EP3240649B1 - Chamber for electrohydraulic forming - Google Patents

Description

Field of the invention

The present invention relates to the field of forming, and more particularly the field of electro-hydraulic forming. The present invention relates to an electro-hydroforming enclosure, in particular for forming parts of small dimensions, according to the preamble of claim 1 (see for example US-A-3,188,844 ).

State of the art

Hydroforming processes are generally used as manufacturing processes, especially for pieces of complex shapes. They consist in using the pressure of a fluid, preferably a liquid, to achieve the plastic deformation of a sheet held in a mold. The fluid then acts on the sheet to make it fit the shape of the mold. This fluid can be pressurized in various ways.

Among the existing hydroforming processes, mention may be made of the electro-hydroforming process, known as the EHF process (from the Anglo-Saxon Electro Hydraulic Forming). This method is a very high-speed deformation forming process which is based on an electrical discharge of a high energy stored in capacitors either between two electrodes placed in a chamber filled with fluid, or in an explosive wire placed in a chamber filled with fluid. When an electric discharge is created in the fluid, a shock wave is generated in said fluid, it propagates and projects the sheet against the mold. The dynamic pressure thus generated on the sheet allows the high-speed deformation of the constituent material which is projected against the mold, thus allowing its shaping.

Such a method allows the forming of sheets but also other parts made of a plastically deformable material. It is used for producing large parts, that is to say parts whose characteristic length is significantly greater than a distance between the two electrodes.

Such a method has many advantages, including obtaining very fine details on the parts, such as for example engravings, the lack of springback, or low manufacturing costs.

• mise en place de la pièce à former dans une enceinte d'électro-hydroformage,
• remplissage d'une chambre creuse dans l'enceinte d'électro-hydroformage par un fluide,
• décharge électro-hydraulique dans le fluide contenu dans la chambre creuse,
• vidange de la chambre creuse,
• retrait de la pièce formée.

However, one of the disadvantages lies in the cycle time required for forming a part, as indicated in the patent. US7493787 . Indeed, in known manner, a forming cycle via the EHF process is broken down into several steps:

• placing the workpiece in an electro-hydroforming enclosure,
• filling a hollow chamber in the electro-hydroforming enclosure with a fluid,
• electro-hydraulic discharge in the fluid contained in the hollow chamber,
• emptying the hollow chamber,
• withdrawal of the formed part.

The steps of filling and emptying the hollow chamber represent the most time-consuming stages.

Presentation of the invention

The present invention is intended to provide an effective solution for forming parts, while reducing the cycle time and ensuring an equivalent result.

This object is achieved according to the invention by an electro-hydroforming enclosure for forming a workpiece with the features of claim 1.

The part is intended to be positioned, in this electro-hydroforming chamber, between the discharge chamber and the forming chamber before activation of said electro-hydraulic discharge system, the activation of said electro-hydraulic discharge system causing the projection and deformation of the piece against the impression of the forming chamber.

By activation is meant the creation, via the electro-hydraulic discharge system, of an electric discharge in the fluid in order to create a shock wave that propagates in the fluid.

The discharge frame is preferably made of a high-strength material, for example a metallic material such as steel, to contain the high pressures generated during activation of the electro-hydraulic discharge system.

According to the invention, the inner wall is covered by two non-metallic coatings.

A coating is a layer that is deposited on the surface of a part, a material, in this case here the inner wall, to confer special properties. The constituent material of the coatings covers the inner wall, partially or totally, but so that said constituent material of the coatings is ultimately integral with this face.

One of the two non-metallic coatings is disposed on the inner wall, around and near the electro-hydraulic discharge system ports in the discharge chamber.

Such a non-metallic coating advantageously avoids the formation of an electric arc between the electro-hydraulic discharge system and the inner wall. Such an electric arc could damage the inner wall, and most importantly, greatly reduce the performance of the electro-hydraulic discharge system, not forming the sheet.

