Mechanical Knowledge

Casting Process and Casting Classification

Casting is the liquid metal smelting meet certain requirements and poured into the mold, the solidified by cooling, after clearing treatment process to obtain castings with a predetermined shape, size and performance. The casting blank is nearly shaped, and the purpose of machining-free or small-scale processing reduces the cost and reduces the time to some extent. Casting is one of the basic processes of the modern manufacturing industry. There are many processes for forming metal materials into desired products, such as casting, forging, extrusion, rolling, drawing, stamping, cutting, powder metallurgy, and the like. Among them, casting is the most basic and most commonly used process.

According to the modeling method, casting can be divided into:

ordinary sand casting, including wet sand, dry sand and chemical hardening sand 3 class.

special casting, press molding can be divided into natural mineral material as a main molding sand special casting material (e.g., investment casting, mud casting, shell casting foundry, vacuum casting, solid casting, ceramic casting Etc.) and special castings with metal as the main casting material (such as metal casting, pressure casting, continuous casting, low pressure casting, centrifugal casting, etc.).

According to materials, Casting can be divided into:

Ferrous metal casting (including cast iron, cast steel) and non-ferrous metal casting (including aluminum alloy, copper alloy, zinc alloy, magnesium alloy, etc.)

According to the mold, Casting can be divided into:

Sand casting and metal casting.

According pouring molten metal process, Casting can be divided into:

1 .gravity casting: refers to the process of the molten metal injected into the mold under gravity a, also known as casting. Generalized gravity casting includes sand casting, metal casting, investment casting, lost foam casting, mud casting, etc.; narrow gravity casting refers to metal casting.

2 .the pressure refers to the process of casting molten metal injected into the mold at the other external forces (excluding gravity) acting in accordance with the magnitude of the pressure, is divided into high-pressure casting (die casting) and a low pressure casting.

Additional knowledge:

1 .precision casting method of casting relative to the conventional casting process terms. It achieves relatively accurate shape and high casting accuracy. It is more common to first make an electrode of the desired blank (with little or no margin left) and then use the electrode to etch the mold body to form a cavity. The wax is then cast by casting to obtain the original wax pattern. A layer of high temperature resistant liquid sand is applied to the wax pattern. After obtaining a sufficient thickness, it is air-dried and then heated to dissolve the inner wax mold to obtain a cavity conforming to the desired blank. Then cast cast iron water in the cavity, peel off after curing, and you can get the finished product with precision manufacturing.

2 should consider when choosing the casting method:

a. Sand casting is preferred

b. The casting method should be compatible with the production batch

c. Modeling method should be suitable for factory conditions

d .To balance the accuracy requirements of castings

3 .the mechanical properties of metal materials mainly refer to: strength, stiffness, hardness, plasticity, toughness and so on.

Casting method


Casting process and characteristics

Precautions and application range (including mold)

Mechanical properties

Sand casting

Sand casting is a traditional casting process that uses sand as the main molding material to make a mold. Sand type is generally made by gravity casting. When there are special requirements, low pressure casting, centrifugal casting and other processes can also be used.


Low cost, simple production process and short production cycle;


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 the sand casting is low; and because the overall property of the sand is soft and porous, the dimensional accuracy of the casting of the sand casting Lower, the surface is also rougher.

Casting accuracy, surface finish, material density, metallographic structure, mechanical properties, etc. tend to be poor, so when these performance requirements for castings are higher, other casting methods should be used .

Sand casting has a wide adaptability, and small pieces, large pieces, simple pieces, complicated parts, single pieces and large quantities can be used. The sand type has a higher degree of refractoriness than the metal type, and thus a material having a higher melting point such as a copper alloy or a ferrous metal is also used in this process .

The mold for sand casting is generally made of wood and is commonly known as wood mold . In order to improve the dimensional accuracy, it is also often used for aluminum alloy molds or resin molds with long service life. Although the price has increased, it is still much cheaper than the mold for metal casting, and the price advantage is particularly prominent in small batches and large parts .

Metal casting

A modern process for producing hollow molds for casting using heat-resistant alloy steel. The metal type can be either gravity casting or pressure casting.


The mold is made of metal and can be used repeatedly many times (several hundred to several thousand times) ; the castings produced by metal type have higher mechanical properties than sand castings. Metal casting has high production efficiency; reduces the causes of defects in castings; simple process, easy mechanization and automation

Disadvantages: metal type manufacturing cost is high; metal type is airtight, and has no retreat, which is easy to cause defects such as insufficient washing of parts, cracking or white cast iron parts ; it is very sensitive to the quality of castings and needs strict control.

When using metal casting, the following factors must be considered together:

Long manufacturing cycle and high cost, not suitable for single piece and small batch production; unsuitable for casting complex shape (especially inner cavity), thin wall and large castings ( metal molds are subject to mold material size and cavity processing equipment, casting equipment) Capacity limitation, so the metal type is not suitable for the production of particularly large castings )

Mold cost is more expensive than sand, cheaper than die casting

Castings produced in metal form have higher mechanical properties than sand castings. The same alloy, the tensile strength can be increased by about 25 % on average, the yield strength is increased by about 20 % on average , and the corrosion resistance and hardness are also significantly improved;

Casting method


Casting process and characteristics

Precautions and application range (including mold)

Mechanical properties

Gravity casting

Refers to the process of injecting molten metal into a mold under the action of gravity of the earth, also known as casting. Generalized gravity casting includes sand casting, metal casting, investment casting, lost foam casting, mud casting, etc.; narrow gravity casting refers to metal casting.

