EP0231677B1 - Method and apparatus for stamping sheet materials with slight elongation - Google Patents

Abstract

Process for press-forming sheet materials and in particular metal sheets on a double-action press of the type comprising a cushion (5) of an elastically yieldable material, the process comprising disposing the sheet on a support, applying a first outer slide (1) on the peripheral portion of the sheet (4), then applying a second central slide (10) on a central portion of the sheet, the process further comprising disposing the peripheral portion of the sheet on a lower blank holder (3) forming a tank for the cushion (5) and having an upper surface (6) for maintaining the sheet and located at a higher level than the level of the working surface (7) of the cushion (5), applying the first outer slide, which has a body (8) having a cross-section less than the lower blank holder (3) and includes on its periphery an upper blank holder (2) cooperative with the lower blank holder (3) for gripping the sheet (4), continuing the descent of the outer slide (1) against the cushion for turning up an edge portion (20) of the sheet and causing the flow of the mass of the cushion and deform the central portion of the sheet in such manner as to impart thereto an area substantially equal to the area of the finished part to be obtained, then displacing the central slide (10) so as to shape the angular volumes and the central portion of the sheet by a final flow of the cushion. The invention also provides a press-forming device of the double-action press type. Application in the press-forming of sheets of large size and small elongation.

Description

The present invention relates to a process and a device for stamping sheet materials with low elongation and more particularly, but not limited to steels with high elastic limit, called HLE.

The stamping of large parts is usually done by drawing with double effect presses, mechanical or hydraulic. These devices essentially comprise a fixed matrix and two independent slides, a central slide, called a plunger carrying a punch, and an external slide reserved for hold-down operations, that is to say sufficient support allowing by reaction the stretching under the punch. The movements are generally as follows: (1) rapid descent of the blank holder which maintains constant pressure on the sheet and thus prevents it from moving; (2) rapid descent of the punch until it comes into contact with the sheet, then (3) slow descent of the punch during the stamping phase, under actual drawing; and (4) rapid ascent of the central slide causing the blank holder to rise.

The implementation of this classic technique which is based on the elongation ability of extra mild steels however quickly finds its limits with HLE steels because of their elongation coefficient which is very seriously reduced (about at least two times lower).

The reduction in the possible elongation makes it practically compulsory to consume metal coming from the peripheral zone under a blank holder.

During the stamping operation, this reduction in the projected surface of the blank requires, to avoid the formation of folds, a blank clamp pressure much higher for HLE steels than for extra mild steels.

This high pressure promotes seizure, and causes rapid wear of the blank holder and the matrix entry spokes.

The second difficulty encountered in the drawing of HLE steels comes from the folding of the sheet in the central part of the stamping during the attack of the punch.

This tendency to wrinkling in the shape of an equal punch is all the more marked the thinner the sheet and the higher its resistance.

To remedy this latter difficulty, the Applicant has proposed in its French patent 8,407,678 a method of stamping on an elastic cushion placed as a support in a double-acting press.

According to this process, at least one active part is provided on the first external slide, the shape of which corresponds to the surface area excess with respect to the volume to be formed, this active part acting on the peripheral part of the sheet itself in contact, by its other side, with the elastic cushion.

This technique allows deep drawing with practically constant thickness and is in particular usable for extra-thin sheets. It is however complicated because it requires the precise determination of the shapes corresponding to the excess surface, by complex mathematical methods and then the rigorous machining of these shapes to constitute the active parts of the first external slide.

These sophisticated methods are very cumbersome to implement and target more particularly extra-thin steels and in particular for deep drawn.

Furthermore, a reversal stamping technique is known which has hitherto been mainly used for deep drawing of steels with high elongation.

This technique consists in deforming in successive stages the sheet blank starting from its periphery, that is to say falling an edge. It allows very large deformations insofar as, at each step, the parameters for reducing the surface under the blank holder act so as to keep the thickness of the sheet substantially constant.

However, this technique would not allow complex shapes to be obtained in the central part of the stamping. In fact, it essentially aims at obtaining deep-drawn stamps of simple shape through the use of successive peripheral deformations produced by the falling of the edges.

