SPI Surface Finish Standard

SPI Mold Finish Standards: A1–D3 Chart & Inspection Guide

Q: Can a drawing specify only “SPI A” or “SPI B”?

It is better to specify the exact grade. A1, A2 and A3—or B1, B2 and B3—are not identical finish levels. A production drawing should identify the required grade and the surface or area where that finish applies.

Q: Can surface roughness Ra replace an SPI finish callout?

Not completely. Ra provides a quantitative roughness measurement, but it does not fully describe gloss, reflection, directional polish marks or the visual character of a finished mold surface. Use the SPI grade for the finish class and add separate roughness requirements only where the project needs them.

Q: Does meeting the SPI grade automatically approve the molded part?

No. SPI verifies the requested tool-surface finish. Molded-part appearance can still be influenced by material behavior, processing, geometry and surface replication. Cosmetic approval should therefore use its own agreed inspection criteria and reference evidence.

Q: What should be added to a drawing besides the SPI grade?

Identify the exact grade, applicable surface and finish boundary. Where appearance approval is important, separately define the cosmetic zone, approved reference sample and any project-specific inspection conditions required for sign-off.

SPI mold finish comparison plaque showing A1 to D3 surface finish grades from mirror polish to textured finish — SPI A1–D3 grades describe mold-surface preparation from high-polish finishes to progressively more textured tool surfaces.

SPI mold finish standards provide a widely used industry reference for specifying how an injection-mold cavity or core surface should be prepared. The A1–D3 system ranges from high-polish A-series finishes to progressively rougher B, C and D surface preparations.

For engineers and buyers, the important question is not simply which finish looks smoother. A complete SPI requirement should identify the required grade, where it applies on the tool, how the surface is prepared, and how the finished mold surface will be inspected before production approval.

SPI and VDI are different mold-finish systems and should not be treated as automatically interchangeable. Their comparison, selection logic and conversion limitations should be reviewed separately rather than using an assumed one-to-one equivalence.

Scope boundary: this guide explains what SPI A1–D3 grades mean, how the corresponding tool surfaces are prepared, how SPI finishes should be called out on drawings, and how tool-side finish inspection should be separated from molded-part appearance acceptance. It does not provide a complete resin selection guide, defect-troubleshooting system, SPI-to-VDI conversion chart, or full mold-acceptance procedure.

SPI A1–D3: Quick Classification

A1–A3 Diamond-polished surfaces used when a high-gloss or mirror-type tool finish is required.

B1–B3 Fine abrasive-paper finishes that produce controlled semi-gloss tool surfaces.

C1–C3 Stone-finished surfaces used for progressively more visible machining texture and reduced gloss.

D1–D3 Dry-blasted surfaces that create progressively heavier matte or textured tool finishes.

Review SPI Finish Feasibility

Share the 3D model, drawing, resin and cosmetic requirements if you need help confirming whether the specified SPI finish is practical before mold steel is finalized.

Standard Definition

What Does the SPI Mold Finish Standard Actually Define?

SPI mold finish standards classify the intended tool-surface finish from A1 through D3. They provide a widely used industry reference for communicating the finish class and associated polishing, abrasive-paper, stone or blasting method expected on the mold surface. They do not, by themselves, define every molded-part appearance requirement or inspection condition.

What SPI Actually Defines

Tool-Side Finish Class: SPI identifies the requested mold-surface finish grade, such as A1, B2, C3 or D2, so the toolmaker knows the intended preparation level.
Associated Preparation Method: the grade is linked to a defined type of finishing route such as diamond polishing, abrasive paper, stone finishing or dry blasting.
Common Specification Language: the A1–D3 system gives designers, buyers and mold suppliers a shared reference when finish requirements are written on drawings and tooling documents.
Tooling Communication Baseline: the SPI callout helps align finish expectations across cavities, inserts and suppliers, but final consistency still depends on the actual finishing process, tool condition and inspection evidence.

