Permanent mold casting

Dr. Dmitri Kopeliovich

Permanent mold casting (gravity die casting) is a casting process involving pouring a molten metal by gravity into a steel (or cast iron) mold.

The permanent mold casting is similar to the sand casting process . In distinction from sand molds, which are broken after each casting a permanent mold may be used for pouring of at least one thousand and up to 120,000 casting cycles with the rate 5-100 castings/hour.

Manufacturing metal mold is much more expensive than manufacturing molds for Sand casting or investment casting process mold. Minimum number of castings for profitable use of a permanent mold is dependent on the complexity of its shape.

Ferrous and no-ferrous metals and alloys are cast by the permanent mold casting process: Aluminum alloys, Copper alloys, Magnesium alloys, zinc alloys, steels and Cast irons.

• Permanent mold casting process
• Design aspects of permanent mold casting
• Advantages and disadvantages of permanent mold casting

• The interior surfaces of the two parts (cope and drag) of a permanent mold are coated with a thin ceramic coating. The mold is preheated before coating to 300-500°F (150-260°C).
• The cores are inserted and installed in the mold assembly.
• The mold is closed.
• The molten metal is poured into the mold.
• After the casting has solidified and cooled down to the desired temperature the mold is opened and the casting is withdrawn from it.
• The gating system is cut away from the casting.
• The finish operations are carried out.

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• Permanent molds are made of Carbon steels, grey cast irons, Graphite (for casting steels and Cast irons) or bronze.
• Since the metallic mold of a permanent casting expands when it is filled with a molten metal and then both the casting and the mold shrinks during cooling the shrinkage allowances taken in the permanent mold design are smaller than those in the Sand casting.
• External cooling (by water or air) may be used for creating desired solidification direction and reducing shrinkage defects and internal stresses.
• Parts of 0.4 lb (0.1 kg) to 150 lb (70 kg) may be cast.
• The section thickness of permanent mold casting may vary in the range 0.1” - 2” (2.5-50 mm).
• The dimensional tolerances are 0.015-0.06” (0.4-1.5 mm) depending on the casting section thickness.
• Allowances of 0.01-0.03” (0.25-0.75 mm) are taken for the dimensions crossing the parting line of the mold.
• The draft angle is commonly 1-3%.
• Permanent cores are commonly used for permanent mold castings, however if a casting has cavity’s shape not allowing a withdrawal of the core it is made of chemically bonded sand or other materials used for preparation of expendable cores. New consumable cores are added after each pour. The process combining permanent mold and consumable parts (cores) are called semi-permanent casting.

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Advantages of permanent mold casting process are determined by relatively high cooling rate caused by solidification in metallic mold:

• Better mechanical properties.
• Homogeneous grain structure and chemical composition.
• Low shrinkage and gas porosity.
• Good surface quality: 40-250 µinch (1-6 µm) R_a.
• Low dimensional tolerances: typically about 0.04” (1 mm}.
• Little scrap process.

Disadvantages of permanent mold casting:

• High cost of the molds.
• Limitations in casting of high melting point metals into metallic molds.
• Intrinsic and complex shapes can not be cast.
• Large parts can not be cast.

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• Casting properties
• Melting furnaces
• Sand casting
• Investment casting
• Die casting
• Centrifugal casting
• Squeeze casting
• Continuous casting
• Horizontal continuous casting in graphite mold
• Continuous casting in traveling mold
• Direct Chill (DC) casting
• Solidification
• Crystallization

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