How to Select Plastic Mold Steel Suitable for Mirror Polishing?

Selecting the appropriate plastic mold steel for mirror polishing primarily depends on the material’s purity, uniformity of microstructure, hardness, and heat treatability.

I. Core Performance Requirements

High Purity: The fewer non‑metallic inclusions (such as sulfides and oxides) in the steel, the more conducive it is to achieving a high‑gloss surface.

Uniform and Fine‑Grained Structure: A homogeneous microstructure helps reduce orange peel or local roughness during polishing. Suitable hardness is generally HRC 48–52.

II. Material Selection Based on Mold Operating Conditions

Mold Hardness Requirements:

Pre‑hardened Steel (HRC 28–45): Suitable for molds that do not require subsequent heat treatment, offering high machining efficiency (e.g., NAK80, 718H).

Quenched and Tempered Steel (HRC 50–60): Suitable for high wear‑resistance requirements (e.g., Corid CP136, CH307), but with greater processing difficulty and higher cost.

Mold Complexity:

Complex Molds (e.g., deep cavities, thin‑walled structures): Pre‑hardened steel is preferred to avoid deformation during heat treatment.

Simple Molds: Quenched and tempered steel can be considered to improve wear resistance.

III. Evaluating Production Volume and Service Life

Small Batch Production: Pre‑hardened steel (e.g., NAK80) can be chosen to save on heat‑treatment costs.

Large‑Batch/Long‑Life Production: Opt for hardened steel (e.g., Corid CP136 heat‑treated to HRC 50–52).

Consider Machining and Maintenance Costs

Pre‑hardened steel eliminates the risk of heat‑treatment deformation but has lower hardness.

Hardened steel offers better wear resistance but requires professional heat treatment and subsequent fine polishing.

IV. Key Considerations

Machining Process: High‑gloss molds require precise machining and strict polishing procedures.

Impact of Heat Treatment: For quenched and tempered steel, heat‑treatment processes must be controlled to avoid oxidation or decarburization, which can affect polishing results.

Post‑polishing Maintenance: High‑hardness mold steel surfaces should be well maintained after polishing to prevent scratches or corrosion.