Last updated: 2026-09-05 · 5 min read

The SPI surface finish chart sorts mold finishes into four families and twelve grades: A grades (A1–A3) are diamond-buffed gloss, B grades (B1–B3) are grit-sanded semi-gloss, C grades (C1–C3) are stone-finished matte, and D grades (D1–D3) are blasted or textured surfaces. The number behind the letter sets how coarse the finish is within its family. Two rules do most of the practical work: name the grade per visible surface on the drawing, and give the finish the draft it needs — the glossier or deeper the surface, the more draft it demands. Get those two right and most finish disputes never happen; get them wrong and the argument arrives at T1, with the samples already in your hands.

Key facts

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  • Four families: A gloss (diamond buff) · B semi-gloss (grit sand) · C matte (stone finish) · D texture (blast or pattern).
  • 1 = finest, 3 = coarsest within each family.
  • Finish grade must be named per surface; “smooth” and “matte” are not specifications.
  • Draft requirement rises with gloss and texture depth — textured faces need noticeably more draft than polished ones.
  • Finish is hand work: grade choice moves cost and lead time, and it interacts with steel choice.

    • SPI surface finish — SPI surface finish — a standardized polish/texture scale from the Society of Plastics Industry. A1 (mirror, 1–3 µin Ra) to D3 (stone texture, 15–30 µin Ra). A1–A2 requires hardened steel (S136, H13). B2 is achievable in aluminum. C2–D3 can be textured in any mold steel. The related guide covers adjacent topics.

    What the SPI finish chart is for

    Surface finish on a molded part is the negative image of the mold surface — whatever the steel carries, the plastic copies, thousands of times a day. Without shared vocabulary, “a nice gloss” means one thing to the buyer, another to the polisher, and the disagreement surfaces at T1 when the samples are already in hand. The SPI chart, from the US plastics industry association, gives both sides twelve named grades so the finish requirement is specified, quoted and accepted against the same words.

    SPI finish grade — one of twelve standard mold surface designations (A1–D3) combining a finishing method with a coarseness level. It is a specification language for tooling surfaces, the finish-side twin of the mold class system.

    The twelve grades at a glance

    Grade Method Result Typical use
    A1 Diamond buff, finest Optical mirror gloss Lenses, light guides, highest-gloss appearance parts
    A2 Diamond buff Very high gloss Glossy housings, chrome-look parts, transparent parts
    A3 Diamond buff, coarsest of A High gloss Visible gloss surfaces at lower cost than A1/A2
    B1 Grit sand, finest Smooth semi-gloss Appearance surfaces without mirror reflection
    B2 Grit sand Medium semi-gloss General visible parts, painted part substrates
    B3 Grit sand, coarsest of B Low semi-gloss Functional visible surfaces, budget-conscious finish
    C1 Stone finish, finest Smooth matte Non-reflective appearance faces
    C2 Stone finish Standard matte Internal visible zones, utility parts
    C3 Stone finish, coarsest of C Rough matte Hidden surfaces that still get basic benching
    D1 Dry blast, fine Fine texture, satin Soft-touch appearance faces
    D2 Dry blast Medium texture Grip surfaces, scratch-hiding faces
    D3 Dry blast, coarse Rough texture Hidden or industrial surfaces

    Two notes keep this table honest. First, commercial texture systems (the patterned textures buyers often specify by pattern number) live alongside this chart — the D family covers blast-type finishes, while named pattern textures are quoted by pattern and depth. Second, the same grade in two factories can differ slightly; that is why the acceptance reference should be agreed as a sample or standard at scope time, not argued at T1.

    Why does the finish decide the draft?

    Draft is the taper that lets the part release from the mold, and the finish changes how hard the surface grips during release. A polished steel face releases easily; a textured face mechanically interlocks with the plastic on every ejection. The practical rules of thumb:

  • A grades: the polish itself helps release, but gloss shows every drag mark — plan at least the mold’s standard draft, and more on deep draws where ejection force concentrates.
  • B and C grades: standard draft ranges work for typical geometry; shallow draft on tall walls still risks scuffing the semi-gloss.
  • D grades and pattern textures: add draft as texture depth grows. The common working rule is roughly one extra degree of draft for every 0.025 mm of texture depth, on top of the base draft the geometry needs. Ignore this and the texture drags, scars or tears at ejection — visible damage on a finish you paid to have.
  • Deep draws multiply everything. The deeper the wall, the more release force, and the more the finish matters. Deep textured parts deserve a draft conversation in the DFM, not after T1.

    • This coupling is why finish belongs in the DFM review, not after it: texture depth changes draft, draft changes the geometry, and geometry changes are free only before the steel is cut — the same asymmetry covered in our DFM checklist.

    What does the finish cost, really?

    Finish is hand work, and hand work does not scale like machining. Each step up the gloss ladder adds polishing hours and raises the steel requirement — an A-grade mirror on soft steel will look right at T1 and degrade within weeks of production, so real A-grade work assumes the steel to hold it. Textures cost differently: the pattern application itself, plus the draft corrections it forces, plus the ejection engineering to protect it. Two consequences for the quotation: finish should be priced per surface so you can see what you are paying for, and finish upgrades mid-project should follow the change-order rule like any other scope movement. The steel side of this pairing is in the steel comparison.

    How do you specify finish on the drawing?

