Tip Processing for Textured Surfaces: Preventing Drag Marks
Key Takeaway: Textured surfaces (VDI 24+) require polished flat tips (Ra ≤ 0.2μm) to prevent drag marks. For deep textures (VDI 33+), switch to air-jet valves for zero-contact ejection.
Why Textured Surfaces Need Special Tips
Textured mold surfaces create mechanical interlocking between the part surface and the cavity texture. During ejection, the pin tip drags across the part surface, leaving a glossy witness mark where the texture is flattened. The deeper the texture, the worse the mark. Standards for surface texturing are defined by VDI 3400 (VDI Society of German Engineers).
Tip Selection by Texture Depth
| Texture Depth (VDI) | Ra (μm) | Recommended Tip | Alternative |
|---|---|---|---|
| VDI 12–20 (fine) | 0.4–1.6 | Polished flat grind (Ra ≤ 0.2μm) | Standard flat grind acceptable |
| VDI 21–27 (medium) | 1.6–6.3 | Polished flat grind + anti-drag coating | Radius tip to reduce shear |
| VDI 28–33 (deep) | 6.3–18 | Polished radius tip or air-jet valve | Recessed pin (below surface) |
| VDI 33+ (very deep) | 18+ | Air-jet valve (zero-contact ejection) | Stripper plate ejection |
Anti-Drag Coating Options
For medium textures where a flat tip is still acceptable, applying a low-friction coating to the tip face reduces drag marks by 50–70%. Common coatings: TiN (gold, μ=0.4), DLC (black, μ=0.1), and PTFE-infused nickel (μ=0.15). DLC coating provides the lowest friction coefficient and is recommended for Class A automotive surfaces.