Patent Application
20100319428
Applicants hereby claim the priority benefits under the provisions of 35 U.S.C. §119, basing said claim of priority on German Patent Application Serial No. 10 2009 030 089.9, filed Jun. 22, 2009. In accordance with the provisions of 35 U.S.C. §119 and Rule 55(b), a certified copy of the above-listed German patent application will be filed before grant of a patent.
The present invention relates to a method for hydroforming hollow profile elements made of a metal material using internal high-pressure hydraulics.
The state of the art is to produce metal components by means of conventional hydroforming methods. Forming times are typically on the order of approx. 1.5 to 3 seconds. These production times are very long compared to so-called high-speed hydroforming (HSH). Hydroforming times using HSH methods are normally well below 0.5 seconds. Cycle times are also very different for high-speed hydroforming. While cycle times for conventional hydroforming are on the order of for example 25 seconds, cycle times for HSH methods are between 6 and 8 seconds.
It is a disadvantage when using internal high-pressure hydroforming on softer materials such as for example aluminum materials because these materials tend to stick or adhere to the forming cavity. This results in aluminum accumulating on the tools. This increases maintenance costs for the hydroforming tools. Despite a theoretically faster forming speed, these advantages may be outweighed by increased tool costs, and repair downtime.
An object of the present invention is therefore to provide a method for internal high-speed hydraulic hydroforming of hollow profile elements made of a metal material, wherein it is possible to prevent method-related material accumulation on the tool surfaces.
This object is attained in a method having the features of patent claim 1.
Advantageous refinements of the present invention are the subject matter of the subordinate claims.
In the present inventive method, a hollow profile element or workpiece that is to be formed is submerged in a dip tank filled with a hydraulic fluid. Thus, the hollow profile is filled not only in the hydroforming station, but in other processing stations as well. More specifically, the workpiece is filled with hydraulic fluid prior to the hydroforming station. This is how the cavity of the hollow profile element is completely filled with liquid prior to the forming process. Thus, the workpiece is transported into the actual hydroforming station through or in the hydraulic fluid. A press that has a top die and a bottom die with a corresponding forming cavity incorporates the hydroforming station. The hollow profile element, filled with hydraulic fluid, is placed in the forming cavity. After the top die has been lowered and the forming cavity is closed, the ends of the hollow profile element are closed using sealing mandrels. At the same time, internal pressure is applied for internal high-pressure hydroforming of the workpiece.
Another aspect of the present inventive method is that a fluid cushion that is provided between the forming cavity and the hollow profile elements, and is maintained in a controlled manner for a period of time as the hydroforming fluid flow decreases. This control provides good lubrication when the sealing mandrels push against the hollow profile element. Therefore, hydraulic fluid is used to form a fluid cushion that must not under any circumstances be removed too rapidly. In contrast to prior high-speed hydroforming, which forms the component as rapidly as possible and brings the component to its final contour as rapidly as possible, in the present inventive method, the final contour of the workpiece is not attained at the beginning of the pushing using particularly high internal pressure, especially in those areas that the sealing mandrel is to push against. Rather, the final contour is not attained until as late a point in time as possible, more specifically, not until the insertion of the sealing mandrel has been concluded. At this moment, the fluid cushion is no longer needed. The fluid cushion should be maintained for as long as the hydroforming process is taking place, especially in those areas where the sealing mandrel pushes against the workpiece. Preferably, the thickness of the fluid cushion should decrease continuously.
It would be best if the workpiece material did not come into direct contact with the tool at all while it is being pushed against. When the method is designed ideally, there is no temporal delay compared to high-speed hydroforming, which does not use such a fluid cushion. This is because the present forming process, or the pushing by the sealing mandrel, does not occur slower when the fluid cushion of the present invention is employed.
It is possible to significantly reduce the friction forces between the workpiece and the forming cavity using the present invention. This has a positive effect on the force that is to be transferred via the sealing mandrel. Accumulations of adhered workpiece material are avoided. The standing time for the tools is increased and overall efficiency is improved.
The inventive method exhibits its advantages in particular with materials that are softer than steel, such as for example aluminum. However, the method is also just as suitable for other metal materials, such as for example steel or even magnesium.
The press used is preferably a transfer press that has automatic transport systems. The press may be either a hydraulically driven press or a mechanically driven press. It is also possible to use presses driven by servo-motors.
