Quality
Diagnosing edge cracks in a drawn clad part
A crack at the rim of a drawn pan is a symptom with five common causes. Working through them in order finds the cause in an afternoon instead of a month of guessing.
At a glance
Specification at a glance
| Step | Check | If it is the cause |
|---|---|---|
| 1. Edge | Burr, micro-cracks, square-cut shear | Deburr or improve the shear; re-blank |
| 2. Anneal | Temper of this lot against the last good one | Ask for the condition and the lot records |
| 3. Draw ratio | Blank diameter divided by cup diameter | Change blank size or accept a two-stage draw |
| 4. Lubricant | Type, quantity, coverage at the flange | Change lubricant or application point |
| 5. Tooling | Die radius, clearance, blank holder pressure | Tool correction |
| 6. Bond | Separation at the crack face | Reject the lot: this is a material fault |
Start at the edge because it is cheapest
A sheared edge with a burr is a stress raiser, and a crack that begins at the rim almost always begins at a burr. Look at the crack face under a lens: a smooth start from a notch points at the edge, a rough start points at the material.
Then compare lots
If the previous lot ran and this one does not, the difference is in the material or the tool, not in the design. Compare the condition, the core source and the coil or sheet lot before changing the drawing.
Draw ratio is a design number
If the ratio exceeds what the structure can take, no lubricant fixes it. The options are a softer anneal, a thinner gauge, a larger blank with a second draw, or a different structure. Those are commercial decisions as much as technical ones.
When it is the bond
If the crack face shows separation between layers, the material has failed rather than the process. That is a lot rejection, not a tooling adjustment, and it should be documented with photographs and the batch reference.
What to send with the complaint
Part drawing, draw ratio, blank size and edge condition, lubricant, press tonnage, the batch reference, and a photograph of the crack face. That set lets a supplier answer in one reply instead of three.
Questions
Common questions
Can a burr really crack a pan?
Yes. A burr concentrates stress exactly where the wall is thinning, which is why deburring is standard for deep-drawn parts.
Does a softer anneal always help?
It helps elongation, but it can make wrinkling worse and flatness harder. The right condition depends on the part, not on a preference.
Is edge cracking a material defect?
Only when the crack face shows layer separation. Otherwise it is a process or edge-condition issue.
How do we prove it to the supplier?
Photographs of the crack face plus the batch reference and the previous lot that worked. A claim with a comparison is hard to dispute.
Should we change grade?
Rarely. Changing the anneal, the edge or the blank size usually fixes it first.