Design and principle of operation of a centrifugal pump
What is a centrifugal pump? The centrifugal pump (see Fig. 27) is a snail-shaped casing in which on an axis with a speed of 500-3000 rpm. The bladed impeller rotates quickly.
Water entering through the suction line through a side opening (pipe) is captured by the blades, set into rotational motion and, thanks to the developing centrifugal force, expelled (driven) from the pump housing along the discharge line at a certain speed and a certain pressure.
At the same time, new portions of water enter through the suction pipes and thus a continuous supply of water is obtained. The location of the suction and discharge branches (pipes) may be different. The pump can have water inlet not only on one side of the wheel, but also on both sides, then a pump with two-way water inlet is obtained.
Rice. 27. Centrifugal pump:
2 - pressure gauge on the discharge pipeline;
3 — tap for filling the pump;
4 — pressure gauge on the suction pipeline;
5 - blade wheel.
Centrifugal pumps are divided into:
— by the number of wheels: single-wheeled or single-stage, multi-wheeled or multistage;
according to the created pressure:
a) low-pressure - with a pressure of up to 20 mm. water Art.;
b) medium-pressure - with a pressure of 20-60 mm of water column;
c) high-pressure - with a pressure of more than 60 mm of water column;
according to the housing connector method:
a) with a horizontal connector;
b) with a vertical connector;
according to the method of supplying water to the wheel:
a) with one-way liquid supply;
b) with two-way liquid supply.
Cantilever centrifugal pumps are manufactured in two modifications:
K - with a horizontal shaft on a separate stand; KM - with a horizontal shaft, monoblock, with an electric motor. Pumps type K and KM are designed for pumping water, as well as other liquids similar to water in specific gravity and viscosity, with temperatures up to 85 ° C and the content of mechanical impurities up to 0.2 mm in size in an amount of no more than 0.1% by weight. volume.
How are centrifugal pumps classified depending on the height of water rise? Depending on the height of water rise, pumps are (conditionally) divided into three groups: low pressure, supplying water to a height of approximately 15 m; medium pressure for supply to a height of approximately 35-40 m and high pressure, lifting water to great heights.
High-pressure centrifugal pumps are usually made with multi-wheels - multi-stage, that is, several impellers are located in one housing sequentially one after the other and are surrounded by guide vanes. Water enters the first wheel through the suction pipe, is carried away by it, goes through the outlet channel to the second wheel, etc.
until it enters the discharge pipe.
What determines the performance of a centrifugal pump? The performance of a centrifugal pump depends on the rotation speed of the impeller and is directly proportional to the rotation speed. If we denote productivity through the letter Q , and the rotational speed of the impeller through the letter n , then we can write
Thus, when the pump speed doubles, the amount of water it supplies also doubles; when the speed increases threefold, the amount of water supplied increases three times, etc.
What fittings and instrumentation are installed on a centrifugal pump? On a centrifugal pump, as a rule, a foot valve and a shut-off device are installed on the suction line; on the discharge line there is a check valve and a shut-off device, as well as a valve for filling the pump with water before starting it and a pressure gauge.
What is the procedure for starting a centrifugal pump? The procedure for starting a centrifugal pump is as follows: inspect the pump, check for the presence of oil in the bearings, then fill the pump and receiving line with water (if it operates on suction), and then check the valve on the pressure pipeline. If the valve on the pressure pipeline is open, then it should be closed before starting, since the pump is started with the valve closed.
Next, you need to check the oil level in the bearings and add oil if necessary. Then turn on the pump. When the pump reaches normal speed, slowly open the valve on the discharge line. When stopping a centrifugal pump, you must first close the shut-off device (valve on the discharge line), and then turn off the electric motor that rotates it.
What is monitored during operation of a centrifugal pump? During operation of the centrifugal pump, monitor the readings of the pressure gauge installed on the discharge line; the condition of the pump bearings; for the electric motor ammeter readings; Check the condition of the pump seals and, if necessary, carefully tighten them slightly.
Centrifugal pump
Centrifugal pumps are one of the most common types of equipment for pumping liquids (and gases). With their help, water is pumped out of wells and boreholes, raised to a considerable height and delivered over long distances through pipes.
Such pumps pump coolant in heating systems and process fluids in industries.
The idea of using centrifugal force to pump liquids belongs to Leonardo da Vinci; the first working examples were created by the French engineer and scientist Denis Papin at the end of the 17th century.
Design features and principle of operation
The design and operating principle of a centrifugal pump have not fundamentally changed since the 17th century. The pump consists of the following parts and assemblies:
Source: https://moy-instrument.ru/masteru/ustrojstvo-i-printsip-dejstviya-tsentrobezhnogo-nasosa.html
Special casting
According to various estimates, foundry technologies are used to produce more parts and workpieces. There is more than one type of foundry technology that makes it possible to produce workpieces of different shapes, sizes, precision and made from various materials.
Scheme of special precision castingZinc sand casting method
There are the simplest technologies for producing blanks, for example, sand casting, and quite complex ones, in particular, especially special precision (precision).
It allows you to create parts that do not require further mechanical or any other processing.
Types of special casting
Casting metal into sand (earth) is associated with certain difficulties; in particular, such production requires a large turnover of molding material.
In addition, the use of this casting method does not always lead to the production of workpieces of the required quality. The development of metallurgical science has led to the emergence of new, special methods of casting metals.
Special methods include pouring metal into molds made of metal, casting into molds made in the form of shells, casting under the influence of centrifugal force and some others.
The main advantage of these special casting methods is that metallurgists began to obtain high-quality parts, reduced the amount of substandard products, and increased production productivity. Of course, the special casting methods put into operation have a positive impact on improving the working conditions of workers and engineers.
