In the field of plastic mold manufacturing, mirror steel (such as S136, NAK80, 420ESR, etc.) is widely used in the production of transparent PET, PC, PMMA and other products due to its excellent polishing performance. The mirror effect of the mold directly determines the appearance texture, light transmittance and market competitiveness of the final products. Mirror polishing is not a simple grinding process, but a systematic work that combines technology and standardization. It needs to integrate steel characteristics, mold structure and product requirements, and control every detail to achieve a flawless, high-gloss mirror effect while extending the service life of the mold. Based on practical production experience, this article details the key polishing points for mirror steel plastic molds, helping practitioners avoid common problems and improve polishing efficiency and quality.

Pre-Polishing Preparation: Lay a Solid Foundation

Seventy percent of the success or failure of mirror polishing depends on pre-polishing preparation. If the mold steel has defects or improper processing, even refined subsequent polishing cannot make up for it. First, ensure the quality of mirror steel, selecting high-quality steel without sand holes, impurities and hard spots. For S136 and 420ESR, the hardness should be controlled at HRC48–52 after quenching and tempering, while NAK80 should be kept at HRC38–42. Uneven hardness will cause uneven force during polishing, leading to orange peel and wave lines.

Secondly, standardize the preliminary processing of the mold cavity. The surface lines after CNC milling and EDM should be uniform, and the depth of EDM marks should be reasonable to avoid being too deep to remove. Reserve a polishing allowance of 0.03–0.05mm during rough and semi-finishing. Excessive allowance reduces efficiency, while insufficient allowance leaves tool marks. Pre-treat the cavity to remove oxide scale, oil stains and rust, ensuring even abrasive action.

Polishing Process: Step-by-Step Operation

The core principle of mirror polishing is "from coarse to fine, gradual refinement". Skipping grits will cause irreparable scratches and orange peel. Use 400# oilstone for rough polishing to remove tool marks and burrs, keeping a consistent direction and uniform force. Then use 800# sandpaper for intermediate polishing, with a crosswise direction to check for residual rough marks, achieving a smooth surface.

For fine polishing, use 1200#–2000# sandpaper, then switch to diamond paste (9μm, 6μm, 3μm, 1μm). Slow down the tool speed and apply light, uniform pressure. Adjust the grinding direction to 90° with the previous step each time to ensure no residual marks.

Tools, Environment and Common Problems

Choose special tools such as wool wheels (for large areas), bamboo and wood chips (for corners). Maintain gentle, uniform force to avoid collapse and scratches, especially for sharp corners.

Polish in a clean, dust-free environment. Clean the mold, tools and hands thoroughly when replacing abrasives to avoid coarse particles causing scratches. Use clean polishing oil and kerosene.

For different steels: slow down polishing for S136/420ESR to avoid scratches; control force for NAK80 to prevent orange peel. Avoid common problems: return to the previous step for orange peel (insufficient hardness/over-force); re-clean and polish for fine scratches; supplement fine polishing for foggy surfaces.

Conclusion

Mirror polishing is a detailed, rigorous work. By focusing on pre-preparation, step-by-step processing, proper tool use and clean operation, we can create high-quality mirror molds. This ensures exquisite products, extends mold life, reduces costs and enhances market competitiveness, making it an essential skill for mold practitioners.