Six Common Casting Methods and Their Advantages and Disadvantages

Casting is a kind of metal hot working technology that has been mastered by human beings for about 6000 years. China has entered the heyday of bronze castings between 1700 and 1000 BC, and the technology has reached a fairly high level. What are the common casting methods? What are their advantages and disadvantages?

1. Ordinary sand casting

The basic raw materials for making sand mold are casting sand and molding sand binder. The most commonly used casting sand is siliceous sand. When the high temperature performance of silica sand cannot meet the requirements for use, zircon sand, chromite ore sand, corundum sand and other special sand shall be used. Clay is the most widely used sand binder, and various dry or semi dry oils, water-soluble silicates or phosphates and various synthetic resins can also be used as sand binder. The external sand mold used in sand casting can be divided into three types according to the binder used in molding sand and the way in which it establishes strength: clay wet sand mold, clay dry sand mold and chemical hardening sand mold.

advantage:

Clay is rich in resources and cheap in price. Most of the used clay wet sand can be recycled after proper sand treatment; Short casting cycle and high work efficiency; The mixed molding sand can be used for a long time; It has a wide range of adaptability. Small pieces, large pieces, simple pieces, complex pieces, single pieces and large batches can be used;

Disadvantages and limitations:

Because each sand mold can only be poured once, the mold will be damaged after the casting is obtained and must be reshaped, so the production efficiency of sand casting is low; The rigidity of the mold is not high, and the dimensional accuracy of the casting is poor; Castings are prone to sand washing, sand inclusion, porosity and other defects.

2. Investment casting

When wax is used as pattern, investment casting is also called “lost wax casting”. Investment casting usually refers to a casting scheme that makes a pattern of fusible materials, wraps several layers of refractory materials on the surface of the pattern to make a mold shell, and then melts the pattern out of the mold shell, so as to obtain a mold without parting surface. After being baked at high temperature, it can be filled with sand for pouring. Since wax materials are widely used to make patterns, investment casting is often called “lost wax casting”. The types of alloys that can be produced by investment casting are carbon steel, alloy steel, heat-resistant alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy and ductile iron.

advantage:

High dimensional accuracy. Generally, it can reach CT4-6 (CT10~13 for sand casting and CT5~7 for die casting); It can improve the utilization rate of metal materials. Investment casting can significantly reduce the processing amount of the formed surface and mating surface of the product, and save the processing time and cutting tool material consumption; It can maximize the similarity between the blank and the parts, and bring great convenience to the structural design of the parts. For castings with complex casting shape, investment casting can produce castings with very complex shape, castings with wall thickness of 0.5mm and weight of less than 1g, and combined and integral castings; It is not limited by alloy materials. Investment casting can be used to cast carbon steel, alloy steel, ductile iron, copper alloy and aluminum alloy castings, as well as castings of superalloys, magnesium alloys, titanium alloys and precious metals. For alloy materials that are difficult to forge, weld and cut, it is particularly suitable for casting by precision casting; High production flexibility and adaptability Investment casting is not only suitable for mass production, but also for small batch production and even single piece production.

Disadvantages and limitations:

The casting size shall not be too large. The cooling rate of the casting is slow due to the complex process. Investment casting has the most complex process and high casting cost among all blank forming methods. However, if the product is properly selected, the part design is reasonable, and the high casting cost is compensated by reducing cutting, assembly, and saving metal materials, investment casting has good economy.

3. Die casting

The principle of die-casting process is to use high pressure to press liquid metal into a precision metal mold cavity at a high speed, and the liquid metal is cooled and solidified under pressure to form a casting. Cold and hot chamber die casting are two basic ways of die casting process. In cold chamber die-casting, the metal liquid is poured into the pressure chamber by manual or automatic pouring device, and then the injection punch moves forward to hydraulic pressure the metal into the cavity. In the hot chamber die casting process, the pressure chamber is perpendicular to the crucible, and the metal liquid automatically flows into the pressure chamber through the feeding port on the pressure chamber. The injection punch moves downward, pushing the liquid metal into the cavity through the gooseneck. After the liquid metal solidifies, the die casting mould is opened, the casting is taken out, and a die casting cycle is completed.

advantage:

The product quality is good. The dimensional accuracy of castings is high, generally equivalent to Grade 6-7, or even Grade 4; Good surface finish, generally equivalent to Grade 5~8; The strength and hardness are relatively high. The strength is generally 25~30% higher than that of sand casting, but the elongation is about 70% lower; Stable size and good interchangeability; Die-casting thin-wall complex castings; High production efficiency, for example, the domestic J Ⅲ 3 horizontal cold air die casting machine can die cast 600~700 times per eight hours on average, and the small hot chamber die casting machine can die cast 3000~7000 times per eight hours on average; The service life of the die-casting mold is long. A pair of die-casting mold and die-casting bell alloy can reach hundreds of thousands of times, even millions of times; It is easy to realize mechanization and automation; The economic effect is excellent. The die castings have the advantages of accurate size, smooth surface, etc. Generally, it is not used directly for machining, or the processing amount is very small, so it not only improves the utilization rate of metal, but also reduces a lot of processing equipment and man hours; The casting price is easy; Combination die casting can be used to make other metal or non-metallic materials. It saves both assembly time and metal.

