Page 55 - MetalForming February 2013

strength steel. The binder area (Fig. 2) requires more force (FF) to pull the higher strength material toward the die opening as the surface area com- presses. And, the material workhardens during the bend/unbend sequence as the workpiece moves over the die radius into the cup wall. However, the material transmitting the pulling force of the punch (FP) also has a higher strength. Therefore, the cup can be successfully drawn to its same depth.
These results have been proven math- ematically and experimentally. Two-piece steel beverage cans are produced by a sequence of draw, redraw, three ironing stages and a bottom-dome forming oper- ation. The DR-9 steel grade is cold-rolled to a thin sheet, annealed and then given another 35-percent cold reduction. The steel forms in the full-hard condition without failure. The secret in making the beverage cans: use of compressive-form- ing processing stages.
Instead of the plastic (permanent) deformation previously discussed, stiff- ness and springback relate to elastic stresses. Pressing down on a flat sur- face, such as a car roof or refrigerator door, with a force less than the yield strength causes an elastic bowing of the sheetmetal. Resistance to the force is the material’s stiffness. Releasing the force allows the elastic bowing to spring back to its initial configuration.
The amount of deformation created by a specific force (stress) depends on the elastic modulus, or Young’s Modulus, of the material. Steel has a high modu- lus (30 million) that creates a relatively stiff panel. Many nonferrous materials, such as aluminum alloys, have an elas- tic modulus of only 10 million. For the same force placed on a panel, an alu- minum alloy will exhibit three times the amount of deflection or reduction in stiffness as steel. Likewise, when the force is released, springback from alu- minum is three times that of steel.
Increasing the yield strength of the material does not change its elastic modulus. It merely allows more panel deflection and springback before the onset of plastic (permanent) deforma- tion. Weight-reduction programs
increase yield strength to offset the reduction in sheet thickness. This may balance load-carrying capacity, but the thinning of the panel reduces its geo- metric stiffness. This limits the amount of panel weight reduction, unless addi- tional design changes are incorporated.
Whether designing a new panel with higher-strength steel or upgrading an existing panel, designers must start by analyzing the stamping to identify the
forming modes, and understand the interactions among adjoining modes. Then we can acquire strain directions and magnitudes for analysis, and make decisions about forming severity and necessary design changes. MF
Keeler, Peter Ulintz and Ed Tarney will present the popular Deep Draw seminar on March 13, 2013 in Livonia (Detroit), MI. Learn more and register to attend at www.pma.org/meetings.
The Science of Forming
Sliding Swing Clamps Clamps
Hollow
Piston Cylinders
Traveling Clamps
Wedge Clamps
Swing Sink Clamps
Ledge Clamps
Pumps
Control Panels
Die Lifters
Success Success
Guaranteed! Guaranteed!
Hilma specializes in die change systems that maximize the output of your spe- cific press. That’s why, if you work with us, you’ll get the best system, and top qual- ity results. We stand behind our products, and provide you with the excellent service you need to remain competitive in today’s manufacturing environment.
Find out the advantages of the HILMA quality guarantee when you work with us!
www.clrh.com
Ellisville, MO 63021 (636) 386-8022 Email: engr@clrh.com
www.metalformingmagazine.com
MetalForming/February 2013 53