Mold Textures Finishes Spi Standards After 22 years of building molds, I’ve learned that surface finish isn’t just about looks—it’s about function, cost, and manufacturing practicality.
The wrong finish can make a part impossible to eject or cost three times as much to tool. Let me break down SPI standards and when to use each.
SPI Finish Classifications The Society of the Plastics Industry (SPI) developed the standard finish system still used today.
Finish Categories
| Category | Finish Type | Ra Range | Polish Method |
|---|---|---|---|
| A | High-gloss | 0-2 Ra | #14, 13, 12 diamond buff |
| B | Standard polish | 2-4 Ra | #6, 5, 4 diamond buff |
| C | Semi-gloss | 4-8 Ra | #400-600 grit paper |
| D | Satin | 8-16 Ra | #320-400 grit paper |
| E | Matte | 16-32 Ra | #240-320 grit paper |
| F | Matte | 32-63 Ra | #180-240 grit paper |
| G | Rough matte | 63-125 Ra | #120-180 grit paper |
Detailed SPI Specifications
| SPI Code | Ra (µin) | Ra (µm) | Polish Sequence |
|---|---|---|---|
| A-1 | 0-1 | 0.00-0.025 | 14, 13, 12, 11, 10 diamond |
| A-2 | 1-2 | 0.025-0.05 | 10, 9, 8 diamond |
| A-3 | 2-3 | 0.05-0.075 | 8, 7, 6 diamond |
| B-1 | 2-4 | 0.05-0.10 | 6, 5, 4 diamond |
| B-2 | 4-6 | 0.10-0.15 | 4, 3 diamond |
| B-3 | 6-8 | 0.15-0.20 | 3, 2 diamond |
| C-1 | 4-8 | 0.10-0.20 | 600, 500, 400 grit |
| C-2 | 8-12 | 0.20-0.30 | 400, 320 grit |
| C-3 | 12-16 | 0.30-0.40 | 320, 280 grit |
| D-1 | 16-24 | 0.40-0.60 | 240, 220 grit |
| D-2 | 24-32 | 0.60-0.80 | 220, 180 grit |
| D-3 | 32-40 | 0.80-1.00 | 180, 150 grit |
| E-1 | 40-63 | 1.00-1.60 | 150, 120 grit |
| F-1 | 63-100 | 1.60-2.50 | 120, 100 grit |
| G-1 | 100-125 | 2.50-3.20 | 100, 80 grit |
Ra = Arithmetic average roughness
Finish Selection by Application
Cosmetic (Class A) Surfaces
| Application | Recommended Finish | Notes |
|---|---|---|
| Automotive exterior | SPI A-1 to A-2 | Show surfaces |
| Consumer visible | SPI A-2 to B-1 | High consumer appeal |
| Appliance exteriors | SPI B-1 to B-3 | Visible consumer areas |
| Medical devices | SPI A-2 to B-2 | Cleanability important |
Functional Surfaces
| Application | Recommended Finish | Notes |
|---|---|---|
| Non-visible interiors | SPI C-1 to C-3 | Standard production |
| Textured surfaces | SPI D-1 to E-1 | Hides flow lines |
| Grip surfaces | SPI D-2 to E-2 | Tactile feedback |
| Textured (heavy) | SPI F-1 to G-1 | Maximum texture |
Engineering Surfaces
| Application | Recommended Finish | Notes |
|---|---|---|
| Sliding surfaces | SPI C-1 to C-2 | Reduce friction |
| Seal surfaces | SPI B-2 to C-1 | Critical sealing |
| Precision fit | SPI A-2 to B-1 | Dimensional control |
Cost Implications
Polish Cost by Finish Level
| Finish | Relative Cost | Cost Multiplier |
|---|---|---|
| SPI D-F (standard) | Baseline | 1.0× |
| SPI C | +30-50% | 1.3-1.5× |
| SPI B | +50-100% | 1.5-2.0× |
| SPI A | +100-200% | 2.0-3.0× |
| SPI A-1 (mirror) | +200-400% | 3.0-5.0× |
Cost Example
| Part Surface | Finish | Polish Cost |
|---|---|---|
| Non-visible (100 in²) | SPI D-1 | $200-300 |
| Interior visible (100 in²) | SPI B-2 | $500-800 |
| Class A exterior (100 in²) | SPI A-1 | $1,500-2,500 |
Finish Cost Factors
| Factor | Impact |
|---|---|
| Surface area | More area = more cost |
