TOOLING · August 29, 2026 · 8 MIN READ

Die Design Standards for Hot Forging: Draft, Fillet Radii, Shrinkage and Parting Line

A hot forging die is not a negative of the finished part. It carries 1.2–1.5 % shrinkage allowance for steel, 3–7° draft on every vertical wall, fillet radii of at least 2 mm inside and 1 mm at the flash land, and a parting line placed where flash removal will not cut a functional surface. Get any one of these wrong and the part is dimensionally correct on the drawing and unusable in the press.

WHERE THIS APPLIES AT FORGEPRESS CHINA

ForgePress China issues die layouts with shrinkage allowance, draft and parting line fixed before cutting; first article 4–8 weeks from die design release. Capacity: HKM3 hot-die forging line 1,000–5,000 t, part weight 0.1–500 kg, maximum envelope 1,200 × 800 mm, as-forged to ISO 8062 class C with critical dimensions machined to ±0.2 mm, process control to ISO 9001, first article 4–8 weeks from die design release.

KEY NUMBERS

Shrinkage allowance: 1.2–1.5 % for carbon and low-alloy steel, 1.5–2.0 % for stainless, 1.0–1.3 % for aluminium.
Draft angle 3–7° per side; 5° is the default, 7° for deep pockets, 3° only for shallow walls under 10 mm.
Inside fillet radius ≥ 2 mm, outside corner radius ≥ 1 mm, flash land thickness 2–4 mm.
Die block insert fit H7/m6; shut-off surfaces need 60–70 % contact on blue check before first heat.

Shrinkage is not one number

The die cavity is cut oversize so the part contracts to nominal dimensions. For carbon and low-alloy steel the accepted range is 1.2–1.5 %, but the correct value depends on the section: a 5 mm rib cools and contracts less in absolute terms than a 60 mm boss on the same part, and a single uniform allowance will make the thin sections oversize and the thick ones undersize.

For parts with section ratios above 3:1, the die is usually cut with a graded allowance — 1.2 % on thin sections, 1.5 % on heavy ones — and the first article is measured to confirm. For stainless the allowance rises to 1.5–2.0 % because of higher thermal contraction and greater springback on ejection.

Material	Linear shrinkage allowance	Where the range comes from
Carbon steel (C45, 20#)	1.2–1.4 %	Lower carbon, lower contraction
Low-alloy steel (40Cr, 42CrMo)	1.3–1.5 %	Alloy content raises contraction
Austenitic stainless (304, 316L)	1.5–2.0 %	High thermal expansion, high flow stress
Aluminium (6061, 6082)	1.0–1.3 %	Low contraction but narrow temperature window
Titanium (Ti-6Al-4V)	0.8–1.0 %	Low expansion, but severe die wear

Values apply to closed-die hot forging with die preheat at 200–300 °C. Confirm with a first-article measurement before committing to production.

Draft angle: the number that decides whether the part leaves the die

Every vertical wall needs draft so the forging can be ejected. The minimum is not a matter of taste: it is set by the ejection force available, the depth of the wall, and the amount of shrinkage grip. As a starting rule, use 5° on walls up to 40 mm deep, 7° on walls deeper than 40 mm, and 3° only on shallow walls under 10 mm where the wall is also a machining allowance.

Draft must be added on the die drawing and then removed in the machining drawing. The most expensive die design error is a forging whose draft is applied in the wrong direction on an internal wall: the part cannot be ejected, and the die must be recut.

Fillet and corner radii: where forging dies crack

Sharp internal corners are stress raisers in the die, not in the part. A die corner below 2 mm radius will heat-check and crack within a few thousand cycles because the thermal gradient concentrates there. The practical minimum is 2 mm inside fillet for a 1–2 kg part, scaled upward with part size: 3 mm for 5 kg, 5 mm for 20 kg.

Outside corners need at least 1 mm so the flash land can be machined without a feather edge, and the flash land itself is normally 2–4 mm thick with a land width of 15–30 mm. Thinner flash builds cavity pressure faster but wears and cracks sooner.

Parting line placement

The parting line should sit on a surface that will be machined, or on a non-functional face, and it should be as close to a single plane as the part allows. A stepped parting line adds die cost and creates a mismatch risk at every step.

Two constraints override aesthetics. First, the parting line must not run across a sealing surface or a bearing seat, because flash leaves a witness mark that will not clean up within the machining allowance. Second, it should be positioned so that the deeper half of the impression is in the lower die, where gravity helps location and where the ejector can reach.

Die construction tolerances worth specifying

Insert fit H7/m6 for shrink-fit assembly, with the shrink temperature documented.
Shut-off surfaces: 60–70 % blue contact before the first heat, 80 % after run-in.
Impression alignment: upper to lower mismatch ≤ 0.10 mm on a 500 mm die.
Flash land thickness tolerance ±0.10 mm; land width ±1 mm.
Ejector clearance 0.15–0.25 mm, with the ejector face flush to ±0.05 mm.

Frequently asked questions

What draft angle do hot forgings need?

Three to seven degrees per side. Use 5° as the default, increase to 7° for pockets deeper than 40 mm or for stainless and high-temperature alloys, and only go to 3° on shallow walls under 10 mm that will be machined afterwards.

How much shrinkage allowance should I add?

For carbon and low-alloy steel, 1.2–1.5 % linear. Apply 1.2 % to thin sections and 1.5 % to heavy sections on parts with section ratios above 3:1, then verify on the first article. Stainless needs 1.5–2.0 %.

Can the parting line be on the machined face?

That is the preferred position, provided the machining allowance is large enough to remove the flash witness mark completely — typically 1.5–3 mm depending on part size. Putting the parting line on a functional surface without allowance is a design error that no die maker can fix.