EP0238099B1 - Method and apparatus for stamping sheet material - Google Patents

Abstract

A sheet of metal of given thickness is press-formed to provide a part having a substantially constant thickness on a double action press. The sheet to be formed is disposed on a support. A first outer slide, or blank holder is applied on the peripheral portion of the sheet. A second central slide is applied on the central portion of the sheet. The peripheral portion of the sheet is formed while allowing it to slide under the blank holder, by displacing relative to said support at least one active part of the outer slide so as to compensate, in certain regions of the finished part, for excess areas of the sheet, for the given thickness of said sheet, relative to the volume of metal to be formed, and displacing simultaneously with the displacement of the part the central slide so as to shape angular volumes of the central portion of the sheet by application of the sheet against the surfaces of the central part of the support.

Description

The present invention relates to a method and a device for stamping sheets, in particular extra-thin steel sheets of large dimensions which can be used for example in the automobile industry, or comprising shapes having relatively closed dihedral angles. A method according to the preamble of claim 1 and a device according to the preamble of claim 5 are known from "Professional technology of working metals in sheets, tubes, profiles", G. Cotant, published by Dunod, Paris, France, 1976 , page 200 to 203.

The development of computerized calculations, particularly among car manufacturers in the field of vehicle body structure design, particularly by analysis methods, such as that known as "finite elements" has made it possible to significantly reduce the time spent designing the parts, while getting as close as possible to their behavior under stress. It is therefore theoretically possible to optimize the shapes and thicknesses of the sheets as a function of the degree of stress.

The lower limit of the thicknesses is nevertheless limited by current stamping techniques, which do not allow the use, in the dimension of the body parts, of steel sheets whose thickness is less than 55/100 mm approximately, because of the tearing which appears in the areas subjected to stretching or of the creasing in the areas subjected to shrinking, of the sheet metal stamping.

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 sliders, a central slider, called a plunger carrying a punch, and an external slider reserved for blank holding operations, that is to say sufficient support allowing by reaction the stretching under the punch. 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 in its ascent.

This classic process is illustrated by patent FR-A-756,767 in which all of the stamping is done by stretching, thus reducing the thickness of the sheet. It appears that this conventional process does not allow stamping of extra-thin sheets (thickness <50/100 of millimeters) because the rigidity of the die and the punch as well as the tightening of the sheet by the blank holder would generate deformations non-homogeneous resulting in certain areas by elongations likely to cause excessive thinning given the small initial thickness of the sheet and in other areas by shrinking tending to cause thickening of the sheet, which lead in practice to the formation of folds due to its low buckling resistance (resistance evolving as a function of the square of thicknesses). In addition, variations in localized stresses due to the shape of the part and the tolerances in the production of the tools cause tears. To remedy the problem of the formation of folds, the above French patent thus proposes means housed in the blank holder which ensure in themselves, an additional drawing of the sheet in the areas where its drawing is insufficient compared to that exerted by the punch in the other zones.

These difficulties are also encountered when producing angular volumes for relatively thicker sheets and obviously aggravated for extra-thin sheets.

There is also known a process called the "Guerrin process" according to which a sheet of sheet metal resting on a mass of elastomer having a high shore hardness of about 90 is formed by means of a punch, taking into account that obtain a precision in obtaining the part, ensure the plating of the sheet metal blank on the punch. However, the main drawback of this method is that it consumes significant energy. In fact, to the energy necessary for the forming of the sheet blank, is added that necessary for the formation of the imprint corresponding to the shape of the part in the mass of elastomer plus the friction of the latter over the entire workpiece surface during the forming operation.

This excludes the possibility of realizing large parts obtained by this process on existing presses.

In addition, this process does not prevent the formation of folds, the pressure gradients generated by the deformation of the elastomer decreasing as we approach the upper surface of the elastomer mass, especially for the production of angular shapes, the sheet being insufficiently held at its periphery due to the work on a single-effect press.

Finally, a forming process is known using a pressurized fluid to form simple shapes of the hemispherical type. However, this technique cannot be used for complicated shapes because it is then necessary to press the sheet against the matrix, which cannot be obtained by this technique.

This is why, in spite of recent significant progress in the mechanical characteristics of steel sheets with high elastic limit, it has not been possible to manufacture bodywork or other parts from extra-thin sheet metal.

The object of the invention is therefore to provide a technology making it possible to stamp metal sheets having angular volumes and in particular extra-thin steel sheets (<50/100 mm), in large series and under competitive economic conditions. . By high elastic limit steel sheets (HLE) is meant steels for which E> 350 MPa.