A so-called transfer bar transports the hollow profile elements or workpieces from station to station. With the present invention, this transfer occurs entirely inside or within a hydraulic fluid bath, i.e., somewhat below the fluid level.
Another process station can be used for a pre-forming station or the like in which the hollow profile element obtains a cross section that is suitable for creating a fluid cushion between the hollow profile element and the forming cavity. For instance, the hollow profile element may be given a wavy cross section, at least in those areas that are to be pushed against. This is so that there are as few points of contact as possible between the hollow profile element and the forming cavity. The goal is to create a defined fluid cushion. Therefore, the cross-sectional contour of the blank can be very different from the contour of the completed part sought through internal high-pressure hydroforming. Thus, the goal of pre-forming the hollow profile element is not to create a contour that is as close as possible to that of the finished product, but rather to create deliberate differences that facilitate forming the fluid cushion.
As a rule, the hollow profile elements or workpieces prepared for internal high-pressure hydroforming are bent, or even just deformed, so that the ends are not completely even against the sealing mandrels. This necessarily results in leaks. In prior hydroforming processes, these leaks would have to be eliminated in a separate production step, wherein the pre-formed components are either compressed or trimmed. However, with the present invention, the increase in pressure during hydroforming is so high, and the corresponding flow is so great, that such leaks at the ends of the hollow profile element can be ignored, and the workpiece considered operably sealed. Therefore, it is not necessary for the end of a hollow profile element to be placed completely flat against the sealing mandrel. Because of the great excess of hydraulic fluid flow, the relatively small quantity of hydraulic fluid that escapes through leaks is negligible. High-speed hydroforming using the present invention can be performed with no problem.
Thus, the intent is to pump a greater volume of hydraulic liquid into the hollow profile element during internal high-pressure hydroforming than can be accommodated therein, in addition to the hydraulic fluid already present in the filled hollow profile element. This is a function of the final contour of the completed part at the end of the internal high-pressure hydroforming process.
Since from the beginning of the present process, the hollow profile element is to be surrounded as completely as possible by a fluid cushion, it is useful for the end of the sealing mandrel to be blunt. In the context of the present invention, a blunt seal shall be construed to be a sealing mandrel having an end face that is disposed perpendicular to the longitudinal direction of the workpiece without projections or depressions that are specially adapted to the inner contour of the hollow profile element. This end face, that is disposed perpendicular to the direction of advancement of the mandrel, extends across a significantly larger area than just the wall thickness of the hollow profile element to be formed. This is specifically because the shape of the hollow profile elements is not close to the final contour of the completed part, but rather is deliberately formed wavy for creating the fluid cushion, and in particular, extends at a distance from the walls of the forming cavity. Some circumferential areas of the hollow profile element are therefore displaced much farther radially outward than other areas during the internal high-pressure hydroforming. The sealing mandrel has a flat, i.e., blunt, positioning surface of a corresponding size, so that no obstructions occur in the area of the sealing mandrel. There are no special sealing mechanisms for reducing the leaks in the transition area between the sealing mandrel and the hollow profile element. This type of leak-tolerant seal has the advantage that the ends of the hollow profile element do not have to be prepared in a particular manner in order to be able to perform high-speed hydroforming using the present method. In addition, the ends of the fully hydroformed hollow profile elements do not have to be cut off. This results in material savings.
It is considered advantageous when the top die displaces less than one twentieth of the quantity of fluid in which the bottom die is disposed when the forming cavity is closed. The ratio of displaced volume to bath or reservoir volume must be selected to be high enough. Very high pressures are used in the present inventive method, and fluid from leaks flows back into the fluid reservoir or bath. The flow from leaks can be damped by a corresponding quantity of hydraulic fluid, so that the hydraulic fluid does not spray out in an uncontrolled manner. To this end, the leak locations are preferably disposed deep under the fluid level of the reservoir. Additional shielding measures are also useful.
Hydraulic fluid is selectively drained or metered from the fluid cushion in a controlled manner from a defined gap disposed between the top die and the bottom die. This gap borders on the forming cavity. In other words, a gap is created in the separating gap between the top die and the bottom die, and it permits exactly enough hydraulic fluid to drain off, so that only when the insertion process for the sealing mandrel has concluded, the fluid cushion has been completely removed. Alternatively, or in addition, grooves can be provided in the forming cavity, and the hydraulic fluid can be selectively drained or metered using these grooves as well. This functionally occurs towards the sealing mandrel where even larger flows from leaks may occur.
