EP2272601B1 - Hydroforming method - Google Patents

Abstract

The method involves immersing a hollow profile (10) into an immersion container (3) filled with hydraulic fluid, and arranging the hollow profile filled with the fluid in a standard cavity of a molding press (1) using an upper die (15) and a lower die (17). Ends of the hollow profile are guided over sealing spikes during application of inner pressure of the press, and a fluid cushion provided between the cavity and the hollow profile is controllably maintained over a time period till guiding of the ends of the hollow profile over the sealing spikes is completed during discharging of the fluid.

Description

The invention relates to a method for hydraulic hydroforming of hollow sections of a metallic material, according to the preamble of claim 1 (see, for example DE-U-20 2007 004 627 ). It is state of the art to produce metallic components by means of conventional hydroforming methods. Typically, the forming times are on the order of about 1.5 to 3 seconds. In comparison with the so-called high-speed hydroforming process (HSH), these production times are very long. The hydroforming times in HSH processes are usually well below 0.5 seconds. The high-speed hydroforming also results in very large differences in the cycle times. While in the conventional hydroforming cycle cycle times are on the order of, for example, 25 seconds, cycle times of between 6 and 8 seconds result in the HSH process.

A disadvantage of the hydroforming of softer materials, such as aluminum materials, is that they tend to smear, so it comes to aluminum accumulations at the tools. This increases the maintenance costs of hydroforming tools. Despite theoretically increased forming speed, these advantages can be at least partially nullified by increased tooling costs.

The invention is therefore based on the object to provide a method for hydraulic hydroforming of hollow sections of a metallic material, with which it is possible to avoid process-related material accumulations on the tool surface.

This object is achieved in a method having the features of patent claim 1.

Advantageous developments of the inventive concept are the subject of the dependent claims.

In the method according to the invention, a hollow profile to be reshaped is immersed in a dip tank filled with a hydraulic fluid. The filling of the hollow profile is thus not only in the hydroforming station, but before. In this way, the cavity of the hollow profile can be completely flooded before forming. The transport into the actual hydroforming station thus takes place in the hydraulic fluid. The hydroforming station used is a press with an upper die and a lower die with a corresponding mold cavity into which the hollow profile filled with hydraulic fluid is inserted. After shutting down the upper die or closing the mold cavity, the ends of the hollow profile are closed by sealing mandrels. At the same time the internal pressure is applied with the goal of hydroforming.

In the method according to the invention is added that between the mold cavity and the hollow profile, an existing fluid cushion is maintained controlled with decreasing amount of fluid over a period of time to good lubrication during the Nachschiebens of the hollow section to provide over the sealing mandrels. It should therefore be used hydraulic fluid to form a fluid cushion, which must not be degraded too quickly. Contrary to the original aim of the high-speed hydroforming to reshape the component as quickly as possible and bring the component quickly to its final contour is provided in the inventive method, this end contour, at least in those areas that are to be nachgeschoben on the sealing mandrel, not at the beginning of Nachschiebens to achieve by particularly high internal pressures, but only at a late date, namely, only when the tracking of a sealing mandrel is completed. At this moment, the fluid cushion is no longer needed. The fluid cushion should be maintained as long as the deformation takes place in particular in the areas pushed in by the sealing mandrel. The thickness of the fluid cushion should decrease continuously.

It would be optimal if the material has no direct contact with the tool during the feeding. With optimized design of the method results over a high-speed hydroforming without such a fluid cushion no time delay, since the forming process or the Nachschieben the sealing mandrel is not slower than without fluid cushion.

With the invention, the frictional forces between the workpiece and the mold cavity can be substantially reduced. As a result, this has a positive effect on the force to be transmitted via the sealing mandrel. Material accumulations are avoided. The service life of the tools is increased and the profitability improved.

The process according to the invention shows its advantages especially in relation to steel softer materials, such. Aluminum. However, the method is also for other metallic materials, such. As steel or magnesium, suitable.

The press used is preferably a transfer press with automatic transport systems. The press can be both a be hydraulically and mechanically driven press. It is also possible to use servo-motor driven presses.

A so-called transfer bar transports the hollow profiles from processing station to processing station. This is preferably done within the scope of the invention entirely within a hydraulic fluid bath, i. in a sense, below a liquid level.

