A die spring is a heavy-duty rectangular-wire compression spring engineered for the brutal environment inside stamping dies, injection molds, and clamping fixtures. Unlike a general-purpose round-wire compression spring, its wire cross-section is rectangular (taller than wide), which packs roughly 30–40% more material into the same coil envelope. That gives you a much higher spring rate and load capacity in the same bore diameter — critical when a die designer has a fixed pocket size and needs to strip a part off a punch with several thousand pounds of force.

Die springs are sold to an ISO 10243 color code that tells you the duty rating at a glance:

• Green — light duty, ~30% max deflection, longest life (2M+ cycles)
• Blue — medium duty, ~25% max deflection
• Red — heavy duty, ~20% max deflection
• Yellow — extra heavy, ~15% max deflection
• Brown — extreme, ~10% max deflection, shortest life

The trade-off is stark: a yellow spring in the same envelope delivers roughly 2× the force of a green one, but you can only compress it 15% of its free length before you start eating into fatigue life. Push a brown spring beyond 10% and it will fail in tens of thousands of cycles instead of millions.

Real-world example: A progressive stamping die running at 200 strokes/minute uses four red die springs to strip a formed bracket off the punch. Each spring is 25 mm OD × 50 mm free length with a rate of 80 N/mm. Compressed 8 mm on each stroke, each delivers 640 N, giving 2,560 N total stripping force. That's 8 mm ÷ 50 mm = 16% deflection — right at the red-duty limit, so the shop stocks spares and swaps them at scheduled PM intervals, not on failure.

Rule of thumb for sizing: Never exceed the color-code's rated deflection percentage, and prefer the next duty level up (with more springs, if needed) rather than pushing a lighter spring to its limit. Doubling the number of medium-duty springs almost always outlives one heavy-duty spring worked hard, because fatigue life scales with the cube of stress amplitude.

Other details that matter: die springs need a guide rod or bore because their high rate makes them prone to buckling if the L/D ratio exceeds ~4. They're wound left-hand by convention (so a right-hand thread doesn't back a retainer off), and the ends are ground square-and-closed so they seat flat under load.