You asked for a smooth, nice-looking part. Then the samples came back shiny, and every fingerprint and tiny dent showed. The mold did exactly what it was told. It just wasn’t told the right finish.
That finish has a name and a code. It’s called an SPI surface finish. It tells your molder how smooth or textured the mold steel should be.
Every part that comes out of the mold copies that surface. So the code you pick shapes how every part looks and feels. In this guide, we explain every SPI grade in plain terms. You’ll see what each one looks like and how to choose the right one for your part.
We’ll start with what SPI finishes are and walk through all 12 grades. Then we’ll cover how finish ties to your material, draft, and defects. Finally, we’ll show you how to pick a finish and put it on your drawing.
SPI surface finish specifications are 12 standard grades that describe how smooth or textured a mold’s surface is. The Society of the Plastics Industry created them. That group is now the Plastics Industry Association. The 12 grades fall into four groups:
In each group, 1 is the smoothest and 3 is the roughest. More polish takes more toolroom time, which raises mold cost.
An SPI surface finish is a grade that sets how smooth or rough the inside of a mold is. The finish lives on the mold steel, not on the plastic. Hot plastic presses against that steel as it fills the mold and takes on its surface. So every part from that mold carries the same look.
SPI is a U.S. standard created by the Society of the Plastics Industry. In 2016, the group changed its name to the Plastics Industry Association. The standard gives designers and molders one shared set of codes. When you write “SPI A-2” on a drawing, your molder knows exactly what you mean.
Words like “glossy” or “smooth” can mean different things to different people. A grade code doesn’t. That’s why it belongs on your drawing instead of a loose description.
A mold finish is not the same as finishing parts after they’re molded:
We set your mold finish while we design and polish the mold. So it pays to choose your grade early, before the steel is polished.
Each SPI grade has a letter and a number. The letter tells you the group, from A (glossiest) to D (textured). The number tells you how fine the finish is within that group. A 1 is the smoothest, and a 3 is the roughest.
| Grade | Finishing Method | Look | Typical Use | Ra (microinches) |
| A-1 | #3 diamond buff | Mirror gloss | Optically clear parts | 0–1 |
| A-2 | #6 diamond buff | High gloss | Highly polished or clear parts | 1–2 |
| A-3 | #15 diamond buff | Gloss | Non-optical lenses | 2–3 |
| B-1 | 600 grit paper | Fine semi-gloss | Medium-polish parts | 2–3 |
| B-2 | 400 grit paper | Medium semi-gloss | Medium-polish parts | 4–5 |
| B-3 | 320 grit paper | Low semi-gloss | Medium- to low-polish parts | 9–10 |
| C-1 | 600 grit stone | Fine matte | Low-polish parts | 10–12 |
| C-2 | 400 grit stone | Medium matte | Low-polish parts | 25–28 |
| C-3 | 320 grit stone | Coarse matte | Low-polish parts | 38–42 |
| D-1 | Dry blast, glass bead | Satin texture | Handheld parts | 10–12 |
| D-2 | Dry blast, #240 aluminum oxide | Dull texture | Grip surfaces | 26–32 |
| D-3 | Dry blast, #24 aluminum oxide | Rough texture | Grip surfaces | 190–230 |
The grades run from a mirror polish at A-1 to a rough blasted texture at D-3. Ra is a roughness number, and lower means smoother. Ra values vary from source to source, so use them as a guide, not a guarantee.
Now that you can read a grade, let’s look at what each group is for.
A finishes are the shiniest grades in the SPI standard. Toolmakers buff the mold steel with diamond paste. A-1 uses grade #3 diamond, A-2 uses #6, and A-3 uses #15. The finer the diamond, the brighter the part.
SPI A finishes are best for:
Not every mold steel can take this kind of polish. High-gloss and clear parts call for hardened steel graded for polishability. These steels take a mirror polish and hold it through a long mold life.
Gloss also shows every flaw. A mirror surface makes sink marks, flow lines, and scratches easy to see. If your part has thick spots that tend to sink, an A finish will put them on display.
A grades take the most polish time in the toolroom. That makes them the most costly finish to put on a mold, and it’s one of the things that drives mold cost. Use them only on surfaces where people will see and judge the shine.
Most molded parts don’t need a mirror. B and C grades give you a clean, even surface with less polish time than an A finish. They’re the working grades behind many everyday products.
B finishes are sanded with grit paper: 600 for B-1, 400 for B-2, and 320 for B-3. The result is a medium shine. Some molders use B-2 as a default when a drawing gives no finish. It removes tool marks and helps parts release from the mold.
C finishes are worked with grit stone in the same 600, 400, and 320 steps. Stone leaves a flat, matte surface. Matte reflects less light, so fingerprints are harder to see.
| SPI B | SPI C | |
| Look | Medium shine | Flat matte, low shine |
| Polish cost | Lower than A grades | Lower than A grades |
| Typical use | Medium-polish parts, such as clips | Low-polish parts, such as keyboards and laptop frames |
Both cost less to polish than A grades. That makes them a common default when looks matter some, but not a lot. In many parts, a B or C grade gives you a good-looking surface without paying for polish no one will notice.
Gloss looks great, but it doesn’t hide much. That’s where texture comes in.
D finishes are the textured end of the SPI chart. Instead of being polished, the mold steel is dry blasted. D-1 uses glass bead. D-2 and D-3 use aluminum oxide, and D-3 is the roughest.
Texture does real work on a part. It hides fingerprints, scratches, and small surface flaws. It also gives you a better grip on handheld products like shop vacs and steering wheels.
SPI isn’t the only texture standard you’ll see on drawings and quotes. VDI 3400 is a German standard, and its textures are made mostly by EDM machining, which shapes steel with electric sparks. Mold-Tech textures are named patterns with set depths. For example, MT-11010 is a light texture about 0.001″ deep.
| Standard | How It’s Made | Common Use |
| SPI D | Dry blast with glass bead or aluminum oxide | Satin to rough blasted textures |
| VDI 3400 | Mostly EDM | Matte to dull textures in numbered grades |
| Mold-Tech | Hand finishing, laser, or chemical etching | Commercial products and patterned textures |
Picking a finish isn’t only about looks. It changes your resin, your draft, and which flaws show. Here’s how each one connects:
A chart only helps once it becomes a clear choice. Start with these five questions for each surface of your part:
You don’t need one finish for the whole part. A single part can mix finishes, such as a glossy front face and a matte inside. Pay for high polish only where people will see it.
Once you choose, put the grade on your drawing. Name the surface along with the grade, so your molder knows exactly where each finish goes.
Sample drawing note: “SPI B-2 on Face A. SPI C-1 on all other surfaces.”
Not sure which grade fits? A DFM review is a good time to check your finish against your resin, draft, and part design before the mold is built.
Your finish is only as good as the shop that builds and polishes the mold. We’re an ISO 9001:2015 certified molder based in Logan. Since 2011, we’ve worked with businesses across Northern Utah and Southern Idaho.
We design, machine, and hand polish molds under one roof. Mold components are fit, polished, and assembled by hand, and we dial in the surface finish for cosmetic parts. Before production starts, we run staged trials during mold sampling and testing. The first parts get checked against your print and for flaws like sink, flash, and warp.
Have a part that needs the right finish? Contact us and request a free quote, or call us at (435) 774-9090 to get things started!
