Optical Injection Mold Steel: S136 vs NAK80 — Which Grade Maintains Mirror Polish Through 500,000 Production Shots?

Most mold steel selection guides compare grades primarily by hardness. For plastic optical molds, hardness is only one factor — mirror polish retention, dimensional stability, and corrosion resistance under optical-grade resin conditions determine whether your precision optical mold, built through careful mold manufacturing and heat treatment control, delivers consistent lens quality from shot 500 through shot 500,000.

For a standard injection mold producing ABS enclosures or PP caps, steel selection is primarily a cost and volume question. P20 for low volumes, H13 for high volumes or abrasive resins, S136 for corrosive materials. The lens surface quality doesn’t enter the equation because the parts don’t require optical transparency.

For a precision optical injection mold, the selection criteria shift entirely. The cavity surface must achieve and maintain SPI A1 mirror finish — Ra ≤ 5nm surface roughness — because any cavity surface defect replicates directly onto the lens optical surface. A scratch on the cavity that would be invisible on a housing panel becomes a visible optical defect on a precision lens.

This changes the steel selection logic in three specific ways:

• Polishability becomes the primary criterion, not just hardness or wear resistance. A steel that is theoretically harder but cannot be polished to Ra ≤ 5nm is disqualified from optical mold use regardless of its wear properties.
• Polish retention over production life matters as much as initial polish quality. A mold that achieves SPI A1 at trial but degrades to SPI A2 at 100,000 shots forces an unplanned maintenance cycle that disrupts production schedules.
• Dimensional stability through production life — not just at first article — determines whether the lens holds its optical specification across the mold’s production lifetime. Steels that creep or distort under repeated thermal cycling shift lens dimensions gradually, causing production drift that SPC monitoring catches but only process tooling changes fix.

S136 is available in standard and ESR (Electro-Slag Remelted) grades. For optical lens molds, standard S136 is not sufficient — ESR grade must be specified. The ESR process passes the steel through a slag-remelting stage that removes sulfide and oxide inclusions down to near-zero levels. These inclusions, invisible in standard steel bar stock, appear as pits during the final polishing stage of precision optical mold manufacturing — pits that no amount of additional polishing can eliminate because they are subsurface voids. Specifying S136 ESR on the tooling purchase order is not optional for optical-grade cavity inserts; it is the minimum specification required to guarantee a pit-free SPI A1 mirror finish.

Both S136 and NAK80 can achieve SPI A1 mirror finish at the start of a mold’s production life. The question that separates them in practice is how long each steel holds that finish before a polishing intervention is required.

Polish retention is determined by surface hardness — harder steel resists the micro-scratching from ejection, handling, and occasional abrasive particles in the resin that gradually degrade the mirror surface. S136, heat treated to HRC 48~52, holds its mirror finish significantly longer than NAK80 at HRC 37–43. In practice: an S136 precision optical mold may require cavity re-polishing at 200,000–300,000 shots, while a NAK80 optical injection mold with the same resin and cycle parameters may require re-polishing at 100,000–150,000 shots.

For a production run of 500,000 parts, this means:

• S136 mold: One or two polishing interventions over the production lifetime
• NAK80 optical injection mold: Three to four polishing interventions over the same production lifetime

Each polishing intervention requires removing the mold from production, an average of 2–5 days of toolroom time, and a re-approval first-article sample run. For a production program running 3 shifts, this is a meaningful difference in total production cost and schedule disruption.

For S136 precision optical molds, heat treatment quality is as important as steel grade selection. A correctly specified S136 ESR insert that is improperly heat treated can perform worse than a properly processed NAK80 insert.

S136 should always be vacuum hardened for precision optical injection mold cavity inserts. Vacuum hardening eliminates surface oxidation during the heating phase, producing a clean, oxide-free surface that requires minimal post-treatment finishing before polishing. Salt-bath hardening produces surface oxidation that must be removed by additional machining — removing stock that was already measured into the cavity dimensions, requiring dimensional verification before final polishing.

S136 is typically double-tempered at 180–220°C after hardening for high-polish optical mold applications, depending on the required hardness and steel supplier’s recommendations. Higher tempering temperatures reduce hardness toward the lower end of the HRC 48~52 range but improve toughness. For optical cavity inserts, toughness is less critical than hardness (impact loading is minimal in service), so tempering at the lower end of the range (200°C) to maximize hardness is appropriate for most precision optical injection mold applications.

When neither option clearly wins — for example, a medical-adjacent application with moderate volume and budget constraint — the correct answer is S136 ESR. The cost of a corrosion incident on a medical optical component, or an unplanned production stop for emergency cavity re-polishing on a precision optical mold, exceeds the steel cost differential many times over. Err toward S136 when the consequence of steel grade failure is high, and toward NAK80 when cost and lead time matter more than long-run polish retention.

ATRMOLD’s mold manufacturing team specifies S136 ESR or NAK80 based on the application requirements established during the DFM review stage — before tooling purchase orders are placed. This steel specification review is included in every optical lens mold project we take on, at no additional cost to the customer.