Press selection - sorting it out: Part III

Editor's Note: This article is one installment of a nine-part article.

Matching Press Characteristics to Your Applications

The following questions—and their answers, provided by industry equipment manufacturers and experts—are intended as a general guide to help you simplify the daunting task of selecting a press or press system.

3. What Other Press Characteristics Should I Look for to Stamp High-strength Materials, Including Stainless? Boron Steel? Aluminum?

HSS, Stainless—High Tonnage, Large Beds. More tooling stations are required stamp difficult-to-form HSS, so you'll need a press with a large bed. Presses with higher tonnage capacity are needed to apply the greater force required to fracture and form the material, Boerger said.

HSS, Stainless—Multistep, Servo, and Hydraulic. To reduce springback while forming titanium alloys, stainless steel, and other HSS, traditionally, stampers have formed the metal progressively in multiple full strokes in a mechanical press progressive-die setup. Alternatively, because of their ability to dwell or apply pressure at any point in the stroke, a servo-mechanical press or a hydraulic press can form the part without a progressive-die setup, by progressively applying more force to the part with each step without the slide traveling through the complete stroke. Good springback reduction results have been achieved, manufacturers said.

Austenitic Stainless Steels—High Blank Holder Force. The high elongation potential of these steels, and therefore good formability, makes them favorable for stretch drawing and deep drawing, according to technical resources at Schuler. "They also exhibit a high degree of cold-hardening. As a result, you need a press with high blank holder force [drawing cushion force] to reduce buckling."

HSS, Stainless, UHSS—Blanking and Deep Drawing, Dual-slide, Triple-slide. The high press force required to blank HSS and UHSS—two to three times the force to deep-draw the same material—is generally acknowledged; however, the severe cutting impact shock during blanking is not as well-known, nor is the greater blank holder force required during drawing to prevent buckling, according to Schuler experts. Because the force needed to draw is less than the force needed to blank UHSS and HSS, problems with accuracy occur when you try to perform both of these actions in the same slide. The stronger blanking force causes a strong reversal of tilting torque from the drawing operation to the blanking and back again, and so on, he said.

[Tip] The problem can be resolved by separating the press into a drawing slide and one or more blanking slides, This type of press is called a dual-slide or triple-slide press.

[Caveat] HSS, Stainless—Transfer Presses. The same tug-of-war between drawing force and blanking force occurs in transfer presses as well, Minster's Cattell said. "Ideally, the whole operation, from coil material to finished part, should be completed on one transfer press. However, in many instances with high-strength steel, this is not possible," Cattell said (see Wrong Use: Bad Investment sidebar).

[Tip] "Because of the higher tonnage needed for blanking high-strength steel, together with the need to nest the blank for coil material savings, it may more practical and economical to use two smaller presses than one very large press," Cattell said. The HSS material can be blanked offline and stacked. The stack of blanks then can be automatically fed into the transfer press," he said.

Stainless, HSS—Warm Forming, Servo. Studies conducted on forming of stainless steels concluded that a Type 304 austenitic stainless steel exhibits a remarkable improvement in drawability when it is formed at elevated temperatures (about 100 degrees to 200 degrees C), said Dr. Taylan Altan, director, Center for Precision Forming.

A servo press's infinite velocity programming capability is very useful for optimizing the warm-forming process," Altan said. "The press ram can be programmed for slow velocity during forming to reduce the strain rate and improve draw formability and a fast return stroke to reduce the cycle time. In addition, a dwell stage can be programmed into the press ram's motion curve to allow time for heating the blank before deep drawing starts," Altan continued.

[Tip] "In fact, one of the most significant advantages of servo press technology is for warm-forming applications; warm forming can take place through heated tools by stopping the slide or dwelling in the stroke," Altan said. "This eliminates the need for heating and transfer systems outside the press."

Boron Steel—Hot Forming, Hydraulic. Stamping complex and structural components of HSS and UHSS in a mechanical press is difficult, requires higher tonnage and can be fraught with problems like springback and material cracking, manufacturers said. Hot forming typical safety related structural components out of boron steel in a hydraulic press resolves some of those problems.

Stamper Cosma Int'l. applies its own hot-forming technique to stamp automotive structural metal components such as A- and B-pillars and roof headers to meet weight reduction goals, while also meeting safety requirements for strength.

Cosma's Kotagiri, explained. "You heat the boron steel to about 900 degrees C, then you transfer it quickly to a press where you form the part while the steel is hot, and then the part is quenched—cooled by water—in the die, he said.

The press line requirements needed to perform the hot-forming process comprises a continuously running roller hearth furnace to heat the blanks up to austenite temperature, a hydraulic hot-forming press usually in the range from 600 to 800 tons with a die cooling system (water-cooling circuits), and the appropriate automation, according to Schuler's Kinzyk "The key is to cool down the parts from an austenite micro structure in the shortest possible time to achieve the desired martensite structure that gives the component high-strength steel material properties of up to 1,500 MPa."

[Caveat] The hydraulic presses used in hot forming must be well-maintained and cannot leak fluid, to prevent fire hazards, Kinzyk said. Also, the hydraulic tank is typically mounted on the opposite side, away from the oven.

Aluminum—Servo, Hydraulic. The programmable slide motion control of servo-mechanical presses and the fully programmable slide stroke of hydraulic presses are well-matched for the formability characteristics of aluminum, experts said.

Generally, aluminum does not have the elongation ability of steel—around 30 percent to deep-draw steel's 45 percent, according to Art Hedrick, president, Dieology. It strains, or stretches, locally. Also, aluminum has more springback than soft draw-quality steel.

Therefore, parts requiring a great deal of stretch in a small area are best made in multiple draw reductions and multiforming operations. A mechanical press progressive-die setup is suitable also.

Aluminum—Drawing, Mechanical. Most pneumatic and hydraulic drawing cushions in mechanical presses cannot produce a clearly defined, reproducible hold-down force, Haller said. Severe impact shock and force peaks can work-harden or mark the soft, impressionable aluminum under the blank holder. It is critical, therefore, to make sure that the press can be equipped with a freely adjustable hydraulic cushion with pre-acceleration to draw and deep-draw aluminum parts.

Composite Materials—Servo, Hydraulic. Press manufacturers said that for composite materials, perforated sheet, and other materials more easily damaged by high-velocity shock than other materials, a servo-mechanical or hydraulic press can be programmed to form the part with multiple, progressive strokes. This eliminates the need for a mechanical press with a progressive-die setup.

Laminations—High-speed. “High-speed presses that can handle blanking and piercing with minimum snap-through shock are required to produce laminations," said Paul Pfundtner, president, Red Stag Engineering & Automation Inc. "Press selection is very limited for this application; it is critical to use a press designed specifically for lamination stamping. The presses must have a deflection of 0.0009 to 0.0019 per inch from a knife edge on a centerline of the bolster to the outside edge of the bolster."