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.

Key Takeaways

| Aspect | Key Information |

SPI Finish Classifications The Society of the Plastics Industry (SPI) developed the standard finish system still used today.

Finish Categories CategoryFinish TypeRa RangePolish MethodAHigh-gloss0-2 Ra#14, 13, 12 diamond buffBStandard polish2-4 Ra#6, 5, 4 diamond buffCSemi-gloss4-8 Ra#400-600 grit paperDSatin8-16 Ra#320-400 grit paperEMatte16-32 Ra#240-320 grit paperFMatte32-63 Ra#180-240 grit paperGRough matte63-125 Ra#120-180 grit paper

Detailed SPI Specifications SPI CodeRa (µin)Ra (µm)Polish SequenceA-10-10.00-0.02514, 13, 12, 11, 10 diamondA-21-20.025-0.0510, 9, 8 diamondA-32-30.05-0.0758, 7, 6 diamondB-12-40.05-0.106, 5, 4 diamondB-24-60.10-0.154, 3 diamondB-36-80.15-0.203, 2 diamondC-14-80.10-0.20600, 500, 400 gritC-28-120.20-0.30400, 320 gritC-312-160.30-0.40320, 280 gritD-116-240.40-0.60240, 220 gritD-224-320.60-0.80220, 180 gritD-332-400.80-1.00180, 150 gritE-140-631.00-1.60150, 120 gritF-163-1001.60-2.50120, 100 gritG-1100-1252.50-3.20100, 80 grit Ra = Arithmetic average roughness

Finish Selection by Application

Cosmetic (Class A) Surfaces ApplicationRecommended FinishNotesAutomotive exteriorSPI A-1 to A-2Show surfacesConsumer visibleSPI A-2 to B-1High consumer appealAppliance exteriorsSPI B-1 to B-3Visible consumer areasMedical devicesSPI A-2 to B-2Cleanability important

Functional Surfaces ApplicationRecommended FinishNotesNon-visible interiorsSPI C-1 to C-3Standard productionTextured surfacesSPI D-1 to E-1Hides flow linesGrip surfacesSPI D-2 to E-2Tactile feedbackTextured (heavy)SPI F-1 to G-1Maximum texture

Engineering Surfaces ApplicationRecommended FinishNotesSliding surfacesSPI C-1 to C-2Reduce frictionSeal surfacesSPI B-2 to C-1Critical sealingPrecision fitSPI A-2 to B-1Dimensional control

Cost Implications

Polish Cost by Finish Level FinishRelative CostCost MultiplierSPI D-F (standard)Baseline1.0×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 SurfaceFinishPolish CostNon-visible (100 in²)SPI D-1$200-300Interior visible (100 in²)SPI B-2$500-800Class A exterior (100 in²)SPI A-1$1,500-2,500

Finish Cost Factors FactorImpactSurface areaMore area = more costGeometry complexityComplex = more costSteel typeHarder steel = harder to polishTexture depthDeep textures = special processReproductionMultiple cavities multiply cost

Finish and Parting Line

Parting Line Finish Strategy LocationRecommended FinishReasonVisible parting lineMatch adjacent finishAppearanceHidden parting lineSPI D-ECost savingsSlider/lifter surfacesSPI C-DWear resistance

Managing Parting Line Visibility Finish AdjacentParting LineVisual ImpactSPI A-1Same finishVisibleSPI A-1SPI BVisibleSPI A-1TexturedMinimally visibleSPI D-1Same finishNot visible

Textured Finishes

Texture Methods MethodDepth ControlConsistencyCostEDM textureGoodExcellent$$Acid etchExcellentGood$Laser textureExcellentExcellent$$$Abrasive blastPoorFair$Print/coatExcellentExcellent$$

Texture Depth and Ejection

| Texture Depth | Release Behavior | Risk Level |

Common Texture Patterns PatternSPI EquivalentApplicationFine pebbleSPI D-1Consumer electronicsCoarse pebbleSPI E-1AppliancesHairlineSPI A-2AutomotiveWood grainSPI F-1Consumer productsCarbon fiberTexture dependentAutomotive

Finish and Material Compatibility

Material-Specific Considerations MaterialRecommended FinishNotesPC (transparent)SPI A-1 to A-3Clarity importantABS (opaque)SPI A-1 to C-1Accepts various finishesPP (low surface energy)SPI B-1 to C-1May require texturePE (low surface energy)SPI B-1 to C-1May require textureGlass-filledSPI B-1 to C-1Texture may show fibers

Finish Effect on Appearance FinishGloss LevelHides ImperfectionsSPI AHighPoorSPI BMedium-HighPoorSPI CMediumFairSPI DMedium-LowGoodSPI ELowBetterSPI F-GVery LowBest

Tool Steel and Finish Capability

Finish by Steel Type Steel TypeBest Finish AchievablePolish DifficultyAluminumSPI A-2 to A-3EasyP20 (pre-hardened)SPI A-1 to A-2ModerateP20 (hardened)SPI A-1ModerateS7SPI A-1ModerateH13SPI A-1Moderate-DifficultD2SPI A-1Difficult

Hardness and Polish HardnessPolish QualityNotes<28 HRC (Al)GoodEasy to polish28-32 HRC (P20)ExcellentStandard production48-52 HRCVery GoodTakes excellent finish54-58 HRCExcellentBest 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 FinishSPI CodeRa (µin)Ra (µm)MirrorA-10-10-0.025High-glossA-2 to A-31-30.025-0.075Standard polishB-1 to B-32-80.05-0.20SatinC-1 to C-34-160.10-0.40MatteD-1 to D-316-400.40-1.00TexturedE-F-G40-1251.00-3.20

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" depth

Finish and Quality Control

Inspection Methods MethodAccuracyUseVisual comparisonSubjectiveInitial approvalTactile (finger)SubjectiveGeneral assessmentProfilometerQuantitativeMeasurementOptical interferometryHigh precisionCritical surfaces

Tolerance Ranges Finish LevelTarget RaToleranceSPI A-10.5 Ra±0.25 RaSPI A-21.5 Ra±0.5 RaSPI B-25 Ra±1 RaSPI C-210 Ra±2 RaSPI D-228 Ra±4 Ra

Troubleshooting Finish Issues

Common Problems ProblemLikely CauseSolutionGloss variationInconsistent polishRe-polish affected areaOrange peelMaterial or mold tempAdjust processSink marks near textureThick sectionRedesign or relocateTexture not reproducingMold texture wornRepair or replaceParting line visibleFinish mismatchMatch finishes

Process Effects on Finish Process ParameterEffect on FinishMold temperatureHigher = higher glossMelt temperatureHigher = higher glossInjection speedFaster = higher glossPacking pressureHigher = better replicationCooling timeAdequate = consistent

Cost-Saving Strategies

Strategic Finish Selection StrategySavingsRiskUse texture instead of polish30-50%May affect designMinimize Class A area20-40%Design constraintUse standard finishes10-30%Limited aestheticsSpecify by Ra, not SPISameNone

Finish Optimization ActionCost ImpactLimit high-polish to visible areasSignificant savingsUse texture for non-critical20-30% savingsMatch all surfaces to lowest grade30-50% savingsPlan texture earlyAvoid 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.