Draft Angle Design for Injection Molding: How Much Taper Your Part Needs

Draft is the slight taper added to the vertical faces of a molded part so it can release cleanly from the mold. It is one of the first things a molder checks on a new design — because a part with too little draft drags against the steel on ejection, scuffs, distorts, or sticks outright. This guide covers how much draft to add and what drives the number.

Why draft exists

As plastic cools it shrinks and grips the core of the mold. When the mold opens, ejector pins push the part off that core — and every vertical wall has to slide across steel to get free. Without taper, that is metal scraping plastic along the full wall height. Draft angles the walls so the part releases almost immediately, cutting friction to near zero.

Rules of thumb

There is no single magic number, but these starting points cover most parts:

• 1 degree per side — a safe general minimum for most vertical faces.
• 0.5 degree — possible on shallow, small, highly polished (SPI A-grade) surfaces.
• 3 degrees or more — for tall walls, shutoffs, and textured surfaces.
• 0 degrees is only realistic on very short features and invites problems — avoid it.

Draft is cheap to add in CAD and expensive to add later, so err toward more wherever cosmetics allow.

Texture changes everything

A textured surface has microscopic undercuts that grip the steel, so it needs far more draft than a polished one. A widely used guideline: add roughly 1 to 1.5 degrees of draft for every 0.025 mm (0.001 in) of texture depth, on top of your base draft. Deep VDI or Mold-Tech textures can demand 3 to 5 degrees or more — confirm the requirement with your texture supplier before finalizing the wall.

Depth and material matter too

• Wall height: the taller the wall, the more total interference at 1 degree — deep ribs and bosses benefit from extra draft.
• Shrinkage: high-shrink semi-crystalline resins grip the core harder and generally want more draft than low-shrink amorphous ones.
• Ribs and bosses: internal features need draft too, and are easy to forget.

What too little draft costs you

• Drag marks and scuffing down the walls as the part scrapes the steel.
• High ejection force that leaves ejector-pin push marks or distorts the part.
• Longer cycles and faster mold wear — and in bad cases, parts that stick in the tool.

Resolve draft during mold design

Draft direction depends on the parting line and pull direction, so it is inseparable from the tool layout — which means it belongs in the mold design and tooling review, before steel is cut. If you are unsure how much draft your part needs or where it should go, our engineers will flag every low-draft face during an initial engineering review — send us your model and we will mark them up before the mold is started.