Walk up to any press tool and you can read its spring specification without a single measurement - the coating colour tells you the load class. Here is how the die spring colour system works and how to use it when specifying or replacing springs.
Why Die Springs Are Colour-Coded
Die springs are built as a standardised system: for a given outer diameter and free length, all load ratings share the same housing bore and guide rod size. That means the only thing distinguishing a light spring from a heavy one of the same size is the wire section inside the coil - invisible at a glance. The colour coating solves this: it encodes the load rating so toolmakers can identify, replace, and audit springs on sight. The convention is standardised in ISO 10243 (compression springs with rectangular section - housing dimensions and colour coding).
The Colour Code
| Colour | Load Rating | Character |
|---|---|---|
| Green | Lightest | Longest available travel, gentlest rate - light stripping and pad pressure |
| Blue | Light | General light-duty work |
| Red | Medium | The general-purpose workhorse |
| Yellow | Heavy | Highest force, shortest available travel |
The trade-off to remember: within one spring size, going up the colour scale buys more force but less usable travel. A heavier spring is not automatically better - it must still deliver the stroke your tool needs.
Why Rectangular Wire Matters
Die springs are wound from rectangular-section chrome-alloy spring steel rather than round wire. A rectangular section fills the annular space between rod and bore far more efficiently, so the spring develops much higher force from the same envelope. Combined with shot peening and presetting during manufacture, this is what lets a compact die spring survive millions of press strokes at forces a round-wire spring of the same size could not deliver.
Selecting a Load Rating
- Fix the envelope first: hole diameter (spring OD) and rod diameter come from your tool design; free length comes from the working stroke plus preload.
- Calculate required force at working deflection - stripping force, pad force, or cushion force.
- Pick the lightest colour that delivers it. Running a lighter spring near its full recommended deflection wears faster than a heavier spring working moderately - but an oversized spring wastes press tonnage and can damage sheet parts.
- Never run any die spring to solid. Keep working deflection within the manufacturer’s recommended limit for the colour class - bottoming a die spring destroys it in short order.
- Replace in sets. When one spring in a group fails, its partners have the same fatigue history. Replace the set, same colour throughout.
Sizes We Stock
We stock all four colour classes ex-Mumbai in outer diameters OD 10, 13, 16, 20, 25, 32, 40, 50 and 63 mm, free lengths 25 - 305 mm, sold per piece. The complete size × colour rate matrix is on the Die Springs tab of our pricelist, and the product details are on our specialty parts page.
Need Die Springs?
Colour-coded die springs in all four load ratings, OD 10 - 63 mm, ex-stock Mumbai with same-week dispatch.
View Die SpringsFrequently Asked Questions
What do die spring colours mean?
The colour identifies the load rating of the spring: green is the lightest load, then blue (light), red (medium), and yellow (heavy). Springs of the same outer diameter and free length are dimensionally interchangeable across colours, so the coating tells you at a glance which duty class is fitted without measuring anything.
Are die spring colour codes universal?
The green-blue-red-yellow progression described here follows the ISO 10243 convention used across Europe and Asia. Be aware that some North American manufacturers use their own colour schemes, so when replacing an unknown spring, confirm the standard it was made to rather than matching colour alone.
Can I mix different colour die springs in one tool?
It is not recommended. Mixing load ratings in one stripper or pad makes the load distribution uneven, which tilts the plate and accelerates wear on guide pillars and the springs themselves. Use one colour class per function, sized so each spring works within its recommended deflection.
Why do die springs use rectangular wire?
Rectangular-section wire packs more material into the same coil envelope than round wire, so the spring delivers substantially higher force from the same housing bore and rod diameter. That is exactly what press tools need: maximum force in a confined, standardised space.