Let's take a closer look at some of these special methods.
Die casting
Pouring into molds made of metal. The essence of this special method is that castings are obtained by pouring melt into metal molds. Such forms are called molds. They are manufactured in two versions - detachable and one-piece.
The first ones consist of several parts; these molds are used to produce complex-shaped castings.
One-piece molds are used for the production of simple castings, etc. For metal molds, cast iron of the SCh grade or alloy steel alloys are used.
The durability of the die is directly influenced by the materials, dimensions of the casting and, of course, the die.
Chill casting process
Engineers have developed and successfully use in practice special methods for extending the operating time of the mold and improving the quality of castings. To do this, special compounds are applied to the working surface of the mold, forming a coating that is resistant to temperature effects from the melt. These materials are applied either using a spray gun or an ordinary brush. For cast iron, it is necessary to apply the lining several times per shift. The paint is applied immediately before pouring the cladding.
Technological process of chill casting
To obtain internal cavities, rods made of steel grade U7 and its analogues are used. In addition to rods made of steel, products made of special sand are also used. Special casting of this type can only be performed in heated equipment.
The operating temperature of the chill mold should be between 200 and 300 degrees Celsius. Warming up the mold will reduce the effect of thermal shock, and during casting there will be no ejection of the melt, which can occur when it enters a cold mold.
Chill casting is used to produce castings from non-ferrous alloys.
Serial and mass production of castings is carried out on equipment that independently, without human intervention, services casting molds, mounts and dismantles cores, and removes castings.
Casting using mechanized equipment allows you to increase production productivity several times.
Meanwhile, chill casting also has a number of disadvantages.
In particular, the production of molds requires a large amount of time, technological difficulties arise when obtaining castings with thin walls, and several others.
Lost wax casting
This method of special casting implies that the castings are obtained in disposable molds, which are obtained by melting models made of low-melting materials, on the surface of which fire-resistant coatings are applied.
This casting method is effective for producing small parts with complex shapes. Moreover, this method is suitable for working with any metals. It is used to produce castings of complex configurations with thin walls.
Lost wax casting
For the production of models, paraffin, wax, etc. are used. Most often, a mixture of paraffin and stearin is used.
To make models, the resulting composition is pressed into molds made of metal. In addition, it is possible to use manual syringes. In this type of production, making castings for one part does not make sense. Therefore, such models are assembled into blocks that have one common riser.
To connect models, feeders (gating passages) are used. The use of such structures increases labor efficiency; in addition, the use of a single gating system leads to material savings. The refractory shell is formed by dipping a block with models into a bath filled with a coating made using ceramics.
After dipping, the blocks are sprinkled with quartz sand and sent to dry, which lasts from 4 to 5 hours. Models are melted in a bath with water heated to a temperature of 70 - 85 degrees. This approach implies that this composition will be returned to re-production in almost its entirety. Forms coated with the composition are calcined at 900 - 950 degrees.
The metal melt is poured immediately after calcination.
The centrifugal filling method is often used. As the shell cools, it cracks and is removed by vibration.
The resulting castings are sent to the machine shop to remove the sprues and flash.
Special pattern casting is used to produce parts with increased precision; its use significantly improves the quality of castings. But the entire casting process of this type is quite rightly considered lengthy and complex. This affects the cost of work performed.
Shell casting
Castings can be obtained using shells with a wall thickness of 8 – 15 mm. For their manufacture, special compounds are used that harden under the influence of heat from models and cores.
Shell casting
Quartz sand is used as the material for the mold. It plays the role of a filler and resin, as a binding component. This mixture is applied to the plate with the model installed on it. Before applying the model mixture, it is necessary to coat the model with a silicone-based emulsion. The result will be a shell model.
This type of technology is used to produce castings weighing 12 – 15 kg. The obvious advantages of this technology include the quality of the castings produced, but at the same time, these forms are characterized by high cost. This is due to the fact that expensive chemicals are used in their production.
By the way, casting into thin-walled one-time molds is a variation of this metal pouring technology.
Centrifugal casting
Special types of casting are not limited to the methods given above. With the centrifugal method of special casting, the melt fills the mold under the influence of centrifugal forces. They occur when the injection mold rotates around an axis, which can be located vertically, horizontally or at an angle to the horizontal.
Centrifugal casting
With this casting method, the inner surface of the casting is formed without coming into contact with the working surface of the mold, and therefore it is called free.
This special technology assumes that the injection molds are made of metal.
Before starting to pour the melt, it must be heated to a temperature of 250 - 350 degrees, then a fire-resistant coating is applied to the working surfaces.
The use of centrifugal processing of the melt makes it possible to obtain the density of the cast metal, the absence of voids and cavities in the body of the resulting part. Under the influence of centrifugal forces.
Centrifugal melt processing allows the production of the following types of parts:
- bushings;
- drums;
- rotor housings and much more.
Centrifugal casting allows for high metal density and the absence of cavities.
As a result, the parts have increased wear resistance. In addition, centrifugal forces displace foreign inclusions and slags from the melt.
Cork or shell casting
Among the special types of casting, there is also one - shell. It is used when working with both non-ferrous and ferrous metals. Casting is performed in shell (crust) molds made from a mixture of quartz sand and bakelite resin, mixed in a ratio of 90 to 10. Bakelite resin polymerizes at a temperature of 300 - 350 degrees.
Casting into cortical (shell) molds
The mixture is applied to the surface of the model, which has been preheated to 220 degrees.
