1. Introduction
Gravity die casting, also known as permanent mold casting, uses gravity—not external pressure—to fill a reusable metal mold with molten alloy.
Although craftsmen experimented with metal molds as early as the 17th century, modern gravity die casting emerged in the late 19th and early 20th centuries alongside advances in iron‑ and steel‑foundry practices.
Today, this process produces millions of high‑integrity components annually, from automotive engine blocks to art‑quality sculptures.
Its enduring popularity stems from a balance of dimensional accuracy, surface finish, and cost efficiency, making it a mainstay in industries that demand consistent quality at moderate volumes.
2. What Is Gravity Die Casting?
Fundamental Principles
At its core, gravity die casting relies on gravitational force to draw molten metal into the mold cavity.
Unlike pressure die casting, which employs hydraulic or mechanical force, gravity casting simply pours the liquid metal at the sprue and lets gravity do the work.
Role of Gravity in Mold Filling
By eliminating high-pressure injection, gravity casting minimizes turbulence and air entrainment, improving soundness.
For instance, pouring aluminum at 700 °C into a preheated steel mold (< 300 °C) creates laminar flow that preserves alloy cleanliness and reduces porosity.
Mold Types: Expendable vs. Permanent
- Expendable (Sand/Plaster) Molds: Used when designers need complex geometry or very low volumes.
- Permanent (Metal) Molds: Fabricated from steel or cast iron, these molds withstand hundreds to thousands of cycles. By contrast, sand molds typically serve only one shot.
Gating and Riser Systems
Effective gating—sprue, runners, gates—and strategically placed risers control fill rate and solidification.
For example, a well‑designed aluminum housing mold might use a bottom‑pour sprue with a tapered runner to achieve a fill time under 2 seconds, followed by a cylindrical riser that compensates for shrinkage.
3. Gravity Die Casting Process Steps
Gravity die casting transforms molten metal into precision components through six tightly controlled stages.
By relying on gravity rather than high-pressure injection, this process delivers excellent part integrity, repeatable dimensions, and fine surface finishes.
Pattern and Mold Preparation
Engineers begin by designing a two‑part mold from H13 tool steel, incorporating 1–3° draft angles to facilitate part ejection.
They machine precise gates, runners, and risers, calibrated to compensate for the 1–2 % linear shrinkage typical of aluminum alloys.
Modern CAD/CAM systems optimize these features to ensure uniform fill and directional solidification.
Mold Preheating and Coating
Before each cast, technicians preheat the mold to 200–300 °C, stabilizing the initial metal skin and reducing thermal shock.
They then apply a thin graphite‑ or zircon‑based refractory coating (10–30 µm thick). This coating:
- Promotes smoother flow into fine details
- Controls cooling rates for consistent microstructure
- Protects mold surfaces, extending die life to up to 2,000 cycles
Metal Melting and Temperature Control
Foundries melt alloys in electric or gas-fired furnaces, holding pouring temperatures within ± 5 °C:
- Zinc alloys: 420 ± 5 °C
- Magnesium alloys: 650 ± 5 °C
- Aluminum alloys: 700 ± 5 °C
Strict temperature regulation ensures optimal fluidity (viscosity ~ 6 mPa·s for aluminum at 700 °C) and prevents cold shuts or misruns.
Pouring Techniques and Flow Rates
Molten metal—typically aluminum or other non‑ferrous alloys—is poured into a pouring basin or runner system that leads directly to the die cavity.
The metal flows under gravity alone, hence “gravity die casting.”
By controlling pour rate and gating geometry, foundries minimize turbulence and air entrapment, resulting in higher‑quality castings.
Filling from the basin’s bottom or via a tilt‑pour setup allows the metal’s meniscus to rise smoothly, driving air out through vents and maintaining laminar flow throughout the cavity.
Solidification, Shake‑Out, and Cleaning
Once filled, the mold remains closed for the solidification interval—5 seconds for thin-walled zinc parts, up to 30 seconds for thicker aluminum sections.
During this time, the metal cools from the mold walls inward, driven by the high thermal conductivity of steel.
