Casting is a manufacturing process that typically involves introducing a material into a mold with a cavity of the desired shape, followed by solidification. The solidified part is called a casting. Through a rigorous historical development process, casting technology has continuously evolved, with its earliest records dating back to 3200 BCE. With the advancement of science and technology and the vigorous development of various industries, casting processes have been continuously expanded, leading to the development of various materials, process-specific castings, and methods suitable for different industries. It plays a crucial role in the automotive industry, aerospace field, machinery manufacturing industry, and construction industry.
Different casting processes have their unique working principles and application scenarios. Common casting processes include sand casting, investment casting, die casting, low-pressure casting, centrifugal casting, permanent mold casting, vacuum die casting, squeeze casting, lost foam casting, and continuous casting. For manufacturers, understanding different casting processes and their characteristics, and selecting the most appropriate casting process and method based on the needs of their own products, is extremely important for improving production efficiency, ensuring product quality, and reducing costs.
Metal casting is a process in which metal is melted into a liquid that meets specific requirements, poured into a mold, and after cooling, solidification, and finishing, a casting with the predetermined shape, size, and performance is obtained. Since casting blanks are close to the final shape, they can achieve the goal of no machining or only minimal machining. This reduces costs and shortens the production cycle to a certain extent, making it one of the fundamental processes in modern machinery manufacturing.
Main materials used: Casting materials are mostly metals that are solid at room temperature but turn liquid when heated, such as copper, iron, aluminum, tin, lead, and steel. Specific alloy materials also include gray cast iron, ductile iron, cast steel, aluminum alloys, magnesium alloys, zinc alloys, etc.
Sand casting is particularly suitable for casting large-sized parts. It involves mixing raw sand, binders, and other auxiliary materials in a certain proportion, creating a sand mold using a pattern, and then pouring molten metal into the mold to form a casting.
Sand casting has an extremely wide range of applications and is compatible with a broad spectrum of materials. Castings made of steel, iron, and most non-ferrous alloys can be produced via sand casting. It is used to manufacture castings such as automobile engine blocks, cylinder heads, and crankshafts, and is also suitable for producing castings of various sizes and shapes in small, medium, or large batches.
Advantages: Strong adaptability, relatively low cost, and wide compatibility with materials.Disadvantages: Low dimensional accuracy and rough surface finish of the parts.
Investment casting usually refers to a process where a mold is made from a fusible material (e.g., wax), the mold surface is coated with several layers of refractory material to form a shell, the fusible mold is then melted out of the shell to create a smooth inner surface for the casting, and after high-temperature baking, the shell is ready for pouring.
Investment casting is particularly suitable for producing small, complex-shaped parts with high precision requirements, such as turbine engine blades, jewelry, and handicrafts. It is also ideal for manufacturing castings that require special properties like airtightness, wear resistance, and corrosion resistance.
Advantages: High dimensional accuracy of parts, good surface quality, and suitability for complex-shaped components.Disadvantages: Complex production process and high cost.
Die casting uses high pressure to force molten metal into a precision metal mold cavity at high speed. The molten metal cools and solidifies under pressure to form a casting. The commonly used injection pressure ranges from 30 to 70 MPa, the mold filling speed is approximately 0.5 to 50 m/s, and the mold filling time is 0.01 to 0.2 seconds.
Die casting was first applied in the automotive industry and instrument manufacturing industry. Later, it gradually expanded to many other industries such as agricultural machinery, machine tool industry, electronics industry, defense industry, computers, medical equipment, clocks, cameras, and daily hardware. It is often used to produce small, thin-walled, complex-shaped castings, such as automotive parts and electronic device housings.
Advantages: High production efficiency, and easy realization of mechanization and automation.Disadvantages: Castings produced are prone to internal defects such as small gas pores and shrinkage cavities; moreover, die castings generally cannot undergo heat treatment, which affects their mechanical properties and corrosion resistance.
Sand-shell casting, also known as shell mold casting, is a expendable mold casting process that uses resin-bonded sand to make molds. The general process includes heating the pattern (usually made of metal), applying a lubricant to the pattern, clamping the pattern into a flask containing a mixture of sand and resin binder, inverting the flask to allow the mixture to coat the pattern, using the heated pattern to partially cure the mixture and form a shell, removing the shell from the pattern, further curing it in an oven, assembling the two halves of the shell to form a complete shell mold, and finally pouring molten metal into the mold cavity.
Shell molding has a wide range of applications. It can be used in the automotive industry, machinery manufacturing industry, aerospace industry, shipbuilding industry, and construction machinery industry.
Advantages: Strong adaptability, high precision, and high production efficiency.Disadvantages: Limited casting size—parts weighing more than 1 ton or less than 40 kg are not suitable for shell molding.
Centrifugal casting involves pouring molten metal into a rotating mold. The molten metal fills the mold and solidifies under the action of centrifugal force. According to the spatial position of the mold’s rotation axis, it can be divided into horizontal centrifugal casting and vertical centrifugal casting.
Centrifugal casting was first used to produce cast pipes. It is now applied in industries such as metallurgy, mining, transportation, irrigation machinery, aerospace, defense, and automobiles. It is often used to produce cylindrical and sleeve-shaped castings made of steel, iron, and non-ferrous alloys, such as centrifugally cast iron pipes, internal combustion engine cylinder liners, and bushings.