Another processing station can be, for example, a preforming station, in which the hollow profile receives a cross-section, which is provided to form a fluid cushion between the hollow profile and the mold cavity. For example, the hollow profile, at least in those areas that are essentially only nachgeschoben, selectively receive a wavy cross section, so there are as few points of contact between the hollow section and the mold cavity. The aim is to create a defined fluid cushion. Therefore, the cross-sectional contour of the preform can differ significantly from the desired by hydroforming contour. The preforming of the hollow profile thus does not have the aim of creating a contour which is as close as possible to the final product, but is intended to have specific deviations therefrom.

In general, the prepared for hydroforming hollow sections are bent or even deformed so that the ends are not completely flat against the Abdichtdornen. This inevitably results in leaks. Either the leaks would have to be eliminated by a separate manufacturing step by upsetting or cutting the preformed components. In the invention, however, such a high pressure increase is realized at a corresponding flow rate that leaks can be neglected and consider the ends of the hollow profile as sealed. Therefore, a hollow profile end must not rest on the sealing mandrel over its entire surface. Due to the high excess of hydraulic fluid, the relatively small amounts of hydraulic fluid that escape through leaks, negligible. The high-speed hydroforming can be done easily.

During hydroforming, therefore, a larger volume of liquid should be specifically pumped into the hollow profile, as can be added in addition to the present in the hollow profile hydraulic fluid in the hollow profile. This refers to the final contour at the end of the hydroforming process, ie the finished product.

Because the hollow profile should initially be as completely as possible surrounded by a fluid cushion, it is expedient to make the end of the sealing mandrel blunt. In the context of the invention, a blunt seal is understood to mean a sealing mandrel having an end face perpendicular to the longitudinal direction without projections or depressions which are particularly adapted to the inner contour of the hollow profile. This standing perpendicular to the feed direction end face extends over a much larger area than the mere wall thickness of the reshaped hollow sections, and that because the hollow sections not close to the final contour, but selectively corrugated to form the fluid cushion and in particular at a distance from the walls of the mold cavity. Some peripheral regions of the hollow profile are therefore displaced much more radially outward than the other during hydroforming. So that it does not come to clamping in the sealing mandrel, the sealing mandrel has a correspondingly large, flat, ie dull, contact surface. It is dispensed with special sealant to reduce the leakage in the transition region between the sealing mandrel and the hollow profile. This type of leak-tight seal has the advantage that the ends of the hollow profile need not be specially prepared to perform the high-speed hydroforming process and that the ends of the finished hydroformed hollow sections need not be cut off. This can save material.

It is considered advantageous if the upper die displaces less than the twentieth part of the fluid quantity in which the lower die is located when closing the mold cavity. The ratio between displaced volume and the bath volume must be chosen sufficiently large. On the one hand, very high pressures occur in the method according to the invention, with leakage flows flowing back into the fluid bath. So that the hydraulic fluid does not spray out in an uncontrolled manner, the leakage flow can be damped by a corresponding amount of hydraulic fluid. For this purpose, the leakage points should be low enough below the fluid level. Additional shielding measures are appropriate.

The controlled discharge of the hydraulic fluid from the fluid cushion can be effected by providing a defined gap between the upper die and the lower die which adjoins the mold cavity. In other words, a gap is set in the parting line between the upper die and the lower die, which just allows enough hydraulic fluid to flow out so that the fluid cushion is completely dismantled at the conclusion of the follow-up operation of the sealing mandrel. Alternatively or additionally, grooves may be provided in the mold cavity, via which the hydraulic fluid is discharged. This is expediently carried out in the direction of the sealing mandrel, since larger leakage flows can occur here anyway.

The hydraulic fluid may be tempered, so that the existing of a metallic material hollow profiles are transformed to a certain extent warm by means of a high-speed hydroforming process. The semi-warm or hot-forming of metal increases the formability. With increasing temperature of the fluid bath, the hollow profile heats up faster, so that the subsequent hydroforming operation can also be carried out accelerated. The heating in the fluid bath has the advantage that the hollow profiles can be conductively heated with a medium contacting the hollow profile directly. This method is more effective than oven heating due to the high thermal conductivity of liquids.

Figur 1

eine schematische Darstellung einer Vorrichtung zur Durchführung des Verfahrens;

Figur 2

einen Querschnitt durch den Formhohlraum einer Hydroformstation;

Figuren 3a bis 3c

einen Teilbereich eines Längsschnitts durch einen Formhohlraum einer Hydroformstation zu drei unterschiedlichen Bearbeitungszeitpunkten;

Figur 4

entsprechend der Darstellung der Figur 3c eine Variante mit Leckagebereichen am Abdichtdorn und

Figur 5

ein Diagramm, bei welchem über der Zeit die Bewegungskurve des Abdichtdorns, der anliegende Druck zur Innenhochdruckumformung und die Dicke des Fluidpolsters aufgetragen ist.

