Mold Texture Standards: SPI, VDI 3400 & Mold-Tech
Mold texture standards are the shared reference systems that let designers, toolmakers, and molders specify a surface finish without ambiguity. Three systems dominate injection molding: SPI surface finish grades for polished and blasted finishes, VDI 3400 for EDM roughness, and Mold-Tech patterns for decorative textures. This hub explains what each system covers, how they differ, and links to the detailed guide for each one.
Mold texture standards at a glance
- SPI: 12 grades (A-1 to D-3) defined by the finishing method, from diamond buffing to dry blasting.
- VDI 3400: a numeric roughness scale (Ref 12 to Ref 45) tied to Ra, used mainly for EDM finishes.
- Mold-Tech: a proprietary library of named texture patterns used for appearance and grip.
- Conversion: the systems describe different things, so cross-references are approximate and must be confirmed on a physical sample.
The three main standard systems
Each system answers a different question. SPI describes how a surface was finished, VDI 3400 describes how rough it is, and Mold-Tech describes what pattern it carries. Choosing the right one depends on whether the requirement is gloss, roughness, or a visual texture.
| System | What it defines | How it is specified | Typical use |
|---|---|---|---|
| SPI surface finish | Finishing method and resulting gloss level | Grade code, for example SPI A-2 or SPI D-2 | Glossy, semi-gloss, matte, and blasted finishes |
| VDI 3400 | Surface roughness on a logarithmic scale | Reference number, for example VDI 27 | Uniform EDM matte finishes on technical parts |
| Mold-Tech textures | Named decorative pattern and depth | Pattern code from the Mold-Tech library | Leather, wood, geometric, and grip textures |
SPI surface finish grades
The SPI system groups finishes into four families: A for diamond-buffed high gloss, B for paper-polished semi-gloss, C for stone-polished matte, and D for dry-blasted textured finishes. Each family has three grades, and the grade number indicates the abrasive used. The grade is set by the process, so two shops following the same grade should produce comparable results.
| Family | Grades | Finishing method | Appearance |
|---|---|---|---|
| A | A-1, A-2, A-3 | Diamond buff: grade #3, #6, and #15 | High gloss to mirror |
| B | B-1, B-2, B-3 | Grit paper: 600, 400, and 320 | Semi-gloss |
| C | C-1, C-2, C-3 | Grit stone: 600, 400, and 320 | Matte, low gloss |
| D | D-1, D-2, D-3 | Dry blast: glass bead #11, oxide #240, oxide #24 | Satin to coarse texture |
Full grade-by-grade guide: SPI surface finish grades.
For the A family in depth, including diamond paste sequence, steel selection, and resin limits for mirror parts: SPI A glossy finish (A-1, A-2, A-3).
For the B family, including paper grit by grade, B-1 compared with A-3, and polishing direction along the draw: SPI B semi-gloss finish (B-1, B-2, B-3).
For the C family, including stone grit by grade, stoned compared with blasted surfaces at the same Ra, and stoning as preparation for texturing: SPI C matte finish (C-1, C-2, C-3).
For the D family, including blast media, blasting parameters, VDI equivalents, and wear on textured tools: SPI D textured finish (D-1, D-2, D-3).
VDI 3400 roughness scale
VDI 3400 is a German guideline widely used in the United States for EDM-textured surfaces. Each reference number maps to an arithmetic mean roughness (Ra) through the relation Ra (µm) = 0.1 × 10^(VDI / 20), so every 6 steps on the scale roughly doubles the roughness. Lower numbers are finer; higher numbers are coarser and need more draft.
| VDI 3400 reference | Ra (µm) | Relative finish |
|---|---|---|
| VDI 12 | 0.40 | Finest EDM finish |
| VDI 18 | 0.80 | Fine |
| VDI 24 | 1.60 | Fine to medium |
| VDI 30 | 3.15 | Medium |
| VDI 36 | 6.30 | Medium to coarse |
| VDI 42 | 12.5 | Coarse |
| VDI 45 | 18.0 | Coarsest on the scale |
Full reference chart and EDM context: VDI 3400.
How the EDM process itself produces these finishes, and how electrode, spark settings, and recast layer affect the result: EDM surface finish.
Measuring the finish: roughness and gloss
A finish grade only works if the result can be checked. Roughness is measured with a profilometer and reported as Ra, Rz or related parameters, and the cutoff and evaluation length must be stated for two measurements to agree. Ra and Rz describe different features of the profile, so one cannot be converted into the other with a fixed factor: Ra, Rz and surface roughness parameters for molds.
Gloss is the second measurement that decides whether a textured or polished part is accepted. It is read in gloss units with a glossmeter at a stated angle, and the same surface gives different readings at 20, 60 and 85 degrees: gloss levels and gloss units.
Mold-Tech textures
Mold-Tech is a commercial texture library rather than a public standard. Its pattern codes are used across automotive and consumer products to call out a specific grain, such as a leather or geometric pattern, together with its depth. Because the pattern is proprietary, the code identifies the look, while the depth drives the draft requirement.
How the catalog is organized and specified: Mold-Tech textures.
Converting between standards
SPI, VDI 3400, and Mold-Tech measure different properties, so there is no exact one-to-one conversion. Cross-reference charts give a practical starting point, for example matching a fine VDI grade to a C or D family SPI finish, but the final match should always be approved on a sample plaque molded in the production resin.
Cross-reference table and limits: SPI, VDI, and Mold-Tech conversion.
SPI mold classes
SPI also publishes a second, unrelated system: mold classes 101 to 105, which rank a tool by expected production life, from Class 101 (one million cycles or more, hardened tool steel) to Class 105 (prototype tools of 500 cycles or fewer). The class is not a finish grade, but it sets the steel and hardness the texture is applied to, which affects how the texture etches, how it wears in production and how it can be repaired: SPI mold classes 101 to 105 and what they mean for texture.
Standards and draft angle
Whatever system is used, rougher or deeper finishes need more draft to release the part without drag marks. Draft is a design decision that must be made together with the finish callout, not after the tool is built.
Depth-to-draft guidance by texture: Draft angle by texture.
Guides in this section
- SPI surface finish grades
- SPI A glossy finish (A-1, A-2, A-3)
- SPI B semi-gloss finish (B-1, B-2, B-3)
- SPI C matte finish (C-1, C-2, C-3)
- SPI D textured finish (D-1, D-2, D-3)
- VDI 3400
- EDM surface finish
- Mold-Tech textures
- SPI, VDI, and Mold-Tech conversion
- Ra, Rz and surface roughness parameters
- Gloss levels and gloss units
- SPI mold classes 101 to 105
To turn these standards into a drawing callout, an approval record and RFQ language in one set of documents, see the Mold Texture Spec Kit.
Technical review: Marcel Dias, Technical Editor, with over 28 years in mold texturing. Read how this content is produced and verified in our methodology. Values are approximate and vary by source; confirm against the primary standard and a physical sample. See our Disclaimer. Last updated: October 23, 2026.