Polishing Requirements for PMMA (Acrylic) Injection Molds

Polishing Requirements for PMMA (Acrylic) Injection Molds

PMMA, also known as acrylic, is highly transparent and sensitive to mold surface quality. Even tiny tool marks, polishing lines, or impurities will cause haze, flow lines, scratches, or uneven light transmission. PMMA mold polishing is one of the most demanding surface treatments in plastic mold manufacturing. This article specifies professional polishing requirements.

1. Mold Steel Selection

Only steels with excellent polishability are suitable for PMMA molds, including NAK80, S136, S136H, and STAVAX. These materials have uniform structure, low impurities, and good corrosion resistance. Steels such as P20 or 718H are not recommended for high‑transparency parts because they tend to show pinholes and orange peel.Hardness should be above HRC 40 to ensure stable polishing and durability.

2. Step‑by‑Step Polishing Process

PMMA molds require strictly progressive polishing:

Rough polishing with oil stones to remove EDM and CNC marks.

Fine sanding from 800# up to 2000# to eliminate previous scratches.

Diamond paste polishing from 3 μm, 1 μm, 0.5 μm to 0.25 μm.

Mirror finishing with soft wool wheels to achieve optical clarity.

Skipping steps causes permanent visible lines in finished parts.

3. Polishing Direction Consistency

Polishing lines must follow the melt flow direction. Crosswise or random polishing causes flow marks, haze, and optical defects. Gate areas require special attention to ensure smooth transition without steps.

4. Surface Roughness Standards

General acrylic parts: Ra ≤ 0.2 μm

High‑transparency parts: Ra ≤ 0.025 μm (optical grade)

Lens‑grade products require ultra‑mirror finish with no visible marks under strong light.

5. Avoiding Common Polishing Defects

Over‑polishing causes orange peel and surface deformation, which is irreversible. Excessive pressure or local overheating must be avoided. Mold surfaces must remain clean to prevent particle embedding.Core and cavity surfaces must be polished to the same level to avoid uneven transparency.

6. Vent and Edge Polishing

Vents are kept at 0.015–0.02 mm depth with smoothly polished edges to prevent flash and burning. Internal radii must be uniformly polished without sharp corners to avoid stress marks and haze.

7. Rust Prevention and Maintenance

Polished mold surfaces are easily stained by fingerprints, moisture, or oil. Regular anti‑rust treatment is required. Even small rust spots destroy transparency. The mold should be protected from impacts to avoid scratches.

8. Surface Treatment for Enhanced Performance

Nitriding or hard chrome plating improves surface hardness, release properties, and wear resistance, ensuring long‑term stable production with consistent transparency.

9. Summary

PMMA mold polishing requires high‑quality steel, standardized step‑by‑step processes, flow‑consistent direction, strict roughness control, and careful maintenance. Only professional polishing can achieve crystal‑clear transparency, flawless appearance, and high optical quality for acrylic parts.