The invention is explained in more detail with reference to exemplary embodiments illustrated in the drawings. It shows:

FIG. 1

a schematic representation of an apparatus for performing the method;

FIG. 2

a cross section through the mold cavity of a hydroforming station;

FIGS. 3a to 3c

a partial region of a longitudinal section through a mold cavity of a hydroforming station at three different processing times;

FIG. 4

according to the representation of Figure 3c, a variant with leakage areas on the sealing mandrel and

FIG. 5

a diagram in which over time the movement curve of the sealing mandrel, the applied pressure for hydroforming and the thickness of the fluid cushion is plotted.

FIG. 1 shows a designed as a transfer press 1 with a hydroforming station. The press 1 comprises a press table 2, on which there is a dip tank 3, which is filled with a hydraulic fluid. Within this dip tank 3 are four processing stations. The first station is a filling station 4. This is followed by a preforming station 5. This is followed by a hydroforming station 6 and finally a finishing station 7. In the processing sequence from left to right, raw material 8 is transported by means of a robot 9 into the filling station 4. There, the raw material 8, which is hollow sections 10, filled with hydraulic fluid or the hollow section 10 runs full of hydraulic fluid. Subsequently, the filled with hydraulic fluid hollow section 10 is transported to the next processing station 5 by means of a schematically indicated transfer bar 11. Transport from workstation to workstation takes place below the fluid level until the hollow section 10, which has been shaped with hydroforming, is finally removed by a further robot 12 at the finishing station 7. The finished parts 13 are stored by the robot 12.

In the preforming station 5 of the hydroforming station 6 and the finishing station 7, the actual processing of the hollow section 10 takes place. For this purpose, the press 1 has a press ram 14, on which corresponding upper dies for the respective processing stations 5, 6 and 7 are arranged. On the upper die 15 of the hydroforming station 6, a piston-cylinder unit 16 is arranged, which serves to press hydraulic fluid into the interior of the hollow profile 10 during the hydroforming. The press 1 is connected in a manner not shown with a pressure control system and a pressure control, in particular, as shown in the DE 10 2005 057 863 B3 is described. The upper die 15 is assigned a lower die 17 in a known manner.

FIG. 2 shows an enlarged view of a cross section through the closed Hydroformstation 6. It can be seen that between the upper die 15 and the lower die 17 within a substantially rectangular in cross-section configured mold cavity 18 a reshaped hollow section 10 is arranged. The hollow profile 10 touches the mold cavity 18 as little as possible, ie only selectively. This is due to the fact that the hollow profile 10 has been reshaped in such a way that a space filled with fluid remains between the mold cavity 18 and the hollow profile 10. In addition, the mold cavity 18 has grooves 20, which in this embodiment are located centrally in the upper die 15 and in the lower die 17 and, as it were, extend in the longitudinal direction of the die cavity 18. These grooves 20 are not intended to serve as a contour for the hollow profile in the hydroforming process, but rather to discharge the hydraulic fluid from the fluid cushion 19 when the internal pressure p in the interior of the hollow section 10 increases during hydroforming and the hollow profile 10 expands. In the lateral region, the discharge of the hydraulic fluid from the fluid cushion 19 via a gap 21, which is located between the upper die 15 and lower die 17. The gap 21 is so narrow that no material of the hollow profile 10 penetrates into the gap 21 during the hydroforming. The same applies to the grooves 20. Of course, it is also possible to provide a plurality of grooves 20 in the upper die 15 and / or lower die 17. The cross section of the grooves and columns is adapted in a special way, in such a way that the hydraulic fluid can flow out of the fluid pads 19 only with reduced flow velocity. The aim is to maintain the fluid cushion 19 controlled with a continuously decreasing amount of fluid over a period of time, namely at least until the tracking of a sealing mandrel is completed.

The in FIG. 2 illustrated cross-section refers to that portion of the hollow section 10, which is only slightly widened by hydroforming, in that it only comes to rest in the mold cavity 18 without being stretched in the sense of a reduction in wall thickness. The actual expansion by internal high pressure takes place in other areas, wherein the cross-sectional contour shown only shows that area which is to be pushed into the said, more dilated areas. The illustrated cross-section is therefore in particular in a sealing mandrel adjacent areas. There, the corresponding grooves 20 and 21 columns should be provided for the fluid cushion 19.

