Injection Mold Steel Selection Guide: P20, H13 & S136

Why Mold Steel Selection Decides Your Part Quality and Tooling Cost

The spec sheet looked fine. The resin was approved. The wall thickness passed DFM¹. But when the first shots came off the press, the surface looked like someone had dragged sandpaper across it. We traced it back to a P20 cavity that had been specified for a 30% glass-filled nylon job. The mold shop had used what they had on the shelf. Three weeks of re-polishing later, the customer had moved to a competitor. That was the last time my team skipped a steel conversation at kickoff.

Most injection mold failures trace back to a mismatch between the steel grade and either the resin’s abrasiveness, its chemical aggression, or the volume demand on the tool. Choosing the wrong steel does not just affect surface quality — it accelerates wear on parting lines, clogs vents with corrosion products, and can halve the tool’s usable life. The cost difference between a right-first-time steel decision and a retrofit repair is typically 3× to 5× the original steel premium.

acero para moldes² comparison blocks” style=”max-width:100%;height:auto;” />

P20 Mold Steel: Best Choice for Budget and Medium-Volume Production

P20 is a pre-hardened chromium-molybdenum acero para herramientas³ delivered at HRC 28–33, which means you can machine it directly without a heat-treat cycle after roughing. That saves 5–10 days on the build schedule and eliminates the distortion risk that comes with post-machining heat treatment. At our factory, we run P20 on roughly 60% of consumer-product tooling where the resin is unfilled ABS, PP, or PE and the annual volume is below 500,000 shots per cavity.

The trade-off is hardness. At HRC 30, P20 will show visible wear on parting lines and gate areas after roughly 300,000–500,000 shots with mildly abrasive resins. For fully unfilled commodity resins, the same tool can reach 800,000–1,000,000 shots before requiring a re-polish or gate repair. P20 also accepts textured finishes (EDM grain, VDI 24–36) reasonably well, but mirror polishing below VDI 12 is difficult because the lower carbon content limits achievable surface hardness.

P20 Mold Steel — Key Properties at a Glance

Propiedad	P20 Value	Practical Implication
Hardness (as-supplied)	HRC 28–33	No post-machining heat treat needed
Tensile strength	~1,000 MPa	Good for standard injection pressures up to 1,400 bar
Max recommended volume	500,000–1,000,000 shots	Depends on resin abrasiveness
Polishability	VDI 12–18 (satin)	Not suitable for optical or Class A mirror finish
Resistencia a la corrosión	Bajo	Requires rust preventive in storage; avoid PVC/POM
Relative tool cost (single cavity)	1.0× (baseline)	Most economical steel grade

One decision point that catches engineers off guard: P20 is often available in a nitrided variant (P20+Ni or P20H) where the surface is case-hardened to HRC 50–55 while the core stays soft. This gives better wear resistance at gates and parting lines without adding a full heat-treat cycle. We specify this variant on P20 tools expected to run 600,000–900,000 shots with lightly filled resins (up to 10% glass fiber).

H13 Tool Steel: The Right Answer for High-Temperature Resins and Abrasive Fills

H13 is a chromium-molybdenum-vanadium hot-work tool steel that reaches HRC 48–52 after vacuum hardening and tempering. The vanadium content forms hard carbides that resist abrasive wear from glass fiber, mineral filler, and carbon fiber reinforcements. When a customer comes to us with a 30% GF nylon 66 part that needs 2 million shots, H13 is almost always the steel we specify — not because it is the only option, but because it hits the best balance of wear resistance, toughness, and machinability at that volume tier.

H13’s other strength is thermal fatigue resistance. The molding temperature for PPS, PEI (Ultem), and LCP often exceeds 300°C. At these temperatures, the repeated thermal cycling of injection and cooling can crack a softer steel at thin ribs or sharp corner radii within 200,000 cycles. H13’s high chromium and molybdenum content reduces thermal expansion coefficient variation, giving it roughly 3× better thermal fatigue life than P20 under identical conditions. In our factory, we run H13 on any mold where the melt temperature exceeds 280°C or the glass-fiber content is above 15%.

