SPI C Finish: Matte Stoned Mold Surfaces C-1, C-2, C-3
SPI C finish is the matte family of the American SPI mold finish standard: three grades, C-1, C-2, and C-3, finished with abrasive stones to a low-gloss surface with a fine directional scratch pattern. C-1 is finished with a 600 grit stone, C-2 with 400 grit, and C-3 with 320 grit. In DME's reference values the roughness runs from about Ra 0.25 µm at C-1 to about 1 µm at C-3, a jump of roughly four times across the family. A C finish costs less than any polished grade, removes machining marks reliably, and suits functional and industrial parts where gloss is not wanted or not needed.
SPI C finish at a glance
- Grades: C-1 (finest), C-2, C-3 (coarsest of the family).
- Method: stoning with bonded abrasive stones; 600 grit for C-1, 400 for C-2, 320 for C-3.
- Roughness (DME reference): about 0.25 to 0.30 µm (C-1), 0.64 to 0.71 µm (C-2), 0.97 to 1.07 µm (C-3).
- Look: low-gloss matte with a fine scratch pattern that follows the stoning direction.
- Best for: internal and functional components, industrial housings, non-cosmetic faces, and a prepared base before other surface work.
What is an SPI C finish?
The SPI system, published by the Society of the Plastics Industry and now maintained by the Plastics Industry Association, groups mold finishes by method: diamond buffing for A, abrasive paper for B, abrasive stone for C, and dry blasting for D. The C family is the first step down from the polished grades. Instead of paper or diamond paste, the toolmaker works the steel with bonded abrasive stones, usually with a light oil, and stops there. The result is clean and even but matte, with a visible scratch pattern running in the direction of the last strokes.
That stoned scratch pattern is what defines the family. It is coarser and more open than the lay of a B finish, and unlike the random surface of a blasted D finish it has a clear direction. For the complete standard and all twelve grades, see the SPI surface finish reference; this page covers the C family only.
The three C grades: C-1, C-2, C-3
The three grades differ in the grit of the final stone. Because each step down in grit leaves a deeper scratch, roughness rises quickly across the family, much faster than it does across the A or B grades.
| Grade | Final abrasive | Ra (µin, DME) | Ra (µm, approx.) | Approx. VDI 3400 position | Typical look |
|---|---|---|---|---|---|
| C-1 | 600 grit stone | 10 to 12 | 0.25 to 0.30 | About VDI 8 to 10 | Fine matte, faint sheen |
| C-2 | 400 grit stone | 25 to 28 | 0.64 to 0.71 | About VDI 16 to 17 | Even matte, no sheen |
| C-3 | 320 grit stone | 38 to 42 | 0.97 to 1.07 | About VDI 20 to 21 | Coarse matte, visible scratch |
The micrometer values are converted from DME's microinch figures, and the VDI positions are calculated with the formula on the VDI 3400 page. C-1 sits below VDI 12, the finest VDI reference grade. C-2 and C-3 fall inside the VDI range, which is why they are often compared with VDI 15 to 21; that comparison is a roughness match only, as the next sections explain. Other published SPI tables give different roughness for the C grades; confirm the final result against a reference plaque.
Stoned versus blasted: C-1 and D-1 at the same Ra
In DME's values, C-1 and D-1 share the same roughness range, about 10 to 12 microinches, and C-2 overlaps D-2 closely (25 to 28 against 26 to 32 microinches). The surfaces are still different. A C finish is cut by a stone moving in one direction, so the peaks and valleys run in parallel lines. A D finish is formed by abrasive particles striking the steel at random, so the surface has no direction at all.
That difference changes how the part looks and behaves:
- Appearance. A stoned surface can show a light streak when viewed along the scratch direction; a blasted surface looks the same from every angle.
- Hiding defects. Blasting diffuses light more evenly, so a D finish usually hides flow lines, sink, and handling marks better than a C finish of the same Ra.
- Release. A stoned scratch that runs in the direction of draw offers little resistance on ejection. A blasted surface has no such alignment and generally needs more draft.