The resin melts and binds the grains of sand together. As a result, a crust 5–7 mm thick is formed on the surface of the model. The mixture hardens when the model is heated to a temperature of 350 degrees. Then, using various devices, the mold is removed from the model. The main advantage of this type of mold is the reduction of allowances and the high accuracy of the resulting castings.
Precision casting
Precision casting is a special method for producing particularly precise castings.
Precision casting equipment
Previously, this method was called lost-wax casting. To perform work using this technology, various substances are used that give the molten metal a number of properties that allow it to accurately fill the mold.
In addition, to perform special casting, molds made of metal and made with increased accuracy requirements are used.
Knockout, shoeing, cleaning and casting control
After the end of the special casting process and the workpiece has cooled to an acceptable temperature, it is removed from the mold and, if necessary, sent to be cleaned of sprues, flash, etc. In addition, quality control of the resulting parts is performed.
The main document on the basis of which quality control of the resulting product is carried out is the working drawing.
Technical control of casting
In addition, technical control service employees must be guided by the requirements of GOST, OST, STP and other regulatory documents, one way or another, related to metal casting.
Source: https://stankiexpert.ru/spravochnik/litejjnoe-proizvodstvo/specialnoe-lite.html
Centrifugal casting technology
Centrifugal casting is a special technology that uses centrifugal forces to form castings by freely pouring molten metal into rotating molds. Using centrifugal casting technology, castings are made from steel, cast iron, as well as alloys based on aluminum, copper, titanium, magnesium, zinc and some other metals.
Since with this method of forming castings, centrifugal forces act on the metal or alloy, which is the starting material, the finished products acquire increased density and improved mechanical characteristics.
Horizontal centrifugal casting
To produce castings using the centrifugal casting method, centrifugal machines with vertical and horizontal axes of rotation are used. They are equipped with investment casting molds and shell molds.
Centrifugal casting with horizontal axis of rotation
This method is the most widely used centrifugal casting method. It consists in the fact that the formation of a casting with a free surface occurs in a field of centrifugal forces. In this case, the inner surface of the mold plays the role of a forming surface.
The melt is poured into the mold from the ladle through a special pouring chute. During the production process, the melt spreads over the inner surface of the mold, and under the influence of centrifugal forces it forms a hollow cylinder.
After the metal or alloy has hardened, the mold stops and the finished product is removed from it.
Centrifugal casting with vertical axis of rotation
In order to obtain a casting on machines that rotate the mold around a vertical axis, molten metal or alloy is poured from above into a rotating mold through a hole located along the axis of rotation of the spindle.
During rotation, the metal or alloy, under the influence of centrifugal forces, tends to the side walls of the mold (mold). Rotation is carried out until the mold hardens completely. As soon as this happens, the mold stops and the casting is removed from it.
One of the characteristic features of those castings that are produced on machines with vertical axes of rotation is that the thickness of their walls is uneven in height: in the lower part they are thicker. This is largely why castings with a small overall height are usually made in this way: rings, flanges, short bushings.
Advantages and disadvantages of centrifugal casting
Using centrifugal casting, it is possible to obtain a casting with a geometrically correct free surface only if the rotation speed is strictly defined (it is determined by such an indicator as the gravitational coefficient). If the rotational speed of the casting is insufficient, then due to shrinkage, both in the vertical and horizontal position of the axis, distortions inevitably occur.
Thus, it can be stated that one of the advantages of centrifugal casting is that it can significantly improve the fillability of the mold with the melt, since it is subject to increased pressure arising under the influence of centrifugal forces. In addition, fewer shells, pores, and various inclusions are formed in castings, and their density increases significantly.
It should also be noted that the advantage of this casting method is also a reduction in metal consumption and an increase in such an indicator as the yield, due to the absence of a gating system. In addition, when centrifugally casting parts and workpieces in the form of pipes and bushings, there is no need to incur costs for technological rods.
As for the disadvantages, centrifugal casting also has them. These include the difficulty of making castings from those alloys that are prone to segregation; inaccuracy in the dimensions of the cavity of castings having free surfaces; increased contamination of casting surfaces with liquids and non-metallic inclusions (because of this, it is necessary to significantly increase the allowances for their mechanical processing).
Source: http://gk-drawing.ru/line-module/casting/centrifugal-casts.php
Centrifugal casting
There are different ways to create castings from non-ferrous and ferrous metals, the three most popular are:
- die casting;
- pouring into sand molds;
- centrifugal casting.
In the first case, pouring is done into collapsible metal molds. In the second - into sand forms. Finally, in the third case, pouring is used into metal forms rotating around a vertical or horizontal axis.
Each option has its own advantages and disadvantages, so the choice of a specific method is made based on the characteristics of the part being cast. Centrifugal casting is used for the production of rotating bodies - pipes, disks, bushings, etc. The main advantages of this method are the speed of casting and economical consumption of metal (there are no gates). In addition, centrifugal castings are of good quality.
Our company carries out centrifugal casting of workpieces with a diameter from 60 to 650 mm, the maximum weight of castings is up to 1 ton. Advantages of working with us:
- wide range of castings;
- high quality casting;
- fast order fulfillment;
- favorable rates.
Production of castings by centrifugal casting
Centrifugal metal casting is one of the most convenient and profitable methods for producing rotating bodies. In a rotating form, under the influence of centrifugal forces, the metal is pressed against its surface, taking the required shape and maintaining it during the cooling process.
This method is used with a wide variety of metals, even centrifugal casting of stainless steel is possible. Centrifugal casting of cast iron is also popular, making it possible to obtain castings that require minimal subsequent processing. But this method is most in demand when working with non-ferrous metals. In particular, we perform centrifugal casting of bronze and brass.
