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Lesson 1.3ShapeChapter 1 · Casting

Permanent mold casting

Metal poured by gravity into a reusable steel mold

Permanent mold casting pours molten metal, usually by gravity, into a mold made of steel or cast iron that is opened and used again thousands of times. Car pistons and many aluminum wheels and cylinder heads are made this way. It sits between sand casting and die casting: better parts than sand, much cheaper tooling than die casting.

Permanent mold casting: a real photo
Photo: U.S. National Archives, Public domain, via Wikimedia Commons
Metal is poured into a reusable steel mold that opens for the next partCross-sections of a two-part metal mold, poured by gravity (as for a car piston)Coat and heatSteel mold halves,preheatedCoating1Close and pourParting lineSteelmoldMoltenmetalSprueCoreheatheatLadle234Open and removeCasting5Mold is used againSolid metal partMolten metalSteel mold, core, ladleCoating spray
Permanent mold casting · coat, pour, openRead the three panels left to right; circled numbers match the steps below.

How it works

  1. Coat and preheat the mold. The two steel halves are sprayed with a thin ceramic coating, which protects them and slows cooling. They are kept hot, typically around 200 to 400 °C, so the metal does not freeze too early.

  2. Set the cores and close. Cores (inserts that form holes and hollows) go in. They can be steel, pulled out after each cycle, or sand, broken out later. The halves clamp together along the parting line.

  3. Pour. Molten metal, most often aluminum, is poured from a ladle into the sprue. Gravity fills the cavity; some machines tilt the mold to fill it gently, and low-pressure machines push the metal up from below.

  4. Let it cool in the mold. The steel draws heat out quickly, so the metal freezes in about a minute or a few minutes. Fast cooling gives fine grains (small crystals) and stronger metal than sand casting.

  5. Open and remove the casting. The halves open, the cores are pulled, and the casting is lifted out. The sprue is cut off, and the mold is ready for the next cycle.

Key facts

Materials
Mostly aluminum and magnesium alloys, also copper alloys (brass, bronze) and some cast iron
Typical parts
Pistons, aluminum wheels, cylinder heads, gear and pump housings, brake calipers, cookware
Part size
Typically from about 100 g to around 50 kg
Precision
About ±0.3 to ±0.5 mm on a 100 mm feature
Surface finish
Fairly smooth: Ra (average roughness) about 2.5 to 6 µm
Batch size it suits
Medium to high volume: a few thousand to hundreds of thousands of parts a year
Tooling and machine cost
Steel molds cost roughly ten thousand to a hundred thousand dollars or more
Speed
A few minutes per part per mold; several molds often run side by side

Strengths and limits

Strengths

  • Better precision and smoother surfaces than sand casting
  • Fast cooling gives fine grain and good strength
  • Few gas holes, so parts can be heat-treated and welded
  • Molds last for thousands to over a hundred thousand parts

Limits

  • Mold is costly, so small batches do not pay off
  • Mostly limited to aluminum, magnesium and copper alloys; steel is too hot for the mold
  • Shapes must come out of a rigid mold, so undercuts need cores
  • Slower than die casting, and walls must be thicker, about 3 to 4 mm minimum

Where it's used

  • AutomotivePistons, cylinder heads, wheels, brake calipers, suspension parts
  • Aerospace and defenseHousings and brackets in aluminum alloys
  • MachineryPump bodies, gear housings, hydraulic parts
  • PlumbingBrass and bronze valve bodies and fittings
  • Home goodsCast-aluminum cookware, lamp and furniture parts

How to recognize it

  • A fairly smooth, slightly matte surface: much finer than sand casting, a little rougher than die casting.
  • A clean, straight parting line where the steel halves met.
  • Walls that are thicker than on a die-cast part, often 4 mm or more.
  • A cut-off stub of the sprue, often on a thick section of the part.

Compare with

Sand casting1.1the mold is made of sand and destroyed each time; cheaper for small batches, but rougher.Die casting1.4also a reusable steel mold, but metal is injected under high pressure, so it is faster and makes thinner walls.Centrifugal casting1.5a spinning metal mold uses force instead of gravity to fill and pack the metal.

Watch it

THORS eLearning Solutions · An animated beginner explanation of gravity permanent mold casting

Densen_casting · A production foundry pouring aluminum into steel molds, cycle after cycle

Kurtz Casting Solutions · How robots run automated gravity casting cells

Kurtz Casting Solutions · The low-pressure variant, where metal is pushed up into the mold from below

See real photos

Remember

  • The mold is steel or iron and is reused thousands of times.
  • Gravity (or low pressure) fills it, not high-pressure injection.
  • Better and stronger than sand castings, cheaper tooling than die casting.

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 lesson1.4Die 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 lesson1.7Semi-solid casting (thixomolding and rheocasting)A half-solid metal slush injected into a steel dieOpen the lesson1.8Single-crystal casting (turbine blades)Slow freezing from one end grows the part as one crystalOpen the lesson

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