SPI Surface Finish Classes Demystified
SPI standards classify mold finishes into four categories with precise roughness (Ra) values and processing methods:
SPI Class A: Diamond Polishing (Optical Clarity)
| Grade | Ra (µm) | Method | Applications |
| A-1 | 0.012-0.025 | #3, 6000-grit diamond paste | Medical lenses, light guides |
| A-2 | 0.025-0.05 | #6, 3000-grit diamond paste | Transparent covers, reflectors |
| A-3 | 0.05-0.10 | #15, 1200-grit diamond paste | Display components |
SPI Class B: Sandpaper Polishing (Semi-Gloss)
| Grade | Ra (µm) | Method | Applications |
| B-1 | 0.05-0.10 | 600-grit sandpaper | Consumer electronics |
| B-2 | 0.10-0.15 | 400-grit sandpaper | Cosmetic packaging |
| B-3 | 0.28-0.32 | 320-grit sandpaper | Industrial housings |
SPI Class C: Stone Polishing (Matte Finish)
| Grade | Ra (µm) | Method | Applications |
| C-1 | 0.35-0.40 | 600-grit stone | Automotive interiors |
| C-2 | 0.45-0.55 | 400-grit stone | Tool grips |
| C-3 | 0.63-0.70 | 320-grit stone | Concealing sink marks |
SPI Class D: Blasting (Textured Surfaces)
| Grade | Ra (µm) | Method | Applications |
| D-1 | 0.80-1.00 | #11 glass beads | Scratch-resistant surfaces |
| D-2 | 1.00-2.80 | #240 aluminum oxide | Anti-slip tools |
| D-3 | 3.20-18 | #24 aluminum oxide | Heavy equipment |
SPI vs. VDI 3400 Comparison
| Parameter | SPI Standard | VDI 3400 Equivalent |
| Optical Finish | A-1 (0.012µm) | VDI 0 (0.025µm) |
| Semi-Gloss | B-1 (0.05µm) | VDI 12 (0.15µm) |
| Matte Texture | C-2 (0.50µm) | VDI 25 (1.0µm) |
| Coarse Texture | D-3 (12.5µm) | VDI 40 (18µm) |
Industry-Specific Applications
Medical Devices
| Requirement | SPI Solution | Benefit |
| Bacterial resistance | A-1 (Ra 0.025µm) | 100% FDA wipe-test pass rate |
| Autoclavable surfaces | B-1 + passivation | 500+ sterilization cycles |
Consumer Electronics
| Requirement | SPI Solution | Benefit |
| Scratch-free bezels | A-3 + hard coating | 5H pencil hardness |
| Fingerprint resistance | D-1 texture | 70% less visible smudges |
BFY Mold’s Precision Process
- Laser Scanning: Detect deviations >0.005mm
- Robotic Polishing: Achieve ±0.01µm Ra consistency
- 3D Validation: Per-part surface mapping
- Electrochemical Maintenance: Restore worn textures
8 Essential SPI Surface Finish FAQs
- Which SPI grade hides sink marks best?
SPI C-2 (Ra 0.45-0.55µm) masks sink marks in glass-filled polymers.
- How does SPI A-2 reduce part rejection?
Reduces flow lines by 60% vs. unpolished molds.
- What’s the lead time for SPI D-2 texturing?
7-10 days for standard molds at BFY Mold.
- Can SPI finishes withstand glass-filled nylon?
Use SPI B-2 minimum; avoid Class A for abrasive materials.
- How to specify SPI on mold drawings?
Annotate: “FINISH: SPI B-1 PER SPI STD-123”.
- Does SPI D-3 improve paint adhesion?
Yes: 5x stronger bonds vs. polished surfaces (ASTM D4541).
- Cost difference between SPI A-1 and B-1?
A-1 adds 25-40% due to manual polishing.
- Restoration of worn SPI finishes?
Electrochemical texturing recovers 95% of the original Ra.
