Definition and classification of casting

Casting is an early metal hot processing technology, which has a history of about 6000 years. China entered the heyday of bronze casting between 1700 BC and 1000 BC.

Definition of casting:

The method is to cast liquid metal into the casting cavity which is suitable for the shape of the part.

Common casting methods include sand casting and precision casting. Detailed classification methods are shown in the table below.

Sand casting:

Casting method for producing casting in sand mold.Steel, iron and most non – ferrous alloy castings can be obtained by sand casting.As the moulding materials used in sand mold casting are cheap and easy to get, and the casting manufacturing is simple, it can be adapted to single part production, batch production and mass production of castings. For a long time, it has been the basic technology in casting production.

Precision casting:

Casting is an early metal hot processing technology, which has a history of about 6000 years.China entered the heyday of bronze casting between 1700 BC and 1000 BC.

Classification of casting methods

Common casting methods and their advantages and disadvantages

1. Common sand mold casting:

The basic raw material for sand mold is casting sand and sand binder.The most commonly used casting sand is siliceous sand. When the high temperature performance of silicon sand cannot meet the requirements of use, special sand such as zircon sand, chromite sand and corundum sand are used.The most widely used sand binder is clay, and various dry oil or semidry oil, water-soluble silicate or phosphate and various synthetic resins can be used as sand binder.

The outer sand mold used in sand mold casting can be divided into three types: wet sand mold, dry sand mold and chemical hardening sand mold according to the binder used in sand mold and the way of establishing strength.

Sand mold casting has been used for centuries for the most popular and simplest types of casting. Sand mold casting is used to make large parts, such as gray cast iron, ductile cast iron, stainless steel and other types of steel.The main steps include painting, mold, core making, molding, melting and pouring, cleaning, etc.

Advantages:

A. clay is rich in resources and cheap in price.After proper sand treatment, most of the used wet clay sand can be recycled and reused.

B. Short cycle time and high efficiency;

C. Long usable time of mixed sand;

D. wide adaptability.Small, large, simple, complex, single, large quantity can be used;

Disadvantages and limitations:

A. Because each sand mold can only be cast once, the mold is damaged after casting and must be remade. B.

C. The stiffness of the mold is not high, and the size accuracy of the casting is poor;

D. The casting is prone to sand washing, sand inclusion and porosity.

2. Investment casting

When waxy material is used for appearance, investment casting is also called “lost wax casting”.Mold casting usually refers to the casting scheme that the shape is made of the fusible material, and the shape is made of several layers of refractory materials on the surface of the shape, and then the shape is melted out of the shape, so that the shape of the non-dividing surface can be obtained, and then the casting scheme can be filled with sand pouring after being roasted at high temperature.Because the appearance widely USES the waxy material to manufacture, therefore often calls melting mold casting “loses the wax casting”.

The alloys produced by the method of investment casting include carbon steel, alloy steel, heat-resisting alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy and nodular cast iron.

Investment casting process

Advantages:

A. High dimensional accuracy.Generally up to ct4-6 (sand casting is ct10-13, die casting is ct5-7);

It can improve the utilization rate of metal materials.Investment casting can significantly reduce the processing amount of forming surface and matching surface of products, saving the consumption of cutting table and cutting tool materials.

B. It can maximize the degree of similarity between the blank and the part, and bring great convenience to the structural design of the part.Casting with complex shapes can produce casting with very complex shapes, casting with a wall thickness of 0.5mm and a weight of less than 1g, or casting combined and integral casting.

C. free from alloy material.Casting method can cast carbon steel, alloy steel, ductile iron, copper alloy and aluminum alloy castings, as well as high-temperature alloy, magnesium alloy, titanium alloy and precious metal castings.For alloy materials which are difficult to forge, weld and cut, it is especially suitable for casting by casting method.

D. High production flexibility and adaptability of strong investment casting are suitable for mass production, small batch production and even single piece production.

Disadvantages and limitations:

The casting size should not be too large.Of all the blank forming methods, the process is the most complex and the casting cost is very high. However, if the products are selected properly, the parts are designed rationally and the high casting cost is compensated for by reducing the cutting processing, assembling and saving metal materials, the investment in the investment casting has good economy.

3. Die casting

The principle of die casting process is to use high pressure to press metal liquid into a precise metal mold cavity at high speed.

Cold and hot chamber die – casting are two basic methods of die – casting process.The metal liquid in the cold chamber die casting is poured into the pressure chamber by manual or automatic pouring device, and then the punch head moves forward to put the metal hydraulic pressure into the mold cavity.In hot chamber die casting process, the pressure chamber is perpendicular to crucible, and the metal liquid flows into the pressure chamber automatically through the feed inlet on the pressure chamber.The injection punch moves downward, pushing the liquid metal through the gooseneck into the cavity.After the metal liquid solidifies, the die-casting mold opens, the casting is removed and a die-casting cycle is completed.

Advantages:

A. good quality.Casting size accuracy is high, generally equivalent to 6~7 levels, even up to 4 levels;Good surface finish, generally equivalent to 5~8 levels;The strength and hardness are higher, the strength is generally 25~30% higher than sand mold casting, but the elongation decreases about 70%.Stable size and good interchangeability;Thin-walled complex castings that can be die-cast;

B. High production efficiency.Machine productivity high, such as domestic J Ⅲ type 3 horizontal cold empty die casting machine die casting of 600 ~ 700 times on average eight hours, small hot chamber die casting machine die casting of 3000 ~ 7000 times on average every eight hours.Die – casting die – casting die – casting, die – casting bell alloy, life up to hundreds of thousands of times, even millions of times;Easy to realize mechanization and automation;

C. good economic results.Due to the precision die – casting size, surface gloss and cleaning.Generally, the machine is no longer used directly, or the processing volume is very small, so it not only improves the metal utilization rate, but also reduces a lot of processing equipment and time.The casting price is easy.Composite die casting may be used for other metallic or non-metallic materials.Save both assembly time and metal.

