Last updated: 2026-09-04 · 5 min read

Mold Steel Hardness Testing — How Do You Verify the Hardness of Mold Steel, What Methods Are Available, and When Should You Reject a Mold Based on Hardness Readings?

Mold steel hardness determines resistance to wear, deformation and surface damage. Verification is simple: compare the hardness reading to the steel mill certificate. Within ±2 HRC — accepted. More than 3 HRC below — reject the cavity; the heat treatment was improper. Three readings (surface, mid-thickness, back) reveal whether hardening was uniform. A spread over 3 HRC means uneven hardening — soft spots will wear faster than hard spots.

Key Facts

Typical hardness specs: P20 HRC 28–32, H13 HRC 48–52, 718 HRC 30–36, S136 HRC 52–56. Acceptance: within ±2 HRC of the steel mill certificate. Reject if more than 3 HRC below.

Three testing methods: Rockwell C (HRC, ±1 HRC, industry standard), Brinell (HB, ±5 HB, for softer pre-hardened steels), Leeb portable (HL, ±2–4 HRC after conversion, for on-mold verification only).

Too soft: premature wear, flash, texture degradation within 50,000–100,000 shots. Too hard: brittle cracking at corners and cooling channel intersections. Uneven hardness: soft spots wear faster than hard spots.

Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

What Are the Testing Methods and Which One Should You Use?

Three methods, each for a different situation:

Method	How It Works	Accuracy	When to Use
Rockwell C (HRC)	Diamond cone indenter, 150 kg load, measures penetration depth	±1 HRC	Industry standard for hardened mold steels (HRC 20–65)
Brinell (HB)	10mm tungsten carbide ball, 3,000 kg load, measures indentation diameter	±5 HB	Softer pre-hardened steels (P20, 718); converts to HRC via table
Leeb portable (HL)	Impact body bounces off surface, measures rebound velocity	±2–4 HRC (converted)	On-mold testing when the cavity insert cannot be removed

Rockwell C is the standard method — it is specified on the steel mill certificate and the mold maker’s material report. The Brinell test is used for pre-hardened steels below HRC 20. The Leeb portable tester is used when the cavity cannot be removed from the mold base — it is handheld and the reading is converted to HRC via an internal table. The Leeb method has a wider error band and should only be used for verification, not for final acceptance.

The Rockwell test leaves a small indentation (about 0.2mm diameter) — on a polished surface (SPI A1–A2), the mark is visible and requires re-polishing. On a textured surface, the indentation is hidden by the texture but may affect uniformity. For polished or textured cavities, use a Leeb tester on the cavity surface (non-destructive) and a Rockwell reading on the back of the insert or witness coupon — the two should agree within ±3 HRC.

What Are the Acceptance Criteria and Where Should You Take the Readings?

Three rules, in order:

Compare to the steel mill certificate. The certificate states the hardness range for the specific heat treatment lot. H13 tempered at 560°C should read HRC 48–52. If the mold maker’s reading is within ±2 HRC of the certificate, the steel is accepted. If more than 3 HRC below, the steel was not properly heat-treated — reject and request re-heat-treatment.
Take readings at three locations on each cavity insert. Hardness is not uniform across a steel block — the surface may be harder (decarburization during heat treatment) and the core may be softer (incomplete hardening). One reading at the cavity surface, one at mid-thickness, one at the back. The three readings should be within 3 HRC of each other. If the spread exceeds 3 HRC, the heat treatment was uneven — the cavity may wear unevenly or crack at soft spots.
Test on a flat, ground surface. The Rockwell test requires a flat, smooth surface (Ra below 0.8 μm) for accurate readings. A rough or textured surface gives false low readings because the indenter does not make full contact. If the cavity surface is textured or polished, take the reading on the back of the insert or on a witness coupon — a small piece of the same steel block that was heat-treated with the cavity.

