A draft angle is the small taper added to the walls of a molded part, parallel to the direction the mold opens, so the part can release cleanly from the mold. Most injection-molded plastic parts use 1–2° of draft per side; textured surfaces need more.
In short: without draft, a cooling part grips the mold and can scuff, stick or distort when it is ejected.
What Is a Draft Angle in Injection Molding?
Draft is measured from the line of draw — the direction the mold halves separate. A perfectly vertical wall has 0° draft; a wall that leans 1° away from vertical has 1° of draft. In CAD it is applied as a feature to every face that runs parallel to the mold-opening direction.
Draft exists because plastic shrinks as it cools. A part shrinks onto the core (the half that forms internal surfaces) and away from the cavity, so a vertical wall ends up clamped against the core steel. A tapered wall breaks contact within the first fraction of ejector travel instead of dragging its full height across the mold surface.
For that reason the core side usually needs more draft than the cavity side. Surface finish matters too: a polished cavity releases more easily than a textured one, whose grain locks into the part.
How Much Draft Angle Do You Need?
| Surface | Typical draft per side |
|---|---|
| Smooth walls, most parts | 1–2° |
| Absolute minimum | 0.5° |
| Frequently recommended range | 1.5–3.5° |
| Textured or grained surfaces | Add 1–1.5° for every 0.001 in (0.025 mm) of texture depth |
| Tall cores, deep ribs and bosses | Increase — the taller the feature, the more draft it needs |
Material affects the answer: semi-crystalline plastics such as nylon and polypropylene shrink more than amorphous plastics such as ABS and polycarbonate, so they grip the core harder.
When Does Draft Angle Matter?
Draft applies to any part formed in a rigid mold: injection molding, die casting and compression molding. Vacuum casting is more forgiving because silicone molds flex, which is one reason cast prototypes can carry less draft than the production part will need.
The features that most often ship without enough draft are ribs, bosses, deep pockets, snap-fit hooks and textured cosmetic faces — all of which are easy to model as vertical walls in CAD.
How to Design a Draft Angle
- Add draft as one of the first features in the model, not the last, so the rest of the geometry follows it.
- Draft both sides of a wall in the same direction to keep wall thickness constant.
- Where a face must be perfectly vertical for fit or function, tell the mold maker up front: zero draft usually means a polished mold surface, a side action, or a slower cycle, and it should be a deliberate choice.
- Check the draft direction against the intended parting line — a face drafted the wrong way becomes an undercut.
A DFM review before tooling is where missing or reversed draft normally gets caught. At NICE Rapid it is the first step of every plastic injection molding project.
Frequently Asked Questions
What is a draft angle in injection molding?
It is the taper built into the walls of a molded part so the part can be pushed out of the mold without sticking. It is measured from the mold-opening direction, and most plastic parts use 1–2° per side.
What happens if a part has no draft?
As the plastic cools it shrinks onto the mold core, so a vertical wall grips the steel. On ejection the part can scuff, whiten, warp or stick in the mold, and the ejector pins may leave marks or punch through thin sections.
Does a textured surface need more draft?
Yes. The texture grips the mold, so a common allowance is 1–1.5° of extra draft for every 0.001 in (0.025 mm) of texture depth — a moderately grained finish can need 3° or more per side.
Related Services at NICE Rapid
- Plastic Injection Molding — every project starts with a DFM review that checks draft, wall thickness and parting-line placement before steel is cut.
- Rapid Tooling — aluminum bridge tooling for early batches, where draft decisions are validated before committing to production steel.
- Request a Quote — send a STEP file and our engineers will flag any faces that need draft.
Sources
- MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — draft of 1–2°, material/dimension/texture dependent; cavity side smaller, core side larger — ocw.mit.edu
- US Patent 9,573,306, Background — "a draft of ½ degrees per side is recommended as minimum with 1.5 degrees to 3.5 degrees per side being frequently recommended" — patents.google.com
- US Patent 5,932,164, Background — textured surfaces: "adding 1° to 1½° of draft angle for each 0.001" of texture or other pattern depth" — patents.google.com
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