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Mold Steel Grades Selection Reference

Mold steel choice decides how long a tool lasts, how well it polishes and how it handles abrasive or corrosive resins. This reference lists common mold materials with typical hardness and uses, helping buyers and designers compare options.

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Grade	Family	Typical working hardness	Typical use in molds	Notes
1018	Low carbon steel	Soft, about 120-160 HB	Mold bases, plates, low-volume simple tooling	Low cost, easily machined, not for cavities that need polish or wear resistance
4140 / 4130 pre-hardened	Alloy steel	About 28-32 HRC	Mold bases, support plates, retainer plates	Good strength for base plates, common standard base material
P20	Pre-hardened tool steel	About 28-32 HRC (pre-hard)	General purpose cavities and cores for low to medium volumes	Good machinability, takes a decent polish, can be nitrided or textured, not for abrasive resins at high volume
P20 modified (718 type)	Pre-hardened tool steel	About 33-40 HRC	Large cavities and cores needing uniform hardness through thick sections	More uniform through-hardness than basic P20, better wear than P20
NAK80	Precipitation hardening steel	About 37-43 HRC	High polish and photo-etch applications	Very clean steel for mirror finish, aged, not to be heat treated further
H13	Hot work tool steel	About 44-52 HRC (heat treated)	Cavities for glass filled resins, high-volume tooling, die cast style inserts	Good toughness and thermal fatigue resistance, requires heat treatment after rough machining
420 stainless	Martensitic stainless	About 48-54 HRC (heat treated)	Corrosion-prone resins such as PVC, optical and medical parts, high polish	Resists corrosion from water and some resin byproducts, polishes well when hardened
S7	Shock resisting tool steel	About 50-58 HRC	Parts subject to impact such as slides, lifters, ejector components	Good toughness, used for tough applications, check suitability for heat treat
A2	Air hardening tool steel	About 57-62 HRC	Wear plates, inserts, shutoffs, gibs	Good wear resistance with moderate toughness, stable in heat treat
D2	High carbon high chromium tool steel	About 58-62 HRC	High-wear inserts, gates and shutoffs for abrasive resins	Very wear resistant but harder to machine and polish, less tough
17-4 PH	Precipitation hardening stainless	About 33-44 HRC depending on condition	Corrosion resistant components, nozzles, some cavities	Good corrosion resistance, heat treatable at low temperature with little distortion
7075-T6 aluminum	Aluminum alloy	About 150 HB	Prototype and low-volume tooling, bridge tooling	Fast to machine and good thermal conductivity, limited wear life and not suited to abrasive resins
QC-10 / mold plate aluminum	Aluminum alloy (cast plate)	About 130-150 HB	Prototype molds, short-run cavities	Stable flat plate, shorter life than steel, mind clamp and ejection loads
Beryllium copper (C17200)	Copper alloy	About 28-40 HRC (aged)	Core pins and inserts in hot spots needing fast heat removal	Very high thermal conductivity versus steel, handle with controls for beryllium dust, consider alternatives such as copper alloys without beryllium

Hardness values are typical ranges and vary with heat treatment and supplier. Confirm the grade, heat treatment and certification with your steel supplier and toolmaker. This table is a starting guide and does not replace the tool design specification.

Keep tool material and shot counts in your ERP

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How to choose a mold steel

Start from expected volume and resin. A prototype or low-volume tool may use aluminum or P20, a high-volume tool with glass filled nylon points to H13 or a harder insert, and a PVC or medical tool needs a corrosion resistant stainless. Next consider surface requirements: optical or high-gloss parts need clean steels that polish well, while textured surfaces need steel that etches uniformly. Finally weigh machining time, heat treat distortion and lead time, since a harder steel costs more to machine and can add weeks.

Match volume and resin abrasiveness to hardness and wear resistance
Specify polish level or texture early, it narrows the steel choice
Use harder inserts only where wear actually occurs

Pre-hardened versus heat treated steels

Pre-hardened steels such as P20 and 718 arrive at working hardness, so there is no heat treat step and less risk of distortion. They suit most medium volume tools. Steels like H13, S7, A2 and D2 are rough machined, heat treated, then finished, which gives higher hardness and wear resistance but adds time and the risk of dimensional change. For tight tolerances, leave stock for finish grinding after heat treatment.

Thermal conductivity and cooling

Steel conducts heat slowly, aluminum much faster, and copper alloys faster still. High conductivity inserts such as beryllium copper let you pull heat out of a thick boss or a deep core pin and cut cycle time. Aluminum tools can run shorter cycles but wear faster. Cooling layout matters as much as material, so combine material choices with conformal or baffled cooling where needed.

Common mistakes

Frequent errors are using P20 for a million-shot glass-filled job, specifying a very hard steel where toughness was needed, and ignoring corrosion from PVC or acetal byproducts. Mixing steels with different thermal expansion at critical shutoffs can cause flash or galling. Record the steel grade, hardness and supplier certificate in the tool record in your ERP, so maintenance and repairs match the original material.

Frequently Asked Questions

What is the difference between P20 and H13?

P20 is a pre-hardened general purpose mold steel at about 28-32 HRC. H13 is a hot work tool steel heat treated to about 44-52 HRC, with better wear and thermal fatigue resistance. H13 suits abrasive resins and high volume, P20 suits lower cost general work.

When should I use aluminum for a mold?

Aluminum suits prototypes, bridge tooling and short runs where fast machining and good heat transfer matter more than wear life. It is not usually the right choice for abrasive filled resins or millions of shots.

Which steel is best for a clear optical part?

Clean, high-polish steels such as hardened 420 stainless or NAK80 type are commonly used. The exact choice depends on the polish grade, volume and resin, so confirm with your toolmaker.

Why is beryllium copper used in molds?

It conducts heat much better than tool steel, so it pulls heat from hot spots and shortens cycle time. It needs safe machining and handling because beryllium dust is hazardous, so some shops use alternative copper alloys.

If you manage tool records, shot counts and maintenance history in an ERP, Netray can help you query them in plain language without moving data outside your network.

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