| Geometry complexity | Complex = more cost |
| Steel type | Harder steel = harder to polish |
| Texture depth | Deep textures = special process |
| Reproduction | Multiple cavities multiply cost |
Finish and Parting Line
Parting Line Finish Strategy
| Location | Recommended Finish | Reason |
|---|---|---|
| Visible parting line | Match adjacent finish | Appearance |
| Hidden parting line | SPI D-E | Cost savings |
| Slider/lifter surfaces | SPI C-D | Wear resistance |
Managing Parting Line Visibility
| Finish Adjacent | Parting Line | Visual Impact |
|---|---|---|
| SPI A-1 | Same finish | Visible |
| SPI A-1 | SPI B | Visible |
| SPI A-1 | Textured | Minimally visible |
| SPI D-1 | Same finish | Not visible |
Textured Finishes
Texture Methods
| Method | Depth Control | Consistency | Cost |
|---|---|---|---|
| EDM texture | Good | Excellent | $$ |
| Acid etch | Excellent | Good | $ |
| Laser texture | Excellent | Excellent | $$$ |
| Abrasive blast | Poor | Fair | $ |
| Print/coat | Excellent | Excellent | $$ |
Texture Depth and Ejection
| Texture Depth | Release Behavior | Risk Level |
|---|---|---|
| <0.0005“ | Easy release | Low |
| 0.0005-0.001“ | Good release | Low-Medium |
| 0.001-0.002“ | Acceptable | Medium |
| 0.002-0.003“ | May require force | Medium-High |
| >0.003“ | Problematic | High |
Common Texture Patterns
| Pattern | SPI Equivalent | Application |
|---|---|---|
| Fine pebble | SPI D-1 | Consumer electronics |
| Coarse pebble | SPI E-1 | Appliances |
| Hairline | SPI A-2 | Automotive |
| Wood grain | SPI F-1 | Consumer products |
| Carbon fiber | Texture dependent | Automotive |
Finish and Material Compatibility
Material-Specific Considerations
| Material | Recommended Finish | Notes |
|---|---|---|
| PC (transparent) | SPI A-1 to A-3 | Clarity important |
| ABS (opaque) | SPI A-1 to C-1 | Accepts various finishes |
| PP (low surface energy) | SPI B-1 to C-1 | May require texture |
| PE (low surface energy) | SPI B-1 to C-1 | May require texture |
| Glass-filled | SPI B-1 to C-1 | Texture may show fibers |
Finish Effect on Appearance
| Finish | Gloss Level | Hides Imperfections |
|---|---|---|
| SPI A | High | Poor |
| SPI B | Medium-High | Poor |
| SPI C | Medium | Fair |
| SPI D | Medium-Low | Good |
| SPI E | Low | Better |
| SPI F-G | Very Low | Best |
Tool Steel and Finish Capability
Finish by Steel Type
| Steel Type | Best Finish Achievable | Polish Difficulty |
|---|---|---|
| Aluminum | SPI A-2 to A-3 | Easy |
| P20 (pre-hardened) | SPI A-1 to A-2 | Moderate |
| P20 (hardened) | SPI A-1 | Moderate |
| S7 | SPI A-1 | Moderate |
| H13 | SPI A-1 | Moderate-Difficult |
| D2 | SPI A-1 | Difficult |
Hardness and Polish
| Hardness | Polish Quality | Notes |
|---|---|---|
| <28 HRC (Al) | Good | Easy to polish |
| 28-32 HRC (P20) | Excellent | Standard production |
| 48-52 HRC | Very Good | Takes excellent finish |
| 54-58 HRC | Excellent | Best finish capability |
Specifying Finishes on Drawings
Drawing Callout Format
FINISH: SPI [LETTER]-[NUMBER] EXAMPLE: FINISH: SPI B-2 ALTERNATIVE: 4-6 Ra (SPI B-2)Finish Specification Guide
| Desired Finish | SPI Code | Ra (µin) | Ra (µm) |
|---|---|---|---|
| Mirror | A-1 | 0-1 | 0-0.025 |
| High-gloss | A-2 to A-3 | 1-3 | 0.025-0.075 |
