EP0165133A1 - Method and apparatus for stamping sheet metal - Google Patents

Abstract

Method for stamping sheets of substantially constant thickness, on a double-acting press, according to which the sheet to be formed is placed on a support (4), a first slide is applied outside the blank holder (1) on the peripheral part of the sheet (7), then a second central slide (2) is applied to the central part of the sheet, characterized in that the peripheral part of this sheet is formed, by allowing it to slide under the blank holder, using at least one active part (11) of the external slide so as to compensate in certain areas of the finished part for the excess surfaces, with unchanged thickness of the starting sheet, relative to the volume to be formed and simultaneously we move the central slide (2) to conform the angular volumes of the central part of the sheet by applying the sheet against the surfaces of the central part of the support (4). The invention also provides a stamping device. Application to the forming of sheets, and in particular of thin steel sheets of large dimensions.

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.

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 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. 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 conventional process is illustrated by patent FR 756,767 in which all of the stamping is done by stretching, thus reducing the thickness of the sheet. It appears that this classic 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). Furthermore. 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 so-called "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 the fact 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, there is known a forming process 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 for stamping 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 HPa.

The subject of the invention is therefore a process for stamping sheets of substantially constant thickness, on a double-acting press, according to which the sheet to be formed is placed on a support, a first slide is applied outside the blank holder 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 this sheet is formed, by letting it slide under the blank holder, using 'at least one active part of the outer slide so as to compensate in certain areas of the finished part for the excess surfaces, with unchanged thickness of the starting sheet, with respect to. volume to be formed and simultaneously and coincidentally, the central slide is moved to conform the angular volumes of the central part of the sheet by applying the sheet against the surfaces of the central part of the support forming the matrix.

According to a variant, an elastic material is chosen as the support material and during the action of the external slide on the sheet so as to compensate ser, always at substantially constant thickness, in certain areas of the finished part the surfaces in excess with respect to the volume to be formed, the mass of the support is caused to creep in order to deform the central part of the sheet so as to give it a substantially surface 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.

This variant which targets more particularly ultra-thin sheets makes it possible to obtain the simultaneity and the coincidence of the forming action of the peripheral part of the sheet in certain zones and the conformation of the central part of the sheet which has been described in the previous embodiment. It should be noted that in this variant, the part of the central slide plays the role of bottom of matrix and the support in flowable material that of punch applying the sheet in the bottom of matrix to form the angular volumes.

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 stamped part desired, increased by the excess peripheral surfaces shaped under blank holder, thus conditioning the maintaining a substantially constant thickness of the sheet of equal stress which will be explained below.

• - 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 ;
• - on prévoit sur la partie active du coulisseau extérieur et sur le support des moyens anti- friction ;
• - le matériau constituant le support est un élastomère à faible dureté shore, par exemple inférieure à 30 et de préférence supérieure à 10 ;
• - après l'opération de formage proprement dite, on provoque une décompression du matériau constituant le support ;
• - on refroidit la masse du matériau de support.

According to other important characteristics:

• - First, the central slide is 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;
• - Provision is made on the active part of the external slide and on the support for anti-friction means;
• - The material constituting the support is an elastomer with low shore hardness, for example less than 30 and preferably greater than 10;
• - After the actual forming operation, the material constituting the support is decompressed;
• - The mass of the support material is cooled.

According to a mixed variant, the elastic material is located in at least one peripheral zone of the support corresponding to at least the active part of the external slide and extending in at least one zone immediately adjacent to the central slide.

In this mixed variant which aims to form relatively thicker sheets with very angular volumes, the support material has a shore hardness preferably between 70 and 100.

This relatively harder elastic material has the disadvantages mentioned above concerning the energy consumed, however since its surface is limited, the energy necessary for the formation of the imprint in the mass of elastomer is also limited in a corresponding proportion making the technically acceptable process.

The invention also relates to a stamping device of the type comprising a support on which the sheet metal to be formed is placed, a first external slide or blank holder and a second central slide, characterized in that the first external slide comprises a active part in relief whose shape corresponds to the excess surface of the sheet with a thickness substantially constant with respect to the volume to be formed, this active part acting on the peripheral part of the sheet simultaneously and in relation to the action of the central slide.

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.

According to a first variant, the support comprises a mass of elastic material occupying the entire surface corresponding to the outer slide and to the central slide.

The elastic support material is preferably easily flowable, for example an elastomer having a shore A hardness of less than 30 and preferably greater than 10, but which can be less than 10 to minimize the energy necessary for its deformation and dissipated in heat form. .

• - le support et la partie active du coulisseau ont des propriétés anti-friction ;
• - 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 rétractées, 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:

• - The support and the active part of the slide have anti-friction properties;
• - 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.

According to a mixed variant, the elastic support material is located in a peripheral zone corresponding to the active part of the external slide. laughing and extending in an area immediately adjacent to the part of the central slide.

In this embodiment, the support material has a shore hardness preferably between 70 and 100.

• - 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 parallélépipède formée selon l'invention ; et
• - les Fig. 12 et 13 en sont deux vues partielles en coupe de la pièce de la Fig. 11.

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.

