Classification of Casting

I. Definition and Classification of Casting

Definition of Casting: Casting is a forming method that involves pouring liquid metal into a casting cavity that matches the shape of the part. After it cools and solidifies, a metal part blank with specific shapes, dimensions, and properties is obtained.

Common casting methods include sand casting and precision casting, and the detailed classification is shown in the following table.

Sand Casting: Sand casting is a casting method that produces castings in sand molds. Steel, iron, and most non-ferrous alloy castings can be obtained using sand casting. Because the molding material used in sand casting is inexpensive and readily available, mold manufacturing is simple, and it can adapt to single-piece production, batch production, and mass production of castings. For a long time, it has been the fundamental process in casting production.

Precision Casting: Precision casting is a collective term for obtaining precise castings using precision molding methods. Its products are precise, complex, close to the final part shape, and can be used directly with little or no machining. It is an advanced process for near-net-shape forming.

What is Casting? 6 Common Casting Methods, 14 Defect Analysis and Control 

Classification of Casting Methods

II. Common Casting Methods and Their Advantages and Disadvantages

1. Ordinary Sand Casting

The basic raw materials for making sand molds are foundry sand and mold binder. The most commonly used foundry sand is silica sand, but when the high-temperature performance of silica sand is insufficient, special sands like zircon sand, chromite sand, and corundum sand are used. The most widely used mold binder is clay, but various dry oils, semi-dry oils, water-soluble silicates, phosphates, and synthetic resins can also be used as mold binders.

The outer mold used in sand casting is classified into three types: clay-bonded wet sand mold, clay-bonded dry sand mold, and chemically bonded sand mold, depending on the binder used and the way its strength is established.

2. Melting and Pouring

Melting and pouring are key steps in the sand-casting process. Metal is melted in a furnace, and then it is poured into the prepared mold cavity. The molten metal fills the mold and takes the shape of the desired casting. This step requires precise control of temperature, metal composition, and pouring speed to ensure high-quality castings.

3. Cooling and Solidification

After the molten metal is poured into the mold, it begins to cool and solidify. This process must be controlled carefully to avoid defects such as shrinkage cavities or porosity in the final casting.

4. Shakeout and Cleaning

Once the casting has cooled and solidified, it is removed from the sand mold in a process called shakeout. The casting is then cleaned to remove any remaining sand or other debris. This may involve mechanical cleaning methods like shot blasting or grinding.

5. Inspection and Quality Control

Casting quality is critical, and inspections are conducted to ensure that the castings meet specified requirements. This can include dimensional checks, visual inspections, and non-destructive testing methods like X-rays or ultrasonic testing.

6. Machining and Finishing

In many cases, castings require additional machining and finishing operations to achieve the final part’s precise dimensions and surface quality. Machining processes like milling, turning, and grinding may be used.

What is Casting? 6 Common Casting Methods, 14 Defect Analysis and Control 

Advantages of Sand Casting:

1. Abundant and inexpensive clay resources.
2. Short mold-making cycles and high efficiency.
3. The mixed molding sand can be used for an extended period.
4. Suitable for a wide range of applications, from small to large parts, simple to complex, single-piece to mass production.

Disadvantages and Limitations of Sand Casting:

1. Lower production efficiency due to the need for remaking molds for each casting.
2. Limited dimensional precision of castings.
3. Susceptible to defects such as sand inclusions, cold shuts, and porosity.

2. Investment Casting

When making wax patterns, investment casting is also known as “lost wax casting.” Investment casting usually refers to a method where a mold is made using a wax pattern, multiple layers of refractory material, and the wax pattern is melted and drained to create a mold cavity, which is then filled with molten metal to form the casting. Investment casting is suitable for a wide range of alloys and can be used to produce various sizes of castings.

What is Casting? 6 Common Casting Methods, 14 Defect Analysis and Control 

Advantages of Investment Casting:

1. High dimensional accuracy, typically ranging from CT4-6 (compared to CT10-13 for sand casting and CT5-7 for die casting).
2. Improved material utilization as minimal machining is required.
3. Ability to produce complex shapes and thin-walled castings.
4. Suitable for a wide range of materials, including challenging alloys.

Disadvantages and Limitations of Investment Casting:

1. Limited to smaller casting sizes.
2. Complex process with relatively high production costs.
3. Not suitable for single-piece or small-batch production.

3. Die Casting

Die casting is a casting process that involves injecting molten metal into a precision metal mold cavity under high pressure. The molten metal quickly solidifies in the mold to form the casting.

What is Casting? 6 Common Casting Methods, 14 Defect Analysis and Control 

Advantages of Die Casting:

1. High-quality products with tight dimensional tolerances (typically CT4-6).
2. High material efficiency as minimal machining is required.
3. Ability to produce thin-walled and complex-shaped castings.
4. High production efficiency with the potential for automation.

