The CAD model looks fine. The geometry passes review. Tooling gets cut.
Then the first shot comes out of the mold — and the part drags, scuffs, or refuses to eject cleanly.
In many cases, the cause is the same: vertical faces with no draft.
What a Draft Angle Actually Is
Draft is a taper applied to the faces of an injection molded part so that those faces are not parallel to the direction the mold opens.
Instead of a perfectly vertical wall, a drafted wall leans slightly — a fraction of a degree to a few degrees — so that as the mold opens, the part can release without the wall dragging against the steel.
Draft is critical to improving moldability. Without it, parts are far more likely to stick, deform, or damage the tooling during ejection.
Why Parts Stick in the Mold
When plastic cools inside a mold, it contracts. The part shrinks onto the core side of the mold — the internal geometry it wraps around. This contraction increases the friction between the part and the steel.
Vertical faces are especially prone to sticking on the core side, because there is no angle to relieve the contact as the part is pushed out. The entire face drags against the mold wall during ejection.
When draft is insufficient, the ejection force required can become excessive. Trying to push the part out under those conditions risks bending ejector pins.
Lack of draft can also cause:
- poor cosmetic finishes from surface drag
- parts that bend, break, or warp from molding stresses during cooling
- damage to the part and, in some cases, to the mold itself
Practical Draft Angle Guidelines
These are practical guidelines used across multiple injection molding manufacturers and design guides. They are starting points, not universal laws — specific requirements depend on material, part geometry, and the manufacturer.
| Situation | Recommended Draft |
|---|---|
| All vertical faces (minimum advised) | 0.5° |
| Most situations | 1° – 2° |
| Shutoff surfaces (minimum) | 3° |
| Light texture (e.g. PM-T1) | 3° |
| Medium / heavier texture (e.g. PM-T2) | 5° or more |
A common rule of thumb: 1 degree of draft per 1 inch of cavity depth. Deeper draws need more draft to compensate for the increased wall contact during ejection.
Even very small draft — as low as 0.25 degrees — can be better than zero, though whether that is sufficient depends on the material, geometry, and manufacturer.
Why Textured Surfaces Need More Draft
A textured surface introduces micro-scale geometry on the mold face. During ejection, those features act like small undercuts — the surface can get hung up in the mold even if the macro geometry would otherwise release cleanly.
This is why textured surfaces require meaningfully more draft than smooth ones. The 3° and 5°+ guidelines in the table above apply specifically to textured conditions.
DuPont's engineering polymers design guide offers a specific rule for texture depth: add 1° of draft per 0.001 inch of texture depth. This is a DuPont source-specific guideline for engineering polymers — not a universal rule — but it illustrates how texture depth and draft are directly related.
The DuPont guide also notes that good draw polishing helps when minimum draft is required, and that both material type and draw depth affect what draft is appropriate.
Why Designers Miss This in CAD
CAD tools validate shape — they check that geometry is mathematically valid, that faces close properly, that a solid is watertight. See what open shells are and why they matter for more on geometry integrity.
But mold release is a manufacturing constraint, not a geometry constraint. A face that is perfectly vertical is geometrically valid. The CAD model will look correct. The STEP file will export cleanly. Nothing in the design file flags the problem.
Draft also gets deferred. Early in design, teams focus on form and fit. Draft gets treated as something to add later — and later often means after tooling review, when changes carry real cost.
The result is the same pattern seen with knife edges and geometry errors that survive export: a model that looks finished, flagged during manufacturing review, requiring redesign at the worst time.
Draft Is Part of Manufacturing Readiness
Geometry integrity is one layer of manufacturing readiness — making sure the model exports correctly, that faces close, that there are no open shells or degenerate edges. Tools like the geometry error checker help with that layer.
DFM checks — draft angles, wall thickness, shutoff geometry — are a second layer. Both need to pass before a part is ready for tooling. Neither replaces the other.
Manufacturing readiness happens before tooling, not during it. Just as CAM toolpath failures get caught late when geometry issues are missed early, draft problems discovered at mold design stage cost more than if they were caught in the design file.
Sources
- Protolabs — Improving Part Moldability with Draft — protolabs.com/resources/design-tips/improving-part-moldability-with-draft
- Protolabs — Plastic Injection Molding Design Guidelines — protolabs.com/services/injection-molding/plastic-injection-molding/design-guidelines
- Xometry — Injection Molding Design Guide — Xometry_DesignGuide_InjectionMolding.pdf
- DuPont — General Design Principles for Engineering Polymers — distrupol.com/General_Design_Principles_for_Engineering_Polymers.pdf