Sand casting, an ancient and widely used casting technique, is the process of pouring molten metal into a sand mold, followed by cooling and solidification to obtain a casting with a predetermined shape, size, and performance. The history of this method dates back to ancient Egypt in 3000 BCE, and it continues to be used extensively around the world.
The working principle of sand casting is relatively simple. First, a sand mold is created that matches the desired shape of the casting, and then the molten metal is poured into the mold. Once the metal cools and solidifies, the mold is removed, yielding the desired casting. This article will detail the process, materials, applications, and advantages of sand casting, among other aspects.
Sand Casting Mold
In sand casting, the mold is a crucial part for forming the geometric shape of the casting; its composition is complex and multifunctional. Through precise design and manufacturing, the mold not only ensures the size and shape of the casting but also effectively prevents various casting defects.
Below we will introduce the components of a sand casting mold, understanding and mastering the basic components can help to understand the process of sand casting more thoroughly.
Composition of Sand Casting Mold
The sand casting mold mainly consists of the following parts:
- Cavity: The cavity is the core part of the mold, and its geometric shape corresponds to the shape of the desired casting. The cavity must be precisely designed and manufactured to ensure the size and shape of the casting.
- Gating System: The gating system is a channel that guides the liquid metal into the cavity and usually includes the main gate, sprue, runners, etc. Its design is crucial for ensuring complete filling and defect prevention.
- Vents: Vents are used to discharge gases produced during the casting process, preventing porosity and gas entrapment defects.
- Support System: The support system maintains the integrity and stability of the mold, typically including supporting sand, strengthening ribs, etc.
- Sand Core: For castings with internal cavities or complex geometric shapes, a sand core is needed. The sand core is placed inside the cavity to form the internal cavity or other complex parts of the casting.
The design and manufacturing of a sand casting mold require precision and attention to detail, especially in the design of the cavity and gating system, taking into account various factors of the casting process.
Sand Casting Mold Sand Materials
Casting sand is the core material for sand casting, and its selection and use directly affect the quality and cost of the casting. Below are common types of casting sand.
- Green Sand: One of the most commonly used casting sands, made from clay, water, and sand particles. It has the advantages of low cost and high adaptability but lower strength and refractory properties.
- Resin Sand: Casting sand using organic resins as binders. Resin sand has higher strength and fire resistance and is suitable for precision casting. However, it is more costly, and some resins may be harmful to the environment.
- Water Glass Sand: Casting sand with water glass as a binder. It has good strength and moderate cost, suitable for manufacturing large castings. The drawback is its high shrinkage, which may cause casting deformation.
- Quartz Sand: Its advantages include high temperature resistance, dimensional stability, chemical stability, suitability for various metals, and non-reactivity with metals. It has good fluidity and abrasion resistance. However, it is heavy and energy-consuming, possibly having environmental issues and a higher cost.
- Chemically Hardened Sand: Casting sand hardened by chemical reactions, such as CO2 hardened sand, phenolic resin sand, etc. These casting sands usually have high strength and dimensional accuracy, suitable for complex casting manufacturing but relatively high in cost and environmental impact.
- Special Casting Sand: Some special casting sands, like zircon sand, chromite sand, etc., are used under special requirements for high-temperature, high-precision casting.
The types of casting sands are diverse, and different casting sands have various pros and cons, suitable for different application scenarios. The choice of molding sand in sand casting must also consider the following factors:
- The appropriate casting sand depends on the type, size, complexity, and required production quantity of the casting. For example, for large castings, high-strength casting sands like water glass sand might be chosen, whereas resin sand might be chosen for precision castings.
- The handling of materials, including proper sand grain grading, binder selection and addition, drying, and hardening of the mold, etc. Different handling methods will directly affect the quality of the mold and the casting effect.
The Process of Sand Casting
The process of sand casting includes mold preparation, metal melting, pouring, cooling and demolding, and post-processing. Mold preparation is the process of making the sand mold, which needs to be designed according to the shape and size of the casting.
Metal melting is the process of heating the selected metal to its liquid state. Pouring involves filling the sand mold with molten metal. Cooling and demolding allow the metal to cool and solidify in the sand mold, which is then removed. Post-processing includes cleaning, grinding, and inspection.
Making the Model
- Creating a model of the desired product: This process uses a reusable model that possesses the same details as the desired final product. The model should account for factors like thermal shrinkage or contraction.
