Draft Angle Is a Per-Face Decision, Not a Table Lookup

A part does not have a draft angle. It has one per face, and the faces that scuff are the ones where two rules disagree.

A note in the corner of a drawing — "draft 1°" — looks efficient and is usually wrong, because it compresses three variables into one number: how much the resin shrinks onto the steel, how rough that surface is, and how deep the draw runs. Change any one of them and the note stops being true for that face.

What the angle is fighting

Draft has one job: overcome the normal force created by shrinkage, plus the friction of the surface itself. That is why the number belongs to a face and a material pair, not to a part.

The ranges most tool shops start from — measured from the draw direction, per side, on steel that can be polished, in an unfilled or lightly filled resin, on a wall without dense ribbing:

• Mirror polish: 0.25–0.75°

• Standard polish: 0.5–1.5°

• Fine texture or matte: 1.0–2.0°

• Deep draw: about +0.5° per 10 mm of depth

The texture band is the one that moves: 1.0–2.0° covers shallow grain, and deeper grain goes past it. These ranges are starting points, not a standard — the grain supplier's minimum for a specific texture number overrides every one of them, and it is written down in the texture specification.

Texture eats draft before the part ever runs

Etching puts depth on top of the nominal surface, and the grain leaves side walls that grip. The conversion used in texture specification is roughly 1° to 1.5° of additional draft per 0.025 mm of etch depth, so a 0.1 mm grain asks for 4° to 6° on its own.

Two things go wrong from there. The angle is computed from the model instead of from the grain number, which is the difference between a wall that releases and one that drags. And the dimension is pushed twice — once by the etch depth, once by the angle added to clear it — with neither move appearing in the tolerance chain on the drawing.

The core side is the side that decides

The part shrinks onto the core and away from the cavity. The core therefore carries the higher normal force and needs the larger angle; symmetric values on both halves are the wrong default.

In a semi-crystalline grade shrinking around 2%, a 60 mm bore closes roughly 0.6 mm onto the core in radius. That interference is what the angle and the polish have to overcome, and it grows with wall thickness and with glass content. POM and glass-filled PA are where this bites hardest: high shrinkage, high modulus, little elongation to absorb the grip.

A part drawn with 0.5° on both halves typically drops out of the cavity and hangs on the core, and by the time it happens the remedy being discussed is spotting, which is the expensive version of the decision that was free at drawing stage.

Depth means contact area, and contact area means force

The step rule — about 0.5° per 10 mm of draw — exists because strip force scales with the contact area, and angle reduces normal force per unit area. It assumes a polished wall.

Texture and depth stack rather than overlap. A 60 mm deep core with a 0.05 mm grain asks for roughly 2° from the texture and another 3° from the depth, which puts the wall near 5° before the material is considered at all. When the part cannot afford that angle, the answer is a different release mechanism — a slide, a lifter, a collapsing core, or a shallower grain — not a compromise number.

Too much draft is a dimensional decision

On a 20 mm tall wall, 1.5° moves the far edge about 0.5 mm. If that edge carries a mating dimension, a snap or a seal groove, the draft has consumed the tolerance, and nothing on the drawing records that it happened.

The way to keep control is to choose the end that matters — the datum face, the mating surface, the seal land — hold it at nominal, and let the draft grow away from it toward the free end. The CAD default of drafting symmetrically about the mid-plane is the worst of the three options, because it moves both ends and leaves neither at nominal.

Where draft is a structure question

Four situations where the number is not the answer:

• A face tolerance-controlled at both ends, such as a boss measured at base and at top.

• Shut-offs and parting-line steps: the two halves have to match, and mismatched draft gives flash on one side and crushed steel on the other.

• A cosmetic wall that must be vertical.

• A wall too short for the angle the texture requires.

In each case the decision is a slide, a lifter, an insert, or a change to the grain. All four are cheap while the drawing is still a drawing, and none of them are cheap after hardening.

Three things to put on the drawing

Mark the draw direction with an arrow. State a minimum draft for each surface class — A-surface, textured, core side — rather than one global note. Put the grain number on the textured faces, because the grain number sets the floor for the angle.

Then require one more thing with the T1 report: the draft map measured from the cut steel, not from the model. A colour map of the actual cavity shows the face that will drag, and it shows it before the tool is spotted by hand for a week trying to find out.