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Lightweighting Challenges

By: Peter Ulintz

Friday, February 01, 2013
 

Automotive manufacturers and their suppliers must innovate in all areas of vehicle design in order to maximize fuel efficiency to meet federal CAFE (corporate average fuel economy) requirements, slated to increase to approximately 56 miles/gal. by 2025. To realize these goals, automakers are working to integrate more lightweight materials into new vehicle designs. This is being achieved through the use of higher tensile-strength steels, allowing automakers to reduce the thickness of stamped metal parts.

As application of higher-strength steels continues to grow in popularity, tier suppliers will become more responsible for engineering final part geometry and tooling for high-strength low-weight stampings. These companies, primarily contract stampers and tool and die shops, generally are well experienced with mild steel and HSLA materials. However, many are not prepared for the challenges presented by these new advanced steels. Assuming applications for higher tensile-strength materials will create a “business as usual” condition for an existing press line can be a very costly mistake.

For many years metal-stamping processes have been narrowly viewed to include only those parameters interacting directly with the stamping: the die (tooling), the sheetmetal blank and any lubricants being used. But even the simplest of forming systems are not always controlled or managed properly, sometimes not at all. Often, for example, stampers ignore the effects of lubrication on the stamping system, especially when they restrict the number of processing lubricants on the shop floor to only one or two types. This practice requires shop-floor personnel to use whatever is on hand and, as a result, lubrication problems can be masked by attempts to adjust tooling or to find fault with the workpiece material.

Occasionally, manufacturers need to run stamping tools on different press lines. While these press lines may operate similarly, the differences in their characteristics can be quite significant, with the potential to negatively impact the stamping process. If metalformers take a similar approach to stamping high-tensile-strength steels, they could face catastrophic failures. The increased forces needed to form, pierce and trim these steels creates significant challenges for pressroom equipment and tooling, including excessive tooling deflection, damaging tipping-moments and amplified vibrations and snapthrough forces.

In short, stamping ultra-high-strength steels impacts the size, strength, power and overall configuration of every major component in a press line, including coil-handling equipment, coil-feed systems, straighteners and presses. These steels, compared to more conventional grades, require significantly more stress to deform, and additional servo-motor power and torque capability may be needed to pull the coil material through the straightener. Additional back tension between the coil-feed and straightening equipment may be required due to the higher yield strength of the steel in the loop as the material tries to “push back” against the straightener or the feed system.

Further, high-tensile-strength steels, due to their correspondingly higher yield strengths, have a greater tendency to retain their coil set. This requires greater horsepower to straighten the material to an acceptable level of flatness. Straightening high-strength steels also may require larger-diameter rolls and wider roll spacing. However, increasing roll diameter and center distances to accommodate high-strength steels limits the range of materials that can be straightened effectively.

Cutting, blanking and piercing stresses produce unloading forces—called snapthrough or reverse tonnage—in stamping presses. Because ultra-high-strength steels require greater stress to blank and pierce compared to HSLA or mild steels, they generate proportionally increased snapthrough forces. High-tensile snapthrough forces introduce large downward accelerations to the upper die half. On every stroke, these forces work to separate the upper die from the bottom of the ram. If the die-clamping system (hydraulic or screw-type) lacks sufficient clamping force, the upper die half could separate from the bottom of the ram on each stroke.

Every stroke of the press expends energy to deform the workpiece, and this energy must be replaced. Therefore, for stamping high-strength steels metalformers must carefully specify (horsepower rating) the main drive motor and the rotational speed of the flywheel. The main motor, with its electrical connection, is the only source of energy for the press and must have sufficient horsepower to supply the demands of the stamping operation. Stampers must properly size the motor and the flywheel to replace the increased energy being expended during each press stroke. The recent proliferation of servo-drive presses is due, in part, to the fact that servo motors can provide full energy—without a flywheel—at working speeds as low as one stroke/min. MF

 

Related Enterprise Zones: Tool & Die

 


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