SPI Surface Finish Standards
SPI surface finish is a standardized system used in injection mold manufacturing to define the surface texture and appearance of plastic parts.
SPI stands for the Society of the Plastics Industry, and the standard is widely used to specify mold surface quality during design and production.
In injection molding, the final part surface is a direct replication of the mold cavity surface. Therefore, SPI standards are applied to mold polishing rather than post-processing.
Learn how this affects molded parts in: Injection Molding Surface Finish Guide
SPI Surface Finish Chart (Full Table)
Below is the complete SPI surface finish classification.
| Grade | Finish Type | Method | Surface Roughness (Ra) | Description |
|---|---|---|---|---|
| A1 | Mirror finish | Diamond buff | ~0.012 μm | Highest quality optical finish |
| A2 | High gloss | Diamond buff | ~0.025 μm | High-gloss surface |
| A3 | Gloss finish | Diamond buff | ~0.05 μm | Standard glossy finish |
| B1 | Semi-gloss | 600 grit paper | ~0.10 μm | Smooth but not reflective |
| B2 | Semi-gloss | 400 grit paper | ~0.15 μm | Moderate finish |
| B3 | Semi-gloss | 320 grit paper | ~0.20 μm | Lower semi-gloss |
| C1 | Matte | 600 grit stone | ~0.35 μm | Fine matte |
| C2 | Matte | 400 grit stone | ~0.45 μm | Standard matte |
| C3 | Matte | 320 grit stone | ~0.70 μm | Rough matte |
| D1 | Textured | Dry blast glass bead | — | Satin texture |
| D2 | Textured | Dry blast | — | Medium texture |
| D3 | Textured | Dry blast | — | Rough texture |
SPI A1, A2, A3 Explained (High-Gloss Finishes)
High-gloss finishes are the most demanding in injection mold polishing.
A1 – Mirror Finish
Achieved by diamond polishing
Used for optical components
Requires high-quality mold steel
A2 – High Gloss
Slightly lower than A1
Common for consumer products
A3 – Standard Gloss
Used for general-purpose glossy parts
These finishes are commonly used in glossy plastic parts
SPI B and C Finishes (Semi-Gloss and Matte)
B Series – Semi-Gloss
Achieved using sandpaper
Balanced between cost and appearance
C Series – Matte
Stone polished
Used to reduce glare and hide defects
These finishes are widely used in automotive interiors and industrial products.
SPI D Finishes (Textured Surfaces)
D finishes are created by sandblasting or chemical etching.
Applications include:
Grip surfaces
Decorative patterns
Anti-slip components
Textured finishes can also help hide molding defects.
How to Choose the Right SPI Surface Finish
Choosing the correct SPI finish depends on several factors.
Product Appearance
High-end products → A1 / A2
Industrial parts → B or C
Functionality
Optical clarity → A1
Anti-glare → C series
Grip → D series
Cost Consideration
Higher SPI grades require:
More polishing time
Higher mold precision
Increased manufacturing cost
SPI Surface Finish and Mold Polishing
SPI standards are directly linked to mold polishing processes.
A grades → diamond polishing
B grades → sandpaper
C grades → stone polishing
D grades → blasting
Common Mistakes When Specifying SPI Finish
Over-Specifying Finish
Using A1 when A2 or B1 is sufficient increases cost unnecessarily.
Ignoring Material Limitations
Some plastics cannot achieve mirror finishes effectively.
Not Considering Mold Design
Poor draft angles or geometry can affect achievable surface finish.
Applications of SPI Surface Finishes
SPI finishes are widely used in:
Automotive
Interior trims, decorative parts
Electronics
Device housings, glossy panels
Medical
Precision components
Consumer Products
Appliances and tools