After reaching a safe handling temperature (~ 150 °C), hydraulic clamps release, and ejector pins push the casting free. Foundries then:
- Remove gates, runners, and risers
- Perform shot blasting or CNC trimming to clear sand, scale, and flash
- Inspect critical dimensions (± 0.1–0.5 mm) and surface quality
Trimming and Final Finishing
In the final stage, technicians trim remaining sprues and flash using band saws, water‑jet cutters, or pneumatic nibblers, reclaiming 90 % of the scrap for remelt. They then:
- Deburr edges via tumbling or manual tools
- Machine high‑precision features—such as bores, flanges, and sealing surfaces—to tolerances as tight as ± 0.02 mm
- Apply surface treatments (e.g., anodizing, bead blasting) to achieve specified finishes (Ra 0.8–3.2 µm)
- Conduct nondestructive testing (X‑ray, dye penetrant) for critical aerospace or automotive parts
4. Materials for Gravity Die Casting
Selecting the right alloy lies at the core of a successful gravity die casting operation.
Each metal brings unique properties—fluidity, freezing range, thermal conductivity—that dictate mold design, process parameters, and ultimately, part performance.
Aluminum Alloys
Popular Grades: A356, A380, B319
- Melting Range: 600–650 °C
- Fluidity: High; flows easily into thin sections (< 3 mm)
- Shrinkage: ~ 1.2 % linear
- Applications: Automotive housings, heat sinks, pump bodies
Key Considerations:
- Aluminum’s excellent thermal conductivity (~ 180 W/m·K) shortens solidification times but risks cold shuts if pour rate lags.
- Adding 7 % silicon (A356) improves fluidity and reduces porosity.
- Preheating molds to 200–300 °C prevents premature freezing in intricate features.
Zinc Alloys
Popular Grades: Zamak 3, Zamak 5
- Melting Point: ~ 385 °C
- Freezing Range: Narrow (~ 5 °C), yielding outstanding fluidity
- Shrinkage: 0.5–0.7 % linear
- Applications: Precision connectors, decorative hardware, small gear blanks
Key Considerations:
- Zinc’s low pour temperature reduces mold wear and energy consumption.
- Narrow freezing range enables faithful reproduction of fine details (< 0.5 mm).
- Designers can specify very thin runners (5–10 mm²) to minimize scrap.
Magnesium Alloys
Popular Grades: AZ91D, AM60
- Pouring Temperature: 650–700 °C
- Density: 1.8 g/cm³ (lightest structural metal)
- Tensile Strength: 200–260 MPa
- Applications: Electronics housings, structural aerospace components
Key Considerations:
- Magnesium oxidizes rapidly; foundries must employ inert‑atmosphere or flux covers.
- High thermal expansion (26 µm/m·K) demands larger pattern allowances (up to 2.5 %).
- Die lifetimes typically run 500–1 000 cycles due to corrosive melt.
Copper and Copper Alloys
Popular Grades: C95400 (Aluminum Bronze), C36000 (Free‑Machining Brass)
- Pouring Range: 1 050–1 200 °C
- Thermal Conductivity: 110–400 W/m·K (depending on alloy)
- Applications: Marine pump impellers, valve components, architectural hardware
Key Considerations:
- Copper alloys’ high melting points require robust die materials (H13 steel) and refractory coatings.
- Alloys with narrow freezing ranges—like silicon bronze—cast more easily than high‑aluminum grades.
- Designers must account for 2–2.5 % shrinkage and incorporate generous risers.
Steels and Cast Irons
Popular Grades: A216 WCB (carbon steel), A217 WC6 (alloy steel), ASTM A536 65‑45‑12 (ductile iron)
- Melting Range: 1 370–1 520 °C
- Cooling Rates: Slow; risk of coarse grains and segregation
- Applications: Pump housings, valve bodies, heavy machinery parts
Key Considerations:
- High pour temperatures demand pre‑heated dies (350–450 °C) and advanced coatings to prevent metal–die reactions.
- Section thickness should exceed 15 mm to avoid hot spots and thermal cracking.
- Ribbing and chill inserts help manage directional solidification in thick sections.
5. The Advantages of Gravity Die Casting
High Dimensional Accuracy and Repeatability
One of the most prominent benefits of gravity die casting is the excellent dimensional accuracy it offers.