What SPI Does Not Define

Molded-Part Cosmetic Approval: a mold surface can meet the specified SPI preparation while the molded part still shows flow marks, splay, gloss variation, weld lines or other appearance issues caused by material or process conditions.
Surface Roughness by Ra Alone: Ra can support measurement, but one roughness value does not fully describe reflection, gloss, polish direction or the complete visual character of an SPI finish.
Inspection Lighting or Viewing Conditions: SPI does not automatically establish the project-specific light level, viewing angle, viewing distance or visual-acceptance environment used for molded-part approval.
Cosmetic Zones: project-defined Zone A, B or C appearance areas are not the same system as SPI A, B, C or D finish families. Cosmetic zones must be defined separately on the drawing or acceptance specification.

Engineering boundary: SPI defines the requested tool-side finish class; it does not replace molded-part cosmetic criteria. When appearance is critical, the SPI grade should be paired with defined cosmetic zones, inspection conditions and, where appropriate, an approved reference plaque or Golden Sample.

A1–D3 Finish Chart

Complete SPI A1–D3 Mold Finish Chart

Same molded housing shown with SPI A2, B2 and D2 finishes to compare gloss, reflection and texture appearance — Molded appearance can vary even when the tool-side SPI grade is correctly prepared, so part approval should not rely on the grade name alone.

The SPI system contains 12 distinct finish grades from A1 through D3. Each grade has its own tool-side preparation method and should be specified individually rather than treating A, B, C or D as one interchangeable finish family.

The chart below focuses on the finish standard itself: preparation method, typical surface character, tool-side inspection focus and drawing callout. Resin selection, defect diagnosis and part-specific cosmetic approval are separate engineering decisions.

Typical appearance terms such as mirror, semi-gloss, matte or textured describe the general finish character. They should not be treated as a substitute for an approved reference plaque or project-specific molded-part acceptance criteria.

SPI A1–D3 Preparation and Inspection Reference

Each grade below is listed separately because the preparation medium and resulting surface character change within every SPI family. The drawing callout should identify the exact grade required on the specified mold surface.

Grade	Tool-Side Preparation	Typical Surface Character	Tool-Side Inspection Focus	Drawing Callout
A1	Grade #3 diamond buff, commonly referenced as approximately 6000-grit diamond.	Super-high-gloss or mirror-type polished surface.	Check polish uniformity, haze, fine scratches, distortion and localized polishing defects under controlled inspection.	SPI A1
A2	Grade #6 diamond buff, commonly referenced as approximately 3000-grit diamond.	High-gloss polished surface with slightly less mirror intensity than A1.	Check gloss consistency, polishing marks, haze and surface uniformity across the specified area.	SPI A2
A3	Grade #15 diamond buff, commonly referenced as approximately 1200-grit diamond.	Normal polished finish with visible gloss but less mirror reflection than A1 or A2.	Check uniform polish level, residual scratches and consistency between adjacent cavity or insert surfaces.	SPI A3
B1	600-grit abrasive paper.	Fine semi-gloss surface.	Check that paper marks are fine, uniform and free from isolated deep scratches or abrupt directional changes.	SPI B1
B2	400-grit abrasive paper.	Medium semi-gloss surface.	Check uniform sheen, consistent abrasive direction and absence of localized over-polishing.	SPI B2
B3	320-grit abrasive paper.	Coarser semi-gloss surface with more visible abrasive character.	Check consistency of the paper finish and confirm that no deeper scratches exceed the intended surface reference.	SPI B3
C1	600-grit stone.	Fine matte stone finish.	Check stone-mark uniformity, direction and removal of deeper machining or polishing defects.	SPI C1
C2	400-grit stone.	Medium matte finish.	Check consistent stone texture and absence of isolated gouges, deep scratches or unfinished areas.	SPI C2
C3	320-grit stone.	Coarser matte stone finish.	Check that the stone pattern is uniform across the specified surface and does not contain abnormal localized marks.	SPI C3
D1	Dry blast using #11 glass bead.	Satin-textured, low-gloss surface.	Check blast uniformity, patchiness, masking boundaries and consistency across the specified tool surface.	SPI D1
D2	Dry blast using #240 aluminum oxide.	Dull textured surface with stronger texture than D1.	Check texture consistency, local over-blast, unblasted patches and boundary control around adjacent surfaces.	SPI D2
D3	Dry blast using #24 aluminum oxide.	Roughest standard SPI textured finish.	Check overall blast uniformity, coarse-texture consistency, masking control and localized surface damage.	SPI D3

Do not collapse the grade: “SPI A” or “SPI B” is incomplete for a production drawing because A1, A2 and A3—or B1, B2 and B3—use different preparation levels. Specify the exact grade and the exact surface or zone where it applies.

