NL1021738C2 - Method and device for reducing crease formation during deep drawing. - Google Patents

Abstract

A method and apparatus for deep drawing a product from a blank ( 5 ), in which, during deep drawing, the blank ( 5 ) is held near its edge by a downholder ( 8 ) against a die ring cooperating with the downholder ( 8 ), thereby preventing, at least reducing wrinkle formation in the blank, by adjusting, at the beginning of deep drawing, the downholder ( 8 ) such that a downholder ( 8 ) force exerted by the downholder ( 8 ) on the edge of the blank is relatively small, and by controlling, during the further course of the process, the downholder ( 8 ) on the basis of a predetermined thickness trend of the mentioned edge during deep drawing and/or a trend or critical value derived from this thickness trend.

Description

Title: Method and device for reducing wrinkle formation in deep drawing.

The invention relates to a method for deep-drawing products from a platinum. More in particular, the invention relates to an improved method for preventing, at least reducing, fold formation in the platine during deep drawing.

During deep-drawing of a platinum between a mold and a stamp cooperating with it, the edges of the platine are held by means of a downholder against a mold ring cooperating with this downholder. It is important here to adjust the downholder in such a way that a downholder force exerted on the edges of the platine is on the one hand sufficiently large to prevent the formation of creases in the edges, but on the other hand is not greater than necessary, because this favors cracking. and moreover leads to high frictional forces I between the platinum and the downholder and mold ring. Such frictional forces require a heavy design of the device, require an unnecessary amount of energy during deep-drawing, lead to wear and shorten the service life of the device.

In the known methods, the required downholder force is often determined by trial and error during a series of tests prior to the actual deep-drawing process. The downholder force found is then multiplied by a safety factor in connection with possible variations in the starting material, such as spread in thickness and composition. As a result, the applied downholder force is ultimately higher than absolutely necessary, with all the aforementioned disadvantages. Moreover, the tests take time, material and money and the outcome depends on the expertise of the person who carries out the tests.

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DE 4038864 discloses a method in which the downholder force (Fn) is preventively chosen to be so high at the start of deep drawing that this force is greater than a counterforce (Fs) to be expected in the platine. During deep drawing, the downholder force is gradually reduced until a start of the formation of folds I is observed. The downholder force is then increased to above I the counterforce expected at that moment in the platine. This is repeated until the downholder force, at the end of the deep-drawing process, is zero. The fold formation is detected with the aid of measuring means 10 which measure the distance between the downholder and the mold. The post-workability of this publication is lacking because it is not explained clearly to those skilled in the art how the expected counterforce should be determined. Moreover, it is remarkable that in the known proposal the exerted downholder force decreases, while, as will be seen below, in the proposal according to the invention the downholder force actually increases in the course of the process.

H With this known method, too, the downholder force is therefore H higher than necessary, because it is increased at the start of the method and H every time fold formation occurs above the counterforce expected in the platinum. Moreover, the height of the downholder force and its proper functioning is highly dependent on the accuracy with which the counterforce can be predicted.

The invention has for its object to provide a method in which at least a number of the drawbacks associated with the known methods are obviated. To that end, a method according to the invention is characterized by the features of claim 1.

With a method according to the invention, the downholder force is initially kept relatively low and only increased when this is necessary, namely with the beginning of pleating. It is important here that this fold formation can be recognized at an early stage. This is achieved in a method according to the invention in that the thickness trend of the edge of the platine during deep drawing or a quantity derived from this thickness trend is taken into account, such as, for example, the speed of the thickness trend. The thickness of the platinum edge gradually increases during deep-drawing, because the diameter of the platine gradually decreases and therefore material in the platinum edge must be accommodated on an increasingly smaller surface. If this thickness increase were not taken into account, then each thickness increase would be seen as starting fold formation and the downholder would be wrongly controlled, whereby not only folds but also said thickness increase would be suppressed, which leads to an unnecessarily high downholder force.

In a method according to the invention, the downholder force is gradually increased during deep-drawing from a relatively low value instead of being reduced from a preventively high value. As a result, the downholder force will always have a value as low as possible, resulting in low frictional forces between the device and the platform. The device can hereby be operated with relatively low forces and relatively little energy and can therefore be of relatively light design. Moreover, with such low forces, the chance of the platinum tearing will be minimal. As a result, less strict requirements can be imposed on the starting material with regard to, for example, the homogeneity of the material, the number of imperfections in the material lattice per unit volume and thickness variations in the platinum. A qualitatively good end product can thus be realized with qualitatively less starting material and correspondingly lower material costs. Moreover, this increases the freedom in materials to be used, making it easier to work with new materials.

