Page 27 - MetalForming December 2010

ing, temperature variations and gradi- ents, and die grinding and sharpening practices.
Design errors often contribute to die failure. Design errors can be classi- fied into two basic groups: those that fail from heat-treatment, and those that fail in service.
Design faults that cause failures from heat treatment include the presence of thick sections adjacent to thin sections, sharp corners, blind holes, stamp marks, fillets with radii that are too small, poorly located or designed grooves or notches, abrupt changes in cross-section and the location of holes that result in thin walls.
Design faults that cause service fail- ures include inadequate fillets in cor- ners, improper clearance (especially if thermal expansion is involved, which is the case with many stamping opera- tions), thin sections and abrupt changes in cross-section.
It is critical to investigate tool design, materials and heat treatment used for a given application. In many instances, tool design and material selection get the most attention from the die shop and press shop, usually because the heattreat process is least understood and almost always outside their direct control.
Finally, it is often beneficial to com- pare a failed component to one that has not failed in order to assess if a fail- ure occurred due to service conditions or an error in the manufacture of the failed component. A highly stressed area in a die detail, for example, may crack or chip in service because a radius specified in the corner of the compo- nent was inadvertently left sharp. This is an example of a manufacturing error. When chlorinated stamping lubricants are used in stamping dies containing solid carbide tool steels, the chlorine will attack the cobalt binding and cause the die component to eventually crack or chip. This is an example of a service- condition failure. In both cases the fail- ure mode, cracking or chipping, was the same but the conditions that caused the failures were different. MF
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