Draft Angle Design Guide — Get the Taper Right Before Steel Is Cut
Draft angles are the tapers added to vertical walls of a mold cavity and core that allow the finished part to be ejected cleanly. Without sufficient draft, the part sticks to the mold, ejector pins push through the wall, and the mold may be damaged. Draft angles are one of the most common oversights that new product developers bring to Chinese mold makers.
This guide covers minimum draft angles by material, by surface texture, and the negotiation points buyers should raise during DFM review.
Minimum Draft Angles by Material
| Material | Minimum Draft (° per side) | Recommended Draft (° per side) |
|---|
| ABS | 0.5 | 1.0 – 1.5 |
| PC (Polycarbonate) | 0.5 | 1.0 – 2.0 |
| PP (Polypropylene) | 0.5 | 0.5 – 1.0 |
| PE (Polyethylene) | 0.5 | 0.5 – 1.0 |
| PA6 (Nylon) | 0.5 | 1.0 – 1.5 |
| POM (Acetal) | 0.5 | 1.0 – 1.5 |
| PMMA (Acrylic) | 1.0 | 1.5 – 2.0 |
| PS (Polystyrene) | 0.5 | 1.0 |
| PC/ABS | 0.5 | 1.0 – 1.5 |
| PBT | 0.5 | 1.0 – 1.5 |
| LCP | 0.3 | 0.5 |
| PEEK | 0.5 | 1.0 – 1.5 |
| TPE / TPR | 1.0 | 2.0 – 3.0 |
| Liquid Silicone Rubber | 3.0 | 5.0 |
Buyer's Tip: A common trick in some Chinese mold factories: when your 3D model has zero draft on vertical walls, they will say "it's fine, we'll make it work." What they mean is they'll add a tiny draft (0.2-0.3°) during CAM that's invisible on your drawing but makes the part 1-2mm smaller at the cavity side. This changes your part dimensions without your approval. Always specify the minimum acceptable draft angle in your RFQ. If your design truly requires zero draft (e.g., a decorative edge), the mold needs special ejection like lifters or collapsing cores — plan for it in both budget and timeline. Adding draft after the mold is cut costs $500-2,000 per steel modification.
Draft by Surface Texture Depth
Textured surfaces dramatically increase the force required to eject a part. The texture "locks" into the steel, requiring more draft to release cleanly.
| SPI Finish | Texture Description | Additional Draft Needed | Total Recommended Draft |
|---|
| SPI-C1 | Light bead blast (600 grit) | 0.5° | 1.5° |
| SPI-C2 | Medium bead blast (400 grit) | 1.0° | 2.0° |
| SPI-D1 | Light EDM texture | 1.0° | 2.0° |
| SPI-D2 | Medium EDM texture | 1.5° | 2.5° |
| VDI-24 | Fine matte | 1.0° | 2.0° |
| VDI-30 | Medium matte | 1.5° | 2.5° |
| VDI-36 | Coarse matte | 2.0° | 3.0° |
| VDI-45 | Rough texture | 2.5° | 4.0° |
| Yick Sang #3 | Deep grain (leather-like) | 3.0° – 5.0° | 5.0°+ |
Core vs. Cavity Draft Requirements
Draft requirements are different on the core side (male) and cavity side (female). The part shrinks onto the core during cooling, so core-side draft must be greater than cavity-side draft — typically 1.5-2x. If the core has only 0.5° draft and the part shrinks 2%, the grip force can exceed 500kg on a 100mm core, making ejection impossible without damage.
Special Cases That Require Extra Draft
- Deep ribs: Ribs deeper than 3x wall thickness need 2-3° draft minimum, regardless of material. Shallow ribs (less than 1.5x wall thickness) can use the standard draft.
- Bosses: Internal and external boss walls both need draft. External boss usually matches part draft (1°), internal needs 0.5° minimum.
- Snap-fits: The leading edge of a snap-fit hook needs enough draft to deflect the mold steel — typically 3-5° on the outer side. Zero-draft snap-fits require lifters in the mold.
- Threads: Molded threads (unscrewing molds) require 0.5-1° draft on thread flanks for release, or the mold must unscrew the core.
- Living hinges: The hinge area itself has zero draft by design, but the walls adjacent to the hinge must have at least 1° draft to avoid tearing during ejection.
When Draft Can Be Reduced or Eliminated
There are limited cases where zero or near-zero draft is acceptable:
- Cavity side of a deep-cup part: The part shrinks away from the cavity wall, so the cavity can have as little as 0.25° draft.
- PTFE-filled materials: The internal lubricant reduces friction, allowing 0.3-0.5° draft even on deep cores.
- High-polish cavity surfaces: A mirror-finished cavity (SPI-A1) has lower friction than EDM-finished, allowing 0.25-0.5° less draft.
- Air-assist or vacuum ejection: Compressed air between part and steel reduces the required draft by approximately 30%.
What This Means for Your Project: Draft angle is often the single biggest DFM issue that causes delays between design sign-off and mold cutting. If your part design lacks sufficient draft, expect the mold maker to request changes — and if they don't, suspect that they're modifying the part geometry without telling you. Before you send the 3D model, run a draft analysis in your CAD software and color-map the draft angle of every surface. Surfaces in red (below minimum draft) should be sent to the mold maker for DFM input, not ignored. For textured surfaces, add an extra 1-2° per side — this cannot be faked by the mold maker and must be in the steel design.
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