Metal casting, as an ancient craft with a history spanning 7,000 years, has evolved into one of today’s most efficient and versatile manufacturing processes. Whereas metal casting was once used to produce items like swords and rings, it now manufactures metal components for industries across the spectrum. Today, casting techniques have overcome limitations in size, weight, and structure, offering near-limitless design possibilities that provide robust support for innovation and development in modern industry.
What is Metal Casting
Metal casting is a process that involves pouring molten liquid metal into a prepared mold. After complete cooling and solidification, the mold is removed to obtain a metal casting with the desired shape and properties. For complex structural components, it stands as one of the primary casting methods. Over time, the casting process has evolved to encompass various techniques, including sand casting, pressure casting, investment casting, and centrifugal casting. The castings produced through these methods have permeated virtually every corner of industry and daily life.
Main Process of Metal Casting
The primary process flow of metal casting includes: manufacturing a master pattern; subsequently creating the required molds for pouring based on the master pattern; then pouring molten metal into the molds; waiting for cooling before removing the shell to obtain the rough casting; finally performing finishing operations as required to produce the target casting.
Pattern Production
Patterns are typically crafted from materials such as wood, metal, or plastic. They replicate the exact structure of the target casting, serving as the basis for all subsequent castings.
Mold Fabrication
Production molds for casting are created based on the pattern, including the internal cores required for the casting.
Pouring Molten Metal
Metal raw materials are heated above their melting point and then rapidly and steadily poured into the mold.
Cooling and Shell Removal
After the molten metal fully cools, the outer shell is removed and cleaned to obtain the rough casting. Disposable molds must be broken to retrieve the casting, while permanent molds feature dedicated openings for extracting the casting.
Finishing and Post-Processing
The rough casting undergoes grinding, polishing, and other machining processes to achieve the desired shape and surface finish.
Different Types of Metal Casting
Based on mold service life, modern metal casting processes can be categorized into two main types: expendable mold casting and non-expendable mold casting. The following provides a detailed overview of the primary metal casting processes.
Expendable Mold Casting
Disposable casting refers to a casting method that utilizes temporary, non-reusable molds for production. Consequently, once the molten metal cools and solidifies, the disposable mold must be destroyed to remove the casting. Common mold materials include resin sand, ceramic shells, and foam materials, making this method more suitable for small to medium-batch casting production.
Sand Casting
Sand casting is the oldest metal casting method, with its techniques traceable to the bronze civilization of China’s Shang Dynasty. As the name suggests, sand casting uses sand (foundry sand) to create molds. The mold consists of an upper sand box and a lower sand box. When joined together, the cavity formed in the middle takes the shape of the desired casting. Molten metal is poured into this cavity through a pouring system. After cooling and solidifying, the sand mold is broken apart to remove the casting.
As one of the most economical and widely used casting processes, sand casting can produce castings ranging in size from a few centimeters to tens of meters and in weight from a few grams to several tons. Moreover, it remains a viable and economical method for producing robust, large castings. Additionally, due to the unique nature of its molds, modifications to sand molds are highly convenient, allowing for prompt adjustments to address any issues. Sand casting imposes minimal restrictions on casting materials, enabling the casting of high-melting-point alloys and making it particularly suitable for producing complex one-off parts. Furthermore, compared to investment casting, sand casting offers a shorter production cycle.
However, due to the unique characteristics of sand casting, it cannot achieve an excellent surface finish, and castings are more prone to defects such as porosity and sand holes. Secondary machining is typically required to attain the desired casting condition.
Sand casting is primarily used for producing automotive engine blocks, large gearboxes, heavy machinery bases, pipe fittings, and similar components.
Investment Casting
Investment casting, also known as lost-wax casting, is currently one of the most widely used and important metal casting methods. It involves pouring molten wax into a mold cavity to create a wax pattern. The wax pattern is then assembled into a tree and coated with ceramic slurry until a ceramic shell of the desired thickness is achieved. After completely melting and removing the wax from the ceramic mold, molten metal is poured into the cavity. Once fully cooled and solidified, the ceramic shell is broken to reveal the investment casting part.
