Gravity Die Casting: Definition, Importance, How It Works, Advantages and Examples

Gravity die casting, a type of permanent mold casting, is an important manufacturing technique that holds a prominent place in modern industry. This method involves the creation of complex metal components by pouring molten metal into a reusable mold. One of gravity die casting’s primary strengths lies in its ability to minimize porosity and air entrapment during casting. This results in components with heightened structural integrity and a reduced likelihood of defects.

This article will discuss the gravity die casting process, highlighting both its advantages and its inherent limitations, as well as give examples of typical industrial applications.  

What Is Gravity Die Casting?

Gravity die casting is a manufacturing process used to produce high-quality metal parts with complex shapes and relatively smooth surfaces. It's a type of casting method that falls under the broader category of die casting processes. The key characteristics differentiating gravity die casting from other casting methods are that (1) only the force of gravity is used to introduce the casting metal into the mold; and (2) a reusable ("permanent") metal mold is used. It is usually made of steel or cast iron.

Why Is Gravity Die Casting Also Called the Lost Wax Process?

Gravity casting is also referred to as lost wax casting because the pattern is made out of wax. Lost wax casting or precision casting is known for its accuracy in part reproduction with tight tolerances. 

How Does Gravity Die Casting Differ From Other Types of Casting?

Gravity die casting distinguishes itself from other casting methods in several key aspects. Gravity die casting is a metal casting process where molten metal is poured into a durable metal mold using gravity. Once the metal cools and solidifies, the mold is opened to retrieve the casting. In sand casting, compacted sand is placed around a wooden or metal pattern, and additional cores are inserted to create internal features. Molten metal is then introduced into the sand molds. The molds are subsequently fractured to extract the castings. In low-pressure die casting, molten metal is introduced into the mold under relatively low pressure, which helps reduce turbulence and air entrapment, resulting in higher-quality castings. In high-pressure die casting, die casting is a high-pressure casting method where molten metal is forced into a mold cavity under high pressure. Investment casting, on the other hand, involves creating a wax pattern that is coated with a ceramic shell. Once the shell is hardened, the wax is melted out, leaving a hollow cavity. Molten metal is then poured into the cavity. In squeeze casting, a precise quantity of molten metal is poured into a die via gravity. Pressure is exerted until the part solidifies. Cores can be employed to craft internal features. To delve deeper into these distinctions, let's compare gravity die casting with these other casting processes using Table 1 below:

What Is the Importance of Gravity Die Casting in Manufacturing?

The importance of gravity die casting in manufacturing lies in its capacity to deliver moderate dimensional accuracy and consistency. This technique enables the creation of complex parts with less energy consumption than high-pressure casting due to gravity being the source of pressure.  

Gravity die casting offers several advantages over other casting methods, such as sand casting, low-pressure casting, high-pressure casting, and investment casting. Unlike sand casting, which is suitable for prototypes and small-volume production, gravity die casting strikes a balance between moderate dimensional accuracy and production volume. It is well-suited for demands ranging from 250 to 50,000 units per annum. Its cast iron or steel molds provide greater accuracy and cost-effectiveness compared to sand casting's wooden or resin patterns.

Which Industries Use the Gravity Die Casting Process?

Gravity die-cast components are used in every industrial domain, including: automotive, marine, defense, architectural and construction applications, electronics, complex machinery, and consumer appliances.

How Does Gravity Die Casting Work?

Gravity casting works by allowing molten metal to flow gently into a permanent mold under the influence of gravity alone. Vents and risers on the mold allow the escape of gasses and the complete filling of the mold with minimal porosity due to trapped gasses. The liquid metal solidifies quickly because the mold is also made of metal, which rapidly conducts heat away from the just-poured casting.

What Is the Step-by-Step Process of Gravity Die Casting?

Listed below is a detailed breakdown of the steps involved:

1. Preheat the Die and Coat the Mold Cavity

Before starting the gravity die casting process, the die (also known as the mold) is preheated to a certain temperature (differs for different metals) to prevent cold spots and ensure uniform solidification of the molten metal. This step helps to prevent thermal shock and guarantee the longevity of the die. Additionally, a release agent or coating is applied to the inner surfaces of the die cavity. This coating helps facilitate the removal of the solidified part once the casting process is complete. Also, dies erode due to molten metal contact, especially in turbulent flow areas. Die coating prevents erosion, maintaining surface integrity. Erosion leads to casting sticking and defects.