The subject of the invention is therefore a process for stamping sheets of substantially constant thickness, on a double-acting press, according to claim 1.

Thus, the invention is based on the principle of the equality of the surfaces of the initial flat sheet metal blank and the shaped blank strictly corresponds to the shape of the desired stamped part, increased by the excess peripheral surfaces shaped under blank holder, thus conditioning the maintenance of a substantially constant thickness of the sheet of equal stress.

The invention also relates to a stamping device according to claim 5.

The active part in relief of the external slide is carried by a movable element in translation independent of the external slide, which cooperates with a complementary shape provided in the support.

• les Fig. 1 à 10 sont des vues schématiques en coupe de trois variantes du dispositif d'emboutissage selon l'invention, au cours des étapes successives du formage d'une pièce;
• la Fig. 11 est une vue en perspective d'une pièce parallèlépipède formée selon l'invention; et
• les Fig. 12 et 13 en sont deux vues partielles en coupe de la pièce de la Fig. 11.
• les Fig. 14 et 15 sont deux vues schématiques montrant respectivement une feuille rectangulaire plane et une pièce parallèlépipède formée de manière classique par pliage.

The invention is set out below in more detail with the aid of the accompanying drawings, which show an embodiment of these drawings:

• Figs. 1 to 10 are schematic sectional views of three variants of the stamping device according to the invention, during the successive stages of the forming of a part;
• Fig. 11 is a perspective view of a parallelepiped part formed according to the invention; and
• Figs. 12 and 13 are two partial sectional views of the part of FIG. 11.
• Figs. 14 and 15 are two schematic views respectively showing a rectangular sheet plane and a parallelepiped part formed in a conventional manner by folding.

According to a first variant shown in FIGS. 1 to 4, 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 slider or blank holder 1 carrying a peripheral part 10 and a central slider 2 forming a punch are in the high position, while the sheet blank 7 to be formed is placed on a support 4 forming a matrix. The peripheral part 10 comprises in its corners active parts in relief 11 carried by elements or candles 30 movable in translation in corresponding housings formed in the blank holder 1. The active parts in relief 11 have an appropriate shape which corresponds to the surface excess in relation to the volume to be formed of the part which it is desired to produce, for example as illustrated in FIG. 11.

The support 4 made of a hard material, for example metallic, forms the matrix and includes a peripheral part 31 in which recessed parts 32 are formed corresponding to the complementary shape of the active parts in relief 11 of the candles 30. The central part 33 of the support 4 constitutes the bottom of the matrix and has the shape of the finished part in its central zone.

The peripheral part 10 of the outer slide 1 has on its outer edges shims 34 bearing on the peripheral part 31 of the support 4 so as to allow, although the sheet metal blank 7 is maintained, the displacement by sliding of the excess material in the cooperating parts 11, 32.

The step in FIG. 1 constitutes the step of placing the sheet blank 7, the external slide of the blank holder 1 being raised as well as the mobile elements 30 and the central slide 2.

Fig. 2 shows the step of bringing the blank holder into contact with the sheet 7, by lowering the outer slide 1 and the punch 2. During this step, the mobile elements 30 do not move and retract into the clamp. blank relative to their initial projecting position, so that the sheet is not stressed by any deformation and only undergoes the tightening operation controlled at its periphery by the blank holder.

Fig. 3 illustrates a stage of active shaping of the part, in which there is a simultaneous descent and in relation to one another of the mobile elements 30 and of the punch 2 so as to gradually absorb the excess surface in certain areas of the part to be formed thanks to the active part 11 and its complementary part 32, while maintaining the sheet metal stretched over the punch by controlled sliding under the blank holder.

At the stage of FIG. 4, the active part of the punch has reached the bottom of the die 33 and the movable elements 30 also have their active parts 11 which cooperate with the complementary parts 32 by enclosing the sheet 7 and thus having absorbed the excess surfaces of sheet relative to to the volume of the finished part to be formed, thus avoiding the formation of folds, shrinking or thinning at any point whatever the thickness of the sheet.

The device shown in Figs. 5 to 9 illustrates a second variant of the method and the device according to the present invention in which the movement relative simulated and coordinated active parts in relief 11 of the peripheral slide 1 and the central slide 2 is obtained.

The device of FIG. 5 in the position before forming, comprises the usual constituent elements of a double-acting press which have already been described and bear the same references as above. The outer slider or blank holder 1 carries a peripheral part 10 forming a matrix which has in its angles an appropriate relief shape 11 formed integrally with the peripheral matrix 10 (this relief shape 11 corresponds to the surface area in excess of the volume to be form the part that you want to make, for example as illustrated in Fig. 11) and its active surface is carefully polished to allow the movement of excess material during forming; this active surface can also be treated to facilitate the sliding of the material.