- Creating models for the sprue and risers: This is the metal delivery system. The model made in step A also includes guiding the metal to the appropriate sprue and risers. This channels the inevitable thermal shrinkage to acceptable regions and manages the flow of metal and necessary gas venting.
Making the Sand Mold
- Creating the sand mold: A high-temperature-resistant refractory material, like sand, is formed around the model. This material must be strong enough to withstand the weight of molten metal during casting, stable against reacting with the metal, but also brittle enough to be easily broken away from the solidified metal after cooling.
- Preparing the sand mold: Sand molds are typically made in two halves, the top half or “cope” and the bottom half or “drag.” Once the sand sets (using traditional/non-machined methods), the halves are separated, and the model is removed. A refractory coating is applied for improved surface finish and to protect the mold from the poured metal’s impact. The two halves are then reassembled, leaving a cavity shaped like the model.
Pouring
Pouring the molten metal into the sand mold: The molten metal is directly poured into the stationary mold. It fills the cavities that define the finished part and the risers. Risers supply usable liquid metal to the casting and are designed to cool and solidify last, ensuring that shrinkage and potential voids concentrate in the risers rather than the desired part.
Vibratory Demolding
The casting (including the desired part and any excess metal needed for its creation) solidifies and cools. In the vibratory demolding process, the sand mold is shattered. Most of the sand used to create the mold is captured, reconditioned, and reused.
Final Operations
- Sprues, runners, and risers are cut from the casting. If necessary, final post-processes, such as sandblasting or grinding, are conducted to finalize the casting dimensions. Sand cast parts typically require some additional machining to achieve final dimensions or tolerances.
- The part might undergo heat treatment to enhance dimensional stability or performance.
- Non-destructive testing might be needed. This could include fluorescent penetrant, magnetic particle, radiographic, or other inspections. Final dimensional checks, alloy test results, and non-destructive tests are verified before shipment.
Sand Casting Materials
Iron-based Alloys
Iron-based alloys are common materials for sand casting, including gray iron, ductile iron, white cast iron, and alloyed iron. These materials are widely used in the automotive, machinery, and construction industries.
Gray Iron: This is the most common type of cast iron, containing 2-4% carbon. It is favored for its good castability and low cost, though it offers lower strength and toughness.
Ductile Iron: This is a high-strength cast iron where the carbon exists in the form of spherical graphite. It has higher strength and toughness than gray iron, but with poorer castability and a higher cost.
White Cast Iron: This type of cast iron is hard but very brittle, with carbon present in the form of cementite. It is typically used in applications requiring high wear resistance, such as grinding plates and cutting tools.
Alloyed Iron: This type of iron is alloyed with other elements like nickel, chromium, and molybdenum to enhance its properties. It possesses higher strength, hardness, and wear resistance than regular cast iron, but at a higher cost.
Steel
This includes carbon steel, alloy steel, and stainless steel. Steel has higher strength and wear resistance but poses greater casting challenges.
Carbon Steel: The most common type of steel, it can be categorized into low carbon steel, medium carbon steel, and high carbon steel based on carbon content. The strength and hardness of carbon steel increase with carbon content, but its toughness and castability decrease.
Alloy Steel: This steel is alloyed with elements like nickel, chromium, and molybdenum to enhance its properties. Alloy steel offers greater strength, hardness, and wear resistance than carbon steel, but at a greater cost.
Stainless Steel: Alloyed with chromium or nickel, this steel offers excellent corrosion resistance. Stainless steel has poorer castability and is more expensive.
Aluminum and Aluminum Alloys
Aluminum and aluminum alloys have low density, light weight, high thermal conductivity, and good castability. They are widely used in the aviation, automotive, and electronics industries. Aluminum alloys can be enhanced in terms of strength, hardness, and wear resistance by adding elements like magnesium, silicon, zinc, and copper
Copper and Copper Alloys
This includes bronze, brass, and copper-nickel alloys. Copper and its alloys have good electrical and thermal conductivity and are corrosion-resistant, making them suitable for the electrical, maritime, and chemical industries.
Bronze: An alloy of copper and tin, bronze boasts good corrosion resistance and castability. It’s commonly used in sculptures and art pieces.
Brass: An alloy of copper and zinc, brass is recognized for its electrical and thermal conductivity. It’s typically used for electrical components and decorations.
Copper-Nickel Alloy: Known for its corrosion and heat resistance, this alloy is often used in the maritime and chemical sectors.
