Magnesium Injection Molding: Thixomolding Is the Real Process

Yes, genuinely, and it is the one metal that is truly injection molded in volume production. The process is called Thixomolding. Magnesium alloy chips are fed into a heated, screw-driven injection barrel that looks and works much like a plastic injection molding machine. The barrel heats the alloy into its semi-solid thixotropic state, a slurry of solid metal globules in liquid, around 580-595°C for AZ91D, and the screw injects that slurry into a steel mold under high pressure. Because the metal is never fully liquefied, there is no molten-metal bath, no fluxing, far less oxidation, and lower porosity than die casting, plus it eliminates the fire and explosion hazard of handling liquid magnesium. The machines are essentially adapted plastic injection molding equipment with fast cycle times producing near-net-shape parts. So unlike every other metal on this list, where injection molding really means MIM or casting, magnesium has a direct, mature, commercially dominant injection-molding process of its own.

AZ91D is the clear workhorse for Thixomolding. It is a magnesium-aluminum-zinc alloy with about 9% aluminum, giving excellent moldability, good strength, and solid corrosion resistance, the D designation indicates the high-purity version controlled for corrosion performance. Its wide semi-solid freezing range makes it forgiving in the Thixomolding barrel, which is why it dominates molded magnesium housings, electronics enclosures, and structural covers. AZ31B, by contrast, is primarily a wrought alloy used as sheet and extrusion; with only about 3% aluminum it has a narrower freezing range that makes it less suited to semi-solid molding, so it is more often formed or machined. WE43 is a high-performance magnesium-yttrium-rare-earth alloy used in aerospace, motorsport, and bioresorbable medical implants for its elevated-temperature strength, but it is more commonly sand cast, gravity cast, or wrought than Thixomolded and is reserved for demanding applications. For most molded magnesium parts, specify AZ91D unless a specific temperature or wrought-property requirement points elsewhere.

Thixomolding offers several real advantages over high-pressure die casting of magnesium. Because the metal is processed semi-solid rather than fully molten, parts exhibit less gas porosity and better dimensional stability, and you can mold thinner walls, down to roughly 0.5-1 mm, with finer detail. Tolerances run about ±0.1-0.15 mm on molded features, and as-molded finishes are often good enough for enclosures without much finishing. A major safety benefit is that the magnesium is never held as a liquid bath, which removes the fire and explosion risk inherent in molten-magnesium die casting. The main tradeoff is capacity: Thixomolding machines are specialized and far fewer shops run them than run die casting, so lead times and supplier availability can be tighter. Cost-wise, for thin-wall, high-quality parts like electronics chassis, Thixomolding frequently wins, while for larger or simpler parts where some porosity is acceptable and die-casting capacity is plentiful, die casting may be cheaper per part. The right choice depends on wall thickness, quality needs, and volume.

Machine magnesium when you have low volumes, prototypes, or parts that demand full density and tight tolerances that molding cannot guarantee. Magnesium is one of the easiest metals to machine, it has very low cutting forces and tolerates extremely high cutting speeds, so material removal is fast and tool wear is minimal, producing fully dense parts with tolerances to ±0.025 mm and excellent surface finishes. The critical caveat is fire safety: magnesium chips and fine dust are flammable and can ignite, so machining requires proper coolant practices (often dry or with specific non-aqueous coolants), good chip management, and fire-suppression measures, which is why you should use shops experienced specifically with magnesium. For high-volume thin-wall enclosures, Thixomolding or die casting will beat machining on cost, but for the first articles, low-volume runs, or any part where porosity is unacceptable, machining is the clean, precise, and fast route. Many programs prototype magnesium parts by machining, then transition to Thixomolding for production once the design is validated.