Introduction to Minimum Draft Angle for Micro-Molded Parts
When features drop below a millimeter, draft becomes the difference between a clean release and a torn edge. The question, “What’s the minimum draft angle for micro-molded parts?” doesn’t have a single number—it depends on resin, surface finish, texture, depth, and ejection method. Below are practical ranges and how to choose them.
Typical draft angles for injection molding
- Polished cavities (no texture): 0.25°–0.5° draft is usually achievable; 0.25° is realistic with LCP and very shallow draws.
- Fine texture (Ra ≤ 0.4 μm / light matte): 0.5°–1.0°.
- Functional textures/laser micro-texture: 1.0°–2.0° (or more with deeper textures).
- Zero-draft? Possible only with slides/lifters/unscrewing, ultra-polish, or sacrificial shutoffs—and carries yield risk. Don’t plan on it for straight pulls.
Why draft matters more at micro scale
- Surface-to-volume blows up: friction and adhesion dominate.
- Short, stiff features: edges scuff easily; any galling shows up as a “big” defect.
- Ejection energy is tiny: pins or air assist have little margin before deformation.
Practical minimums by material (straight-pull, polished steel)
| Material | Typical Micro Draft | Pushing the Limits | Notes |
|---|---|---|---|
| LCP | 0.3°–0.5° | 0.25° | Low shrink, high stiffness; great for micro features. Orient. can make release directional. |
| PC / PC-ABS | 0.5°–1.0° | 0.5° | Tends to scuff; sensitive to heat/hold-up. |
| PP | 0.5°–1.0° | 0.5° | Good slip; watch for draw depth vs. cooling time. |
| PEI | 0.5°–1.0° | 0.5° | High-temp resins; coatings help reduce drag. |
| PA12 (Nylon) | 0.5°–1.0° | 0.5° | Moisture affects release; control conditioning. |
| TPE/TPU | 1.0°–2.0° | 0.75° | Soft parts stick; texturing increases needed draft. |
How surface finish changes the answer
| Cavity Finish | Examples | Add this Draft |
|---|---|---|
| Mirror / SPI A1–A2 | Diamond polish, optics | 0° add (use table baseline) |
| Fine matte / EDM-polish hybrid | Ra ≈ 0.2–0.4 μm | +0.25°–0.5° |
| Light texture / bead blast | Cosmetic matte | +0.5°–1.0° |
| Functional micro-texture | Laser grip, micro-channels | +1.0°–2.0° (or per texture depth) |
Design levers that let you go smaller
- Keep draws shallow. Short features release with less draft.
- Coat the steel. DLC, TiN, CrN reduce adhesion and scuff.
- Use directional draft. More draft opposite knit lines/ejector pins.
- Break edges. Micro-chamfers (0.02–0.05 mm) at parting lines reduce galling.
- Vent properly. 0.005–0.02 mm micro-vents stop burn that roughens walls.
- Ejection strategy. Favor air-assist/sleeves/stripper plates over small pins that bruise parts.
- Thermal balance. Polished micro cores need tight ΔT to avoid stick-slip.
When zero-draft is (barely) acceptable
- Unscrewing threads, slides/lifters, or collapsible cores where steel moves away.
- Disposable sacrificial shutoffs (tooling inserts you plan to refinish).
- Ultra-short features with mirror polish and low-shrink resin (e.g., LCP).
Caveat: expect higher maintenance and lower yield. Treat zero-draft as an exception, not a spec.
Micro features: ribs, bosses, and holes
- Ribs: 0.25–0.5° (LCP at 0.25°; most others 0.5°). Keep rib thickness ≤ 0.5–0.6× wall.
- Bosses/tubes: 0.5–1.0° on OD and ID; add venting at core tips.
- Through-holes/slots: 0.5° typical; EDM texture inside holes demands more.
- Lenses/optics: draft as low as 0.1–0.25° is doable with SPI A1; protect from pins.
Validation checklist (use before you cut steel)
- Draft map from CAD (color-plot) with min ≥ target + 0.1° safety.
- Call out finish per surface (SPI/VDI/Ra) and apply the add-on draft.
- Simulate ejection (interference, pin sizing, ejector mark risk).
- Agree on steel coatings and polish sequence in the DFM.
- Plan for first-article metrology (optical CMM) to verify draft angles.
Need Help Minimizing Draft Angles for your Micro-Molded Parts?
Plan on 0.25–0.5° for polished straight-pull features with the right resin (LCP at the low end), and 0.5–1.0°+ as soon as you add texture, depth, or softer materials. Going below that is possible—but it all depends, and we’re always up for a challenge.