The present invention aims to obtain deep-drawn parts of medium depth, but large areas such as automobile parts, the central parts of which are practically never forms of revolution but represent complex shapes which cannot be developed.

However, these complex shapes cannot be produced in a single pass with a metal punch without the risk of folds and breakage of the stamped part.

The present invention relates to a process for stamping sheet materials, in particular sheet metal, on a double-acting press of the type comprising a cushion made of an elastic material according to which the sheet metal to be formed is placed on a support, a first slide is applied. outside on the peripheral part of the sheet, then a second central slide is applied to the central part of the sheet, characterized in that the peripheral part of the sheet to be formed is placed on a lower blank holder forming a container for the cushion of elastic material and whose upper face for holding the sheet is located at a level higher than that of the working face of the elastic cushion, the first external slide is applied, the body of which has a section smaller than that of the lower blank holder and which has at its periphery an upper blank holder cooperating with the lower blank holder to tighten the sheet, the outer slide is continued to descend against the elastic cushion to drop an edge of the sheet blank and cause the mass of the elastic cushion to creep in order to deform the central part of the sheet so as to give it a surface substantially equal to the surface of the finished part to be obtained, then the central slide is moved to conform the angular volumes and the central part of the sheet by final creep of the support.

The essential characteristic of the invention resides in the falling of a peripheral edge which has the object of which is to reduce the volume of metal to be shrunk and consequently leads to a reduction in the pressure corresponding to the punch, that is to say in the case of elastoforming on an elastic cushion according to the present invention, to a reduction in the pressure in this cushion.

In fact, in the method of the invention, the surface of the central slide in contact with the sheet plays the role of the bottom of the matrix and the elastic cushion of flowable material that of a punch applying the sheet in the bottom of the matrix to form the angular volumes.

Since the tensile and compressive stresses are substantially equal, the risk of wrinkling is substantially reduced.

For a deep-drawn having in the central part forms with sharp angles or small details, an external slide is then used whose working face in contact with the sheet, forming a peripheral matrix, includes active parts as described in the patent FR 8,407,678 in combination with the edge falling operation according to the present invention. It should be noted that these active parts form a relief that is both convex and concave according to the most advantageous technical arrangement for the part considered.

It will be recalled that these active parts determine in the peripheral part of the sheet metal adjacent to the fallen edge disposed under the outer slide of the shapes which compensate in certain areas of the finished part the excess surfaces with thickness substantially unchanged of the starting sheet, relative to the volume to be formed.

These active parts can also be placed in the bottom of the matrix carried by the central slide when they correspond to areas of the finished part which are intended to be eliminated by subsequent cutting.

• - on amène dans un premier temps le coulisseau central dans une position dans laquelle il limite la déformation de la partie centrale de la tôle sous l'effet du fluage du matériau de support;
• - le matériau constituant le support est un élastomère à faible dureté Shore, par exemple inférieur à 30 Shore 00;
• - après l'operation de formage proprement dite, on provoque une décompression du matériau constituant le support;
• - on refroidit la masse du matériau de support.

According to other characteristics of the invention:

• - The central slide is first brought into a position in which it limits the deformation of the central part of the sheet under the effect of the creep of the support material;
• - The material constituting the support is an elastomer with low Shore hardness, for example less than 30 Shore 00;
• - After the actual forming operation, the material constituting the support is decompressed;
• - The mass of the support material is cooled.

The subject of the invention is also a stamping device of the type comprising a support on which the sheet metal to be formed is placed, a first external slide, a second central slide and a cushion made of an elastic material, characterized in that the support is constituted by a lower peripheral blank holder forming a container for the cushion of elastic material, and the sheet holding surface of which is situated at a level higher than that of the elastic cushion, the external slide having a body of section smaller than that of the lower blank holder to penetrate the latter by making a falling edge of the sheet blank and reach the cushion of elastic material and cause its creep, the outer part of the outer slider carrying an upper peripheral blank holder cooperating with the blank holder lower to tighten the sheet, and the central slide carrying a bottom of matrix.

