Magnesium Die Casting Design Rules: Walls, Ribs, Draft, and Bosses

Magnesium rewards designers who use its castability — and punishes parts converted from aluminium by find-and-replace. These are the rules we apply in DFMA reviews of AZ91D and AM60B parts. Design to them and the die will cooperate.

The numbers

• Walls — 1.0–1.5 mm for most structural walls; down to 0.8 mm over short flow lengths. Keep walls uniform — magnesium fills fast and freezes faster, so abrupt thickness jumps become porosity.

• Ribs — Rib thickness 60–80% of the adjoining wall; height up to 5× wall. Prefer more shallow ribs over few tall ones — that's where the stiffness compensation lives.

• Draft — 1.5–2° on external faces, 2–3° internal; magnesium's low die affinity lets you run about a degree less than aluminium, which your cosmetic faces will appreciate.

• Bosses — Wall-thickness-matched boss walls with a connecting rib to the nearest wall. Cored holes over drilled wherever possible.

• Corners — Radii ≥ 1× wall at every junction; sharp internal corners concentrate both stress and shrinkage.

The rule that outranks the others

Every steel fastener location is a corrosion decision. Specify isolation washers or coated inserts at the design stage — a galvanic cell designed out costs nothing; one patched after field returns costs a program. (We've written a full playbook on this — see the fastener article.)

Get your part checked against these rules

A rules list generalises; your geometry doesn't. Send three values — current mass and alloy, stiffness- or strength-driven, thinnest wall — and we'll return a bounded magnesium estimate within one working day. No CAD file, no NDA.

Email the three values, or send a sanitised STEP file for a full DFMA read in 3–4 working days — including 'stay in aluminium' if that's the answer. https://www.exclusivemagnesium.com/learnmore/magnesium-weight-savings-calculator