SPI B Finish: Semi-Gloss Mold Surfaces B-1, B-2, B-3
SPI B finish is the semi-gloss family of the American SPI mold finish standard: three grades, B-1, B-2, and B-3, polished with abrasive paper to a clean, lightly reflective surface. B-1 is finished with 600 grit paper, B-2 with 400 grit, and B-3 with 320 grit. In DME's reference values the roughness runs from about Ra 0.05 µm at B-1 to about 0.25 µm at B-3. A B finish gives most of the visual quality of a polished part at a fraction of the cost of diamond buffing, which is why it is the usual choice for appliance exteriors, consumer covers, and housings that should look clean but do not need a mirror.
SPI B finish at a glance
- Grades: B-1 (finest), B-2, B-3 (coarsest of the family).
- Method: polishing with abrasive paper; 600 grit for B-1, 400 for B-2, 320 for B-3.
- Roughness (DME reference): about 0.05 to 0.076 µm (B-1), 0.10 to 0.13 µm (B-2), 0.23 to 0.25 µm (B-3).
- Look: semi-gloss with a fine, directional polishing line.
- Best for: appliance exteriors, consumer covers, handles, and visible housings without optical requirements.
What is an SPI B 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 B family sits between the mirror A grades and the matte C grades. The cavity is polished with abrasive paper rather than diamond paste, so the surface is smooth and reflective but not optical, and it keeps a faint line from the direction of the last polishing stroke.
That line, called the lay, is the practical signature of the B family. It is what separates a B-1 from an A-3 even where the two have similar roughness, and it is something the drawing can control. For the complete standard and all twelve grades, see the SPI surface finish reference; this page covers the B family only.
The three B grades: B-1, B-2, B-3
The three grades differ in the grit of the final paper, and therefore in sheen and in how visible the polishing line is.
| Grade | Final abrasive | Ra (µin, DME) | Ra (µm, approx.) | Approx. VDI 3400 position | Typical look |
|---|---|---|---|---|---|
| B-1 | 600 grit paper | 2 to 3 | 0.05 to 0.076 | Below VDI 0 | Bright semi-gloss, fine line |
| B-2 | 400 grit paper | 4 to 5 | 0.10 to 0.13 | About VDI 0 to 2 | Clean satin gloss |
| B-3 | 320 grit paper | 9 to 10 | 0.23 to 0.25 | About VDI 7 to 8 | Low gloss, visible line |
The micrometer values are converted from DME's microinch figures, and the VDI positions are calculated with the formula on the VDI 3400 page. All B grades sit below VDI 12, the finest VDI reference grade, so a B finish cannot be specified as a VDI number. Other published SPI tables give somewhat different roughness for B-3 in particular; confirm the final result against a reference plaque.
B-1 versus A-3: same roughness, different surface
In DME's values, B-1 and A-3 share the same roughness range, about 2 to 3 microinches. They still do not look the same. An A-3 is buffed with diamond compound in overlapping strokes until no line remains, so it reflects evenly in every direction. A B-1 is finished with 600 grit paper, which leaves a fine, uniform line in the direction of the last stroke. Under a light source the B-1 shows a soft streak along that direction, while the A-3 gives a clean reflection.
This is the clearest example of why SPI is defined by method and appearance rather than by Ra alone. If the part only needs a bright, clean surface, B-1 delivers it with less labor and on a less demanding steel. If it needs an even, line-free reflection, the A family is the right call; see SPI A glossy finish.
How an SPI B finish is produced
A B finish is reached by working the cavity through progressively finer abrasives until the target paper grit has removed every mark left by the step before. The usual sequence is:
- Machine and clean. The cavity is machined to size; any EDM or tool marks are removed first, since paper will not hide them.
- Stone the surface. Abrasive stones level the steel and remove machining marks, leaving an even base.
- Paper in steps. The surface is worked with abrasive paper, finishing at 320 grit for B-3, 400 for B-2, or 600 for B-1. Each step must fully remove the scratches of the previous, coarser one.
- Set the final direction. The last strokes are made in one direction, usually in line with the direction the part ejects, so the lay runs along the draw.
- Clean and inspect. The cavity is cleaned and checked under even light against a reference plaque.
Because the method stops at paper, a B finish takes noticeably less polishing time than an A finish, and the time does not grow as steeply from B-3 to B-1 as it does inside the A family.
Why the polishing direction matters
The lay of a B finish is a design parameter, not a side effect. When the final strokes run in the direction of draw, the fine lines are parallel to the movement of the part on ejection, which reduces drag and keeps the lines from catching. On a cosmetic face, a lay that runs in one clean direction also reads as deliberate, while lines that cross or change direction read as a defect. For both reasons, drawings for visible B surfaces often state the polishing direction along with the grade.