It is possible to produce a wide variety of castings ; one of the most popular options is centrifugal casting of pipes. In addition, we can produce a variety of bushings, disks, shafts, bearings, liners, flanges, and any other rotating bodies. Centrifugal bronze casting makes it possible to obtain high-quality castings in any required quantity and in the shortest possible time, which makes this technology very attractive.
In some cases, centrifugal injection molding may be used to produce the highest quality castings. Such castings have increased strength; they are free of cavities, slag and other inclusions.
Centrifugal casting at competitive prices!
You can always use our services and order the production of the castings you are interested in using the centrifugal casting method. We use aluminum and tin bronzes and brass for casting. All products we manufacture from non-ferrous alloys are certified and fully comply with GOST requirements.
We guarantee the high quality of our products, fast fulfillment of your order and very competitive prices. We work in Yekaterinburg, you can place an order for the production of the castings you need right now!
- 8 (800) 222-79-79 (multi-channel)
- (343) 345-79-79 (multi-channel)
Source: http://promts.su/centrobejnoe_lite
Centrifugal casting
Production LLC "MASHTECHTSENTR" specializes in the production of castings obtained by centrifugal casting and finished products based on them for enterprises in the papermaking, metallurgical, engineering, petrochemical industries, as well as for the energy and oil and gas complex.
Available capacities make it possible to produce centrifugally cast pipes from carbon, alloy, stainless, heat-resistant, heat-resistant steels and alloys and cast iron with an outer diameter from 73 to 1500 mm, a length of up to 8500 mm, a wall thickness of 7 to 140 mm and a weight of up to 27 tons.
The basis of the nomenclature series is:
- Reaction pipes intended for the manufacture of coils of tube furnaces for ammonia, hydrogen, ethylene and carbon disulfide production plants, as well as braided pipes welded from heat-resistant pipes used in the equipment of petrochemical plants;
- Pipes for deep oil refining units (hydrocracking, hydrotreating, catalytic cracking, etc.);
- Pipes for steam pipelines for various purposes (energy, oil refining, etc.);
- Radiant tubes (U-shaped, W-shaped, dead-end);
- Kiln rollers and continuous caster rollers;
- Retorts of various modifications;
- Jackets of rollers producing paper and cardboard;
- Various types of bushings;
- Sleeves for compressors, presses and other equipment;
- Blanks for crane drums.
And more than a dozen items of various technological equipment based on pipe blanks in cast form or with mechanical processing on the outer and (or) inner surface for a variety of industries.
Centrifugal casting method
Centrifugal casting is a method for producing metal parts for various purposes.
Its distinctive feature is that the filling of the mold with the melt and its solidification occur strictly directionally in the field of action of centrifugal forces. The cast metal is very dense, especially in the outer layers.
The centrifugal casting method provides high mechanical properties, additional product life, minimal allowances for machining and reduced costs.
Centrifugal casting method
The essence of the centrifugal casting method is that in a rotating vertical or horizontal plane of a cylindrical mold, under the influence of centrifugal forces, the internal and external outlines of the casting are formed.
Using the centrifugation method, shaped castings are produced in molds located on a rotating faceplate. Under the action of centrifugal forces, the mold is filled with melt entering through the vertical center riser and the horizontal feeders communicating with it.
Casting calculator
Source: https://www.mashtc.ru/produktsiya/otlivki/tsentrobezhnoe-lityo/
Centrifugal casting. The essence of the method. Basic operations and scope of use
The principle of centrifugal casting is that the filling of the mold with the melt and the formation of the casting occur when the mold rotates around a horizontal, vertical or inclined axis, or when it rotates along a complex trajectory. This achieves an additional effect on the melt and solidifying casting of the field of centrifugal forces. The process is implemented on special centrifugal machines and tables.
Most often, two variants of the method are used, in which the melt is poured into a mold with a horizontal or vertical axis of rotation. In the first option, castings are produced - bodies of rotation of small and large lengths, in the second - bodies of rotation of short lengths and shaped castings.
The most common method is to cast hollow cylindrical castings into metal molds with a horizontal axis of rotation. According to this method (Figure 6.1), casting 4 is formed in a field of centrifugal forces with a free cylindrical surface, and the inner surface of the mold serves as the forming surface.
Melt 1 from ladle 3 is poured into a rotating mold 5 through a pouring chute 2. The melt spreads over the inner surface of the mold, forming a hollow cylinder under the action of a field of centrifugal forces. After the metal has solidified and the mold has stopped, the casting 4 is removed.
This method is characterized by the highest technological yield (TVG = 100%), since there is no metal consumption for the gating system.
Figure 6.1 – Scheme for obtaining a casting by rotating the mold around a horizontal axis: 1 – melt; 2 – filling chute; 3 – ladle; 4 – casting; 5 – form
When producing castings with a free parabolic surface when the mold rotates around a vertical axis (Figure 6.2), the melt from ladle 1 is poured into mold 2, mounted on a spindle 3 driven by an electric motor 4. Melt 5, under the influence of centrifugal and gravitational forces, is distributed along the walls of the mold and hardens, after which the rotation of the mold is stopped and the hardened casting 6 is removed from it.
Figure 6.2 – Scheme for producing castings by rotating the mold around a vertical axis: 1 – ladle; 2 – shape; 3 – spindle; 4 – electric motor; 5 – melt; 6 – casting
Castings with an internal surface of complex configuration are produced using rods (Figure 6.3, a) in molds with a vertical axis of rotation. This is how, for example, the crowns of gear wheels are cast.