Thus, the dimensions of the discharge chamber can be reduced without fear of such an electric arc. The reduced dimensions of the discharge chamber then advantageously make it possible to reduce the volume fluid necessary for filling said discharge chamber. As a result, the cycle time required to perform a forming process using such an electro-hydroforming chamber is greatly reduced and the production rate is greatly increased.

Such an electro-hydroforming enclosure is particularly suitable for producing small parts, such as for example a USB key body ("Universal Serial Bus" in English terminology), embellished for example fine engravings.

According to preferred embodiments, to further reduce the risk of electric arc, the coating is a coating made of an electrically insulating material.

According to preferred embodiments, the electro-hydraulic discharge system comprises two electrodes intended to be connected to an electrical energy storage unit.

According to preferred embodiments, the electro-hydraulic discharge system comprises an explosive wire intended to be connected to an electrical energy storage unit.

According to preferred embodiments, the electro-hydraulic discharge system comprises an explosive wire connected between two electrodes.

The invention also relates to an electro-hydroforming machine comprising an electro-hydroforming chamber according to the invention and an electrical energy storage unit connected to the electro-hydraulic discharge system.

Presentation of figures

• La figure 1 représente une vue en coupe d'une enceinte d'électro-hydroformage selon un mode de réalisation préféré de l'invention,
• La figure 2 illustre une vue en coupe d'une enceinte d'électro-hydroformage selon un autre mode de réalisation préféré de l'invention,
• La figure 3 illustre un corps de clé USB gravé au moyen d'une enceinte d'électro-hydroformage selon l'un des modes de réalisation préférés de l'invention.

The features and advantages of the invention will appear more clearly in the light of the following examples of implementation, provided in FIG. simple illustrative title of the invention, with the support of Figures 1 to 3 , in which :

• The figure 1 represents a sectional view of an electro-hydroforming enclosure according to a preferred embodiment of the invention,
• The figure 2 illustrates a sectional view of an electro-hydroforming enclosure according to another preferred embodiment of the invention,
• The figure 3 illustrates a USB key body etched by means of an electro-hydroforming chamber according to one of the preferred embodiments of the invention.

Detailed description of an embodiment of the invention

An electro-hydroforming enclosure 10 for forming a workpiece 50 according to one embodiment of the invention is illustrated on the figure 1 . The parts to be formed may be flat or, alternatively, tubular. The pieces can also be preformed by conventional stamping techniques.

This electro-hydroforming chamber is used as part of a conventional forming process which will be recalled later.

The electro-hydroforming chamber 10 is made in two parts. The electro-hydroforming enclosure 10 comprises a first part, called the discharge frame 20, and a second part, called the matrix 30. The discharge frame 20 may represent an upper part of the electro-hydroforming enclosure (according to FIG. the orientation of the figures) and the matrix 30 may represent a lower part, as illustrated in the figure. Alternatively, and without departing from the scope of the invention, it is conceivable that the discharge frame 20 represents a lower portion of the electro-hydroforming enclosure (according to the orientation of the figures) and the matrix 30 represents an upper part. Also as a variant, the first part may represent a left part of the electro-hydroforming chamber (according to the orientation of the figures) and the second part may represent a right part of the electro-hydroforming chamber (according to FIG. the orientation of the figures) or vice versa.

The discharge frame 20 has an inner wall 21 defining a discharge chamber 22.

The die 30 has a forming chamber 32 intended to face the discharge chamber 22 when the discharge frame 20 and the die 30 are assembled.

The discharge frame 20 and the die 30 are removable relative to each other so as to allow insertion and removal of the workpiece 50 to be formed.

Said piece to be formed is disposed at an interface 33 between the die 30 and the discharge frame 20, and held in position hermetically. Once in position in the electro-hydroforming enclosure, the part to be formed separates the forming chamber 32 from the discharge chamber 22.

In the example of the figure 1 , the piece to be formed is a piece of flat shape. In the example of the figure 2 the workpiece is a tubular workpiece.

The forming chamber 32 has, facing the part to be deformed, an impression 31 corresponding to the shape that the workpiece must take after deformation.

