What Are the Commonly Used Steel Materials for Injection Molds?

金型鋼とは？なぜ工具寿命を決定するのか？

金型鋼材は、射出成形金型のコアとキャビティを加工する構造材料である。適切な選択は、金型寿命、表面仕上げ品質、サイクルタイム、生産期間全体の金型コストを決定する。

金型鋼材の選択は、CNC機械に手を付ける前に確定される。鋼材が発注され荒削りされた後、材質を変更するにはブロックを廃棄してやり直す必要があり、通常$3,000～$15,000の損失となる。選択を誤ったエンジニアは一度だけ代償を払う。正しく選択したエンジニアは、次の50万ショットまで鋼材のことを考えなくて済む。

中核となるトレードオフは単純です：硬度の高い鋼材は寿命が長く耐摩耗性に優れますが、加工や修復のコストが高くなります。 プレハードン鋼¹P20のような鋼材はHRC 28–36程度 — 高速フライス加工が可能な柔らかさでありながら、ほとんどの熱可塑性樹脂に十分な硬度を備えています。H13やS136のような通焼入れ鋼は熱処理後にHRC 45–55に達し、より多くの ³ 時間と注意が必要ですが、摩耗性樹脂や光学透明性が必要な場合の唯一の実用的選択肢です。

金型鋼材の選択を左右する4つの特性: ² （耐摩耗性）、靭性（耐割れ性）、 ⁴ （樹脂との化学的適合性）、および加工性（キャビティ加工と修復実行のコスト）です。単一の鋼材がこれら4つ全てを最大化することはできません — あらゆる選択は、特定の樹脂、成形数、表面仕上げ要件に合わせて調整されたトレードオフです。

P20：汎用金型の業界標準材

P20は、HRC 28–36で出荷される予備焼入れ低合金鋼で、追加熱処理なしで加工可能です。世界中の量産射出成形金型の約60%を占めています。あらゆるカテゴリーで最高の鋼材ではないものの、すべての面で十分な性能を持つためです。

P20はほとんどの汎用熱可塑性樹脂を問題なく扱います：ABS、PP、PE、PC、標準ナイロングレードはすべて、鋼材を侵食することなく成形可能です。プレハード化された状態は、標準的な超硬工具で直接CNCフライス加工が可能であり、微小な損傷をクラックを生じずに溶接修復でき、特殊な研磨なしにRa 0.4–0.8 µm範囲の表面仕上げを達成できることを意味します。最大400,000ショットまでの成形数では、P20が費用最適な選択肢です。

P20が不向きな場合：ガラス含有率20–30%以上の摩耗性樹脂では、20万ショットまでにP20キャビティが顕著に摩耗し、寸法変動や表面劣化を引き起こします。PA66-GF30やPEEKを加工する場合、P20は適していません。また、P20は耐食性がなく、PVCの塩化水素ガスにより生産開始数週間でキャビティ表面にピットが発生します。

718H（別名P20+Ni）はニッケル改質バリアントで、標準P20より二つの点で優れる：研磨性の向上（Ra 0.2 µmまで）とわずかに高い靭性。透明ABS成形や高キャビティ金型での表面均一性向上が必要な場合、P20より約8～12%高価だが検討の価値がある。

H13：大量生産および摩耗性樹脂向けの定番材

H13は熱間工具鋼で、真空焼入れ・焼き戻し後にはHRC 46–54に達します。この硬度では、ガラス繊維強化樹脂による摩耗に対してP20よりも約3倍耐性があります — これは当社の社内生産プログラムにおいてPA66-GF30およびPOMを使用して実証された知見です。

H13の主な利点は熱安定性です。クロム・モリブデン・バナジウム合金組成により、高温下での軟化に抵抗します。これは高速サイクルや、280°Cを超えるバレル温度を必要とする熱感受性樹脂にとって重要です。P20とは異なり、H13は連続多シフト生産中もクリープやキャビティ変形なく硬度を維持します。

H13におけるトレードオフは加工コストです。この鋼材は焼きなまし状態（HRC ~18–22）で加工され、その後真空熱処理に送られるため、製造工程が長く高コストになります：荒削りCNC → 中仕上げ → 熱処理 → 仕上げ研削 → 微細部の放電加工。同等のP20金型に比べ、金型コストは25–40%高くなると予想されます。

H13が適しているのは以下の場合です：(1) 樹脂に20%以上のガラスまたは鉱物充填材が含まれる、(2) 予想生産数が50万ショットを超える、(3) サイクル温度が常時280°Cを超える、(4) 部品に厳しい寸法公差が必要で、P20では長期生産で維持できない場合。

