Tooling by Design
Stamping problems related to higher strength materials are not limited to deep-drawing processes nor are they limited to the new advanced high strength steels (AHSS). The problems associated with higher strength materials have als been present; for the most part we just ignored them—mostly because we could. But the new AHSS materials have pushed the limits of our stamping operations to a point where the problems can no longer be ignored.
Stamping higher strength materials can affect the size, strength, power and overall configuration of every major piece of the press line, including coil-handling equipment, coil-feed systems, straighteners and presses. But rarely are the stamping presses, coil-feed lines or coil-straightening equipment considered to be part of the overall forming system, except that the tooling and steel coils must physically fit into the equipment. This means we must change how we view the stamping process. The stamping die represents only one component of the process but some companies consider the die to be the process. That means any problems associated with a stamped part must be corrected in the die, which also means the rest of the process is being ignored.
Higher strength materials require more stress to deform. This means the feed system may require additional servo-motor power and torque capability to pull the coil material through the straightener. Additional back tension between the coil feed and straightening equipment also may be required due to the higher yield strength of the material in the loop as the material tries to “push back” against the straightener or the feed system.
Higher strength materials, due to their 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. It also might require a straightener with larger diameter rolls and wider roll spacing to work the stronger material more effectively. The larger rolls and journals and broader center distances safeguard the straightener from potential damage caused by the higher stresses. But keep in mind that increasing roll diameter and center distances on straighteners to accommodate higher strength steels may limit the range of materials that can be effectively straightened.
Cutting, blanking and piercing stresses all produce unloading forces in stamping presses called “snapthrough” or “reverse-tonnage.” Because higher strength materials require greater stress to cut and pierce compared to mild steels, they generate proportionally increased snap-through forces. High tensile snap-through forces introduce large downward accelerations to the upper die half. These forces work to essentially separate the upper die from the bottom of the ram on every stroke. If the hydraulic die-clamping system or screw clamping system lacks sufficient clamping force, the upper die half could separate from the bottom of the ram on each stroke, causing fatigue to the upper die-mounting fasteners.
Other factors to consider:
Tool Breakage—Resulting from the increased stress required to form parts from grades that may have double and triple the strength of mild steels.
Tool Wear—Such that hardened inserts and shear edges can wear out during a single production run.
Part Quality—Problems arising from the increased springback associated with the higher strengths and increased process sensitivity to material properties.
The Center for Automotive Research (CAR), a consulting partner with Auto-Steel Partnership that maintains a working relationship with the tool and die industry, assembled tooling coalitions to tackle common tooling and metalforming problems associated with the new AHSS materials. The coalition of North American tooling shops identified additional challenges, including:
• The absence of any detailed metallurgical steel data early in die development to adequately support forming simulations and ensure continuity of material throughout die development and tryout.
• Numerous die recuts required due to extensive springback not predicted by forming simulation. Some tools are being recut as many as 10 times, driving up tool cost and reducing quality.
• Dimensional buy-off criteria being too difficult for AHSS. The unattainable goal of 100-percent print compliance and dimensional Cpk levels of 1.33 places nearly every tool program into crisis mode, resulting in late delivery.
The tooling coalition identified five general recommendations for improving tooling, stamping processes and the product design for AHSS stamped parts:
Springback prediction accuracy in computer simulations and springback management on the shop floor for AHSS grade materials still requires much improvement.
Product design features such as radii and part shape greatly affect springback, including twist and curl. Anticipating the impact of these product features during the tooling design phase (with improved simulation codes) can help reduce springback problems.
Tool and die standards for HSLA and mild-steel stampings are not adequate for AHSS. Special tool standards accounting for the higher forming stresses and wear characteristics of AHSS alloys need to be adopted.
Material availability is a significant problem affecting simulation accuracy, tool design and tool tryout, especially when specified as a last-minute change.
Tool buyoff standards are historically based on part-print compliance and Cpk results. These requirements are usually challenging enough for HSLA stampings and proven to be much too difficult for AHSS stampings. A functional buyoff procedure, rather than the pursuit of print dimensions or unachievable Cpk, is needed, especially for tools capable of achieving high levels of repeatability but not part-print specification.
Assuming a business-as-usual approach to higher-strength steel applications with an existing press line can be a very costly mistake. Some high-strength stamping processes can easily push press-line performance requirements well beyond the capabilities and limitations for which they were originally designed. How long will you ignore the demands placed on your stamping processes? MF
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