Compare P20 and 718H pre-hardened mold steel to choose the right plastic mold steel for cavities and cores, and learn the hardness and polish details to specify.
P20 and 718H are the two most widely used pre-hardened plastic mold steels, and because both arrive already hardened to a usable level, mold makers can machine cavities and cores without a later quench. The two are close cousins, but 718H is a refined, higher-hardness development of the basic P20 family, and the difference matters for large molds, polished surfaces, and long-run tooling. This guide helps you choose the right one for the mold at hand.
How 718H Relates to P20
P20 is the original pre-hardened mold steel, typically supplied around 28 to 32 HRC, with good machinability and uniform hardness that suits medium-sized molds. 718H is an upgraded version of the same family, refined for higher hardness around 33 to 38 HRC and better through-hardening in larger sections. The higher hardness gives better wear resistance for long production runs, and the cleaner, more uniform structure of 718H improves polishability and etching for textured mold surfaces.
When 718H Is the Better Choice
For large molds where the core and cavity must hold hardness through a thick section, and for high-volume production runs where the mold will see millions of cycles, 718H is the safer specification. Its higher hardness extends mold life, its improved polishability matters for glossy cosmetic parts, and its better dimensional stability under thermal cycling reduces the risk of distortion in long runs. When the part requires a mirror finish or a fine etched texture, the cleaner structure of 718H delivers a more consistent surface.
When P20 Is Sufficient
For smaller molds, prototype or short-run tooling, and general-purpose plastic parts where surface finish is not critical, P20 remains an excellent and economical choice. It machines easily, is widely stocked, and its hardness is adequate for moderate production volumes. Because P20 is produced in larger volumes, both price and lead time are usually more favorable than 718H. For non-cosmetic or short-life tooling, specifying P20 avoids paying for capability the mold will not use.
What to Specify on the RFQ
State the grade clearly, distinguishing P20 from 718H by full designation, and specify the supplied hardness range, since both names cover a band rather than a single value. For polished or textured surfaces, request a grade with controlled cleanliness and confirm the polishability requirement. For large molds, ask about through-hardness uniformity in the section size you need, because a soft core in a thick block will wear unevenly. Finally, confirm the machining allowance and whether the supplier can supply cut blocks to near-net size to reduce your roughing time.