The central slide 2 carries a bottom of matrix 2a and is in the raised position, while the sheet blank 7 to be formed is placed in the center of a support 4 disposed in a container 3 (tray). In this case it should be noted an inversion of the matrix-punch roles which will appear in the description below of this embodiment. Indeed, the support made of an easily flowable elastic material plays the role of punch by deformation.

The support 4 consists of an elastomer having a shore hardness less than 30 and preferably less than 10, 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 natural rubber foam having a shore hardness of about 15 and having a very short deformation time of the order of 1 s. It is also possible to use other conventional gels or foams, preferably having great plasticity, for example silicone elastomers, or cellular foams, the cells of which can be filled with liquid.

It is also possible to use a composite support consisting of a substantially parallelepipedal mass based on a silicone elastomer having a shore hardness of 10 to 20, covered on its upper face and on all or part of its lateral faces with a relatively thin skin (for example 10 to 15 mm) and of a more resistant and harder material such as a shore hardness silicone 50 or teflon having favorable antifriction properties.

Retractable members 5 (inflatable candles or bladders) protrude into the elastomer serving as a support 4, and their inserted volume represents approximately the expansion volume of the elastomer after forming.

The support 4 is covered with a plastic sheet 8, for example of "Teflon", interposed between the sheet 7 and the elastomer, which can be optionally glued or welded to the elastomer and which has the main purpose to facilitate the sliding of the sheet during forming, but it can also serve to protect the elastomer as indicated in the preceding composite structure.

The support 4 includes conduits 6 allowing the circulation of a cooling fluid such as compressed air. Other conduits 9, in particular when compressed air is used, can be used to take off from the finished part. For cooling the support mass 4, provision may also be made embedded metal wires or a metal powder filler improving thermal conductivity.

Fig. 6 shows the pre-forming step of the part. The blank-holding slide 1 is lowered carrying the peripheral matrix 10. The latter comes into contact with the sheet blank 7 which reacts by compressing the elastomer support 4. The elastomer, under the effect of this action of peripheral compression acts by creep on the central zone of the sheet blank and causes its deformation.

The swelling of the central part of the sheet blank is limited by the matrix bottom 2a fixed to the central slide 2 in order to avoid uncontrolled erratic deformations due to the anisotropy of the metal or to forms of asymmetrical parts. According to one of the characteristics of the invention, the descent of the blank holder slide 1 carrying the peripheral matrix 10 is limited by adjustable mechanical stops 12, 16 so that the deformation in its central part of the sheet blank gives a surface which is substantially equal to that of the finished part to be obtained.

Fig. 7 represents the final shaping stage of the part. The central slide 2 carrying the bottom of the die 2a descends to its low position and causes the final forming of the central part of the sheet 7, preformed during the previous operation.

The compression stresses due to the support of the bottom of the matrix 2a on the top of the sheet are transformed by the action of the elastomer 4, 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 matrix 2a and, cause the displacement of this sheet throughout 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. These conditions are necessary in the particular case of forming extra-thin sheets.

Fig. 8 represents the step of decompression of the elastomer support 4 by withdrawal of the candles 5. This operation aims to avoid the deformation of the stamped part by expansion reaction of the elastomer.

Fig. 9 represents the step of releasing the formed part 7, by simultaneous ascent of the two slides 1 and 2 carrying the dies. To limit the heating of the elastomer support 4, especially when working in series, compressed air is circulated in the conduits 6. The support 4 can also be cooled during the previous decompression step ( Fig. 8). Furthermore, compressed air is sent through the conduits 9 to allow the part 7 to take off.

According to a mixed variant shown in FIG. 10 the outer slider or blank holder 1 is of the type illustrated by the first variant, namely, it has active parts in relief 11 carried by elements movable in translation in housings formed in the blank holder 1.

Opposite these moving parts 11, are formed cells 40 filled with a relatively hard elastomeric material 41 in which will form the complementary imprint of the active parts in relief 11 acting through the sheet 7 which will thus keep in the corners the shape corresponding to the surface excess of the sheet compared to the volume to be formed.

During the forming of the central part of the sheet blank under the effect of the central slider 2 forming a punch, the mobile elements 30 descend simultaneously and in relation to the movement of the punch so as to obtain the same effect as that described for the first variant .

