Humanity has been using metals and their alloys for several millennia. At first, metals were found in the form of nuggets and placers; later, prehistoric tribes learned to process metal-containing ores. A proven method of producing metal products was casting in earthen molds.
Arrowheads and swords, agricultural implements and tools, utensils and decorations were cast. Over the millennia since then, man has invented many new material processing techniques and casting methods, including injection molding, gasification molds and powder metallurgy. The ancient method has also been preserved, but is used mainly in sculpture workshops and artistic crafts.
Features of metal casting
Compared to other materials, such as wax or plaster, metal casting differs in several ways. The first of them is the high temperature of transition from solid to liquid state. Wax, plaster and cement harden at room temperature. The melting point of metals is much higher - from 231 °C for tin to 1531 °C for iron. Before you can start casting the metal, it must be melted. And if tin can be melted in a clay bowl on a simple fire made from nearby branches, then to melt copper, not to mention iron, you will need a specially equipped furnace and prepared fuel.
Tin and lead - the softest and most fusible metals - can even be cast into wooden dies.
For casting more refractory metals, molds made from a mixture of sand and clay will be required. Some metals, such as titanium, require metal molds for casting.
After pouring, the product needs to cool. Reusable dies are disassembled, disposable molds are destroyed, and the casting is ready for further machining or use.
Metals for pouring
Black metals
In the metallurgical industry, a distinction is made between non-ferrous and ferrous metals. Blacks include iron, manganese, chromium and alloys based on them. This includes all steels, cast irons and ferroalloys. Ferrous metals account for more than 90% of the world's consumption of metal alloys. Steel is used to produce hulls and parts of vehicles from scooters to supertankers, building structures, household appliances, machine tools and other industrial equipment.
Cast iron is an excellent metal for casting large, strong, durable structures that are not subject to bending or twisting stresses.
Non-ferrous metals, in turn, depending on their physical properties, and above all, specific gravity, are divided into two large groups
Light non-ferrous metals
This group includes aluminum, titanium, magnesium. These metals are less common than iron and are more expensive. They are used in those industries where it is necessary to reduce the weight of a product - the aerospace industry, the production of high-tech weapons, the production of computing and telecommunications equipment, smartphones and small household appliances.
Titanium, due to its excellent interaction with the tissues of the human body, is widely used for prosthetics of bones, joints and teeth.
Heavy non-ferrous metals
These include copper, tin, lead, zinc and nickel. They are used in the chemical industry, production of electrical materials, electronics, transport - wherever sufficiently strong, elastic and corrosion-resistant alloys are required.
Noble metals
This group includes gold, silver, platinum, as well as rarer ruthenium, rhodium, palladium, osmium, and iridium.
The first three have been known to man since prehistoric times. They were rarely (relative to copper and iron) found in nature and therefore served as a means of payment, material for valuable jewelry and ritual objects.
With the development of civilization, gold and platinum retained their role as a means of accumulating wealth, but they became very widely used in industry and medicine due to their unique physical and chemical properties.
Metal casting methods
The main metal casting methods are as follows:
Traditional method
The metal enters the mold under the influence of gravity. Sand-clay or metal matrices are used. The disadvantage of the method is the high labor intensity of making molds and other operations, difficult working conditions and low environmental friendliness
Low pressure casting
The model is removed from the mold, its parts are assembled together and created. The form is pricked with thin sharp needles to ensure gas removal. They make a casting, wait for it to cool,
A split mold, called a mold, is made from metal parts. Matrix parts are produced by casting or, if high surface quality and dimensional accuracy are required, by milling. The molds are lubricated with non-stick compounds and filled.
After cooling, the molds are disassembled, the castings are removed and cleaned. The metal matrix can withstand up to 300 operating cycles.
The model is made not of wood or wax, but of a low-melting and gasifying material, mainly polystyrene. The model remains in the mold and evaporates when the metal is poured.
Advantages of the method:
- the model does not need to be extracted from the matrix;
- you can make models of as complex castings as you like; complex and composite molds are not needed;
- The complexity of modeling and molding has been significantly reduced.
Gasification casting is becoming increasingly popular in modern metallurgical industries.
Casting molds
The most ancient type of molds are those made from sand-clay molding mixture, or “earth.” Historically, centers of metallurgy arose near the deposits of sands that were ready in their composition for casting, for example, near the world famous Kasli iron plant. Mixtures are divided into coating and filling mixtures.
To construct any matrix, a model is required - a life-size mock-up of the future product, but somewhat larger in size - equal to the amount of casting shrinkage.
The model is placed in the center of the formwork, or flask, and a layer of coating mixture is applied to it - heat-resistant and plastic. Then they begin to fill the flask layer by layer, carefully tamping each layer, with the filling mixture. The requirements for filling mixtures are much lower than for coating mixtures - they must withstand the pressure of the poured metal, maintaining the configuration of the casting, and ensure the release of melting gases. Afterwards, the model is removed from the mold and the melt is poured in its place.
For castings of complex configurations, with intricate details and internal cavities, composite models and molds made of several parts are used.
Casting is also carried out in metal molds. They are used for large runs of cast parts, in cases where high dimensional accuracy and low surface roughness of the casting are required, as well as for some metals that are active in a heated state. The melting temperature of the mold material must be significantly higher than the temperature of the melt being cast.
Application area
Various casting methods have their own advantageous areas of application.
Thus, sand casting is used for single castings or small series. The method, proven over thousands of years, is gradually disappearing from industrial enterprises, but continues to be used in arts and crafts and in sculpture workshops.
Metal casting is used in cases where it is required
- large quantities of castings;
- high dimensional accuracy;
- high surface quality.
Metal casting is also popular in the jewelry industry and in the production of metal jewelry.
Injection molding is increasingly used by enterprises focused on the quality of their products, monitoring the environment, labor safety and the efficient use of material and energy resources.
Casting using gasified models is used in cases where large quantities of castings are planned, high precision and labor savings are required.
