About Gravity Die Casting 

Gravity die casting, otherwise called permanent mold casting, is a method for fabricating high-performance elements from various low-temperature metals. Gravity casting varies from die casting in that the molten metal is infused into the mold without the application of extra force. In opposition to die casting, gravity die casting can employ metal and sand cores to generate more complex designs known as semi-permanent mold casting.
 
Gravity die casting presents numerous benefits in contrast to other casting approaches, such as shell casting, vacuum casting, squeeze casting, and lost foam casting. Dissimilar to shell casting, which is appropriate for initial models and low-volume manufacturing, gravity die casting achieves a compromise between moderate dimensional precision and production quantity. It is highly suitable for requirements ranging from 300 to 40,000 units per annum. Its cast aluminum or copper molds offer higher accuracy and cost-efficiency compared to shell casting’s plastic or fiberglass molds.
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How Does Gravity Die Casting Work?

Gravity die casting is a method that distinguishes itself from die casting in that it solely depends on gravity to fill the mold with molten metal. This technique is ideal for those seeking rapid processing and mass production. The die is constructed of steel and can endure high temperatures to melt aluminum, zinc, or other alloys. Through just three steps, the gravity die casting process enables manufacturers to produce precision parts at a high speed.

  • Preheat the Die and Apply Mold Cavity Coating
    Before commencing the gravity die casting procedure, the die (also referred to as the mold) is preheated to a specific temperature (varying for different metals) to avoid cold spots and guarantee the uniform solidification of the molten metal. This step serves to prevent thermal shock and ensure the durability of the die. Furthermore, a release agent or coating is applied to the internal surfaces of the die cavity. This coating facilitates the removal of the solidified component once the casting process concludes. Additionally, dies undergo erosion due to contact with molten metal, particularly in areas of turbulent flow. Die coating averts erosion, maintaining surface integrity. Erosion leads to the sticking of castings and the occurrence of defects.
Example of a gravity die casting mold

Example of a gravity die casting mold

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  • Pour Molten Metal into the Cavity
    Once the die is preheated and coated, it is firmly clamped together. Molten metal, commonly 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 solely under the force of gravity, thus earning the name “gravity die casting.” The meticulously controlled pouring process helps minimize turbulence and air entrapment, resulting in castings of superior quality.
  • Enable the Metal to Solidify
    As the molten metal is introduced into the mold cavity, it cools rapidly. Solidification commences initially when the molten metal comes into contact with the cooler surfaces of the mold and progresses inward towards the center of the casting cross-section. Molten metal cools promptly within the mold cavity, solidifying first at the cooler mold surfaces and advancing inward. Preheated molds ensure consistent solidification rates, preventing premature solidification in thin sections that could impede metal flow. The time of solidification varies with the type of metal being cast and the design of the part. Mold preheating offers benefits such as avoiding thermal shock, maintaining the temperature of the molten metal for proper flow, and reducing thermal stress in the solidifying casting.
  • Open the Die and Extract the Part
    After the molten metal has completely solidified within the die cavity, the clamps holding the die together are released. The die is opened, exposing the solidified casting. The application of the release agent in the initial stages assists in the effortless removal of the casting from the die.
  • Machine the Part if Necessary
    Once the casting is removed from the die, it might undergo additional processing, such as machining, to attain the desired final shape and surface finish. Machining might be requisite to eliminate excess material at the mold parting line, create features that cannot be accomplished through casting alone, or ensure tight tolerances. The decision to machine the part depends on its intended application and the requisite level of precision.

What Materials Are Available for Gravity Die Casting?

