Selection of Optical Injection Mold Steels

The selection of mold steel is critical for producing high‑precision optical plastic components such as lenses, light guides, and sensor housings. Unlike standard injection molds, optical molds require exceptional mirror‑polishing performance, corrosion resistance, dimensional stability, and wear resistance. The right steel must be chosen based on the optical requirements, plastic material characteristics, and production volume.

1. Key Requirements for Optical Mold Steel

Optical plastics such as PMMA, PC, and COP typically require a surface roughness of Ra ≤ 0.002 μm, with no orange peel, pinholes, or flow marks. To achieve this, the mold steel must meet several core requirements:

Superior mirror‑polishing capability: The steel must have minimal impurities and a dense, uniform microstructure to avoid polishing defects.

Excellent dimensional stability: A low and stable coefficient of thermal expansion ensures the mold cavity maintains its precision during high‑temperature injection molding.

Good corrosion resistance: Protection against acidic gases (from materials like PC) and chemical cleaning agents prevents surface pitting or rust.

High wear resistance: Resistance to erosion from molten plastic, especially for glass‑fiber reinforced materials.

Good machinability: Ease of precision machining (CNC, EDM, laser) to minimize polishing time.

2. Main Types of Optical Mold Steel and Their Characteristics

Optical mold steels are commonly divided into three categories: pre‑hardened, quenched and tempered, and age‑hardening steels.

Pre‑hardened mirror steels are delivered with a predetermined hardness, eliminating the need for further heat treatment. They offer low processing costs and minimal deformation risk. Typical grades include 718H and P20HH. 718H, produced via electroslag remelting (ESR), offers a mirror finish of Ra 0.008 μm and is ideal for light guides and optical covers. P20HH provides a more cost‑effective solution for general optical applications.

Quenched and tempered mirror steels undergo heat treatment to achieve higher hardness and superior mirror‑polishing properties. S136 and its variant S136H are stainless steels with excellent corrosion resistance and a mirror finish of Ra 0.001 μm. S136 is suitable for high‑precision lenses and prisms. STAVAX ESR offers even higher purity and is used for ultra‑precision optical molds.

Age‑hardening mirror steels are rough‑machined first and then hardened via low‑temperature aging, resulting in minimal deformation (within μm levels). NAK80 is the preferred choice for light guide molds due to its exceptional dimensional stability and mirror‑polishing capability. NAK55 offers higher hardness and wear resistance, making it suitable for high‑volume production.

3. Selection Principles for Optical Mold Steel

The selection of optical mold steel depends on four main factors: precision requirements, plastic material, production volume, and cost.

For high‑precision components such as camera lenses and fiber optic connectors, quenched and tempered steels (S136, STAVAX ESR) or age‑hardening steels (NAK80) are preferred. For general‑purpose optical parts, pre‑hardened steels (718H, P20HH) provide a cost‑effective solution.

Material characteristics also influence selection. PMMA and PC require steels with excellent mirror‑polishing and corrosion resistance. Glass‑fiber reinforced plastics demand harder, more wear‑resistant steels like NAK55 or hardened S136. Plastics that generate acidic gases require stainless steel to prevent corrosion.

Production volume is another key factor. Pre‑hardened steels are suitable for low‑volume or custom production, while high‑volume applications benefit from the durability of quenched and tempered or age‑hardening steels.

Cost considerations are essential. Pre‑hardened steels are the most economical, followed by age‑hardening steels, with quenched and tempered steels being the most expensive.

4. Usage Considerations for Optical Mold Steel

Proper processing and maintenance are essential to maximize mold performance and lifespan. High‑precision machining equipment (5‑axis CNC, high‑precision EDM) should be used to avoid surface stress. EDM‑affected layers must be completely polished away. Heat treatment should be performed by specialized vendors to minimize deformation. Regular cleaning and rust prevention are necessary to maintain the mirror finish, especially after exposure to acidic residues.

Summary

Selecting the right optical mold steel requires balancing precision, material compatibility, production volume, and cost. Pre‑hardened steels are suitable for medium‑precision, cost‑sensitive applications. Quenched and tempered steels excel in high‑precision and corrosive environments. Age‑hardening steels are ideal for ultra‑precision molds with complex cavities. By considering these factors and implementing proper maintenance, manufacturers can ensure high‑quality optical products and optimal mold longevity.