Skip to content
MecAgentMecAgent
Lesson 1.7ShapeChapter 1 · Casting

Semi-solid casting (thixomolding and rheocasting)

A half-solid metal slush injected into a steel die

Semi-solid casting injects metal that is neither fully liquid nor solid, but a slush of tiny solid grains in liquid. That slush flows smoothly into a steel die, traps little air and shrinks less, so parts come out denser than ordinary die castings. Magnesium laptop and camera bodies and some aluminum car parts are made this way.

Metal is injected half solid, like a slush, so it fills the die gentlyCross-section of a thixomolding machine for magnesium; the rheocasting route is belowDriveturns, then ramsMagnesium chipsTurning screwHeatersSemi-solid slurry:solid grains in liquidSteel dieThin-walledpart12345Rheocasting (aluminum)Liquid aluminum is stirred or cooled until it is partly solid,then the slurry is shot into a die-casting machine (lesson 1.4).Thixocasting does the same from reheated solid billets.Solid metal (chips, grains)Heat and liquid metalMachine and die
Semi-solid casting · thixomolding machine and rheocasting routeThe top drawing shows thixomolding, the bottom strip rheocasting; circled numbers match the steps below.

How it works

  1. Feed solid chips. Thixomolding starts with small magnesium chips, not molten metal. They fall from a hopper into a heated barrel, under a blanket of argon gas to stop them burning.

  2. Heat and stir into a slush. Heaters and the turning screw warm and shear the chips until they are partly melted, typically around 5 to 60 percent solid. Shearing breaks the solid into round grains, so the slush flows like a thick paste.

  3. Gather a shot. The screw carries the slush forward and a measured amount collects at the front of the barrel.

  4. Inject. The screw rams forward like a plunger and pushes the slush through the nozzle into a closed steel die. Because it is thick and partly solid, it fills smoothly instead of splashing.

  5. Cool and eject. The part freezes in seconds, the die opens and ejector pins push it out, as in die casting.

Rheocasting reaches the same slush from the other side: liquid aluminum is stirred or cooled until it is partly solid, then cast in a die-casting machine. Thixocasting reheats solid aluminum billets to the same mushy state.

Key facts

Materials
Thixomolding: magnesium alloys. Rheocasting and thixocasting: mainly aluminum alloys
Typical parts
Laptop, camera and phone housings, power-tool bodies, car brackets, steering parts, telecom housings
Part size
From a few grams to several kilograms
Precision
About ±0.05 to ±0.2 mm, similar to die casting
Surface finish
Smooth: Ra (average roughness) about 1 to 2 µm
Batch size it suits
Tens of thousands to millions of parts
Tooling and machine cost
Similar to die casting: dies from tens of thousands of dollars, machines from hundreds of thousands
Speed
Roughly 20 seconds to a minute or two per shot

Strengths and limits

Strengths

  • Few pores, so parts can be heat-treated and are stronger than die castings
  • Less shrinkage, so tighter tolerances and thinner walls
  • Lower process temperature, so dies last longer
  • Thixomolding avoids handling a pot of molten magnesium

Limits

  • Process control is harder: the solid fraction must stay just right
  • Fewer suppliers and less know-how than ordinary die casting
  • Machines and special feedstock (chips, billets) cost more
  • Mostly limited to magnesium and aluminum alloys

Where it's used

  • ElectronicsLaptop, camera and phone housings in magnesium
  • AutomotiveBrackets, steering-wheel frames, suspension and engine-mount parts
  • TelecomHousings and heat sinks for radio units
  • Tools and sportsPower-tool housings, bicycle and archery parts

How to recognize it

  • Very thin, even walls on a light metal housing, often magnesium (feels lighter than aluminum).
  • A smooth surface, usually painted, with small ejector-pin marks like a die casting.
  • On a cut section, almost no pores; under a microscope, round grains instead of tree-like crystals.
  • From the outside it is hard to tell from die casting: check the maker's data.

Compare with

Die casting1.4injects fully liquid metal; cheaper and more common, but more porous.Injection molding2.1the thixomolding machine works much like a plastic injection molding machine, with a screw and a barrel.Metal injection molding9.2also injects through a screw, but a mix of metal powder and binder that is later sintered.

Watch it

North American Die Casting Association · An industry overview of semi-solid processes, with diagrams

The Limiting Factor · A long explainer of thixomolding and why magnesium suits it

Daseon Industry Group Ltd. · A thixomolding machine in operation

Comptech AB · A rheocasting cell producing aluminum parts

See real photos

Remember

  • The metal goes in as a slush, partly solid and partly liquid.
  • Slush fills the die calmly, so parts have fewer pores than die castings.
  • Thixomolding uses magnesium chips and a screw; rheocasting stirs liquid aluminum.

More in Casting

All 100 processes
1.1ShortSand castingMetal poured into a cavity packed in sand; the sand is broken awayOpen the lesson1.2Investment casting (lost wax)Wax copy coated in ceramic, melted out, then filled with metalOpen the lesson1.3Permanent mold castingMetal poured by gravity into a reusable steel moldOpen the lesson1.4Die castingMetal shot into a steel die at high pressureOpen the lesson1.5Centrifugal castingMetal flung against the wall of a spinning moldOpen the lesson1.6Lost foam castingMetal poured onto a foam copy buried in loose sandOpen the lesson1.8Single-crystal casting (turbine blades)Slow freezing from one end grows the part as one crystalOpen the lesson

Try it

Put AI to work in your CAD.

Model parts, generate drawings and automate the repetitive work inside your CAD software.

Download (Free)Book a demo