Investment casting is renowned for its exceptional dimensional accuracy and outstanding surface finish. Since it employs a single-piece forming process, the produced castings are net-shaped or near-net-shaped, eliminating parting lines present in sand castings. Consequently, investment castings require minimal or no post-processing before use. Furthermore, investment casting imposes no restrictions on casting materials and offers exceptional design freedom, enabling the production of parts with highly complex geometries. Additionally, since all wax patterns are produced from the same master pattern, castings from the same batch exhibit high consistency—a critical advantage for mass production.
However, the superior casting quality and complex process entail higher costs for investment casting. There are also limitations on casting size and weight, preventing the production of larger castings. Furthermore, investment casting struggles with parts featuring small, deep cavities. The use of cores adds additional challenges to the process.
Investment casting is commonly used for aerospace engine blades, medical implants, intricate automotive components, and marine equipment.
Non-expendable Mold Casting
Non-expendable mold casting, also known as permanent mold casting, refers to a casting method utilizing reusable metal molds. This eliminates the need to create new molds for each casting, offering significant advantages in mass-producing identical metal castings. It also delivers superior surface finish and dimensional accuracy.
Die Casting
Die casting is one of the primary and most widely used metal casting processes for manufacturing high-quality complex parts in industrial applications. The molds employed consist of a fixed die and a moving die, with the moving die incorporating an ejection system. Before injecting molten metal, operators spray lubricant into the mold cavity to regulate mold temperature and facilitate part removal. The two mold halves are then tightly closed, and molten metal is forcibly injected into the cavity under pressure. This pressure is maintained to ensure proper cooling and solidification. The mold is then opened to eject the casting and separate it from the scrap, yielding the desired component.
The primary die casting methods are categorized into hot-chamber die casting and cold-chamber die casting. In hot-chamber die casting, the injection plunger is directly connected to the metal furnace and submerged in the molten metal, automatically filling the mold cavity. This method is therefore more suitable for metals with low melting points. Cold-chamber die casting employs a separate furnace, requiring manual transfer of molten metal into the injection chamber, enabling the casting of metals with higher melting points.
Die casting produces parts with exceptional dimensional accuracy and surface finish, and thin-walled designs are more readily achievable. With only four primary steps, the process operates at a faster pace.
However, die casting molds and equipment are highly expensive, resulting in high initial costs. This makes it more suitable for high-volume production. Gas residues often remain in the molten metal during casting, leading to minor porosity on the surface or within the casting. Additionally, die casting imposes certain limitations on casting materials, with highly fluid metals being more suitable for this process.
Key application areas for die casting include automotive engine blocks, transmissions, motor housings, and electronic product enclosures.
Centrifugal Casting
Centrifugal casting is a casting process that utilizes centrifugal force generated by rotation to shape molten metal against the inner walls of a mold. It is specifically designed for producing hollow, rotationally symmetrical castings. During casting, molten metal is directly poured into a rapidly rotating mold. Under the influence of centrifugal force, the molten metal spreads evenly along the walls, ultimately forming a hollow casting.
Centrifugal casting is the optimal choice for specific components, such as thin-walled cylinders. It excels at producing standard-sized castings, particularly pipes. Most pipe fittings—including water and gas conduits—are manufactured via centrifugal casting due to their seamless surfaces and exceptional structural strength. These castings exhibit dense microstructures, typically low defect rates, and superior mechanical properties. Centrifugal casting eliminates the need for cores and risers when producing pipe-type castings, enhancing material utilization.
However, centrifugal casting is severely limited in the shapes it can produce, being suitable only for simple rotationally symmetric hollow forms. It is also applicable to only certain alloys. Impurities tend to concentrate in the central region during casting, requiring removal through machining.
Centrifugal casting is categorized into horizontal and vertical types. Horizontal centrifugal casting is primarily used for producing longer cylindrical components, such as pipes. Vertical centrifugal casting is mainly employed for manufacturing shorter disc-shaped components, including discs and flanges.
Common Defects of Metal Casting
Porosity
During casting, if gases within the molten metal are not promptly expelled, porosity may form on the surface or within the casting as it cools and solidifies. Porosity can also develop under conditions of excessive environmental humidity. The presence of porosity reduces the strength of the casting.
