VDI 3400 GRADE GUIDE · PHYSICAL COMPARISON
VDI 21 is a useful checkpoint when a team is moving from a finer appearance toward a more visible matte texture. Treat the plaque as a selection aid, not a guarantee of part gloss.
This Ra figure matches the widely published Charmilles/Agie-derived VDI 3400 comparison chart, cross-checked here against an independent EDM reference-standard supplier's published specification (see citations below), not only against this site's own material. It is an indicative comparison value, not a licensed-standard quotation, gloss measurement, texture-depth conversion, guaranteed molded appearance, or contractual tolerance. If another document (for example a downloadable PDF) shows a different rounded figure, treat this page as the controlling reference until confirmed otherwise with your supplier.
How to evaluate VDI 21
Mid-range finishes benefit from side-by-side comparison because neighboring grades can be mistaken in photographs. VDI 21 is often specified as a balance between grip and a clean appearance. Whether that balance holds on the finished part depends heavily on draft angle and ejection design, not on the roughness figure alone.
Before you specify this grade
Review the parting direction, draw depth, resin, and texture location with the moldmaker; approve a molded trial before freezing the callout.
- Mark exactly which faces receive the texture and note the direction of draw.
- Name the physical master and confirm its material and condition.
- Approve appearance on a molded sample using the production resin and color.
- Define the measurement method and acceptance limits if Ra is contractual.
Will the chosen texture still release and meet appearance needs on the actual tool and part geometry?
Compare adjacent grades
Look at the reference plaques together rather than relying on a phone or monitor image. The physical plaque helps align design, tooling, purchasing, and inspection; a production-material trial remains necessary for final appearance approval.
Need the physical reference?
Review the available VDI reference plaques and confirm which scale or material version covers your target grade before ordering.
See VDI reference plaques View the complete VDI 12–45 guide
Reference and limitations
The VDI official catalog describes VDI 3400 as a guideline on electrical discharge machining. The Ra value here is reproduced as an indicative supplier comparison value, not represented as a certification or a complete statement of the licensed guideline. Published charts can vary in rounding. Agree the applicable reference and measurement procedure before contractual use.
Supplier information only. No VDI endorsement or certification is implied. Validate the selected finish on the actual tool and molded part.
Content last reviewed: 2026-09-24.