    1. Mark the appearance faces. Only the surfaces that are actually seen need finish grades; unmarked faces fall to standard benching. Deciding this explicitly saves money in both directions.
    2. Name the grade per face. “A2 on the top face, B2 on the sides, D1 on the grip zone” — per surface, not per part.
    3. For textures, name pattern and depth. The texture system, the pattern number and the depth range, with the draft addition the texture implies.
    4. Set the acceptance reference. A master sample, a texture plaque or a named standard the parts will be compared against — agreed before T1, referenced in the acceptance criteria.
    5. Put all of it in the RFQ. Finish is one of the blocks that makes quotes comparable; the full document is in the RFQ checklist.

    How do you choose between gloss and texture?

    When the product team has not made up its mind, the finish decision usually collapses into “what looks best” — and the mold pays for the ambiguity later. Four questions settle it before the RFQ goes out:

  • What does the face do? Optical and light-managing surfaces push toward A grades; grip, touch and handling surfaces push toward texture; everything else should argue from cost, not preference.
  • What happens to it in service? Gloss shows every scratch, fingerprint and flow mark; texture hides minor defects and wear marks. A part that lives in a pocket or a tool bag has a different durability requirement than one behind glass.
  • What does the resin allow? Filled grades and some pigments fight mirror gloss; the finish ambition and the material choice must be reconciled before quoting, because each constrains the other.
  • What is the acceptance test? If you cannot describe how the finish will be judged at delivery — a sample, a plaque, a gloss range — the decision is not finished. The acceptance reference is part of the specification, not a detail for T1.

    • Mixed strategies are normal and often correct: gloss where the product wants reflection, texture where hands and wear live, matte everywhere else. The requirement is simply that each choice is named per surface, with the draft to support it, before the price is asked.

    Which mistakes show up most often?

  • One grade for the whole part. Parts usually need two or three grades on different faces; a single blanket grade either over-pays on hidden faces or under-specifies visible ones.
  • Texture without extra draft. The most common ejection damage story: beautiful texture specified, standard draft kept, drag marks delivered.
  • Mirror gloss on the wrong steel. The finish is quoted and delivered, then fades in production because the steel underneath could not hold it.
  • No acceptance sample. Without an agreed reference, every finish conversation at T1 is opinion against opinion. With one, it is measurement.
  • Upgrading finish after steel cutting. Going from B to A grades mid-build means re-polishing hardened steel or remaking inserts — a change order in the literal sense.

    • Finish conversations go two ways. Buyers who say “make it look nice” get whatever the bench decides, and argue about it at T1 when the samples are already in hand. Buyers who hand me a drawing with a grade per face, a texture depth where needed and the draft to support it get exactly what they asked for, priced per surface before any steel moves. The chart exists so the second conversation is easy — use it.

    — Arthur, Tooling Engineer at aCatMould

    Frequently asked questions

    What is the most common SPI finish grade?

    B and C grades dominate general tooling — most parts need a clean semi-gloss or matte on visible faces and standard benching elsewhere. A grades belong where gloss is a product requirement, not a default ambition; D grades where texture serves grip, touch or defect hiding. Start from what the face must do, then pick the family.

    Can different faces carry different grades?

    Yes — that is the correct way to specify. Grade per face, with unmarked faces falling to the stated default. It keeps cost where the requirement is.

    How much draft does a textured surface need?

    Base draft for the geometry plus roughly one extra degree per 0.025 mm of texture depth is the common working rule. Deeper patterns and taller walls push the number up; ask the factory to state its draft plan per textured face in the DFM.

    Does the resin affect the finish result?

    Noticeably. The same A2 polish reads differently in ABS, PP or a filled grade; fillers and pigments change gloss behavior, and some resins will simply not mirror levels. That is another reason the finish spec should travel together with the resin grade in the RFQ — judged as a pair, not in isolation.

    Is the SPI chart used outside North America?

    It is understood worldwide as the shared vocabulary, but Chinese and European toolrooms may quote equivalent methods — polishing numbers, blast media and pattern systems — instead of letter grades. Translation is straightforward: method plus coarseness, stated per surface, either way.

    Can a finish be corrected after molding starts?

    Downgrades are usually possible — repolishing or blasting an existing surface. Upgrades often are not, because the steel may not support them. Decide upward before the steel is cut; the change-order economics are unforgiving in that direction.

    Key takeaways

    1. Twelve grades in four families: A gloss, B semi-gloss, C matte, D texture — 1 finest, 3 coarsest within each family.
    2. Specify grade per visible surface, name texture pattern and depth, and leave unmarked faces at a stated default.
    3. Draft rises with finish severity: gloss needs clean release geometry, texture needs added draft roughly one degree per 0.025 mm of depth.
    4. Finish is hand work paired with steel capability — price it per surface and match A-grade ambition to steel that can hold it.
    5. Agree the acceptance reference before T1 so finish is judged by measurement, not opinion — a master sample, a texture plaque or a named gloss range, written into the acceptance criteria.

    If your part carries appearance faces that must survive the argument at T1, send us the geometry and the finish requirement per face. You will get a written DFM with draft corrections, a line-item quotation with finish priced per surface, and an acceptance reference you can hold us to — from a named engineer.

    Sources
    Plastics Industry Association — SPI mold standards and surface finish sets
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions
    MatWeb — material property database for resin and steel datasheets

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet