Injection Molding Surface Finish: SPI, VDI and Mold-Tech

Concept render

Injection molding surface finish is set by the mold surface. SPI grades run from A (diamond-polished gloss) through B (paper) and C (stone matte) to D (blasted texture). VDI 3400 numbers and Mold-Tech codes describe roughness and textures. Deeper texture needs more draft. Polish and texture can change the tool price, not the part price, and texturing adds lead time.

The finish on a plastic housing is decided inside the mold. Whatever the steel looks like, every part copies it. On a wearable or tracker, the finish sets how the product looks, how it shows fingerprints, how much draft the walls need and how long the tool takes. This guide covers the finish systems in molder quotes, the finish for each face of a device housing, and what to check before the tool is cut.

Values come from the pages listed under Sources, read on November 6, 2026. Each is that company's guideline for its own process. No Duobay price, no test result.

What are the injection molding surface finish standards?

Molders use three common systems, plus their own house codes, and one callout does not always translate into another. Protolabs describes two paths. SPI and its own PM finishes are mostly applied by hand through polishing, stoning or bead blasting. Mold-Tech textures are applied after the cavity is milled, by chemical etching or with a five-axis laser (Protolabs texture guide, as of November 2026).

Sources: Xometry, An Overview of Injection Molded Surface Finishes (updated May 14, 2025), Protolabs texture guide and Protolabs surface finishes, all as of November 2026.

What do the SPI surface finish grades mean?

The SPI system names the abrasive used on the steel. Fictiv sums it up: diamond abrasive for a glossy A finish, grit paper for semi-gloss B, stone for matte C, and dry blasting for textured D (Fictiv SPI guidelines, dated November 11, 2022, as of November 2026). The table gives Xometry's method and roughness range (Ra) for each grade.

Source: Xometry finishes overview, as of November 2026. Fictiv lists the same abrasives for each grade.

Xometry says a part typically gets SPI B-2 unless you specify otherwise, to help part release and remove tool marks. Xometry lists A-1 and A-2 for high polish or transparent parts, and says transparent parts are typically polished on both sides. Protolabs lists three SPI grades in its standard menu, SPI-A2, SPI-B1 and SPI-C1, next to its own codes PM-F0, PM-F1, PM-F2, PM-T1 and PM-T2 (Protolabs surface finishes, as of November 2026). Protolabs describes its SPI-A2 as a Grade #2 diamond buff, while Xometry's table lists A-2 as Grade #6, 3000 grit, so ask each shop what its grade means.

How do VDI surface finish numbers compare with SPI?

VDI 3400 is a VDI guideline. Its listing on vdi.de gives the English title Electrical Discharge Machining (DEM): Definitions, processes, application (German title Elektroerosive Bearbeitung; Begriffe, Verfahren, Anwendung), issue date June 1975, available in German (VDI 3400 listing, as of November 2026). We did not read the full text, which is sold. Molders use its numbers as a roughness scale, mainly produced by EDM per Xometry. A higher number means a rougher surface.

Source: Xometry finishes overview, as of November 2026. Selected rows; the last groups VDI 39, 42 and 45.

Xometry gives two near matches: VDI 12 is equivalent to SPI C-1, and SPI D-3, VDI 33 and MT-11020 look very similar. Treat other cross-references as approximate, and approve the grade on a sample.

What is a Mold-Tech texture?

Mold-Tech is the texture catalog of Standex Engraving Mold-Tech. Protolabs says Mold-Tech textures can go on steel or aluminum molds. Codes are MT- plus five digits, for example MT-11010 (sand-like), MT-11120 (smooth concrete) and MT-11555 (wood panel) (Protolabs texture guide, as of November 2026). Xometry says the catalog has four series, A to D, and Series A is the most used.

Source: Xometry finishes overview, Mold-Tech Series A rows, as of November 2026.

Deeper texture needs more draft, and the minimum for each texture depth is covered in our injection molding draft angle guide.

The Standex Mold-Tech Texture Technical Data Sheet adds conditions that a single ratio leaves out (Standex Mold-Tech data sheet, as of November 2026). Its draft ratio is for walls the part shrinks away from. Areas that shrink onto the steel need 2 to 3 times the draft. Shutoffs on textured walls need more draft or special work, and slides and lifters are judged by their angle of action. Texture depth can be reduced in chosen areas without a visible hard line, to head off ejection problems. The same draft guide covers plain walls, ribs and bosses.

Which finish fits each face of a device housing?