Disadvantages and Limitations of Die Casting:

1. Restricted to specific alloys.
2. Unsuitable for large casting sizes.
3. High initial tooling and equipment costs.
4. Limited flexibility for small-batch or single-piece production.

4. Metal Mold Casting (Permanent Mold Casting)

Metal mold casting, also known as permanent mold casting, is a casting method that utilizes reusable metal molds made from materials like cast iron or steel. These molds can be used for multiple casting cycles, making it a cost-effective choice for high-volume production of castings.

What is Casting? 6 Common Casting Methods, 14 Defect Analysis and Control

Advantages of Metal Mold Casting:

1. Good reusability, reducing material and mold preparation time.
2. High-quality castings with fine surface finishes (typically Ra 6.3µm).
3. Tight dimensional tolerances (usually IT12-IT14).
4. Low labor intensity, good working conditions, and potential for mechanization and automation.

Disadvantages and Limitations of Metal Mold Casting:

1. High mold manufacturing cost and longer lead times.
2. Less suitable for small-batch or single-piece casting.
3. Limited to simpler shapes in some cases.

5. Low-Pressure Casting

Low-pressure casting is a method in which liquid metal is injected into the mold cavity at relatively low pressures (0.02-0.06MPa). This process is used to create high-quality castings, particularly for aluminum and magnesium alloys.

What is Casting? 6 Common Casting Methods, 14 Defect Analysis and Control 

Advantages of Low-Pressure Casting:

1. Suitable for various alloys and casting sizes.
2. Low turbulence during filling, reducing defects like gas porosity.
3. Excellent for casting thin-walled, complex parts.
4. High metal utilization (90-98%).

Disadvantages and Limitations of Low-Pressure Casting:

1. Limited mold life, especially with high-melting-point alloys.
2. Not cost-effective for small-batch production.
3. Can be challenging to manage the oxidation and slag formation of molten metal during the process.

These are the main methods and their advantages and disadvantages in the casting process.

6. Centrifugal Casting

Centrifugal casting is a casting method in which molten metal is poured into a rotating mold and solidifies into a desired shape under the influence of centrifugal force.

Classification of Centrifugal Casting Based on the spatial position of the axis of rotation of the mold, centrifugal casting can be divided into two common types:

1. Horizontal Centrifugal Casting: Centrifugal casting where the axis of rotation of the mold is in a horizontal state or has a small angle with the horizontal line (<4°).
2. Vertical Centrifugal Casting: Centrifugal casting where the axis of rotation of the mold is in a vertical state.

Centrifugal casting with a large angle between the mold’s rotation axis and both the horizontal and vertical lines is called tilted-axis centrifugal casting, but it is rarely used.

Advantages:

• When producing hollow rotating castings using centrifugal casting, it is possible to eliminate the need for cores, gating systems, and risers.
• Due to the centrifugal force generated during rotation, denser metal is pushed toward the outer wall, while less dense gases and impurities move toward the free surface, resulting in directional solidification from the outside to the inside. This leads to good shrinkage conditions, dense casting structures, and good mechanical properties.
• Suitable for casting “bimetal” bushings and bearings, such as embedding a thin layer of copper lining inside a steel bushing, which can save the cost of expensive copper material.
• Good filling ability.
• Eliminates or reduces consumption related to gating systems and risers.

Disadvantages and Limitations:

• The free surface inside the casting is rough, with large dimensional errors and poor quality.
• Not suitable for alloys with significant density segregation (such as leaded bronze) and aluminum, magnesium, and other alloys.

Three, Casting Defects and Their Control Methods

There are many types of casting defects, and the causes of defects are complex. They are related not only to casting processes but also to factors such as the properties of casting alloys, alloy melting, and molding materials. Therefore, when analyzing the causes of casting defects, it is necessary to start from specific situations, conduct comprehensive analysis based on the characteristics, locations, process parameters, and molding materials related to the defects, and then take appropriate technical measures to prevent and eliminate them.

1. Misrun Localized incompleteness of casting, often occurring in thin-walled sections, farthest from the gating system, or at the top of the casting. The edges of the incompleteness are smooth and shiny and do not stick to sand. Causes:

• Low pouring temperature, slow or intermittent pouring.
• Small cross-sectional area of the sprue or runner.
• Low carbon and silicon content in molten iron.
• Excessive moisture or coal dust content in molding sand, causing excessive gas generation, or too high clay content, leading to poor permeability.
• Insufficient height of the upper sand mold, insufficient iron pressure.

