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Casting Process

Casting is an ancient manufacturing method that can be traced back to 6000 years ago in our country. With the development of industrial technology, the quality of large castings directly affects the quality of products. Therefore, casting occupies an important position in the machinery manufacturing industry. The development of casting technology is also very rapid, especially at the end of the 19th century and the first half of the 20th century, many new casting methods, such as low pressure casting, ceramic casting, continuous casting, etc., have been perfected and practicalized in the second half of the 20th century. .

The casting process is a casting method that uses raw sand with certain properties as the main modeling material.

The technology and method of producing castings by applying relevant casting theory and system knowledge. Including casting process, casting system, feeding system, vent, chilling system, special casting process, etc.

The content of the casting process design includes: the design of the casting process drawing, the design of the casting drawing, the design of the mold assembly drawing and the production of process cards, etc.

Casting is a process in which molten metal is poured into a mold and cooled and solidified to obtain parts with the desired shape and performance. Casting is a commonly used manufacturing method with low manufacturing cost and great process flexibility. It can obtain complex shapes and large castings. It occupies a large proportion in mechanical manufacturing. For example, machine tools account for 60 to 80%, automobiles account for 25%, and tractors account for 25%. 50~60%.

Due to the increasing requirements for casting quality, casting precision, casting cost, and casting automation, casting technology is developing towards precision, large-scale, high-quality, automation and cleanliness. For example, in recent years, my country has been using precision casting technology and continuous casting technology. The rapid development of casting technology, special casting technology, casting automation and casting molding simulation technology.

The main casting process includes: metal smelting, model manufacturing, pouring solidification and demoulding cleaning. The main materials used in casting are cast steel, cast iron, cast non-ferrous alloys (copper, aluminum, zinc, lead, etc.).

The casting process can be divided into sand casting process and special casting process.

Special casting

Special casting processes include centrifugal casting, low pressure casting, differential pressure casting, pressurized casting, plaster casting, ceramic casting, etc.

High Pressure Die casting

Pressure casting refers to the process of injecting molten metal into the mold under the action of other external forces (excluding gravity). Die casting in a broad sense includes pressure casting and vacuum casting of die casting machine, low pressure casting, centrifugal casting, etc.; pressure casting in a narrow sense refers to metal die casting of die casting machine, referred to as die casting. These casting processes are the most commonly used in non-ferrous metal casting and the lowest relative price.

Metal mold casting

Metal mold casting is a modern process of hollow casting molds made of metals (heat-resistant alloy steel, ductile iron, heat-resistant cast iron, etc.).

The metal mold can be either gravity casting or pressure casting. The metal mold can be used repeatedly. Every time the molten metal is poured, a casting is obtained. The life is very long and the production efficiency is very high. Metal castings not only have good dimensional accuracy and smooth surface, but also have higher strength than sand castings and are less likely to be damaged when the same molten metal is poured. Therefore, in the mass production of medium and small non-ferrous metal castings, as long as the melting point of the casting material is not too high, metal casting is generally preferred. However, metal mold casting also has some shortcomings: because heat-resistant alloy steel and the processing of making hollow cavities on it are relatively expensive, the cost of metal molds is expensive, but the overall cost of die casting molds is cheaper. too much. For small batch production, the mold cost allocated to each product is obviously too high, which is generally not easy to accept. And because the metal mold is limited by the size of the mold material and the capacity of the cavity processing equipment and the casting equipment, it is also helpless for particularly large castings. Therefore, metal mold casting is rarely used in the production of small batches and large parts. In addition, although the metal mold adopts heat-resistant alloy steel, its heat-resistant ability is still limited. Generally, it is mostly used for casting of aluminum alloy, zinc alloy, and magnesium alloy. It has been less used in copper alloy casting, but used for ferrous metal casting It’s even less.

Die casting

Die casting is a metal mold pressure casting performed on a die casting machine, which is the most efficient casting process.

