Steel Polishing Requirements for Transparent Injection Molds

The surface quality of transparent plastic parts (such as PMMA, PC, and PET) is directly determined by the finish of the mold cavity. The polishing performance of the mold steel is therefore a critical factor in achieving a high-gloss, defect-free surface. This document outlines the core requirements for mold steel used in transparent part applications, focusing on surface finish, material properties, and practical selection criteria.

1. Core Polishing Requirements for Transparent Molds

1.1 Surface Roughness Standards

Transparent molds require a mirror finish. Standard transparent parts typically demand a surface roughness of Ra 0.02~0.04 μm, while optical-grade components (e.g., lenses, light guides) require a finish of Ra ≤ 0.008 μm to ensure optimal light transmission and prevent scattering. The final surface must be free of visible defects such as pits, pinholes, orange peel, or scratches.

1.2 Uniformity and Consistency

The steel must have a homogeneous microstructure to ensure uniform polishing across the entire cavity. This is particularly important for complex geometries with deep ribs or narrow gaps, where inconsistent polishing can lead to visible gloss differences on the final part.

1.3 Balance of Polishability and Wear Resistance

High-quality mold steel should polish efficiently, allowing quick removal of machining marks. Simultaneously, the polished surface must exhibit excellent wear resistance to withstand molten plastic flow and release agent erosion during mass production, maintaining the mirror finish over time.

2. Key Material Properties Influencing Polishing

2.1 Steel Purity

High purity is essential to avoid polishing defects. Non-metallic inclusions (sulfides, oxides) are harder than the matrix and tend to fall out during polishing, leaving pits. Premium steels require low sulfur (≤0.005%) and phosphorus (≤0.01%) content, with inclusion ratings ≤ 1.0, typically achieved through vacuum melting or electroslag remelting.

2.2 Microstructure Uniformity

A fine, uniform grain structure (grain size ≥ Grade 8) prevents "orange peel" or grain boundary attack during polishing. Segregated or coarse structures can cause uneven material removal. Additionally, pre-hardened steels must have a uniform hardness distribution (≤±2 HRC) to ensure consistent polishing rates.

2.3 Hardness Compatibility

The ideal hardness range for transparent mold steel is HRC 48~54. Hardness below HRC 40 is prone to scratching and wear, while hardness above HRC 58 increases brittleness and polishing difficulty, potentially causing micro-cracks.

2.4 Corrosion Resistance

Transparent plastics like PC can decompose into acidic byproducts at high temperatures. Combined with release agents, this creates a corrosive environment. Therefore, the steel must resist rust and pitting to maintain the mirror surface. Stainless steel grades or surface treatments (chrome plating, nitriding) are often employed.

3. Comparison of Common Mold Steels

3.1 Stainless Steels (S136, S136H)

S136 is a benchmark for optical molds. Its high purity allows it to reach a Ra 0.008 μm finish. S136H is a pre-hardened version (HRC 48~50) offering excellent corrosion resistance and polishability, ideal for PC and PMMA parts in high-volume production.

3.2 Pre-hardened Steels (NAK80, 718H)

NAK80 is an age-hardened steel (HRC 40~45) known for its fine structure and excellent polishability (Ra 0.01~0.02 μm). It is cost-effective and suitable for standard transparent parts. 718H (HRC 28~32) offers moderate polishability (Ra 0.02~0.04 μm) at a lower cost, suitable for non-critical applications.

3.3 Hot Work Steels (H13)

H13 (HRC 48~52) offers good heat resistance and wear resistance but has lower purity than stainless steel. It is typically used for high-temperature PC molding where optical perfection is not the primary requirement.

4. Polishing Process & Selection Recommendations

4.1 Process Guidelines

Polishing must follow a step-by-step refinement process (e.g., 800#→2000# grit paper, followed by W7→W1.5 diamond paste). Skipping steps can embed deep scratches. High-purity steels may benefit from a combination of mechanical and chemical polishing.

4.2 Selection Guide

Optical Grade (Lenses, Light Guides): Choose S136 for ultimate clarity.

Standard Transparent Parts (Covers, Housings): Choose NAK80 or 718H for a balance of quality and cost.

High-Temperature Applications: Choose H13 for thermal stability.

Corrosive/High-Humidity Environments: Choose S136H for rust resistance.