Another advantage over the known methods is that a counterforce to be expected in the platine does not have to be determined prior to deep drawing. Neither do time-consuming and costly tests have to be carried out to determine a suitable initial retainer force, which is of great advantage in particular for relatively small product series.

Tests carried out by the applicant have shown that if, during deep-drawing, fold formation in the edge of the platine is prevented in the manner described above, both folds in the flange of the deep-drawn end product (so-called primary folds) and folds in the wall of the deep-drawn end product (so-called secondary folds) can be effectively prevented. In this description, the 'edge of the platine' is understood to mean the part of the platine that is located between the downholder and the mold ring.

In a first preferred embodiment, a method according to the invention is characterized by the features of claim 2.

H In this embodiment, a H parameter relevant for creasing is measured and the hold-down force is increased when this H-measured parameter exceeds a predetermined critical trajectory or value H, which trajectory or value is based on, or at least takes into account, H the platinum thickness increase discussed above, or a quantity derived from this thickness increase.

The downholder force can be increased step by step, with a preset step size, but can also be controlled, for example, by means of control algorithms known per se, such as for instance a proportional, integrating and / or differentiating control, to a desired value, wherein the measured parameter is again below the specified critical quantities (trajectory or value).

In a further elaboration of the first preferred embodiment, a method according to the invention is characterized by the features of claim 3.

H The parameter relevant for creasing can be, for example, the downholder opening, defined as the perpendicular distance between the H downholder and the mold ring. In that case, the thickness variation or the maximum thickness increase of the platinum edge can be entered as a critical range, respectively critical value. After all, if the downholder opening becomes larger than the instantaneous thickness, or the final maximum thickness of the platinum edge, during deep-drawing, this indicates the formation of folds. The thickness trend can be determined theoretically, with the aid of suitable simulation programs, but it can also be measured once, before the deep drawing of a new series of products. To this end, a constant downholder force is set, with no creasing or cracking occurring and the downholder opening is measured during deep drawing. Since no creasing occurs, it can be assumed that the measured downholder opening is substantially equal to the thickness increase of the platinum. It is thereby assumed that the measured thickness increase is independent of the set downholder force, or at least that the influence of the height of the downholder force on the measured thickness increase is negligibly small, within the working area 20 of the downholder force of importance for the present application.

In a further elaboration of the first preferred embodiment, a method according to the invention is characterized by the features of claim 4.

Instead of or in addition to the downholder opening, the speed at which this opening changes can also act as a critical quantity. This speed can also be determined by simulation or a test measurement in the manner described above. In this case, the speed trend can function as a critical trajectory or a speed value that is highest measured during this course can function as a critical value. The downholder opening speed trend is more sensitive than the downholder opening trend, whereby crease formation can be recognized at an even earlier stage.

Of course, both signals, the downholder opening path I and the downholder opening speed path, can also be used next to each other, wherein the critical value, or the critical path I that is first exceeded, is decisive for the control of the downholder.

In a second preferred embodiment, a method according to the invention is characterized by the features of claim 7.

With this method, it is not the downholder force but the downholder position that is controlled based on a predetermined thickness profile of the platine. This variation in thickness can be simulated or measured in the same way as discussed above H. On the basis of this thickness profile H, the downholder can always be adjusted during deep-drawing, such that the downholder opening is substantially equal to the predetermined H instantaneous thickness of the platine. Such a method according to the H invention offers the advantage that during deep drawing the H downholder opening and speed do not have to be measured and compared with a predetermined critical value. Moreover, even with this embodiment, the downholder force will be minimal, or at least not greater than necessary, with all the aforementioned advantages.

Of course, a combination of the two preferred embodiments according to the invention is also possible, wherein primarily the position of the downholder is controlled on the basis of a predetermined thickness profile of the platine and subsequently the downholder opening trend and / or the downholder opening speed trend H are measured so that certain thickness profile does not entirely correspond to the actual thickness profile (for example due to H variations in the starting material or hysteresis in the downholder movement) H 30 can be corrected by increasing the downholder force.

The invention further relates to a device for deep-drawing a product from a platinum, suitable for applying a method according to the invention.

