EP1197273A1

EP1197273A1 - An apparatus and method to perform hydroforming, hydromechanical forming or mechanical forming

Info

Publication number: EP1197273A1
Application number: EP00870228A
Authority: EP
Inventors: Michel Vermeulen
Original assignee: Sidmar SA
Current assignee: Sidmar SA
Priority date: 2000-10-10
Filing date: 2000-10-10
Publication date: 2002-04-17

Abstract

The present invention is related to a device used for forming an object, in particular from a sheet or tube, said device comprising at least one die (1), said die having at least one alcove (11) in the inner wall (13) of said die, said alcove (11) being connected to a hydraulic supply system (16), said system supplying a fluid flow between said material (3) and said die (1), characterised in that said alcove (11) is partially filled by a porous zone (12), said porous zone representing a restrictor in said fluid flow, said porous zone being placed flush with said die wall (13), so that no interruption of said die wall occurs.

Description

Field of the invention

The present invention is related essentially to devices used for forming objects, in particular from flat sheet material or tubes by a hydroforming (HF) process, a mechanical process or a hydro-mechanical forming process.

State of the art

In standard mechanical forming processes, such as deep drawing, a sheet is pushed against a die by a punch, in order to take on the form of the punch and of the die's inside wall. In hydro-mechanical forming, a hydraulic pressure is locally added to this mechanical force, in order to facilitate the deformation process. Finally, hydroforming (HF) techniques exist, wherein a tube or a sheet of a given material, such as metal, composite or plastic is deformed solely by applying a hydraulic pressure inside a hollow die, pushing the tube or sheet against said die so that said tube or sheet takes on the form of said die's interior surface. Any fluid can be used, e.g. a mixture of oil and water, suitably treated for high-pressure applications, i.e. anti-corrosion, lubricating additives for pumps, piping and drainage.
A problem in all three types of forming processes, is caused by the friction forces between the object to be formed and the die. As the mechanical or hydraulic pressure reaches very high values, these friction forces become excessive, inhibiting the sliding of the material along the die wall and eventually leading to cracks in the material. This problem is particularly acute in hydroforming when it comes to acquiring small radii in the components. Moreover, the use of extremely high pressures in HF is not attractive for economic reasons. Places of high friction also rapidly lead to excessive tool wear.
In general HF praxis, the outer surfaces of the component to be formed are lubricated in order to reduce friction and wear. The use of special fluids can improve the situation to some extent. However, in the critical zones, one is not able to bring and/or maintain the fluid where it is needed during the HF process.
Other solutions have therefore been proposed. One of these solutions is described in document EP-A-771598. This document describes the use of ring grooves around tubes to be formed by an HF process. From these grooves, fluid is pressed into the gap between the tube material and the die. In that document, the emphasis is put on the fact that the fluid pressure in the ring grooves can be constant or controlled in different ways.
Another document, DE-A-19751413, proposes a set of recesses in an HF die wall at each zone where friction needs to be reduced. These recesses are supplied with fluid under a sufficient pressure so that said fluid is pressed between the die and the object to be formed.
The problem of both cited documents lies in the fact that recesses need to be made in the die wall. Especially in the case of hydroforming, this leads to the problem of material being pushed into these recesses by the hydroforming pressure, resulting in an unwanted deformation, which would obstruct the sliding of the material along the die wall. These types of recess can therefore not be used in the zones of the die where high forces are applied perpendicular to the material to be deformed.

Aims of the invention

The present invention aims to provide a device and a method, in particular for hydroforming, wherein friction is reduced in the zones where it is needed the most, i.e. where the actual deforming is taking place.
The present invention equally aims to use the device and method according to the invention in conventional mechanical forming techniques such as deep drawing techniques.

