Shank-reduction (束杆) dies are among the most stress-intensive tools in cold heading. The extrusion ratio determines whether the material flows smoothly or cracks under excessive deformation. Getting this number right is essential for die longevity and part quality.
The extrusion ratio (λ) is the ratio of the original cross-sectional area to the reduced cross-sectional area. It quantifies how much the material is being compressed during the shank-reduction process.
λ = A₀ / A₁ = D₀² / D₁²
Where A₀ = original area, A₁ = reduced area, D₀ = original diameter, D₁ = reduced diameter
Consider an M10 flange bolt where the shank is reduced from Ø10 mm to Ø8 mm:
This is a moderate reduction — well within the safe range for most carbon and alloy steels.
| Material | Safe λ Range | Max λ (single pass) |
|---|---|---|
| Low carbon steel (SWRCH 8K, 10K) | 1.2 – 1.8 | 2.0 |
| Medium carbon steel (SWRCH 35K, 45K) | 1.1 – 1.5 | 1.6 |
| Alloy steel (SCM, SNCM) | 1.1 – 1.4 | 1.5 |
| Stainless steel (SUS304, SUS316) | 1.1 – 1.3 | 1.4 |
If the required reduction exceeds the safe λ for the material, consider:
At BENC MOULD, we engineer shank-reduction dies with the extrusion ratio as the primary design parameter. The taper angle, land length and surface finish are all optimised based on the calculated λ and the specific material being formed. This approach consistently delivers die life exceeding 500,000 hits on standard carbon steel applications.
Send us your bolt drawing — we’ll calculate the optimal die geometry for your application.