In the further subclaims, further advantageous embodiments of a method and device according to the invention are described.

To clarify the invention, an exemplary embodiment of a deep-drawing device and a method according to the invention will be further elucidated with reference to the drawing. Therein: FIG. 1 schematically a deep-drawing device according to the invention;

FIG. 2 a diagram in which the course of the lower upper limit of the permissible downholder force is plotted as a function of the deep-drawing ratio;

FIG. 3 a test measurement with a downholder force, for determining a critical value and / or trajectory where creasing occurs;

FIG. 4 a measurement in which the downholder opening and the downholder force are plotted against time, with and without control according to the invention;

FIG. 5A a simulation of the speed development of the downholder opening (v0) during deep drawing;

FIG. 5B a simulation of the downholder force (Fn), controlled on the basis of the speed of figure 5A;

FIG. 6 a photograph of a deep-drawn product, with and without control according to the invention; and FIG. 7 a deep-drawn product with primary and secondary folds.

Figure 1 shows a device 1 for deep-drawing products, with which fold formation in those products can be reduced and preferably completely suppressed. In this description, crease forming is understood to mean the formation of both primary creases 21 in one

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flange 6 of the deep-drawn product 15, as secondary folds 22 in wall parts 25 of the product 15, as illustrated in figure 7. The device and method according to the invention focuses on suppressing both types of folds.

The device 1 comprises a mold 2 provided with a mold opening 3 and a stamp 4 arranged above this mold opening 3.

This punch 4 can be moved into the opening 3 with the aid of suitable positioning means 7, as shown in Figure 1 on the right-hand side.

A platinum 5 of starting material placed over the opening 3 is thereby forced into the opening 3 and deformed between the walls of this opening 3 and the punch 4 into a desired end product 15.

As can be seen from a comparison of the left and right half of Fig. 1, during this deep drawing, the starting diameter D0 of the platine 5 gradually decreases, which is accompanied by an increase in the thickness d of the edge 6 and can further lead to creasing.

In order to prevent the edge 6 from folding and rising during deep-drawing, an annular downholder 8 is arranged around the punch 4, which annular holder 8 can be moved with suitable positioning means 9 towards a mold ring 10 extending around the die opening 3 while clamping the edge 6 of the platform 5. A downholder force Fn exerted by the H downholder 8 on the edge 6 will increase H as the downholder opening s0, defined as the perpendicular distance between the downholder 8 and the mold ring 10, increases.

During deep drawing, the downholder force Fn must remain between two extreme limits, a lower limit FN.min and an upper limit Fornax. These limits are shown graphically in Figure 2, as a function of the deep-drawing ratio. When the downholder force Fn falls below the minimum limit FN.min, creases will occur in the product, while when the maximum limit FN.max is exceeded, cracks will arise in the product. In practice, the downholder force Fn is therefore set at a 'safe' distance 9 between the two limits, so that a certain safety margin is present, in connection with possible spread in the starting material (composition, thickness, etc).

In a method according to the invention, on the other hand, the downholder force Fn is held as close as possible to the lower limit FN.min. This will minimize the chance of cracking. Moreover, this will also limit frictional forces Fw that occur during deep-drawing between the platinum 5 and the downholder 8 and the die ring 10, since these frictional forces are proportional to the downholder force Fn. Lower frictional forces ensure that less lubricant can suffice, that the deep-drawing process can take place under lower forces, with less work and that the entire arrangement can be made lighter.

In order to keep the downholder force Fn as low as possible, it is set according to the invention to a low value when the deep-drawing commences and is only increased when creasing occurs. The downholder force is then increased to a value required to suppress the detected folds. Thus, the downholder force Fn will always not be greater than necessary to suppress the instantaneous fold formation.

For this method to function properly, it is important to be able to detect fold formation at an early stage. It has been found that both the downholder opening trend s0 and its derivative over time, the downholder opening speed trend v0, are useful measurement signals for that purpose. For these signals, s0 and v0, a critical value can be determined in a manner still to be described, in the event of exceeding of which there is talk of pleating.