Die casting machines are divided into hot chamber die casting machines and cold chamber die casting machines. The hot chamber die casting machine has a high degree of automation, less material loss, and higher production efficiency than the cold chamber die casting machine. However, due to the heat resistance of the machine parts, it can only be used for the production of low melting point materials such as zinc alloy and magnesium alloy. Aluminum alloy die castings, which are widely used today, can only be produced on cold chamber die casting machines due to their high melting point. The main feature of die casting is that molten metal fills the cavity under high pressure and high speed, and forms and solidifies under high pressure. The disadvantage of die casting is: because the molten metal fills the cavity under high pressure and high speed, it is inevitable The air in the cavity is wrapped in the inside of the casting to form subcutaneous pores, so aluminum alloy die castings are not suitable for heat treatment, and zinc alloy die castings are not suitable for surface spraying (but paint can be sprayed). Otherwise, the pores inside the casting will expand when heated during the above treatment, which will cause the casting to deform or bubbling. In addition, the machining allowance of die-casting parts should also be smaller, generally around 0.5mm, which can reduce the weight of the casting and reduce the amount of cutting to reduce the cost. It can also avoid penetrating the dense layer of the surface and exposing the subcutaneous pores. The artifact is scrapped.

Investment casting

Lost wax casting is now known as investment casting. It is a casting process with little or no cutting. It is an excellent process technology in the foundry industry and has a wide range of applications. It is not only suitable for casting of various types and alloys, but also produces castings with higher dimensional accuracy and surface quality than other casting methods, and even complex, high-temperature resistant, and difficult-to-process castings that are difficult to cast by other casting methods. Can be cast by investment casting.

Investment casting is developed on the basis of ancient wax casting. As an ancient civilization, China is one of the earliest countries to use this technology. As far back as hundreds of BC, the ancient working people of our country created this lost wax casting technology, which was used to cast various fine patterns and characters. Zhong Ding, utensils and other products, such as the tomb of Zeng Hou Yi in the Spring and Autumn Period. The base of the tomb of Zenghou Yi’s tomb is composed of a number of intertwined dragons, which are connected end to end and staggered up and down to form a hollowed-out multilayer moiré pattern. These patterns are difficult to produce by ordinary casting techniques, but are cast by the lost wax method. The craftsmanship can take advantage of the lack of strength and easy carving of paraffin. Ordinary tools can be used to carve crafts made of paraffin the same as the tomb of Zenghou Yi to be obtained, and then a pouring system, coating, dewaxing, and pouring can be added. You can get the exquisite tomb plate of Zeng Hou Yi.

The practical application of modern investment casting methods in industrial production was in the 1940s. The development of aviation jet engines at that time required the manufacture of heat-resistant alloy parts such as blades, impellers and nozzles with complex shapes, precise dimensions and smooth surfaces. Because heat-resistant alloy materials are difficult to machine, and the shape of the parts is complex, it is impossible or difficult to manufacture by other methods. Therefore, it is necessary to find a new precision molding process. The improvement of modern investment casting methods has achieved important development on the basis of ancient craftsmanship. Therefore, the development of the aviation industry has promoted the application of investment casting, and the continuous improvement and perfection of investment casting has also created favorable conditions for the aviation industry to further improve its performance.

my country began to apply investment casting to industrial production in the 1950s and 1960s. Since then, this advanced casting process has been greatly developed, and has been widely used in aviation, automobiles, machine tools, ships, internal combustion engines, gas turbines, telecommunications equipment, weapons, medical equipment, and knives, and other manufacturing industries. Manufacturing of fine art.

The so-called investment casting process is simply to use fusible materials (such as wax or plastic) to make a fusible model (referred to as investment or model), apply several layers of special refractory coatings on it, and then dry and harden it. After forming an integral shell, use steam or hot water to melt the model from the shell, then put the shell in the sand box, fill the mold with dry sand around it, and finally put the mold in the roasting furnace through high temperature Roasting (if a high-strength mold is used, the mold shell after demolding can be directly fired without modeling). After the mold or mold is fired, molten metal is poured into it to obtain a casting.

The dimensional accuracy of investment castings is relatively high, generally up to CT4-6 (CT10~13 for sand casting and CT5~7 for die casting). Of course, due to the complex process of investment casting, there are many factors that affect the dimensional accuracy of castings, such as molds. The shrinkage of the material, the deformation of the investment mold, the linear change of the shell in the heating and cooling process, the shrinkage of the alloy and the deformation of the casting during the solidification process, etc., so the dimensional accuracy of ordinary investment castings is high, but its Consistency still needs to be improved (the size consistency of castings with medium and high temperature waxes should be improved a lot).

When pressing the investment mold, the pressure type with high surface smoothness of the cavity is used, so the surface smoothness of the investment mold is also relatively high. In addition, the shell is made of a refractory coating made of a special high-temperature resistant adhesive and refractory material, which is coated on the investment mold, and the inner surface of the cavity that is in direct contact with the molten metal has a high smoothness. Therefore, the surface finish of investment castings is higher than that of general castings, generally reaching Ra.1.6~3.2μm.

