Polishing Mold Steel: What Decides the Finish Before the Polisher Starts

Polishability is a material property

When a cavity will not take a mirror finish, the fault is usually in the steel, not the polisher. The controlling factors, in roughly the order they matter:

1. Cleanliness — non-metallic inclusions (sulphides, oxides, silicates) are the primary limiter. Under the polishing abrasive, inclusions either tear out and leave pits, or stand proud and leave comet tails. Steels intended for mirror work are produced to high cleanliness — ESR (electroslag remelted) grades such as 1.2083 ESR or premium P20 variants exist precisely to shrink and distribute inclusions.
2. Hardness and its uniformity — softer steel polishes slowly and smears; more importantly, hardness variation across the surface (soft spots, decarburized skin, mixed microstructure) polishes at different rates and produces a wavy, uneven lustre. Through-hardened or quality prehardened steels with even hardness take the most uniform finish.
3. Microstructure — a fine, uniform martensitic structure polishes best. Banding, coarse carbides (as in high-carbon cold-work steels like D2) and retained austenite all print through at high finishes; D2 can be polished well, but never as easily as a clean martensitic stainless.

The SPI finish scale

The plastics industry grades cavity finishes on the SPI (Society of the Plastics Industry) scale:

Class	Description	Typical route
A	Mirror / optical	Diamond buff to #3,000+ grit equivalent; lens-quality (A-1) demands the cleanest steels
B	High polish, near-mirror	Fine grit papers to about #600–800
C	Medium polish	Stones and papers in the #320–600 range
D	Matte / textured	Dry blast (glass bead, aluminium oxide)

Each class subdivides (A-1, A-2, A-3...). The practical lesson: specify the finish when choosing the steel, not after machining. Asking for SPI A-1 on a sulphur-bearing free-cutting steel (1.2312) or a heavily banded block is asking for the impossible — the inclusions cap the outcome regardless of labour.

Orange peel: causes

Orange peel — a pebbly, dimpled texture that appears as polishing approaches mirror level — has several distinct causes that need different responses:

• Over-polishing soft or inhomogeneous steel: prolonged work on a surface with hard and soft micro-regions erodes them at different rates. The fix is upstream (better steel), not more polishing.
• Too much pressure or heat with soft buffs, which drags the surface rather than cutting it.
• Polishing a surface whose previous machining/EDM layer was not fully removed — the damaged skin responds unevenly. Each polishing step must completely erase the previous step's scratches before moving finer; skipping grades is the most common labour error.

Practical sequence

Progress through stones and papers in order, cleaning between steps so coarse particles never contaminate a fine stage; polish in alternating directions so scratches from the previous grade are visible until gone; and agree the finish standard — SPI class and a comparison plaque — before work starts, because "mirror" without a class is a dispute waiting to happen.