Because the process uses machined, reusable metal molds, parts consistently achieve tighter tolerances compared to expendable mold methods like sand casting.
- Typical tolerances: ±0.1 mm for small features; ±0.3 mm for larger dimensions
- Reproducibility: Ideal for long runs of identical components
This repeatability reduces the need for post-casting machining and ensures compatibility in assemblies—critical for automotive, aerospace, and precision-engineered parts.
Superior Mechanical Properties
Gravity die casting produces components with a denser, more uniform microstructure due to controlled solidification and relatively slow filling rates.
This minimizes gas entrapment and cold shuts.
- Higher strength-to-weight ratios
- Improved elongation and fatigue resistance
- Reduced porosity compared to sand or pressure die casting
For example, aluminum alloy castings produced via gravity casting can achieve tensile strengths of 180–280 MPa,
depending on the alloy and process control, often exceeding the properties of equivalent sand castings by 20–40%.
Enhanced Surface Finish
The smooth interior surfaces of metal molds—especially when coated with graphite or ceramic-based release agents—produce cleaner and smoother as-cast surfaces.
- Surface roughness: Typically in the range of Ra 1.5–3.2 µm
- Reduced need for grinding or polishing in many applications
- Better base for coatings, plating, or painting
This is particularly beneficial in decorative components and applications requiring sealing surfaces or precise fits.
Cost Efficiency in Medium-Volume Production
Compared to investment or sand casting, gravity casting offers faster cycle times and lower labor intensity once tooling is amortized.
- Cycle times: 2–6 minutes per part, depending on size and wall thickness
- Mold longevity: 1,000–10,000 cycles depending on alloy and care
For production runs above 1,000 units, the reduced unit cost begins to offset initial mold investment, often resulting in 30–50% lower per-part costs over the entire production cycle.
Environmentally Friendly Process
Gravity die casting produces less waste than many casting alternatives:
- Reusable molds reduce the need for expendable materials like sand or wax.
- Metal yield is higher (up to 90–95%), minimizing scrap.
- Many foundries now use electric furnaces, reducing carbon footprint.
Additionally, there are fewer emissions and less need for extensive ventilation systems compared to sand or investment casting with organic binders or wax burnout.
Versatility in Part Design
Although more limited than pressure die casting in terms of intricate geometries, gravity casting still supports a broad range of part types:
- Wall thicknesses from 3 mm to 50 mm
- Features such as bosses, ribs, and undercuts (with cores)
- Mold inserts and multiple cavities for higher efficiency
The method also accommodates multiple alloys, including high-strength aluminum, copper, and magnesium-based formulations.
Shorter Lead Times for Reorders
Once a mold has been developed, the repeatability of the gravity casting process allows manufacturers to quickly respond to reorder demands.
Lead times for repeat production runs can be reduced by up to 50% compared to single-use mold processes.
6. The Disadvantages of Gravity Die Casting
High Initial Tooling Cost
Perhaps the most significant drawback of gravity die casting lies in the upfront investment in tooling.
The permanent metal molds, usually made of heat-resistant tool steel like H13, require high-precision machining and robust construction to withstand repeated thermal cycling.
- Typical mold cost: $5,000–$50,000 depending on complexity and part size
- Lead time for tooling: 4–8 weeks or longer for intricate molds
For low-volume or prototype production, this cost can be prohibitive, making alternative methods like sand or investment casting more economical.
Limited Design Flexibility
Gravity die casting imposes more geometric constraints than some other casting processes:
- Parts require draft angles (typically 1–3°) to facilitate ejection.
- Undercuts and complex internal geometries are difficult or expensive to achieve without using sand or soluble cores.
- Thin-walled or intricate features (<3 mm) may not fill completely, especially in alloys with poor fluidity.
Not Suitable for All Alloys
While gravity die casting performs well with many non-ferrous alloys—especially aluminum, magnesium, and copper-based alloys—it is not ideal for materials with narrow solidification ranges or low castability:
- Steel and cast iron are rarely gravity die cast due to their high melting points and aggressive oxidation, which cause mold damage and rapid wear.
- Alloys prone to hot tearing or gas porosity (e.g., high-silicon bronzes) may require advanced gating and venting systems, increasing cost and complexity.
