The definition of casting and the definition of classified casting: casting liquid metal into a casting cavity that is suitable for the shape of the part, after it has cooled and solidified, a forming method for obtaining a blank of a metal part having a certain shape, size and performance. The common casting methods are sand casting and precision casting. The detailed classification is shown in the table below. SAND CASTING: Sand Casting -- a method of producing castings in a sand mold. Steel, iron, and most non-ferrous alloy castings can be obtained by sand casting. Because the molding materials used in sand casting are cheap and easy to get, and the casting is easy to make, it can adapt to single piece production, batch production and mass production of casting. INVESTMENT CASTING: Investment Casting is a general term for the process of obtaining precise castings by precise moulding. Its products are precise, complex, close to the final shape of parts, can be used directly without or very little processing, is a near net shape of the advanced technology.

Common casting methods and their advantages and disadvantages. The basic raw materials for making sand mold in common sand mold casting are casting sand and mold sand binder. The most commonly used foundry sand is silica sand, silica sand high temperature performance can not meet the use of zircon sand, chromite sand, corundum sand and other special sand. The most widely used molding sand binder is clay, but also can use a variety of dry oil or semi-dry oil, water-soluble silicate or phosphate and various synthetic resin as molding sand binder. According to the binder used in molding sand and the way of setting up its strength, the outside sand used in sand casting can be divided into three kinds: clay wet sand mold, clay dry sand mold and chemical hardening sand mold.

Sand casting is the most popular and simplest type of casting and has been used for centuries. Sand casting is used to make large components, such as gray iron, ductile iron, stainless steel and other types of steel processes such as sand casting. The main steps include painting, molding, core making, molding, melting and pouring, cleaning and so on. Process Allowance: The so-called Process Allowance, that is, the surface of the casting that needs to be machined, should set aside a certain amount of processing allowance in advance, its size depends on many factors, such as the type of casting alloy, molding method, casting size and the position of machining surface in the mold. Pitch: In order to make the pattern easy to remove from the mold, the vertical wall perpendicular to the parting surface is added pitch pitch. Casting Fillet: In order to prevent the casting from stress and crack at the joint and corner of the wall, to prevent the sharp corner of the mold from damage and sand hole, the casting wall and corner should be designed as a fillet when designing the casting. Core head: in order to ensure the core in the casting positioning, fixation and exhaust, pattern and core to design the core head. Shrinkage allowance: due to the cooling shrinkage of the casting after pouring, this portion of the shrinkage dimension is added to the pattern.

Advantages: rich in clay resources, low price. Most of the used green clay sand can be recovered and reused after being treated with proper sand. The moulding period is short and the working efficiency is high. The mixed green sand can be used for a long time, disadvantages and limitations: Because each sand mold can only be cast once, the mold is damaged after obtaining the casting and must be re-molded, so the productivity of sand mold casting is relatively low; The casting is easy to produce defects such as sand washing, sand inclusion and air hole. 2. Investment casting with wax pattern, investment casting also known as ‘Lost Wax Casting’. In investment casting, a pattern is usually made of a fusible material, covered with several layers of Refractory on the surface of the pattern, and then the pattern is melted out of the pattern, thus obtaining a mold with no parting surface, casting scheme of filling sand and pouring after high temperature roasting. Because the pattern widely uses the wax material to manufacture, therefore often calls the investment casting “Loses the wax casting”. The alloys available for 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.

Advantages: high dimensional accuracy. Generally up to CT4-6(SAND CASTING FOR CT10 ~ 13, DIE CASTING FOR CT5 ~ 7) , can improve the utilization of metal materials. Investment casting can significantly reduce the amount of processing on the forming surface and mating surface of the product, save the processing time and the consumption of cutting tool materials, and maximize the similarity between the blank and the parts, it brings great convenience to the structural design of parts. Investment casting casting with complicated shape can produce very complicated shape casting, also can cast the casting with thickness of 0.5 mm and weight less than 1G, also can cast combined and whole casting, not limited by alloy material. Investment casting can cast carbon steel, alloy steel, ductile iron, copper alloy and aluminum alloy castings, but also cast high temperature alloy, magnesium alloy, titanium alloy and precious metals and other materials casting. For Alloy materials that are difficult to forge, weld and cut, investment casting is particularly suitable; flexible and adaptable investment casting is suitable for mass production, small batch or even single piece production. Disadvantages and limitations: Casting size can not be too large process complex casting cooling speed. Investment casting is the most complex of all the blank forming methods, and the casting cost is very high. However, if the products are properly selected and the parts are properly designed, the high casting cost is compensated by reducing cutting, assembling and saving metal materials, so the investment casting has good economy.

3. The principle of die casting technology is to use high pressure to press liquid metal into the cavity of a precision metal mold at high speed.

