Casting process

The process of pouring molten metal into a mold cavity tailored to the shape and size of the part, allowing it to cool and solidify to obtain a blank or part, is commonly known as liquid metal forming or casting.

Process Flow: Liquid metal → Mold filling → Solidification shrinkage → Casting.

Process Characteristics:

1. Can produce parts with arbitrarily complex shapes, especially those with complex internal cavities;

2. High adaptability, no restrictions on alloy types, and virtually no restrictions on casting size;

3. Wide range of material sources, scrap can be remelted, and equipment investment is low;

4. High scrap rate, lower surface quality, and poor working conditions.

A. Sand Casting: A casting method that produces castings in sand molds. Steel, iron, and most non-ferrous alloy castings can be obtained using sand casting.

Process Flow:

Technical Characteristics:

1. Suitable for producing blanks with complex shapes, especially those with complex internal cavities;

2. Wide adaptability and low cost;

3. For some materials with poor plasticity, such as cast iron, sand casting is the only forming process for manufacturing their parts or blanks.

Applications: Castings such as automobile engine cylinder blocks, cylinder heads, and crankshafts.

B. Investment Casting: This usually refers to a casting process where a pattern is made from a fusible material, several layers of refractory material are coated onto the pattern surface to form a shell, the pattern is then melted and removed from the shell, resulting in a mold without a parting surface. After high-temperature firing, sand can be filled and poured. It is often called "lost-wax casting."

Advantages:

1. High dimensional and geometric accuracy;

2. High surface roughness;

3. Capable of casting complex shapes, and the alloys used are not limited.

Disadvantages: Complex process, higher cost.

Applications: Suitable for producing small parts with complex shapes, high precision requirements, or those difficult to machine, such as turbine engine blades.

C. Die Casting: This method uses high pressure to force molten metal at high speed into a precision metal mold cavity. The molten metal cools and solidifies under pressure, forming a casting.

Advantages:

1. High pressure and fast flow rate of the molten metal during die casting;

2. Good product quality, dimensional stability, and good interchangeability;

3. High production efficiency, and the die casting mold can be used many times;

4. Suitable for mass production, resulting in good economic benefits.

Disadvantages:

1. Castings are prone to small porosity and shrinkage cavities;

2. Die castings have low plasticity and are not suitable for operation under impact loads or vibration;

3. For high-melting-point alloys, the mold life is short, affecting the expansion of die casting production.

Applications: Die castings were initially used in the automotive and instrument industries, and later expanded to various industries such as agricultural machinery, machine tools, electronics, defense, computers, medical devices, watches, cameras, and daily hardware.

D. Low-pressure casting: This method involves filling a mold with molten metal under relatively low pressure (0.02–0.06 MPa) and allowing it to crystallize under pressure to form a casting.

Process Flow:

Technical Features:

1. The pouring pressure and speed are adjustable, making it suitable for various molds (such as metal molds, sand molds, etc.) and casting various alloys and sizes of castings;

2. Bottom-filling ensures smooth molten metal filling without splashing, avoiding gas entrapment and erosion of the mold walls and core, thus improving the casting yield;

3. Crystallization under pressure results in dense, well-defined, and smooth castings with high mechanical properties, particularly advantageous for casting large, thin-walled parts;

4. Eliminates the need for feeding risers, increasing metal utilization to 90%–98%;

5. Low labor intensity, good working conditions, simple equipment, and easy mechanization and automation.

Applications: Primarily for traditional products (cylinder heads, wheel hubs, cylinder frames, etc.).

E. Centrifugal Casting: A casting method in which molten metal is poured into a rotating mold, and solidified under centrifugal force to fill the mold and form the desired shape.

Process Flow:

Advantages:

1. Almost no metal consumption due to the gating and riser systems, increasing the yield rate;

2. No core is needed when producing hollow castings, significantly improving the metal filling capacity when producing long tubular castings;

3. High casting density, fewer defects such as porosity and inclusions, and high mechanical properties;

4. Facilitates the manufacture of composite metal castings such as cylinders and sleeves.

Disadvantages:

1. Limited application in the production of irregularly shaped castings;

2. Inaccurate inner diameter, rough inner surface, poor quality, and large machining allowance;

3. Castings are prone to gravity segregation.

Applications: Centrifugal casting was first used in the production of cast pipes. It is now widely used domestically and internationally in industries such as metallurgy, mining, transportation, irrigation and drainage machinery, aerospace, defense, and automobiles to produce steel, iron, and non-ferrous carbon alloy castings. Among these, the production of castings such as centrifugal cast iron pipes, internal combustion engine cylinder liners, and bushings is the most common.