The melt from the ladle through the filling hole and riser 1 enters the central cavity of the mold 2, made by rods 3 and 4, and then under the action of centrifugal forces through slot feeders into the working cavity of the mold.
In this case, the excess metal in the central cavity of mold 5 acts as a profit, providing power to the casting during solidification.
Small shaped castings can be produced by centrifugal casting in sand molds (Figure 6.3, b). Mold parts 1 and 2 are installed on a centrifugal table and secured to it. If necessary, use rods 4. Working cavities 3 must be located symmetrically relative to the axis of rotation to ensure balancing of the shape.
The melt is poured through a central riser, from which it enters the mold cavities through radial channels. The technological yield of castings using this method approaches the yield of castings in sand molds.
With centrifugal casting, you can use sand, metal, shell and volumetric ceramic, as well as combined molds.
Figure 6.3 – Scheme for obtaining shaped castings: a – gear crown: 1 – riser; 2 – central cavity of the mold; 3 and 4 – rods; 5 – profit; b – small shaped castings: 1 – lower half-mold; 2 – upper half-form; 3 – working surface of the mold; 4 – rod
Features of casting formation
The peculiarity of the formation of castings using the centrifugal casting method is that the filling of the mold with metal and the solidification of the casting occur in the field of action of centrifugal forces, many times greater than the force of gravity.
Under these conditions, if solid particles come into contact with the wall of the mold, they are pressed against the wall and no longer float up. This is the basis for the use of bulk coatings for metal molds during centrifugal casting.
The action of centrifugal forces must also be taken into account when designing slag retention and casting feeding systems, for example, when producing steel shaped castings by centrifugal pouring into sand molds.
Features of cooling and solidification of castings in the field of centrifugal forces
When producing castings with a free surface, the melt is cooled unevenly in volume in the mold. Part of the heat is removed from the melt through the wall of the mold and its lid, and part is removed by convection and radiation from the free surface.
The amount of heat transferred into the air space from the free surface of the casting is significant. The air in the casting cavity is involved in the rotation process and is in constant motion. Along the axis of rotation, portions of cold air replace the heated air.
More intense air circulation is observed in the case of rotation of the mold with the melt around a vertical axis due to the natural rise of hot air upward.
Such uneven cooling, especially of thick-walled castings, leads to the emergence of convective flows in the melt: the cooled and denser melt moves from the free surface into the solidifying casting, and the hot and less dense melt moves outward. Therefore, convective flows in the melt circulate in the radial direction (Figure 6.4, a).
Under centrifugal casting conditions, this phenomenon is observed even with a small difference in temperature and metal density, since the forces acting in this system increase in proportion to the magnitude of the gravitational coefficient.
This promotes directional solidification of the casting in the radial direction, which is more pronounced the greater the angular velocity of rotation of the mold.
During directional solidification from the walls of the mold, the front of the crystals growing in the radial direction is under significant excess pressure of the melt, caused by the action of the field of centrifugal forces. As a result, the crystals grow in the direction of the incoming melt (Figure 6.4, b), so they are slightly inclined to the side in the direction of rotation.
The pressure developed during rotation of the melt promotes its penetration into the intercrystalline spaces, which improves the nutrition of the solidifying casting and increases its density. The free surface of the melt solidifies last, and with a horizontal axis of its rotation, the shape of the free surface remains geometrically correct - cylindrical.
Figure 6.4 – Scheme of the occurrence of convective flows (shown by curly arrows) in a rotating solidifying melt (a) and diagram of the crystalline structure of the casting (b): l0 – depth of location of shrinkage porosity; The arrow shows the direction of rotation of the mold.
Foreign particles (gases, slag, etc.), the density of which is less than the density of the melt, float to the free surface of the melt at high speed during centrifugal casting. This leads to the need to assign large allowances for processing the free surfaces of castings, which is a disadvantage of this casting method.
Thus, with directed solidification, it is possible to obtain castings with a dense body structure, without shrinkage defects and foreign inclusions. However, centrifugal forces contribute to directional solidification only in cases where the crystals of the solid phase released on the free surface have a higher density than the density of the rest of the melt.
For most cast alloys this condition is met. There are two exceptions:
- when the alloy hardens with increasing volume, for example, gray cast iron;
- when the crystals released from the liquid metal are enriched with alloy components that have a lower density than the remaining melt. This phenomenon is observed, for example, during the solidification of hypereutectic silumins. In this case, when the silicon content in silumins is more than 11.7%, the primary crystals are enriched with silicon, the density of which is less than the density of aluminum. If these lighter crystals nucleated and grew on a free surface, then they will remain there. If crystals nucleate in a supercooled melt, they float due to the difference in the densities of the melt and the solid phase. As a result, the casting hardens from the walls of the mold and from the side of the free surface, and towards the end of solidification, due to lack of nutrition, shrinkage pores are formed inside the casting. In this case, the faster the mold rotates, the more intensely the crystals are carried to the free surface and the deeper the shrinkage porosity is located.
Shrinkage porosity under the free surface is also observed during the manufacture of thick-walled castings (Figure 6.4, b). In thin-walled castings of large length, the depth of the location of the zone of shrinkage porosity l is less. This is explained by the ratio of cooling rates from the outer and inner surfaces of the casting. The lower the cooling rate of the inner surface of the casting and the greater the cooling rate on the outer surface, the smaller the depth l.
The cooling rate of the casting can be controlled. So, from the outside, this is achieved by changing the thickness of the layer or the thermophysical properties of the refractory coating, changing the cooling rate of the mold. On the inner surface side, bulk refractory materials or exothermic mixtures can be used for this purpose.