Choose a finish for each face. Protolabs says the A side of the mold typically forms the outside of the part and the B side carries most of the ejector and retention pins, and it can put different finishes on each side (Protolabs surface finishes, as of November 2026). The rows below are our reading of the sources for common device faces.

Sources: Fictiv SPI guidelines, Xometry, Protolabs surface finishes and Protolabs texture guide, all as of November 2026.

Gloss, matte and fingerprints

Fictiv says handheld products benefit from a dull finish that hides fingerprints or a texture that will not slip, and that C-1 to C-3 make fingerprints less noticeable than a high polish (Fictiv SPI guidelines, as of November 2026). That suits a pendant, a band or a remote. We found no source that measures scratch visibility by grade, so rub and handle sample plaques before you decide. Protolabs offers a sample plaque showing the seven most popular Mold-Tech textures (Protolabs texture guide, as of November 2026).

Resin and color change the result

Protolabs says harder materials and darker colors show texture more, and fillers reduce it: unfilled nylon picks up a finish better than 30% glass-filled nylon (Protolabs surface finishes). Its texture guide adds that, in most cases, texture gives the best cosmetic result in darker materials (Protolabs texture guide). Both as of November 2026. Review resin and finish together. Our PC vs ABS vs PC/ABS guide covers the resin side.

Knit lines also change how texture looks where two flow fronts meet. Our weld lines guide explains where they form on an enclosure and how to keep them off a textured front face.

Marking each face on the drawing

For a custom finish, Protolabs asks for a color-coded image that marks each finish, surfaces with at least 0.015 in (0.38 mm) of feature height to define the edges, and a CAD model that already has the required draft (Protolabs, as of November 2026). Name the system and grade on each face, for example SPI A-2 on the light pipe and MT-11010 on the top shell.

How does the finish choice change tooling time and cost?

The finish shows up mainly in the tool price. Xometry says it does not alter the part price and can change the tool price (Xometry). Fictiv says finishes that take longer to produce, use more expensive abrasives or need more complex processes cost more (Fictiv). Both as of November 2026.

• Texture adds days. A 2019 Protolabs post says mold texturing typically adds nine business days to a Protolabs mold order (Protolabs finishing options, as of November 2026). Ask your molder for its figure.
• Texture needs a clean base. The Standex data sheet asks for 500 emery under textures shallower than 0.0008 in (20 µm), 400 emery up to 0.004 in (100 µm) and 320 emery above that, with all EDM marks removed.
• Changes are harder once texture is on. Protolabs says a face that already has PM-T1 or PM-T2 needs one or both mold halves re-machined before it can take a further texture.
• Prototype and production tools can differ. Standex has textured steels such as P-20 and H-13 as well as aluminum, and says tool material can significantly affect texture appearance. Approve the texture again on parts from the production tool. Our injection mold tooling guide compares tool materials.

So freeze the walls, add draft for the texture you expect, mold first parts plain, and texture once the design stops changing. Protolabs advises deciding on texture after design iteration is complete, and says texture can often be added to a finished mold when geometry and the existing surface roughness permit (Protolabs texture guide, as of November 2026). For how the mechanical work fits into the build, see how the mechanical work is organized.

What should you ask your hardware team about surface finish?

Take this list to your mechanical engineer and to the molder that quotes the tool. Record each answer with its source.

• Which finish system and grade is called out on each face, and where does each callout come from?
• For each texture, what is its depth, and which draft rule did we use: the Mold-Tech data sheet, Xometry or the texturing shop?
• Does any textured wall shrink onto the steel, or sit at a shutoff, and has the texturing shop checked its draft?
• Which resin and color will the part ship in, and do the molder's material ratings support that finish?
• Will the prototype tool be aluminum and the production tool steel, and who approves the texture match between them?
• Which sample plaques or first-shot parts will we sign off as the finish reference?

If you want the finish callouts for your housing checked against draft, resin and tool plan, tell us about your device.

Sources. All pages accessed November 6, 2026. Molder guidelines change, so confirm current values with the shop that builds your tool.

• Xometry, An Overview of Injection Molded Surface Finishes (published October 7, 2020, updated May 14, 2025) (link)
• Protolabs, Mold Texture Standards and Finishes (link)
• Protolabs, Injection Molding Surface Finishes (link)
• Protolabs, Using Finishing Options for Injection-Molded Parts (June 12, 2019) (link)
• Fictiv, SPI Guidelines for Injection Mold Surface Finish (November 11, 2022) (link)
• Standex Engraving Mold-Tech, Texture Technical Data Sheet (no date printed) (link)
• VDI, VDI 3400 standard listing (link)