The discharge frame 20 and the matrix 30 are preferably made of a metallic material, for example steel, in order to present a structural resistance of the respective chambers (discharge chamber 22 and forming chamber 32) and to contain the high pressures generated at the the moment of an electro-hydraulic discharge, during the forming process, because the voltage during an electro-hydraulic discharge can reach several tens of kilovolts.

The discharge chamber 22 is intended to be filled with an incompressible fluid, preferably a liquid, for example water.

A water supply conduit 23 is provided in the discharge frame 20 to allow the discharge chamber 22 to be connected to a tank (not shown) containing water and to supply said discharge chamber 20 with water.

A water discharge conduit (not shown) is provided in the discharge frame 20 to allow the discharge chamber 22 to be connected to a tank and to drain the water out of said discharge chamber into the tank.

In an alternative embodiment, the water supply duct 23 and the water discharge duct are one and the same duct allowing the supply and the emptying of the water in / out of the discharge chamber to / from a single tank.

The forming chamber 32 is in turn preferably under vacuum.

A duct (not shown) is made in the die 30 to allow the forming chamber 32 to be connected to a vacuum pump (not shown). However, as an alternative or in the absence of means to achieve this vacuum, it may also leave the forming chamber 32 in an atmosphere and provide vents allowing the evacuation of air during the forming process.

In a preferred embodiment, the electro-hydroforming enclosure 10 and the discharge chamber 22 have a substantially cylindrical geometric shape.

However, without departing from the scope of the invention, the electro-hydroforming chamber 10 and the discharge chamber 22 may have any geometric shape. More particularly, the discharge chamber 22 may preferably have a geometric shape such that the inner wall 21 reflects the shock wave, obtained during the electro-hydraulic discharge, in the direction of the workpiece 50. For example, a part top of the inner wall may have a conical shape, as illustrated on the figure 2 .

The electro-hydroforming enclosure 10 further comprises an electro-hydraulic discharge system 40.

In the non-limiting example illustrated on the figure 1 the electro-hydraulic discharge system 40 has two separate electrodes 41.

Each electrode 41 passes through the discharge frame 20. A first end 42 of each electrode is positioned inside the discharge frame 20, in the discharge chamber 22. A second end 43, placed on the outside of the discharge frame 20, is connected, via a power cable, to an electrical energy storage unit (not shown).

Each electrode 41 is preferably covered with an envelope 44 of electrically insulating material to electrically isolate them from the metallic material constituting the discharge frame 20.

The electrodes 41 are disposed in the electro-hydroforming enclosure 10 so as to create an inter-electrode distance d ₁ , between the first ends 42 of the two electrodes 41. In a known manner, this inter-electrode distance d ₁ makes it possible to define the power of the shock wave generated during the electro-hydraulic discharge, in terms of amplitude and duration.

Depending on the complexity of the shape to be obtained for the part to be formed and / or the material constituting the part to be formed, the inter-electrode distance d ₁ is increased or reduced, which modulates the energy reached during the electro-hydraulic discharge. and influences the power of the shock wave.

In one embodiment, the inter-electrode distance d ₁ can be adjusted by conventional adjustment means (not shown), such as for example a nut system, since the adjustment operations do not damage the electrodes 41.

The electrodes are also arranged, relative to the workpiece, so as to maintain a distance d ₂ between the electro-hydraulic discharge location and the workpiece. This distance d ₂ contributes to forming the workpiece by direct wave.

The electrical energy storage unit, to which the two electrodes 41 are connected, comprises among others at least one capacitor. The various components of the electrical energy storage unit are known to those skilled in the art in their form and operation and are not described in more detail in the present description.

The electro-hydroforming enclosure assembly and the electrical energy storage unit form an electro-hydroforming machine.

The inner wall 21 of the discharge frame 20 has in part a non-metallic coating 24.

The coating 24 is a layer deposited against all or part of the inner wall 21. The coating 24 partially covers the inner wall 21 and is secured to it by appropriate means.

Preferably, the coating 24 is chosen to have a thickness e sufficient to eliminate the risk of arcing between the first end 42 of an electrode 41 and the metal discharge frame.