The FIGS. 3a-c show the course of the hydroforming process. Shown is a longitudinal section through the mold cavity 18 of FIG. 2 , In FIG. 3a it can be seen that the sealing mandrel 22 is retracted into the mold cavity 8. Hydraulic fluid is pumped into the interior of the hollow profile 10 via a channel 23 and a pressure p is built up. In FIG. 3b the sealing mandrel 22 has been moved in the direction of arrow P1 to track the hollow section 10. In a region of the mold cavity 18, not shown, a bulge is provided in which the hollow section 10 through Hydroforming is to be pressed. To avoid Materialwandstärkenreduzierung there, material is nachgeschoben end. In these retarded regions of the hollow profile 10, the fluid cushion 19 is maintained. Small amounts of the hydraulic fluid can flow out of the fluid cavity via grooves 20 in the upper die 15 and the lower die 17 in the direction of the sealing mandrel 22 from the mold cavity 18. In a manner not shown that also applies to the gap 21 between the upper die 15 and lower die 17. The hollow section 10 floats during this phase of deformation, ie while the sealing mandrel 22 is displaced by the path W, so to speak in the hydraulic fluid and is of the fluid pads 19 worn. Only at or after completion of the Nachschiebevorgangs the hollow section 10 comes to the mold cavity 18 to the plant, as shown in Figure 3c. The internal pressure p has the hollow profile 10 expanded so far that the fluid cushion 19 has been reduced. It can be seen that the hollow profile 10 in this case has not penetrated into the grooves 20. In the illustrated position, only the Nachschiebevorgang or the tracking of the sealing mandrel 22 is completed. Meanwhile, the widening of regions of the hollow profile 10 which are not shown in detail can still be continued because the internal pressure p is still applied even when the fluid cushion 19 has been broken down.

FIG. 4 shows a hollow profile 10, the end of which does not lie flush against the sealing mandrel 22. The circled area L shows that the end face of the hollow profile 10 in the vicinity of the upper die 15 extends at a distance from the sealing mandrel 22. There leaks occur. Due to the fact that a very large amount of fluid is conveyed via the channel 23 in the sealing mandrel 22, the pressure p can nevertheless be applied to hydroforming. The area L is shown exaggeratedly large. In practice, in the region L, not so much hydraulic fluid would escape that the desired forming pressure P could not be achieved. The method according to the invention can therefore also be carried out if there are leakage flows in the region of the sealing mandrel 22. Therefore, the sealing mandrel 22 may have a perpendicular to the feed direction end face without additional sealing means, which in the would be introduced hollow profile 10 to be formed. As a result, even with larger fluid pads or larger distances of the preformed hollow profile 10 can be ensured by the wall of the mold cavity, that the hollow profile transverse to the sealing mandrel 22, ie in the expansion, can move without jamming.

Essential in the method according to the invention is the speed with which the fluid cushion is broken down. This will be explained with reference to the diagram of Figure 5. The curve K1 represents the pressure curve of a press 1 over time with an electronically or hydraulically controlled pressure system for the hydroforming according to the known prior art. The pressure build-up starts at zero and rises above the working point A of the hydroforming process up to the top dead center B1 Curve K1. Subsequently, the pressure drops over the pressure drop point C1 to the point D1 again.

The curve K2 shows the way, ie the stroke, of a mechanical press. In the press used here, in which the upper die is formed with an additional piston-cylinder unit 16 for generating pressure, the press stroke, after it has reached its bottom dead center B1, continues in the direction of not shown in the diagram top dead center on the Pressure Drop Points C1 and D1. Between the bottom dead center B1 of the press stroke, the curve K1 and the pressure drop point C1, the press is still locked. From the pressure drop point C1, the press opens, the pressure is reduced and the press 1 opens the hydroforming tool in point D1. The top dead center, not shown, OT is traversed without time delay. The curve K2 shows the pressure curve of a press, as shown in the DE 10 2005 057 863 B3 is described. The local press a pressure control system and a pressure control of at least one piston-cylinder-spring unit are provided, wherein the press is provided with a further device for additional manufacturing operations. In the time window of the print plateau B1-B2 additional manufacturing operations are performed.

The curve K3 is a movement curve of the sealing mandrel 22. In the first movement phase in the range 0 to R, relatively little resistance is afforded by the hollow profile to be formed. In this period, there is a sufficient fluid cushion between the hollow profile and the mold cavity. The section RS is normally the critical path of the entire movement curve, because in this area the hydraulic fluid quickly flows out of the fluid cushion, since the hollow section 10 applies in this phase to the mold cavity. In the phase ST, the sealing mandrel 22 holds its position until it is finally returned again (TD ₂ ).

