The mold is the biggest single check you'll write in an injection molding project — and the steel it's cut from decides how long it lasts, how much maintenance it demands, and whether shot #300,000 still looks like shot #1. Here's how we spec P20, 718H, and S136, and when the premium grade actually pays for itself.
Buyers compare mold quotes on price and lead time, but the steel grade buried in the spec sheet is what determines whether that tool is a one-time asset or a recurring expense. A cheaper steel isn't a discount — it's a shorter loan. The cost comes back as flash on parts, unplanned repair downtime, and a tool that dies with orders still on the books.
P20: the prototyping economy grade
P20 is the baseline pre-hardened mold steel, supplied at roughly HRC 28–32. It machines fast and keeps tooling cost down, which is exactly what you want for a mold that doesn't need to live long.
The catch is wear. At that hardness, the parting line degrades early, and a worn parting line means flash — thin fins of plastic squeezed out of the seam that add a trimming operation to every part. P20 also has essentially no corrosion resistance, so it needs disciplined rust-prevention between runs. Plan on a working life under ~300,000 shots.
Where it belongs: prototype tools, bridge tooling, low-volume structural parts, and big parts with loose tolerances — think bins and pallets, not enclosures.
718H: the volume workhorse
718H is a higher-purity, vacuum-degassed evolution of the same family, pre-hardened to HRC 33–38. Because it ships pre-hardened, there's no quench step — and no risk of heat-treatment distortion in a finished cavity.
That extra hardness buys uniform wear resistance through the cross-section and a typical life of 500,000–800,000 shots with proper care. Corrosion resistance is only moderate, though: 718H tools still need regular anti-rust maintenance, especially in humid climates or between production intervals.
Where it belongs: the broad middle of real production — consumer electronics housings, automotive interior parts, durable goods at serious volume.
S136: the stainless flagship
S136 is a high-chromium stainless tool steel, hardened after machining to HRC 48–52. It's a different class of tool: engineered for 1,000,000+ shots under proper maintenance, with hardness that keeps parting lines crisp deep into the tool's life.
The chromium is the quiet hero. S136 is highly resistant to rust and corrosion, which matters twice: once for the plant environment (humid air, condensation from chilled cooling lines), and once for the resin itself — some plastics release mildly corrosive gases when they're melted. A stainless cavity shrugs that off, and the anti-rust ritual that eats downtime on carbon steels mostly disappears.
Where it belongs: optical and transparent parts, medical devices, and any resin carrying corrosive additives — flame retardants, UV stabilizers, PVC.
The comparison at a glance
| Metric | P20 | 718H | S136 |
|---|---|---|---|
| Hardness (HRC) | 28–32 | 33–38 | 48–52 hardened |
| Typical tool life | < 300k shots | 500k–800k shots | 1M+ shots |
| Corrosion resistance | Poor | Moderate | Excellent |
| Maintenance overhead | High | Medium | Very low |
| Dimensional stability over life | Fair | Good | Excellent |
| Tooling cost | Lowest | Moderate | Highest |
Tool-life figures are typical ranges with proper maintenance — part geometry, resin abrasiveness, and molding conditions all move them.
A worked example: UV-stabilized ABS
Here's how this plays out on a real part class we handle often: fountain and aquatic components molded in ABS with UV stabilizers.
Two things gang up on the mold. First, the UV additive package can release mildly corrosive volatiles at melt temperature, slowly attacking a carbon-steel cavity. Second, these parts often get secondary CNC operations — threading, drilling, port-facing — and machining fixtures assume the molded blank is dimensionally identical every time.
Run that job in P20 and the cavity wears and pits early. The molded parts drift dimensionally, the CNC fixtures stop lining up, and the defect rate climbs in the machining step — a mold problem masquerading as a machining problem. In 718H the tool holds up but earns its keep in maintenance discipline. In S136, both failure modes are largely off the table.
This isn't hypothetical — we run exactly this combination in production. See how the mold-plus-machining split played out in our hybrid molding + CNC nozzle case study.
How to choose
P20
Cheap, fast to cut, and fine for a tool that won't see 300k shots. Don't ask it to do production duty.
718H
The cost-performance pick. Budget for a preventative maintenance routine — cleaning and anti-rust protection between runs.
S136
Highest upfront cost, lowest cost per shot. The premium buys near-zero rust maintenance and dimensional precision that lasts.
The volume logic here is the same one that governs whether to mold at all: amortize the upfront cost over realistic lifetime volume, not the first purchase order. And whichever steel you land on, the mold can only be as good as the part design going into it — these five DFM mistakes will hurt in any grade.
Speccing a mold? Tell us the resin, the lifetime volume, and what happens to the part after molding. Every tooling quote we send names the steel grade and why — so you're comparing tools, not just prices.