Steel and polymer considerations
A B finish is far less demanding on the steel than a mirror A finish. A pre-hardened, general-purpose steel such as P20 takes a B finish well, which is a large part of why B is so widely used on production tooling. Harder and more homogeneous steels still hold the finish longer and resist the fine scratches that dull a semi-gloss surface over a long run, so they are worth considering for high-volume visible parts.
On the polymer side, amorphous resins such as ABS, PC, and PC/ABS reproduce a B cavity closely and give a clean semi-gloss. Semi-crystalline resins such as PP and PE come out duller and can show the polishing line less sharply. Glass-filled and mineral-filled grades raise surface roughness and reduce gloss, so a B-1 cavity may read closer to a B-3 on a filled part. Approve the finish on a first-shot sample in the production resin and colour.
Draft angle for SPI B finishes
A B finish adds very little texture depth, so it needs little extra draft. The draft angle by texture page gives 0.5 to 1.0 degree as a minimum for polished SPI A and B surfaces, on top of what the geometry needs. Running the lay in the direction of draw helps the part release at the low end of that range. Deep walls, walls that shrink onto a core, and high-shrink resins still need more.
Where B finishes are used
| Grade | Typical parts | Why |
|---|---|---|
| B-1 | High-gloss consumer parts, premium covers and bezels | Close to polished gloss at a lower cost than an A finish |
| B-2 | Appliance exteriors, housings, panels | Clean, lightly reflective look that hides minor handling marks |
| B-3 | Handles, frames, internal visible parts, prototypes of polished parts | Smooth to the touch with low reflection and low cost |
The common thread is a part that is seen and handled, where the surface should look finished and consistent but where optical clarity or a perfect reflection is not the point.
Common problems with paper-polished mold finishes
| Problem | Probable cause | Prevention |
|---|---|---|
| Cross scratches visible on parts | Coarser marks not removed before the final grit | Complete each paper step before moving to the next |
| Lay in the wrong direction | Final strokes not set along the draw | State the polishing direction on the drawing |
| Drag lines on walls | Too little draft, or lay across the draw | Adequate draft and lay in the direction of ejection |
| Orange peel or pitting on the steel | Over-polishing, or inclusions in the steel | Stop at the specified grit; use a clean, homogeneous steel |
| Gloss differs between cavities | Different final grit or technique per cavity | Polish all cavities to the same sequence and plaque |
How to specify an SPI B finish on a drawing
Write the grade on each surface that receives it, for example "SPI B-2", rather than one note for the whole part. Three additions make the callout complete:
- Polishing direction. State the direction of the final strokes, usually "in direction of draw", on cosmetic faces.
- Reference. Name the reference plaque or the Ra the finish will be checked against, since published B values vary by source.
- Approved part. Record a first-shot sample in production resin and colour as the appearance standard.
Choosing between B-1, B-2, and B-3
Use B-1 when a part should look nearly polished but the budget, the steel, or the lead time does not support an A finish. Use B-2 as the general-purpose semi-gloss for appliance and consumer housings, where it balances appearance, cost, and resistance to visible handling marks. Use B-3 for parts that should be smooth and clean but not shiny, or as a lower-cost step when a matte C or textured D finish would look too rough. If the part must hide fingerprints and flow lines rather than show a clean sheen, move to the C or D families; see SPI D textured finish.
Frequently asked questions
What is an SPI B finish?
The semi-gloss family of the SPI mold finish standard: three grades, B-1, B-2, and B-3, polished with abrasive paper to a clean, lightly reflective surface.
What grit is used for B-1, B-2, and B-3?
B-1 is finished with 600 grit paper, B-2 with 400 grit, and B-3 with 320 grit.
What Ra is an SPI B-2 finish?
DME's guide lists about 4 to 5 microinches, roughly 0.10 to 0.13 µm. Other published tables differ slightly, so confirm against a reference plaque.
Is B-1 the same as A-3?
They share a similar roughness in DME's values, but A-3 is diamond buffed without a visible line, while B-1 is paper polished and shows a fine directional line.
Can P20 steel take an SPI B finish?
Yes. A B finish is well within what pre-hardened P20 can reach and hold, which is one reason B is common on production tools.
How much draft does an SPI B finish need?
About 0.5 to 1.0 degree as a minimum on top of the draft the geometry needs, with the polishing direction running along the draw.
Can an SPI B finish be expressed as a VDI grade?
No. All B grades are smoother than VDI 12, the finest VDI reference grade, so they sit below the scale used on drawings.
Is an SPI B finish cheaper than an A finish?
Yes. Paper polishing is faster than diamond buffing and works on less demanding steels, so a B finish costs less and shortens the tooling schedule.
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.