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 which bear on the peripheral part 31 of the support 4 so as to allow, although the blank of t8le 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 represents the step of bringing the blank holder into contact with the plate 7, by lowering the outer slider 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 biased 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 there 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 simultaneous and coordinated relative movement of the active parts in relief 11 of the peripheral slide 1 and of 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 metal blank 7 to be formed is placed in the center of a support 4 disposed in a container 3 (tray). In this case, a reversal of roles should be noted. satrise-poincon which appears 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 greater 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.

One can also use a composite support consisting of a substantially parallelepipedic 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 silicone of shore hardness 50 or of teflon having favorable antifriction properties.

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

The support 4 is covered with a sheet of plastic material 8, for example of "Teflon", interposed between the t8le 7 and the elastomer, which can be optionally glued or welded to the elastomer and which has the main purpose of facilitating the sliding of the sheet during forming, but it can also be used to protect the elastomer as indicated in the previous 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, it is also possible to provide embedded metal wires or a metal powder charge improving the thermal conductivity.

Fig. 6 represents the step of pre-forming 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 bottom of the matrix 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 the matrix 2a and, cause the displacement of this sheet in the entire 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 the forming of the 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, in particular when working in series, compressed air is circulated in the conduits 6. The cooling of the support 4 can also be performed 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 movable 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 area of the t81e in excess of 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 embodiment according to the invention a part made of steel t81e HLE E = 6 ₀ kg / mm ² of 40/100 mm, having substantially the shape of a rectangular parallelepiped of 1.5 m2 of surface . 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. Thereafter 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.

But, to make this piece 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 for producing the angles of a stamped volume described above constitutes 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 fabile 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 fabulous initial thickness;
• - avoid stress concentrations; and
• - carry out the forming of 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 it is conceivable 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 (20)

1. A method of stamping sheets of substantially constant thickness, on a double-acting press, according to which the sheet to be formed is placed on a support (4), a first external slide or blank holder (1) is applied to the peripheral part of the sheet (7), then a second central slide (2) is applied to the central part of the sheet, characterized in that the peripheral part of this sheet is formed by allowing it to slide under the blank holder using at least one active part (11) of the external slide so as to compress in certain areas of the finished part the excess surfaces, with unchanged thickness of the starting sheet, relative to the volume to be formed and simultaneously we move the central slide (2) to conform the angular volumes of the central part of the sheet by applying the sheet against the surfaces of the central part (33) of the support (4). 2. Method according to claim 1, characterized in that the support (4) is an elastic material and during the action of the active part (11) of the outer slide on the sheet so as to compensate, at a substantially constant thickness of the sheet, in certain areas of the finished part the surfaces in excess with respect to the volume to be formed, the mass of the support is caused to creep 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 (2) is moved to conform the angular volumes and the central part of the sheet by final creep of the support. 3. Method according to claim 2, characterized in that firstly brings the central slide (2) in a position in which it limits the deformation of the central part of the sheet under the effect of the creep of the material of support. 4. Method according to any one of claims 2 and 3, characterized in that there is provided on the active part (10) of the outer slide (1) and on the support (4) anti-friction means. 5. Method according to any one of claims 2 to 4, characterized in that the material constituting the support (4) is an elastomer with low shore hardness. 6. Method according to claim 5, characterized in that the elastomer has a shore hardness less than 30 and preferably greater than 10. 7. Method according to any one of claims 2 to 6, characterized in that, after the actual forming operation, one causes a decompression of the material constituting the support (4). 8. Method according to any one of claims 2 to 7, characterized in that the mass of the support material is cooled. 9. Method according to any one of the preceding claims, characterized in that the support comprises an elastic material only in a peripheral zone corresponding to the active part (11) of the external slide (1) and extending in at least an area immediately adjacent to the central slide. 10. Method according to claim 9, characterized in that the elastic material has a shore hardness preferably between 70 and 100. 11. Stamping device of the type comprising a support (4) on which the sheet metal (7) to be formed is placed, a first external slide or blank holder (1) and a second central slide (2), characterized in that the first external slide (1) comprises at least one active part (11) in relief whose shape corresponds to the excess surface, of substantially constant thickness, of the sheet relative to the volume to be formed, this active part acting on the peripheral part of the sheet simultaneously and in relation to the action of the central slide (2). 12. Device according to claim 11, characterized in that the active part in relief of the external slide is carried by an element (30) movable in translation independent of the external slide (1), which cooperates with a complementary shape (32,41) formed in the support (4). 13. Device according to claim 11, characterized in that the support comprises a mass of elastic material occupying the entire surface corresponding to the outer slide (1) and the central slide (2). 14. Device according to claim 13, characterized in that the support material (4) is an elastomer having a shore hardness preferably in-, fbrieure at 30 and greater than 10. 15. Device according to any one of claims 11 to 14, characterized in that the elastic support material is located in a peripheral zone (40) corresponding to the active part of the external slide and extending in an zone immediately adjacent to the part of the central slide. 16. Device according to claim 15, characterized in c _P that the support material has a shore hardness preferably between 70 and 100. 17. Device according to any one of claims 11 to 16, characterized in that the support and the active part of the slide have anti-friction properties. 18. Device according to any one of claims 11 to 17, characterized in that there are provided members (5) which initially project into the mass of the support material (4) and which, in a second time can be retracted, after the forming operation. 19. Device according to any one of claims 11 to 18, characterized in that there are provided cooling means (6) in the mass of material constituting the support (4). 20. Device according to any one of claims 11 to 19; characterized in that there are provided means for taking off (9) of the finished part of the support material (4).

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