The deep-drawing device 1 of figure 1 is therefore provided with measuring means 11 with which said downholder opening trend s0 and / or downholder opening speed trend v0 can be measured. These measuring means 11 can for instance comprise an optical, capacitive or magnetic sensor. The measuring means 11 are connected to a control 12, which is provided with means for comparing the measuring signals with a critical value or critical trajectory set for those signals, and which control 12 is further adapted to detect the critical quantities mentioned above moving the downholder 8 towards the mold 2 with the aid of the positioning means 9. These positioning means 9 can for instance comprise a piston-cylinder assembly, an electrically driven screw spindle, a piezoelectric element or the like.

The critical values can be determined prior to deep drawing H of a new product series during a test measurement, by measuring the downholder opening trend s0 during this test as shown in figure (3). It can clearly be seen that the downholder opening s0 initially has a substantially constant value (path I-II) and then gradually increases with a constant slope (path II-III) corresponding to a constant downholder opening speed v0. From point III, the measured downholder opening trend s0 shows a kink and this trend increases faster, which indicates fold formation P. On the basis of this measurement, therefore, the trajectory I-III can be implemented in the control 12 as the desired critical downholder opening trajectory. Instead, buckling point III can also be entered as the critical H downholder opening value. If a downholder opening curve s0 measured during H deep-drawing now exceeds the critical range or the H-critical value, then the thickness increase is greater than may be expected on the basis of H of the test measurement, which indicates fold formation.

In a somewhat modified variant of the method described above for determining the critical values, the deep-drawing test measurement can also be carried out with such a high downholder force Fn that creasing or cracking will not occur. As a result, it can be assumed that a downholder opening trend s0 measured during this test fully corresponds to the thickness increase I of the edge 6 of the platine 5. This measured downholder opening trend can therefore be implemented as a critical path in the control 12. Instead of this, the maximum measured thickness increase can also be entered as a critical I value.

Similarly, during said test measurement, in addition to or instead of the downholder opening, it is also possible to measure the speed at which this downholder opening changes. This measured velocity variation 10 or a velocity value that is highest measured during this measurement can be entered in the control 12 as a critical range or critical value for the downholder opening velocity.

When a downholder opening speed v0 measured during deep drawing exceeds this critical range or this critical value, this indicates a formation of folds, since the downholder opening increases faster than might be expected on the basis of the normal increase in thickness.

It should be noted, incidentally, that the critical speed value can also be determined by determining the slope of trajectory II-III in Figure 3. Figure 4 shows a measurement of the downholder opening s0 and the downholder force Fn over time, wherein the thinly drawn lines represent the course without control according to the invention, while the thickly drawn lines represent the course when the downholder force Fn is driven on the basis of the in FIG. 3 determined critical downholder opening. It can clearly be seen that the hold-down force Fn from the critical value (buckling point III) determined in Figure 3 is increased stepwise and thus effectively suppresses fold formation (see thin-drawn s0 line) (see thick-drawn s0 line).

It is furthermore clear from Figure 4 that the downholder force 30 initially starts relatively low and is only increased when this is actually required. The force can be increased step by step, with I predetermined step size or increased proportionally to the measured deviation I. Other control techniques known per se are also possible, wherein use is made, for example, of integrating and / or differentiating actions. The device can also be provided with an H self-learning control, wherein the step size of the force Fn is initially set by an operator and the control itself adjusts this value in the course of the process on the basis of feedback measurement data.

Figures 5A, B show a simulation in which the downholder force is regulated on the basis of the measured opening speed v0 and a previously determined critical speed value, which in the case shown was approximately 1 x 10 -4.

It can clearly be seen how the downholder force Fn is increased stepwise, each time the downholder opening speed exceeds the critical value. It can also be seen that the increase in the H holding-down force Fn is greater as the critical value is exceeded.

Figure 6 shows on the left side a flange 6 'of a product 15', H manufactured by a conventional deep-drawing method (see the thin drawn lines in Figure 4), while in addition, on the right-hand side, a product 15 is shown which is manufactured with a method according to the invention (see the lines in figure 4). It is clear that the product according to the present method has virtually no folds.

It will be clear that the critical values for the downholder opening and the opening speed depend on the starting material and the desired end product. These values are therefore preferably re-determined prior to each new product series. This can be done by means of a test measurement as described above. With sufficient material and process data, these values can also be simulated with the aid of software packages known per se, so that test measurements no longer have to be carried out at all.

