When the drawn part is essentially a square tube, severe cracking can occur near the center of the straight side walls.

When the general drawing shape is a square tube, near the center of the straight side wall, there may be a large-scale drawing cracking phenomenon.

Cause Analysis

Part Geometry

1. Draw height too large:

Reducing the draw depth can prevent cracking, but rule out every other cause first. Only after all other conditions check out clean and the problem persists should you reduce the draw height, and add a separate drawing-and-sizing operation.

2. Die fillet radius too small:

For square-tube deep drawing, the usual way to prevent loose metal and sagging on the straight side walls is to use drawbeads, but drawbeads leave bead marks on the part.

When you draw without drawbeads, run a die gap slightly smaller than the material thickness and keep the corner radius tight. If the part cracks before reaching the forming limit, increase the radius slightly and run a trial draw.

Stamping Conditions

1. Excessive blank holder force:

A fully burnished flange surface indicates the blank holder force is too high. Reduce it to the point where the material flows in freely but the flange does not wrinkle.

2. Poor die surface lubrication:

To keep the material flowing in, check the type and amount of lubricant.

Die Problems

1. Poor die surface finish and fit:

If the part cracks and the flange surface is scratched at the same time, polish the die surface with a grinding wheel until the material flows easily.

2. Die gap too small:

If the side wall is burnished and cracked, the wall is being ironed too thin, so adjust the die gap.

3. Poor drawbead position and shape:

Drawbeads that exert too much force frequently cause rupture, so reduce the bead force.

4. Poor die precision:

Low die precision lets the punch shift off center, and the parallelism and squareness between the punch and die suffer. Checking the die in advance prevents these problems during tryout.

5. Insufficient blank holder rigidity:

When the blank holder lacks rigidity, only a few cushion pins take the load, and the part stretches violently and cracks.

6. Poor press precision:

A press that is out of tolerance produces the same defects as an imprecise die. Bring the press to a high-precision state before running tryouts.

Material

1. If the defect comes from insufficient material tensile strength or oversized grain, change the material grade. 2. If defects come from insufficient sheet thickness, increase the thickness.

①. When the defect is caused by insufficient tensile strength of the material and excessive grain size, it is necessary to change the material
②. When defects occur due to insufficient thickness of the plate, it is necessary to increase the thickness of the plate.