A stamping die that cracked after 50,000 hits. The die was a progressive die for a 1.5 mm steel bracket. The die used heavy-duty die springs (rectangular section) for stripper plate return. The springs were rated at 30% compression. The customer had them at 35% compression (near the limit). At the bottom of the stroke, the stripper plate compressed the springs fully. The tonnage spiked. The die shoe cracked. The issue: the spring rate was too high for the stripper travel. At full compression, the spring force exceeded the press tonnage. The die absorbed the overload. This is about die spring selection and why spring curve matters more than rated force.
The spring force curve
A die spring isn’t a constant-force device. The force increases linearly with compression: F = k × x. The spring rate k is specified by the manufacturer. For a yellow die spring (light duty), k = 150 N/mm. For a blue spring (medium), k = 300 N/mm. For a red spring (heavy), k = 600 N/mm. The force at any compression is the rate times the deflection.
The stripper plate must strip the part from the punch. The stripping force is about 5-10% of the blanking force. For a 1.5 mm steel part with a 50 mm perimeter: blanking force = 0.7 × t × L × τ = 0.7 × 1.5 × 157 × 350 = 57,800 N. Stripping force = 5% = 2890 N. The springs must provide at least this force when the stripper contacts the stock (at the start of the stroke).
The die had 4 red springs (600 N/mm each). At 10 mm initial compression: F = 4 × 600 × 10 = 24,000 N. That’s 8x the stripping force. Way over. At the bottom of the stroke, the springs compress another 15 mm (total 25 mm). F = 4 × 600 × 25 = 60,000 N. The press was a 40-ton (40,000 N) mechanical press. The springs alone needed 60 kN at the bottom. The press couldn’t deliver. The crankshaft stalled. The die absorbed the impact. The shoe cracked.
What was changed
1. Switched to yellow springs (lower rate). Four yellow springs at k=150 N/mm each. At 10 mm preload: F = 4 × 150 × 10 = 6000 N. That’s 2x the stripping force (2890 N). At bottom (25 mm): F = 4 × 150 × 25 = 15,000 N. Well within the 40 kN press capacity. The press cycled smoothly. The die didn’t crack. The lower rate springs provide enough stripping force without overloading the press at bottom dead center.
2. Reduced the preload. The original springs were preloaded to 10 mm. The stripping force at contact was 24 kN. The part was being stripped too hard — it deformed. The yellow springs at 5 mm preload: F = 4 × 150 × 5 = 3000 N. Just above the 2890 N stripping force. The part stripped cleanly without deformation. The preload should be just enough to strip — not 8x more.
3. Used nitrogen springs. For new designs, nitrogen springs (gas springs) provide near-constant force over the stroke. The force doesn’t spike at the bottom. A 5000 N nitrogen spring provides 5000 N from 5 mm to 80 mm stroke. No rate increase. The press sees a constant load. The die doesn’t crack. The nitrogen spring costs 5x a die spring but eliminates the bottoming spike. For progressive dies with high stroke count, nitrogen springs are the standard.
The die spring color code
| Color | Load class | Max compression | Use for |
|---|---|---|---|
| Blue (light) | Light duty | 40% of free length | Stripper, knockout |
| Yellow (medium-light) | Medium-light | 35% | Stripper, ejection |
| Red (medium-heavy) | Medium-heavy | 30% | Heavy stripper, pressure pad |
| Green (heavy) | Heavy | 25% | Draw pad, thick material |
The compression limit is the free length × the percentage. For a 50 mm long yellow spring: max compression = 50 × 35% = 17.5 mm. Running the spring at 25 mm (as the original did) exceeds the limit. The spring takes a set. It loses force. The die then runs under-sprung. The cracked die was a symptom of over-compressed springs that had lost their designed rate.
The press tonnage check
At the bottom of the stroke, the press must deliver: blanking force + stripper force + cushion force. For the cracked die: blanking 58 kN + stripper 60 kN = 118 kN. The press was 40 kN. It was 3x overloaded. The press stalled. The die absorbed the shock. The die spring selection should be checked against the press capacity before the die is built. A die that needs 118 kN needs a 160 kN press, not a 40 kN one.
The spring rule: select springs for the stripping force at contact, not for the rated force at full compression. The cracked die wasn’t overloaded by the stamping — the red springs spiked to 60 kN at bottom. Use yellow springs with just enough preload to strip. For progressive dies, nitrogen springs eliminate the bottoming spike. Always sum blanking + stripper + cushion against press tonnage before cutting the die.