Shrinkage Defects
Shrinkage defects include shrinkage porosity and shrinkage cavities. It refers to jagged or linear defects appearing on the casting surface when volume contraction during the cooling of molten metal is not adequately compensated by incoming material. Shrinkage cavities refer to large, concentrated, irregular voids forming in the last solidified areas of the casting.
Mould Material Defects
Defects caused by mold materials primarily occur in sand casting. Key issues include sand inclusions (where loose sand particles enter the casting) and sand inclusions (where molten metal infiltrates through cracks in the mold surface).
Pouring Metal Defects
Casting metal defects encompass insufficient filling, cold shuts, and inclusions. Insufficient filling occurs when molten metal fails to fill the mold, leaving voids. Cold shuts result from incomplete fusion between two molten metal streams during pouring, creating demarcation lines or cracks on the casting surface. Both insufficient filling and cold shuts significantly reduce material strength near the defect. Inclusions refer to non-metallic impurities within the casting, compromising structural integrity.
Metallurgical Defects
Metallurgical defects are categorized into hot cracks and hot spots. Hot cracks occur when metal fractures during cooling due to unyielding stress, typically manifesting as irregular fissures. Hot spots arise from uneven cooling within large castings, causing slower-cooling regions to contract abnormally and form irregular linear patterns.
Materials for Metal Casting
Due to the diverse processes involved in metal casting, the range of casting materials is also highly complex. Below, I will introduce several of the most commonly used metal casting materials.
Cast Iron
Cast iron is inexpensive, offers excellent casting properties, and possesses good wear resistance and compressive strength. However, it is brittle in nature and lacks sufficient toughness.
Stainless Steel
Stainless steel is renowned for its outstanding corrosion resistance. It also offers good strength. Its unique hygienic properties make it a favorite in hygiene-sensitive applications.
Aluminum Alloy
Aluminum is the material of choice for lightweight production due to its outstanding lightness and corrosion resistance. It also offers good strength and casting properties.
Copper Alloy
Copper alloys are renowned for their excellent electrical and thermal conductivity. Bronze alloys provide outstanding strength and corrosion resistance; brass alloys offer superior machinability.
Zinc Alloy
Zinc alloys exhibit exceptional fluidity, making them ideal for casting thin-walled components. They offer good ductility, excellent surface finish, and ease of casting.
To learn more about casting materials or select the appropriate alloy for your casting project, click “A Guide to Choosing the Right Alloy for Your Castings”.
Applications of Metal Casting
Due to the virtually unlimited possibilities in terms of size, weight, and structural complexity offered by metal casting, its applications are exceptionally diverse. Below are common application areas for metal castings.
Household Appliances: Cookware, air conditioner compressors, range hood housings, etc.
Automotive: Engine blocks, transmission housings, wheel hubs, etc.
Construction: Piping, valves, structural components, flanges, etc.
Heavy machinery: Components for heavy equipment in petroleum, agricultural, mining, and other industrial sectors.
Railway: Brake components, suspension system parts, etc.
Marine: Load-bearing elements for offshore platforms, ship fittings, etc.
Aerospace: Turbine blades, engines, etc.
Medical: Medical devices, surgical implants, etc.
Why Choose Metal Casting
High Consistency: Metal casting enables mass production with consistently high quality.
Complex Structures: The casting process can produce intricate components.
Design Freedom: Metal casting offers almost unlimited design flexibility for castings.
Size & Weight Constraints: Metal casting imposes minimal restrictions on casting dimensions and weight, making it suitable for manufacturing both small and large items.
Single-piece Construction: The metal casting process achieves castings of a single piece, significantly reducing machining allowances.
Material Selection: Metal casting offers extensive freedom in material selection.
Conclusion
As an efficient and economical method of metal fabrication, metal casting holds an undeniably vital position in modern industry. When considering different metal casting processes, it is essential to select the appropriate casting procedure based on the specific requirements of your casting project. Should you require assistance with casting, please feel free to contact us—Pingheng Machinery. With 26 years of hands-on experience in the investment casting field, we are confident that our extensive casting expertise and solid problem-solving capabilities will provide you with the most satisfactory solutions.