Summary of the invention

The present invention is related to a device used for forming an object, in particular from a sheet or tube, said device comprising at least one die, said die having at least one alcove in the inner wall of said die, said alcove being connected to a hydraulic supply system, said system supplying a fluid flow between said material and said die, characterised in that said alcove is partially filled by a porous zone, said porous zone representing a restrictor in said fluid flow, said porous zone being placed flush with the die wall, so that no interruption of said die wall occurs.
Said porous zone may be formed by a piece of porous material, said material may be metallic or non-metallic.
According to another embodiment of the present invention, said porous zone consists of a plurality of slots.
According to still another embodiment of the present invention, said porous zone consists of a plurality of bores.
A device according to the present invention may be a device used for hydroforming of objects, or a device used for mechanical or hydro-mechanical forming of objects.
One embodiment of the invention is related to a device, wherein at least one separate hydraulic system is used for supplying fluid to said porous zones.
Another embodiment of the present invention is related to a device for hydroforming or hydro-mechanical forming, wherein the hydraulic system used for supplying the hydroforming pressure or the hydraulic pressure during hydro-mechanical forming is also used to supply the fluid to said porous zones.
The present invention is equally related to a method to perform hydroforming using a device according to the invention, said method being characterised in that a plurality of porous zones is activated by supplying them with fluid, said activating of said zones taking place in a sequence during the deformation process.

Short description of the drawings

Fig. 1 represents a schematic view of an HF installation according to the prior art.
Fig. 2 illustrates the problem of acquiring a small radius by hydroforming.
Fig. 3a represents a schematic view of an HF installation according to the present invention.
Fig 3b represents a detailed view of a device according to the present invention.
Fig. 4 represents a schematic view of the flow through the device according to the present invention.
Fig. 5 represents an alternative restrictor in the fluid flow according to the invention.
Fig. 6 represents a restrictor consisting of a plurality of slot inserts.
Fig. 7 represents a restrictor consisting of a plurality of bores with small diameter.
Fig. 8 represents an installation according to another embodiment of the invention.
Fig. 9 represents an exemplary case of a device according to the present invention.

