Earing is the wavy top edge you see on a drawn cup: the wall rises in a few places and stays low in the others, at fixed angles around the circumference. It looks like a tooling problem. It is not. It is the sheet telling you that its properties are not the same in every direction in the plane.
What causes earing in deep drawn cups?
Earing comes from planar anisotropy: because the strip was rolled, its yield strength and its resistance to thinning are not the same in every direction in the plane of the sheet. When the flange is drawn inward, the directions that resist thinning more pull in less, so the wall rises there and stays low elsewhere. The result is a cup that is level on average and uneven in reality, with ears and troughs at fixed angles to the rolling direction. It is not a tooling fault, and it is not random. Every drawn cup has some earing; the real questions are how much, whether the trim allowance was planned for it, and whether the height tolerance on the drawing was set with the ear height in mind. Those are decided at quotation, not at tryout.
Why it happens
Rolling aligns the grain structure, so yield strength and the plastic strain ratio change with direction within the plane of the sheet. In the flange, the material is compressed in the hoop direction as it flows inward. Where the sheet resists thinning, less material flows into the wall, and the wall ends up higher. That is why earing appears as ears and troughs at consistent angles to the rolling direction rather than as random variation around the circumference - the pattern is a property of the coil, not of the die.
What it costs you
- Trim height. The ears have to be cut off, so the blank has to be sized for the tallest point - and that scrap is real material on every order.
- Height tolerance. If the height tolerance on the drawing is tighter than the ear height, the part cannot be made without a trim operation, and that operation belongs in the quoted route.
- Wall thickness variation. The directions that end up as ears are the directions that resist thinning, so the walls between the ears are thinner. This is why wall thickness needs a stated measurement position.
- Fit and appearance. On a visible or sealing surface an un-trimmed ear is a fit problem, not a cosmetic one.
What actually moves it
The first lever is material, not tooling. A different grade, or a different coil from the same grade, changes the ear pattern and the ear height - which is why the material specification is asked for and not only the drawing. The second lever is the process: blank diameter, blank holder force and the draw sequence all shift how much earing the part finishes with. The third is trim allowance, planned at quotation. What does not work is fighting earing with tooling alone.
Key point
Earing is planar anisotropy showing up as a wavy top edge at fixed angles to the rolling direction. Plan a trim allowance, set the height tolerance with the ear in mind, and change the material or the draw sequence before you change the die.
Frequently asked questions
Is earing a defect?
It is a normal consequence of drawing an anisotropic sheet, so every drawn cup has some. It becomes a defect when the height tolerance or the trim allowance on the drawing does not account for it.
How many ears should I expect?
Commonly four, at roughly 45 degrees to the rolling direction, or two at 0 and 90 degrees, depending on the material and its texture. The pattern is a property of the coil rather than of the tool.
Can a different lubricant remove earing?
No. Lubrication changes friction, the forming load and the surface, but the ear pattern comes from the sheet's directional properties. Changing the material or the draw sequence is the lever that moves it.
Does earing affect wall thickness?
Yes. The directions that end up as ears resist thinning most, so the walls between the ears are thinner. That is one of the reasons wall thickness has to be specified with a measurement position.