Mirror-Polished Mold Steels for Optical and Cosmetic Applications

Polish is a property of the steel, not of the polish schedule

A standard P20 block can be polished to a mirror finish in a tool shop. It takes weeks of hand work, the finish varies slightly from spot to spot, and after the first production run the surface looks consistent but the underlying inclusion structure means the polish wears unevenly. A NAK80 or S136H block, by contrast, arrives mirror polished at the mill, with a documented Ra value, and that finish locks during the tool build rather than being chased with abrasives.

The difference is inclusion structure. Pre-polished grades are produced to a cleaner cleanliness specification, with a finer grain and fewer non-metallic inclusions that would show through a polished surface. The mill polishes every block to a specified Ra value (typically Ra 0.01 μm or better) and documents it, so the tool builder receives a block whose surface finish is the starting point, not the end result.

The two pre-polished workhorses

NAK80 (Nippon Koshuha proprietary, sometimes referred to as "pre-hardened mirror steel") is pre-hardened to roughly 40 HRC and delivered mirror polished. It is a general-purpose pre-polished grade used for cosmetic automotive and appliance parts, optical lenses, clear housings and any application where polish consistency is a specification. Corrosion resistance is modest — a standard block in a standard humidity will eventually show rust, so storage and protection after every run are required.

S136H is the pre-hardened variant of S136 stainless, hardened to roughly 48–50 HRC and delivered mirror polished. It is the premium choice when the application also needs corrosion resistance — a cosmetic part that runs a mildly corrosive resin, an optical lens that sits in a humid environment, or a medical or food-contact cosmetic component. The higher hardness means the mill polish is more durable and the tool survives more production cycles without re-polishing.

Ra values, and what the specification means

Ra 0.01 μm is the normal target for pre-polished grades. Ra (arithmetic mean roughness) is the average deviation of the surface from its mean plane, measured over a specified sampling length. A standard P20 polished by hand reaches roughly Ra 0.02–0.03 μm. Pre-polished grades start at Ra 0.01 μm or better.

Ra is a one-number summary. For optical applications that require consistent surface quality across a large cavity, Ra has to be measured at multiple points on the cavity, and the range across those points has to be within specification. The mill certificate for pre-polished grades documents Ra measured at multiple spots on each block, which is why the grade is the starting point and not the polish schedule.

Draft, polish and cosmetic specifications

A mirror polish on a cavity does not change the friction between resin and steel — if anything it reduces friction, which means less drag during ejection. That reduces the required draft angle: a mirror-polished wall needs less draft than a textured or EDM wall. The practical rule: 0.5° minimum draft for a mirror-polished cavity, with more draft where the draw is deep.

The honest mistake to avoid: specifying both a mirror polish and a deep texture or EDM detail in the same cavity. The texture drags more than the mirror area, and the draft requirement becomes a compromise that usually satisfies neither.

Polish repair after production

A mirror finish wears over time from resin flow, from the parting line clamp pressure and from the vent land gases that escape over the cavity surface. The visible symptom is a dull patch where the finish has worn. A standard hand polish on a pre-polished grade does not restore the mill finish because the underlying inclusion structure is now exposed — the hand polish follows the inclusion pattern and produces a non-uniform surface.

The correct repair is a controlled abrasive schedule that matches the original mill process: a fine diamond paste run on a polishing jig that maintains the original surface geometry. This is a process to be performed by a tool maker who has experience with pre-polished grades, not a hand-polish fix by the moulding shop.

Common mistakes

• Polishing NAK80 or S136H at the tool shop "because we always do". The mill polish is consistent and is the reason the grade exists.
• Specifying a mirror polish on a standard P20 block because "we need a mirror". The weeks of hand work cost more than the premium of a pre-polished grade and produce an inferior, non-uniform finish.
• Forgetting draft on a mirror-polished cavity. Even a mirror surface needs some draft, just less than a textured one.