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SPI / SPE Mold Finish Standards (A1–D3): Chart, Applications & Inspection Checklist

SPI mold finish standards define the visual and tactile quality of injection-molded surfaces. Ranging from A-1 high-gloss mirror polish to D-3 textured blast finishes, these grades provide a universal language for engineers to specify appearance and functional requirements.

This engineer-ready guide covers the complete SPI/SPE finish chart, including diamond buffing, sandpaper, stone, and bead blast methods. Learn how to specify finishes on drawings and avoid cosmetic disputes during inspection.

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High-precision steel mold cavity showing SPI A1 mirror diamond buff finish vs textured surfaces

What is SPI / SPE Mold Finish Standard?

Surface Finish Definition

SPI mold finish standards are a visual and tactile grading system used to define the surface quality of injection molds. Ranging from mirror polishes (Grade A) to coarse blast textures (Grade D), these criteria ensure consistent cosmetic appearance and functional performance across global manufacturing supply chains.

Unlike mechanical roughness values (Ra) which provide a mathematical average of surface height, SPI is a process-driven standard. It specifies the exact method of preparation—whether using diamond buffing, paper, stones, or glass beads—to achieve a specific visual result. Ra can be a helpful secondary metric, but it cannot fully capture the light reflection and directional clarity defined by SPI grades.

The final outcome is influenced by more than just the polishing tool. Engineering variables such as the choice of mold steel (e.g., S136 vs. P20), geometric accessibility (the ability to reach deep ribs or corners), and molding parameters like melt temperature significantly affect how the plastic replicates the mold surface.

Expert Note: Always define cosmetic zones on your drawings to ensure polishing effort is focused where visual impact is highest.

High-precision injection mold steel with SPI A1 mirror finish and diamond buff polishing process

SPI A vs B vs C vs D — What’s the Difference?

The core distinction between SPI finish grades lies in the polishing medium used: Grade A uses diamond buffing for high-gloss mirror surfaces; Grade B uses grit paper for semi-gloss; Grade C uses stones for matte finishes; and Grade D uses dry blasting for textured, non-reflective surfaces.

Mirror Finish

Diamond Buffing

Semi-Gloss

Grit Paper

Matte Finish

Polishing Stone

Textured

Dry Blasting

Review the technical parameters and Ra values in the Full SPI Finish Chart Below ↓

SPI Finish Grade Table (A1–D3)

Pro Tip: To minimize cosmetic disputes, explicitly specify "SPI Grade + Cosmetic Zone + Inspection Conditions + Reference Plaque" on your technical drawings.

SPI Grade	Finishing Method	Media Used	Typical Ra (µin / µm)	Visual Appearance & Application
A-1	Super High Mirror	#3 Diamond Buff	0.5 - 1 / 0.012 - 0.025	Lens-quality mirror polish. For high-end optical parts.
A-2	High Mirror	#6 Diamond Buff	1 - 2 / 0.025 - 0.05	High-gloss reflective. Clear bezels and cosmetic housings.
A-3	Normal Mirror	#15 Diamond Buff	2 - 3 / 0.05 - 0.075	Glossy finish. Standard for transparent components.
B-1	Semi-Gloss	600 Grit Paper	3 - 5 / 0.075 - 0.125	Fine finish, removes all tool marks. Consumer electronics.
B-2	Medium-Gloss	400 Grit Paper	5 - 7 / 0.125 - 0.175	Uniform semi-gloss. General exterior housings.
B-3	Low-Gloss	320 Grit Paper	10 - 12 / 0.25 - 0.3	Smooth matte. For non-cosmetic visible parts.
C-1	Matte Stone	600 Stone	12 - 15 / 0.3 - 0.4	Dull matte finish. Interior functional components.
C-2	Medium Stone	400 Stone	25 - 28 / 0.6 - 0.7	Rough matte. Good for part release and mold hiding.
C-3	Coarse Stone	320 Stone	38 - 42 / 0.9 - 1.0	Visible tool/stone marks. For hidden structural parts.
D-1	Fine Blast	#11 Glass Bead	30 - 35 / 0.8 - 0.9	Uniform satin finish. Anti-glare and light diffusion.
D-2	Medium Blast	#240 Oxide	45 - 55 / 1.1 - 1.4	Classic textured matte. Hides fingerprints/scratches.
D-3	Coarse Blast	#80 Oxide	130 - 160 / 3.3 - 4.0	Heavy texture. For grip or non-slip surfaces.

How to Specify SPI Finish on Drawings

Ambiguous finish callouts are the #1 cause of cosmetic disputes. Use these copy-ready engineering notes to define exactly how your parts should be finished and inspected.

Grade: High Cosmetic / Clear

SURFACE FINISH TO BE SPI A2 IN COSMETIC ZONE A. INSPECT UNDER 5000K DIFFUSE LIGHT, 45° VIEWING ANGLE, 30–50 CM DISTANCE. REFERENCE PLAQUE REQUIRED FOR ACCEPTANCE.

