If you've spent any time sourcing mold steel, you've probably run into this exact debate on a quote sheet: NAK80, P20, or 1.2738? They all get lumped together as "plastic mold steel," but treating them as interchangeable is where a lot of tooling budgets go sideways.
Let's start with what 1.2738 actually is. It's a pre-hardened chromium-nickel-molybdenum steel, typically supplied at 28-32 HRC, and it's known for having a very even hardness profile through thick sections. That last part matters more than people give it credit for. On a large mold block, say anything over 300mm thick, hardness can drop off noticeably from surface to core with lesser grades. 1.2738 was basically engineered to fight that problem, which is why it's such a common choice for big injection molds, especially ones with deep cavities or thick cross-sections.
So where does that leave NAK80? NAK80 is the premium option in this comparison. It polishes to a mirror finish better than almost anything else on the market, and it takes texture beautifully, which is why you'll see it specified for optical parts, cosmetic housings, and anything where surface finish is the whole ballgame. The tradeoff is cost and, honestly, lead time in some regions. If your part doesn't need that level of polish, you're paying for a capability you won't use.
P20 sits at the other end. It's been the default mold steel for decades because it's cheap, widely available, and good enough for a huge range of applications, particularly lower-volume or prototype tooling. But P20's hardenability through-thickness isn't as consistent as 1.2738, and it doesn't handle abrasive or glass-filled resins nearly as well over long production runs.
Here's how we'd actually think about the decision. If you're running high-volume production with standard engineering resins, and your tool has thick sections or a complex cavity where uniform hardness is critical, 1.2738 is usually the sweet spot between cost and performance. If the part is cosmetic, needs SPI A-1 or A-2 finish, or involves fine texture like leather grain or wood grain, NAK80 earns its price premium. And if you're building a bridge tool, a low-volume run, or something where the mold might only see a few thousand cycles before it's retired, P20 is often the smarter spend.
One thing we'd caution against: don't pick based on habit. A lot of shops default to P20 because "that's what we've always used," even on jobs that are now running higher volumes or tighter tolerances than they used to. Revisiting that assumption on your next tool build can genuinely extend mold life and reduce maintenance downstream.
If you're not sure which grade fits your specific geometry and resin, it's worth having that conversation with your steel supplier before the tool design is locked in, not after.