——From optical mirror to leather grain, one chart tells you exactly how shiny to polish
1. Why “roughness” decides both LOOKS & YIELD
In the molding world, every 0.01 µm drop in cavity Ra lifts yield by 2–4 %.
If a glossy-transparent part sees Ra > 0.05 µm, “cloudy spots” appear;
if a textured surface falls below 1.0 µm it feels sticky.
Polishing is therefore a numbers game—never a matter of gut feeling.
2. The universal language: SPI grade × Ra look-up table
Таблица
SPI Grade How we get it Typical Ra / µm Visual result Typical application A-1 6000# diamond paste 0.012 – 0.025 Mirror / optical clear Lenses, light-guides, clear packaging A-2 3000# diamond paste 0.025 – 0.05 High gloss / sharp reflection Automotive lamps, phone housings A-3 1200# diamond paste 0.05 – 0.10 Bright / slight reflection Cosmetic caps, appliance skins B-1 600# sand paper 0.05 – 0.10 Semi-bright / fine satin Key tops, inner frames B-2 400# sand paper 0.10 – 0.15 Matte / visible scratches Hidden brackets, ribs B-3 320# sand paper 0.28 – 0.32 Dull / brushed look Tool shells, bases C-1 600# stone 0.35 – 0.40 Flat matte Car interior, handles C-2 400# stone 0.45 – 0.55 Light orange-peel Small household devices C-3 320# stone 0.63 – 0.70 Heavy orange-peel Industrial bins D-1 Glass bead #11 0.80 – 1.00 Velvet Buttons, remote controls D-2 Al₂O₃ #240 1.00 – 2.80 Fine texture Housings, grips D-3 Al₂O₃ #24 3.20 – 18.0 Coarse leather Tool boxes, door panels Figures are from the July-2025 revision of the SPI standard, rounded to two decimals.
3. Un-written rules for precision molds
- Cavity surface ≤ 0.2 µm
Even if the final part will be textured, polish first to Ra 0.1-0.2 µm to guarantee dimensional fidelity and draft consistency.- Parting line ≤ 0.8 µm
Rougher faces collect plastic dust and peen over time, causing flash; below 0.4 µm you pay for shine you don’t need.
- Ejector area 0.4 – 0.8 µm
Too glossy → vacuum drag & stress-whitening; too rough → ejector pin galling. Tests on ABS show minimum ejection force at Ra 0.6 µm.- Cooling channel walls ≤ 1.6 µm
Guarantees turbulent flow, improves cooling by 8-12 % and retards fouling.
4. Plastic-specific mirror requirements
Таблица
Resin Recommended SPI Reason PC, PMMA A-1 Any scratch > 0.05 µm scatters light into “fog lanes” ABS, PC/ABS A-2 High-gloss paint substrate; Ra 0.03 µm is enough POM, PA66 B-1 Crystalline shrinkage high; too shiny reveals sink TPE/TPU C-2 Orange-peel hides knit line, improves grip PP house-ware C-3 Cost-driven; allows heavy peel, conceals weld
5. Four steps to reach 0.01 µm class
- Start with clean steel: H13 ESR, FS136, M333 — inclusion rating ≤ 1.0, no “fish-eyes” after polish.
- Stress-free heat treat: Deep-cryo –196 °C plus double temper → retained austenite < 2 %.
- Diamond paste ladder: 1200# → 3000# → 6000# → 8000#, cross-hatch 45°, spindle ≤ 1500 rpm.
- Class 1000 clean-room: 1 µm dust landing on cavity = permanent pit; apply anti-rust coating within 30 min.
6. Take-away
Surface roughness in precision injection molds is not “the shinier the better” but “just right”—balancing resin replication, appearance, ejection, cost and cycle time.
Print the SPI vs Ra chart, pin it above your polishing station, and let every stroke be data-driven.
That’s how you mass-produce “micron-level beauty” shot after shot.
We are a Chinese leading company in the manufacture of tooling, stamping, injection molding, CNC machined parts. If you wish to learn more about this subject, please do not hesitate to contact us.