Thus, the peculiarities of casting formation during centrifugal casting are associated with both great advantages and disadvantages.
The advantages of this method include: the possibility of improving the filling of molds with the melt under the influence of pressure developed by centrifugal forces; increasing the density of castings due to a reduction in the number of shrinkage pores, cavities, gas, slag and non-metallic inclusions; reduction of metal consumption and increase in yield, due to the absence of a gating system in the manufacture of castings such as pipes, rings, bushings or a reduction in the weight of the gates in the manufacture of shaped castings; elimination of costs for rods in the manufacture of castings such as bushings and pipes.
The disadvantages of this method are: difficulties in obtaining castings from alloys prone to segregation; contamination of the free surface of castings with non-metallic inclusions; inaccuracy of dimensions and the need for increased allowances for processing the free surfaces of castings, caused by the accumulation of non-metallic inclusions in the casting material near this surface and deviations in the accuracy of the dose of the melt poured into the mold.
The highest technical and economic indicators of the centrifugal casting method are achieved when producing hollow cylindrical castings of various sizes and weights (length up to several meters and weight up to several tons): pipes for various purposes made of cast iron, steel, non-ferrous and special alloys; bushings and liners for stationary and transport diesel engines; rolling bearing rings, etc. Centrifugal casting has become widespread for the manufacture of bimetallic products, products from alloys with low fluidity and high surface tension, when it is necessary to obtain thin-walled castings with complex geometry and surface microrelief. These include, for example, turbine disks with blades, castings for artistic and jewelry purposes.
- ← Section 5.6
- Section 6.2 →
Source: https://uas.su/books/spesialmethodsforcasting/61/razdel61.php
Centrifugal metal casting
Centrifugal casting . This method, borrowed from dental technology, has been used in jewelry for many decades. Every jewelry company should use this casting method.
The production of original wax models and the process of obtaining metal samples from them using this type of casting open up wide opportunities for producing highly artistic jewelry of any complexity.
For example, new centrifugal casting machines cast 50 rings per pour.
The centrifugal casting method has the following advantages:
1) the ability to produce castings from models of any complex shape in a relatively simple way;
2) a significant reduction in the possibility of formation of shrinkage cavities;
3) reduction in metal consumption due to the lack of profits, the need for which disappears.
Manual centrifugal lost wax casting. Let's look at an example of casting a massive elephant 30 mm high (Fig. 1).
Rice.
1. Elements of a mold for centrifugal casting: a - flask with holes; b - wax model; c - wire frame
Making the model. First, a wire frame of the wax model is made from the same material used for casting. A model is made of wax around this frame. For modeling, you can use red or blue wax, used in dental technology. The wax is heated and the model is sculpted using a heated metal spatula. The finished model is smoothed with a scraper.
For smoothing, you can very quickly carry the model through the flame.
At the same time, it melts slightly, and smoothing is done easily and quickly. To reduce shrinkage when making large products such as this model, it is recommended to use casting pins made of brass or copper wire with a diameter of 1 to 1.5 mm and a length of 10 to 20 mm and small ball pins.
The pins are heated and inserted a few millimeters into the wax model. At a distance of 3-4 mm from the model, wax balls (separable profits) are placed on the pins. The cross-section of the sprue channels formed by the pins can be increased by coating the pins with wax. After the wax is melted from the mold, the casting pins are removed, and the cavities formed by the balls act as gains during the shrinkage of the metal.
In addition, gases are removed through the channels formed by the casting pins, thereby reducing the possibility of pore formation.
The elephant's four legs are extended by wax pins that converge in a wax ball. These pins and ball perform the same role as casting pins.
Molding. The finished model is degreased by immersion in alcohol so that the molding compound fits tightly to the model. A spherical stand is also made from wax, into which injection molding pins are pressed, as shown in Fig. 1, b. The distance between the model and the stand should be as short as possible so that the melt does not need to travel such a long way. The model, in addition, must be placed so that all its parts are located in the direction of casting.
Preparations used in dental technology are used as molding mass. The molding mass is diluted in distilled water, thoroughly stirred and the air is removed from it by vacuum. The molding mass should be fluid, creamy, and without lumps. Apply it to the model with a soft brush. At the same time, no air bubbles should appear on it.
This first layer is powdered, which prevents dehydration and prevents cracking as the mold dries. After 2-3 minutes, another layer of molding compound is applied and this is continued until the wax model is covered with a layer about 5 mm thick. The model prepared in this way is placed in a flask and filled with molding mass. The surface of the flask has many holes, thanks to which the mass dries faster.
When filling, it should be wrapped in paper so that the molding compound does not leak out. The distance between the model and the wall of the flask should be about 10 mm; to the base of the mold - 15 mm.
Drying and calcination. The finished form should not be left idle for a long time. When the molding compound has set, the wax hemisphere and injection pins are removed.
When drying and melting the wax model, the mold is set in a normal oven or in an enameling oven at a low temperature so that the hole faces down and the wax can flow out. After slowly drying the mold, it is calcined.
Heating during calcination must be carried out slowly, otherwise the mold may collapse. The heating temperature is increased until the sprue channels are heated to a cherry-red color.
Filling. The heated mold is placed on a centrifuge plate, and the metal is poured, which must be melted with a small amount of borax. For jewelers who rarely deal with centrifugal casting, the most affordable is a tabletop centrifuge (Fig. 2). Its design is so simple that you can easily make it yourself. When casting large-sized products, which include the above example, an appropriate counterweight is placed on another plate.
