluid cushion should be reduced
substantially. The curve K5 is therefore flatter than the curve
K4.
[0041] In the foregoing description, it will be readily appreciated
by those skilled in the art that modifications may be made to the
invention without departing from the concepts disclosed herein.
Such modifications are to be considered as included in the
following claims, unless these claims by their language expressly
state otherwise.
* * * * *