So when a drawing calls for "about 0.3 µm matte", the choice between C-1 and D-1 is really a choice between a directional and a non-directional surface. For the blasted family, see SPI D textured finish.
SPI C versus VDI at the same roughness
The same logic applies to VDI. A C-2 cavity and a VDI 16 to 17 cavity can measure close to the same Ra, but the VDI surface is produced by spark erosion and consists of small overlapping craters with no direction, while the C-2 surface is a field of parallel scratches. The VDI surface diffuses light more evenly and wears differently. Where a supplier or a drawing offers both, treat the cross-reference as a starting point for sampling, not as an equivalence. The SPI, VDI and Mold-Tech conversion page sets out how far these comparisons can be trusted, and EDM surface finish covers how VDI surfaces are produced.
How an SPI C finish is produced
A C finish is reached by stoning the cavity through progressively finer grits until the target stone has removed every mark left by the step before. The usual sequence is:
- Machine and remove recast. The cavity is machined or eroded to size. Any EDM recast layer and deep tool marks are removed first, since a fine stone will only cut slowly through them.
- Rough stone. Coarser stones level the steel and take out cutter marks and EDM craters.
- Stone in steps. The surface is worked through finer stones, finishing at 320 grit for C-3, 400 for C-2, or 600 for C-1. Each step must fully remove the scratches of the previous, coarser stone.
- Set the final direction. The last strokes are made in one direction, normally in line with the direction of ejection.
- Clean and inspect. The cavity is cleaned of oil and abrasive residue and checked under even light against a reference plaque.
Because the work stops at the stone, a C finish takes much less bench time than a B or A finish. For many tools the C sequence is also the first part of the work toward a polished grade, which is why the same stones appear at the start of a B or A finish.
SPI C as a base for texturing
Stoning is also the usual preparation for a cavity that will receive a chemical or laser texture. Machining marks and EDM craters left in the steel can show through a fine texture after it is applied, so the surface is first brought to an even, stoned condition. In that case the C grade is not the final finish of the part; it is the starting surface for the texture. When the drawing calls for a texture such as those in Mold-Tech textures, the texturing supplier normally states the preparation it needs, and that requirement takes priority over a generic C callout.
Steel and polymer considerations
A C finish places little demand on the steel. Pre-hardened P20, through-hardened tool steels, and most aluminum mold alloys all take a C finish without difficulty, and inclusions or hard spots that would spoil a polished surface rarely show on a stoned one. That tolerance is part of why C is a common default for non-cosmetic faces on production tools.
On the polymer side, a C cavity gives a dull, even surface on most resins. Semi-crystalline resins such as PP, PE, and POM soften the scratch pattern further, while amorphous resins such as ABS and PC copy it more sharply, so the directional streak is easier to see on them. Glass-filled grades add their own roughness and can make the stoning direction harder to read. Approve the finish on a first-shot sample in the production resin and colour.
Draft angle for SPI C finishes
The scratch depth of a C finish is small but no longer negligible, especially at C-2 and C-3. The draft angle by texture page gives 1.0 to 2.0 degrees as a minimum for SPI C surfaces, on top of what the geometry needs. Use the low end of that range for C-1 with the stoning direction along the draw, and the high end for C-3, for deep walls, and for walls that shrink onto a core. A scratch pattern that runs across the draw on a low-draft wall is a common cause of drag marks.
Where C finishes are used
| Grade | Typical parts | Why |
|---|---|---|
| C-1 | Visible industrial parts, tool and equipment housings, low-gloss covers | Clean matte appearance at low cost, smoother than any VDI grade |
| C-2 | Internal components, brackets, frames, non-cosmetic faces | Removes machining marks quickly without any cosmetic polishing |
| C-3 | Hidden structural parts, ribs and cores, fixtures, short-run tools | Lowest-cost finished surface that still releases cleanly |
The common thread is a part where function, cost, and lead time come first. A C finish is the right answer when the surface needs to be clean and consistent but nobody is asking it to shine.