Disadvantages and limitations:

During die casting, due to the high filling speed of liquid metal in the mold cavity and unstable flow pattern, the general die casting method is adopted, and the castings are easy to produce air holes and cannot be heat treated; For castings with complex concave, die casting is more difficult; High melting point alloy (such as copper and ferrous metal), low die casting life; It is not suitable for small batch production. The main reason is that the manufacturing cost of die casting mold is high, the production efficiency of die casting machine is high, and small batch production is not economical.

4. Metal mold casting

Also called hard mold casting, it is a casting method that liquid metal is poured into the metal mold to obtain castings. Casting mold is made of metal and can be used repeatedly (hundreds to thousands of times), also known as permanent mold casting. Generally, the metal mold is made of cast iron and cast steel. The inner cavity of the casting can be either a metal core or a sand core. There are many kinds of metal mold structures, such as horizontal parting, heavy straight parting and composite parting. The vertical parting is convenient for opening the inner gate and taking out the casting; Horizontal parting is often used to produce thin-walled wheel castings; The upper half of the compound parting is formed by connecting the two halves of the vertical parting with hinges, and the lower half is a fixed horizontal bottom plate, which is mainly used for the casting of more complex castings.

advantage:

It has good reusability and can be “one type with multiple castings”, saving modeling materials and modeling time. Because of the strong cooling ability of the metal mold to the casting, the structure of the casting is dense and the mechanical properties are high. The dimensional accuracy of castings is high, and the tolerance grade is IT12-IT14; The surface roughness is low, Ra is 6.3m. The metal mold casting does not use sand or uses less sand, which improves the working conditions.

Disadvantages and limitations:

The manufacturing cost of the metal mold is high, the cycle is long, and the process requirements are strict. It is not suitable for the production of single small batch castings. It is mainly suitable for the mass production of nonferrous alloy castings, such as aluminum pistons, cylinder bodies, cylinder heads, oil pump housings, and copper alloy bearing shells and sleeves for aircraft, automobiles, internal combustion engines, motorcycles, etc. The ferrous alloy castings are also limited to medium and small castings with simple shapes.

5. Low pressure casting

Low pressure casting is a method to make liquid metal fill the mold under low pressure (0.02~0.06MPa) and crystallize under pressure to form castings. Pour the melted liquid metal into the heat preservation crucible, install the sealing cover, lift the liquid pipe to connect the liquid metal with the mold, lock the mold, slowly inject dry compressed air into the crucible furnace, and the liquid metal, under the effect of gas pressure, fills the cavity along the lift pipe and the pouring system from bottom to top, and crystallizes under pressure. After the casting is formed, remove the pressure in the crucible, and the liquid metal in the lift pipe drops back to the metal level in the crucible.

Open the mold and take out the casting.

advantage:

The rising speed and crystallization pressure of molten metal during pouring can be adjusted, so it can be used for casting various alloys and castings of various sizes in different molds (such as metal mold, sand mold, etc.); The bottom injection mold filling is adopted, the liquid metal mold filling is stable, without splash, which can avoid the entrapment of gas and the scouring of the mold wall and core, and the casting has fewer defects such as porosity and slag inclusion, thus improving the qualification rate of the casting; The casting crystallizes under pressure, with dense structure, clear contour, smooth surface and high mechanical properties, which is particularly beneficial to the casting of large and thin wall parts; The feeding riser is omitted, and the metal utilization rate is increased to 90%~98%; Low labor intensity, good working conditions, simple equipment, easy to realize mechanization and automation.

Disadvantages and limitations:

The life of the riser is short, and the liquid metal is easy to oxidize and produce slag inclusion during the heat preservation process. It is mainly used to cast some aluminum alloy and magnesium alloy castings with high quality requirements, such as cylinder block, cylinder head, crankcase, aluminum piston of high-speed internal combustion engine and other thin-walled parts.

6. Centrifugal casting

Centrifugal casting is a kind of casting method in which molten metal is poured into a rotating mold, and the mold is filled and solidified under the action of centrifugal force. According to the position of the rotational axis of the mold in space, the common centrifugal casting can be divided into two types: horizontal centrifugal casting: centrifugal casting when the rotational axis of the mold is horizontal or the angle between the rotational axis of the mold and the horizontal line is very small (<4 °). Vertical centrifugal casting: When the rotational axis of the mold is vertical, the centrifugal casting is called vertical centrifugal casting. Centrifugal casting with inclined axis is called centrifugal casting when the rotational axis of the mold has a large angle with the horizontal and vertical lines, but it is rarely used.

advantage:

When the hollow rotary casting is produced by centrifugal casting, the core, gating system and riser can be omitted; Due to the centrifugal force generated by the liquid metal during rotation, the metal with high density is pushed to the outer wall, while the gas and slag with low density move to the free surface to form directional solidification from outside to inside, so the feeding conditions are good, the casting structure is dense, and the mechanical properties are good; It is convenient to cast “bimetallic” bearing sleeves and bearing shells. If a thin layer of copper bushing is cast in the steel sleeve, the more expensive copper material can be saved; Good filling capacity; Eliminate and reduce the consumption of gating system and riser.

Disadvantages and limitations:

The inner free surface of the casting is rough, with large size error and poor quality; It is not applicable to alloys with large density segregation (such as lead bronze) and aluminum, magnesium and other alloys.