S136および420ステンレス：腐食や表面欠陥が許容できない場合

S136（AISI 420ステンレス鋼相当、Uddeholm呼称）は、13.6%のクロム含有量を持つマルテンサイト系ステンレス工具鋼で、熱処理後HRC 50–52を実現します。その主な機能は耐食性であり、工場の湿度だけでなく、PVC、POM、難燃樹脂の分解時に発生する酸に対する耐性も備えています。

S136が必要な3つのシナリオ：PVCの加工（バレル温度で塩酸を発生）、210°C以上でのPOM-CまたはPOM-Hの成形（ホルムアルデヒドガスが標準鋼材を侵食）、Ra ≤ 0.025 µmの鏡面研磨を必要とする光学グレード透明部品の製造です。S136は微細な炭化物組織により鏡面仕上げを達成・維持できますが、P20やH13では不可能です。

S136はP20よりもキログラム当たり50～70%高価であり、厳格な加工プロトコルを必要とします：溶接前の事前加熱、熱処理後の制御冷却、微細割れを防ぐための専用EDMパラメータなどです。しかし、240°CでPMMAを成形し、>92%の光透過率が要求されるレンズ金型では、代替材料はありません。このコストは任意ではなく、仕様の対価です。

重要な区別：S136は塩浴法よりも真空焼入れを好みます。真空焼入れはより清浄な表面酸化皮膜と優れた寸法安定性をもたらし、熱処理後の仕上げ加工余裕を削減します。発注書に真空焼入れを指定することは杓子定規ではありません。それは最終的な鏡面品質に直接影響します。

718HとNAK80：複雑形状用の焼入れ済み代替材

718H（718またはP20+Niとも表記）とNAK80（P21グレード）は、標準P20と完全焼入れ鋼種の間を埋めるプレハード鋼です。両方ともHRC 33–38で納入され、機械加工後の熱処理を必要とせず、リードタイムを1～2週間短縮し、複雑形状での熱処理歪みリスクを排除します。

718Hはニッケル含有量（約1%）により結晶粒組織が微細化され、標準P20よりも優れた研磨性を達成します。クリアABS、透明PC/ABSブレンド、またはSPI A3–B1仕上げを必要とする部品には、718Hがプレハード鋼の選択肢となります。NAK80はさらにAl、Cu、Sを添加することで時効硬化能力と優れたテクスチャ加工性を実現し、テクスチャ表面や粒目エッチングされた自動車内装パネルに最適です。

鋼材選定の意思決定フレームワーク：適切なグレードの選び方

鋼材選択の決定は、成形する樹脂と予想される生産量という2つの入力から連鎖的に導かれます。その他のすべて（予算、リードタイム、表面仕上げ）は、これら2つの軸を中心に調整されます。

A common mistake is selecting steel grade in isolation from the cooling system design. P20’s thermal conductivity of 29–36 W/(m·K) is already adequate for most cycle time targets — but if you’re switching to H13 (32–34 W/(m·K)) or S136 (24–28 W/(m·K)), review your cooling channel placement. S136’s lower thermal conductivity can increase cycle time by 5–10% versus P20 if channels are not relocated closer to the cavity wall.

A second factor engineers overlook is lead time. P20 is widely stocked at mold steel distributors across China, the US, and Europe — typical delivery is 3–5 business days. H13 in large block sizes (above 200mm thick) may require 10–15 days from Daido or Finkl. S136 (Uddeholm) has the longest lead time: 2–4 weeks for custom-cut blocks. If your project timeline is tight, confirm steel availability before finalizing the grade specification. Switching from S136 to 718H due to lead time adds 20–25% cost but can save 2–3 weeks of schedule.

Volume projection accuracy is the most underappreciated risk in steel selection. Customers who specify P20 based on a 200K shot estimate and then extend the program to 800K shots will face a mid-production cavity regrind or replacement. If there is real uncertainty about final production volume, bias toward H13. The incremental cost is 25–40% higher upfront, but it eliminates the risk of a $15,000–$40,000 cavity rebuild at year two of the program when demand unexpectedly grows.

Budget pressure often pushes engineers toward P20 when S136 or H13 is the right call. A decision framework that acknowledges real cost: if the mold is expected to run more than 800,000 shots and the resin is even mildly abrasive, the incremental cost of H13 over P20 typically breaks even at 300,000 shots through reduced regrind and rework costs. Model the total cost of ownership, not just the upfront tooling quote.

One more steel selection variable engineers undervalue: weld repairability. P20 can be welded with standard MIG or TIG process without preheat above 150°C. H13 requires 300–400°C preheat, careful interpass temperature control, and post-weld temper — each repair step adds cost and extends tooling downtime. S136 requires even more precise thermal management to avoid martensite cracking. For molds expected to undergo frequent engineering changes or cavity repairs, the weld repair protocol matters as much as the base hardness specification.