Best for: Bezel, Lens, High-gloss housings.

Grade: Textured Matte

SPI D2 FINISH ON EXTERIOR MATTE SURFACES. NO DIRECTIONAL TOOLING OR BLASTING LINES PERMITTED. SURFACES MUST BE CLEANED AND FREE OF MEDIA RESIDUE AFTER BLASTING.

Best for: Industrial covers, non-slip grips.

Grade: Functional Matte

INTERNAL SURFACES TO BE SPI C2 (STONE FINISH). ALL TOOLING MARKS TO BE REMOVED. MINIMUM DRAFT OF 1.5° REQUIRED TO ENSURE CLEAN RELEASE WITHOUT DRAG MARKS.

Best for: Internal ribs, structural chassis.

Understanding "Cosmetic Zone Marking"

Don't over-specify. Mark your drawings with Zone A (High Visibility), Zone B (Occasional Visibility), and Zone C (Hidden/Internal). This allows the toolmaker to focus polishing hours on the critical areas, reducing both cost and lead time.

Tip: Use a thick dashed line on your 2D print to delineate where the high-gloss finish ends and the functional finish begins.

Engineering drawing showing cosmetic zone marking for injection mold finish

Ready to finalize your drawing notes?

Ensure your finish callouts are manufacturable and cost-effective.

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Inspection & Acceptance: Avoiding Cosmetic Disputes

Checklist Summary

How to inspect SPI mold finishes correctly and achieve consistent sign-off:

Use 5000K-6500K diffuse daylight sources.
Position part at a 45-degree viewing angle.
Maintain a 30-50 cm (12-20 in) inspection distance.
Compare against a master Reference Plaque.
Allow 5-10 seconds of observation per zone.
Define "Cosmetic Zone A" vs. non-visible areas.

Light & Angle

Inspection must occur under 5000K-6500K diffuse light. Use "raking light" at a 45° angle to highlight scratches or directional polish marks that are invisible under direct overhead lighting.

Distance & Time

Standard observation is at arm’s length (30–50 cm). Avoid "microscopic auditing" for standard cosmetic parts; limit viewing time to 10 seconds to simulate real-world consumer interaction.

Reference Plaque

The most critical tool. A certified physical plaque provides a tangible "Go/No-Go" baseline that Ra meters cannot replicate, especially for textured Grade D finishes.

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Why specifying "SPI A2" alone leads to disputes

SPI describes the process (the polishing medium), not the final part appearance. Without defining the steel type (e.g., S136 vs P20) and inspection lighting, the same "A2" process can look drastically different due to steel purity and molding temperature delta.

Special Note: Transparent & Optical Parts

For clear parts (PC, PMMA, PET), SPI finish is only one factor. You must explicitly define Haze (%) and Optical Distortion limits.

Haze: Measures light scattering; mirror polish does not guarantee clarity.
Distortion: Caused by uneven wall thickness or cooling delta, regardless of A1 polish.

Optical clarity inspection for transparent injection molded parts with A1 mirror finish

SPI Finish vs. Ra: Should You Use Roughness Values?

Engineering Position: Ra (Roughness Average) should be treated as a secondary verification tool, not the primary acceptance standard for cosmetic injection molded surfaces.

The Limitation of Ra in Cosmetics

Ra measures the arithmetic mean of surface profile heights, but it fails to capture the Visual Reflection and Optical Clarity of a part. Two surfaces can have identical Ra values yet appear completely different to the human eye.

Directionality: A stone finish (Grade C) has random peaks, while a paper finish (Grade B) has linear scratches. Ra often ignores these directional patterns that define gloss.
Reflectivity: Ra cannot distinguish between a "hazy" mirror and a "crisp" mirror (Grade A), as it doesn't account for the microscopic slope of surface peaks.

Material Replication & Fidelity

A mold steel polished to an SPI A-2 standard will yield different Ra values depending on the resin used. Higher viscosity resins or those with Glass Fiber (GF) fillers do not replicate the mold surface at 100% fidelity.

Steel Grade: High-chrome steels (S136/420) achieve better SPI results than P20 at the same Ra level due to carbide distribution.
Resin Delta: Amorphous materials (PC, ABS) replicate polish better than semi-crystalline materials (PP, PE).

Technical analysis of surface roughness Ra vs SPI mold finish standards for injection molding

Common Problems: Why Mirror Polish Still Shows Defects

An SPI A-1 polish on the steel doesn't always guarantee a perfect part. Understanding whether a defect originates from the toolroom (polishing) or the injection floor (process) is critical for troubleshooting.

Tooling-Induced Defects

Polishing Physics

Orange Peel Result of over-polishing or using excessive pressure, causing the steel's grain structure to "ripple."