Figure 1 below shows the metal mold:

2. Pour Molten Metal Into the Cavity

Once the die is preheated and coated, it is securely clamped together. Molten metal, typically aluminum or other non-ferrous alloys, is then poured into a pouring basin or runner system that leads to the die cavity. The molten metal enters the mold cavity under the force of gravity alone, hence the name "gravity die casting." The carefully controlled pouring process helps minimize turbulence and air entrapment, resulting in better-quality castings. Figure 2 illustrates the pouring of molten metal into the mold:

3. Allow the Metal To Solidify

As the molten metal is introduced into the mold cavity, it cools rapidly. Solidification starts first when the molten metal contacts the cooler surfaces of the mold, and progresses inward toward the center of the casting cross-section. Molten metal cools quickly in the mold cavity, solidifying first at the cooler mold surfaces and progressing inward. Preheated molds ensure consistent solidification rates, preventing premature solidification in thin sections that could block metal flow. Solidification time varies with metal type and part design. Mold preheating benefits include avoiding thermal shock, maintaining molten metal temperature for proper flow, and reducing thermal stress in solidifying casting. The time required for solidification depends on the type of metal being cast and the design of the part.

4. Open the Die and Remove the Part

After the molten metal has fully solidified within the die cavity, the clamps holding the die together are released. The die is opened, revealing the solidified casting. The application of the release agent during the initial steps aids in the easy removal of the casting from the die. 

5. Machine the Part if Needed

Once the casting is removed from the die, it may undergo further processing, such as machining, to achieve the desired final shape and surface finish. Machining might be necessary to remove excess material at the mold parting line, create features that cannot be achieved through casting alone, or ensure tight tolerances. The decision to machine the part depends on its intended use and the required level of precision.

What Materials Are Suitable for Gravity Die Casting?

A number of materials are compatible with gravity die casting. Some of the most common are listed below: 

1. Steels

While steel can be cast using the gravity die casting process, it's predominantly utilized for creating permanent molds rather than being the casting material itself. Typically, cast iron or steel serves as the mold material due to its durability. Steel's higher melting point compared to common non-ferrous casting materials leads to rapid die wear, making steel die casting generally impractical. However, certain low-alloy steels with relatively lower melting points can be used for gravity die casting applications where specific mechanical properties are required.

2. Magnesium

Magnesium alloys are well-suited for gravity die casting due to their low density, good strength-to-weight ratio, and excellent machinability. Magnesium castings offer advantages in industries where lightweight components are crucial, such as the aerospace and automotive sectors. However, proper handling and safety precautions are essential due to the flammability of magnesium during melting. To safeguard the molten alloy from oxygen exposure, either a flux or a protective gas cover is essential. This leads to two primary systems for protecting the molten magnesium and its alloys: flux and fluxless methods.

3. Aluminum Alloys

Aluminum and its alloys are the most commonly used materials for gravity die casting due to their relatively low melting point, good fluidity when molten, and favorable mechanical properties as a finished product. These make it an ideal choice for a wide range of applications. Aluminum gravity castings are found in automotive parts, consumer electronics, and various industrial components. To learn more, see our guide on Aluminum Alloy.

4. Cast Irons

Cast iron materials, known for their excellent wear resistance, heat resistance, and damping properties, can also be cast using gravity die casting. However, due to their higher melting points and generally more complex casting processes, cast irons are less common in gravity die casting compared to other materials.

5. Copper Alloys

Copper alloys are suitable for gravity die casting, particularly when parts require good electrical conductivity or thermal properties. Copper's high melting point and greater thermal conductivity can pose challenges during the casting process, but with careful control of the casting parameters, high-quality copper alloy castings can be achieved.

6. Zinc

Zinc and zinc alloys are widely used in gravity die casting due to their relatively low melting points, excellent casting fluidity, and good natural surface finish they produce. Zinc castings find applications in a number of industries, including: electronics, decorative hardware, and automotive components. The ability to reproduce the fine details of a mold makes zinc a popular choice for decorative items.

How Long Does It Take To Make a Gravity Die Casting?

The duration of the gravity die casting process can vary widely depending on several factors, including: the material being cast, the complexity of the part's design, and the size of the casting. The entire gravity die casting process typically takes from a few minutes for simpler castings to several hours for larger or more intricate parts. Cycle times can be optimized through careful design of the casting process parameters, die design, and material selection, in order to balance production efficiency with casting quality.

How Accurate Is Gravity Die Casting?

Die casting has a comparatively high dimensional accuracy when compared to processes like sand casting. However, it is of only middling accuracy when all possible casting methods are considered. Although the accuracy is material-dependent, it is typically in the range of 0.1 mm for the first 2.5 cm and 0.02 for every centimeter to follow. 

How Much Does Gravity Die Casting Cost?

