What’s The Best VDI Surface Finish For Your Injection Moulded Part?

When it comes to surface finishing options for plastic injection molding, you’ve likely come across the Verein Deutscher Ingenieure (VDI) finish. Like the SPI standard, VDI is a mold finish—but with a key difference: it focuses on surface roughness rather than the degree of polish.

This article covers the standard VDI mold finishes we offer, recommended finish for each material, and results you can expect to get the most out of injection moulding.

What’s the VDI surface finish standard?

The VDI (or VDI 3400) surface finish standard includes 12 official grades: 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, and 45 — with 12 representing the smoothest finish and 45 the roughest. Each grade corresponds to a specific surface roughness value (Ra, in micrometers) and is commonly used to specify EDM-textured finishes in injection molding. The VDI finish standard chart below outlines all twelve grades, as well as the typical finishing methods and a VDI-to-Ra conversion for practical reference.

VDI vs SPI finish

SPI standard is the primary U.S. reference for mould polishing (focus on “SMOOTH“). On the other hand, VDI 3400 is more widely used in Europe and, increasingly, worldwide, (focus on “ROUGH“). Although these standards serve different purposes—SPI for smooth polishing and VDI for rougher textures—they share some comparable grades. Below is a conversion chart providing a rough match between VDI 3400 and SPI finishes for practical reference.

VDI 3400 draft considerations

Some VDI grades (rougher finishes) can increase friction and make part ejection more difficult. To ensure better mold release, it’s important to adjust draft angles according to the specific material (e.g., ABS, PC, PA) and surface texture. Refer to the chart below for general draft angle recommendations.