Slower Production Rates than Pressure Die Casting
Although gravity die casting is faster than sand or investment casting, it is significantly slower than high-pressure die casting (HPDC):
- Cycle time: 2–6 minutes per part for gravity die casting
- Cycle time: 20–60 seconds per part for HPDC (aluminum/zinc)
As a result, gravity die casting is not always the best choice for very high-volume production, where pressure casting may offer better economies of scale despite higher machine and tooling costs.
Limited to Certain Part Sizes
Although gravity die casting can produce medium to large parts, it is generally not suitable for extremely large components (>30 kg or >1 m in dimension),
due to the limitations of mold handling, clamping force, and uniform filling by gravity alone.
In such cases, sand casting or low-pressure die casting may be more effective.
7. Applications of Gravity Die Casting
Automotive Industry
The automotive sector is one of the largest consumers of gravity die cast components, driven by the industry’s demand for lightweight, durable, and geometrically precise parts.
Common applications include:
- Engine components: Cylinder heads, timing covers, valve bodies
- Transmission housings and clutch casings
- Suspension parts and steering knuckles
- Brackets and mounts for sensors and assemblies
Aerospace and Aviation
In the aerospace sector, gravity die casting is used for structural components that must maintain performance under extreme stress and temperature variation.
Typical gravity-cast aerospace parts:
- Support brackets and hinges for airframe structures
- Pump housings and compressor covers
- Heat-resistant covers for engine accessories
Industrial Equipment and Machinery
Industrial manufacturers utilize gravity die casting for its durability, reliability, and production efficiency when making medium-volume runs of mechanical parts.
Examples include:
- Pump bodies and impellers
- Valve casings, manifolds, and pipe fittings
- Hydraulic housings and actuator mounts
- Electric motor enclosures and fan blades
Marine and Desalination Equipment
The marine industry favors gravity casting for producing parts that demand corrosion resistance and strength in harsh, saltwater environments.
Gravity cast marine parts include:
- Heat exchangers and water-cooled engine parts
- Pump components and fluid control devices
- Propeller blades and nozzles
- Deck fittings and gear housings
Electronics and Electrical Systems
For electrical systems requiring thermal and electrical conductivity, gravity casting enables the production of components with minimal internal defects and high dimensional fidelity.
Typical applications:
- Bus bars and electrical terminals
- Connector blocks
- Enclosures for power distribution units
- Cooling plates for power electronics
Architectural and Decorative Hardware
Gravity die casting is well-suited to ornamental and structural elements where aesthetic quality and dimensional consistency are essential.
Common architectural uses:
- Balusters, railings, and door handles
- Lighting fixtures and lamp housings
- Faucets and decorative fittings
8. Gravity Die Casting Compared to Other Forms of Casting
To understand gravity die casting’s advantages and limitations, it’s essential to compare it with other widely used casting methods: pressure die casting, investment casting, centrifugal casting, and squeeze casting.
Each method serves distinct purposes based on design complexity, mechanical properties, cost, and production volume.
Gravity Die Casting vs. Pressure Die Casting
Fundamental Difference:
- Gravity die casting relies solely on gravity to fill the mold.
- Pressure die casting forces molten metal into the die cavity under high pressure (typically 10–150 MPa).
Comparison:
| Criteria | Gravity Die Casting | Pressure Die Casting |
|---|---|---|
| Mold Type | Permanent metal mold | Steel die (usually more complex) |
| Metal Flow | Gravity-fed (low turbulence) | Pressure-forced (faster, can be turbulent) |
| Alloy Suitability | Aluminum, copper, magnesium | Zinc, aluminum, magnesium (not suitable for copper) |
| Part Integrity | Better metallurgical quality (less porosity) | Higher risk of porosity |
| Surface Finish | Good, but not as smooth as pressure casting | Excellent surface quality |
| Cost | Moderate tooling and cycle costs | High tooling cost but very fast cycles |
| Typical Applications | Medium-volume structural components | High-volume, thin-walled precision parts |
Conclusion:
Gravity die casting is ideal for medium-batch production where higher structural integrity is prioritized over surface finish or speed.
Pressure die casting suits high-volume, complex geometry parts requiring tight tolerances and superior finish.