Cavity surface area also influences steel cost more than most engineers realize. A 300×200mm cavity block in S136 can cost $2,000–$4,000 in raw steel before a single tool path is run. At P20 pricing, the same block runs $600–$900. On a 32-cavity hot runner tool, that $3,000 difference per block compounds quickly. For high-cavity tools where only a subset of cavities are high-wear, consider using S136 for the first few cavities while running P20 on the remainder — a hybrid approach that reduces total tooling cost by 20–30% while maintaining surface quality where it matters.

Mold steel selection also interacts with the cooling system design. Conformal cooling channels machined by metal AM (additive manufacturing) or gun-drilling are only economical in P20 and H13; S136’s high chromium content makes it more challenging for deep-hole drilling with standard equipment. If your cooling design requires channel diameters below 6mm at depths beyond 150mm, verify your toolmaker’s equipment capability against the specified steel grade before committing to the design.

表面処理オプション：窒化処理、PVD、ハードクロム

Surface treatment extends the working life of any mold steel by adding a wear-resistant layer without changing the bulk material. The three most common treatments in injection mold practice are gas nitriding, PVD (Physical Vapor Deposition) coating, and hard chrome plating.

Gas nitriding penetrates 0.1–0.3mm into the steel surface, creating a nitrogen-diffused case hardness of HRC 65–70 without dimensional change. It works on P20 and H13; it does not work well on stainless grades because the chromium oxide layer blocks nitrogen diffusion. Nitriding adds approximately $200–$800 per cavity set and extends P20 tool life in mildly abrasive conditions by 40–80%.

PVD coatings (TiN, TiAlN, CrN) apply a 2–5 µm hard layer at temperatures below 500°C, preserving the base steel’s temper. TiN is gold-colored, HRC 80+, and improves wear and release simultaneously. CrN provides better corrosion resistance than TiN. PVD is the preferred treatment for slides, cores, and other moving inserts that experience cyclic contact stress — it reduces galling risk and can be reapplied 2–3 times before the insert must be replaced.

Hard chrome plating (0.02–0.1mm thickness) is the legacy option — cheaper than PVD, but it’s being phased out in regulated markets due to hexavalent chromium toxicity. In applications where PVD is not available locally, hard chrome remains viable but requires more frequent inspection for micro-cracking under cyclic load.

ZetarMoldの鋼材調達と品質基準

ZetarMold sources mold steel exclusively from verified mills: Uddeholm (Sweden) for S136, ASSAB for 718H and 718S, and Daido (Japan) or Finkl Steel (USA) for H13. P20 is sourced domestically from Baoshan Iron & Steel (Baosteel) with incoming material inspection on each batch.

Every steel block goes through incoming hardness verification (±2 HRC tolerance) and ultrasonic testing for internal inclusions before being released to the CNC department. Steel that fails ultrasonic inspection is returned — regardless of lead time pressure. A cavity that cracks from an inclusion at 50,000 shots costs more than the three-day delay of getting a clean block.

Our heat treatment is performed in-house or at a certified partner using vacuum furnaces to avoid decarburization. Temper temperature and cycles follow Uddeholm and ASSAB’s published datasheets — not approximations. For S136, we verify final hardness via Rockwell testing at three cavity zones (gate area, end-fill, and parting line) and document the results in the mold data pack provided to the customer.

For customers who supply their own steel, we require mill certificates with heat number traceability, incoming hardness verification, and ultrasonic certification. We’ve declined to proceed with un-certified customer-supplied steel on three occasions in the past two years — in each case, the customer later discovered inclusions in the block during machining. The inspection overhead is not bureaucracy; it is loss prevention.

よくある質問

射出成形用金型に最適な鋼材は何ですか？

P20 is the best default for general production molds running standard thermoplastics (ABS, PP, PC) up to 400K shots. H13 (HRC 46–54) is the right choice for glass-filled engineering resins or volumes above 500K shots, where P20 wears too fast. S136 (420 stainless) is mandatory for optical-grade parts requiring mirror polish and for corrosive resins like PVC or POM. 718H fills the gap between P20 and H13 for complex geometries that cannot tolerate heat treat distortion. There is no single best — match the steel to the resin, volume, and surface finish specification.

