Page 38 - MetalForming April 2009

Tooling Technology TOOLING BY DESIGN
PETER ULINTZ
Peter Ulintz has worked in the sheetmetal-forming industry since 1978. His background includes tool and die making, tool and process engineering, engineering management and product devel- opment. Peter also operates the website ToolingbyDesign.com, a source for the transfer of modern metalforming and tool-and-die technology, and which promotes the use of “Performance-Based Die Engineering Strategies.”
Peter speaks at PMA seminars and roundtables focusing on tool and die design, die maintenance, deep drawing, stamping simula- tion, tooling for stamping high- strength steels and problem solv- ing in the press shop.
Peter Ulintz pete.ulintz@toolingbydesign.com www.toolingbydesign.com
Springback, sidewall curl and panel twist all have their origins in unbal- anced stresses in the formed part. These may be inherent in the product design due to nonsymmetrical geometry and cutouts, rapid changes in cross- section, or unequal flange lengths. They may be equally inherent to the forming operation due to the number of highly interactive process parameters. These include die-process lubrication, die- polishing techniques, blankholder forces, blank positioning, and broken or worn draw beads, just to name a few.
Some compensation for springback is routinely designed into most forming processes to limit or reduce the number of additional over-bending or restrike operations. The method and magnitude of compensation usually depends on the die designer’s experience with sim- ilar parts, materials and processes. The arrival of new higher-strength steels dramatically changes the old approach to springback compensation because previous experience with these new materials does not exist.
Today, simulation codes often are employed to study relationships between product geometry, die geometry, mate- rial properties, friction conditions and springback. The goal of springback sim- ulation is to provide both compensation direction and magnitude for die tryout. But research has exposed several weak- nesses relating to the accuracy of spring- back prediction in sheetmetal- forming simulations, especially
when higher-strength steels are involved. For example:
• Springback results are shown to be very sensitive to anisotropy values (delta-r).
• Very different springback results have been observed when using differ- ent workhardening rules.
• Friction coefficients are shown to be highly influential in springback calculations.
• Material models that consider strain-rate sensitivity (m) values predict different springback magnitudes and modes than rate independent models.
With the increasing use of higher- strength steels and the inherent spring- back problems that come with them, recent research has intensified to achieve improved accuracy in both forming and springback predictions. Still, current springback results provide very useful data for die process planning and initial springback compensation.
Because springback is a major concern in higher-strength applications, it must be addressed as early as possible in the design phase. Springback can be mini- mized in the product design phase by:
• Avoiding right or acute angles (Fig. 1).
• Using large open-wall angles to allow for over-bending and springback. • Avoiding large transition radii
between two walls.
• Using opened-end stampings
instead of closed-end stampings.
• Using stiffeners, darts, step flanges, etc., to prevent the release of elastic
stresses.
• Designing punch radii as sharp as
formability and product design will
Springback in High-Strength Steel Stampings “Compensation is Not Commensurate with Experience”
36 METALFORMING / APRIL 2009
www.metalformingmagazine.com
Avoid Preferred
Fig. 1—Avoid right or acute angles when form- ing higher-strength steels.