The biggest advantage of investment casting is that because investment castings have high dimensional accuracy and surface finish, machining work can be reduced, but only a small amount of machining allowance can be left on the parts with higher requirements, and even some castings only have Leave a grinding and polishing allowance, and it can be used without mechanical processing. It can be seen that the use of investment casting method can save a lot of machine tool equipment and processing man-hours, and greatly save metal raw materials.

Another advantage of the investment casting method is that it can cast complex castings of various alloys, especially high-temperature alloy castings. For example, the blade of a jet engine, its streamlined profile and cooling cavity, can hardly be formed by machining technology. Production by investment casting process can not only achieve mass production, ensure the consistency of castings, but also avoid the stress concentration of residual knife marks after machining.

Lost foam casting

Lost foam casting technology (EPC or LFC) is to use foam plastic to make a solid mold with the same structure and size as the part. It is dipped and coated with refractory bonding paint, dried and then molded with dry sand, vibrated and compacted, and then poured The molten metal vaporizes and disappears the pattern by heating, and obtains a casting method for metal parts with the same shape as the pattern. Lost foam casting is a new technology with almost no margin and precise forming. It does not need to close the box to take the mold, and uses binder-free dry sand to reduce pollution. It is considered to be the most likely green casting process in the 21st century. technology.
There are mainly the following types of lost foam casting technologies:

1. Pressure lost foam casting technology

The pressure lost foam casting technology is a new casting technology that combines the lost foam casting technology and the pressure solidification crystallization technology. It is in the pressure pouring with the sand box, after the molten metal is poured to make the foam disappear, the pressure pouring is quickly sealed, and A casting method in which gas at a certain pressure is introduced to solidify and crystallize the molten metal under pressure. The characteristic of this casting technology is that it can significantly reduce the casting defects such as shrinkage cavity, shrinkage porosity and pores in the casting, increase the density of the casting, and improve the mechanical properties of the casting.

2. Vacuum low pressure lost foam casting technology

Vacuum low pressure lost foam casting technology is a new casting technology developed by combining negative pressure lost foam casting method and low pressure anti-gravity casting method. The characteristics of vacuum low pressure lost foam casting technology are: it combines the technical advantages of low pressure casting and vacuum lost foam casting, completes the filling process under controllable air pressure, and greatly improves the casting filling ability of the alloy; compared with die casting, the equipment The investment is small, the cost of castings is low, and the castings can be strengthened by heat treatment. Compared with sand casting, castings have high precision, low surface roughness, high productivity, and good performance; under the action of anti-gravity, the direct gate becomes a shortened channel and the pouring temperature The loss of the liquid alloy is small, and the liquid alloy is fed and solidified under a controllable pressure. The casting system of the alloy casting is simple and effective, the yield is high, and the structure is dense; the casting temperature of the vacuum low pressure lost foam casting is low, and it is suitable for a variety of non-ferrous alloys.

3. Vibration lost foam casting technology

Vibration lost foam casting technology is to apply a certain frequency and amplitude of vibration during the lost foam casting process to solidify the casting under the action of the vibration field. Because the metal solution is vibrated for a certain period of time during the solidification process of the lost foam casting, the vibration force makes the liquid The relative movement between the phase and the solid phase causes the dendrites to break and increase the crystalline core in the liquid phase, so that the final solidification structure of the casting is refined, the feeding is improved, and the mechanical properties are improved. This technology uses the ready-made compact vibrating table in the lost foam casting, through the mechanical vibration generated by the vibrating motor, makes the molten metal nucleate under the dynamic excitation, and achieves the purpose of refining the structure. It is a simple operation, low cost, and environment-free Methods of contamination.

4. Semi-solid lost foam casting technology

Semi-solid lost foam casting technology is a new casting technology that combines lost foam casting technology and semi-solid technology. Because the feature of this process is to control the relative proportion of liquid-solid phase, it is also called transformation-controlled semi-solid forming. This technology can increase the density of castings, reduce segregation, improve dimensional accuracy and casting performance.