In an alternative embodiment of a method according to the invention, the downholder force is not controlled on the basis of a measured start of fold formation (feedback control), but the position of the downholder and thus the downholder opening is regulated according to a predetermined trajectory, such that said downholder opening corresponds to the expected thickness profile of the plate 5 (forward control). The expected thickness variation 10 can be determined in the same way as described above with reference to Fig. 3. Such a forward regulation has the advantage that during deep-drawing itself no downholder opening or speed need be measured, so that with a very simple device , without measuring means and without advanced control will suffice.

The invention is by no means limited to the exemplary embodiments shown in the description and the drawing. Many variations thereof are possible within the scope of the invention as set forth in the claims.

For example, deep-drawing can take place in one or more steps depending on, among other things, the distance that is deep-drawn and the thickness of the starting platinum. Furthermore, the forward position control and feedback force control can be combined, whereby the force control can act correctively on the position control.

These and many variations are understood to fall within the scope of the invention as set forth in the claims.

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Claims (11)

Method for deep-drawing a product from a platinum, wherein the platinum is held by a downholder during its deep-drawing near a rim holder against a mold ring cooperating with the downholder, whereby fold formation in the platinum is prevented, at least H 5 reduced by controlling the downholder, characterized in that the downholder (8) is set at the start of deep-drawing such that a downholder force exerted by the downholder (8) on the edge (6) of the platine (5) (Fn) is relatively small and further control of the downholder (3) takes place on the basis of a predetermined thickness profile of the edge (6) mentioned during deep drawing and / or a profile or critical value derived from this thickness profile. 2. Method as claimed in claim 1, characterized in that during the deep-drawing a parameter relevant for the shaping of the fold is measured H and compared with a critical value at which shaping occurs, which critical value is predetermined based on the thickness variation of the platinum edge (6) and / or a signal derived from this thickness variation, and wherein, if the measured parameter exceeds or threatens to exceed this critical value, the downholder (8) is controlled such that a downholder (8) places on the edge (6). ) exercised downholder force 20 (Fn) increases and the measured parameter falls below the critical value. Method according to claim 2, characterized in that the parameter relevant for creasing is the downholder opening (s0), defined as the perpendicular distance between the downholder (8) and the mold ring (10), and the critical value is the predetermined thickness profile or the predetermined maximum thickness increase is of the edge (6). A method according to claim 2, characterized in that the parameter relevant to creasing is the speed (v0) at which the downholder opening changes, and the critical value is the predetermined speed profile or the predetermined maximum speed at which the platinum edge during deep drawing in thickness increases. A method according to any one of claims 2-4, characterized in that the critical value is measured during a test session, prior to deep drawing. Method according to one of claims 2-4, characterized in that the critical value is simulated with the aid of a dynamic model of the platine (5) and the deep-drawing process. The method of claim 1, wherein the position of the downholder (8) is controlled according to a predetermined path such that the downholder opening (s0), defined as the perpendicular distance between the downholder (8) and the mold ring (10), during deep-drawing substantially corresponds to a predetermined thickness profile of the edge (6) to be expected during deep-drawing. Device for deep-drawing a product from a platinum (5), comprising a downholder (8), a mold ring (10) cooperating with it for holding an edge (6) of the platin (5) during deep-drawing, a control (12) provided with means for storing therein a desired downholder opening course (s0), downholder opening speed course (v0) and / or a critical value derived therefrom, and positioning means (9) for moving the downholder (8), the control (12) is adapted to control the positioning means (9) such that the displacement of the downholder (8) is in accordance with the stored downholder opening course (s0), downholder opening speed course (v0) and / or a critical value derived therefrom. 9. Device as claimed in claim 8, wherein the control (12) is adapted to control the positioning means (9) such that a distance between the downholder (8) and the mold ring (10) is in accordance with the stored downholder opening trend (s0) or a critical value derived therefrom I. Device as claimed in claim 8 or 9, wherein measuring means (11) are provided for measuring the downholder opening trend (s0), the downholder opening speed trend (v0) and / or a critical value derived therefrom and wherein the control (12) comprises means for comparing I signals measured with the measuring means (11) with a downholder opening course (s0), downholder opening speed course (v0) and / or a critical value derived therefrom, and wherein the control (12) is arranged on this basis to control the downholder (8) such that the displacement, speed or critical value of the downholder (8) derived therefrom is in accordance with the stored downholder opening course, downholder opening speed course and / or the critical value derived therefrom. Device according to one of claims 8 to 10, characterized in that the measuring means (11) comprise a contactless sensor, for example an optical, capacitive or magnetic sensor.