Gravity Die Casting vs. Investment Casting (Lost Wax)
Fundamental Difference:
- Gravity die casting uses a reusable metal mold.
- Investment casting uses a single-use ceramic mold formed around wax patterns.
Comparison:
| Criteria | Gravity Die Casting | Investment Casting |
|---|---|---|
| Detail Reproduction | Moderate, limited by metal mold machining | Excellent—complex, intricate designs possible |
| Surface Finish | Good (Ra ≈ 3–6 μm) | Superior (Ra ≈ 1.5–3 μm) |
| Tooling Cost | Moderate initial die cost | High pattern/tooling costs per part |
| Production Volume | Best for medium to high volumes | Best for low to medium volumes |
| Tolerances | ±0.3–0.5 mm typical | ±0.1–0.3 mm achievable |
| Alloy Flexibility | Aluminum, copper, magnesium | Most metals including steels, superalloys |
Conclusion:
Gravity die casting is more cost-effective for medium to large production runs with moderate complexity. Investment casting is better for small runs with high precision and detail.
Gravity Die Casting vs. Centrifugal Casting
Fundamental Difference:
- Gravity die casting uses stationary molds and fills them with gravity.
- Centrifugal casting spins the mold to force metal outward into the cavity.
Comparison:
| Criteria | Gravity Die Casting | Centrifugal Casting |
|---|---|---|
| Best Geometry | Flat, prismatic, or moderately complex parts | Cylindrical, symmetrical parts |
| Porosity Levels | Low (especially with bottom filling) | Very low—impurities are pushed to the center |
| Mechanical Properties | Good grain structure | Excellent grain refinement and density |
| Applications | Housings, brackets, pump bodies | Bushings, pipes, rings, liners |
Conclusion:
Use gravity die casting for versatile shapes and moderate to high production volumes. Choose centrifugal casting for rotationally symmetrical parts demanding exceptional structural integrity.
Gravity Die Casting vs. Squeeze Casting
Fundamental Difference:
- Squeeze casting combines die casting with high pressure during solidification.
- Gravity die casting does not use any applied pressure.
Comparison:
| Criteria | Gravity Die Casting | Squeeze Casting |
|---|---|---|
| Solidification Control | Moderate | Excellent—pressure reduces porosity |
| Mechanical Strength | Good | Very high—near-forging quality |
| Tooling Complexity | Medium | High—needs precise control of pressure |
| Alloy Types | Mainly non-ferrous | Aluminum, magnesium, composites |
| Cost | Lower | Higher equipment and cycle cost |
Conclusion:
Gravity die casting is more economical and simpler to implement. Squeeze casting is chosen when exceptional strength and ductility are required, often replacing forged components.
9. Conclusion
Gravity die casting remains a versatile, cost‑effective, and reliable technique for medium‑volume production of moderately complex metal parts.
By leveraging gravity’s gentle flow, precise mold design, and tailored process controls, manufacturers achieve a compelling mix of surface quality, dimensional precision, and mechanical integrity.
As advanced simulation, hybrid molding, and new alloy developments gain traction, gravity die casting will continue to evolve—maintaining its central role in high‑value manufacturing.
At LangHe, we stand ready to partner with you in leveraging these advanced techniques to optimize your component designs, material selections, and production workflows.
ensuring that your next project exceeds every performance and sustainability benchmark.
FAQs
How does gravity die casting differ from high-pressure die casting?
Unlike high-pressure die casting, which forces molten metal into a mold using hydraulic pressure, gravity die casting relies solely on gravity for mold filling.
As a result, gravity die casting operates at lower pressures, has slower fill rates, and generally results in fewer porosity-related defects.
However, it is less suited for highly complex or thin-walled parts compared to high-pressure die casting.
How long does a gravity die last?
Die life varies based on the alloy cast and mold material. For aluminum, a high-quality steel die (e.g., H13) may last between 10,000 to 100,000 cycles.
Proper maintenance, mold coating, and preheating can significantly extend die lifespan.
Can gravity die castings be heat-treated?
Yes. One of the key advantages of gravity die casting over high-pressure die casting is that the castings are generally free of internal gas entrapment, making them suitable for heat treatment processes like T6 for aluminum alloys.