The development of mass production of castings has led to the improvement of known and the development of new special casting methods. The foundry is faced with the task of producing castings with their shape and dimensions as close as possible to the shape and dimensions of the finished part, while the most labor-intensive machining operation should be limited only to finishing and grinding. This can be achieved by improving and introducing special, more accurate casting methods such as chill casting, pressure casting, centrifugal casting, lost wax casting, shell casting, etc.
When producing precision castings in one-time molds, mechanical processing of the castings is eliminated or reduced. Such casting methods include casting in shell molds, lost wax casting, casting in plaster and glass molds, casting in polystyrene foam models.
In semi-permanent molds (from fireclay, metal-ceramics, graphite), several dozen and even hundreds of castings can be obtained without their destruction.
Several thousand castings with very precise dimensions can be produced in a metal mold. Metal mold casting includes die casting, centrifugal casting, injection molding and etc.
5.1.1. Sand casting.
For the production of large-sized parts of complex shapes, in small-scale and individual production, sand casting is used. Figure 4.1 shows an example of the sequence of manufacturing a casting, a valve body, in a sand mold. Based on the drawing of the part, a drawing of the casting is developed (Fig. 1a). In the model shop, a model is made from wood or metal, consisting of two or more parts, depending on the design features of the part, ensuring its removal from the molding sand. The model simulates the external contours of the part and the seats of the rod (signs 1), with which the rod is fixed in the mold. In the molding shop, one half of the model is installed on a model plate, with the bottom one fixed to it. mold chamber 4.
The flask is a rectangular box and is part of the casting mold. The mold, with the model inside, is filled with molding sand and compacted. I remove the flask from the plate, turn it over 180 0 Fig.5.1.c and install the second half of the model with gating system 2, as well as the upper flask 3. Upper flask 3,
Fig.5.1 pour the molding mixture and compact it.
In the core box Fig. 5.1d, a rod is made in Fig. 5.1d, simulating the internal cavity of the cast workpiece and the shape
sign, i.e. the place where it is fixed in the form. The material used is a core mixture, from which the core is formed.
The upper flask is removed, the model of the part and the gating system is removed from both halves of the mold, being careful not to damage the integrity of the molded mixture. Install rod 6 in Fig. 5.1.e into the lower half of the mold and cover it with the upper half of the mold. The cavity formed between the core and the molding mixture of the upper and lower half molds is filled with molten metal through the gating system.
Fig.5.2
After the metal has hardened, the mold is disassembled and the casting is removed. The cast blank is cleaned of the molding sand, the cores are knocked out, the sprues are cut off and cleaned. The form can consist of two or several flasks. In Fig. 5.2. shows the molds for obtaining a cast pulley blank. The formation of the external contours of the part is carried out in the following sequence.
The molding of the lower part of the workpiece is carried out in the lower flask 3, which
installed on the model plate. A model is fixed to the model plate, which imitates the casting up to the parting plane of the flask. A filling frame is installed on the flask and the flask is filled with molding sand. The molding mixture is compacted by pressing, shaking or using special machines, sand-throwing or sand-shooting machines.
After molding, the flask is carefully removed from the model plate and turned over 180 0 . The model must have such a shape that the molding sand is not destroyed when removing the model from the flask, i.e. the necessary slopes are provided. Install the model of bushing 4, riser 6, buttress 5, Fig. 5.2.a, and form the upper half-mold.
After compacting the molding sand, the upper half-mold is removed, a model of the riser and abutment is removed from it, and a casting model is removed from the lower half. Before assembling the mold halves, rods 1 and 2 are installed, which serve to form a central hole and an annular recess in the casting. The rods are made from special molding mixtures that provide greater gas permeability, strength, and non-stick properties.
In a single production, the same part can be made in three flasks, the parting planes of which pass along the end surfaces of the pulley. With this molding, the manufacture of one of the rods 2 is eliminated. The model of the sleeve 4 and flange 8 is made detachable so that they can be removed from the molding sand during the process of disassembling the flask and removing the model. The middle flask 10 ensures the production of an annular recess for the pulley.
5.1.2. Casting into metal molds.
Casting in metal molds (chill) has advantages over casting in sand molds: the cost of the casting process and the complexity of mechanical processing of cast blanks are reduced; the mechanical properties of alloys and labor productivity increase. This method is mainly used in
serial and large-scale production. The disadvantage of this method is the high complexity of manufacturing a metal mold.
Figure 5.3 shows the design of the mold, consisting of two halves (1 and 4). The working cavity (10) imitates the external contours of the cast workpiece, and the sand rods (5) imitate the internal cavities and holes.
Similarly, as in sand molds, channels for the gating system (8) and vents for removing gases are provided in the chill mold. To coordinate the two halves of the mold relative to each other, pins (15 and 3) are installed, which fit into the guide holes of the second half of the mold. The resulting cast billet is pushed out of the mold Fig.5.3 pushers through the holes (9). The mold is secured on the work table with tides (7). The die can withstand a greater number of pours, depending on the temperature of the alloy being poured. The design of the cast part must have a relatively simple form, allowing the two halves of the die to be separated after the metal of the cast piece has solidified. Otherwise, it is necessary to provide space in the mold for installing additional sand rods that form a complex surface.
5.1.3. . Lost wax casting.
This method makes it possible to produce castings using one-time models (melted, burnt-out, soluble) in multi-layer, one-piece, fire-resistant forms. Parts obtained by this method may not require subsequent machining, have a very complex configuration and high surface quality. The method is quite labor-intensive and it is advisable to use it in the manufacture of parts with complex and labor-intensive machining, when using materials that are difficult to machine. The essence of the method is as follows. To obtain a model according to the casting drawing Fig. 5.4a, a metal or plastic mold is made Fig. 5.4.b, usually detachable, with channels for the gating system. Melted in an oven Fig. 5.4 into a low-melting alloy consisting of 50% paraffin and 50% stearin, poured into a mold Fig. 5.4 d .
Fig.5.4.
The hardened model Fig. 4.4.d is removed from the mold and assembled into a block Fig. 5.4f consisting of several models connected by a common gating system.