H13 Tool Steel — Key Properties at a Glance

Propiedad	H13 Value	Practical Implication
Hardness (heat-treated)	HRC 48–52	High wear resistance at gate and parting line
Tensile strength	~1,600 MPa	Maneja altas presiones de inyección y fuerzas de cierre
Max recommended volume	1,000,000–2,000,000+ disparos	Ideal para producción de alto volumen con resinas abrasivas
Polishability	VDI 6–12 (semi-brillante)	Buena pero no acabado de espejo grado óptico
Resistencia a la corrosión	Moderado	No adecuado para PVC; aceptable para la mayoría de las otras resinas
Relative tool cost (single cavity)	1,5×–2,0× P20	Costo más alto de acero + tratamiento térmico, menor mantenimiento por disparo

Una nota práctica desde el taller: H13 requiere un ciclo adecuado de liberación de tensiones después del mecanizado preliminar y antes del endurecimiento final. Omitir este paso es la causa más común de fractura de moldes H13 que observamos en talleres de herramientas que intentan reducir el tiempo de entrega. El ciclo de liberación de tensiones (550–600°C durante 2 horas por cada 25 mm de grosor de sección) añade 2–3 días pero previene la distorsión durante el endurecimiento en vacío. Hemos visto herramientas fracturarse en las bases de nervios dentro de las primeras 50,000 inyecciones cuando se omitió este paso — una factura de reparación de $15,000+ que hace que el ahorro de tiempo parezca absurdo.

S136 Stainless Mold Steel: Mandatory for Corrosive Resins and Optical Surfaces

S136 (equivalente al acero inoxidable modificado AISI 420) contiene aproximadamente 13% de cromo, lo cual proporciona protección contra corrosión por capa pasiva de óxido frente a gases ácidos de PVC, retardantes de llama halogenados, POM (acetal) y algunos poliuretanos. La resistencia a la corrosión no es solo cosmética — el ataque ácido en la superficie del cavidad crea micro-picaduras que se transfieren directamente a la superficie de la pieza, generando defectos que no pueden pulirse sin re-mecanizar la cavidad.

El S136 también alcanza la mayor pulibilidad de cualquier acero para moldes común, llegando a VDI 0-3 (acabado espejo, Ra 0,01-0,02 µm) cuando se procesa correctamente. Esto es esencial para lentes ópticas, guías de luz, carcasas de dispositivos médicos y cualquier pieza que requiera transparencia cosmética Clase A. En nuestra fábrica, usamos S136 exclusivamente para todas las herramientas de dispositivos médicos, piezas transparentes de PC y PMMA, y cualquier aplicación que involucre resinas de PVC o POM. La superficie pulida a espejo en una cavidad de S136 puede mantenerse durante 500.000-1.000.000 de disparos con protocolos adecuados de desmoldeo y limpieza.

Acero para Moldes Inoxidable S136 — Propiedades Clave de un Vistazo

Propiedad	Valor S136	Practical Implication
Hardness (heat-treated)	HRC 48–52	Misma dureza que el H13, mejor resistencia a la corrosión
Chromium content	~13%	La capa de óxido pasiva resiste los gases ácidos del PVC, POM, FR-ABS
Pulibilidad máxima	VDI 0–3 (espejo, Ra 0,01 µm)	Requerido para lentes ópticos, guías de luz, carcasas transparentes
Resistencia a la corrosión	Alto (acero inoxidable)	Adecuado para PVC, POM, resinas de grado médico
Max recommended volume	500,000–1,000,000 shots	Ligeramente menor tenacidad que el H13 en altos números de ciclos
Relative tool cost (single cavity)	2.0×–2.8× P20	Costo de material más alto; compensado por menor frecuencia de reparación superficial

La compensación con el S136 es la tenacidad. Es ligeramente más frágil que el H13 al mismo nivel de dureza, lo que significa que es más susceptible a astillarse en bordes de nervaduras delgadas (por debajo de 0,5 mm) o en ranuras profundas y estrechas. Diseñamos moldes de S136 con una relación mínima nervadura-profundidad de 1:6 (espesor de nervadura:profundidad) y añadimos un radio de 0,3 mm en todas las esquinas internas afiladas. Estos cambios de diseño añaden 2-4 horas de mecanizado pero previenen fallos por fragilidad en servicio. Para el diseño de moldes de inyección⁴ equipo, este es un punto crítico de DFM.

Steel Grade Comparison: P20 vs H13 vs S136 Side by Side

Cuando los ingenieros nos preguntan qué acero usar, la respuesta casi siempre se reduce a tres preguntas en este orden: ¿Cuántos disparos? ¿Qué resina? ¿Qué acabado superficial? La tabla siguiente codifica nuestros 20 años de experiencia en fábrica en una comparación directa. Observe que ningún grado es universalmente superior — cada uno ocupa un nicho operativo distinto.

