VDI surface finish guide for mold textures, Ra values and draft angles
What VDI surface finish means on a drawing
VDI surface finish usually refers to the VDI 3400-style numerical scale used to specify mold cavity texture, especially EDM or spark-eroded textures for injection molded plastic parts. In drawing practice, a callout such as VDI 24 or VDI 30 tells the moldmaker the target texture grade and the approximate roughness level. Lower numbers are smoother; higher numbers are rougher and more matte.
VDI is not only a cosmetic preference. It can affect part appearance, mold release, required draft angle, inspection method and, in some cases, resin selection. A clear VDI callout should identify the grade, the affected surfaces, the acceptance reference and any material or draft constraints.
The public VDI listing identifies VDI 3400 as a guideline titled Electrical Discharge Machining, with a publication date of June 1975 and 25 pages. In moldmaking, the term is closely associated with the VDI/Charmilles roughness comparison plaques used to communicate repeatable EDM texture levels. For more finishing context across machining and molding, see the Mechmeld surface finishing section.
How the VDI 3400 grade scale works
The VDI grade number is a compact way to describe a textured mold surface. It is commonly paired with roughness values such as Ra in micrometers, and many charts also show Rz or a peak-to-valley value. The values below are approximate figures compiled from commonly used VDI 3400 conversion charts. They are useful for design discussions and quoting, but they should not replace a physical texture plaque or an agreed inspection plan for critical cosmetic parts.
| VDI grade | Typical Ra | Approximate Rz or peak-to-valley value | Typical visual description |
|---|---|---|---|
| VDI 12 | 0.40 µm | About 1.5-1.6 µm | Fine low polish |
| VDI 15 | 0.56 µm | About 2.3-2.4 µm | Low polish, slightly more visible texture |
| VDI 18 | 0.80 µm | About 3.2-3.3 µm | Satin or fine bead-blast look |
| VDI 21 | 1.12 µm | About 4.5-4.7 µm | Light dull texture |
| VDI 24 | 1.6-1.62 µm | About 6.3-6.5 µm | Common matte molded texture |
| VDI 27 | 2.2 µm | About 9-10.5 µm | Noticeable matte texture |
| VDI 30 | 3.2 µm | About 12.5 µm | Coarser dull texture |
| VDI 33 | 4.5 µm | About 17.5-18 µm | Heavy matte texture |
| VDI 36 | 6.3 µm | About 24-25 µm | Heavy rough texture |
| VDI 39 | 9 µm | About 34-36 µm | Very rough dull texture |
| VDI 42 | 12.6 µm | About 48-50 µm | Very heavy texture |
| VDI 45 | 18 µm | About 69-76 µm | Extremely rough texture for special use |
Grades in increments of three are the ones most often seen on comparison plaques, although some conversion charts list every grade from 0 to 45. In everyday product design, the team usually selects from a small set of visual samples rather than specifying every intermediate number.
VDI, SPI and Mold-Tech are not the same thing
VDI, SPI and Mold-Tech are often discussed together because they all describe mold surface appearance, but they are not interchangeable. VDI is mainly a numerical roughness and EDM texture language. SPI, historically associated with the plastics industry, groups finishes by polishing, paper, stone and blasting methods. Mold-Tech refers to proprietary decorative texture patterns, often used when the desired appearance is leather-like, grained or branded rather than simply matte.
| System | Primary use | What it communicates well | Main limitation |
|---|---|---|---|
| VDI surface finish | EDM-style mold texture and roughness callouts | Texture grade and approximate Ra/Rz level | Does not fully define gloss, pattern direction or decorative grain |
| SPI finish | Polished, papered, stoned and blasted mold finishes | General finishing method and smoothness class | Conversion to VDI is approximate, not exact |
| Mold-Tech texture | Decorative molded surface patterns | Specific visual grain or pattern from a texture library | Often requires proprietary samples and approval plaques |
Because these systems describe different surface characteristics, an SPI-to-VDI chart should not be treated as a guaranteed substitute. Two surfaces can have similar Ra values but look different because the lay, crater shape, gloss and peak distribution are different. A VDI 24 EDM texture and a chemically etched texture with similar roughness may not match under production lighting.
Ra and Rz explain roughness, but not the whole appearance
Ra is the arithmetic average roughness value of a profile. It is widely used because it is easy to compare, but it reduces peaks and valleys to one average number. That makes it less sensitive to isolated deep valleys or high peaks. Rz is based on profile height over sampling lengths and gives a stronger indication of peak-to-valley texture. Modern profile roughness practice is covered by the ISO 21920 family, while many shops still refer to earlier DIN EN ISO 4287 and ISO 4288 terminology in legacy drawings and measuring guides.
This distinction matters because VDI texture acceptance can still be disputed when a single Ra reading appears close. Roughness readings are influenced by measurement length, filter settings, stylus tip radius, measurement direction and where the probe crosses the texture. On a cosmetic surface, gloss and light scattering may be as important as the numeric roughness. For critical exterior parts, the safer approach is to combine a VDI grade with a physical sample plaque, visual acceptance criteria and a defined measurement method.
Draft angle and material effects
Texture makes molded parts harder to release because the microscopic peaks and valleys in the cavity surface create small undercut-like features. As plastic cools, it shrinks onto the core or cavity. If the wall has insufficient draft, the part can drag across the texture during ejection, causing scratches, whitening, scuffing, sticking or distortion. This is why a rougher VDI surface finish usually requires more draft than a polished surface.