| Standard polish | B-1 to B-3 | 2-8 | 0.05-0.20 |
| Satin | C-1 to C-3 | 4-16 | 0.10-0.40 |
| Matte | D-1 to D-3 | 16-40 | 0.40-1.00 |
| Textured | E-F-G | 40-125 | 1.00-3.20 |
Multiple Finish Specification
SURFACE FINISH:
- A-Surfaces (Visible): SPI A-2
- B-Surfaces (Secondary): SPI B-2
- C-Surfaces (Hidden): SPI C-1
- Texture (Where Indicated): Laser Texture, 0.0015" depthFinish and Quality Control
Inspection Methods
| Method | Accuracy | Use |
|---|---|---|
| Visual comparison | Subjective | Initial approval |
| Tactile (finger) | Subjective | General assessment |
| Profilometer | Quantitative | Measurement |
| Optical interferometry | High precision | Critical surfaces |
Tolerance Ranges
| Finish Level | Target Ra | Tolerance |
|---|---|---|
| SPI A-1 | 0.5 Ra | ±0.25 Ra |
| SPI A-2 | 1.5 Ra | ±0.5 Ra |
| SPI B-2 | 5 Ra | ±1 Ra |
| SPI C-2 | 10 Ra | ±2 Ra |
| SPI D-2 | 28 Ra | ±4 Ra |
Troubleshooting Finish Issues
Common Problems
| Problem | Likely Cause | Solution |
|---|---|---|
| Gloss variation | Inconsistent polish | Re-polish affected area |
| Orange peel | Material or mold temp | Adjust process |
| Sink marks near texture | Thick section | Redesign or relocate |
| Texture not reproducing | Mold texture worn | Repair or replace |
| Parting line visible | Finish mismatch | Match finishes |
Process Effects on Finish
| Process Parameter | Effect on Finish |
|---|---|
| Mold temperature | Higher = higher gloss |
| Melt temperature | Higher = higher gloss |
| Injection speed | Faster = higher gloss |
| Packing pressure | Higher = better replication |
| Cooling time | Adequate = consistent |
Cost-Saving Strategies
Strategic Finish Selection
| Strategy | Savings | Risk |
|---|---|---|
| Use texture instead of polish | 30-50% | May affect design |
| Minimize Class A area | 20-40% | Design constraint |
| Use standard finishes | 10-30% | Limited aesthetics |
| Specify by Ra, not SPI | Same | None |
Finish Optimization
| Action | Cost Impact |
|---|---|
| Limit high-polish to visible areas | Significant savings |
| Use texture for non-critical | 20-30% savings |
| Match all surfaces to lowest grade | 30-50% savings |
| Plan texture early | Avoid expensive rework |
Checklist
Finish Specification
- Class A surfaces identified
- Finish level for each surface
- Parting line treatment planned
- Texture requirements specified
- Measurement method defined
- Acceptance criteria documented
Tool Construction
- Steel selection appropriate for finish
- Polish sequence specified
- Texture method selected
- Polish marks mapped
- Quality checkpoints defined
Validation
- First article inspection complete
- Surface finish measured
- Visual approval obtained
- Process parameters optimized
- Document approved standard
The Bottom Line Surface finish is part engineering specification, part aesthetic choice, and part cost decision.
The right finish depends on visibility, function, and budget. The SPI standards give you a common language. The application tells you what’s needed. And the cost analysis tells you what’s practical. Don’t over-specify finishes you don’t need. Don’t under-specify where it matters. Match the finish to the application. That’s how you build molds efficiently.