When a MIM part requires a textured surface — EDM spark erosion, chemical etching, sand blasting, or machined grain patterns — the effective draft angle must increase to compensate for the texture depth.
The rule: Add 1.0° of draft for every 0.025 mm of texture depth. Why texture requires more draft: Mold texturing creates undercuts at the microscopic level. When the part cools and shrinks onto the mold core, these micro-undercuts resist ejection. Without sufficient draft, the textured surface tears, part sticks, or the texture is damaged after a few thousand shots. Draft angle by texture type and depth:| Texture Type | Typical Depth | Draft (Polished Surface) | Draft (Textured) | Minimum Draft |
|---|---|---|---|---|
| Fine matte (blasted, VDI 15-24) | 0.005-0.015 mm | 0.5° | 1.0-1.5° | 1.0° |
| Medium grain (VDI 24-30) | 0.015-0.030 mm | 0.5° | 1.5-2.0° | 1.5° |
| Coarse grain (VDI 30-36) | 0.030-0.050 mm | 0.5° | 2.0-3.0° | 2.0° |
| Deep etching (leather, wood grain) | 0.050-0.100 mm | 0.5° | 3.0-5.0° | 3.0° |
| EDM spark texture (rough) | 0.020-0.050 mm | 0.5° | 2.0-3.0° | 2.0° |
For a standard fine-blasted finish: minimum 1.0° draft. For a deep leather or wood grain texture: minimum 3.0° draft. Always specify both the texture standard and the required draft angle on the part drawing — the mold maker needs both to build a cavity that releases the part without damage.
Design rule: if the part has a cosmetic textured surface on a vertical wall, either the texture must be limited to shallow (VDI < 24) or the draft must increase to 2-3°. Many engineers discover this only after the tool is built — and pay for a mold modification.