Detailed description of the invention

Figure 1 shows a schematic view of a hydroforming device, comprising a die 1. The internal pressure p_i is exerted on the die's interior, said pressure being provided by a hydraulic supply system 2. The pressure p_i causes a sheet of material 3 to take on the form of the die's interior wall. The sheet is clamped between die and blankholder 4. A problem occurs when it comes to pushing the material into the corners of the die and acquiring a sharp radius there.
Figure 2 shows an enlarged view of one of the corners of the die shown in figure 1. A high pressure is needed to give the sheet such a sharp radius. At a certain value of this pressure, the friction forces between the sheet and the die are so high that sliding of the sheet along the die wall is no longer possible, so that the material can only be pushed into the corner by stretching the available material between the contact points 5 and 6. This may easily lead to cracking of the material. Otherwise, insufficient component radii can be obtained. As was explained above, existing methods of supplying fluids between the material and the die show some clear disadvantages.
Figure 3a shows the same device, with means 7 for lubricating and maintaining the gap between the sheet and the die, according to a preferred embodiment of the present invention. Figure 3b shows a detailed view of said means 7. At least one alcove 11 is produced in the die wall. Into said alcove fits a porous insert 12, which is placed flush with the inner die wall 13 in such a way that no discontinuity appears in said wall 13. Above the insert, a portion of the alcove 11 is left unoccupied, forming a cavity 14. The alcove 11 is then connected through a channel 15 with a fluid supply system 16. A flow of fluid is maintained through the pores of the porous insert and into the gap 17 between the die 1 and the sheet 3, thereby maintaining a small lubricating gap between said die and sheet. This results in very low friction.
Advantageously, the die wall is now smooth and uninterrupted by the alcove 11, so that no material can be pushed into the alcove. This solves the problem of earlier described installations. Moreover, this kind of tool offers the added advantage of making optimal use of the hydrostatic bearing principle. This is explained in more detail on the basis of figure 4.
This figure 4 represents a porous insert 12 according to the invention and an equivalent hydraulic circuit corresponding to the fluid flow through said insert and the gap 17 between the die 1 and the material 3 to be formed. The cavity 14 is equivalent to a hydraulic capacitance C_C, with a given resistance R_C in parallel. After this cavity 14, the fluid flow is restricted by two resistances in series: R_r, defined by the porous insert 12, and R_g, defined by the gap 17.
When a fluid flow Q_g at a given pressure p_c is sent into the cavity 14, this system becomes self-regulating as a consequence of the hydrostatic bearing principle. When the gap 17 becomes smaller, the resistance R_g increases, leading to an automatic increase of the pressure between the insert 12 and the sheet 3. This effect only occurs in the presence of two restrictors in series, in other words in the presence of an additional restrictor besides the one formed by the gap 17 between the sheet and the die. In prior art devices, this added restrictor R_r is not present, or it may be present outside the die, so that a recess in the die wall remains necessary. The present invention, as already described, solves the problem of a non-smooth die wall. Moreover, since the porous insert represents the necessary second restrictor in the fluid flow, the present invention allows at the same time to use the hydrostatic bearing principle to its full advantage.
The porous insert can be made of any porous material, metallic (e.g. bronze, iron, etc.) or non-metallic (e.g. ceramic). The bearing material should have suitable compression and tensile strength and appropriate roughness properties: R_a should be sufficiently low and the skewness R_sk should be negative. The insert can have any form suitable for its location: square, round disc, annular cylindrical rings,...
Besides a porous insert, other means are possible to create a restrictor close to the sheet, without creating a discontinuity in the die wall. One of these options consists of a number of so-called 'slot entries'. This type of restrictor consists of a number of narrow slots, with given dimensions such as length 1, width w and slot thickness d. From these dimensions, the hydraulic resistance can be easily calculated. Figure 5 shows an embodiment whereby a number of these slots 18 have been made in the die wall. Figure 6 shows the cross section along the line AA' with the slots 18. Alternatively, a separate insert, containing these slots may be placed in the alcove, analogous to the porous insert 12. The thickness d of the slot entries must be sufficiently lower than the sheet thickness t, in order to avoid deformation of the sheet into the slot itself and consequently to avoid sliding obstruction of the sheet along the die wall.
Another option is the use of a set of bores 19 with small diameters in the die wall. The frontal section in this type of restrictor would look similar to the one shown in figure 5. Figure 7 shows the section along AA' for this type of restrictor. The number of bores may be much larger than shown on this drawing. Also, the bores may be placed at non-equal distances from each other. It is however essential that they result in a homogenous flow resistance over the entire surface. A restrictor of this type may equally be obtained by placing a separate insert, containing these bores. The diameter a of the bores must once again be sufficiently lower than the sheet thickness t to avoid the sheet being 'pushed' into the bores, which would cause an obstruction to the sliding of said sheet along the die wall.
To summarise, the present invention is related to a forming device, comprising one or more porous zones through which fluid is supplied and which are flush with the die wall. Said porous zones can be either separate inserts made of porous material, inserts containing slots or bores, or simply slots or bores in the die wall. In all cases, the porosity must be such that the die wall is smooth on all locations, and that no deformation of the material into the pores, slots or bores can occur.
To simplify the design, one insert or restrictor zone may be supplied with fluid from several supply lines, so that this one insert may serve to lubricate several locations (see references 47 and 48 or 49 and 50 in fig. 9).
In all cases, a drain channel must be present in the die, so that the fluid can be evacuated from the space between the material and the die. This drain is not shown in figures 3a and 8. It is however shown in figure 9 (see number 60).
A forming device according to the invention can be a hydroforming device such as shown in figure 3a, or a device used for mechanical forming (such as a deep drawing device) or hydro-mechanical forming, The porous zones and their hydraulic supply systems are equally applicable in these latter types of devices.
In the case of HF or hydro-mechanical forming, the pressure control of the fluid flow into the hydrostatic inserts according to the invention can be realised separately, as in figure 3a, by applying a different hydraulic supply system 16 for each insert 12. The pressure control can alternatively be integrated in the hydraulic system of the hydroforming or hydromechanical forming process, as shown in figure 8. In the latter case, a simple shut-off valve 20 in the path to the inserts has to be used to stop the flow when the material is not yet present at the location of the insert. It is advised to open this valve during a stepping down or a low-pressure period of the internal HF pressure. In case of many 'high friction spots' on the die, simultaneous or sequential action of the appropriate valve(s) has to be realised depending on the specific application. Said sequential action forms the basis of the method according to the present invention, described hereafter.
In the case of mechanical forming, wherein no hydraulic pressure is used for the actual deforming, at least one separate hydraulic system will have to be installed to activate the porous zones. In all types of devices, any variant of separate or combined hydraulic systems may be designed within the scope of the present invention.
The present invention is equally related to a method to perform a forming technique on a sheet or a tube, such as hydroforming. The method is characterised by the fact that the inserts 12 or more generally 'porous zones' (thereby including the slot entry and bore-type inserts), are activated in a given sequence during the deformation process, so that fluid is supplied to the area's where the friction forces are the highest. The porous zones are activated by turning on the independent supply systems 16 in the embodiment of figure 3a, e.g. by using active control valves to control said supply systems. In the embodiment of figure 8, the porous zones are activated by operating the valves 20. The sequence will depend on the object to be formed, and may be controlled on the basis of different principles of which three are mentioned here:

by time; i.e. at a specific moment during the deformation cycle.
by pressure (in particular for HF), i.e. at specific values of the hydroforming pressure. This option requires the use of pressure sensors in contact with the HF fluid.
by proximity sensors, sensing the position of the sheet material approaching the die wall.