Appearance is not defined by SPI alone: the tool-side grade establishes the preparation class, but molded-part appearance can still change with resin, processing, geometry and the project-specific inspection environment.

Tool-Side Preparation

How Are SPI A1–D3 Mold Finishes Produced?

SPI finish grades are created on the mold surface through four main preparation families: diamond polishing, abrasive-paper finishing, stone finishing and dry blasting. The exact grade determines the finishing medium or level within that family, while the quality of the result still depends on the condition of the underlying tool surface and the consistency of the finishing process.

A1–A3

Diamond Polishing

Process family: progressively fine diamond polishing is used to create the A-series high-polish surfaces.
Surface objective: remove visible polishing scratches and increase surface reflection as the grade moves toward A1.
Control focus: underlying tool marks, local over-polishing, haze, distortion and scratch consistency become increasingly visible at high polish levels.

B1–B3

Abrasive-Paper Finishing

Process family: fine abrasive paper is used to establish the controlled B-series surface character.
Surface objective: produce a repeatable semi-gloss finish without taking the tool surface to a diamond-polished mirror condition.
Control focus: abrasive direction, isolated deeper scratches and inconsistent local polishing should be controlled across the specified surface.

C1–C3

Stone Finishing

Process family: abrasive stones are used to create the C-series matte tool finish.
Surface objective: establish a controlled stone pattern with progressively coarser surface character from C1 to C3.
Control focus: stone marks should remain uniform and deeper machining scratches, gouges or unfinished patches should not remain within the specified area.

D1–D3

Dry Blasting

Process family: dry blasting with the specified media creates the D-series textured mold surface.
Surface objective: produce a low-gloss or progressively rougher texture by changing the blasting media associated with the specified grade.
Control focus: blast coverage, masking boundaries, local over-blast and texture consistency require particular attention.

What Must Be Controlled During Mold-Surface Finishing?

The final SPI grade is only as consistent as the surface preparation beneath it. A finer finishing medium cannot reliably hide deeper tool marks, damaged steel or inconsistent preparation left from an earlier step.

Underlying Surface Condition Milling, EDM or previous polishing marks should be reduced to a level compatible with the requested final finish.

Scratch Progression Deeper scratches from a previous preparation stage should not remain after moving to a finer finishing level.

Surface Uniformity Adjacent areas should show a consistent preparation pattern without localized over-polishing, patchiness or unfinished zones.

Edges and Geometry Sharp features, shut-offs, parting surfaces and local details require controlled finishing so geometry is not unintentionally altered.

Masking and Boundaries Blasted or locally specified surfaces should have clearly controlled transitions where one finish ends and another begins.

Clean Inspection Surface Residue, compound, dust or contamination can distort the visual assessment of the finished tool surface.

Preparation Method vs Final SPI Grade

The grade identifies the finish target.

The drawing should specify the exact SPI grade—such as A2, B2, C1 or D2—rather than simply calling for “polished,” “matte” or “textured.” The grade communicates the expected tool-surface preparation more clearly than a generic appearance description.

The workshop sequence may include multiple preparation steps.

A mold surface often requires earlier machining marks or scratches to be reduced before the final SPI preparation can be produced. Those intermediate steps support the result but are not the same as the final grade callout.

Final verification should focus on the finished surface.

The completed tool surface should be checked for uniformity, residual scratches, patchiness, boundary control and other visible preparation defects that are inconsistent with the specified finish reference.

Engineering rule: an SPI grade defines the requested finish class and its associated preparation method; it should not be interpreted as one universal, mandatory workshop polishing sequence for every toolmaker. The final requirement is the correctly prepared and verified mold surface at the specified grade.

Drawing Specification

How Should SPI Mold Finishes Be Specified on Drawings?

Injection mold drawing example showing SPI finish callouts applied to defined cosmetic surfaces and zones — A useful SPI callout identifies the exact grade and where it applies; appearance zones and molded-part approval conditions are defined separately.