In a preferred embodiment, to reduce its thickness e, the non-metallic coating 24 is made of an electrically insulating material.

Preferably, the coating 24 is chosen from a material with a very high dielectric strength, greater than 20 kV / mm.

In an exemplary embodiment, when the voltage reached during the electro-hydraulic discharge is 100kV, and the material chosen for the coating 24 has a dielectric strength of 20kV.mm ^-1 , then the coating will have a thickness of 5mm.

The coating is also subjected to stresses related to the impact of the shock wave against the inner wall. The coating has a tensile strength, preferably greater than 20 MPa.

• un polyethylene haute densité (PEHD) ;
• un polyTétraFluoroEthylène (PTFE) ;
• un polyamide, tel que le polyamide 6 (PA6) ;
• un polycarbonate (PC) ;
• un polychlorure de vinyle (PVC) ;
• un Polyether ether ketone (PEEK) ;
• un polyuréthane (PU).

In preferred exemplary embodiments, the coating material is a plastic, for example:

• high density polyethylene (HDPE);
• polytetrafluoroethylene (PTFE);
• a polyamide, such as polyamide 6 (PA6);
• polycarbonate (PC);
• polyvinyl chloride (PVC);
• a polyether ether ketone (PEEK);
• a polyurethane (PU).

In other exemplary embodiments, the coating material is a ceramic, such as, for example, porcelain.

The coating may also be composed of a combination of these materials.

Each electrode 41 passes through the discharge frame 20 at the non-metallic coating 24 of the inner wall 21.

Although an electric arc can propagate by ramping along the envelope 44 of an electrode 41 and propagate towards the discharge chamber 22, the risk of electric arc at the junction of the insulators (envelope 44 of the electrode 41 and insulating coating 24 of the inner wall 21) is strongly attenuated during the electro-hydraulic discharge between the electrodes 41. Indeed, the pressure wave compresses the electrode-envelope assembly in the direction of the electrode. In response, the electrode-casing assembly radially deforms in expansion at the insulating coating. This deformation increases the contact pressure between the insulators, and closes the passage for an electric arc potential.

According to the invention, the inner wall is covered by two non-metallic coatings.

The inner wall 21 illustrated figure 2 is covered with two nonmetallic coatings 24, 25. The nonmetallic coating 24, located at the two electrodes, is chosen from a material with a higher dielectric strength than the second coating 25, in order to reinforce the structural and insulating nature of the frame. 20, near the electro-hydraulic discharge.

Such an electro-hydroforming enclosure 10, from the non-metallic coating 24 of all or part of the inner wall 21, advantageously allows the production of a discharge chamber 22 of small volume, for example preferably less than 1 liter, more preferably less than 0.5 liter. This small volume allows rapid filling of the discharge chamber, of the order of 5 seconds.

It is thus conceivable to perform several electro-hydraulic discharges per minute, for example at least two electro-hydraulic discharges per minute, preferably six electro-hydraulic discharges per minute.

Such an electro-hydroforming enclosure 10 is particularly suitable for producing small-sized parts, such as, for example, a USB key body 80 embellished, for example, with fine engravings 81, as illustrated on FIG. figure 3 .

The present invention is not limited to the preferred embodiments described above as examples and to the variants mentioned. It also relates to the variants that are not outside the scope of the invention as defined in the claims.

In particular, as illustrated figure 2 , the electro-hydraulic discharge system 40 may present as an alternative to the two electrodes, an explosive wire 46. The explosive wire is known to those skilled in the art in its operation and will not be described in more detail in the present description.

In this variant, a passage duct 26 is formed in the discharge frame 20, passing through it at the level of the non-metallic coating 24 of the internal wall 21, in order to allow the routing of the explosive wire 46 in the discharge chamber 22.

The explosive wire 46 is preferably positioned in the center of the discharge chamber, vis-à-vis the non-metallic coating of the inner wall.

The thickness of the coating 24 is also a function of the energy generated during the electro-hydraulic discharge.