The metal gravity casting equipment is relatively simple, the mold and equipment investment is relatively small, the casting process is relatively simple, and the casting yield is high, so it is widely used in the production of various colored castings. However, metal casting also has the disadvantages of low metal utilization rate, difficulty in casting thin-walled complex castings, and lower casting microstructure density and pressure casting.

Widely used in the production of various colored castings

However, metal casting also has the disadvantages of low metal utilization rate, difficulty in casting thin-walled complex castings, and lower casting microstructure density and pressure casting.

High pressure casting (die casting)

The main feature is that the molten metal fills the cavity under high pressure and high speed, and is formed and solidified under high pressure .

It is a metal type pressure casting on a die casting machine, which is the most productive casting process at present.


High productivity, high precision of castings, smooth surface ( IT11~13 , R a 3.2-0.8um ), and can directly cast very thin ( 0.5mm ) pieces or with small holes (aperture 0.7mm ), thread (pitch) 0.75mm ) castings. The casting cools quickly and crystallizes under pressure, so the grain is fine, the surface is tight, and the strength and hardness of the casting are high.


1. The cost of the die-casting machine is high and the preparation time is long. It is not suitable for unit price and small batch production.

2 , the casting material is limited, currently not suitable for the casting of high melting point alloys such as steel and cast iron. 3 , casting and condensation speed is too fast, easy to have pores, shrinkage holes and shrinkage.

1 , since the molten metal at a high pressure, high speed filling process of the cavity, the cavity inevitably entrapped in the air inside the casting, formation of pinholes, it should not be heat-treated aluminum die casting, die casting zinc alloy surfaces should Spray (but can be painted). Otherwise, the internal pores of the casting will expand upon heating as the above treatment heats, causing the casting to deform or bubble.

2 , the mechanical cutting allowance of die-casting parts should also be smaller, generally about 0.5mm , which can reduce the weight of castings, reduce the amount of cutting processing to reduce costs, and avoid penetrating the dense layer of the surface, revealing the subcutaneous pores, resulting in The workpiece is scrapped.

3 , since the die casting member loose inside, ductility, toughness is poor, not suitable for manufacturing a member to withstand impact loads. 4 , casting wall thickness is uniform, and 3 ~ 4mm thin-walled castings are appropriate, the maximum wall thickness should be less than 6 ~ 8mm , to prevent shrinkage holes and other defects.

4. Avoid machine addition to prevent internal holes from being exposed.

Crystallization under pressure, so the crystal grain is small, the surface layer is tight, and the strength and hardness are high.

Poor plasticity and toughness due to loose interior of die-cast parts

Low pressure casting

Low-pressure casting is a method in which a molten metal is injected into a cavity at a low pressure of 0.02 to 0.07 M p a and solidified under pressure to obtain a casting . Low pressure casting because of the low pressure used


1. Adapt to molds of different materials. The liquid metal is filled smoothly and the exhaust is good, which reduces the possibility of casting slag and pores.

2 , under pressure, and top-down directional solidification, the casting structure is dense, strength, air tightness.

3 , the pouring system is simple, no riser, the metal utilization rate is as high as 90 % ~ 95 %;

4. The surface quality of the casting is higher than that of the metal casting ( IT12-14 , R a12.5-3.2um ) , which can produce thin-walled castings with a wall thickness of 1.5-2mm . Low pressure casting equipment costs less than pressure castings.


The life of the riser tube is short, and the liquid metal is prone to oxidation and slag during the heat preservation process, and the productivity is lower than that of the pressure casting.

1 . Filling liquid metal under pressure, can improve the fluidity of the molten metal, the casting moldability is good, favors the formation of a clear outline of the casting smooth surface is more advantageous for large thin-walled castings molded; 2 . The casting is crystallized and solidified under pressure, and can be fully filled, so the casting structure is dense and the mechanical properties are high; 3 . The process yield of the molten metal is improved, and in general, no riser is required, so that the yield of the molten metal is greatly improved, and the yield is generally up to 90%. 4 . Good working conditions; high production efficiency, easy to achieve mechanization and automation, is also a prominent advantage of low pressure casting.

5 . Low-pressure casting has a wide range of applications for alloy grades and can be used in a variety of cast alloys. It is not only used for casting non-ferrous alloys, but also for cast iron and cast steel. Especially for the oxidizable non-ferrous alloy, it shows its superior performance, that is, it can effectively prevent the metal liquid from generating oxidative slag during the casting process.

6 . Low pressure casting has no special requirements for mold materials

The casting is densely organized and has high mechanical properties.

Squeeze casting

Referred to as extrusion.

Squeeze casting has in common with both pressure casting and low pressure casting: the effect of its pressure causes the casting to shape and compact, making the casting dense. Total points is different: no squeeze casting runner, and the larger the size of the casting, thick some suffered flow resistance than small, so small shaped pressure than the desired pressure casting, pressure casting for main Compact and compact.

1 , squeeze casting can be cast large thin-walled member;

2. When squeezed, the static pressure of the liquid metal gradually increases, which can better shrink the microvoids of the dendrites. In addition, since the liquid metal continuously flows and washes along the crystallization, the free growth of the dendrites is prevented, and the crystal structure of the casting is refined. Therefore, the extrusion casting can cast a large-area high-quality thin-wall aluminum casting and complicated. Hollow thin-walled parts.

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