A drawing should specify more than “polished,” “matte” or “SPI finish.” The requirement should identify the exact SPI grade—such as A2, B1, C2 or D1—and clearly show which mold surface or molded-part area the callout applies to.

For simple parts, one finish note may be sufficient. For parts with multiple cosmetic or functional surfaces, the drawing should separate the affected areas so the mold supplier does not have to interpret where one finish ends and another begins.

Finish notes should also be separated from molded-part cosmetic approval. The SPI grade defines the tool-surface preparation; visual inspection conditions, Golden Samples and defect limits belong to the project acceptance criteria.

From Ambiguous to Production-Ready SPI Callouts

Ambiguous Generic Finish Note

FINISH: POLISHED

This does not identify the SPI grade, the applicable surface or the expected finish reference. Different suppliers may interpret it differently.

Better Exact SPI Grade

FINISH: SPI A2

This identifies the requested finish grade, but it can still be ambiguous when only selected surfaces require the finish.

Production Ready Grade + Defined Surface

SURFACES MARKED "F1": SPI A2

This links the exact finish grade to a clearly identified surface, reducing interpretation risk during tool finishing and inspection.

What Should an SPI Finish Drawing Callout Include?

Not every project needs every item below, but critical cosmetic programs should separate the tool finish requirement from the molded-part approval conditions.

Drawing Item	What It Should Define	Why It Matters
Exact SPI Grade	Specify A1, A2, B2, C1, D2 or another exact A1–D3 grade.	“SPI A” or “SPI B” does not identify the required preparation level.
Applicable Surface	Identify the face, cavity region, insert or drawing-marked area where the finish applies.	Prevents the finish from being applied to unrelated functional or hidden surfaces.
Finish Boundary	Define where one finish ends and another begins when multiple finishes exist on the same part.	Reduces ambiguity during polishing, masking, blasting and final tool inspection.
Cosmetic Zone	Identify project-specific visible or critical zones separately from the SPI finish grade.	Cosmetic importance and SPI preparation are two different engineering classifications.
Reference Standard	Where necessary, reference an approved finish plaque, texture reference or agreed Golden Sample.	Provides a common visual baseline beyond the written grade alone.
Inspection Conditions	Define project-specific lighting, viewing distance, angle or other conditions only when required for molded-part approval.	These conditions are not automatically defined by the SPI grade.

Do not confuse cosmetic zones with SPI finish families: a project-defined “Zone A” cosmetic surface does not mean “SPI A-series,” and “Zone B” does not mean “SPI B-series.” Cosmetic zones describe the importance or visibility of a molded-part area; SPI A/B/C/D identifies the mold-surface preparation family.

Inspection-condition boundary: requirements such as a specific light level, viewing angle or viewing distance should be identified as project-defined inspection conditions, not presented as universal SPI requirements. The SPI callout controls the tool finish; the project acceptance document controls how the molded part is visually approved.

Once the SPI grade, affected surface and related approval notes are agreed, record them in the injection mold specification sheet so the requirement remains traceable through tool build, trial and final mold acceptance.

Finish Inspection

Tool-Side SPI Finish Inspection vs Molded-Part Appearance Acceptance

A mold can meet the specified SPI finish and still produce a molded part that does not meet the customer's cosmetic expectations. The reason is that tool-side finish inspection verifies how the mold surface was prepared, while molded-part appearance acceptance evaluates what is actually visible on the produced plastic part.

These two checks should be connected but not treated as interchangeable. Tool inspection focuses on polish, stone, paper or blast uniformity, scratches and finish boundaries. Part inspection may additionally consider gloss variation, reflection, visible flow-related effects and the agreed visual reference.

For cosmetic programs, the inspection scope should therefore identify the exact surface being reviewed and the evidence used for sign-off rather than relying on the written SPI grade alone.

Two Different Inspection Questions

Tool-Side Verification

Does the Mold Surface Match the Specified SPI Grade?

Finish method: confirm the specified polishing, paper, stone or blasting class.
Surface uniformity: check for inconsistent polish, stone marks, blast patchiness or unfinished areas.
Residual defects: review scratches, haze, gouges or preparation marks inconsistent with the required grade.
Finish boundaries: confirm that masking and transitions match the drawing-defined area.
Cavity consistency: compare equivalent surfaces where multiple cavities or inserts should carry the same finish.

