SPI surface finish standards: how to choose a mold polish grade
What do SPI grades A1-A3, B1-B3, C1-C3 and D1-D3 mean, and which grade is enough for which part and material?
The finish quality of a mold cavity/core surface is described using the SPI (Society of the Plastics Industry) classification. Four main groups, each split into three sub-grades (1 is the finest, 3 the coarsest).
The four main groups
- SPI A (mirror polish): Polished with diamond paste (A1 finest, A3 coarser). Used for optical lenses, cosmetic packaging lids, and products where high visible gloss is required. The most expensive and labor-intensive grade.
- SPI B (fine stoning/sanding): Finished with sandpaper (B1 around 600 grit, B3 around 320 grit), giving a matte but smooth surface. The standard choice for general consumer products.
- SPI C (stone/brush texture): Textured with a grinding wheel or brush; a slightly matte surface with good scratch tolerance (internal mechanism parts, hidden surfaces).
- SPI D (bead/sand blast or EDM texture): The coarsest texture; used for an intentional texture (leather-look, slip-resistant grip surface) or to cut cost.
What drives the choice?
- Visibility: Visible, touched surfaces justify a finer grade; hidden/internal surfaces are well served by B or C.
- Material: Filled plastics (glass fiber, talc) don't hold a very fine polish (A1) well — fiber bloom can appear on the surface over time. Unfilled PC or acrylic are good candidates for grade A.
- Draft angle: On deep parts with shallow draft, a coarser texture (B/C) can reduce friction and ease demolding; a glossy A surface sometimes increases sticking.
- Cost/time: A1 polishing can take days per cavity; grade B finishes in hours. Asking for a finer grade than needed inflates budget for no benefit.
Practical note
The surface grade should be written on the technical drawing before the mold order is placed; a verbal request like "make it shiny" leads to interpretation gaps and cost disputes later.
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