The assembled block is immersed in a fireproof suspension, sprinkled with dry sand and air dried
The operation is repeated several times until a mold with a thickness of 5-8 mm is obtained. Fig.5.4.g. The paraffin model from the resulting block is melted with hot air at 120-150 0 C, steam or hot water. The mold thus obtained is calcined, and it turns into a durable ceramic shell. In Fig. 5.4. The technological sequence of manufacturing a casting mold is presented.
The mold is filled with molten metal (Fig. 5.4.h) and after the casting has hardened, it is knocked out of the mold, destroying the ceramic shell. To completely clean the ceramic mold, the castings are treated with an alkaline solution and washed in hot water.
A casting is a workpiece obtained by pouring molten metal, plastic, ceramic materials, etc. into a casting mold. After solidification, the casting retains the configuration of the mold cavity. The casting method can be used to produce products of complex configurations that are difficult or impossible to obtain by other types of processing - forging, stamping, welding.
Casting is one of the economical ways to produce blanks and parts of complex shapes, large and small sizes.
The technological process for producing metal castings consists of the following main operations:
1) production of casting molds;
metal smelting;
pouring metal into a mold;
solidification of the metal and cooling of the casting;
heat treatment of the casting;
casting quality control;
delivery of the casting for machining.
Each of the listed operations is complex and multi-transition in nature. The implementation of each operation must ensure a high level of casting quality in all respects, including dimensional accuracy and surface cleanliness, favorable metal structure, as well as the absence of external and internal casting and metallurgical defects.
In the foundry industry, more than 50 types of casting are used to produce metal castings, which differ from each other in the material of the casting mold, in the method of feeding the poured metal into it, in dimensional accuracy, in the cleanliness of the surface of the castings, in productivity and the degree of complexity of the technological process.
A casting mold is a device designed to pour metal and form a casting (Fig. 3.1). It consists of a working cavity (8) and a gating system. In Fig. 3.1 The casting mold is divided into upper and lower halves. The body of the workpiece is directly formed in the working cavity, therefore its configuration and dimensions must correspond to the outlines and dimensions of the casting being manufactured. In this case, it is necessary to take into account that the dimensions of the working cavity must exceed the dimensions of the casting by the amount of casting shrinkage of the metal. In turn, the dimensions of the casting must be larger than the dimensions of the part by the amount of technological allowance removed during machining. Thus, the final dimensions of the working cavity of the casting mold include the corresponding dimensions of the parts, allowances for machining and for casting shrinkage of the metal.
There may be various holes, cavities and recesses inside some castings, as well as on their outer surface. To carry them out, when assembling the mold, appropriate ceramic or metal elements are installed in it, sewn on with rods (7). The rods are removed from the casting during knockout, leaving behind the necessary holes or recesses in it.
Figure 3.1 - Scheme of the casting mold
1 - bowl (funnel) 2 - riser. 3 - choke, 4 - slag catcher, 5 - feeder, 6 - side income, 7 - rod, 8 - working cavity
The gating system serves to supply metal into the working cavity and feed the casting during the crystallization process. It includes a bowl (funnel) (1), a riser (2), a throttle (3) that regulates the filling speed and prevents air from suction into the riser, a slag catcher (4) located in the upper half of the mold to retain non-metallic inclusions, a feeder (5) , feeding metal into the working cavity directly or, as in this case, through a side income (6). The profit feeds the casting body during cooling and crystallization of the metal and prevents the formation of shrinkage cavities in it. Profits can be top or side.
In the foundry industry, two groups of types of casting have developed: casting in sand-clay molds and special types of casting.
Casting in sand-clay molds is the simplest and most common method of producing cast blanks. Each mold is used to produce one casting. For the manufacture of such forms, the materials used are molding mixtures consisting of a sand base, to which certain amounts of clay and water are added as binding materials.
In addition, non-stick additives are introduced into the mixture in the form of ground coal, marshallite (pulverized quartz), fuel oil and other substances that help improve the quality of the casting (sawdust, sulfate-alcohol stillage).
The materials for making rods are rod mixtures, consisting mainly of sand bound with special substances - fasteners (linseed oil, sulfate stillage, dextrin, rosin, etc.).
A casting mold usually consists of two halves of molds, produced separately by hand or machine: lower and upper. Each of the semi-forms is made in special metal boxes without bottoms and lids, called flasks. When assembling the mold, the flasks are placed on top of each other and fastened together.
In flasks filled with molding sand, the working cavities for casting are obtained using halves of a split model, the shape and dimensions of which correspond to the shape and calculated dimensions of the working cavity. Assembling a casting mold from mold halves - flasks - is carried out after removing the halves of the models and installing the cores in the lower mold half. The cores are produced in special devices - core boxes - and undergo mandatory drying.
The assembled mold, consisting of fastened flasks, is poured through the gating system using a special ladle and remains in place until crystallization is completed and the casting body cools. Then the flasks are unfastened, and the casting is knocked out of the mold using a special installation. Then the casting is trimmed and cleaned from the gating system with profits, the remains of the molding and core mixtures are removed, and the surface of the casting is cleaned from various defects. After this, the casting is subjected to heat treatment, which aims to eliminate the coarse-grained, dendritic structure of the metal, casting stresses and prepare the casting metal for machining.
The disadvantages of casting in sand-clay molds are: low dimensional accuracy and surface cleanliness, leading to large allowances for machining, as well as low productivity and poor sanitary and hygienic working conditions due to high dust and noise in the workplace.
The most common specialty casting types are:
chill casting;
centrifugal casting;
injection molding;
shell casting;
Lost wax casting.
Chill casting is a method of producing castings in metal molds - chill molds. These molds are made of cast iron or steel. Filling the mold with the alloy and its hardening occurs without any external influence. The main advantages of chill casting are: the molding process is eliminated, favorable cooling conditions are provided, ease of removal of castings from the mold, high dimensional accuracy and surface cleanliness of the casting, as well as fine grain of the casting metal, which reduces the metal consumption of products and increases the strength of the metal
The use of malleable metal forms made from sheet steel packages, as well as thin-walled water-cooled forms in which the working cavity is made in the form of a replaceable stamping, is promising.
When chill casting of thin-walled body parts made of aluminum and magnesium alloys, vacuum suction is used. To produce large-sized thin-walled castings, the so-called “book” molding method is used, when pouring is done into an open mold, followed by squeezing when the mold halves are closed.
