Achieving Mirror-Finish Polishing on Cosmetic Packaging Molds
Cosmetic packaging (such as compact powder cases, lipstick tubes, perfume caps, and lotion pump caps) demands extremely high cavity surface quality, typically requiring SPI-A1 (highest mirror finish) or SPI-A2 (high-gloss mirror finish), with surface roughness Ra ≤ 0.012 μm (A1) or Ra ≤ 0.025 μm (A2). The surface must be completely free of any visible machining marks, orange peel, haze, and capable of clearly reflecting a human face.
Below is the most mature and practical mirror polishing process route in the industry (applicable to the most common cosmetic mold steels: NAK80, S136, Stavax ESR):
| Step | Operation | Tools/Consumables | Key Notes |
|---|---|---|---|
| 1 | Rough milling → Finish milling → Heat treatment | Carbide tools, CBN tools | After finish milling: Ra 0.4~0.8μm; achieve required hardness (NAK80 pre-hardened 48–50HRC; S136 quenched & tempered 52–54HRC) |
| 2 | Precision grinding (flatness & perpendicularity) | Surface grinder + diamond wheel | Ensure cavity geometry accuracy ≤0.005mm, surface Ra ≤0.2μm |
| 3 | Fine EDM (finish spark) | Copper or graphite electrodes | Use 0.1~0.15mm finish regime, VDI 18–22 (Ra 0.4~0.8μm), leave minimal stock for polishing (≤0.02mm) |
| 4 | Rough polishing (remove EDM recast layer & spark patterns) | Oil stones: 180# → 320# → 600# → 1000# → 1500# | Completely remove white layer and spark marks; thoroughly clean and polish in 90° cross directions after each grit |
| 5 | Medium polishing (diamond compound) | 15μm → 9μm → 6μm → 3μm → 1μm diamond paste + hardwood sticks/bamboo sticks | Polish uniformly, clean thoroughly after each grade, 90° cross direction; mirror effect begins to appear |
| 6 | Fine polishing (ultra-fine) | 1μm → 0.5μm → 0.25μm diamond or alumina paste + soft wood sticks/wool wheels | Key areas: light & even pressure using wool wheel + electric polisher |
| 7 | Ultimate mirror (SPI-A1 level) | Imported 1/0 (0.25μm) or 1/4μm diamond paste + black wool/felt wheel OR OP-S suspension + pitch wheel (top-tier) | Speed 800~1200 rpm, pressure 0.5~1 kg, polish 10–20 min to reach SPI-A1 |
| 8 | Ultrasonic cleaning + rust prevention | Ultrasonic cleaner + WD-40 or mold rust inhibitor | Completely remove residual compound; otherwise, fogging or pitting will occur during production |
| 9 | Apply protective film or ion nitriding/PVD (optional) | 3M blue protective film or PVD CrN/TiN coating | Common for high-end lipstick tubes and perfume caps (ultra-high gloss + wear resistance) |
Key Techniques and Taboos for Cosmetic Mold Mirror Polishing
Absolute Taboos
- Never use sandpaper directly for mirror polishing Sandpaper leaves uneven scratches; no amount of subsequent polishing can achieve A1.
- Must thoroughly clean after every diamond compound change Large particles mixing in cause “comet tails” or orange peel.
- Polishing direction must be 90° crossed Only proceed to the next grit after previous scratches are completely eliminated.
Difficult Areas
- Deep cavities, thin ribs, LOGO details → Use custom-shaped hardwood sticks, bamboo, or copper-wrapped diamond paste
- Japanese “wave-shaped” polishing needles (MoldMaster, OSG, Dijet) are highly effective
- Most failure-prone spots: → R-corner dead zones (use round sticks smaller than the radius) → Parting line seams (EDM must be uniform before polishing) → Around ejector pins (protect with copper foil before polishing)
Quick A1 Verification Methods
- Black background: completely black with no haze
- Phone flashlight at 45°: no bright spots
- Can clearly reflect facial features
Recommended Consumable Brands (easily available in China)
- Diamond paste: Swiss Merard, Japan Nihon Kenshi, Taiwan Jingling
- Oil stones: Japan AAA, Naniwa, Ceratone
- Pitch wheel + OP-S: German OSBORN (essential for top-tier A1)
- Wool wheels: 3M black/purple high-speed wool wheels
As long as the sequence 180# oil stone → 15μm → … → 0.25μm → pitch wheel + OP-S is strictly followed, even an average polishing technician can consistently achieve cosmetic-grade mirror finish. Top-tier contract manufacturers for Dior, YSL, and Chanel essentially use this exact flow, only upgrading the final step with more expensive German pitch wheels and imported OP-S suspension to push the mirror quality one level higher.