Complete guide to SPI surface finish standards for injection molds, covering gloss levels, surface roughness values, polishing techniques, and texture selection for different applications.
The Society of the Plastics Industry (SPI) defines six grades of surface finish for injection molds, ranging from high-gloss diamond-polished surfaces to coarse stone finishes.
A-1 (100 percent mirror): Surface roughness Ra 0.012 micrometers. Achieved with diamond grits through 3000 mesh.
A-2 (90 percent mirror): Ra 0.025 micrometers. Diamond polish through 2000 mesh.
A-3 (80 percent mirror): Ra 0.050 micrometers. Diamond polish through 1200 mesh.
Applications: Optical lenses, clear parts where transparency is critical, decorative consumer products
B-1 (600 grit paper): Ra 0.100 micrometers.
B-2 (400 grit paper): Ra 0.125 micrometers.
B-3 (320 grit paper): Ra 0.200 micrometers.
Applications: Automotive interior parts, appliance housings, general consumer goods with moderate appearance requirements
C-1 (600 stone): Ra 0.280 micrometers.
C-2 (400 stone): Ra 0.350 micrometers.
C-3 (320 stone): Ra 0.450 micrometers.
Applications: Industrial parts, under-hood automotive components, parts where appearance is secondary to function
D-1 through D-3: Various textured finishes created by sandblasting or chemical etching.
Applications: Interior trim parts, anti-slip surfaces, parts requiring matte finish or tactile texture
| Application | Recommended SPI Grade | Typical Ra |
|---|---|---|
| Optical lenses | A-1 | 0.012 |
| Clear cosmetic parts | A-3 | 0.050 |
| Automotive interior | B-1 to B-2 | 0.100 to 0.125 |
| Appliance housings | B-3 | 0.200 |
| Industrial parts | C-2 to C-3 | 0.350 to 0.450 |
| Hidden surfaces | C-3 | 0.450 |