Chill casting produces castings from cast iron, steel, aluminum, copper, magnesium and other alloys.
On automatic lines, cast iron frames and bearing shields for electrical machines are produced in metal molds. The molds for pouring bearing shields consist of two water-cooled halves with a vertical parting plane.
To cast electric motor frames with a rotation axis height of 112 mm, facing molds are used. The method of casting into a lined chill mold is that a layer of disposable sand-resin lining with a thickness of 4...8 mm on a thermosetting binder is applied to the working surface of the chill mold, preheated to 200°C. In individual die zones, the thickness of the lining can be greater or less depending on the cooling conditions of various parts of the castings.
Casting in a lined die has the following advantages compared to casting in a clean (unlined) die:
Increased durability of chill molds (up to 30,000 fills);
Exclusion of casting annealing operations from the technological process;
Simplified design of the mold (it can be manufactured with cast working sockets that do not require machining, which reduces the cost of the mold);
Possibility of producing castings with protruding parts and deep cavities;
Possibility of obtaining precise castings of any configuration;
Obtaining optimal conditions for solidification of castings.
Centrifugal casting is casting into rapidly rotating metal molds, in which molten metal, subjected to centrifugal forces, is thrown against the walls of the mold and solidifies, forming a casting. In this way, short (Fig. 3.2, a) or long (Fig. 3.2, b) bodies of rotation are cast. This casting method is widely used in industry, especially for producing hollow castings with a free surface of cast iron and steel pipes, rings, bushings, shells, etc. with 12th grade accuracy. Metal molds are installed on centrifugal casting machines. Depending on the position of the axis of rotation of the molds, horizontal and vertical machines are distinguished. Castings produced by centrifugal casting have increased density in the outer layer. The advantages of centrifugal casting are the same as those of chill casting, but the quality of the internal surface due to shrinkage phenomena is worse than the external one. To obtain an internal cavity in cylindrical castings, rods are not required, which saves resource costs for their manufacture.
a - short bodies of rotation, b - long bodies of rotation, 1 - shape, 2 - groove
Figure 3 2- Scheme of centrifugal casting
Injection molding can be divided into two types:
1) metal injection molding;
injection molding of polymer materials.
Metal injection molding is a method of producing castings from alloys of non-ferrous metals and steels of some grades, when a liquid melt is introduced into a closed metal mold under significant pressure (30...100 MPa) and crystallizes while remaining under pressure (Fig. 3.3). This makes it possible to bring the dimensions and shape of the casting as close as possible to the dimensions and shape of the finished part, which makes it possible to reduce or eliminate their subsequent machining. The strength of castings made by this method is 30% higher than the strength of castings made by casting into earth molds.
A- supply of metal to the antechamber, b- injection of metal into the working cavity, V- knockout of the casting, 1 - chamber, 2 - piston. 3 - mold
Figure 3.3 - Injection molding scheme
The following factors can affect the accuracy of injection molding castings:
Dimensions and complexity of the casting design;
Casting wall thickness;
Direction of metal shrinkage;
Precision mold manufacturing;
The degree of wear of the mold.
Maximum dimensional accuracy is achieved for those casting elements that are in one half-mold or formed by stationary parts of the mold. The dimensional accuracy of castings, depending on the design of the molds, is allowed within 0.03...0.18 mm.
Injection molding is performed on casting machines with cold and hot pressing chambers. Casting molds are made of steel. One mold can contain several working cavities that are fed simultaneously. Such multi-cavity molds make it possible to produce more than 20 castings in one pour. The productivity of casting machines is up to 600 pours (pressings) per hour. Injection casting is the highest quality, accurate, clean and productive type of casting. This method is widely used in serial and mass production for the manufacture of small parts of complex shapes. Modern automatic injection molding machines for castings weighing up to 300 g provide a productivity of up to 6000...8000 castings per hour. The surface roughness of the workpieces is R a = 2.5...0.32 µm.
Steel injection molds have complex configurations and high costs.
Previously, when injection molding electrical machine housings from aluminum alloys, molds with a vertical split were used. In them, the body of the electric machine, terminal box and paws were cast separately and then, after machining, assembled.
Currently, molds are used that have four horizontal connectors (Fig. 3.4). The body is cast into them along with the paws and the terminal box. The arrangement of the ribs is vertical-horizontal.
A four-compartment mold makes it possible to cast housings with thinner and higher ribs, which improves the cooling of the machine and reduces the mass of the housing by 15... 25%.
Most aluminum castings for low-power electrical machines are produced by injection molding on special casting machines (Fig. 3.5, A).
Figure 3.4 – Sketch of a four-compartment mold
5 – fixed plate, 6 – cylinder, 7 – crucible, 8 – piston,
9 – liquid metal, 10,11 – parts of the mold, 12 – cavity
Figure 3.5 – Injection molding machine (a) and casting diagram (b)
All metals can be cast. But not all metals have the same casting properties, in particular fluidity - the ability to fill a casting mold of any configuration. Casting properties depend mainly on the chemical composition and structure of the metal. Melting temperature is important. Metals with low melting points are easy to industrially cast. Of the common metals, steel has the highest melting point. Metals are divided into ferrous and non-ferrous. Ferrous metals are steel, ductile iron and cast iron. Non-ferrous metals include all other metals that do not contain significant amounts of iron. For casting, alloys based on copper, nickel, aluminum, magnesium, lead and zinc are used, in particular. ALLOYS.
Black metals.
Become.
There are five classes of steels for industrial casting: 1) low-carbon (with a carbon content of less than 0.2%); 2) medium-carbon (0.2–0.5% carbon); 3) high-carbon (more than 0.5% carbon); 4) low alloyed (less than 8% alloying elements) and 5) highly alloyed (more than 8% alloying elements). Medium-carbon steels account for the bulk of ferrous metal castings; Such castings are, as a rule, industrial products of a standardized grade. Various types of alloy steels are designed to achieve high strength, ductility, toughness, corrosion resistance, heat resistance and fatigue strength. Cast steels are similar in properties to forged steel. The tensile strength of such steel ranges from 400 to 1500 MPa. The mass of castings can vary over a wide range - from 100 g to 200 tons or more, the thickness in section - from 5 mm to 1.5 m. The length of the casting can exceed 30 m. Steel is a universal material for casting. Due to its high strength and ductility, it is an excellent material for mechanical engineering.