Immediately after pouring the mold, the cord is pulled, and the pipe on which the rocker arm with the plate is attached begins to rotate at high speed around a fixed axis. The advantage of this method is ease of maintenance, high rotation speed and safety. After several training sessions, the same success can be achieved with a manual centrifuge (Fig. 3), although in this case the result often depends on subjective reasons, and the risk of casting defects increases.
Rice. 3. Manual centrifuge
Casting on centrifugal machines . Let's consider the process of producing cast medals with a double-sided relief using a centrifugal machine (Fig. 4). To obtain a wax model, you will need to make a rubber mold.
Making a standard model. Since rubber vulcanization occurs at elevated temperatures, the standard (sample) model cannot be made of wood, plastic or low-melting metals. In addition, during the vulcanization of rubber, a small amount of nitric acid is released, so the standard should not be made from silver and its alloys, as well as from 333 gold alloys.
585 gold alloy is very suitable for making a model standard, especially if its surface is coated with rhodium. The surface of the standard must be carefully processed and polished. It is better to spend a few extra minutes processing this sample than to waste hours later finishing a batch of castings.
In conclusion, you should pay attention to the fact that the casting, due to metal shrinkage, turns out to be somewhat smaller and further decreases in size during mandatory finishing. Therefore, the standard should have slightly larger dimensions than the finished model.
Rice. 4. Scheme of the process of obtaining products by centrifugal casting
Manufacturing of elastic mold . For this purpose, a flask is required, similar to a foundry for sand casting, with guide pins. Molding hole is optional.
The flask with guide pins is placed on a smooth base plate with the base down, for example glass, and filled with plasticine, into which the model standard is pressed halfway (Fig. 4, 1, 2). Install a second flask and fill it with gypsum solution. After the plaster has hardened, the mold is turned over, the plasticine is removed, and the standard remains in the plaster mold.
Several indentations are made in the plaster, which should later become the guide protrusions of the rubber mold. Raw rubber is cut into small pieces, which are used to fill the upper half of the mold (Fig. 4, 3). The flasks are clamped between two steel plates that overlap the walls of the flasks.
The rubber is vulcanized for 30 minutes at a temperature of 150° C in a gas oven or, better, in a special press with two plates heated by electric current (Fig. 4, 4, 5). The plaster is broken, removed and the model and rubber half-mold are thoroughly cleaned.
Now the finished rubber half-mold is insulated with talcum powder, and the second half of the mold is filled with pieces of raw rubber, after which the mold is placed so that the finished rubber half-mold is at the bottom. The rubber of the second half-mold is vulcanized and both parts of the rubber mold are obtained (Fig. 4, 6). All that remains is to separate the parts of the mold from each other, separate them from the flasks and remove the model (Fig. 4, 7). After this, the sprue channel is cut.
The advantage of elastic rubber molds is their ease of manufacture, easy removal of wax models from them and their durability.
Disadvantages include the impossibility of manufacturing spiral-shaped cavities and obtaining very thin sections. The thickness of the casting wall should not be less than 0.4 mm, since when pressing wax, the mold must be clamped and therefore the walls of the mold halves can be connected tightly.
Other methods for making elastic molds. When making molds for a simple model, it is enough to place its standard between two rubber plates of appropriate thickness and vulcanize them under pressure. The standard is pressed into the softened rubber mass.
The disadvantage of this method is that both rubber plates and the mold must be cut to remove the standard and then cast the wax model.
In Germany, a special rubber molding paste is made, into which the metal model can be easily molded.
This paste hardens when cold without vulcanization.
Obtaining wax models. A wax injector is used to fill rubber molds with wax (Fig. 4, 8). The tank contains molten wax, the constant temperature of which is maintained using electric heating.
The necessary pressure in the tank is created by compressed air, the latter presses the wax through the fitting into the sprue channel of the mold. Liquid wax can be introduced into the mold centrifugally using a metal centrifuge.
In this case, the walls of the rubber mold must be insulated with eucalyptus oil or a mixture consisting of one part water and one part glycerin before each filling with wax.
Preparing the mold and pouring . As schematically shown in Fig. 4, 9, the finished wax models are placed around a wax riser, to which they are connected using wax pins. soldered onto a wax base and mounted on a rubber base, the cross-section of which can be seen in the same figure.
Next, the block of wax models is processed in the same way as described above: degreased, covered with a brush with molding compound, placed in a cylindrical flask and filled with evacuated molding compound. After the molding mass has hardened, the flask is placed in a muffle furnace (Fig. 4, 10) and the wax is melted for an hour at 200 ° C. Then the temperature is increased to 1000° C and the mold is maintained at this temperature for another 15 minutes.
After this, it is removed from the oven, placed in a centrifugal machine (Fig. 4, 11) and poured. The finished block of cast medals with sprues, obtained after knocking out the flask, is shown in Fig. 4, 12.
Source: https://www.shipmodeling.ru/tooling/spuncasting
Centrifugal Casting Equipment
Metal molds for centrifugal casting are called chill molds or molds. The centrifugal method is also used for pouring titanium, bronze, cast iron, steel and other alloys into one-time molds.
With this casting method, the alloy is poured into a heated rotating mold (Fig. 1). It begins to rotate under the influence of centrifugal forces and hardens. The still hot casting is removed from the mold, the mold is cooled to the optimum temperature (200...300 °C), a heat-insulating coating is applied to its working surface, and the process is repeated.
Rice. 1. Schemes of centrifugal casting
Three centrifugal casting schemes are possible. For any design, the axis of rotation of the mold can be horizontal, vertical or inclined.