Preventive Measures:

• Increase pouring temperature and accelerate pouring to prevent intermittent pouring.
• Increase the cross-sectional area of the runner and sprue.
• Adjust the composition of the melt after the furnace and increase the carbon and silicon content.
• Improve venting in the mold, reduce coal dust in the molding sand, and control the organic material content.
• Increase the height of the upper sand flask.

2. Short Pour Incomplete filling of the upper part of the casting, with the liquid iron level in the pouring basin being at the same level as the liquid iron level in the casting. The edges of the incompleteness are slightly circular. Causes:

• Insufficient liquid iron in the ladle.
• Narrow runner, excessive pouring speed. When the molten iron overflows from the pouring cup, the operator mistakenly believes that the mold is already full and stops pouring prematurely.

Preventive Measures:

• Accurately estimate the amount of liquid iron in the ladle.
• For castings with narrow runners, slow down the pouring speed to ensure that the mold is filled.

4. New Technologies and Trends in Automobile Casting
5. Trends in Sand Molding Technology The development of the sand molding process has gone through several stages: manual compacting of loose molds, compaction through shaking and pressing, high-pressure compaction with micro-vibration, air-impact compaction, and static pressure compaction. The essence of static pressure molding technology is “air-impact pre-compaction + compaction.” It offers several advantages, including clear mold contours, high and uniform surface hardness, minimal mold release angle, high pattern plate utilization, low equipment wear, high mold surface smoothness, and low mold rejection rate. Therefore, it is the latest and most advanced molding technology and has become the mainstream compaction technology today.

Currently, well-known manufacturers of static pressure molding equipment abroad include KW in Germany, HWS, and SAVELLI in Italy. Most domestic automobile casting manufacturers use equipment manufactured by HWS or KW, such as FAW Foundry Company, Dongfeng Automotive Casting Factory, Shanghai Shengdeman Foundry Company, Huadong Taikesi, Shanxi Sanlian, Guangxi Yuchai, Wuxi Diesel Engine Factory, and others.

2. Trends in Near-Net-Shape Technology Casting Forming Technology Lost foam casting, also known as gasification pattern casting, solid casting, or shapeless cavity casting, is hailed as the “casting technology of the 21st century” and the “green project of casting” in the casting industry. This process involves the use of a binder-free dry sand and vacuum extraction technology. In China, more than a hundred enterprises use this technology to produce three major categories of castings: box-type, pipe-fitting valve bodies, and heat-resistant and wear-resistant alloy steel castings, with a total output exceeding 100,000 tons. In the future, this technology will increasingly use rapid manufacturing and simulation and simulation technology to shorten the production preparation cycle and achieve quick casting production.

Precision Investment Casting Technology Precision investment casting processes include water glass shell-making, composite shell-making, and silica sol shell-making. Materials used for automotive products include carbon steel, alloy steel, non-ferrous alloys, and nodular cast iron. High-alloy steel and superalloy materials are also used abroad. Domestic melting equipment includes ordinary and high-frequency furnaces, while foreign countries use vacuum furnaces, tilting furnaces, and high-frequency furnace technology. The future development trend of precision investment casting technology is to bring products closer to commodities. Traditional precision castings are only used as blanks and are no longer suitable for rapid market changes. The complexity and quality of products will continue to increase, and research and development methods will become stronger. Specialization and collaboration will begin to emerge, with the application of CAD, CAM, and CAE becoming the main technologies for product development.

3. Trends in Core Making Technology Currently, there are three core making processes for automobile casting, with the main processes being hot core box core making, shell core making, and cold core box core making. Traditional resin or oil sand core making has been phased out. Cold core box core making technology, in particular, is the dominant trend. Companies such as FAW Foundry Company, Dongfeng Automotive Casting Factory, Shanghai Shengdeman Foundry Company, Huadong Taikesi, and Shanxi International Foundry Company all use cold core box core making technology.
4. Trends in Cast Iron Melting Technology Currently, there are two main methods for iron casting melting technology, namely, the dual-process melting process with large hot blast furnaces and medium-frequency induction furnace technology. The medium-frequency furnace technology produced by companies like Inductotherm and Pillar in the United States has gained increasing attention due to its maturity and prominent advantages in terms of cleanliness, environmental friendliness, energy efficiency, safety, and effectiveness. FAW Foundry Company and Dongfeng Automotive Corporation use medium-frequency furnace and insulation furnace technology produced by Inductotherm and Pillar. Various additives, such as inoculants, nucleating agents, spheroidizing agents, and other products, have been developed and applied, forming a commercial, standardized, and series of products.
5. Pressure Die Casting Technology for Alloy Cylinder Blocks and Cylinder Heads Aluminum alloy is the fastest and most widely used lightweight metal in automobiles due to its excellent properties such as lightweight, high strength, and corrosion resistance. Initially, aluminum alloy was only used for some non-impact parts. However, by reinforcing alloy elements, the strength of aluminum alloy has significantly increased. Its lightweight and excellent heat dissipation characteristics make it suitable for engine pistons, cylinder blocks, and cylinder heads operating in harsh environments. The core technology of aluminum alloy cylinder blocks and cylinder heads can improve material quality control through purification, refinement, fine-graining, and transformation, ensuring consistent and stable quality of aluminum castings.

