Draft Angle Guide
for Silicone & Rubber Molding
Insufficient draft causes part damage during demolding, mold wear, and increased scrap rates. Here’s what your drawing needs to specify.
What Is Draft Angle?
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Draft angle is the slight taper applied to vertical walls of a molded part, allowing it to release cleanly from the mold after cure. In silicone and rubber molding, the part shrinks onto (or away from) the mold core during cure, making adequate draft angle critical for clean demolding. Without sufficient draft, parts tear or crack during ejection — especially soft silicones (Shore A 10–30). The mold surface also gets scratched, reducing tool life and part quality over time. |
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Recommended Draft Angles by Process
| Process | Minimum Draft | Recommended | Notes |
|---|---|---|---|
| Compression Molding (HTV) | 1° | 2–3° | Textured surfaces need extra draft |
| LSR Injection Molding | 0.5° | 1–2° | Low-modulus silicone releases easily |
| Rubber Compression | 1.5° | 3–5° | Rubber sticks more than silicone |
| Overmolding | 1° | 2–3° | Apply to both substrate and overmold faces |
Why Draft Angle Matters
Part DamageWithout sufficient draft, parts tear or crack during ejection — especially soft silicones (Shore A 10–30). |
Mold WearForced ejection scratches cavity surfaces, reducing tool life and part quality over time. |
Core LockingSilicone shrinks onto core pins. Zero-draft cores lock the part and prevent ejection. |
Design Guidelines
- Inner walls (cores) need more draft than outer walls — silicone shrinks onto cores. Use 3–5° on cores, 1–3° on cavities.
- Textured surfaces: Add 1–2° extra draft per 0.1mm of texture depth. Deep textures may need 5–7°.
- Soft silicones (Shore A <40): More flexible, can tolerate slightly less draft — but are also easier to tear. Use 2–3° minimum.
- Hard silicones (Shore A >60): Stiffer; release more easily but are less forgiving of undercuts. Use 1.5–3°.
- Side-actions: If geometry requires side-actions, draft on sliding surfaces is critical to avoid binding and flash.
Mold Design for Draft
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The mold cavity must be cut with the draft angle built in. The image shows a precision steel mold with visible draft angles on the vertical walls, ejector pins, and core inserts. Key mold design considerations: inner walls (cores) need more draft than outer walls because silicone shrinks onto cores. Textured surfaces need extra draft per 0.1mm of texture depth. Side-actions require their own draft on sliding surfaces to prevent binding. |
Special Cases
Undercuts with DraftSmall undercuts (<0.5mm) can sometimes be snapped out if silicone is soft (Shore A <40) and draft is present. Increases reject rate. |
Overmolding DraftWhen overmolding silicone onto a rigid insert, the insert acts as a core. The insert must have draft, or silicone will shrink onto it and tear. |
Zero Draft FeaturesIf a feature must be truly vertical (0° draft), highlight it on the drawing. Options: post-mold machining, coining, or side-action. |
How to Specify Draft Angle on Your Drawing
“ZERO-DRAFT SURFACES: HIGHLIGHTED IN RED — DISCUSS WITH ENGINEER.”
Other DFM Guides:
| Wall Thickness → | Undercut → | Parting Line → | Shrinkage & Tolerance → |
Not Sure About Your Draft Angles?
Upload your drawing — we’ll review draft angles and flag issues before tooling.