What is a draft angle in thermoforming?
A draft angle (also called a taper angle or release angle) is the angle between the vertical wall of a thermoformed part and the vertical axis of the mould. It is expressed in degrees. A wall with 0° draft is perfectly vertical; a wall with 3° draft tapers outward by 3° from vertical.
Draft angles exist for one reason: to allow the formed part to release from the mould. When a hot thermoplastic sheet is drawn over or into a mould, it cools and shrinks. Without draft, the cooled part grips the mould surface and cannot be removed without tearing or permanent distortion.
Unlike injection moulding — where the mould opens and the part is ejected — thermoforming relies on the part sliding off the mould under its own weight or with light mechanical assistance. Draft angles make this possible.
Male vs female moulds: why draft requirements differ
The direction of shrinkage determines how much draft is needed. This is the single most important factor in draft angle selection:
| Mould Type | How Part Shrinks | Minimum Draft | Recommended Draft | Typical Use |
|---|---|---|---|---|
| Female (cavity) | Away from mould wall | 1° | 2–3° | Packaging trays, covers, enclosures |
| Male (core/plug) | Onto mould surface | 3° | 5–7° | Automotive liners, bath products, deep containers |
| Female, textured | Away from wall | 3° | 5–7° | Automotive interior panels, consumer products |
| Male, textured | Onto surface | 5° | 7–10° | Truck bed liners, heavy-duty covers |
Draft angle requirements by material
Different thermoplastics have different shrinkage rates and surface friction characteristics. Higher-shrinkage materials grip moulds more aggressively and need more draft:
| Material | Shrinkage Rate | Min Draft (Female) | Min Draft (Male) | Notes |
|---|---|---|---|---|
| ABS | 0.4–0.7% | 1–2° | 3–5° | Low shrinkage, easy to release |
| HDPE | 1.5–3.0% | 3–5° | 5–8° | High shrinkage — needs generous draft |
| PP | 1.0–2.5% | 2–4° | 5–7° | Moderate shrinkage, slightly waxy surface |
| PC | 0.5–0.7% | 1–2° | 3–5° | Low shrinkage, similar to ABS |
| PETG | 0.2–0.5% | 1° | 2–3° | Very low shrinkage — easiest to release |
| HIPS | 0.4–0.7% | 1–2° | 3–5° | Similar to ABS |
| TPO / TPE | 1.0–2.0% | 3–5° | 5–8° | Flexible — can release with less draft but grips textures |
Texture depth and additional draft
Textured surfaces grip the mould mechanically — the texture peaks interlock with the mould surface as the part shrinks. This requires additional draft beyond the smooth-surface minimum. The industry rule is:
Texture draft rule
Add 1° of draft per 0.025mm of texture depth (Ra value). A medium texture of 0.05mm Ra needs +2° extra draft. A deep texture of 0.1mm Ra needs +4° extra draft.
| Texture Grade | Depth (Ra) | Additional Draft | Total Draft (Male) |
|---|---|---|---|
| Smooth / polished | < 0.01mm | 0° | 3–5° |
| Fine texture (leather grain) | 0.025mm | +1° | 4–6° |
| Medium texture (pebble) | 0.05mm | +2° | 5–7° |
| Coarse texture (truck liner) | 0.1mm | +4° | 7–10° |
| Very coarse (aggressive grip) | > 0.15mm | +6°+ | 9–13° |
Draft angle formula and calculation
Draft angle is calculated from the offset at the base of the wall relative to the wall height:
// Draft angle formula
draft_angle = arctan(offset / depth)
// Example: 200mm deep wall, 10mm offset at base
draft_angle = arctan(10 / 200) = 2.86°
// Reverse: given 3° draft and 200mm depth, what is the offset?
offset = depth × tan(3°) = 200 × 0.0524 = 10.5mm
Most CAD packages (SolidWorks, CATIA, NX, Fusion 360) have a built-in draft analysis tool that colour-codes surfaces by draft angle. Green = sufficient draft, red = insufficient. Run this analysis before sending drawings to the mould maker.
Common draft angle failures and fixes
✗ Part tears on demould
Cause: Zero or negative draft on male mould — part shrinks onto mould and grips
Fix: Add minimum 3–5° draft to all vertical walls. Apply mould release agent as a temporary fix while redesigning.
✗ Part distorts when removed
Cause: Insufficient draft causing the part to drag and stretch as it's pulled off
Fix: Increase draft by 2–3°. Consider switching from male to female mould geometry if the application allows.
✗ Textured surface tears on release
Cause: Texture draft not accounted for — smooth-surface draft applied to textured mould
Fix: Add 1° per 0.025mm of texture depth. Polish the mould surface between the texture peaks to reduce friction.
✗ Part releases but has witness marks
Cause: Barely sufficient draft — part scrapes mould wall during release
Fix: Increase draft by 1–2°. Polish mould surface to Ra < 0.4μm. Apply PTFE-based mould release.
Frequently asked questions
What is the minimum draft angle for thermoforming?
The minimum draft angle for thermoforming is 1–2° for smooth surfaces on female moulds. For male moulds, 3–5° is the practical minimum. For textured surfaces, add 1° per 0.025mm of texture depth — a medium texture (0.05mm) needs at least 2° extra draft, so 5–7° total. Parts with draw ratios above 1:1 need 7–10° to release cleanly.
Why do thermoformed parts need draft angles?
Thermoformed parts shrink onto the mould as they cool. Without draft, the part grips the mould surface and cannot be removed without tearing or distortion. Draft angles create a slight taper so the part can slide off the mould as soon as vacuum is released. Male moulds need more draft than female moulds because the part shrinks onto a male mould and away from a female mould.
How do I calculate the draft angle for a thermoformed part?
Draft angle = arctan(offset / depth). For a 200mm deep part with 5mm offset at the base, draft = arctan(5/200) = 1.43°. Most CAD tools calculate this automatically. As a rule of thumb: 1° draft per 25mm of depth is a safe starting point for smooth ABS on a female mould. Double this for male moulds and add 1° per 0.025mm of texture depth.
Do male and female moulds need different draft angles?
Yes. Female moulds (cavity moulds) need less draft — typically 1–3° — because the part shrinks away from the mould wall as it cools. Male moulds (core moulds) need more draft — typically 3–7° — because the part shrinks onto the mould and grips it tightly. For the same part geometry, a male mould requires approximately 2× the draft of a female mould.