One major drawback of gravity die casting is that it can be quite costly. However, giving an exact estimate of what the process will cost is rather difficult, since the cost will differ on a project-by-project basis. The cost of a die casting project is influenced by quite a few factors. The complexity of the part being cast plays a crucial role. Intricate designs, thin walls, and undercuts can lead to higher costs due to the need for specialized tooling and longer production times. 

Due to the unique nature of each project, accurate cost estimates are typically obtained through consultations with specialized casting companies or manufacturers, who can provide quotes based on specific project details and requirements. The tooling cost for gravity die casting is typically between $10,000 to $100,000. 

Is Gravity Die Casting Cheaper Than Sand Casting?

It depends. Sand casting has lower tooling costs but higher unit production costs compared to gravity die casting. However, sand-cast products are not as dimensionally accurate as their gravity die-cast counterparts. To learn more, see our guide on What is Sand Casting.

What Are the Advantages of Gravity Die Casting?

There are a lot of advantages associated with gravity die casting, including:

1. Generates high-quality non-ferrous castings characterized by a smooth surface finish and good dimensional precision. 
2. Consistently yields non-ferrous castings of good quality. The use of durable steel molds contributes to part-to-part dimensional stability over time.
3. Provides a cost-efficient approach for manufacturing non-ferrous castings on a substantial scale (250 to ­50,000 per annum). It is more cost-efficient than pressure casting at lower volumes. While the initial investment in tooling and equipment might appear high compared to sand casting, the cost per part diminishes as production volumes rise. 
4. Results in less scrap compared to other casting methods, owing to its ability to create non-ferrous castings with minimal defects and porosity.
5. Can work with an extensive array of materials, encompassing such non-ferrous materials as aluminum, brass, bronze, zinc, magnesium, and copper, among others. 

What Are the Disadvantages of Gravity Die Casting?

While there are a lot of advantages associated with die casting, there are also some drawbacks to consider, including:

1. Has distinct requirements that impose limitations on design flexibility. These molds are designed to allow molten metal flow driven solely by gravitational forces, in contrast to LPDC and HPDC, which employ external pressure for mold filling. Achieving specific design features, like thin walls and intricate geometries, can be challenging within the confines of this method.
2. Necessitates a moderately high initial investment in terms of tooling and equipment, being roughly between $10,000 and $100,000. This is much lower than pressure die casting which can go up to $300,000, but also significantly higher than sand casting which is roughly between $2,000 and $40,000. 
3. In comparison to alternative casting methods, gravity die casting typically involves longer lead times. Fabrication of the permanent molds can take up to several months. 
4. Is most suitable for producing relatively small aluminum castings. The method is less appropriate for manufacturing larger components. This is because, at the largest sizes, the permanent mold approach becomes "uneconomical." 

What Are Examples of Gravity Die-Cast Products?

Some examples of products that can be manufactured using gravity die casting include:

1. Automotive parts such as: engine blocks, cylinder heads, pistons, intake manifolds, and transmission housings
2. Industrial machinery parts such as: gearboxes, pump housings, and valve bodies
3. Aerospace components like: aircraft engine parts, brackets, and housings
4. Decorative and functional lighting fixtures, such as: lamp bases, housings, and ornamental components
5. Parts for household appliances, including: washing machine components, oven parts, and refrigerator components
6. Kitchen utensils such as pots and spoons
7. Enclosures for electronic devices, including: computer chassis, audio equipment housings, and server rack components

What Is the Lifespan of Gravity Die-Cast Products?

The lifespan of gravity die-cast products can vary significantly due to factors such as: material selection, casting quality, environmental conditions, maintenance practices, applied load, and surface treatments. The choice of material used in the casting process can influence how long the product will last because of the differences in corrosion resistance and mechanical properties. The quality of the casting process itself, including attention to detail and quality control, contributes to the product's longevity. There is no fixed lifespan for gravity die-cast products, as their longevity results from the interplay of these factors. The molds, however, are expected to last about 100,000 cycles on average before they need to be replaced. 

How Durable Are Gravity Die-Cast Products?

In general, properly designed gravity die castings produced with adequately maintained equipment according to a well-established procedure can exhibit high durability and withstand a wide range of conditions. The durability of these products does, however, depend on several factors, including: the material used, the quality of the casting process, the design of the product, the environmental conditions in which they are used, and the level of maintenance and care they receive.

Are Gravity Die-Cast Products Heat Resistant?

It depends. Whether or not gravity die-cast products are heat resistant depends mainly on the relationships between the service temperature and the material used to make the casting. Heat resistance can also be influenced by the casting process parameters and any subsequent factors including: the type of material used for casting and any subsequent heat treatment or surface treatments applied to the products. For example, gravity die casting is often used for casting materials like: aluminum, zinc, and magnesium alloys. These alloys can have varying degrees of heat resistance based on their composition. The heat resistance of a gravity die casting product can also be enhanced through heat treatments such as solution heat treatment and aging. 