During the forming process, the cavity pressure (inside alcoves 11) must be higher than the internal hydroforming pressure (in the case of HF) in order to guarantee a positive flow of fluid through the insert in the direction of the gap.
In the case of a steadily increasing internal pressure, said internal pressure may be lowered at regular intervals, so that a lubrication at the insert locations may be effectuated at the same regular intervals.
When the internal pressure is sinusoidal, the capacitor action of the alcove 11 (and its connecting lines), along with an efficient design of the porous insert system can cause a periodic lubrication of the gap between sheet and die wall during each stepping down cycle of the (internal) pressure loop.
In the case of a shock wave HF process - which is known in the art - the internal pressure may be lowered before the actual shock wave takes place, so that the lubrication may take place just before the actual shock wave.

Description of a preferred embodiment of the invention

Figure 9 illustrates an example of a hydroforming device and method according to the present invention. The device shown exhibits cylindrical symmetry around the axis 30. A sheet 31 is being formed by way of an internal pressure p_i inside the die 40. One flat porous insert 41 is present on the top surface, and one cylindrical ring insert 42 is present around the curved surface. Several cavities 43, 44, 45, 46 are present close to the porous inserts, said cavities being connected via supply lines 47, 48, 49, 50 respectively to the hydraulic supply system (not shown). The cavity 44 is ring shaped and supplied with fluid through the supply line 48. A drain tube 60 is present to allow the fluid to be evacuated from the space between the die and the object to be formed. A proximity sensor 61 is present.
The deformation process is subdivided into different phases, each phase being characterised by the lubrication action of a different supply line. Three states of the object during deformation are labelled 31, 32 and 33. During the deformation process different supply lines of fluid can be activated in a given sequence, in order to acquire an optimal flow of the sheet material into the sharp corners of the die. In phase 1, the sheet is deformed into state 31 by applying an internal pressure while activating the central supply line 47 in order to let the sheet material slide easily along the flat top surface of the die cavity. In phase 2, the material is further deformed with the additional help of the supply line 50, making the sheet slide more easily along the die's side wall. As a result, the sheet is deformed into the state 32. Finally, the supply lines 48 and 49 are simultaneously activated, allowing the sheet to slide into the sharp corner of the die cavity and to acquire the final state 33.

Claims

A device used for forming an object, in particular from a sheet or tube, said device comprising at least one die (1), said die having at least one alcove (11) in the inner wall (13) of said die, said alcove (11) being connected to a hydraulic supply system (16), said system supplying a fluid flow between said material (3) and said die (1) , characterised in that said alcove (11) is partially filled by a porous zone (12), said porous zone representing a restrictor in said fluid flow, said porous zone being placed flush with said die wall (13), so that no interruption of said die wall occurs.
A device according to claim 1, wherein said porous zone is formed by a piece of porous material.
A device according to claim 2, wherein said porous material is metallic.
A device according to claim 2, wherein said porous material is non-metallic.
A device according to claim 1, wherein said porous zone consists of a plurality of slots (18).
A device according to claim 1, wherein said porous zone consists of a plurality of bores (19).
A device according to any one of claims 1 to 6, wherein said device is used for hydroforming of objects.
A device according to any of the claims 1-6, wherein said device is used for mechanical forming of objects.
A device according to any of the claims 1 to 6, wherein said device is used for hydro-mechanical forming of objects.
A device according to any one of claims 7 to 9, wherein at least one separate hydraulic system is used for supplying fluid to said porous zones.
A device according to claim 7 or 9, wherein the hydraulic system used for supplying the hydroforming pressure or the hydraulic pressure during hydro-mechanical forming is also used to supply the fluid to said porous zones.
Method to perform hydroforming using a device according to claims 1-11, said method being characterised in that a plurality of porous zones is activated by supplying them with fluid, said activating of said zones taking place in a sequence during the deformation process.