Mold Polishing Grades

Mold Polishing Grades

Polishing standards for injection molds typically follow the SPI (Society of the Plastics Industry) guidelines. These standards classify surface finishes—from high-gloss mirror to matte textures—based on surface roughness (Ra) and gloss level. Below is a summary of the 12 primary SPI polishing grades.

SPI Polishing Grades

Extended Mirror Finishes (Beyond SPI A1)

In highly demanding applications—such as optical lenses—finer mirror finishes are specified beyond standard SPI A1:

• A0: Ra ≤ 0.008 μm

• A1: Ra ≤ 0.016 μm

• A3: Ra ≤ 0.032 μm

• A4: Ra ≤ 0.063 μm

Polishing Methods & Material Selection by Grade

Achieving these finishes requires specific processes and appropriate mold steel selection.

A-Grade (Glossy Finish)

• Process: Use diamond paste polishing. Begin with coarse oilstones (e.g., #400) to remove machining marks, then progressively refine with finer oilstones (#600, #1000), followed by sandpaper, and finally diamond paste for mirror finish.

• Materials: High-hardness, dense steels such as S136 (54 HRC) or 8407 (52 HRC) are recommended for optimal gloss.

B-Grade (Semi-Gloss Finish)

• Process: Sandpaper polishing, starting from coarser grits (e.g., #320) and progressing to finer ones (#400, #600, #800). Change polishing direction by 45°–90° when switching grits to fully eliminate prior scratches.

• Materials: Pre-hardened steels like 718H offer a cost-effective solution.

C-Grade (Matte Finish)

• Process: Oilstone grinding, progressing from coarse to fine (e.g., #320 → #400 → #600).

• Materials: 718H is well-suited.

• D-Grade (Textured Finish)Process: Dry abrasive blasting using media such as glass beads or aluminum oxide at varying grit sizes to achieve satin to rough textures.

• Materials: 718H meets typical requirements.

Practical Polishing Tips & Considerations

1. Follow “Coarse-to-Fine” Sequence Strictly

   Never skip intermediate steps. Jumping directly to ultra-fine abrasives will leave deeper scratches unremoved, compromising surface quality.

2. Thorough Cleaning Between Steps

   When changing sandpaper grits or transitioning to diamond paste, clean the surface meticulously with 100% cotton and alcohol to prevent cross-contamination from residual particles.

3. Control Pressure and Direction During Final Polishing

   Use light pressure with fine papers (#1200, #1500), polish in two passes with direction changes to avoid surface burns or micro-scratches.

4. Consider Plastic Material Compatibility

   Not all resins suit high-gloss finishes. ABS and PC perform well with A-grade polishing, while PP and PE are better suited for matte or textured surfaces. Also, avoid prolonged use of #1500+ sandpaper on pre-hardened steels to prevent surface overheating.

How to Select the Appropriate Polishing Grade

Choose the most cost-effective grade based on the part’s functional and aesthetic requirements:

• A0–A1: Optical lenses, transparent high-gloss components (e.g., camera covers, light guides).

• A2: Consumer electronics housings, appliance panels—where visual appeal matters but perfection isn’t critical.

• A3: Secondary visible parts or internal components requiring smoothness at lower cost.

• B-Grade: Structural or non-cosmetic parts needing basic surface refinement.

• C & D Grades: Functional surfaces requiring fingerprint resistance, grip enhancement, or mold-line concealment (e.g., automotive interiors, tool handles).

  ---