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Flexforming expedites automotive prototype sheet-metal parts.

Flexforming is a high-pressure process widely used by the aerospace industry to precision form complex-shaped parts from difficult-to-form sheet metals. It has been brought down to earth by European automakers. SAAB, for example, uses the process to form stampings for preproduction passenger-car prototypes, Figure 1.

Karl Binder, a major producer of prototype stampings for the European auto industry, will install a new flexforming press later this year. BMW has ordered a press that, when delivered next year, will be used to form both prototype and short-run stampings, and at least one US automaker is keenly interested in the process and currently is evaluating flexformed stampings produced in Europe.

Flexforming muscle

Traditionally, automakers have used one of two methods to form prototype stampings: hammer form them from flat blanks by hand, or form blanks on a conventional press, using simple punch-and-die tooling made from low-cost tool materials. Both methods require long leadtimes and are expensive.

Manual hammer forming is slow. Forming a complex shape from a flat blank may take more than a week.

Press forming is faster once tooling is available. But, designing, building, and trying out tooling can take as long as three months. Moreover, the geometry of prototype press-formed stampings usually must be corrected manually.

In contrast, flexforming offers three major benefits that make it exciting to automakers:

* Simple, low-cost tooling can be designed, built, and tried out in only three weeks.

* Flexformed stampings are so accurate that they don't require manual hammer forming to correct part geometry.

* Lower tooling costs and elimination of manual correction make flexforming much less expensive than manual or conventional press forming. For example, producing a prototype inner quarter panel, Figure 2, by flexforming reduced tooling costs 91 percent and part cost 90 percent.

An added benefit is that mechanical properties of flexformed prototypes are more like those of production stampings. Manual hammer forming sets up stresses that increase part strength, so results of crash testing could be misleading.

How it works

Flexforming, Figure 3, requires only one hard tool--a male form block or a female cavity die. This tool, which defines the stamping shape, may be made of epoxy plastic, kirksite, aluminum, or other low-cost tooling material.

Pressure to form the part is applied through a flexible diaphragm mounted at the bottom of a cylinder filled with hydraulic fluid. A blank is positioned on top of the hard tool, which then is shuttled into the forming zone, directly under the fluid cell. Then additional fluid is pumped into the cell. Pressure increases and the flexible diaphragm expands, exerting uniformly high pressure over the entire area of the blank.

The work material flows smoothly to fit the contours of the hard tool. At maximum pressure, the material takes a permanent set. Pressure then is released. The tool is shuttled out of the press, and the stamping is removed.

One forming cycle takes from one to two minutes. Time depends one size and configuration of the stamping, material thickness, and amount of pressure required to achieve sharp definition of workpiece geometry.

Maximum pressures range from 14,500 to 20,000 psi. At high pressures materials act more formable than at lower pressures. As a result, tool contours are precisely reproduced in the stamping, radii are more sharply defined (Figure 4), and springback variations decrease. Stampings are completely formed in one or two operations, using the same hard tool.

Complex prototype stampings that must be formed in two pieces on conventional presses often can be flexformed in one piece, obviating a welding operation. Also, the flexible diaphragm applies pressure more evenly, resulting in smoother material flow. There are no pinch points to trap flowing material.

Since stampings are completely flexformed on the same tool, there's no need to design and build expensive progressive dies, which may be required when stampings are formed on conventional tooling. This minimizes tooling costs as well as leadtime.

Tooling tryout time also is reduced. In the case of conventional punch-and-die tooling, mating surfaces of the punch and die are both shaped to match the stamping contour. The punch and die must be made carefully to ensure that mating surface contours are identical so they can be aligned during setup. This is very time consuming.

Press particulars

Figure 5 illustrates a flexforming press loaded and unloaded from both ends. When formed parts are shuttled out one end, blanks are shuttled in the other end.

For safety, the forming zone is totally enclosed by a massive cylindrical frame, reinforced with hundreds of miles of high-tensile-strength steel wire. The wire is wrapped around the cylindr under tension, holding it under compression even when the highest available press force (106,000 metric tons in this case) is applied. This makes fatigue failure from repeated high pressure cycling, extremely unlikely.

Flexforming presses have large capacities. The press illustrated can handle parts as large as 63" X 134", making it possible to form even the largest body stampings in one piece.

Also, a number of smaller stampings can be formed at one time. These need not be identical. The flexible diaphragm is capable of applying even pressure to all stampings in the forming zone, regardless of their size or shape. Simultaneous forming of several workpieces increases productivity and reduces forming cost per stamping.

Production translatable

When conventional punch-and-die tooling is used to make prototype stampings, successful tooling designs often can be translated directly into production tooling designs, thus reducing time to perfect production tooling. Originally, some automakers doubted that flexforming tools could be used as a basis for production tooling designs, particularly since flexforming uses only one hard tool and doesn't require mechanical hold-down devices.

Experience shows, however, that close to 90 percent of the design of flexforming tooling is translatable. Skilled tool designers, guided by well-established principles of good tooling design, have no trouble bridging the 10-percent gap.

It's unlikely that automakers will adopt flexforming for long-run production. The usual two to three minute forming cycles are just too long. There is, however, considerable interest in using flexforming for pilot and short-run production.

Manual hammer forming often is used to produce prototype stampings in quantities from 1 to 100 or so. Conventional forming, using simple tooling, is used for prototype and pilot production runs from about 5 to 500; production (steel) tooling is used for runs of 500 or more.

Flexforming covers a range from 1 to approximately 5000 stampings. For prototype production, its decisive advantage usually is shortened leadtime because of reduced tooling design, build, and tryout times. For pilot and short-run production, low tooling cost may be the decisive advantage.

Regardless of the quantity required--from 1 to 5000--the high quality of flexformed stampings, made possible by high forming pressure, is perceived as a primary benefit by automakers.

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Author:Johannisson, Tom; Nilsson, Stefan K.
Publication:Tooling & Production
Date:Oct 1, 1985
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