  1. Steels
    Although steel can be cast through the gravity die casting process, it is mainly employed for fabricating permanent molds rather than serving as the casting material itself. Commonly, cast iron or steel is utilized as the mold material because of its durability. The higher melting point of steel in comparison to common non-ferrous casting materials results in rapid die wear, rendering steel die casting generally unfeasible. Nevertheless, specific low-alloy steels with relatively lower melting points can be utilized for gravity die casting applications where particular mechanical properties are necessary.
  2. Magnesium
    Magnesium alloys are highly appropriate for gravity die casting because of their low density, favorable strength-to-weight ratio, and outstanding machinability. Magnesium castings provide advantages in industries where lightweight components are of vital importance, such as the aerospace and automotive fields. However, proper handling and safety precautions are indispensable due to the flammability of magnesium during melting. To protect the molten alloy from exposure to oxygen, either a flux or a protective gas cover is essential. This gives rise to two primary systems for safeguarding the molten magnesium and its alloys: flux and fluxless methods.
  3. Aluminum Alloys
    Aluminum and its alloys are the most frequently utilized materials for gravity die casting because of their comparatively low melting point, good fluidity when molten, and favorable mechanical properties as the final product. These characteristics make it an ideal option for a wide range of applications. Aluminum gravity castings are found in automotive parts, consumer electronics, and diverse industrial components. To acquire more knowledge, refer to our guide on Aluminum Alloy.
  4. Cast Irons
    Cast iron materials, renowned for their exceptional 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 complicated casting processes, cast irons are less prevalent in gravity die casting compared to other materials.
  5. Copper Alloys
    Copper alloys are suitable for gravity die casting, especially when parts demand good electrical conductivity or thermal properties. Copper’s high melting point and greater thermal conductivity can present challenges during the casting process, but with meticulous control of the casting parameters, high-quality copper alloy castings can be accomplished.
  6. Zinc
    Zinc and zinc alloys are extensively employed in gravity die casting due to their relatively low melting points, excellent casting fluidity, and the good natural surface finish they generate. Zinc castings find applications in several industries, including electronics, decorative hardware, and automotive components. The capacity to replicate the fine details of a mold makes zinc a popular choice for decorative items.

What Are the Alternative Techniques Compared to Gravity Die Casting?

While gravity die casting is a commonly adopted method for manufacturing non-ferrous components, there exist several alternative approaches, each presenting distinctive pros and cons depending on the specific demands of the project. The principal alternatives are as follows:
Gravity die casting part-2
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Investment Casting: Investment casting proves to be a feasible alternative when high precision and elaborate detailing are necessary. This process is applicable 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 component. Investment casting provides a streamlined and automated filling process that reduces errors and the requirement for extensive post-processing. However, it can be more costly to set up due to the complexity of the process.
 
Sand Casting: Sand casting offers an alternative for generating three-dimensional hollows and elaborate molds, outperforming the capabilities of gravity die casting. This approach entails creating a mold by compacting sand around a pattern, which is subsequently removed to leave a cavity. Molten metal is poured into the cavity to create the desired shape. Sand casting is highly versatile and can manage more complex designs. It is also the most economical, although it has longer production cycles in contrast to other methods.
High-pressure Die Casting: Die casting carried out under high pressure involves forcing molten metal into a mold cavity. This procedure is highly suitable for fabricating parts with complex geometries, tight tolerances, and smooth finishes. The rapid production cycle and minimal post-processing needs render it perfect for high-volume manufacturing. Nevertheless, it might not be appropriate for all shapes and sizes, and the tooling expenses can be substantial.
 
Centrifugal Casting: This method entails rotating the mold at high speeds while pouring molten metal into it. The centrifugal force propels the metal towards the mold walls, leading to a denser and more uniform casting. It is frequently utilized for cylindrical components such as pipes and tubes.
 
Squeeze Casting: Squeeze casting is a hybrid process that integrates elements of die casting and forging. Molten metal is poured into a mold cavity, and then a force is exerted on the mold to further compress the metal as it solidifies. This results in enhanced mechanical properties compared to traditional casting.
 
Low-Pressure Die Casting: In the low-pressure die casting process, molten metal is introduced into a mold cavity at a lower pressure. The mold is typically preheated, and a pressurized furnace or crucible is employed to inject the metal into the mold cavity. This method is renowned for its capacity to diminish turbulence and gas entrapment during filling, leading to enhanced casting quality. Low-pressure die casting is frequently selected for producing components that demand superior structural integrity compared to gravity die casting.

FAQs

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.

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. 

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.

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  Sand 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. 

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

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.

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.

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. 

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.