Exact draft depends on part depth, wall geometry, resin shrinkage, ejection layout, mold steel, texture process and molder capability. Supplier guidance commonly starts around 0.5 degrees on vertical faces, uses 1-2 degrees for many ordinary molded walls, and increases draft significantly for textured surfaces. Some design guides require about 3 degrees for light texture and 5 degrees or more for heavier texture. VDI-specific draft charts often show a gradual increase from fine textures such as VDI 12-18 to coarse textures such as VDI 36-45.
| Texture range | Typical design interpretation | Draft planning note |
|---|---|---|
| VDI 12-18 | Fine low-polish to satin texture | May work with modest draft if geometry and resin are favorable |
| VDI 21-27 | Common matte visible texture | Plan additional draft early, especially on tall ribs and side walls |
| VDI 30-36 | Coarse dull or heavy matte texture | Often needs substantial draft and careful ejection review |
| VDI 39-45 | Very heavy texture | Use only with molder approval, generous draft and sample validation |
Material selection also changes the final surface. Glass-filled materials can look rougher or more fibrous because the filler affects the skin layer. Stiffer materials may release differently from softer grades. Polycarbonate, acrylic and other appearance-sensitive resins can show drag marks more readily if draft or mold temperature is poorly controlled. For this reason, VDI texture should be selected together with resin, wall thickness, gate location and expected production volume, not after the tool design is finished.
How to specify VDI surface finish clearly
A good drawing callout removes ambiguity for the toolmaker, molder and quality team. It should identify the surface, grade and acceptance method. It should also state whether the texture is required in the mold cavity only, on the molded part, or on a specific side of the part. If different surfaces use different finishes, mark the boundaries clearly. Avoid placing texture across shutoffs, sealing lands, optical windows or tight sliding interfaces unless the function has been reviewed. See also: CNC Machining.
- Use a clear grade callout, such as VDI 24, rather than only saying matte finish.
- State the affected surfaces using drawing zones, CAD face colors or surface labels.
- Define whether the value is for the tool surface, molded part appearance or both.
- Reference a physical texture plaque or approved sample part when appearance is critical.
- Include roughness units if numeric Ra or Rz limits are part of inspection.
- Confirm draft angle before the mold cavity is textured.
- Define polishing or pre-texture requirements because the base mold finish affects the final look.
- Specify any no-texture zones around parting lines, clips, bosses, ribs, sealing areas and logos.
Example drawing language can be simple: visible exterior surfaces to VDI 24 texture, matched to approved texture plaque; maintain minimum draft agreed with molder; no texture on sealing land. The exact wording should follow the company drawing standard, but it should always connect the VDI grade with a real acceptance reference.
Inspection, procurement and common mistakes
For procurement, the risk is not only choosing the wrong grade. It is assuming that every supplier interprets the same grade in the same way. One shop may quote an EDM texture, another may quote blasting, and another may propose chemical etching to achieve a similar roughness. These processes can produce different gloss and pattern character. First article approval should therefore include visual review under agreed lighting, roughness measurement when required, and comparison to the approved plaque or sample.
Common mistakes include converting SPI to VDI too casually, placing coarse texture on walls with too little draft, measuring only Ra when Rz or visual texture matters, approving a color chip but not a texture sample, and texturing the mold before dimensional or sink-mark corrections are complete. Another frequent error is choosing a heavy VDI grade to hide molding defects. Texture can reduce the visibility of minor flow marks, but it cannot fix poor gate design, trapped gas, weld-line weakness, sink or inconsistent packing.
For production tools, agree on the acceptance sequence before final texturing. Many teams first cut and sample the mold with a simpler finish, correct dimensional and molding issues, and then apply the final texture. That sequence reduces the risk of polishing away texture or reworking a finished cosmetic cavity.
Frequently asked questions
Is VDI surface finish the same as VDI 3400?
In most molding discussions, yes. The phrase VDI surface finish normally means a VDI 3400-style texture grade, such as VDI 18, VDI 24 or VDI 30. Strictly speaking, the official VDI 3400 guideline is associated with electrical discharge machining, while the molding industry often uses VDI 3400 as shorthand for a practical EDM texture scale.
What VDI finish is commonly used for a matte plastic part?
VDI 21 to VDI 30 is often considered for matte visible plastic surfaces, with VDI 24 and VDI 27 frequently appearing in design discussions. The right choice depends on resin, wall geometry, color, gloss target, draft and sample approval. Designers should review a physical texture plaque rather than choosing a grade only from a table.
Can I convert SPI finish directly to VDI?
You can use SPI-to-VDI charts for early comparison, but the conversion is approximate. SPI describes finishing method and polish class, while VDI describes a numerical texture grade and roughness range. Similar Ra values do not guarantee the same gloss or visual texture.
Does a higher VDI number mean a smoother finish?
No. A higher VDI number means a rougher texture. VDI 12 is much finer than VDI 30, and VDI 45 is extremely rough compared with common satin or matte molded finishes.
Should VDI be inspected on the mold or the molded part?
Both may be relevant, but they are not identical. The mold surface defines the texture source, while the molded part shows the final appearance after resin, color, processing and shrinkage effects. For cosmetic parts, approve the molded sample under defined lighting and also keep the mold texture reference on record.