Cold and Hot Chamber Die Casting are two basic ways of die casting technology. In Cold Chamber Die Casting, liquid metal is poured into the chamber by hand or automatic pouring device, and then the injection punch moves forward to press the liquid metal into the cavity. In the hot chamber Die Casting Process, the Die Chamber is perpendicular to the crucible, and the metal liquid flows into the die chamber automatically through the feed hole on the die chamber. The injection punch moves downward, pushing the liquid metal through the gooseneck tube into the cavity. After the liquid metal solidifies, the die casting mould is opened and the casting is taken out to complete a die casting cycle.

Advantages: good product quality. High precision casting size, generally equivalent to 6 ~ 7, or even up to 4; good Surface finish, generally equivalent to 5 ~ 8; High Strength and hardness, strength generally than sand casting increased by 25 ~ 30% , but elongation, reduced by about 70% ; dimensional stability, good interchangeability; die-casting thin-walled Complex Castings; high production efficiency. For example, the domestic J III 3 horizontal Cold Air die-casting machine can die-cast 600 ~ 700 times in 8 hours on average, and the small hot chamber die-casting machine can die-cast 3000 ~ 7000 times in 8 hours on average, it has a life span of one hundred thousand or even one million times; it is easy to mechanize and automate; and it has excellent economic results. Due to the precision of die casting size, surface pan-finish and other advantages. Generally no longer mechanical processing and direct use, or processing volume is very small, so not only improve the metal utilization, and reduce a large number of processing equipment and man-hours; Combination die casting can be used with other metal or non-metal materials. It saves both assembly time and metal.

Defects and limitations: Due to the high speed of liquid metal filling cavity and unstable flow state, the general die casting method is adopted, which can not be heat treated because of the easily produced blowhole High melting point alloy (such as copper, Ferrous) , die-casting die life is low; it is not suitable for small batch production, the main reason is that die-casting die manufacturing cost is high, die-casting machine production efficiency is high, small batch production is not economic. 4. Metal Mold Casting, also called hard mold casting, is a casting method of pouring liquid metal into metal mold to obtain castings. Mold is made of metal, can be used many times (hundreds to thousands of times) , also known as permanent mold casting. Of a general construction, the mold is made of cast iron and cast steel. The inner cavity of the casting can be made of either metal or sand cores. The structure of the metal type has many kinds, such as horizontal type, heavy straight type and compound type. Among them, the vertical parting is easy to open the Ingate and take out the castings; the horizontal parting is often used to produce the thin-walled wheel-shaped castings; the upper half of the compound parting is made of the two halves of the vertical parting connected by hinges, the lower half is a fixed horizontal base plate, which is mainly used in the casting of more complicated castings.

The technological characteristics of permanent mold casting: The rapid heat conduction and non-yielding of permanent mold make the casting easy to produce defects such as insufficient pouring, cold shut, crack and white. In addition, the metal repeated hot metal liquid erosion, will reduce the service life, should adopt the following auxiliary process measures. PREHEATING METAL MOLD: preheating metal mold before casting, can slow down the cooling capacity of the mold, conducive to the filling of molten metal and, cast iron graphitization process. PREHEAT CAST IRON TO 250 ~ 350 °C in Metal Mould and 100 ~ 250 °C in Non-ferrous metal. BRUSH PAINT: In order to protect the metal type and convenient exhaust, usually in the surface of the metal type spray brush refractory coating layer, so as to avoid metal, type of direct metal liquid erosion and thermal effect. Because adjusting the coating layer thickness can change the cooling rate of each part of the casting, and is conducive to the gas discharge in the metal mold. Different alloys should be sprayed with different coatings. In the case of Aluminium Alloy Castings, a coating made of zinc oxide powder, talcum powder and water glass shall be sprayed; in the case of Grey Iron Castings, a coating made of graphite powder, talcum powder, fireclay powder, peach glue and water shall be applied. CASTING: the thermal conductivity of the metal mold is strong, so the casting temperature of the alloy should be 20 ~ 30 °c higher than that of the sand mold. Generally, aluminum alloy is 680 °C ー740 °C, cast iron is 1300 °C ー1370 °C, tin bronze is 1100ー1150 °C. Upper limit for thin-walled parts and lower limit for thick-walled parts. The Wall thickness of the Iron Castings shall not be less than 15mm to prevent the white structure.