According to a variant, the external slide comprises on its lower face forming a peripheral matrix at least one active part in convex or concave relief, the shape of which corresponds to the excess surface of the sheet, of substantially constant thickness, with respect to the volume to be formed.

According to another embodiment of this variant, the central slide carries a matrix bottom comprising at least one active part in concave or convex relief corresponding to the excess surface of the sheet, with thickness substantially constant with respect to the volume to be formed.

The elastic support material is preferably easily flowable, for example an elastomer having a Shore 00 hardness of less than 30.

• - il est prévu des organes qui, dans un premier temps, sont en saillie dans la masse du matériau de support et qui dans un second temps, peuvent être rétractés, après l'opération de formage, pour provoquer une décompression dudit matériau;
• - il est prévu des moyens de refroidissement dans la masse du matériau constituant le support:
• - il est prévu des moyens de décollage de la pièce finie du matériau de support.

According to other characteristics:

• - There are provided members which, initially, protrude into the mass of the support material and which, in a second step, can be retracted, after the forming operation, to cause decompression of said material;
• - cooling means are provided in the mass of the material constituting the support:
• - Means are provided for removing the finished part from the support material.

• - les Fig. 1 à 5 sont des vues schématiques en coupe du dispositif d'emboutissage selon l'invention, au cours des étapes successives du formage d'une pièce;
• - les Fig. 6 et 7 sont des vues schématiques en coupe de deux modes de réalisation d'une variante du dispositif d'emboutissage selon l'invention représentée uniquement à l'étape préliminaire de mise en place de la tôle à emboutir.

The invention is set out below in more detail with the aid of the appended drawings, which show two embodiments of these drawings:

• - Figs. 1 to 5 are schematic sectional views of the stamping device according to the invention, during the successive stages of the forming of a part;
• - Figs. 6 and 7 are schematic sectional views of two embodiments of a variant of the stamping device according to the invention shown only in the preliminary step of placing the sheet to be stamped.

According to a first variant shown in FIGS. 1 to 5 the device of FIG. 1 in position before forming, comprises the usual constituent elements of a double-acting press, and therefore only the part relating to the invention is shown.

An external slide 1 carries at its external part an upper peripheral blank holder 2 which cooperates with a lower peripheral blank holder 3 forming a support on which there is a sheet to be stamped.

The lower peripheral blank holder 3 forms a container in which is housed an elastic cushion 5 occupying the entire surface of this container. The sheet holding surface 6 is located at a level higher than the upper working face 7 of the elastic cushion.

The outer slider 1 comprises a body 8 whose outer section is less than the inner section of the lower blank holder 3, so that the underside 9 of the outer slider opposite the sheet 4, can, in the absence of this last, enter the lower blank holder 3 to reach the cushion 5 of elastic material and cause its creep.

The central slide 10 carries a bottom of the matrix 11, the peripheral part of the matrix being constituted by the lower face 9 of the external slide 1.

The outer slider 1 and the central slider 10 are actuated in synchronism as will be seen below and play, by their lower faces 9 and 11, the role of matrix, the cushion 5 of elastic material playing the role of punch during operation.

The cushion 5 made of elastic material consists of an elastomer having a Shore 00 hardness less than 30, a very important characteristic residing in the rapid return time of the material (preferably less than 1 second) to its initial shape. One can, for example, use a silicone-based material.

The bottom of the matrix (11) carried by the central slide is made of a material easy to machine or to shape such as a plastic material, in particular a polyurethane, polyepoxy, a polyester, a concrete, a concrete with resin added, a composite material, these materials possibly being loaded with fibers, in particular glass; or hardwood such as boxwood.

Retractable members 12 (inflatable candles or bladders) protrude into the elastic cushion 5, and their inserted volume represents approximately the expansion volume of the elastomer after forming.

The cushion 5 comprises conduits 13 allowing the circulation of a cooling fluid such as compressed air. Other conduits 14, in particular when compressed air is used, can be used to take off from the finished part. For the cooling of the mass of the cushion 5, it is also possible to provide embedded metal wires or a metal powder charge improving the thermal conductivity.