5. Lost foam shell casting technology

Lost foam shell casting technology is a new type of casting method that combines investment casting technology and lost foam casting. The method is to coat several layers of refractory material on the surface of a foamed plastic pattern made with a foaming mold that has the same shape as the part. After it is hardened and dried, the foamed plastic pattern is burned and gasified to disappear to form a shell, which is baked. , And then pouring to obtain a new type of precision casting method for castings with higher dimensional accuracy. It has the characteristics of large pattern size and high precision in lost foam casting, as well as the advantages of encrustation precision and strength in investment precision casting. Compared with ordinary investment casting, it is characterized by low cost of foam mold material, convenient pattern bonding and combination, easy gasification and disappearance, and overcomes the problem of investment deformation caused by easy softening of investment casting mold material, and can produce relatively Large-size complex castings of various alloys

6. Lost foam suspension casting technology.

Lost foam casting technology is a new type of practical casting technology that combines lost foam casting technology and suspension casting. The technical process is that after the molten metal is poured into the mold, the foamed plastic shape is vaporized and mixed in the suspending agent of the riser model (or the suspending agent is placed in a specific position of the pattern, or the suspending agent is combined with EPS to make a foam pattern. ) Physical and chemical reaction with molten metal to improve the overall (or part) structure and performance of the casting.

Due to the low cost, high precision, flexible design, clean and environmentally friendly, and suitable for complex castings, the lost foam casting technology conforms to the general trend of casting technology development in the new century and has broad development prospects.

Fine-grain casting

The principle of fine-grain casting technology or process (FGCP) is to strengthen the nucleation mechanism of the alloy by controlling the ordinary investment casting process, so that the alloy forms a large number of crystalline cores during the casting process, and prevents the grains from growing, thereby obtaining an average grain size For uniform, fine, isotropic, equiaxed crystal castings with a size less than 1.6μm, the typical fine crystal castings have a grain size of ASTM 0~2. While fine-grain casting refines the crystal grains of the castings, it also reduces the size of primary carbides and strengthening phase γ’in the superalloy, and improves the morphology. Therefore, the outstanding advantage of fine-grain casting is to greatly increase the low-cycle fatigue life of castings at medium and low temperatures (≤760℃), and significantly reduce the dispersion of mechanical properties of castings, thereby increasing the design tolerance of castings . At the same time, the technology also improves the tensile properties and durability of castings to a certain extent, and makes the castings have good heat treatment performance.

Fine-grain casting technology can also improve the machining performance of high-temperature alloy castings, and reduce the potential danger of machining cracks in screw holes and sharp edges. Therefore, this technology can expand the application range of investment castings to the original use of forgings, thick plate machined parts and forging-casting assemblies. In the precision casting production of aero-engine parts, it is common to use fine-grained castings to replace some forgings or to use fine-grained ingots as forging billets.

Short process

The “short-flow” casting process uses blast furnace molten iron directly into the electric furnace to raise temperature and adjust the composition. After modification, the casting is poured, eliminating the need for pig iron ingots to remelt into molten iron. It is an energy-saving and efficient , Cost-reducing casting production method is one of the optimization technologies promoted by the Foundry Association.

The “short process” process has been well applied in Shandong and other provinces. Among the 72 national high-quality cast iron base pilot enterprises announced not long ago, 12 enterprises in Shandong have adopted the “short process”. It has played a very important role in strengthening the construction of foundry pig iron bases and optimizing the development of foundry industry clusters, and will promote the foundry industry to a higher level.

Sand casting

Sand casting is a traditional casting process that uses sand as the main modeling material to make molds.Sand casting mainly adopts gravity casting process:

Gravity casting refers to the process of injecting molten metal into a mold under the action of the earth’s gravity, also known as casting. Gravity casting in a broad sense includes sand casting, metal casting, investment casting, mud casting, etc.; gravity casting in a narrow sense refers specifically to metal casting.

Gravity casting is generally used for sand molds, and low pressure casting, centrifugal casting and other processes can also be used when there are special requirements. Sand casting has a wide range of adaptability, small pieces, large pieces, simple pieces, complex pieces, single pieces, and large quantities can be used. The molds used for sand casting were mostly made of wood in the past, and they are commonly called wooden molds. The disadvantage of wooden molds is that they are easily deformed and damaged; in addition to single-piece sand castings, aluminum alloy molds or resin molds with higher dimensional accuracy and longer service life can be used. Although the price has increased, it is still much cheaper than the mold for metal casting. In the production of small batches and large parts, the price advantage is particularly prominent. In addition, sand molds have higher refractoriness than metal molds, so materials with higher melting points such as copper alloys and ferrous metals also use this process. However, sand casting also has some shortcomings: because each sand mold can only be poured once, the mold is damaged after the casting is obtained and must be reshaped, so the production efficiency of sand casting is low; and because the overall nature of sand is soft However, it is porous, so the dimensional accuracy of sand castings is lower, and the surface is rougher.

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