Malleable cast iron.
There are two main classes of ductile iron: regular grade and pearlitic. Castings are also made from some alloyed malleable cast irons. The tensile strength of ductile iron is 250–550 MPa. Its fatigue resistance, high rigidity and good machinability make it ideal for machine tools and many other mass production applications. The mass of castings ranges from 100 g to several hundred kilograms, and the cross-sectional thickness is usually no more than 5 cm.
Cast iron.
Cast irons include a wide range of alloys of iron with carbon and silicon containing 2–4% carbon. There are four main types of cast iron used for casting: gray, white, bleached and half-cast. The tensile strength of cast iron is 140–420 MPa, and some alloy cast iron is up to 550 MPa. Cast iron is characterized by low ductility and low impact strength; among designers it is considered a fragile material. The mass of castings ranges from 100 g to several tons. Foundry iron castings are used in almost all industries. Their cost is low and they can be easily processed by cutting.
Nodular cast iron.
Spherical inclusions of graphite give cast iron ductility and other properties that distinguish it favorably from gray cast iron. The spherical shape of graphite inclusions is achieved by treating cast iron with magnesium or cerium immediately before casting. The tensile strength of nodular cast iron is 400–850 MPa, ductility is from 20 to 1%. True, nodular cast iron is characterized by low impact strength of the notched sample. Castings can have both large and small thickness in cross-section, weight - from 0.5 kg to several tons.
Non-ferrous metals.
Copper, brass and bronze.
There are many different copper-based alloys suitable for casting. Copper is used in cases where high thermal and electrical conductivity is required. Brass (an alloy of copper and zinc) is used when an inexpensive, moderately corrosion-resistant material is desired for a variety of general purpose products. The tensile strength of cast brass is 180–300 MPa. Bronze (an alloy of copper and tin, to which zinc and nickel can be added) is used in cases where increased strength is required. The tensile strength of cast bronzes is 250–850 MPa.
Nickel.
Copper-nickel alloys (such as Monel metal) have high corrosion resistance. Nickel-chromium alloys (such as Inconel and nichrome) are characterized by high thermal resistance. Molybdenum-nickel alloys are highly resistant to hydrochloric acid and oxidizing acids at elevated temperatures.
Aluminum.
Cast products made of aluminum alloys have recently been increasingly used due to their lightness and strength. Such alloys have fairly high corrosion resistance and good thermal and electrical conductivity. The tensile strength of cast aluminum alloys ranges from 150 to 350 MPa.
Magnesium.
Magnesium alloys are used where the requirement for lightness comes first. The tensile strength of cast magnesium alloys is 170–260 MPa.
Titanium.
Titanium, a strong and lightweight material, is melted in a vacuum and cast into graphite molds. The fact is that during the cooling process, the titanium surface can become contaminated due to a reaction with the mold material. Therefore, titanium cast into shapes other than those made from machined and pressed powder graphite is heavily contaminated on the surface, which manifests itself in increased hardness and low flexural ductility. Titanium casting is used primarily in the aerospace industry. The tensile strength of cast titanium is over 1000 MPa with a relative elongation of 5%.
Rare and precious metals.
Castings from gold, silver, platinum and rare metals are used in jewelry, dental technology (crowns, fillings); some parts of electronic components are also made by casting.
CASTING METHODS
The main casting methods are: static casting, injection molding, centrifugal casting and vacuum casting.
Static fill.
Most often, static filling is used, i.e. pouring into a fixed mold. With this method, molten metal (or non-metal - plastic, glass, ceramic suspension) is simply poured into the cavity of a stationary mold until it is filled and held until it solidifies.
Injection molding.
A casting machine fills a metal (steel) mold (which is usually called a mold and can be multi-cavity) with molten metal under a pressure of 7 to 700 MPa. The advantages of this method are high productivity, high surface quality, accurate dimensions of the cast product and minimal need for machining. Typical metals for die casting are zinc, aluminum, copper and tin-lead alloys. Due to their low melting point, such alloys are highly technological and allow for close dimensional tolerances and excellent casting characteristics.
The complexity of the configuration of castings in the case of injection molding is limited by the fact that the casting can be damaged when separated from the mold. In addition, the thickness of the products is somewhat limited; more preferable are products with a thin section, in which the melt solidifies quickly and evenly.
There are two types of injection molding machines - cold chamber and hot chamber. Hot chamber pressing machines are mainly used for zinc-based alloys. The hot pressing chamber is immersed in molten metal; under a slight pressure of compressed air or under the action of a piston, liquid metal is forced out of the hot pressing chamber into the mold. In cold chamber casting machines, molten aluminum, magnesium or copper alloy fills the mold under pressure from 35 to 700 MPa.
Castings obtained by injection molding are used in many household appliances (vacuum cleaners, washing machines, telephones, lamps, typewriters) and very widely in the automotive industry and in the production of computers. Castings can weigh from several tens of grams to 50 kg or more.
Centrifugal casting.
In centrifugal casting, molten metal is poured into a sand or metal mold that rotates around a horizontal or vertical axis. Under the influence of centrifugal forces, the metal is thrown from the central sprue to the periphery of the mold, filling its cavities, and solidifies, forming a casting. Centrifugal casting is economical and for some types of products (axisymmetric such as pipes, rings, shells, etc.) is more suitable than static casting.
Vacuum filling.
Metals such as titanium, alloy steels and high-temperature alloys are melted in a vacuum and poured into multiple molds, such as graphite, placed in a vacuum. This method significantly reduces the gas content in the metal. Ingots and castings produced by vacuum casting weigh no more than several hundred kilograms. In rare cases, large quantities of steel (100 tons or more), smelted using conventional technology, are poured in a vacuum chamber into molds or foundry ladles installed in it for further casting in air. Large metallurgical vacuum chambers are evacuated by multi-pump systems. The steel obtained by this method is used for the manufacture of special products by forging or casting; this process is called vacuum degassing.
