Page 29 - MetalForming June 2010

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 suffi- cient 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 associ- ated with the higher strengths and increased process sensitivity to materi- al properties.
The Center for Automotive Research (CAR), a consulting partner with Auto- Steel Partnership that maintains a work- ing relationship with the tool and die industry, assembled tooling coalitions to tackle common tooling and metal- forming problems associated with the new AHSS materials. The coalition of North American tooling shops identi- fied additional challenges, including:
• The absence of any detailed metal- lurgical steel data early in die develop- ment 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 improv- ing 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 spring- back, including twist and curl. Antici- pating the impact of these product fea- tures 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 ade- quate 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.
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Tool buyoff standards are histori- cally 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 stamp- ings. A functional buyoff procedure, rather than the pursuit of print dimen- sions 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 appli- cations with an existing press line can be a very costly mistake. Some high- strength stamping processes can easily push press-line performance require- ments well beyond the capabilities and limitations for which they were origi- nally designed. How long will you ignore the demands placed on your stamping processes? MF
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METALFORMING / JUNE 2010 27