Die Springs

Rectangular-wire die springs for mould, die and general compression work. Rectangular section stores more energy in the same installed space than round wire, which is why moulds use it — and it is also why die springs are less forgiving of over-compression than an ordinary coil spring.

Colour codes are not universal — check the standard first

Two systems are in common use and they disagree. Under the Japanese system (JIS, the one most Asian-made springs follow) yellow is extra light, blue is light, red is medium, green is heavy and burgundy is extra heavy. Under ISO 10243, used by most European die sets, green is light, blue is medium, red is heavy and yellow is extra heavy. Yellow and green are exact inversions between the two; blue and red shift a class. A yellow spring is the lightest in the box under one standard and the heaviest under the other. Never order by colour alone — order by outside diameter, free length and required load, and confirm which standard the supplier is working to.

Maximum compression depends on the life you need

Published maximum compression figures are quoted against a cycle count, and the two numbers are often confused. For a JIS extra-light (yellow) spring, roughly 50% of free length is the figure for about 300,000 cycles, but for one million cycles it drops to about 40%. For JIS light (blue) the same pair is about 40% at 300,000 cycles and about 32% at one million. If you are speccing for a long-run production tool, design to the one-million-cycle figure. Springs that break early in service have usually been sized against the short-life number.

Sizing in practice

Work from the load you need at the working stroke, not from free length. Preload matters: a spring installed with some initial compression gives a more consistent return force through the stroke, but that preload counts towards the compression limit. Leave room for the solid height. Where several springs share a plate, the plate will tilt if their rates differ, so keep them the same batch and the same series rather than mixing colours to reach a total force.

Why die springs fail

In order of frequency: over-compression against the wrong cycle figure; a bore or guide rod that is too tight, so the spring rubs and fatigues at the contact point; heat, since rate falls and relaxation rises as the mould temperature climbs; and side loading from a plate that does not run square. A spring that has taken a permanent set — shorter free length than when new — has already been over-compressed and will not recover.

What to send with an enquiry

• Outside diameter, inside diameter or rod diameter, and free length
• Working stroke, preload and the load you need at the working point
• Expected cycle life and operating temperature
• Which standard your colour reference follows, if you quote one
• Quantity and destination