Advantages: Great advantages in controlling production costs—little to no metal is consumed in the gating and riser systems, thus improving process yield; castings produced have high density, few defects such as gas pores and slag inclusions, and excellent mechanical properties.Disadvantages: Certain limitations in producing irregularly shaped castings.
Above, we have learned about some common casting processes and their application fields. In addition, there are some less common casting processes that also play important roles in various industries. Let’s take a look at these special casting processes.
Permanent mold casting refers to a forming method where molten metal is filled into a metal mold under the action of gravity, then cooled and solidified in the mold to obtain a casting. The mold is made of metal and can be used thousands of times.
Permanent mold casting has a wide range of applications. It is suitable for mass production of complex-shaped aluminum alloys, magnesium alloys, and other non-ferrous metal alloys, as well as for producing iron and steel castings and ingots.
Advantages: The permanent mold has high thermal conductivity and heat capacity, leading to fast cooling speed and dense casting structure. For the same alloy, its tensile strength can be increased by approximately 25% on average, yield strength by about 20% on average, and corrosion resistance and hardness are also significantly improved.Disadvantages: The permanent mold itself is not permeable, so certain measures must be taken to discharge the air in the mold cavity and the gas generated by the core.
Vacuum die casting is an advanced die casting process that removes gas from the die casting mold cavity during the die casting process to eliminate or significantly reduce gas pores and dissolved gases in die castings, thereby improving the mechanical properties and surface quality of die castings.
Vacuum die casting has a wide range of applications. It is suitable for scenarios requiring high internal and surface quality of castings, such as automotive engine parts and aerospace components.
Advantages: Can eliminate or reduce internal gas pores in die castings, improve the mechanical properties and surface quality of die castings, and enhance coating performance.Disadvantages: The mold sealing structure is complex, making manufacturing and installation difficult, thus increasing costs.
Squeeze casting is a method where liquid or semi-solid metal is solidified and flowed under high pressure to form and directly obtain parts or blanks. It is widely used in the production of various alloys, such as aluminum alloys, zinc alloys, copper alloys, and ductile iron.
Advantages: Can eliminate internal defects such as gas pores, shrinkage cavities, and shrinkage porosity, and improve the density and mechanical properties of castings.Disadvantages: For castings with overly complex shapes, the design and manufacturing of molds are difficult.
Lost foam casting, also known as full-mold casting, uses a cluster of patterns made of paraffin or foam plastic that has a size and shape similar to the desired casting. After brushing a refractory coating and drying, the pattern cluster is buried in dry quartz sand and vibrated to form the mold. Molten metal is poured under negative pressure to vaporize the pattern, and the molten metal occupies the pattern’s position; after solidification and cooling, a casting is formed.
Lost foam casting is very suitable for producing precision castings of various sizes with complex structures. It has no restrictions on alloy types or production batches, such as gray cast iron engine blocks and high-manganese steel elbows.
Advantages: High precision of produced castings, no need for sand cores (reducing machining time), lower investment and production costs, small initial investment, and small floor space.Disadvantages: The vaporization of the plastic pattern during pouring may produce toxic gases, causing environmental pollution.
Continuous casting is a highly advanced casting method. Its principle involves continuously pouring molten metal into a special metal device called a crystallizer. The liquid metal cools and solidifies in the crystallizer to form a casting with a shell of a certain thickness, which is then continuously pulled out from the other end of the crystallizer to obtain a casting of any length or a specific length. According to the spatial arrangement of the crystallizer’s axis, it can be divided into vertical continuous casting ingots and horizontal continuous casting ingots.
Continuous casting has a wide range of applications. In the iron and steel industry, it is an important process for steel production and can be used to produce steel ingots, slabs, etc. In the non-ferrous metal industry, it is also used to cast ingots, billets, cast pipes, and other long castings with uniform shapes made of non-ferrous metals such as copper alloys, aluminum alloys, and magnesium alloys.
Advantages: Good structural performance, high metal recovery rate, and simplified process.Disadvantages: Limited application range and large equipment investment.
In addition to metal casting and special casting, there are other casting processes such as resin casting and plaster casting.
Resin casting is a plastic casting method where liquid synthetic resin is injected into a mold, and after the resin hardens, a casting (plastic part) is obtained. Commonly used thermosetting resins are mixed with a curing agent (polymerization catalyst) and undergo a polymerization reaction at room temperature and normal pressure to achieve curing.
Plaster casting is a type of investment casting that uses plaster as the casting material. First, a plastic foam or wax sacrificial pattern of the desired part is made, then plaster is poured around the pattern to form a mold. After burning out the pattern, molten metal is injected into the mold cavity to obtain a metal part.
Selecting a casting process requires a series of rigorous steps:
Analyze the technical requirements of the part and its manufacturing feasibility. Understand the specific requirements and characteristics of the part to provide a basis for subsequent process selection.
Consider factors such as production tasks and requirements, as well as the production conditions of the factory and workshop, and select appropriate casting and molding methods. Different casting methods are suitable for different scenarios—for example, sand casting is a commonly used method for producing castings of various alloys, while permanent mold casting is suitable for manufacturing castings with high dimensional accuracy.
In addition, the technical characteristics of the casting method should also be considered. For instance, squeeze casting can eliminate defects such as internal gas pores and has low surface roughness and high dimensional accuracy; lost foam casting offers advantages such as precise casting size and shape, high surface finish, and elimination of sand cores. In short, selecting a casting process requires comprehensive consideration of multiple factors, and only through careful analysis and comparison can the most suitable casting process be determined.
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