Open Mold: The later the open mold, the greater the shrinkage of the casting in the metal mold, take out the use of difficult, and easy to produce large casting, the internal stress and cracks. In general, the casting temperature is 700 ~ 950 °C, and the casting time is 10 ~ 60 seconds after pouring. Advantages: compared with sand mold casting, metal mold casting has the following advantages: good reusability, can be “One type of multi-casting”, saving molding materials and molding time. Because of the strong cooling ability of the metal mold to the casting, the structure of the casting is compact and the mechanical properties are high. The castings have a high dimensional accuracy, tolerance class IT12 ~ IT14, and a low Surface roughness Ra of 6.3 m. No or little sand is used in metal mould casting, which improves the working conditions. Disadvantages and limitations: METAL MOLD manufacturing costs high, long cycle, strict process requirements, not suitable for the production of single small batch castings, mainly for non-ferrous alloy castings, mass production, such as aircraft, cars, internal combustion engines, motorcycles and other Aluminum Piston, cylinder block, cylinder head, oil pump shell and copper alloy bearings, bushings and so on. For Black Alloy castings, also limited to the shape of the relatively simple medium and small castings. 5. Low pressure casting low pressure casting is a method that liquid metal is filled with mold at low pressure (0.02 ~ 0.06 MPA) and crystallized at pressure to form a casting.

PRINCIPLE DIAGRAM OF LOW PRESSURE CASTING PROCESS: 1. The working principle of low pressure casting process is as follows: 1. The molten metal is poured into an insulated crucible, a sealing cap is fitted, a riser connects the molten metal to the mold, the mold is locked, and the molten metal is slowly Compressed air into the crucible, the mold cavity is filled from bottom to top along the riser and gating system, and crystallized under pressure. The pressure in the crucible is removed after the casting is molded, and the liquid metal in the riser is returned to the liquid metal level in the crucible. Open the mold and remove the casting.

Advantages: The rising speed and crystallization pressure of liquid metal can be adjusted during pouring, so it can be used for casting various alloys and castings of various sizes in various moulds (such as metal molds, sand molds, etc.) , no splash phenomenon, can avoid entrainment of gas and the erosion of the mold wall and core, the casting has fewer defects such as air hole and slag inclusion, which increases the qualified rate of the casting, high Mechanical Properties, especially for the large thin-walled casting, without feeding riser, the metal utilization rate can be increased to 90% ~ 98% , low labor intensity, good working conditions, simple equipment, easy to realize mechanization and automation. Disadvantages and limitations: lift pipe life is short, and in the heat preservation process of metal oxidation and easy to produce slag. Mainly used to cast some high-quality aluminum and magnesium alloy castings, such as cylinder block, cylinder head, crankcase and high-speed internal combustion engine Aluminum Piston and other thin-walled parts. 6. Centrifugal casting is a casting method in which liquid metal is poured into a rotating mold and then solidified by filling the mold under the action of Centrifugal Force.

Centrifugal casting can be divided into two kinds according to the position of the rotating axis in space: Horizontal Centrifugal Casting: when the rotating axis is horizontal or the angle between the rotating axis and the horizontal axis is very small (& Lt; 4 °) . Vertical Centrifugal Casting: when the rotating axis of the mold is in a vertical state, the centrifugal casting is called vertical centrifugal casting. Centrifugal casting with a large angle between the mold rotating axis and the horizontal and vertical axis is called inclined axis centrifugal casting, but it is seldom used.

1,16—— Ladle 2,14 -- Mold 3, 13. As a liquid metal 4-stage and 5-stage casting 7-stage electromotor 8-stage casting system 9-type 10-type core 11-type 12-type 15-type casting 17-end cover advantages: when hollow revolving casting is produced by centrifugal casting, the core, gating system and riser can be dispensed with; as a result of the centrifugal force generated during the rotation of the liquid metal, the dense metal is pushed to the outer wall, while the less dense gas and slag move toward the free surface, it is easy to cast “BIMETAL”Bush and Bush, such as casting a thin layer of copper Bush in the Steel Bush, which can save the more expensive copper material Good filling capacity; eliminate and reduce the consumption of gating system and riser. Defects and limitations: Casting inside the free surface rough, size error, poor quality; not suitable for density segregation of large alloys (such as lead bronze) and aluminum, magnesium and other alloys.

The application of non-destructive testing (NDT) in casting is more and more extensive. Fluorescent magnetic powder is often used to detect surface cracks of important parts, the nodularization rate of ductile iron is measured by ultrasonic or audio frequency, and the Matrix structure (pearlite content) of casting is measured by eddy current. In order to meet the requirements of important parts detection, some of the above three detection instruments are combined into an automatic detection line. 10. The development trend of green foundry technology “Green foundry”is to make the whole life cycle of foundry products from design, manufacture, packing, transportation, use to end-of-life treatment, which has the least negative impact on the environment and the highest resource efficiency. Foundry Industry has always been regarded as high energy consumption, high pollution industry, to continue to develop new energy-saving, clean, low-emission, low-pollution foundry materials to put into production. 

Source: Heat Treatment ECOSPHERE

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