Fixing A Fracture Without Creating A Buckle Or Ghost Line

A body-side Outer develops a split in the C-pillar and header intersection area. In a frantic move to eliminate the split someone adjusts the equalizers on the inner binder or “pillow” to get more metal to flow in that area. The split goes away, and production starts back up. The split, which was caused by high localized strain, might have went away but this move could have caused another issue, a buckle on the class A surface. There have been many other ways die makers have tried to correct this same situation. They might have ground the beads on the inner and/or outer binder, while others may have just changed the Nitrogen pressure on the inner pillow (if constructed this way).

Reducing the internal binder force may resolve the split problem, but we can’t let the restraining forces between the inner and outer binders become too unbalanced. As inner part restraining forces are reduced to eliminate the split and the outer binder forces remain unchanged, material will begin to be displaced over the door opening radius (Point A). The resulting metal movement (green line in the below photo) will produce a ghost, shadow or double draw line (depending on your facility’s terminology) which may go unnoticed until parts are painted after assembly.

To prevent material displacement over the product radius , it’s logical to consider rebalancing the binder forces. But lowering the outer binder forces is accompanied by a reduction in the overall work hardening and dent resistance of the door header area. Without adequate stretch in this region of the part, the draw depth and length of line differential between the door opening and shallower “C” pillar may produce a buckle or low in the area shown by the blue area above. 

In this case it may be necessary to create a restraining force by roughening the door opening radius (above the fracture area) as the internal binder restraining force is reduced. By creating a less polished product radius, friction will locally increase allowing for the necessary increase in material displacement from the door opening cutout without reducing the outer binder restraining forces.

If metal displacement over the door opening radius still is evident, a smaller product radius at the door opening line may be required. This change will increase bending restraining forces and friction, preventing material from being drawn over the product radius onto the header (product show) area. Care during welding and radius refinishing are required to to prevent weld line show through on final product.

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