The most widely used scheme is I. It produces hollow cylindrical castings without rods. Machines with a horizontal axis of rotation (Fig. 1, a) are used for casting long bodies of rotation: the length is 3 times greater than the diameter. The free surface of the casting is a cylinder. The free surface of a casting is a surface that does not come into contact with the walls of the mold, but only with air.
On machines with a vertical axis of rotation (Fig. 1, b), short bodies of rotation are obtained due to the difference in thickness along the height of the casting. The free surface is a paraboloid. The higher the casting, the greater the difference in thickness.
Schemes II and III, in which there is no free surface, are used less frequently; in them, centrifugal forces are used to increase the density of castings or improve the filling of thin-walled castings.
The centrifugal casting method according to Scheme I allows the use of free-flowing, dry, binder-free fire-resistant heat-insulating coatings of molds instead of non-stick paints to cover the walls of the mold. Therefore, before pouring, sand is introduced into the rotating mold, which is distributed by centrifugal forces over the working surface in an even layer.
The rotation speed of the mold is selected from the conditions for obtaining a casting of the correct geometric shape and creating centrifugal forces necessary for the optimal process of solidification of the casting. The minimum number of revolutions will be at which there is no sprinkling - separation of metal drops from the flow and their fall, i.e. the centrifugal force on the free surface is slightly greater than the force of gravity.
Rice. 2. Scheme for producing a cast iron pipe using the centrifugal method : a - initial position; b - pouring cast iron; c - end of filling; d - removing the casting and returning the machine to its original position.
In Fig. Figure 2 shows the pipe casting process. In the initial position (Fig. 2, a), the filling tray 4 is inserted into the farthest part of the mold 2, which is mounted on rollers inside the protective casing 3 and is driven into rotation by drive 1.
After the metal from the transfer ladle 5 along tray 4 fills the lowest part of the mold 2 (Fig. 2, b), it is shifted to the left using a trolley together with the drive and the metal falls into another part of it.
This continues until it is completely filled (Fig. 2, c, d).
The centrifugal method produces large castings from alloy steels for rolling pipes, bushings and rims from antifriction alloys, grinding bodies from white cast iron, liners (automobile and tractor engines) from alloy cast iron, pressure and drain cast iron pipes, propellers (according to scheme II, Fig. 1), parts made of heat-resistant and titanium alloys.
Rice. 3. Vertical centrifugal casting machine TsBM-05
1. Vertical centrifugal casting machines
The vertical centrifugal casting machine TsBM-05 (Fig. 3) is equipped with a vibration damping mechanism and has a smooth control of the die rotation speed from 350 to 1500 min-1. Dimensions of melted products: outer diameter from 80 to 500 mm, inner diameter from 50 to 450 mm, casting height up to 400 mm
2. Horizontal centrifugal casting machines
The MTsVR centrifugal casting machine (Fig. 4) is designed for casting cast iron rollers of rolling mills. Has a horizontal axis of rotation.
For pouring, molten metal is transported from the furnace on a trolley 19 in pouring ladles 1 and poured into a volumetric dosing ladle 3, the work of which is supervised by an operator located in a protective cabin 2. From the volumetric dosing ladle through a pouring device 4, the metal is fed into a cast iron chill mold 12 located in the mold 7.
The chill mold together with the mold rotates at a speed of up to 1000 min-1 using a drive device 9. For free removal of the casting from the chill mold after it has cooled, the front 10 and rear 15 covers are used, protected from the effects of molten cast iron by graphite-chamotte inserts 11 and 14
Rice. 4.
Centrifugal casting machine MTsVR : 1 - ladle (drum) pouring; 2 — pourer’s cabin; 3 — volumetric dosing bucket; 4 - filling device; 5 - fixed support; 6 — protective casing; 7 - mold; 8 — movable support; 9 - drive device; 10 — front cover; 11, 14 — graphite-chamotte inserts; 12 — cast iron mold; 13 — steel mold; 15 — rear cover; 16 — air preparation unit; 17 — mechanism for moving the drive device; 18 - bed; 19 - trolley
The machine can cast parts with a diameter from 230 to 960 mm and a length of up to 1200 mm. If a bushing-type part is cast, the hole must be at least 90 mm.
The absence of inclination of the die axis in the vertical plane is due to the fact that relatively short blanks are cast on the machine. With an increase in the ratio of the length of the workpiece to its diameter, the frame of the centrifugal casting machine is made inclined with an adjustable angle of inclination of the mold from 4 to 90°.
Rice. 5. Model 4986 Double Rotor Centrifugal Casting Machine
In addition to single-position machines, two-position and multi-position (carousel) machines are produced (see Fig. 1, b, diagram III). In Fig. Figure 5 shows the Model 4986 two-rotor centrifugal casting machine. It is designed to produce cast iron grinding balls and steel bearing rings. It has a horizontal axis of rotation of the chill molds.
The machine operates in semi-automatic mode with manual pouring of liquid metal. After heating the chill molds with gas burners, it operates in the following cycle:
- both halves of the mold are painted with special heat-insulating paint, the coating is dried;
- the halves of the mold are closed;
- the filling tray is inserted into the mold, the metal is poured in a strictly dosed portion;
- the tray is removed from the chill mold, and the chill mold continues to rotate until the castings are completely solidified;
- the rotation of the centrifuge stops, the front half of the mold is retracted and the casting cluster is removed;
- The die halves are cleared of blockages and painted.
Then the cycle repeats.
The machine is equipped with a pneumatic system for pushing the casting from the mold into the receiving tray, a three-stage V-belt transmission for the mold rotation drive, a mechanism for supplying and removing the receiving tray, and water cooling of the mold body.
Source: https://extxe.com/3034/oborudovanie-dlja-centrobezhnogo-litja/
