Injection Molding
Draft Angle in Injection Molding: Why 1 Degree Saves You Money
Straight walls grip the mold. A small draft helps the part release, protects the tool and shortens the cycle.
Key takeaways
- Draft is a slight taper on every wall that runs in the direction of mold opening.
- 1° is a good default. 2° or more for textured surfaces.
- Zero draft causes drag marks, stress whitening and short shots.
- Draft costs almost nothing in CAD and saves a lot on the tool.
What draft actually is
Draft is a small taper added to walls that run parallel to the direction the mold opens. Instead of a perfectly vertical wall, the wall leans by a degree or two.
It is measured from the direction of pull, not from the parting line. A wall with 1° of draft is 1° away from straight.
Why the mold needs it
Plastic shrinks onto the core as it cools. That shrinkage grips the steel tightly. When the mold opens, the part has to slide off that core.
With straight walls, the whole surface fights the ejection at once. With draft, contact breaks progressively from the edge inward. The part releases with far less force.
- Less ejection force means less wear on ejector pins and cores.
- The part leaves the tool without dragging, so cosmetic surfaces stay clean.
- Cycle time drops because the part comes out sooner and cooler.
- Thin walls are less likely to crack or stress-white during ejection.
Typical values
| Surface | Minimum draft | Comfortable draft |
|---|---|---|
| Smooth vertical wall | 0.5° | 1° to 2° |
| Light texture (MT 11010) | 1.5° | 2° |
| Medium texture | 2° | 3° |
| Deep texture | 3° or more | 4° to 5° |
| Inside walls and ribs | 0.5° | 1° per side |
| Bosses (outer) | 0.5° | 1° |
| Shut-off surfaces | 0° allowed | Keep vertical for sealing |
What happens with no draft
- Drag marks. Scratches that run along the wall where the plastic rubbed the steel.
- Stress whitening. A pale haze around corners and ribs after ejection.
- Short shots. The cavity does not fill because the part is stuck before packing finishes.
- Broken cores. Thin core pins snap when the part grips them.
- Longer cycles. The operator waits longer and ejects more slowly.
How to add draft in CAD
Model draft as part of the design, not as an afterthought. Most CAD tools have a draft feature that rotates faces around a neutral plane.
- Pick the parting line first. Everything else depends on it.
- Draft the walls, then the ribs and bosses.
- Check the inside corners. A drafted rib is narrower at the tip, which can weaken it.
- Re-run a wall thickness check after drafting. Draft changes section thickness.
- Draft text and logos too. A vertical letter wall will drag.
Common mistakes
- Drafting only the outside walls and forgetting the core side.
- Using 1° on a deeply textured surface, where 2° to 3° is needed.
- Adding draft after the DFM review, which forces a tool change.
- Forgetting that draft on a tall wall moves the top edge further than on a short wall.
Common questions
Is 1 degree of draft always enough?
For a smooth wall, usually. For textured or deep walls, use more. The deeper the wall, the more draft you need.
Does draft change the part size?
Yes. The dimension at the parting line stays put, and the far end moves. Tell your supplier which end is the nominal size.
Can I skip draft on a prototype tool?
You can sometimes, with a soft tool and a release agent. Do not carry that habit into production tooling.
The bottom line
Draft is one of the cheapest decisions in the whole project. Add 1° as a default, 2° on texture, and check it before the DFM review rather than after.