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What do SPI mould finishes A-1 to D-3 mean?
SPI finishes are a standard set of twelve mould polish grades from the US plastics industry association. Grade A is diamond-buffed (mirror to high gloss), B is polished with grit paper, C with stones, and D is blasted for a satin or dull matte look. The lower the letter and number, the smoother and more expensive the finish.
The finish on the part comes from the surface of the mould steel, so the grade is specified for the mould. The scheme was set up by the Society of the Plastics Industry, now the Plastics Industry Association, which is why the grades carry the SPI name.
The twelve grades
| Grade | Finishing method | Typical look or use | Published Ra (µm) |
|---|---|---|---|
| A-1 | Grade #3 diamond buff | Optical, mirror | 0.012 to 0.025 |
| A-2 | Grade #6 diamond buff | High gloss | 0.025 to 0.05 |
| A-3 | Grade #15 diamond buff | High to medium gloss | 0.05 to 0.10 |
| B-1 | 600 grit paper | Medium polish | 0.05 to 0.10 |
| B-2 | 400 grit paper | Medium polish | 0.10 to 0.15 |
| B-3 | 320 grit paper | Medium to low polish | 0.28 to 0.32 |
| C-1 | 600 stone | Low polish | 0.35 to 0.40 |
| C-2 | 400 stone | Low polish | 0.45 to 0.55 |
| C-3 | 320 stone | Low polish | 0.63 to 0.70 |
| D-1 | Dry blast, glass bead | Satin | 0.80 to 1.00 |
| D-2 | Dry blast, #240 oxide | Dull matte | 1.00 to 2.80 |
| D-3 | Dry blast, #24 oxide | Rough matte | 3.20 to 18.00 |
Roughness values are from Xometry's finish guide. DME's mould finish guide gives the same order in microinches, for example about 0 to 1 for A-1, 10 to 12 for C-1 and 190 to 230 for D-3, along with the blasting distance, pressure and time used for each D grade. Different references round these differently, so use them to compare grades rather than as inspection limits.
Not every resin takes every finish
A mirror finish on the mould only gives a mirror part if the resin reproduces it. Upmold's mould surface sheet rates common resins against each grade on a scale from 1 (good) to 4 (not acceptable). It rates acrylic good across the A grades, while ABS, nylon, polystyrene and polyethylene are marked not recommended or not acceptable at A-1, and polypropylene and polyethylene rate good from B-3 all the way to D-3. The same sheet notes that glass-filled resins show a rougher surface because the filler shows through, and that high gloss needs a mould surface at or above the resin's glass transition temperature.
Finish affects draft and ejection
- Polished faces can grip the part through vacuum, and Eastman's guide warns against polishing drafted walls smoother than needed; it suggests SPI 5 or lower for ejection and a finer polish only for appearance.
- Textured faces need extra draft. Precision Molded Plastics' published texture chart lists Mold-Tech textures with depths from about 0.0001 in to 0.010 in and minimum draft from 1 degree up to 15 degrees for the deepest grain.
- The finer the grade, the more hand polishing the toolmaker has to do, so A grades add the most cost and time.
How to choose
- Use the coarsest finish that meets the look you need; reserve A grades for lenses and true high-gloss faces.
- Specify finish face by face. Inside faces and hidden walls rarely need more than a C or D finish.
- Choose the resin and finish together, and check the resin can reproduce the gloss you want.
- If the part will be textured, pick the texture early and design the draft to its chart.
- Ask for a finish sample plaque in your resin when appearance is critical.
Roughness values are published reference figures. Confirm the finish and texture with the toolmaker and any texture supplier.
Questions buyers ask
Is SPI the same as VDI?
No. VDI 3400 is a separate German scale used mainly for spark-eroded textures. Mould suppliers publish cross-reference charts, but the two scales measure different kinds of surface.
What finish do prototype moulds use?
Many rapid-tooling suppliers offer a limited menu, often an as-machined finish plus a few SPI polishes and light bead blasts. Check which grades are standard and which cost extra.
Does a matte finish hide flaws?
It can hide flow lines and minor sink better than high gloss. It still needs adequate draft, or the texture will scuff during ejection.
Can the finish be changed after the mould is made?
Making a finish rougher, for example by blasting or texturing, is generally easier than making it smoother. Changing finish can also change the draft a face needs.
Sources
- Xometry, An overview of injection molded surface finishes
- DME (via Upmold), Mold tool finish guide (PDF)
- Precision Molded Plastics, Plastic finishes and textures (PDF)
- Eastman, Keys to successful molding (PDF)
- Upmold, Mold surface classifications (PDF)