VDI 18 Surface Finish: Ra Reference & Plaque Guide

VDI 3400 GRADE GUIDE · PHYSICAL COMPARISON

At this point, do not choose by the grade label alone. Surface appearance on the molded part depends on the agreed sample, resin, pigment, and process route.

Published grade on this site VDI 18 Indicative comparison value Ra 0.80 µm

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 18

Coarser finishes should be reviewed with the moldmaker for geometry, draft, and release considerations. VDI 18 marks the shift from a fine, silk-like feel toward a visibly matte surface. Where that transition reads on a given part depends on resin and color, so validate it on the intended material rather than the grade number alone.

Before you specify this grade

Compare VDI 15, 18, and 21 on the same plaque and document which physical master is approved.

  • 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.
Question to settle with your team:
Does the molded sample read as the intended satin or matte appearance under real viewing conditions?

Compare adjacent grades

Compare VDI 15Compare VDI 21

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