SPI mold polish (A-1 to D-3)

SPI mold finishes are standard grades for the polish on an injection mold's cavity, which every molded part copies. They run from A-1, a mirror finish from grade 3 diamond buff, through B (grit paper) and C (stone) to D-3, a coarse dry blast. A-1 and A-2 are needed for clear optical parts; B-2 and C-1 are common for general parts. Finer grades add tooling cost.

Add it to a part

Applies to: Injection molded thermoplastics and TPEs; masters for urethane casting
Adds: 3 days
Cost: Low

Specs

SPI mold polish (A-1 to D-3) specifications
A-1 / A-2 / A-3 (diamond buff)	About 0.5–4 µin Ra (0.012–0.10 µm); optical and high gloss
B-1 / B-2 / B-3 (grit paper)	About 2–13 µin Ra (0.05–0.32 µm); semi-gloss
C-1 / C-2 / C-3 (stone)	About 14–28 µin Ra (0.35–0.70 µm); low gloss
D-1 / D-2 / D-3 (dry blast)	About 32–700 µin Ra (0.8–18 µm); matte
Tool steel for A-1	Hardened, clean steel such as 420 stainless or H13 at 48–52 HRC
Draft	Polished walls release at about 0.5–1°; matte and textured walls need more

Call out on the drawing

SPI / PLASTICS Industry Association mold finish grades A-1 through D-3
ASME B46.1 or ISO 21920 for Ra verification

Materials it suits

Common on

Questions

What SPI finish should I specify?

SPI B-2 or C-1 for most non-cosmetic parts, since they release well and hide minor flaws. SPI A-2 or A-1 only for clear lenses, light pipes and gloss show surfaces. SPI D grades or a mold texture for matte cosmetic surfaces that must hide scratches and sink.

Why do high-polish finishes cost more?

A-grade finishes need hardened, inclusion-free tool steel and many hours of hand polishing through progressively finer diamond compounds, then careful protection in production. A-1 can add days to the tool build and significant cost on a large cavity.

Does a glossy finish hide defects?

No, it shows them. High gloss highlights sink, flow lines, knit lines and scratches. Matte finishes such as SPI C-1, D-2 or a mold texture hide them.

What SPI finish do clear parts need?

A-1 or A-2 on the optical faces of both cavity and core, in a hardened stainless tool steel, with a clear resin such as polycarbonate or acrylic dried properly before molding.