Servo-Mechanical Presses Rise to the Challenges of Hot Stamping

April 1, 2018

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Hot-forming (or press-hardening) technology continues to play a growing role in the stamping of structural automotive parts from advanced high-strength steels (AHSS). For example, automotive suppliers forming boron-alloyed steels heat blanks to 900 C prior to forming, to achieve a fully austenitic state. The heated blanks then remain in the press with the die (water-cooled) closed until the steel temperature drops to 150 C. This process yields a fully martensitic steel with a tensile strength in the neighborhood of 1500 MPA or more.

The Role of Servo Presses

A servo-mechanical press serves as the heart of a hot-stamping installation at Batz, a European automotive-parts supplier. The installation has demonstrated significant energy savings and improvements in part quality, according to Batz officials.

A critical success factor during hot stamping is the ability of the press to keep the die closed during the cooling process, requiring the slide to stop and stay at bottom dead center (BDC). Here’s where the unique capabilities of servo-mechanical presses shine—in particular the ability to provide force control even while the ram remains stationary.

Much work has been done in recent years to evaluate the force stability of servo-mechanical presses at BDC, as well as motor-torque performance and the required restarting torque.

Having tested press performance over varying closing speed and press tonnage, researchers have found that presses are able to maintain position control to a dynamic accuracy of less than 0.01 mm; a peak motor torque of less than 3 percent of maximum tonnage; and a starting torque similar at BDC and top dead center—no additional force required during press restarts. (A key press-design enhancement that allows repeatable stop-start cycles at full tonnage at BDC: proper lubrication of the bronze bushing within the press crown.)

These findings verify that press conditions at BDC are well suited to a continuous hot-stamping process, with the required control of ram position and force on the part and die without a measurable increase in power consumption.

Case Study: Automotive-Parts Supplier Batz

While hydraulic presses originally got the call for hot-stamping lines, the Spanish automotive supplier Batz can testify to the benefits provided by servo-mechanical presses. It has used a 15,000-kN Fagor servo-mechanical press (3.6 by 2-m bed) for hot stamping since March 2016—for die tryout and production. The company uses hot stamping to produce primarily safety-related parts from boron steels, and employs production lines outfitted with robotic destackers, roller or walking-beam furnaces, robotic unloading to conveyors, and laser or press trimming.

With two years under its belt using the servo-mechanical press in hot-stamping die set-up and preserial production, Batz now has a clear idea as to press performance in this application. Energy savings was an immediately identified result.

“Energy consumption was one logical and expected advantage,” explains Andoni Alonso, hot-forming business manager for Batz. “With hydraulic presses in our facilities, we were used to their high energy consumption. With this servo-mechanical press, energy expenses have dropped dramatically and this is something to take into account nowadays.”

More surprising was the equipment’s effect on part quality.

“During the set-up of new dies, we realized that we could more easily achieve higher-quality parts than in our conventional hydraulic presses,” Alonso says. “We attribute this in part to the higher drawing speed of the servo-mechanical press. The servo control also allows us to simulate the end user’s press conditions, which ensures that part quality at a customer’s facility will be comparable to ours.”

Alonso advises that proper training is necessary to gain the full advantages of this hot-stamping technology, a lesson learned during Batz’s adoption of the new equipment.

“Operating mechanical presses versus hydraulic presses presents some differences,” he says, “so it was necessary to train personnel in understanding the data-introduction system for the servo-mechanical press. Once proper training was achieved, we realized that the system attains the same force control and even greater flexibility than we had been accustomed to when using hydraulic presses.” MF