CASTING MOLDS
Casting molds are divided into multiple and one-time (sand) molds. Multiple forms are metal (molds and chill molds), or graphite or ceramic refractory.
Multiple forms.
Metal molds (molds and chill molds) for steel are usually made from cast iron, sometimes from heat-resistant steel. For casting non-ferrous metals such as brass, zinc and aluminum, cast iron, copper and brass molds are used.
Molds.
This is the most common type of multiple casting molds. Most often, molds are made of cast iron and are used to produce steel ingots at the initial stage of production of forged or rolled steel. Molds belong to open casting molds, since the metal fills them from above by gravity. “Through” molds are also used, open both at the top and bottom. The height of the molds can be 1–4.5 m, the diameter – from 0.3 to 3 m. The thickness of the casting wall depends on the size of the mold. The configuration can be different - from round to rectangular. The mold cavity expands slightly upward, which is necessary for removing the ingot.
The mold, ready for pouring, is placed on a thick cast iron plate. As a rule, molds are filled from the top. The walls of the mold cavity must be smooth and clean; When pouring, you need to make sure that the metal does not spill or splash onto the walls. The poured metal hardens in the mold, after which the ingot is removed (“stripping the ingot”). After the mold has cooled, it is cleaned from the inside, sprayed with molding paint and used again. One mold allows you to produce 70–100 ingots. For further processing by forging or rolling, the ingot is heated to a high temperature.
Kokili.
These are closed metal casting molds with an internal cavity corresponding to the configuration of the product, and a gating (pouring) system, which are made by machining in a cast iron, bronze, aluminum or steel block. The chill mold consists of two or more parts, after connecting which only a small hole remains at the top for pouring molten metal. To form internal cavities, gypsum, sand, glass, metal or ceramic “rods” are placed in the mold. Chill casting produces castings from alloys based on aluminum, copper, zinc, magnesium, tin and lead.
Chill casting is used only in cases where it is required to obtain at least 1000 castings. The lifespan of the chill mold reaches several hundred thousand castings. The chill mold goes into scrap when (due to gradual burnout from the molten metal) the surface quality of the castings begins to deteriorate intolerably and the calculated tolerances for their dimensions are no longer met.
Graphite and fireproof molds.
Such forms consist of two or more parts, when connected, the required cavity is formed. The form can have a vertical, horizontal or inclined parting surface or can be disassembled into separate blocks; this makes it easier to remove the casting. Once removed, the mold can be reassembled and used again. Graphite molds allow hundreds of castings, ceramic - only a few.
Graphite multiple molds can be made by machining graphite, and ceramic ones are easy to form, so they are significantly cheaper than metal molds. Graphite and refractory molds can be used for recasting in case of unsatisfactory die castings.
Fireproof molds are made from porcelain clay (kaolin) and other highly refractory materials. In this case, models made of easily machined metals or plastics are used. Powdered or granular refractory is mixed with clay in water, the resulting mixture is shaped and the casting mold blank is fired in the same way as bricks or dishes.
One-time forms.
Sand casting molds are subject to far fewer restrictions than any other mold. They are suitable for producing castings of any size, any configuration, from any alloy; they are the least demanding on the design of the product. Sand molds are made from a plastic refractory material (usually siliceous sand), giving it the desired configuration so that the poured metal, upon solidification, retains this configuration and can be separated from the mold.
The molding mixture is obtained by mixing sand with clay and organic binders in water in a special machine.
When making a sand mold, it is provided with an upper sprue hole with a “bowl” for pouring metal and an internal gating system of channels to supply the casting with molten metal during the solidification process, since otherwise, due to shrinkage during solidification (typical of most metals), voids may form in the casting (shrinkage cavities).
Shell forms.
These molds come in two types: low melting point material (gypsum) and high melting point material (fine silica powder). A gypsum shell mold is made by mixing gypsum material with water with a fastener (quick-hardening polymer) to a thin consistency and lining the casting model with such a mixture. After the mold material has hardened, it is cut, processed and dried, and then the two halves are “paired” and poured. This casting method is only suitable for non-ferrous metals.
Lost wax casting.
This casting method is used for precious metals, steel and other alloys with a high melting point. First, a mold is made that matches the part being cast. It is usually made of low-melting metal or (machined) brass. Then, by filling the mold with paraffin, plastic or mercury (then frozen), a model for one casting is obtained. The model is lined with fireproof material. The shell-shaped material is made from a fine refractory powder (for example, silica powder) and a liquid binder. The layer of fire-resistant cladding is compacted by vibration. After it hardens, the mold is heated, the paraffin or plastic model melts and the liquid flows out of the mold. Then the mold is fired to remove gases and, when heated, is filled with liquid metal, which flows by gravity, under compressed air pressure or under the influence of centrifugal forces (in a centrifugal casting machine).
Ceramic forms.
Ceramic molds are made from porcelain clay, sillimanite, mullite (aluminosilicates) or other highly refractory materials. In the manufacture of such forms, models from easily machined metals or plastics are usually used. Powdered or granular refractory materials are mixed with a liquid binder (ethyl silicate) to a gelatinous consistency. The newly made mold is flexible so that the model can be removed from it without damaging the mold cavity. Then the mold is fired at a high temperature and filled with a melt of the desired metal - steel, a hard brittle alloy, an alloy based on rare metals, etc. This method makes it possible to produce molds of any type and is suitable for both small-scale and large-scale production.
All metals can be cast. But not all metals have the same casting properties, in particular fluidity - the ability to fill a casting mold of any configuration. Casting properties depend mainly on the chemical composition and structure of the metal. Melting temperature is important. Metals with low melting points are easy to industrially cast. Of the common metals, steel has the highest melting point. Metals are divided into ferrous and non-ferrous. Ferrous metals are steel, ductile iron and cast iron. Non-ferrous metals include all other metals that do not contain significant amounts of iron. For casting, alloys based on copper, nickel, aluminum, magnesium, lead and zinc are used, in particular. ALLOYS.
Black metals.
