Gravity Die Casting -Permanent Mold Casting Explained

Gravity die casting (also called permanent mold casting or tilt casting) is a metal casting process in which molten metal is poured into a reusable steel or iron die under the force of gravity alone -with no external pressure applied during filling. The die is machined from tool steel or cast iron and can be reused thousands of times, unlike sand molds which are destroyed after each pour.

Gravity die casting sits between sand casting and high-pressure die casting in terms of tooling investment, dimensional accuracy, and production rate.

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How Gravity Die Casting Works

1. Die preparation -the die halves are heated to 200-250°C and coated with a refractory release agent
2. Metal pour -molten aluminum (or other alloy) is poured by ladle or tilted automatically from a furnace into the die cavity
3. Solidification -metal solidifies under gravity; risers feed the casting as it contracts
4. Die open and extraction -the die opens and the casting is removed by hand or robot
5. Degating and finishing -risers and gates are cut off; the casting is inspected

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Gravity Die Casting vs High Pressure Die Casting

Parameter	Gravity Die Casting	High Pressure Die Casting (HPDC)
Injection force	Gravity only	10-75 MPa
Fill speed	Slow (turbulence-free)	Fast (10-100 m/s at gate)
Porosity level	Very low -no entrained gas	Higher (gas entrapment risk)
Heat treatment	Yes -T6 possible without blistering	Restricted -porosity causes blistering
Wall thickness (min)	3- mm	1.2 mm (aluminum)
Dimensional tolerance	CT6-CT8	CT4-CT6 (better)
Surface finish	Ra 3.2-3.3 μm	Ra 1.6-2.2 μm (better)
Cycle time	2- min	30-120 s (faster)
Tooling cost	Lower than HPDC	Higher
Optimal volume	500-10,000 pieces/year	10,000-millions
Complex geometry	Limited (no thin walls, less detail)	High (thin walls, fine detail)

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When to Choose Gravity Die Casting

Gravity die casting is the preferred process when:

• T6 heat treatment is required -gravity-cast parts have near-zero gas porosity, so they can be solution heat treated without blistering. This enables tensile strengths of 280-310 MPa in A356-T6 vs ~170 MPa as-cast.
• Large cross-sections or thick walls -parts above 6 mm average wall are often better served by gravity casting, which avoids the shrinkage porosity that HPDC produces in thick sections
• Low-to-medium volumes -tooling cost is 40-50% lower than equivalent HPDC tooling
• Pressure-tight requirements without vacuum HPDC -gravity cast aluminum A356 reliably meets helium leak test requirements for hydraulic and pneumatic components

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Common Gravity Die Casting Alloys

Alloy	Standard	Tensile (as-cast)	Tensile (T6)	Best Use
A356	ASTM B26	160 MPa	280-310 MPa	General structural, automotive
A357	ASTM B26	170 MPa	320-340 MPa	High-strength structural
LM25 (UK equiv)	BS 1490	160 MPa	280 MPa	General engineering
319	ASTM	185 MPa	-	Engine components

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Gravity Die Casting Applications

• Automotive: cylinder heads, inlet manifolds, brake calipers (where T6 treatment required)
• Hydraulic: pump housings, valve bodies, manifold blocks requiring pressure tightness
• Industrial: gearbox covers, bearing housings, large brackets
• Lighting: large heat sink bodies where HPDC cycle time is not critical

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Does KastMfg Offer Gravity Die Casting?

KastMfg specializes in high-pressure die casting (HPDC) and vacuum-assisted HPDC for the full range of aluminum, zinc, and magnesium alloys. For customers requiring gravity die casting, we can advise on process selection and partner referrals.

If your application requires T6 heat treatment for maximum mechanical properties but also demands finer features or thinner walls than gravity casting can provide, ask us about vacuum-assisted HPDC -which approaches gravity casting in porosity level while retaining HPDC dimensional precision.

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Frequently Asked Questions

Is gravity die casting the same as sand casting?

No. Sand casting uses expendable molds (destroyed after each pour). Gravity die casting uses permanent, reusable steel or iron dies. Gravity die castings have better surface finish, tighter tolerances, and faster cycle times than sand castings.

Why is gravity die casting sometimes called "permanent mold casting"?

Both terms refer to the same process. "Permanent mold" emphasizes the reusable steel die, while "gravity die casting" emphasizes that the metal fills under gravity without applied pressure.

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KastMfg Manufacturing Co., Ltd. Email: yaoqingpu1983@gmail.com | Phone: +86 138 1403 4409 No.6, Rungu Road, Nanjing, China