P20とH13金型鋼の違いは何ですか？

P20 is pre-hardened at HRC 28–36, delivered ready to machine with no heat treatment required. It is cost-effective, easy to weld-repair, and adequate for most unfilled thermoplastics up to 500K shots. H13 is a hot-work tool steel that must be vacuum-hardened to HRC 46–54 after roughing — a process that adds 1–2 weeks lead time and 25–40% cost. In return, H13 delivers roughly 3× better wear resistance against glass-filled resins and maintains dimensional stability well past 1 million shots. Choose P20 for standard volume; choose H13 when abrasion or longevity demands it.

射出成形金型にS136ステンレス鋼はいつ使用すべきですか？

S136 is required in three situations: processing corrosive resins that generate acidic off-gas during molding (PVC releases HCl, POM releases formaldehyde), producing optical-grade transparent parts that need Ra ≤ 0.025 µm mirror polish (lens molds, display covers), and molds stored in humid environments without consistent preventive maintenance. Its 13.6% chromium content provides corrosion resistance that P20 and H13 cannot match. Expect to pay 50–70% more in raw steel cost and 35–40% more in machining time versus P20, but for these use cases, S136 is not optional.

How does mold steel hardness affect surface finish quality?

Higher steel hardness allows finer polishing and holds that finish through more production cycles. P20 at HRC 30 is polishable to Ra 0.4–0.8 µm (SPI B2 range), suitable for semi-gloss consumer parts. H13 at HRC 50 reaches Ra 0.1 µm (SPI A3) with careful bench polishing. S136 at HRC 52 achieves Ra ≤ 0.025 µm (SPI A1, mirror grade) — required for PMMA lenses and optical PC parts. Each step finer requires progressively finer diamond abrasive sequences and 2–4× more polishing labor, adding $500–$3,000 per cavity depending on cavity size and geometry.

窒化処理やPVDコーティングは、より硬い金型鋼へのグレードアップを代替できるでしょうか？

For mildly abrasive resins (10–20% GF) and moderate production volumes under 300K shots, gas nitriding a P20 cavity can extend service life meaningfully — it adds HRC 65+ surface hardness for $200–$800 per cavity set versus $3,000–$8,000 to upgrade to H13. PVD coatings (TiN, CrN) work similarly for sliding components. However, these surface treatments are 2–5 µm thick and 0.1–0.3mm deep respectively. For heavy abrasion with >30% GF content, or for volumes above 500K shots, the treatment layer wears through and the softer base steel erodes underneath. Surface treatments complement good steel selection; they do not replace it.

ZetarMoldは標準生産工具にどのような金型鋼を使用していますか？

ZetarMold defaults to P20 (Baosteel certified) for standard production molds running unfilled thermoplastics at volumes up to 500K shots. For glass-filled engineering resin programs including PA66-GF30, POM, and PBT-GF30, we specify H13 sourced from Daido or Finkl Steel with vacuum hardening to HRC 48–52. Optical and corrosion-sensitive applications receive Uddeholm S136 with in-house hardness verification and ultrasonic inspection before cavity machining begins. All incoming steel batches undergo hardness spot-check and ultrasonic testing for internal inclusions — regardless of supplier certification documents provided.

金型鋼材の選択は射出成形サイクルタイムにどのように影響しますか？

Thermal conductivity is the key variable linking steel grade to cycle time. P20 conducts heat at 29–36 W/(m·K); H13 at 32–34 W/(m·K) is roughly comparable. S136, however, runs at 24–28 W/(m·K) — about 15–20% lower than P20. On a thin-wall part where cooling time accounts for 60–70% of total cycle time, switching from P20 to S136 without repositioning coolant channels can increase cycle time by 5–10%. Compensate by reducing coolant channel-to-cavity wall distance from the standard 15mm to 10–12mm, maintaining adequate channel diameter to prevent flow restriction.

1. pre-hardened steel: Pre-hardened steel is a mold steel that has been heat-treated to a working hardness (typically HRC 28–40) before delivery, eliminating the need for post-machining heat treatment. ↩

2. hardness: Hardness is a material property measured on the Rockwell C (HRC) or Brinell (HB) scale that indicates resistance to permanent surface deformation; higher HRC values mean greater wear resistance but reduced toughness. ↩

3. EDM： EDM (Electrical Discharge Machining) refers to a manufacturing process that removes material from a workpiece using controlled electrical discharges, commonly used to machine hard mold steels into precise cavity shapes. ↩

4. corrosion resistance: Corrosion resistance is defined as a material’s ability to withstand oxidation, chemical attack, and moisture-induced degradation; measured by weight-loss tests or salt-spray hours in mold steel contexts. ↩

5. thermal conductivity: Thermal conductivity is measured in W/(m·K) and refers to a material’s ability to transfer heat; higher values in mold steel result in faster cycle times by improving heat extraction from the molded part. ↩