DefinitionA 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.
Explained
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.
Key takeaways
- Draft angle is the taper on molded walls parallel to the mold-opening direction, enabling clean ejection.
- Most injection-molded plastic parts need 1–2° of draft per side; textured surfaces require extra.
- The core side usually needs more draft than the cavity side because the part shrinks onto the core.
- Semi-crystalline materials such as nylon and polypropylene need more draft than amorphous materials such as ABS.
- Add draft early in CAD, keep wall thickness constant, and verify direction against the parting line to avoid undercuts.
Related terms
References
- MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — MIT OpenCourseWare
- US Patent 9,573,306, Background — patents.google.com
- US Patent 5,932,164, Background — patents.google.com