P20 vs H13 vs S136 — Matriz de Comparación Completa

Criterion	P20	H13	S136
Dureza (HRC)	28–33 (pre-endurecido)	48–52 (tratado térmicamente)	48–52 (tratado térmicamente)
Mejor para volumen	Hasta 500,000 disparos	500K–2M+ disparos	Hasta 1 millón de disparos
Resistencia a la abrasión	Low–Medium	Alta	Medium–High
Resistencia a la corrosión	Bajo	Moderado	Alto (acero inoxidable)
Capacidad de pulido espejo	Medio (VDI 12–18)	Buena (VDI 6–12)	Excelente (VDI 0–3)
Vida a fatiga térmica	Bien	Excelente	Bien
Resinas recomendadas	ABS, PP, PE, PS (sin carga)	Nylon con GF, PPS, PEI, LCP, PC	PVC, POM, FR-ABS, PMMA, PC óptico
Tratamiento térmico post-mecanizado	No requerido	Required (vacuum)	Required (vacuum)
Lead time impact	Shortest (+0 days)	Moderate (+5–10 days)	Moderate (+5–10 days)
Relative cost index	1.0×	1.5×–2.0×	2.0×–2.8×

A common trap: engineers see that H13 and S136 have the same HRC range and conclude they are interchangeable. They are not. H13’s vanadium carbides make it superior against abrasive wear; S136’s 13% chromium makes it immune to corrosive attack. Running PVC in an H13 mold will produce pitting corrosion within 50,000 shots. Running 40% GF nylon in an S136 mold will cause accelerated surface wear because S136 lacks the vanadium carbide phase that gives H13 its abrasion edge.

How to Select Mold Steel in 3 Steps: A Practical Decision Framework

Step 1: Qualify the Resin Chemistry

The first filter is always chemistry. Check the resin’s technical data sheet for off-gas products, recommended processing temperature, and any corrosion warnings. PVC, halogen-containing flame retardants, acetal (POM), and moisture-absorbing engineering resins that decompose at high temperatures all produce acids during processing. Any resin that generates HCl, HBr, HF, or formaldehyde off-gases during normal processing requires S136 or at minimum 2316 stainless steel. No exceptions — we have never seen P20 or H13 survive 200,000 shots in a PVC application without visible cavity pitting.

For optical or medical applications, even if the resin is not chemically aggressive, the surface finish requirement drives you to S136. Polycarbonate for light guides, PMMA for lenses, and COC/COP for medical vials all require Ra values below 0.025 µm — a level only achievable with S136 and a skilled polisher with 6–8 hours on the cavity surface.

Step 2: Establish the Production Volume Target

Volume defines how much wear resistance you need to pay for. If the program is a bridge tool for 50,000–100,000 shots while the production tool is being built, P20 is almost always the right call — the savings on tooling cost fund the production tool. If the program is 3 million shots over 5 years, H13 or S136 is not optional: rebuilding a P20 mold at 500,000 shots would cost more in downtime and re-tooling than the original price differential. At our factory, the break-even calculation for upgrading from P20 to H13 generally favors H13 at annual volumes above 250,000 shots per cavity for abrasive resins.

Step 3: Match Surface Finish to Steel Grade

Surface finish requirements flow directly from the part specification. SPI A1/A2 (mirror) requires S136. SPI B1 (semi-gloss) can use H13. SPI C1/C2 (matte) can use P20. When the part drawing calls for ‘as-molded gloss’ without specifying SPI grade, ask for clarification before quoting steel — the difference between a vague ‘glossy’ spec and an SPI A1 spec can add $4,000–$6,000 to a single-cavity tool cost. In our DFM reviews, we always confirm the surface finish grade before selecting steel because clients often do not realize their ‘shiny part’ requirement translates into optical-grade polishing work.

Real-World Cost and Timeline Impact of Steel Grade Choices

Numbers help settle arguments about steel grade faster than theory. Here is actual data from our factory’s recent tooling builds, anonymized for client confidentiality. These figures represent single-cavity family tools for consumer and industrial parts with a surface area of approximately 150 cm² per cavity.

Tooling Cost vs Steel Grade (Single-Cavity Tool, 150 cm² Cavity Area)

Grado de acero	Material Cost (steel only)	Total Tool Cost	Plazos de entrega	Recommended For
P20	$400–$800	$8,000–$15,000	4–6 weeks	Prototype, low-volume, unfilled commodity resins
H13	$800–$1,600	$12,000–$22,000	6–8 weeks	High-volume, abrasive resins, high-temp engineering plastics
S136	$1,200–$2,400	$15,000–$28,000	6–8 weeks	PVC, POM, optical, medical, FR resins

The lead time difference reflects both heat treatment scheduling and the additional EDM and polishing work required for H13 and S136. Vacuum hardening furnaces at most tool shops run on a batch schedule — if your tool misses the weekly run, you add 5–7 days. This is why we advise clients to commit to steel grade in the first week of tooling kickoff, not after the mold has been roughed out in whatever steel was on the shelf.