Molded-Part Acceptance

Does the Produced Part Meet the Agreed Appearance Standard?

Visible appearance: evaluate gloss, reflection, texture replication and other project-defined cosmetic expectations.
Inspection zone: apply the correct acceptance criteria to the relevant visible or critical part area.
Reference evidence: compare against an approved plaque, Golden Sample or boundary sample where one has been established.
Inspection environment: use the agreed lighting, distance and viewing orientation where the project requires controlled visual review.
Approval decision: record whether the molded appearance meets the defined customer or project acceptance baseline.

What Should Be Controlled During Appearance Inspection?

Controlled conditions improve repeatability when visual approval matters, but these conditions should be established by the project or customer requirement—not presented as universal SPI rules.

Lighting Define a repeatable light source or inspection environment when gloss, reflection or subtle appearance variation must be judged consistently.

Viewing Position Establish the project-specific viewing distance and orientation where visual sign-off could otherwise become subjective.

Reference Sample Use an approved finish plaque, Golden Sample or boundary sample when the written SPI grade alone cannot define the visual limit.

Inspection Zone Confirm which part surfaces are included in cosmetic review and which are functional or non-visible areas with different acceptance needs.

SPI Finish Inspection and Acceptance Evidence

The evidence should match the question being answered. Tool-side finish evidence confirms surface preparation; molded-part evidence confirms the released cosmetic result.

Review Item	Tool-Side Inspection	Molded-Part Acceptance
Primary Question	Was the specified SPI finish correctly produced on the mold surface?	Does the molded part meet the agreed appearance requirement?
Main Reference	SPI grade, tool-surface reference and drawing-defined finish area.	Customer criteria, approved sample, Golden Sample or visual acceptance specification.
Typical Evidence	Surface comparison, cavity review, polishing or blasting verification and documented finish condition.	Molded samples reviewed under agreed visual conditions with acceptance status recorded.
Main Failure Question	Is the observed issue caused by incomplete or inconsistent tool-surface preparation?	Does the visible result fail the approved appearance limit, regardless of whether the tool finish itself is correct?

Important boundary: passing SPI tool-surface inspection does not automatically approve the molded part. Material behavior, processing, geometry and surface replication can influence the final appearance even when the tool finish itself is correctly prepared.

Full mold-acceptance handoff: this section only separates SPI finish verification from molded-part appearance approval. Overall mold buy-off, dimensional release, functional checks, documentation and production-readiness criteria belong in the injection mold acceptance criteria process.

Scope Boundaries

What Do SPI Mold Finish Standards Not Replace?

SPI A1–D3 defines a tool-surface finish reference, but several related engineering questions belong to different standards or specialist review paths. Keeping those boundaries separate prevents a finish callout from being used as a substitute for material selection, defect diagnosis, roughness measurement or mold-design decisions.

Related Standard

SPI Does Not Replace VDI—or Create an Automatic Conversion

SPI and VDI are separate mold-surface finish systems. A project may use either system, but one grade should not be assumed to have a perfect one-to-one equivalent in the other system based only on visual appearance or one roughness value.

Stay on this page for SPI A1–D3 definitions, preparation, drawing callouts and inspection boundaries.

Compare SPI vs VDI mold finish standards →

Material Boundary

SPI Grade Does Not Select the Resin

The specified tool finish does not determine whether a resin is suitable for the part. Material behavior can influence surface replication, gloss stability, wear, shrinkage, release behavior and production consistency, but those are material-selection questions rather than SPI grade definitions.

Do not use a simple “resin = SPI grade” table as a substitute for part-specific material and DFM review.

Review injection molding material selection →

Troubleshooting Boundary

SPI Finish Does Not Diagnose Molded-Part Defects

A correctly prepared SPI surface does not identify the root cause of splay, weld lines, flow marks, blush, sink, warpage or other molded-part defects. These symptoms may come from material, process, mold, venting, cooling, gate or part-design conditions.

Finish question: was the tool surface prepared correctly? Defect question: why is the molded symptom occurring?

