VDI 3400 surface finish is commonly used in mold discussions to identify an EDM texture grade and a reference roughness. A grade number alone is not a complete cosmetic specification. Identify the textured faces, material and color, the approved appearance reference, and how the mold and molded part will be accepted.
What the VDI number means
The DIN Media listing for VDI 3400:1975-06 identifies a technical guideline on electrical discharge machining: definitions, processes and application. A VDI finish callout is a project specification; it is not, by itself, proof that a plastic part meets a European legal or product-safety requirement.
In the references below, the higher grade corresponds to a larger Ra value. Ra describes average profile roughness. It does not fully describe the surface pattern, gloss, color or the appearance of the final resin. Review those requirements separately when two adjacent molded parts must match.
Selected VDI-to-Ra references
The following values are selected reference points from Meusburger's EDM graphite technical guidance. They are not a reproduction of the complete VDI guideline or a guarantee of achievable finish on every tool.
| VDI grade | Reference Ra (µm) | Use in a specification |
|---|---|---|
| 20 | 1.0 | Confirm the reference sample and measurement conditions. |
| 22 | 1.26 | Check appearance in the specified resin and color. |
| 24 | 1.6 | Identify the textured faces and untextured boundaries. |
| 27 | 2.2 | Review the tool process and release requirements. |
Meusburger relates achievable surface quality to electrode grade and machine technology. Do not treat these reference values as a tolerance band, or infer an Rz acceptance limit by multiplying Ra by a fixed factor. If Rz or another parameter is required, state it explicitly with the applicable measurement specification.
Choose texture using the actual part requirements
Start with the purpose of each surface: visible housing, grip area, assembly interface or an area that will receive printing or coating. Mark these areas on the drawing or a controlled CAD view. Specify where the texture stops so that mating faces, seals and dimensional references are not left open to interpretation.
For appearance approval, request an agreed sample made with the intended resin and color. Protolabs' surface-finish guidance discusses material sample plaques and identifying separate finish areas. A tool sample or a photograph can help a discussion, but it does not establish how every resin will look in the production geometry.
Do not select a texture on the assumption that it will remove fingerprints, conceal all sink marks or eliminate weld lines. Define any cleaning, wear or chemical-exposure requirement separately. If appearance is critical, agree on the viewing conditions and acceptable defects before tool finishing.
Draft and release need a design review
Draft must be assessed with the mold-opening direction, textured wall depth, resin, shrinkage and ejection arrangement. There is no single minimum angle that can be assigned to every VDI 24 or VDI 36 part. Protolabs' draft guidance explains why texture and geometry affect release; its PM-T1 and PM-T2 examples refer to its own bead-blast finishes, not universal VDI conversion rules.
For a deep housing, review inner and outer walls separately. Identify low-draft areas, ribs, side actions and surfaces that may drag during ejection. Agree on any geometry change before the tool is textured, and assess the molded trial parts for scuffing and shiny streaks.
An example of a reviewable texture request
This is an illustrative request, not a customer production case: “Apply VDI 24 reference texture to the marked external faces of the housing. Leave the gasket seat and locating features untextured. Confirm draft feasibility using the specified resin and CAD revision. Approve appearance against sample S-01 before final texture acceptance.”
Add the actual resin grade, color, sample identifier and revision. If numerical roughness acceptance is required, also agree on the parameter, allowed range, measurement locations and procedure. Keep mold roughness acceptance separate from molded-part cosmetic approval so that a passing tool measurement does not substitute for the required appearance check.
What to include in your enquiry
- Drawing and CAD revision with textured areas and boundaries marked.
- Resin grade, color and intended application.
- VDI reference grade and approved physical sample, when available.
- Draft constraints, mold-opening direction and critical interfaces.
- Cosmetic criteria, measurement requirements and requested inspection records.
See our injection molding surface finishes guide for related finish choices. Send the drawing and texture requirements for a project-specific review; achievable finish, process and inspection scope must be confirmed for the component.
Technical references checked on 29 September 2026: DIN Media's VDI listing, Meusburger's EDM guidance and the linked Protolabs design guidance.
