Achieving the right surface finish on molded parts is crucial for both aesthetics and functionality. The finish influences part appearance, texture, release properties, and even durability. Two widely used standards—SPI (Society of Plastics Industry) and VDI (Verein Deutscher Ingenieure)—define surface textures for molds.
In this post we highlight the VDI standard. This standard (VDI 3400) defines textured finishes using Electrical Discharge Machining (EDM) ranging from VDI 12 (smooth) to VDI 45 (rough look). Ideal for technical parts, automotive applications, and grips where aesthetics or tactile properties matter.
| Ra | Rz | |
|---|---|---|
| VDI 3400-12 | 0.40 | 1.5 |
| VDI 3400-15 | 0.56 | 2.4 |
| VDI 3400-18 | 0.80 | 3.3 |
| VDI 3400-21 | 1.12 | 4.7 |
| VDI 3400-24 | 1.60 | 6.5 |
| VDI 3400-27 | 2.24 | 10.5 |
| VDI 3400-30 | 3.15 | 12.5 |
| VDI 3400-33 | 4.50 | 17.5 |
| VDI 3400-36 | 6.30 | 24.0 |
| VDI 3400-39 | 9.00 | 34.0 |
| VDI 3400-42 | 12.50 | 48.0 |
| VDI 3400-45 | 18.00 | 69.0 |
- Ra, or average roughness (measured in µm), is the most frequently used surface texture parameter. It offers a general measure of the height of the texture across a surface. More precisely, Ra is the average of how far each point on the surface deviates in height from the mean height.
- Rz, or the Ten-point Height of Irregularities,(measured in µm) represents the average difference between the five tallest peaks and the five deepest valleys in a surface’s texture. Unlike Ra, which focuses on overall roughness, Rz offers a clearer view of extreme variations by highlighting outlier peaks and valleys.
Every VDI finish comes with its own advantages and constraints in terms of compatibility with part geometry. In many cases an additional draft angle will be necessary. We’ll explore that in more detail later on..
Selecting the right finish depends on the part’s function—whether it’s a sleek consumer product, an industrial component, or a high-grip surface. A well-chosen surface finish enhances durability, reduces defects, and optimizes molding efficiency.
Need help picking the right finish for your project? Let’s talk!
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