F. Gravity die casting: This refers to a forming method in which liquid metal is filled into a metal mold under gravity and then cooled and solidified within the mold to obtain a casting.

Process Flow:

Advantages:

1. Metal molds have high thermal conductivity and heat capacity, resulting in rapid cooling, dense casting structure, and mechanical properties approximately 15% higher than sand castings;

2. It can obtain castings with high dimensional accuracy and low surface roughness values, and good quality stability;

3. Because sand cores are not used or are used very little, it improves the environment, reduces dust and harmful gases, and lowers labor intensity.

Disadvantages:

1. Metal molds themselves are not permeable, and certain measures must be taken to remove air from the mold cavity and gases generated by the sand core;

2. Metal molds have no flexibility, making them prone to cracking during solidification;

3. Metal mold manufacturing has a longer cycle time and higher cost. Therefore, it only shows good economic benefits in large-scale batch production.

Applications: Metal mold casting is suitable for mass production of complex-shaped non-ferrous alloy castings such as aluminum and magnesium alloys, as well as for producing castings and ingots of ferrous metals.

G. Vacuum Casting: An advanced die casting process that eliminates or significantly reduces porosity and dissolved gases in die castings by removing gas from the die casting mold cavity during the die casting process, thereby improving the mechanical properties and surface quality of the die castings.

Advantages:

1. Eliminates or reduces porosity inside die castings, improving mechanical properties and surface quality, and enhancing plating performance;

2. Reduces back pressure in the mold cavity, allowing the use of lower specific pressures and alloys with poorer casting properties, and potentially enabling the die casting of larger castings with smaller machines;

3. Improves filling conditions, allowing for the die casting of thinner castings.

Disadvantages:

1. Complex mold sealing structure, difficult manufacturing and installation, resulting in higher costs;

2. If not properly controlled, the effects of vacuum die casting are not very significant.

Extrusion casting: A method of directly obtaining parts or blanks by solidifying and flowing liquid or semi-solid metal under high pressure. It has advantages such as high utilization of liquid metal, simplified processes, and stable quality, making it an energy-saving metal forming technology with promising applications.

Process Flow:

Direct Extrusion Casting: Spraying coating, pouring alloy, mold closing, pressurizing, holding pressure, depressurizing, mold parting, blank demolding, and repositioning;

H. Lost Foam Casting (also known as solid foam casting): A new casting method in which paraffin or foam models similar in size and shape to the casting are bonded together to form a model cluster, coated with refractory coating and dried, then embedded in dry quartz sand and vibrated to create the model. Under negative pressure, the metal is poured, causing the model to vaporize, the liquid metal to occupy the model's position, and solidification and cooling to form the casting.

Process Flow: Pre-foaming → Foaming molding → Coating immersion → Drying → Molding → Pouring → Sand removal → Cleaning

Indirect Extrusion Casting: Spraying coating, mold closing, feeding, filling, pressurizing, holding pressure, depressurizing, mold parting, blank demolding, and repositioning. Technical Features:

1. Eliminates internal defects such as porosity, shrinkage cavities, and shrinkage porosity;

2. Low surface roughness and high dimensional accuracy;

3. Prevents casting cracks;

4. Facilitates mechanization and automation.

Applications: Can be used to produce various types of alloys, such as aluminum alloys, zinc alloys, copper alloys, and ductile iron.

Technical Features:

1. High casting precision, no sand core, reducing processing time;

2. No parting surface, flexible design, high degree of freedom;

3. Clean production, no pollution;

4. Reduces investment and production costs.

Applications: Suitable for producing various sizes of relatively precise castings with complex structures, regardless of alloy type or production batch size. Examples include gray cast iron engine housings and high manganese steel bent pipes.

I. Continuous Casting: An advanced casting method. Its principle is to continuously pour molten metal into a special metal mold called a crystallizer. The solidified (shelled) casting is continuously pulled out from the other end of the crystallizer, allowing for castings of arbitrary or specific lengths.

Technical Features:

1. Rapid cooling of the metal results in dense crystallization, uniform structure, and good mechanical properties;

2. Saves metal and increases yield;

3. Simplifies processes, eliminating molding and other steps, thus reducing labor intensity and significantly reducing the required production area;

4. Continuous casting production is easily mechanized and automated, improving production efficiency.

Applications: Continuous casting can be used to cast long castings with unchanged cross-sectional shapes, such as ingots, slabs, bars, and tubes, from steel, iron, copper alloys, aluminum alloys, and magnesium alloys.