Become.
There are five classes of steels for industrial casting: 1) low-carbon (with a carbon content of less than 0.2%); 2) medium-carbon (0.2–0.5% carbon); 3) high-carbon (more than 0.5% carbon); 4) low alloyed (less than 8% alloying elements) and 5) highly alloyed (more than 8% alloying elements). Medium-carbon steels account for the bulk of ferrous metal castings; Such castings are, as a rule, industrial products of a standardized grade. Various types of alloy steels are designed to achieve high strength, ductility, toughness, corrosion resistance, heat resistance and fatigue strength. Cast steels are similar in properties to forged steel. The tensile strength of such steel ranges from 400 to 1500 MPa. The mass of castings can vary over a wide range - from 100 g to 200 tons or more, the thickness in section - from 5 mm to 1.5 m. The length of the casting can exceed 30 m. Steel is a universal material for casting. Due to its high strength and ductility, it is an excellent material for mechanical engineering.
Malleable cast iron.
There are two main classes of ductile iron: regular grade and pearlitic. Castings are also made from some alloyed malleable cast irons. The tensile strength of ductile iron is 250–550 MPa. Its fatigue resistance, high rigidity and good machinability make it ideal for machine tools and many other mass production applications. The mass of castings ranges from 100 g to several hundred kilograms, and the cross-sectional thickness is usually no more than 5 cm.
Cast iron.
Cast irons include a wide range of alloys of iron with carbon and silicon containing 2–4% carbon. There are four main types of cast iron used for casting: gray, white, bleached and half-cast. The tensile strength of cast iron is 140–420 MPa, and some alloy cast iron is up to 550 MPa. Cast iron is characterized by low ductility and low impact strength; among designers it is considered a fragile material. The mass of castings ranges from 100 g to several tons. Foundry iron castings are used in almost all industries. Their cost is low and they can be easily processed by cutting.
Nodular cast iron.
Spherical inclusions of graphite give cast iron ductility and other properties that distinguish it favorably from gray cast iron. The spherical shape of graphite inclusions is achieved by treating cast iron with magnesium or cerium immediately before casting. The tensile strength of nodular cast iron is 400–850 MPa, ductility is from 20 to 1%. True, nodular cast iron is characterized by low impact strength of the notched sample. Castings can have both large and small thickness in cross-section, weight - from 0.5 kg to several tons.
Non-ferrous metals.
Copper, brass and bronze.
There are many different copper-based alloys suitable for casting. Copper is used in cases where high thermal and electrical conductivity is required. Brass (an alloy of copper and zinc) is used when an inexpensive, moderately corrosion-resistant material is desired for a variety of general purpose products. The tensile strength of cast brass is 180–300 MPa. Bronze (an alloy of copper and tin, to which zinc and nickel can be added) is used in cases where increased strength is required. The tensile strength of cast bronzes is 250–850 MPa.
Nickel.
Copper-nickel alloys (such as Monel metal) have high corrosion resistance. Nickel-chromium alloys (such as Inconel and nichrome) are characterized by high thermal resistance. Molybdenum-nickel alloys are highly resistant to hydrochloric acid and oxidizing acids at elevated temperatures.
Aluminum.
Cast products made of aluminum alloys have recently been increasingly used due to their lightness and strength. Such alloys have fairly high corrosion resistance and good thermal and electrical conductivity. The tensile strength of cast aluminum alloys ranges from 150 to 350 MPa.
Magnesium.
Magnesium alloys are used where the requirement for lightness comes first. The tensile strength of cast magnesium alloys is 170–260 MPa.
Titanium.
Titanium, a strong and lightweight material, is melted in a vacuum and cast into graphite molds. The fact is that during the cooling process, the titanium surface can become contaminated due to a reaction with the mold material. Therefore, titanium cast into shapes other than those made from machined and pressed powder graphite is heavily contaminated on the surface, which manifests itself in increased hardness and low flexural ductility. Titanium casting is used primarily in the aerospace industry. The tensile strength of cast titanium is over 1000 MPa with a relative elongation of 5%.
Rare and precious metals.
Castings from gold, silver, platinum and rare metals are used in jewelry, dental technology (crowns, fillings); some parts of electronic components are also made by casting.
CASTING METHODS
The main casting methods are: static casting, injection molding, centrifugal casting and vacuum casting.
Static fill.
Most often, static filling is used, i.e. pouring into a fixed mold. With this method, molten metal (or non-metal - plastic, glass, ceramic suspension) is simply poured into the cavity of a stationary mold until it is filled and held until it solidifies.
Injection molding.
A casting machine fills a metal (steel) mold (which is usually called a mold and can be multi-cavity) with molten metal under a pressure of 7 to 700 MPa. The advantages of this method are high productivity, high surface quality, accurate dimensions of the cast product and minimal need for machining. Typical metals for die casting are zinc, aluminum, copper and tin-lead alloys. Due to their low melting point, such alloys are highly technological and allow for close dimensional tolerances and excellent casting characteristics.
The complexity of the configuration of castings in the case of injection molding is limited by the fact that the casting can be damaged when separated from the mold. In addition, the thickness of the products is somewhat limited; more preferable are products with a thin section, in which the melt solidifies quickly and evenly.
There are two types of injection molding machines - cold chamber and hot chamber. Hot chamber pressing machines are mainly used for zinc-based alloys. The hot pressing chamber is immersed in molten metal; under a slight pressure of compressed air or under the action of a piston, liquid metal is forced out of the hot pressing chamber into the mold. In cold chamber casting machines, molten aluminum, magnesium or copper alloy fills the mold under pressure from 35 to 700 MPa.
Castings obtained by injection molding are used in many household appliances (vacuum cleaners, washing machines, telephones, lamps, typewriters) and very widely in the automotive industry and in the production of computers. Castings can weigh from several tens of grams to 50 kg or more.
Centrifugal casting.
In centrifugal casting, molten metal is poured into a sand or metal mold that rotates around a horizontal or vertical axis. Under the influence of centrifugal forces, the metal is thrown from the central sprue to the periphery of the mold, filling its cavities, and solidifies, forming a casting. Centrifugal casting is economical and for some types of products (axisymmetric such as pipes, rings, shells, etc.) is more suitable than static casting.