Is Gravity Die Casting an Effective Mass-Production Tool?

Yes, gravity die casting is considered an effective mass-production tool for producing metal components with moderate dimensional accuracy, consistent quality, and relatively short cycle times. It is particularly suitable for producing medium to large quantities of parts. 

While gravity die casting is effective for many mass production scenarios, it might not be the best choice for all types of parts. Some parts with requirements for complex geometries or specialized material properties might be better suited to other manufacturing methods. 

What Are the Alternatives to Gravity Die Casting?

While gravity die casting is a popular method for producing non-ferrous components, there are several alternatives available, each offering unique advantages and disadvantages based on the specific requirements of the project. The main alternatives are listed below:

1. High-pressure Die Casting: Die casting is a high-pressure casting method where molten metal is forced into a mold cavity under high pressure. This process is well-suited for producing parts with complex shapes, tight tolerances, and smooth surfaces. The rapid production cycle and minimal post-processing requirements make it ideal for high-volume manufacturing. However, it might not be suitable for all shapes and sizes, and tooling costs can be significant.
2. Low-Pressure Die Casting: Low-pressure die casting is a process in which molten metal is introduced into a mold cavity at low pressure. The mold is typically preheated, and a pressurized furnace or crucible is used to inject the metal into the mold cavity. This method is known for its ability to reduce turbulence and gas entrapment during filling, resulting in improved casting quality. Low-pressure die casting is often chosen for producing parts that require better structural integrity compared to gravity die casting.
3. Sand Casting: Sand casting provides an alternative for creating three-dimensional hollows and intricate molds, surpassing the capabilities of gravity die casting. This method involves creating a mold by packing sand around a pattern, which is then removed to leave a cavity. Molten metal is poured into the cavity, creating the desired shape. Sand casting is versatile and can handle more intricate designs, it is also the cheapest even though it has longer production cycles compared to other methods.
4. Investment Casting: Investment casting is a viable alternative when high precision and intricate detailing are required. This process is suitable for both small and large production runs. It involves creating a wax or plastic pattern, coating it with a ceramic shell, and then melting out the pattern to leave a cavity. Molten metal is poured into the cavity, resulting in a highly detailed and dimensionally accurate part. Investment casting offers a streamlined, automated filling process that reduces errors and the need for extensive post-processing. However, it can be more expensive to set up due to the intricacies of the process.
5. Centrifugal Casting: This method involves rotating the mold at high speeds while pouring molten metal into it. Centrifugal force pushes the metal towards the mold walls, resulting in a denser and more uniform casting. It is often used for cylindrical parts like pipes and tubes.
6. Squeeze Casting: Squeeze casting is a hybrid process that combines elements of die casting and forging. Molten metal is poured into a mold cavity, and then a force is applied to the mold to further compress the metal as it solidifies. This results in improved mechanical properties compared to traditional casting.

What Is the Difference Between Gravity Die Casting and Die Casting?

Die casting is a general term for casting processes using permanent (reusable) molds. There are several variations of die casting methods, each with its own unique characteristics and applications. Three main types of die casting processes are gravity die casting, low-pressure die casting, and high-pressure die casting.

Gravity die casting involves the use of a preheated and coated reusable metal mold (die). Molten metal is introduced into the die cavity under the influence of gravity alone, allowing it to naturally flow and fill the mold's contours. Gravity die casting is well-suited for creating parts with complex shapes and aesthetic requirements.

Low-pressure die casting is a process in which molten metal is introduced into a mold cavity at low pressure. The mold is typically preheated, and a pressurized furnace or crucible is used to inject the metal into the mold cavity. This method is known for its ability to reduce turbulence and gas entrapment during filling, resulting in improved casting quality. Low-pressure die casting is often chosen for producing parts that require better structural integrity compared to gravity die casting.

High-pressure die casting relies on the use of high pressure to force molten metal into a mold cavity made of steel. The mold usually consists of two halves that are clamped together. The high-pressure injection ensures rapid and efficient filling of the mold. This method is widely employed in various industries such as: automotive, aerospace, industrial machinery, and consumer goods. High-pressure die casting is known for its high production rates, tight tolerances, and the capability to produce large quantities of parts with consistent quality. However, die casting is less suitable for creating structural parts as the process results in poorer mechanical strength compared to gravity die casting. 

To learn more, see our guide on Die Casting Manufacturing.

Summary

This article presented gravity die casting, explained it, and discussed how it works and its advantages. To learn more about gravity die casting, contact a Xometry representative.

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