What Draft Angle Actually Does
Draft is the taper cut into vertical mold walls so the part releases from the steel. Without it, the plastic shrinks onto the core as it cools and grips. Ejector pins push and the part drags. You get scuff marks, deformation, or a part stuck hard enough to need a pry bar.
Every vertical surface in the mold needs draft. The only question is how much. That depends on the surface finish, the resin, and whether the wall is on the core side or the cavity side.
The Basic Rules
For smooth, polished surfaces: 1 degree per side is the minimum that works reliably. Half a degree can work on shallow draws with easy resins, but it leaves no margin. If anything varies — a little more shrinkage, a slightly rougher polish — the part starts dragging.
For textured surfaces, the rule scales with texture depth. The industry standard is 1 degree of additional draft for every 0.001 inch (0.025 mm) of texture depth. A light matte texture (0.001″ deep) needs about 1° extra. A heavy leather grain (0.004–0.005″ deep) needs 4–5° extra on top of the base draft.
This is why texture decisions must come before mold design is finalized. Adding a deep texture to a wall designed with 1° draft leaves two options. Re-cut the steel for more draft, or accept drag marks on every part.
Core Side vs. Cavity Side
Plastic shrinks as it cools. On an outside wall (cavity side), shrinkage pulls the plastic away from the steel. Less grip, less draft needed.
On an inside wall (core side), shrinkage pulls the plastic tighter onto the steel. More grip, more draft needed. Internal walls, deep ribs, and tall bosses on the core side are where parts get stuck.
Rule of thumb: add at least 1° extra draft to core-side walls compared to cavity-side walls with the same finish. For deep core features — tall standoffs, deep ribs — go further.
Resin Matters
High-shrinkage resins (PP, PE) pull away from the cavity and grip the core harder. They need generous core-side draft. Low-shrinkage resins (ABS, PC, glass-filled grades) grip less aggressively. But they are less forgiving of drag marks. A scuff on a PC cosmetic surface is scrap. The same scuff on a hidden PP surface might pass.
Glass-filled resins are abrasive. They polish the mold steel over time, which actually reduces grip. But they demand more draft upfront because the fibers increase ejection friction.
Thermoplastic elastomers (TPE) and soft-touch materials grip like rubber bands. Double the normal draft minimums. If the design allows, 3–5° on TPE overmold surfaces is not excessive.
Where Designers Get It Wrong
Zero-draft walls with texture. The most common DFM flag we raise. A designer applies a leather grain to a vertical wall drawn at 0° because it looked fine in CAD. In steel, that texture locks the part in. Either add 4–5° draft or drop the texture on that wall.
Draft in the wrong direction. Draft must open toward the parting line — the taper gets wider as the mold opens. We see parts where the draft narrows toward the parting line, creating a permanent undercut. The mold cannot open without destroying the part. Always check draft direction against the pull direction.
Inconsistent draft on mating surfaces. Two parts that snap or press together need matching draft. If the housing has 2° and the cover has 0.5°, the assembly gap varies along the wall. Match draft angles on any surfaces that mate.
Forgetting draft on ribs and bosses. Small features get overlooked. A 0.5 mm thick rib, 20 mm tall, with no draft will not eject cleanly. It flexes, drags, and eventually breaks off in the mold. Ribs need 1–2° per side minimum. Tall thin ribs need more.
How to Check Draft on Your Drawing
Before sending a file for quoting, run a draft analysis in your CAD software. Every major CAD package has one — it color-codes surfaces by draft angle against a specified pull direction. Set the threshold to 1° and look for red.
Pay special attention to:
- Deep pockets and wells (material buckets, housings)
- Tall standoffs and bosses on the core side
- Ribs taller than 3× their thickness
- Any wall getting texture — verify against the texture depth rule
- Undercut-adjacent walls near sliders and lifters
What We Flag in DFM
When we review your 3D file, draft is one of the first checks. Our report calls out every surface below 1°. Every textured wall without adequate draft for its texture depth. Every core-side feature that needs more taper than drawn. We suggest specific angle changes — not “add more draft” but “increase to 3° on these four walls.”
Fixing draft on paper costs nothing. Fixing it in steel after T1 means welding,
re-machining, re-polishing, and re-texturing. Typically $500 to $2,000 per area and
a week of schedule, though actual costs vary by region and mold complexity. Send the
file before the design is frozen and the DFM catches it free.