Diagnose injection molding defects →

Metrology Boundary

SPI Grade Is Not the Same as Ra or Dimensional Tolerance

Surface roughness measurements such as Ra can provide useful quantitative information, but one roughness value does not completely describe gloss, reflection, directional polish marks or visual appearance. Dimensional tolerances are also a separate requirement from the mold-surface finish class.

Use SPI to define the tool finish class. Use the applicable metrology and tolerance requirements for dimensional or roughness-control questions.

Review injection molding tolerance standards →

When the Finish Decision Becomes a Mold-Design Decision

Surface finish should not be reviewed in isolation when it changes release, tooling or production risk.

If the required finish affects draft, part release, steel choice, shut-offs, gating, venting, cosmetic zoning or another mold-design decision, review those interactions before steel cut rather than trying to solve them through the SPI callout alone. Use the injection mold design decision guide to route the issue to the appropriate DFM review path.

What this SPI guide owns: A1–D3 grade definitions, tool-side preparation methods, drawing callouts, finish boundaries and the distinction between tool-surface inspection and molded-part appearance acceptance. Related material, defect, tolerance and mold-design decisions are intentionally handled by their specialist guides.

Engineering rule: use the SPI grade as one controlled tooling specification—not as a universal answer for resin behavior, molded-part defects, dimensional tolerance, surface roughness or the complete cosmetic-approval system.

SPI Finish FAQ

SPI Mold Finish Standards: Frequently Asked Questions

These questions clarify the most common specification and approval boundaries around SPI A1–D3. They focus on the finish standard itself rather than replacing material selection, defect troubleshooting or the complete mold-acceptance process.

What is the difference between SPI A1, A2 and A3?

All three are A-series diamond-polished finishes, but they use different finishing levels. A1 is the finest of the three, followed by A2 and then A3. The drawing should specify the exact grade rather than using the generic note “SPI A.”

Can a drawing specify only “SPI A” or “SPI B”?

It is better to specify the exact grade. A1, A2 and A3—or B1, B2 and B3—are not identical finish levels. A production drawing should identify the required grade and the surface or area where that finish applies.

Can surface roughness Ra replace an SPI finish callout?

Not completely. Ra provides a quantitative roughness measurement, but it does not fully describe gloss, reflection, directional polish marks or the visual character of a finished mold surface. Use the SPI grade for the finish class and add separate roughness requirements only where the project needs them.

Does meeting the SPI grade automatically approve the molded part?

No. SPI verifies the requested tool-surface finish. Molded-part appearance can still be influenced by material behavior, processing, geometry and surface replication. Cosmetic approval should therefore use its own agreed inspection criteria and reference evidence.

What should be added to a drawing besides the SPI grade?

Identify the exact grade, applicable surface and finish boundary. Where appearance approval is important, separately define the cosmetic zone, approved reference sample and any project-specific inspection conditions required for sign-off.

Can SPI and VDI grades be converted directly?

SPI and VDI are different finish systems, so a direct one-to-one conversion should not be assumed from appearance or Ra alone. For the comparison limits and selection logic, see SPI vs VDI mold finish standards .

Approval boundary: the SPI grade controls the mold-surface finish specification. Molded-part cosmetic acceptance should still be documented through the drawing, specification sheet or project-specific approval criteria.

Drawing & Finish Review

Review Your SPI Finish Specification Before Tool Steel Is Finalized

If your drawing already includes SPI A1–D3 requirements, we can review whether the exact grade, affected surfaces, finish boundaries and inspection expectations are defined clearly enough for tooling release. If the finish also creates a material, draft or mold-design concern, that issue can be routed into the appropriate DFM review.

SPI Grade Review Confirm that the exact A1–D3 grade is stated rather than a generic finish family.

Surface & Zone Review Check where the finish applies and whether cosmetic zones are separated from SPI finish families.

Callout Review Check drawing notes, finish boundaries and specification traceability before tool release.

Inspection Boundary Separate tool-side finish verification from molded-part cosmetic acceptance requirements.

Review SPI Finish Feasibility

Share the 3D model, 2D drawing, resin and appearance requirements. The review identifies specification conflicts before tooling release and does not replace final physical cosmetic validation.