The temperature of the hydraulic fluid can be controlled so that the hollow profile elements, comprised of a metal material, can be somewhat hot-formed during the present high-speed hydroforming method. Semi-hot-forming and hot-forming of metal increases formability. As the temperature of the fluid bath increases, the hollow profile element heats faster, so that the subsequent internal high-pressure hydroforming operation can also be performed at an accelerated pace. Heating the hollow profile elements in the fluid bath has the advantage that the hollow profile elements can be heated conductively with a medium that is in direct contact with the hollow profile element. This method is more effective than furnace heating because liquids conduct heat so well.
These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings.
The invention is explained in greater detail in the following using the exemplary embodiments depicted in the drawings.
a through 3c depict a segment of a longitudinal section through a forming cavity of a hydroforming station at three different times during the processing.
For purposes of description herein, the terms “upper”, “lower”, “right”, “left”, “rear”, “front”, “vertical”, “horizontal” and derivatives thereof shall relate to the invention as oriented in
The hollow profile element 10 is actually processed in the pre-forming station 5, the hydroforming station 6, and the final station 7. To this end, the press 1 has a press ram 14. Appropriate top dies 15 for each of the stations 5, 6 and 7 are arranged on the press ram 14. A piston/cylinder unit 16 is arranged on the top die 15 of the hydroforming station 6, and presses hydraulic fluid into the interior of the hollow profile element 10 during the internal high-pressure hydroforming. The press 1 is connected (not shown in greater detail) to a pressure control system and pressure regulator, such as that described in DE 10 2005 057 863 B3. A bottom die 17 is associated with the top die 15 in a known manner.
The cross-sectional view of
a-c illustrate how the hydroforming method proceeds. A longitudinal section through the forming cavity 18, similar to
The speed with which the fluid cushion is dissipated or removed is essential in the present inventive method, and shall be explained using the graph in
Curve K2 depicts the path, i.e., the stroke, of a mechanical press. In the press that is used here, the top die is embodied with an additional piston/cylinder unit 16 for producing pressure. After it has reached its bottom-most dead point B1, the press stroke moves back in the direction of the top dead point (cannot be shown in the graph) through pressure drop points C1 and D1. The press is still held down between the lower dead point B1 of the press stroke, the curve K1, and the pressure drop point C1. The press moves up starting at pressure drop point C1, the pressure is removed, and the press 1 opens the hydroforming tool at point D1. The top dead point OT (not shown) is passed through without a temporal delay. The curve K2 depicts the pressure progression for a press as is described in DE 10 2005 057 863 B3. In that press, a pressure control system and a pressure regulator made of at least one piston/cylinder/spring unit are provided. The press is provided with another apparatus for additional production operations. Additional production operations are performed in the time window for the pressure plateau B1-B2.
The curve K3 is a movement curve for the sealing mandrel 22. In the first movement phase, in the zero to “R” range, the hollow profile element that is to be formed provides relatively little resistance. There is an adequate fluid cushion between the hollow profile element and the forming cavity during this period of time. The R-S segment is normally the critical segment for the entire movement curve because, in this range, the hydraulic fluid drains out of the fluid cushion rapidly since the hollow profile element 10 is positioned against the forming cavity in this phase. In the S-T phase, the sealing mandrel 22 holds its position until it is finally withdrawn (T-D2).
The curve K4 illustrates the thickness of a fluid cushion. It can be seen that the thickness decreases relatively quickly between the point “G” and the point “H”, and in particular approaches zero, before the sealing mandrel has completely passed through the R-S range in the movement curve. This means that the fluid flows out rapidly. There is increased friction between the workpiece and the tool, which can lead to material adhesion and the disadvantages discussed above. In accordance with the present invention, it is provided that the thickness of the fluid cushion decreases at a significantly slower rate, as is illustrated by the curve K5. It can be seen that the sealing mandrel has already passed through the R-S range, while the thickness of the fluid cushion has not even decreased 50 percent. It is only at point “J”, which is temporally after the end of the sealing mandrel insertion process, that the thickness of the fluid cushion approaches zero. However, at this point in time, there is no more friction between the workpiece and the tool, so that the fluid cushion is no longer needed. Thus, what is critical is that the point “J” for curve K5 on the time axis be located to the right of the point “S”, wherein point “S” denotes the end point for the sealing mandrel insertion process.
The curves depicted in
In the foregoing description, it will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims by their language expressly state otherwise.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 030 089.2 | Jun 2009 | DE | national |