With the development of China’s automobile industry, especially the rapid increase in domestic passenger cars and the growth in the export of automotive components, there will be significant growth in aluminum castings for automobiles. There are mainly two aluminum cylinder head forming processes: gravity casting represented by Europe and the United States and low-pressure casting represented by Japan and South Korea. Companies such as Shanghai Pierburg, Nanjing Taikesi, and others use gravity casting machines from Italy’s FATA to produce aluminum cylinder heads. Low-pressure casting processes are mainly represented by Japan and South Korea, with companies like Dongfeng Nissan Engine Division Aluminum Die-Casting Workshop, Guangdong Zhaoqing Foundry Company, and Tianjin Toyota Casting Company using low-pressure casting machines from companies like Japan’s New Dong.

6. Semi-Solid Die Casting Technology Semi-solid technology originated in the United States and is considered one of the most promising material forming and processing technologies of the 21st century. Alumax was the first company to convert this technology into productivity. The size of the aluminum alloy automotive brake master cylinder blank produced is close to the size of the part, with machining accounting for 13% of the casting weight. In contrast, traditional metal mold castings have machining allowances of up to 40% of the casting weight. Since the 1980s, European countries and others have conducted extensive research and application work on semi-solid applications.
7. Trends in Cast Iron Materials Thin-Walled High-Strength Gray Iron Technology Gray iron castings are widely used in automobiles due to their low cost and good casting performance. With the increasing demand for lightweight automotive technology, the growth and development of gray iron may be affected. Therefore, the development and application of thin-walled, high-strength cylinder blocks and cylinder head casting technology will be a trend.

Spheroidal Graphite Cast Iron Technology Spheroidal graphite cast iron combines the strength of nodular cast iron with the toughness of gray cast iron. It has properties such as vibration resistance, good heat dissipation, and casting performance similar to gray iron. It also exhibits good plasticity and heat fatigue resistance. It can address the issue of thermal fatigue cracks in high-power engine cylinder heads. The vast potential market for ductile iron is in the automotive industry, with its main products being engine cylinder blocks and high-power diesel engine cylinder head castings. As automobiles become lighter and the specific power (kW/displacement) increases, the operating temperatures of cylinder blocks and cylinder heads continue to rise, with many areas operating at temperatures above 200°C, where the strength of aluminum alloys drops significantly. In such conditions, ductile iron has a significant advantage.

Ductile Iron Technology Ductile iron is still experiencing significant development in the automotive market due to its high strength, high toughness, and low cost. The automotive casting industry mainly focuses on four product technology trends: cast pearlite-ferrite high-strength truck and passenger car crankshafts, cast ferrite pearlite high elongation automotive exhaust pipes and axle housing castings, heat-resistant ductile iron materials such as high-silicon molybdenum and medium-silicon molybdenum, and high-nickel ductile iron used for producing crankshafts and other products. In addition to the above, automotive foundries have also produced as-cast nodular cast iron camshafts.

8. Trends in Computer Applications in the Casting Process As automobile casting technology rapidly advances, the use of rapid prototyping technology, computer simulation and modeling, three-dimensional modeling, and numerical control technology is becoming increasingly widespread. Rapid prototyping technology is used not only to create prototypes for new product development but also to directly assemble sand molds. Simulation modeling is becoming one of the forefront areas of international automotive casting. The application of software such as Magma and HUACAST for the simulation analysis of casting filling, solidification temperature fields, and flow fields for new products allows for the prediction and analysis of casting defects.
9. Casting Inspection Technology Non-destructive testing technology is being widely used, with fluorescent magnetic particle testing often used for surface crack detection in critical components. Ultrasonic or acoustic testing is used to assess the nodularity of ductile iron, and eddy current testing is used to examine the matrix structure (pearlite content) of castings. To meet the testing requirements for critical components, some manufacturers combine the above three testing instruments into an automated testing line.
10. Trends in Green Casting Technology “Green casting” aims to minimize the negative environmental impact and maximize resource efficiency throughout the entire product lifecycle, from design, manufacturing, packaging, transportation, use, to disposal. The casting industry has traditionally been considered a high-energy and high-pollution industry, and there is a continuous need to develop new casting materials that are energy-efficient, clean, low-emission, and low-pollution for production use.