Draft Angle: The 2° Rule That Decides Whether Your Injection-Molded Part Ships or Sticks

Draft angle is the slight taper built into a part's vertical walls so it releases from the mold instead of dragging against it. Skip it, or get the amount wrong, and the part doesn't fail in the design review, it fails on the factory floor, stuck in the tool.

What draft angle actually is

Every injection mold opens in one direction, and any wall running parallel to that direction of pull needs a small taper, angled so the part gets narrower as it moves away from the parting line. Without that taper, the wall is perfectly vertical, and a perfectly vertical wall has full surface contact with the steel all the way down, which means friction all the way down when the mold tries to let go of it.

The 2° rule, and where it actually comes from

1 to 2 degrees per side is the standard starting point for a smooth-walled thermoplastic part, and it's repeated so often it gets treated as a law. It isn't one. It's a safe default for a shallow, polished, unremarkable wall, the number that actually applies to your part depends on three things: how deep the wall is (a taller wall needs the same angle to clear far more total steel), what the surface finish is (texture changes the math entirely, covered below), and how flexible the material is (a rigid engineering polymer needs more help releasing than a part with some natural give).

Texture changes the number, not just the surface

A textured finish is cut directly into the mold's steel, and that texture has its own depth, sitting slightly proud of wherever the "smooth" wall would have been. A draft angle that comfortably clears a polished wall can still snag on texture, because the texture itself is what's catching, not the underlying surface. A common guideline in mold design adds roughly 1 extra degree of draft for every 0.001 inch of texture depth, per side. Specify a heavy leather-grain or bead-blast texture without revisiting the draft, and the 2° default stops being safe.

What actually happens at 0°

Nothing subtle. A wall with no draft has to be forced off the core, so the ejector pins push harder than they're supposed to, which can deform thin walls, crack brittle ones, or leave visible witness marks exactly where the pins made contact. Cycle time goes up because the part doesn't release cleanly on the first attempt. And it's not a one-time cost: every cycle that drags metal-on-plastic wears the mold a little, so a draft problem discovered after tooling is cut doesn't just cost the current run, it keeps costing for the life of that tool.

The mistake: adding draft after the wall is already modeled

Draft is easy to bolt on at the end in CAD, a quick "add 2 degrees to everything" pass right before the file ships. That's exactly how it gets missed on the features that actually need a different number: a deep rib, a textured boss, a tall sidewall next to a shallow one. Draft angle needs to be part of the wall's geometry from the first sketch, checked per-feature against its actual depth and finish, not applied as a blanket rule at the end and hoped to be enough.

Start at 2°, keep checking

1 to 2 degrees is where to start, not where to stop checking. Match the angle to the wall's real depth, add for texture, and build it into the geometry from the start, not as a pass applied right before the file goes to the mold shop.