The curve K4 illustrates the thickness of a fluid cushion. It can be seen that the thickness between the point G and the point H decreases relatively rapidly and in particular goes to zero, before the sealing mandrel has completely passed through the region R-S of the movement curve. This means that the liquid drains off quickly. There is increased friction between the workpiece and the tool and the disadvantages discussed. According to the invention, it is provided that the thickness of the fluid cushion is reduced much more slowly, as shown by the curve K5. It can be seen that the sealing mandrel has already passed through the region R-S, while the fluid cushion has not even been reduced to 50% of its thickness. Only at the point J, which is temporally after the end of the Nachführvorgangs the sealing mandrel, the thickness of the fluid cushion goes to zero. At this time, however, no friction between the workpiece and the tool takes place, so that the fluid cushion is no longer needed. It is therefore crucial that the point J of the curve K5 lies on the time axis on the right side of the point S, which marks the end point of the Nachführvorgangs the sealing mandrel.

The illustrated curves are purely schematic. In practice, curves may result instead of the drawn straight line. Essential is the statement that the speed with which the hydraulic fluid flows out of the fluid cushion should be reduced. Therefore, the curve K5 is flatter than the curve K4.

Reference numerals:

1 -

Press

2 -

press table

3 -

plunge pool

4 -

filling station

5 -

preform station

6 -

Hydroforming station

7 -

finishing station

8th -

raw material

9 -

robot

10 -

hollow profile

11 -

transfer bars

12 -

robot

13 -

precast

14 -

press ram

15 -

upper die

16 -

Piston-cylinder unit

17 -

lower die

18 -

mold cavity

19 -

fluid cushion

20 -

groove

21 -

gap

22 -

sealing mandrel

23 -

channel

K1

Curve

K2 -

Curve

K3 -

Curve

K4 -

Curve

K5 -

Curve

L -

leakage area

P -

internal pressure

P1 -

arrow

W -

path

Claims (9)

1. Method for hydroforming hollow profiles made from a metal material, comprising the following steps:

   1.1. a hollow profile (10) to be hydroformed is immersed in an immersion container (3) filled with a hydraulic fluid;

   1.2. the hollow profile (10) filled with the hydraulic fluid is placed into a cavity (18) of a press (1) comprising a hydroforming station (6), said press having an upper die (15) and a lower die (17);

   1.3. the ends of the hollow profile (10) are fed in via sealing mandrels (22) during application of the internal pressure, characterised in that

   1.4. a fluid cushion (19) which exists between the cavity (18) and the hollow profile (10) is maintained in a controlled manner as the quantity of fluid decreases, for a period of time ending at the same time as or later than the time at which the feeding-in of a sealing mandrel (22) is completed.
2. Method according to claim 1, characterised in that the hydroforming station (6) is part of a transfer press (1) which comprises at least one further processing station (5), and wherein the transporting of the hollow profiles (10) from the processing station (5) to the hydroforming station (6) takes place within the hydraulic fluid.
3. Method according to claim 1, characterised in that, during the hydroforming, a larger volume of fluid is pumped into the hollow profile (10) than can be accommodated in the hollow profile (10) in addition to the hydraulic fluid present in the hollow profile (10).
4. Method according to one of claims 1 to 3, characterised in that the further processing station (5) is a pre-forming station in which the hollow profile (10) is given a cross-section intended to create a fluid cushion (19) between the hollow profile (10) and the cavity (18).
5. Method according to one of claims 1 to 4, characterised in that, during the closing of the cavity (18), the upper die (15) displaces less than one-twentieth of the quantity of fluid in which the lower die (17) is located.
6. Method according to one of claims 1 to 5, characterised in that, during the hydroforming, hydraulic fluid from the fluid cushion (19) is drained off in a targeted manner out of a gap (21) of defined width which is arranged in the separating join between the upper die (15) and the lower die (17).
7. Method according to one of claims 1 to 6, characterised in that hydraulic fluid is drained off via grooves (20) in the cavity (18) which extend in the direction of a sealing mandrel (22).
8. Method according to one of claims 1 to 7, characterised in that the hydraulic fluid is heated to a temperature above room temperature.
9. Method according to one of claims 1 to 8, characterised in that the metal material used is an aluminium, steel or magnesium alloy.

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