Vacuum filling.
Metals such as titanium, alloy steels and high-temperature alloys are melted in a vacuum and poured into multiple molds, such as graphite, placed in a vacuum. This method significantly reduces the gas content in the metal. Ingots and castings produced by vacuum casting weigh no more than several hundred kilograms. In rare cases, large quantities of steel (100 tons or more), smelted using conventional technology, are poured in a vacuum chamber into molds or foundry ladles installed in it for further casting in air. Large metallurgical vacuum chambers are evacuated by multi-pump systems. The steel obtained by this method is used for the manufacture of special products by forging or casting; this process is called vacuum degassing.
CASTING MOLDS
Casting molds are divided into multiple and one-time (sand) molds. Multiple forms are metal (molds and chill molds), or graphite or ceramic refractory.
Multiple forms.
Metal molds (molds and chill molds) for steel are usually made from cast iron, sometimes from heat-resistant steel. For casting non-ferrous metals such as brass, zinc and aluminum, cast iron, copper and brass molds are used.
Molds.
This is the most common type of multiple casting molds. Most often, molds are made of cast iron and are used to produce steel ingots at the initial stage of production of forged or rolled steel. Molds belong to open casting molds, since the metal fills them from above by gravity. “Through” molds are also used, open both at the top and bottom. The height of the molds can be 1–4.5 m, the diameter – from 0.3 to 3 m. The thickness of the casting wall depends on the size of the mold. The configuration can be different - from round to rectangular. The mold cavity expands slightly upward, which is necessary for removing the ingot.
The mold, ready for pouring, is placed on a thick cast iron plate. As a rule, molds are filled from the top. The walls of the mold cavity must be smooth and clean; When pouring, you need to make sure that the metal does not spill or splash onto the walls. The poured metal hardens in the mold, after which the ingot is removed (“stripping the ingot”). After the mold has cooled, it is cleaned from the inside, sprayed with molding paint and used again. One mold allows you to produce 70–100 ingots. For further processing by forging or rolling, the ingot is heated to a high temperature.
Kokili.
These are closed metal casting molds with an internal cavity corresponding to the configuration of the product, and a gating (pouring) system, which are made by machining in a cast iron, bronze, aluminum or steel block. The chill mold consists of two or more parts, after connecting which only a small hole remains at the top for pouring molten metal. To form internal cavities, gypsum, sand, glass, metal or ceramic “rods” are placed in the mold. Chill casting produces castings from alloys based on aluminum, copper, zinc, magnesium, tin and lead.
Chill casting is used only in cases where it is required to obtain at least 1000 castings. The lifespan of the chill mold reaches several hundred thousand castings. The chill mold goes into scrap when (due to gradual burnout from the molten metal) the surface quality of the castings begins to deteriorate intolerably and the calculated tolerances for their dimensions are no longer met.
Graphite and fireproof molds.
Such forms consist of two or more parts, when connected, the required cavity is formed. The form can have a vertical, horizontal or inclined parting surface or can be disassembled into separate blocks; this makes it easier to remove the casting. Once removed, the mold can be reassembled and used again. Graphite molds allow hundreds of castings, ceramic - only a few.
Graphite multiple molds can be made by machining graphite, and ceramic ones are easy to form, so they are significantly cheaper than metal molds. Graphite and refractory molds can be used for recasting in case of unsatisfactory die castings.
Fireproof molds are made from porcelain clay (kaolin) and other highly refractory materials. In this case, models made of easily machined metals or plastics are used. Powdered or granular refractory is mixed with clay in water, the resulting mixture is shaped and the casting mold blank is fired in the same way as bricks or dishes.
One-time forms.
Sand casting molds are subject to far fewer restrictions than any other mold. They are suitable for producing castings of any size, any configuration, from any alloy; they are the least demanding on the design of the product. Sand molds are made from a plastic refractory material (usually siliceous sand), giving it the desired configuration so that the poured metal, upon solidification, retains this configuration and can be separated from the mold.
The molding mixture is obtained by mixing sand with clay and organic binders in water in a special machine.
When making a sand mold, it is provided with an upper sprue hole with a “bowl” for pouring metal and an internal gating system of channels to supply the casting with molten metal during the solidification process, since otherwise, due to shrinkage during solidification (typical of most metals), voids may form in the casting (shrinkage cavities).
Shell forms.
These molds come in two types: low melting point material (gypsum) and high melting point material (fine silica powder). A gypsum shell mold is made by mixing gypsum material with water with a fastener (quick-hardening polymer) to a thin consistency and lining the casting model with such a mixture. After the mold material has hardened, it is cut, processed and dried, and then the two halves are “paired” and poured. This casting method is only suitable for non-ferrous metals.
Lost wax casting.
This casting method is used for precious metals, steel and other alloys with a high melting point. First, a mold is made that matches the part being cast. It is usually made of low-melting metal or (machined) brass. Then, by filling the mold with paraffin, plastic or mercury (then frozen), a model for one casting is obtained. The model is lined with fireproof material. The shell-shaped material is made from a fine refractory powder (for example, silica powder) and a liquid binder. The layer of fire-resistant cladding is compacted by vibration. After it hardens, the mold is heated, the paraffin or plastic model melts and the liquid flows out of the mold. Then the mold is fired to remove gases and, when heated, is filled with liquid metal, which flows by gravity, under compressed air pressure or under the influence of centrifugal forces (in a centrifugal casting machine).
Ceramic forms.
Ceramic molds are made from porcelain clay, sillimanite, mullite (aluminosilicates) or other highly refractory materials. In the manufacture of such forms, models from easily machined metals or plastics are usually used. Powdered or granular refractory materials are mixed with a liquid binder (ethyl silicate) to a gelatinous consistency. The newly made mold is flexible so that the model can be removed from it without damaging the mold cavity. Then the mold is fired at a high temperature and filled with a melt of the desired metal - steel, a hard brittle alloy, an alloy based on rare metals, etc. This method makes it possible to produce molds of any type and is suitable for both small-scale and large-scale production.