In another alternative embodiment, the electro-hydraulic discharge system 40 may comprise an explosive wire between two electrodes.

In this variant, a passage duct is made in an electrode, to allow the routing of the explosive wire between the two electrodes in the discharge chamber.

An example of an electro-hydraulic forming process from the electro-hydroforming enclosure 10 is now described.

To form a part 50 by electro-hydroforming, the method comprises a first step of positioning, in the electro-hydroforming enclosure 10, the workpiece.

The piece 50, for example initially flat, is positioned between the discharge frame 20 and the matrix 30. The part 50 is disposed in the electro-hydroforming enclosure 10 so as to be opposite the impression 31, and to separate the discharge chamber 22 from the forming chamber 32.

The workpiece is held in position and in the electro-hydroforming chamber, so as to seal the forming chamber with respect to the discharge chamber.

The method then comprises a step of filling the discharge chamber with water.

Water is introduced into the discharge chamber via the water supply conduit 23 until it is filled.

The method then comprises a step of electro-hydraulic discharge in the fluid contained in the discharge chamber.

One way to accomplish this step is to quickly discharge the at least one capacitor from the electrical energy storage unit.

The electro-hydraulic discharge system is activated.

In the variant of the electrodes, an electric arc is created between the electrodes, creating a bubble in the water.

In the variant of the explosive wire, the wire introduced into the discharge chamber explodes by vaporization, creating a bubble in the water.

This bubble collapses and releases its energy in the form of a shock wave, which propagates in the water and throws the piece against the impression of the forming chamber at very high speed (several hundred m / s) , causing its deformation and formatting. The voltage reached during the discharge is of the order of a few tens of kV.

In the case of tubular parts, the parts are deformed by radial expansion, instead of being deformed by stamping.

At the end of this step, the piece is formed.

The method then comprises a step of emptying the discharge chamber.

Water is pumped from the discharge chamber to the tank via the water drain.

The electro-hydroforming enclosure 10 is then open at the interface 33, releasing access to the forming chamber, from which the formed part is extracted.

The above description clearly illustrates that by its different characteristics and advantages, the present invention achieves the objectives it has set for itself. In particular, it offers an electro-hydroforming chamber suitable for forming small parts. It advantageously has an inner wall having two non-metallic coatings such that the dimensions of the discharge chamber can be significantly reduced, allowing a reduction in the volume of liquid required for the forming process. The cycle time is greatly reduced.

Claims (5)

1. Electrohydraulic forming chamber (10) for forming a part (50), comprising:

   - a first portion, called discharge frame (20), comprising an internal wall (21) delimiting a discharge chamber (22) intended to receive a volume of fluid,

   - a second portion, called die (30), comprising a forming chamber (32) having an imprint (31) intended to complement the form that the part has to take after deformation,

   - an electrohydraulic discharge system (40),

   said part (50) being intended to be positioned between the discharge chamber (22) and the forming chamber (32) before an activation of said electrohydraulic discharge system, the activation of said electrohydraulic discharge system causing the projection and deformation of the part (50) against the imprint (31) of the forming chamber (32), characterised in that the internal wall (21) is covered with two non-metallic coatings (24, 25), and in that the first non-metallic coating (24) situated at the electrohydraulic discharge system is chosen in a material with a greater dielectric strength than the second coating (25).
2. Electrohydraulic forming chamber (10) according to claim 1, wherein the coating (24) is a coating produced in an electrically insulating material.
3. Electrohydraulic forming chamber (10) according to one of the preceding claims, wherein the electrohydraulic discharge system (40) comprises two electrodes (41) intended to be linked to an electrical energy storage unit.
4. Electrohydraulic forming chamber (10) according to any of the preceding claims, wherein the electrohydraulic discharge system (40) comprises an explosive wire intended to be linked to an electrical energy storage unit.
5. Electrohydraulic forming machine comprising an electrohydraulic forming chamber (10) according to any of claims 1 to 4 and an electrical energy storage unit linked to the electrohydraulic discharge system.

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