FIG. 2 represents the step of falling off the edge 20 of the part which is housed in the annular space 21 formed outside the body 8 of the external slide 1 as indicated in FIG. 1

In the step illustrated in FIG. 2, the outer slide 1 is lowered, carrying the peripheral matrix 9. The latter comes into contact with the sheet blank 4 whose peripheral part is progressively clamped between the upper 2 and lower 3 blank clamps to avoid its scalloping.

During its descent, the peripheral matrix 9 forms a fallen edge 20 of the sheet blank and simultaneously compresses by reaction the elastomer cushion 5. The latter under the effect of this peripheral compression action acts by creep on the central area of the blank of sheet metal and causes its deformation.

The swelling of the central part of the sheet blank is limited by the die bottom 11 fixed to the central slide 10 in order to avoid uncontrolled erratic deformations due to the anisotropy of the metal or of the shapes of asymmetrical parts. The descent of the outer slide 1 carrying the peripheral matrix 9 is limited in such a way that the deformation in the central part of the sheet blank gives a surface substantially equal to that of the finished part to be obtained.

FIG. 3 represents the stage of final shaping of the part. The central slide 10 carrying the bottom of the matrix 11 descends to its low position and causes the final forming of the central part of the sheet 4, preformed during the previous operation.

The compression stresses due to the support of the bottom of the matrix 11 on the top of the sheet are transformed by the action of the elastomer cushion 5, acting on the opposite face of the sheet, into tensile stresses exerted on the entire surface of the sheet not compensated by the presence of the bottom of the matrix 11, and cause the displacement of this sheet throughout the available volume.

These stresses (compression, traction) thus tend to cancel each other (at yield near the elastomer) thus allowing the final production of the part with a minimum variation in thickness.

FIG. 4 represents the step of decompression of the elastomer cushion 5, by withdrawal of the candles 12. The aim of this operation is to avoid deformation of the stamped part by expansion reaction of the elastomer.

FIG. 5 represents the step of releasing the formed part 21, by simultaneous ascent of the opdeux slides 1 and 10 carrying the dies. To limit the heating of the elastomer cushion 5, in particular when working in series, compressed air is circulated in the conduits 13. The cooling of the elastomer 5 can also be carried out during the previous step of decompression (Fig. 4). Furthermore, compressed air is sent through the conduits 14 to allow the part 21 to take off.

According to a variant illustrated in FIG. 6, the outer slide 1 carries a peripheral matrix 9 which has in its angles an appropriate shape in convex relief, that is to say forming a projection 22 formed integrally with the peripheral matrix 9 (this relief shape 22 corresponds to the surface area in excess of the volume to be formed of the part that is to be produced) its active surface is carefully polished so as to allow the displacement of the excess material during forming; this active surface can also be treated to facilitate the sliding of the sheet.

According to another embodiment of the variant of FIG. 6, illustrated in FIG. 7, the external slide 1 carries a peripheral matrix 9 in which are concave active parts, that is to say hollow 23, which play the same role as the parts 22 of FIG. 6, the arrangement of which is chosen for reasons of optimization of the stamped part. Thus the active parts 23 may possibly be arranged in areas of the stamped, which will be eliminated by cutting in the finished part. It is also possible to place active parts 24 having the functional role defined below in the bottom of the matrix 11 carried by the central slide 10 when these active parts 24 are located in central zones of the stamping which will be eliminated by cutting in the finished part or when the latter corresponds to the part of the sheet metal located essentially under the outer slide.

This variant is more especially intended for the production of complex shapes with sharp angles of the central part of. the stamped.

Stamping by turning as described above with its falling edge makes it possible to reduce the pressure necessary for forming the sheet by reversing the traditional punch cycle. The pressure which was exerted only for the forming of the sheet on the equivalent of the matrix entry radii, is applied after this inversion on the entire central surface of the stamped.

The pressure necessary for this pre-forming of the sheet is very low (value comprised at most between 10 and 20 bars (1 and 2 MPa).