Fig. 11 shows an exemplary embodiment according to the invention of a piece of sheet steel HLE E = 60 kg / mm² of 40/100 mm, having substantially the shape of a rectangular parallelepiped of 1.5 m² in area. Two sectional views along lines 12 and 13 show that the excess surfaces of material, during the forming of the corner 13, have been displaced towards the base 13a of the corner of the part in a manner corresponding to the relief shapes. 11 of the peripheral matrix 10. Subsequently an operation of trimming the part makes it possible to eliminate the undesirable edges 15 and to give it its final shape.

Indeed, we can make a rectangular parallelepiped (as shown in Fig. 11) starting from a flat rectangular sheet (shown in Fig. 14) by folding (Fig. 15) on the condition of cutting a square 50 to each corner of the sheet.

However, to produce this part by stamping, this square 50 exists and represents excess material. However, the aim of the present invention is to avoid the formation of any excess thickness or thinning which would risk generating folds or tears, in particular in ultra-thin sheets having in particular deep angular volumes.

The technique of making the angles of a stamped volume described above is a solution to this problem.

Indeed, the stamping die has at each of its angles a suitable shape in relief 11 which will form in hollow at the base of each of the angles of the part to obtain a progressive concave shape of surface equivalent to the excess of material , for example, the square 50 mentioned in the previous paragraph (forming a rectangular parallelepiped).

The compression stresses due to the excess material generated by the shape of the part is therefore compensated by equivalent tensile stresses generated by the shape of the tool (absorption of the excess material).

• de limiter au maximum l'amincissement du flan de tôle malgré sa faible épaisseur initiale;
• d'éviter les concentrations de contraintes; et
• de réaliser le formage des angles sans formation de plis.

The method according to the invention thus allows:

• limit as much as possible the thinning of the sheet blank despite its initial low thickness;
• avoid stress concentrations; and
• to form the angles without forming folds.

In addition, the device according to the invention is adaptable to existing double-acting presses.

The invention may prove to be particularly useful for the production of ultra-thin sheet steel of automobile body parts, of aircraft fuselage, etc .; but it can also be applied to the stamping of thicker sheets of various metals.

In addition, it can be noted, in the case of the embodiment of the stamping on an elastomer cushion, that the surface condition of the face of the sheet in contact with the elastomer is fully preserved, so that it is possible to form sheets previously coated without injuring the surface coating film during stamping, even if it is a film of paint, glue or any other organic coating.

Claims (10)

1. A method for stamping sheets with a substantially constant thickness on a double action press, according to which the sheet to be shaped is disposed on a support (4), a first outer press ram or blank holder (1) is applied to the peripheral portion of the sheet (7), then a second central press ram (2) is applied to the central portion of the sheet, and the peripheral portion of this sheet is shaped by allowing it to slide under the blank holder by means of at least one active part (11) of the outer press ram, characterized in that one compensates in some zones of the finished part the excess surfaces having the same thickness of the original sheet, in relation to the space to be shaped increased by the excess peripheral surfaces shaped under the blank holder, and simultaneously one displaces the central press ram (2) to shape the angular spaces of the central portion by applying the sheet against the surfaces of the central portion (33) of the support (4).
2. A method according to claim 1, characterized in that provision is made for anti-friction means on the active portion of the outer press ram (1) and on the support (4).
3. A method according to claim 1, characterized in that the support (4) comprises a resilient material in a peripheral zone corresponding to the active part (11) of the outer press ram (1) and extending at least in a zone directly adjacent to the central press ram.
4. A method according to claim 3, characterized in that the resilient material has a Shore hardness, preferably comprised between 70 and 100.
5. A stamping apparatus of the type comprising a support (4) whereon the sheet (7) to be shaped is placed, a first outer press ram or blank holder (1) and a second central press ram (2), the first outer press ram (1) comprising at least one active part (11) in relief, characterized in that the shape of the active part (11) makes it possible to displace the excess surfaces of material, which corresponds to the excess surface, with a substantially constant thickness, of the sheet in relation to the surface of the space to be shaped, this active part cooperating with a complementary shape (32, 41) arranged in the support (4) and acting in relation to the action of the central press ram (2).
6. An apparatus according to claim 5, characterized in that the active part in relief of the outer press ram is carried by an element (30) that is movable in translation and independent of the outer press ram (1).
7. An apparatus according to claim 5, characterized in that the support (4) and the active part of the press ram have anti-friction properties.
8. An apparatus according to any one of claims 5 to 7, characterized in that provision is made for means (9) for separating the finished part from the support (4).
9. An apparatus according to any one of claims 5 to 8, characterized in that the support (4) comprises a resilient material in a peripheral zone corresponding to the active part (11) of the outer press ram (1) and extending in a zone directly adjacent to the central press ram.
10. An apparatus according to claim 9, characterized in that the support material has a Shore hardness preferably comprised between 70 and 100.

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