Last updated: 2026-09-02 · 5 min read

Mold Steel Polish Grades — How Do SPI A1, A2, B1, B2, C1 and D3 Finishes Affect Part Quality and Mold Cost?

SPI finish grades define the surface roughness of the mold cavity — A1 (mirror polish, Ra 0.01–0.025 μm), A2 (semi-mirror, Ra 0.025–0.05 μm), B1 (fine stone, Ra 0.05–0.10 μm), B2 (medium stone, Ra 0.10–0.20 μm), C1 (fine grit, Ra 0.20–0.35 μm) and D3 (coarse grit, Ra 0.35–1.0 μm). The finish grade affects the part appearance, the ejection force, the mold cost and the maintenance requirement.
A mirror-polished cavity (A1) produces a glossy part that releases easily but costs $2,000–5,000 more to polish than a medium stone finish (B2).

Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. See also related guide.

Key Facts

  • Polish cost increases exponentially with grade: B2 (medium stone) is the baseline; B1 adds $500–1,000; A2 adds $1,000–2,500; A1 adds $2,000–5,000. The cost difference is labor — mirror polishing requires 20–40 hours of skilled hand work per cavity.
  • Steel grade matters: P20 polishes to B1–B2 maximum; H13 polishes to A2; S136 and NAK80 polish to A1. The steel must be specified for the required finish — requesting A1 polish on P20 steel will not produce a mirror surface regardless of the polishing effort.
  • Texture vs. polish: a textured surface (EDM, laser etch, chemical etch) is the opposite of a polished surface — texture hides flow lines and ejection marks but requires 3–5° more draft angle. The choice between polish and texture depends on the part’s cosmetic requirement and the resin being molded.

    • Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. The related guide covers adjacent topics.

    SPI Finish Grade Comparison

    Grade Ra (μm) Process Cost Adder
    A1 (mirror) 0.01–0.025 Diamond paste, hand polish +$2,000–5,000
    A2 (semi-mirror) 0.025–0.05 Diamond paste, hand polish +$1,000–2,500
    B1 (fine stone) 0.05–0.10 Fine grit stone, hand +$500–1,000
    B2 (medium stone) 0.10–0.20 Medium grit stone, hand Baseline
    C1 (fine grit) 0.20–0.35 Glass bead blast -$200–500
    D3 (coarse grit) 0.35–1.0 Sandblast or EDM -$500–1,000

    Steel Grade and Polish Compatibility

  • P20 (pre-hardened): maximum achievable finish is B1–B2. P20 has inclusions that limit the polish quality — requesting A1 or A2 on P20 will not produce the specified finish. P20 is suitable for non-cosmetic parts and short-run molds.
  • H13 (through-hardened): achievable finish is A2. H13 is used for engineering molds that require a good surface finish — the steel is cleaner than P20 and can be polished to a semi-mirror finish. H13 is the standard for molds running abrasive or corrosive resins.
  • S136 / S136-ESR (stainless): achievable finish is A1. S136-ESR (electro-slag remelted) is the cleanest steel available — it polishes to a true mirror finish. S136 is required for optical parts, medical devices and transparent components. NAK80 (pre-hardened stainless) also achieves A1 with less polishing labor.

    • What This Means for Your Quotation

    The part drawing should specify the surface finish requirement for each surface — cosmetic surfaces (A1–B1), non-cosmetic surfaces (B2–C1) and hidden surfaces (D3 or as-machined). The quotation should itemize the polish cost by cavity — so the buyer understands the cost impact of the finish specification. A mold with four A1 cavities costs $8,000–20,000 more in polish labor than the same mold with four B2 cavities — and the buyer should know this before approving the mold design.

    Frequently Asked Questions

    Can a mold be re-polished if the surface degrades during production?

    Yes — but the re-polish removes material from the cavity, which changes the part dimensions slightly. A mold that has been re-polished multiple times may produce parts that are out of tolerance on the affected surfaces. The mold maintenance schedule should include surface inspection and re-polish before the finish degrades to the point where it affects part quality or ejection.

    How does the finish affect ejection?

    A polished surface reduces the ejection force — the part releases more easily from a mirror cavity than from a textured cavity. However, a mirror surface can create a vacuum effect on flat parts, which requires air ejection or stripper plates to break the vacuum. A textured surface provides air channels that prevent vacuum lock but increases the ejection force due to the surface roughness.

    What is the difference between SPI polish and EDM finish?

    EDM (electrical discharge machining) produces a textured surface with a roughness of Ra 0.5–3.0 μm — the surface has a characteristic “as-spark” texture that must be removed by polishing if a smooth finish is required. EDM is used for complex geometries that cannot be machined by conventional methods; the EDM surface is then polished to the specified finish. The deeper the EDM cut, the more polishing labor is required to remove the EDM recast layer.

    Key Takeaways

    1. SPI finish grades range from A1 (mirror, Ra 0.01–0.025 μm) to D3 (coarse, Ra 0.35–1.0 μm) — the polish cost increases from baseline at B2 to +$5,000 at A1.
    2. Steel grade limits the achievable finish: P20 to B1–B2, H13 to A2, S136/NAK80 to A1 — the steel must be specified for the required finish.
    3. The quotation should itemize the polish cost by cavity — so the buyer understands the cost impact of the finish specification and can make the decision based on the part’s cosmetic requirement.

    Send aCatMould the part drawing with the surface finish specification for each surface, and the quotation itemizes the polish cost by cavity — so the finish decision is based on the part’s cosmetic requirement and the mold cost is transparent.

    Sources
    MoldMaking Technology — SPI finish grades and mold polishing
    Plastics Technology — surface finish specification for injection molds
    SPE — SPI surface finish standards and mold steel selection

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