This low pressure regime thus makes it possible to form large areas and to create a new double-effect tool technique with an elastomer matrix of low Shore hardness which can be adapted to existing body presses.

The first phase of the stamping operation consists in making the peripheral edges (Fig. 2) fall from the blank by the external slider 1.

The first phase simultaneously sees the realization of the curved preforming surface which causes the expansion of the sheet and thus prevents the formation of folds.

• 1 par le volume d'élastomère déplacé, ce qui définit la surface courbe de la tôle;
• 2° par la proimité du poinçon qui évite la déformation erratique de la tôle, et ordonne la pré-déformation;
• 3° par la retenue périphérique du flan de tôle qui est ajustée afin de limiter l'expansion bi-axiale de la tôle à une valeur faible 3 à 5%, cette valeur évitant la formation de plis.

It is important to note that this expansion is limited in several ways:

• 1 by the volume of elastomer displaced, which defines the curved surface of the sheet;
• 2 ° by the proximity of the punch which avoids the erratic deformation of the sheet, and orders the pre-deformation;
• 3 ° by the peripheral retention of the sheet blank which is adjusted in order to limit the bi-axial expansion of the sheet to a low value 3 to 5%, this value avoiding the formation of folds.

The curvature of the surface being mainly obtained by the multidirectional bending of the sheet, which allows during the shaping operation second and last phase of the stamping operation which will modify these bends, a very great potential for rearrangement of shapes .

Although the description has been made with reference to the forming of sheets, that is to say generally metallic thin plates, it should be understood, however, that the method according to the present invention is in no way limited to this application, as this is clearly indicated in the first two lines of the description. Thus the method of the present invention can be implemented with thin plates, in particular of plastic.

• - un polybutène dont les propriétés sont décrites dans l'ouvrage "Matières Plastiques" Chimie-applications de Jean BOST, p 244-245
• - un polyéthylène, un polyéthylène chloré, un polypropylène, un PVC:
• - un PVC chloré, une résine ABS (acrylonitrile, bu- taniène, styrène), un polycarbonate, polyphénylène oxyde, polysulfone, chlorotrifluoroéthylène, cellulose acétate, cellulose acétate butyrate, polyacetal, phenoxy, nylon 6, nylon 66. Les propriétés de ces matières plastiques sont instamment décrites dans "Polymers Engineering and Science, Mars 1971, volume 11, n° 2 page 106. Il faut également entendre par matières plastiques des matériaux composites éventuellement chargés.

Among the plastics formable by the process of the present invention, non-limiting examples will be cited:

• - a polybutene, the properties of which are described in the book "Plastic Materials" Chemistry-applications by Jean BOST, p 244-245
• - a polyethylene, a chlorinated polyethylene, a polypropylene, a PVC:
• - a chlorinated PVC, an ABS resin (acrylonitrile, butane, styrene), a polycarbonate, polyphenylene oxide, polysulfone, chlorotrifluoroethylene, cellulose acetate, cellulose acetate butyrate, polyacetal, phenoxy, nylon 6, nylon 66. The properties of these materials plastics are urgently described in "Polymers Engineering and Science, March 1971, volume 11, no. 2 page 106. Plastics should also be understood as possibly charged composite materials.

Thus, in the present description, the term "sheet metal" must be given a general meaning of a thin plate of sheet material without wanting to limit its scope to metallic products.

The process of the present invention can also be used for thermoforming sheet materials. In this case, the materials can be previously heated to a temperature which does not degrade the material constituting the elastic cushion.

Claims (16)

1. A process for the press-forming of sheet materials, more particularly metal sheets, on a double action press of the kind comprising a cushion (5) of a resilient material, wherein a first outer slide (1) is applied to the peripheral portion of the sheet (4), whereafter a second central slide (10) is applied to the central portion of the sheet, characterized in that the peripheral portion of the sheet to be formed is disposed on a lower blank holder (3) which forms a tray for the cushion (5) of resilient material and whose upper surface (6) for maintaining the plate is located at a level higher than that of the working surface (7) of the resilient cushion (5); the first outer slide is applied, whose body (8) has a cross-section smaller than that of the lower blank holder (3) and whose periphery comprises an upper blank holder (2) cooperating with the lower blank holder (3), to grip the sheet (4); the descent of the outer slide (1) against the resilient cushion is continued to produce the turning up of an edge (20) of the sheet blank, and the mass of the resilient cushion is caused to flow, to so deform the central portion of the sheet as to impart thereto a surface substantially equal to the surface of the finished part to be obtained; whereafter the central slide (10) is displaced to shape the angular volumes and the central portion of the sheet by the final flow of the support.

2. A process according to claim 1, characterized in that the peripheral portion of the sheet (4) adjacent the turned-up edge (20) is formed by means of at least one active portion of the peripheral die (9) borne by the outer slide so as to compensate in certain zones of the finished part for the excess areas for an unchanged thickness of the initial sheet, relative to the volume to be formed.

3. A process according to claim 1 charaterized in that the central portion of the sheet (4) is formed by means of at least one active portion (24) of the bottom of the die (11) borne by the central slide (10), to compensate in certain zones of the finished part for the excess areas for a substantially unchanged thickness of the initial sheet, in relation to the volume to be formed during the simultaneous displacement of the central slide (10).

4. A process according to any of claims 1 to 3, characterized in that in a first stage the central slide (10) is moved into a position in which it limits the deformation of the central portion of the sheet as a result of the flow of the support material.

5. A process according to any of claims 1 to 4, characterized in that the material forming the resilient cushion (5) is an elastomer of low Shore hardness.

6. A process according to claim 5, characterized in that the elastomer has a Shore hardness of less than 30.

7. A process according to any of claims 1 to 6, characterized in that after the actual forming operation, the material forming the resilient cushion (5) is decompressed.

8. A process according to any of claims 1 to 7, characterized in that the mass of the material forming the resilient cushion is cooled.

9. A press-forming device of the kind comprising a support on which the sheet (4) to be formed is placed, a first outer slide (1), a second central slide (10), and a cushion (5) of a resilient material, characterized in that the support is formed by a lower peripheral blank holder (3) which forms a tray for the cushion of resilient material and whose surface (6) maintains the sheet which is situated at a level higher than the level of the working surface (7) of the resilient cushion (5), the outer slide (1) comprising a body (8) having a cross-section smaller than that of the lower blank holder (3) to enter the latter and produce a turning up of the edge (20) of the sheet blank and reach the cushion of resilient material, which it causes to flow, the outer portion of the outer slide bearing an upper peripheral blank holder (2) cooperating with the lower blank holder (3) to grip the sheet, the central slide bearing a die bottom (11).

10. A device according to claim 9, characterized in that the outer slide (1) comprises on its lower surface forming a peripheral die (9) at least one active portion (22) in convex or concave relief whose shape corresponds to the excess surface of the sheet for a substantially constant thickness, in relation to the volume to be formed.

11. A device according to claim 9, characaterized in that the central slide (10) bears a die bottom (11) comprising at least one active portion (24) in concave or convex relief whose shape corresponds to the excess area of the sheet for a substantially constant thickness, in relation to the volume to be formed.

12. A device according to any of claims 9 to 11, caracterized in that the material of the resilient cushion (5) is an elastomer having a Shore hardness A preferably less than 30. 1

13. A device according to any of claims 9 to 12, characterized in that the die bottom (11) borne by the central slide is made from a material which is easy to machine or shape, such as a plastics material, more particularly a polyurethane, a polyepoxy, a polyester, a concrete, a concrete to which resin has been added, a composite material, such materials possibly being charged with fibres, more particularly glass fibres, or a hard wood such as boxwood.

14. A device according to any of claims 9 to 13, characterized in that members (12) are provided which in a first stage project into the mass of the resilient cushion (5) and which in a second stage can be retracted after the forming operation.

15. A device according to any of claims 9 to 14, characterized in that cooling means (13) are provided in the mass of material forming the resilient cushion.

16. A device according to any of claims 9 to 15, characterized in that means (14) are provided for detaching the finished part from the resilient cushion (5).

Images