What causes a metal stamping part to crack or tear?
Tearing in sheet metal forming occurs when the tensile stress exerted on the blank exceeds the material’s ultimate tensile strength. Instead of flowing smoothly into the die cavity, the metal gets restricted, over-stretches, and fractures. As a master toolmaker at Kailai, I resolve tearing by balancing material flow—reducing blank holder force, polishing die radii, and optimizing lubrication to keep the metal safely within its Forming Limit Diagram (FLD).
On the press floor, tearing is the exact opposite of wrinkling, but the result is the same: absolute scrap. When a deep-drawn shell splits at the bottom radius or a high-strength steel bracket cracks along a bend line, you cannot simply weld it back together. Tearing means the material has structurally failed.
While inexperienced operators might blame “bad coils of steel,” a seasoned toolmaker knows that 90% of tearing issues are rooted in die setup and tooling geometry. If your current Metal Stamping partner is struggling with high scrap rates due to split parts, here is the 8-point diagnostic checklist we use to cure tearing and restore perfect material flow.
1. Excessive Blank Holder Force (Tonnage Too High)
The Shop-Floor Cause
If wrinkling is caused by a loose binder, tearing is caused by a binder that is locking the material down too aggressively. If the blank holder force (cushion pressure) is too high, the metal cannot slide into the die cavity. As the punch continues to travel downward, the material is forced to stretch beyond its physical limits, resulting in a catastrophic tear, usually near the punch radius.
The Toolmaker’s Solution
The immediate fix is to bleed off the pressure. We systematically reduce the press cushion tonnage or adjust the nitrogen gas springs in the die. The goal is to find the “sweet spot” where the pressure is just high enough to prevent wrinkling, but light enough to allow the blank to flow inward.
2. Die Entry Radius Too Sharp
The Shop-Floor Cause
The die entry radius ($R$) is the cliff the metal must slide over to enter the cavity. If this radius is machined too small or is too sharp, it acts almost like a cutting blade. The material is forced to make a brutal 90-degree turn under high tension, creating massive friction that locks the flow and rips the metal.
The Toolmaker’s Solution
We must increase and polish the die radius. By grinding a larger, smoother radius and polishing it to a mirror finish, we drastically reduce the friction coefficient. This allows the sheet metal to glide smoothly over the edge and into the draw cavity without initiating a fracture.
3. Draw Beads Are Too Aggressive
The Shop-Floor Cause
Draw beads are physical brakes on the binder designed to slow down material flow. However, if the draw beads are too tall, too sharp, or placed too closely together, they lock the material completely. The punch pulls, the beads hold, and the metal simply tears in half at the flange.
The Toolmaker’s Solution
We shallow out the draw beads. Toolmakers will carefully grind down the height of the bead profile or increase the radius of the bead groove to reduce the bending severity. We tune the beads until they provide just enough drag to keep the panel taut without causing a fracture.
4. Insufficient Lubrication (Dry Friction)
The Shop-Floor Cause
Unlike wrinkling (where too much oil causes slipping), tearing is frequently caused by a lack of lubrication. If the blank is dry, or if the wrong type of stamping fluid is used, the friction between the sheet metal and the tool steel spikes. The material galls, sticks to the die, and tears under the tension.
The Toolmaker’s Solution
We upgrade the tribology. This means applying a high-performance drawing compound specifically formulated for high-pressure boundary lubrication. We ensure the lubricant is accurately applied to the high-friction zones (like the die radius and punch corners) to facilitate smooth plastic deformation.
5. Punch Radius is Too Small
The Shop-Floor Cause
Sometimes the tear happens exactly at the bottom of the drawn part. This is usually because the radius on the nose of the punch is too sharp. A sharp punch radius localizes all the stretching stress into a very small area, causing the material to thin out rapidly and split open like a punctured balloon.
The Toolmaker’s Solution
We modify the punch geometry. By enlarging the punch radius, we distribute the stretching forces over a larger surface area. This prevents localized thinning and allows the material to yield safely across the entire bottom profile.
6. Incorrect Material Grain Direction
The Shop-Floor Cause
Sheet metal is rolled at the steel mill, giving it a distinct “grain direction” (like wood). If a part is designed so that a sharp bend runs exactly parallel to this grain direction, the metal is highly susceptible to splitting open along the grain lines.
The Toolmaker’s Solution
We alter the strip layout. By rotating the blank layout by 45 or 90 degrees on the coil, we force the bends to cross the grain direction rather than run parallel to it. This simple DFM (Design for Manufacturing) adjustment dramatically increases the material’s bending strength.
7. Poor Blank Edge Quality (Micro-Cracks)
The Shop-Floor Cause
When a blank is sheared or punched, the cutting action leaves a work-hardened edge with microscopic tears (burrs). During a heavy forming or flanging operation, the edge of the material stretches. Those micro-tears act as stress concentrators, instantly propagating into massive visible cracks.
The Toolmaker’s Solution
We fix the edge before we form the part. As part of our advanced Precision Metal Stamping workflow, we utilize automated deburring immediately after blanking. Removing the hardened burr “heals” the edge, vastly improving the material’s elongation capacity and preventing edge-splitting.
8. Inadequate Die Clearance
The Shop-Floor Cause
In deep drawing, the gap between the punch and the die cavity is critical. As metal is drawn into a cup shape, the material in the upper walls naturally thickens due to compressive forces. If the clearance between the punch and die is too tight to accommodate this natural thickening, the die “pinches” the wall, locking it in place until the bottom rips out.
The Toolmaker’s Solution
We re-machine the die cavity to increase the clearance. A master toolmaker will calculate the expected material thickening (often adding 10% to 15% of the material thickness to the gap) to ensure the metal can flow deeply without being pinched.
Stop Paying for Scrapped Parts
Tearing is not an unavoidable cost of doing business; it is a mechanical failure that can be engineered out of the process. By applying these eight precision adjustments, the toolmakers at Kailai eliminate the root causes of material fracture. If your projects require deep draws, high-strength alloys, and zero compromises, partner with a Precision Metal Stamping expert who knows exactly how to control the flow.