Pitting / Pinholes Caused by non-metallic inclusions in low-purity steel being pulled out during diamond buffing.

Scratches / Swirl Marks Evidence of contaminated diamond paste or skipping grit stages too quickly during preparation.

Process-Induced Defects

Molding Dynamics

Flow Lines & Weld Lines Velocity changes or cold material fronts meeting; magnified by high-reflectivity mirror surfaces.

Splay (Silver Streaks) Moisture in resin or material degradation appearing as "steam" marks on the glossy finish.

Burn Marks & Gate Blush Compressed gas (venting issues) or high shear stress at the gate causing localized dullness.

Eliminate cosmetic uncertainty before production

Our engineering team provides deep-level analysis of steel purity and gate locations to ensure your mirror finish remains flawless throughout the production run.

Request a finish plaque sample before production Ask for steel recommendation (P20 vs H13 vs 420/S136)

Best SPI Finishes by Application: Quick Selection

Transparent & Optical Parts

A2 / A3

Typical Parts: Display lenses, light pipes, clear bezels, transparent housings.
Requirement: Maximum light transmission and minimal haze.
Steel Recommendation: S136 or 420 high-purity ESR stainless steel to avoid pitting.

Consumer Aesthetics

B1 / B2

Typical Parts: Consumer electronics, exterior trim, appliance front panels.
Requirement: Smooth semi-gloss appearance that hides minor molding artifacts.
Advantage: Much lower maintenance cost than mirror-finish Grade A tooling.

Functional & Hidden Parts

C1 / C2

Typical Parts: Internal structural ribs, chassis, non-visible battery covers.
Requirement: Uniform part release and tool mark removal without extra polishing cost.
Draft Tip: Standard stone finishes facilitate easier part ejection in deep cavities.

Anti-Glare & Tactile Surfaces

D1 / D2 / D3

Typical Parts: Industrial grips, automotive dashboards, matte buttons.
Requirement: Diffuse light reflection and improved scratch resistance.
Caution: Requires increased draft angles (typically 1°–1.5° per SPI grade level).

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When NOT to Use: Engineer’s Field Notes

Avoid Grade A on Glass-Filled (GF) Resins: GF fibers will cause rapid "micro-scratching," destroying the mirror polish within a few thousand cycles.

Avoid Grade D on Low-Draft Ribs: Blasted textures create high friction. Using Grade D on ribs with < 1.5° draft will lead to part sticking and drag marks.

Avoid Mirror Polish on High-Stress Zones: High-gloss surfaces act as a "magnifying glass" for weld lines and sink marks.

Don't Specify Grade A for Internal Components: It adds unnecessary lead time and cost without any functional or visual benefit to the end-user.

Downloadable Templates (Engineer-Ready)

Standardize your production and quality control with our professional toolkits. These resources are designed for seamless integration into your toolroom and drafting workflows.

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SPI Finish Chart (Excel/PDF)

Complete technical data including Ra values, polishing media, and appearance descriptions for Grades A1–D3.

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Drawing Callout Notes (Copy/Paste)

Pre-written engineering notes and cosmetic zone templates to ensure your drawing specs are unambiguous.

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Inspection Checklist (One-Page)

A structured walkthrough for QC teams to verify surface quality under correct lighting and distance parameters.

Download SPI Chart + Checklist Get Drawing Note Templates

Engineering documentation and SPI finish reference chart for injection mold production

Optional: Sanity-Check Your Finish Callouts

Want a second set of eyes on your print? Send your drawing for a quick feedback review. We'll flag potential manufacturability issues with your finish specs and draft angles.

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FAQ (Engineer-Focused)

What is the most cost-effective SPI finish for internal parts?

For internal components, SPI C-1 or C-2 (Stone Finish) is most efficient. It removes all tool marks and aids in part release without the added cost and lead time of secondary diamond buffing.

Can Ra values replace SPI grades on a drawing?

Ra is a helpful secondary metric but cannot capture reflection and clarity. SPI grades specify the process (media used), ensuring visual consistency that a mathematical average height (Ra) cannot guarantee.

Which SPI finish is best for avoiding glare on housings?

SPI D-1 or D-2 (Bead Blast) is recommended. These textures diffuse light, significantly reducing glare while improving the part's resistance to visible fingerprints and scratches.

How do I prevent "Orange Peel" on mirror finishes?

Orange peel is caused by over-polishing or using low-purity steel. Ensure you specify high-purity ESR stainless steel (S136/420) and limit polishing pressure to maintain the steel's grain integrity.

Related Manufacturing Capabilities

Injection Molding

High-volume production with precision tolerance control and automated QC systems.

Expert mold making and surface polishing

Mold Making & Polishing

In-house toolroom equipped for high-gloss mirror polishing and custom texturing.

DFM & Material Selection

Pre-production engineering support to optimize draft, wall thickness, and finish feasibility.

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