Frequently Asked Questions About Injection Mold Steel Selection?

¿Cuál es el mejor acero para moldes para piezas plásticas transparentes?

S136 stainless mold steel is the industry standard for transparent injection molded parts. It achieves mirror-polish surface finish (SPI A1/A2, Ra ≤ 0.025 µm) required for optical-grade clarity in PC, PMMA, and COC resins. The 13% chromium content also resists staining from mold release agents that could cloud a polished cavity surface. P20 and H13 can reach SPI B1 but cannot sustain the Ra 0.01–0.02 µm values needed for lens or light-guide applications. For medical optical parts, S136H (pre-hardened variant at HRC 38–42) provides a faster build schedule while still meeting polishability requirements.

¿Puedo utilizar acero P20 para un molde de producción de alto volumen?

P20 can handle production volumes up to 500,000–1,000,000 shots for unfilled commodity resins (ABS, PP, PE, PS) at injection pressures below 1,200 bar. Above these volumes, or when running filled resins with more than 10% glass fiber, P20 will develop visible wear at gate areas and parting lines that transfers to parts as flash or dimensional drift. For annual volumes above 500,000 shots per cavity with any abrasive resin, upgrading to H13 typically pays for itself within the first year by eliminating mid-production cavity repairs. Always confirm resin type and annual volume before fixing the steel grade.

¿Qué acero para moldes debo usar para el moldeo por inyección de PVC?

PVC moldeo por inyección requires S136 (or 2316) stainless mold steel without exception. During processing, PVC decomposes slightly and releases hydrochloric acid (HCl) gas, which attacks non-stainless cavity steel surfaces within 50,000–100,000 shots, causing pitting corrosion that destroys surface finish and creates part defects. S136’s 13% chromium forms a passive oxide layer that resists HCl attack. All cooling lines, core pins, and slides in contact with the PVC melt zone should also be made from stainless steel or chrome-plated to prevent internal corrosion. Even short-run PVC prototype tools should use S136 — HCl damage accumulates rapidly.

¿Cómo afecta la dureza del acero de molde a la calidad del acabado superficial?

Harder steels (HRC 48–52, H13 and S136) can be polished to finer surface finishes than softer steels (HRC 28–33, P20) because the high carbide content of hardened steels allows the polishing abrasive to produce a uniform, scratch-free surface. P20 at HRC 30 has softer matrix regions that tear during diamond polishing, limiting achievable Ra to approximately 0.05–0.10 µm (SPI B1). S136 at HRC 50 can reach Ra 0.01 µm (SPI A1) with progressive diamond polishing from 6 µm down to 0.25 µm grit. H13 at HRC 50 reaches Ra 0.03–0.05 µm (SPI A2–B1). Hardness also determines how long the polished finish lasts under production conditions.

¿Es mejor el H13 o el S136 para un molde de dispositivo médico?

S136 is the preferred choice for medical device molds in most cases. Medical parts typically require mirror-polished surfaces for cleanability and cosmetic inspection compliance, and many medical resins — including LDPE for drug packaging, PC for device housings, and various drug-eluting polymers — may contain additives that cause corrosion in non-stainless steels. S136 satisfies both the surface finish requirement (VDI 0–3) and corrosion resistance needed in a medical production environment. H13 is used in medical tooling only when the part geometry has thin ribs or deep cores where S136’s slightly lower toughness creates chipping risk, and where the resin is not corrosive.

---

1. DFM: DFM (Design for Manufacturability) refers to an engineering review process applied before tooling begins, identifying features in a part design such as thin walls, sharp corners, or insufficient draft that would cause molding defects or increase tool cost. ↩

2. mold steel: Mold steel refers to a category of tool steels used to fabricate injection mold cores and cavities, selected based on hardness (HRC), polishability, corrosion resistance, and thermal fatigue strength for the target production volume and resin type. ↩

3. tool steel: Tool steel is a category of carbon and alloy steel specifically formulated for the manufacture of cutting and forming tools, including injection mold cavities, measured in hardness on the Rockwell C (HRC) scale, typically ranging from HRC 28 to HRC 65 depending on application. ↩

4. injection mold design: Injection mold design is an engineering discipline that defines the cavity geometry, gating system, cooling circuit layout, and ejection mechanism of a mold, directly determining cycle time, part quality, and tool life. ↩