Disadvantages and limitations:

A. During die casting, due to the high filling rate of liquid metal cavity and the unstable flow state, general die casting method is adopted, which can easily produce pores and cannot conduct heat treatment.

B. It is difficult to die-cast complex castings with inner pits;

C. High melting point alloy (such as copper, ferrous metal), low die casting life;

D. It is not suitable for small batch production, which is mainly due to the high manufacturing cost of die-casting mold, high production efficiency of die-casting machine and uneconomical small-batch production.

Metal mold casting

Also known as hard mold casting, it is a casting method by pouring liquid metal into metal mold to obtain casting.Mold is made of metal and can be used many times (hundreds to thousands of times), also known as permanent casting.

Generally, the metal mold is made of cast iron and cast steel.The inner cavity of the casting can be used both metal core and sand core.There are many metal types, such as water bisection, bisection and composite.Vertical parting is convenient to open the gate and take out the casting;Horizontal parting is mostly used to produce thin – walled wheel castings.The upper half of the composite parting is composed of two vertical parting plates joined by hinges, while the lower half is a fixed horizontal base plate, which is mainly used in the casting of more complex castings.

Technological features of metal mold casting:

The speed of thermal conductivity of metal type is fast and non-yielding, which leads to defects such as insufficient casting, cold insulation, cracks and white mouth.In addition, the metal type is repeatedly subjected to the erosion of hot metal liquid, which will reduce the service life.

Advantages:

A. Compared with sand mold casting, metal mold casting has the following advantages:

B. Good reusability, “one type multiple casting”, saving moulding materials and time.

C. Due to the strong cooling ability of the metal mold to the casting, the casting has compact structure and high mechanical performance.

D. High dimensional accuracy and tolerance grade of casting: IT12 ~ IT14;The surface roughness was low, Ra was 6.3m.

E. Metal mold casting without sand or less sand, improving labor conditions.

Disadvantages and limitations:

The metal type has high cost, long cycle and strict process requirements, and is not suitable for the production of single small batch castings. It is mainly suitable for the mass production of non-ferrous alloy castings, such as aluminum piston, cylinder body, cylinder cover, oil pump shell, copper alloy bearing shell and axle sleeve for aircraft, automobile, internal combustion engine, motorcycle, etc.For the black alloy castings, only the middle and small castings with simple shapes are allowed.

5. Low-pressure casting

Low-pressure casting is a method for filling casting of liquid metal under low pressure (0.02 ~ 0.06MPa) and crystallizing under pressure to form casting.

Schematic diagram of low-pressure casting process: 1 — greenhouse 2 — crucible 3 — liquid raising tube 4 — gas storage tank 5 — mold

The operating principle of low-pressure casting is shown below.The melting crucible pour liquid metal heat preservation, mount airtight cover, lift catheter with mold, the metal liquid lock casting, slowly to the crucible furnace into the dry compressed air, liquid metal by the effect of gas pressure, from bottom up along the lift tube and pouring system full of cavity, and crystallization under pressure, the casting molding retreat to the pressure inside the crucible, the lift tube of metal liquid drop back to the metal surface in the crucible.Open the mold and remove the casting.

Advantages:

A. The rising speed and crystallization pressure of the metal liquid can be adjusted during casting, so it can be applied to various casting types (such as metal mold, sand mold, etc.), casting various alloys and castings of various sizes;

B. The bottom injection mold is adopted, and the metal liquid filling is stable and free of spillage, which can avoid the entrainment of gas and scour the mold wall and core.

C. The casting is crystallized under pressure, with compact structure, clear profile, smooth surface and high mechanical properties, which is particularly favorable for casting of large thin-walled parts;

D. Eliminate the feeding and shrinkage risers, and increase the metal utilization rate to 90% ~ 98%;

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

Disadvantages and limitations:

The service life of the riser pipe is short, and the metal liquid is easy to oxidize and produce slag during the heat preservation process.It is mainly used for casting some high quality aluminum alloy and magnesium alloy castings, such as cylinder block, cylinder cover, crankcase and aluminum piston of high speed internal combustion engine.

6. Centrifugal casting

Centrifugal casting is a casting method in which molten metal is poured into a rotating mold and filled with cast mold and solidified under the action of centrifugal force.

Classification of centrifugal casting

According to the position of the rotating axis of the cast mold in space, common centrifugal casting can be divided into two types:

Horizontal centrifugal casting, casting the rotation axis in the level of state or with horizontal Angle is small (< 4 °) with centrifugal casting.

Vertical centrifugal casting: vertical centrifugal casting is called vertical centrifugal casting.

Centrifugal casting with a large Angle between a cast rotary shaft and horizontal or vertical lines is called inclined shaft centrifugal casting, but it is rarely used.

Advantages:

A. When the hollow rotary body casting is produced by centrifugal casting, the mold core, casting system and risers can be omitted;

B. Due to the centrifugal force generated by the liquid metal during rotation, the denser metal is pushed to the outer wall, while the denser gas and slag move to the free surface to form directional solidification in the outward and inward direction.

C. It is convenient to pour “bimetal” bush and bearing shell. If a thin layer of copper bushing is cast inside the steel bushing, it can save copper materials at a higher price;

D. Good mold filling ability;

E. Eliminate and reduce consumption of pouring systems and risers.

Disadvantages and limitations:

A. Rough inner free surface of casting, large size error and poor quality;

B. not applicable to alloys with high density segregation (such as lead bronze) and alloys such as aluminum and magnesium.hook abrator application 6


Post time: Sep-26-2018
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