Magnesium and Magnesium Alloys
Magnesium and magnesium alloys have an even lower density and lighter weight, with good castability. They’re frequently utilized in the aviation and automotive sectors to reduce weight.
Zinc and Zinc Alloys
Zinc and its alloys have good fluidity, making it possible to cast intricate shapes, but they possess lower strength and heat resistance. They’re often used in die casting for toys, decorations, and electrical components.
High-temperature Alloys
Mainly employed in aviation engines and gas turbines that operate at high temperatures, these alloys typically contain elements like nickel, chromium, cobalt, and molybdenum. They’re appreciated for their excellent heat resistance and antioxidation properties.
Applications of Sand Casting
Sand casting is widely used across various industries, including automotive, aerospace, construction, mechanical manufacturing, and more. Below are the applications of sand casting in different fields:
Automotive Industry
Sand casting is extensively used in the automotive industry, including engine parts (such as cylinder bodies, cylinder heads, crankshafts, flywheels, etc.), suspension system components, brake system parts, etc. As sand casting can create large and complex-shaped castings, it is highly suitable for automotive parts manufacturing.
Mechanical Manufacturing
Sand casting is also widely used in the manufacture of various mechanical devices, such as pumps, valves, gears, bearings, turbines, etc. These components usually need to withstand high pressure and high temperature, thus requiring high-strength, wear-resistant casting materials.
Aerospace Industry
In the aerospace field, sand casting is used for manufacturing engine components, structural parts, missile components, etc. These components usually need to operate under extreme conditions, thus requiring high-performance casting materials like high-temperature alloys.
Construction Industry
In the construction industry, sand casting is used to make various architectural hardware, such as door and window accessories, pipe fittings, decorations, etc. These components usually need good corrosion resistance and aesthetics, often using non-ferrous metals like copper and aluminum alloy
Energy Industry
In the energy industry, sand casting is used to manufacture various power generation equipment components, such as wind turbine blades, gas turbine turbines, nuclear power station pressure vessels, etc. These components usually need to withstand high pressure and high temperature, thus requiring high-strength, heat-resistant casting materials.
Railways and Ships
In the fields of railways and ships, sand casting is used to manufacture various transportation equipment components, such as wheels, bearings, crankshafts, propellers, etc. These components usually need to withstand high pressure and high-impact loads, thus requiring high-strength, tough casting materials.
Sand Casting VS Investment Casting
Sand casting and investment casting have many similarities, and the table below summarizes the adaptability of sand casting and investment casting in different aspects.
Sand Casting | Investment Casting | |
Cast Size | Suitable for large castings | Suitable for small and medium castings |
Shape & Complexity | Suitable for simple shapes | Suitable for complex shapes and fine details |
Precision Requirement | Lower accuracy and surface quality | Higher dimensional accuracy and surface quality |
Quantity | Higher production efficiency for large quantities | Suitable for small quantities or single pieces |
Material | Suitable for various metals | Suitable for stainless steel, aluminum alloys, magnesium alloys, etc. |
Cost & Time Consideration | Lower equipment and material costs, shorter production cycle | Higher equipment and material costs, longer production cycle |
Advantages and Disadvantages of Sand Casting
As a common casting method, sand casting has many advantages, suitable for various industrial applications. Here are some main benefits:
- High flexibility: Sand casting can be used for various sizes and shapes of castings, from a few grams to several tons, whether for small or large-scale production.
- Cost-effective: Compared to other casting processes, sand casting has relatively lower initial equipment and mold costs, making it more economical for custom parts and small-batch production.
- Wide material compatibility: Almost all alloys can be cast with the sand casting method, including iron, steel, aluminum, copper, etc.
- Large design freedom: Sand casting can achieve complex geometric shapes, including internal channels and external structures, which may be difficult in other casting processes.
- Strong adaptability: The sand casting process can be adjusted and optimized according to product needs, with excellent adaptability.
- Production efficiency: Compared to some other casting processes, sand casting is usually easier to implement, more conducive to automation, thereby improving production efficiency.
- Recyclability: The sand molds used in casting can be recycled and reused, thereby reducing material costs and environmental impact.
- Technological maturity: Sand casting has a long history and widespread industrial application, thus there is a wealth of experience and expertise available for reference and learning.
However, sand casting also has some drawbacks, such as possible porosity, inclusions, surface roughness, etc. But with proper process control and quality inspection, these issues can typically be resolved.
