If you have ever sent the same mold drawing to three suppliers and gotten back three wildly different prices, you are not alone. It is one of the most common sources of confusion for buyers sourcing injection molds from China. The instinct is to assume someone is padding their margin — but in most cases, the real answer is hiding in a single spec line: the mold steel grade.
Steel typically accounts for only 20–30% of a mold’s total cost, yet it is the single biggest factor in how long that mold will last, how it handles your specific resin, and how well it holds the surface finish your product needs. Choosing the right grade — not simply the most expensive one — is what actually controls cost and performance.
Here are the three questions that should drive steel selection on every mold quote.
How many shots does the mold need to produce?
Expected production volume is the starting point, because it determines how much wear and fatigue the mold needs to withstand over its working life.
| Expected shot count | Typical steel |
|---|---|
| Under 100,000 shots | P20 / 1045 / aluminum |
| 100,000 – 500,000 shots | P20 / 718H |
| 500,000 – 1,000,000 shots | NAK80 / hardened tool steel |
| Over 1,000,000 shots | S136 / H13 + surface coating |
Reference point: the SPI-SPE mold classification system grades tools from 101 to 105 based on expected service life — useful when comparing quotes that specify different steels for what looks like the same part.
What resin are you molding?
The material running through the mold matters as much as the volume. Some resins are abrasive or chemically aggressive, and a steel that works fine for one material can wear out — or corrode — rapidly with another.
| Resin type | Recommended steel |
|---|---|
| PVC / flame-retardant grades / POM | S136 required (corrosion resistance) |
| Glass-filled PA (PA+GF) | Hardened + coated H13 |
| High-temperature resins (PEEK, LCP) | H13 / 1.2344 |
| General-purpose resins (PP, PE, ABS) | P20 / 718H is sufficient |
Glass-filled resin behaves like sandpaper running through the tool — on pre-hardened steel, the gate area can wear through in as few as 50,000 shots. PVC also releases hydrochloric acid as it decomposes under heat, which will pit ordinary mold steel over time; this is why S136, not just any hardened steel, is the standard for PVC tooling.
What does the finished part need to look like?
The third factor is cosmetic and dimensional requirement. A higher price tag does not automatically mean a better match — the goal is pairing the steel to what the part actually demands.
| Part requirement | Recommended steel |
|---|---|
| Optical / mirror-finish surfaces | S136 ESR / NAK80 |
| Deep textured / leather-grain surfaces | 718H (tough, resists chipping) |
| High precision + complex geometry | Pre-hardened steel |
NAK80 polishes beautifully to a mirror finish, but it offers little corrosion resistance. If a part needs both an optical surface and corrosion resistance, NAK80 alone is the wrong call.
A cost-saving tip most buyers don’t ask for: many molds don’t need the same steel grade everywhere. Using a premium steel in the cavity (where cosmetic surface quality matters most) and a tougher, more economical steel in the core and structural inserts can reduce overall tooling cost by 15–20% without sacrificing part quality. Surface treatments — nitriding, PVD, TD coating — can also extend mold life by 2–3x on the same base steel, often a better return than upgrading to a pricier grade.
Before you sign: watch for “P20-type” steel
One wording trick shows up often enough in mold contracts to call out on its own. A quote or contract that lists the cavity steel as P20-equivalent or P20-type — rather than simply P20 — is not necessarily describing certified P20. It can mean an unbranded substitute that approximates P20’s hardness but skips the composition and heat-treatment consistency a certified mill grade guarantees, which quietly undercuts the shot-count expectations in the table above.
Before signing, ask the supplier for the steel mill or brand (for example Assab, Daido, or a verifiable domestic equivalent) rather than accepting a generic “type” description. It costs nothing to ask, and it closes the gap between what a quote implies and what actually gets machined.