Common problems with stoned mold finishes
| Problem | Probable cause | Prevention |
|---|---|---|
| Deep scratches or EDM craters remain | Coarser marks not removed before the final stone | Complete each stone step before moving to the next |
| Drag marks on walls | Too little draft, or scratch pattern across the draw | Draft per the C guidance and final strokes along the ejection direction |
| Uneven matte between areas | Different stone grit or pressure on different faces | Use the same final stone and technique across the cavity |
| Rounded edges and lost detail | Stoning across sharp edges and fine features | Protect edges and work small features with shaped stones |
| Streak visible on cosmetic face | Directional scratch seen under raking light | Move to a blasted D finish or a texture where the face is visible |
How to specify an SPI C finish on a drawing
Write the grade on each surface that receives it, for example "SPI C-2", rather than one note for the whole part. Three additions make the callout complete:
- Stoning direction. State the direction of the final strokes, usually "in direction of draw", on walls with low draft.
- Reference. Name the reference plaque or the Ra the finish will be checked against, since published C values vary by source.
- Purpose. If the C surface is a preparation for texturing, say so, so the toolmaker works to the texturing supplier's requirement rather than to a final cosmetic standard.
Choosing between C-1, C-2, and C-3
Use C-1 when a part is visible but should read as matte, and a semi-gloss B-3 would look too shiny. Use C-2 as the general-purpose finish for internal and non-cosmetic faces, where it gives a clean surface in little bench time. Use C-3 for hidden parts, structural features, and short-run tools, where the only requirements are a clean release and a surface free of machining marks. If the matte face must also hide flow lines and handling marks, a blasted D finish usually does that better than a C finish of similar roughness. If the part needs a light sheen instead, step up to SPI B semi-gloss finish.
Frequently asked questions
What is an SPI C finish?
The matte family of the SPI mold finish standard: three grades, C-1, C-2, and C-3, finished with abrasive stones to a low-gloss surface with a fine directional scratch pattern.
What grit is used for C-1, C-2, and C-3?
C-1 is finished with a 600 grit stone, C-2 with a 400 grit stone, and C-3 with a 320 grit stone.
What Ra is an SPI C-1 finish?
DME's guide lists about 10 to 12 microinches, roughly 0.25 to 0.30 µm. Other published tables differ, so confirm against a reference plaque.
Is C-1 the same as D-1?
They share the same roughness range in DME's values, but C-1 is stoned and has a directional scratch pattern, while D-1 is blasted with glass bead and has no direction.
What VDI grade is equivalent to SPI C?
By roughness, C-2 sits near VDI 16 to 17 and C-3 near VDI 20 to 21, while C-1 is smoother than VDI 12. The surfaces differ in structure, so treat the match as approximate.
How much draft does an SPI C finish need?
About 1.0 to 2.0 degrees as a minimum on top of the draft the geometry needs, with the low end for C-1 and the high end for C-3 and deep walls.
Is an SPI C finish cosmetic?
C-1 can be used on visible industrial parts, but the directional scratch can show under raking light. For cosmetic matte faces, a blasted D finish or a texture usually looks more uniform.
Why are cavities stoned before texturing?
Machining marks and EDM craters can show through a fine texture, so the steel is first brought to an even stoned surface. The texturing supplier usually states the exact preparation.
Normative References & Sources
- Plastics Industry Association (formerly the Society of the Plastics Industry), SPI mold finish designations: plasticsindustry.org
- DME, Micro Finish Mold Tool Finish Guide, SPI finish designations with approximate Ra: dme.net
- ISO 1302, indication of surface texture on drawings: iso.org
Written and technically reviewed by Marcel Dias, Technical Editor, with over 28 years in mold texturing. Roughness and draft values are approximate and vary by source and process; verify against the primary standard and a physical sample before production. See our Disclaimer. Part of the Mold Texture Standards section. Last updated: October 9, 2026.