Condition	What Happens	Fix
Too soft (more than 3 HRC below spec)	Premature wear: cavity dimensions drift, parting line flash, texture degrades within 50,000–100,000 shots	Re-heat-treatment: $500–2,000 per insert plus risk of distortion. If already polished or textured, re-machining from scratch.
Too hard (more than 3 HRC above spec)	Brittle steel: cracks at sharp corners, ejector pin holes, cooling channel intersections under thermal cycling	Re-tempering: heat to lower temperature to reduce hardness. Less risky than re-hardening — no full austenitization, less distortion.
Uneven (spread more than 3 HRC)	Soft spots wear faster than hard spots, creating an uneven cavity surface	Re-heat-treatment (full cycle). If mild (2–3 HRC spread), use with increased inspection frequency — measure dimensions every 25,000 shots instead of 50,000.

Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. The related guide covers adjacent topics.

Hardness verification is a low-cost check ($100–300 per mold) that prevents expensive failures. The hardness check should be part of every mold acceptance package.

Key parameter identified — critical variable confirmed against spec.

Tolerance within achievable range — commercial ±0.15mm or tighter as needed.

Material matched to application — resin grade selected for end-use environment.

Process window validated at T1 — parameters locked after first trial.

Inspection method defined — CMM, visual, or functional test specified.

Frequently Asked Questions

How often should you check mold steel hardness?

At three points in the mold life. First, after heat treatment — before the cavity is machined, the mold maker verifies hardness against the steel mill certificate. Second, after mold build — the customer or a third-party inspector verifies hardness on the finished insert (back surface or witness coupon). Third, during mold life — if the mold shows premature wear (flash, dimensional drift, texture degradation), a hardness check determines whether the steel has softened from thermal cycling or whether the wear has another cause.

For standard molds, the one-time check at mold build is sufficient. For molds running abrasive resins (GF30+, mineral-filled) or operating at high cycle temperatures, periodic checks every 200,000–500,000 shots detect softening before it causes quality problems.

Can you check hardness on a textured or polished surface without damaging it?

The Rockwell test leaves a visible indentation on polished surfaces (SPI A1–A2) — the mark requires re-polishing to remove. On textured surfaces, the indentation is hidden by the pattern but may affect texture uniformity.

The recommended approach: use a Leeb portable tester on the cavity surface (non-destructive, no visible mark) and a Rockwell reading on the back of the insert or witness coupon. The Leeb reading has a wider error band (±2–4 HRC) but is non-destructive. The two readings should agree within ±3 HRC. For final acceptance, the Rockwell reading on the back or coupon is the authoritative value.

Does hardness change during the mold’s life?

Yes — the most common change is softening (loss of 1–3 HRC over 500,000–1,000,000 shots) caused by temper back. The repeated thermal cycling (mold at 80–120°C, melt at 200–320°C) acts as a low-temperature temper that gradually reduces hardness. Softening accelerates when the mold runs hot (above 150°C) or when the steel was tempered at a low temperature during the original heat treatment.

The fix is preventive: specify a higher tempering temperature during the original heat treatment. H13 tempered at 600°C (HRC 46) is more resistant to temper back than H13 tempered at 540°C (HRC 50), even though the initial hardness is lower. The more stable microstructure resists softening over the mold’s life.

Key Takeaways

Rockwell C (HRC) is the standard; Leeb portable for on-mold verification only.

Acceptance: within ±2 HRC of steel mill certificate; reject if more than 3 HRC below.

Three readings per insert (surface, mid, back) — spread must be within 3 HRC.

Test on flat ground surface — not on textured or polished surfaces.

Hardness verification is a low-cost check that prevents expensive failures — premature wear, cavity cracking, flash at the parting line.
The hardness check should be part of the mold acceptance package: the mold maker provides the readings on the material report, and the customer verifies them before the mold ships. If the hardness is out of spec, the mold should not be accepted until the steel is re-heat-treated and re-verified.