SPI Mold Finish Standards for Injection Molding
A practical reference for SPI A, B, C, and D mold finishes, surface roughness, and material compatibility in plastic injection molding.
Understanding SPI Mold Finish Standards
SPI Finish Categories
A quick overview of the four SPI finish categories, from mirror-polished surfaces to coarse textured finishes.
Glossy
Diamond Buff
Highly polished surfaces ranging from high gloss to mirror-like finishes.
Grades: A-1, A-2, A-3
Semi-Glossy
Grit Paper
Smooth, polished surfaces with less reflection than SPI A finishes.
Grades: B-1, B-2, B-3
Matte
Stone
Low-gloss and matte surfaces created using increasingly coarse polishing stones.
Grades: C-1, C-2, C-3
Textured
Dry Blast
Satin to coarse textured surfaces produced using glass bead or aluminum oxide blasting.
Grades: D-1, D-2, D-3
SPI Mold Finish Standards
| SPI Finish | Finishing Method | Typical Finish | Typical Surface Roughness, Ra (µm) |
|---|---|---|---|
| A-1 | Grade #3 Diamond | Lens / Mirror – requires 420 SS material | 0.012–0.025 |
| A-2 | Grade #6 Diamond | High Polish Parts | 0.025–0.05 |
| A-3 | Grade #15 Diamond | High Polish Parts | 0.05–0.10 |
| B-1 | 600 Grit Paper | Medium Polish Parts | 0.05–0.10 |
| B-2 | 400 Grit Paper | Medium Polish | 0.10–0.15 |
| B-3 | 320 Grit Paper | Medium–Low Polish | 0.28–0.32 |
| C-1 | 600 Stone | Low Polish Parts | 0.35–0.40 |
| C-2 | 400 Stone | Low Polish Parts | 0.45–0.55 |
| C-3 | 320 Stone | Low Polish Parts | 0.63–0.70 |
| D-1 | Dry Blast Glass Bead | Satin Finish | 0.80–1.00 |
| D-2 | Dry Blast #240 Oxide | Dull Finish | 1.00–2.80 |
| D-3 | Dry Blast #24 Oxide | Dull Finish | 3.20–18.0 |
Glossy Finish — SPI A
| Material | A-1 | A-2 | A-3 |
|---|---|---|---|
| ABS | Average | Average | Good |
| Polypropylene (PP) | Not Recommended | Average | Average |
| Polystyrene (PS) | Average | Average | Good |
| HDPE | Not Recommended | Average | Average |
| Nylon | Average | Average | Good |
| Polycarbonate (PC) | Average | Good | Excellent |
| Polyurethane (TPU) | Not Recommended | Not Recommended | Not Recommended |
| Acrylic | Excellent | Excellent | Excellent |
Semi-Glossy Finish — SPI B
| Material | B-1 | B-2 | B-3 |
|---|---|---|---|
| ABS | Good | Good | Excellent |
| Polypropylene (PP) | Good | Good | Excellent |
| Polystyrene (PS) | Excellent | Excellent | Excellent |
| HDPE | Good | Good | Excellent |
| Nylon | Good | Excellent | Excellent |
| Polycarbonate (PC) | Good | Good | Average |
| Polyurethane (TPU) | Not Recommended | Average | Average |
| Acrylic | Good | Good | Good |
Matte Finish — SPI C
| Material | C-1 | C-2 | C-3 |
|---|---|---|---|
| ABS | Excellent | Excellent | Excellent |
| Polypropylene (PP) | Excellent | Excellent | Excellent |
| Polystyrene (PS) | Excellent | Excellent | Excellent |
| HDPE | Excellent | Excellent | Excellent |
| Nylon | Excellent | Excellent | Excellent |
| Polycarbonate (PC) | Average | Not Recommended | Not Recommended |
| Polyurethane (TPU) | Good | Good | Good |
| Acrylic | Good | Good | Good |
Textured Finish — SPI D
| Material | D-1 | D-2 | D-3 |
|---|---|---|---|
| ABS | Excellent | Excellent | Good |
| Polypropylene (PP) | Excellent | Excellent | Excellent |
| Polystyrene (PS) | Excellent | Excellent | Good |
| HDPE | Excellent | Excellent | Excellent |
| Nylon | Excellent | Excellent | Good |
| Polycarbonate (PC) | Excellent | Not Recommended | Not Recommended |
| Polyurethane (TPU) | Excellent | Excellent | Good |
| Acrylic | Average | Average | Average |
Looking for SPI Mold Classifications & Standards?
SPI surface finish standards define the texture and appearance of mold surfaces and molded parts, while SPI mold classifications define the expected construction, durability, and service life of the tooling.
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