The composition of the molding sand for casting bronze. Precision Lost Wax Casting at Home: Technology, Advantages and Disadvantages

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mold making

Molding materials and mixtures

Forming materials. Molding materials used for the manufacture of molds and cores are divided into the following groups: sands, binders, non-stick, highly refractory, special and auxiliary.

Cesks (quartz, clay) were formed as a result of the destruction of rocks (granite, basolite, etc.); they consist of grains of the mineral quartz (Si02) 0.06-1.6 mm in size with an admixture of clay and other minerals (iron oxides, feldspars). Quartz has a high hardness and high refractoriness (melting point 1713 °C).

Quartz sands contain up to 2% clay and a small amount of impurities, clay sands contain up to 50% clay. According to the clay content, clayey sands are divided into lean (2-10%), bold (10-20%), fatty (20-30%) and very fatty (30-50% clay).

Binders: molding clay, water glass, sulphite stillage, various binders, ethyl silicate, pulverized bakelite, etc.

Molding clay has high fire resistance (melting point 1750-1787 ° C) and consists of very small (0.001 mm) mineral particles, which form sticky solutions when interacting with water.

Liquid glass, sulfite stillage, binders are introduced into mixtures, non-stick paints and other compounds to give them strength.

Non-stick materials (graphite, pulverized quartz, talc, coal, etc.), as well as foundry paints prepared from them, rubbing pastes are applied to the surface of molds and cores in order to prevent molding materials from burning to the surface of castings. Graphite and pulverized quartz are used as powder and in the preparation of paints and rubbing. Coal is added to the composition of molding sands.

Highly refractory materials (chamotte, chromium iron ore, zircon, magnesite, asbestos, etc.) are used in the manufacture of foundry molds and cores for very large and massive castings from alloyed (stainless, heat-resistant, etc.) steels, as well as reusable molds.

Special materials - cast iron shot, caustic soda, formalin, sawdust, peat, etc. Cast iron shot is used in the manufacture of castings by a special casting method as a filler. Sawdust, peat, etc. are introduced into mixtures to increase gas permeability and compliance of dried molds and rods.

Auxiliary materials - model powders, separating liquids, glue, etc. Modeling powders and separating liquids are used in the manufacture of molds and cores so that when removing the model from the mold, as well as the core from the core box, do not damage their surface. Glue is used in the assembly of rods and molds for gluing halves.

The main properties of molding materials: thermal conductivity, heat capacity, gas permeability, strength, fluidity, etc.

molding mixtures. A wide variety of molding sands are currently used in foundries. The choice of composition of mixtures is determined by the nature (weight, dimensions, shape, type of alloy) of the castings being produced, as well as the type of molds used (raw, dry, surface dried, chemically hardening).

Depending on the purpose, the mixtures are divided into facing, filling and single.

The facing mixture is of the highest quality and is used to cover the working surface of the mold in direct contact with the molten metal. The thickness of the facing mixture layer depends on the type and nature of the casting (15-50 mm).

The filling mixture is poured over the facing, has less strength and gas permeability and is cheaper. The filling mixture is prepared by processing the used molding sand with the addition (3-5%) of fresh materials (sand and clay).

A single mixture makes up the entire volume of the mold and is used in machine molding, on automatic machines in the conditions of mass production of small and thin-walled castings. It differs from the filler mixture by a high content of fresh materials and better physical and mechanical properties.

Core mixtures. The composition and properties of core mixtures are determined mainly by the class of cores produced.

Responsible cores of the first class are made from core mixtures consisting entirely of quartz sand with the addition of binders. Large rods are made from cheaper core mixtures, they very often include a used mixture (20-35%), and the binder is molding clay, sulfite stillage, and sawdust as an organic additive.

Core mixes should have the same properties as molding mixes. But considering that most of the rod (except signs) is exposed to high temperature and pressure of the metal poured into the mold, they are made with higher strength, gas permeability, ductility and fire resistance.

The composition of core mixtures most often includes pure quartz sand from 70 to 100%;, refractory clay or bentonite, and various kinds of binders. Such mixtures have high gas permeability up to 120, strength up to 0.55 in the raw state and up to 12 kg/cm2 in the dry state. In recent years, liquid self-hardening mixtures with good technological properties have been widely used for the manufacture of rods.

Regeneration of spent mixtures. Waste mixtures that accumulate in the trimming and cleaning department (from core knockout, from cleaning machines), spillage collected from the floor of the molding and core departments, drying chambers, etc. are subjected to regeneration. Such a mixture contains a lot of dust, ash from burnt sawdust and coal , pieces of rods and molds, various metallic and non-metallic inclusions, as well as up to 60-80% of sand grains suitable for further use. To extract sand grains from this mixture, it is subjected to processing: kneading clods, magnetic separation, screening and dedusting.

Obtaining high-quality castings largely depends on the quality of molding materials and mixtures from which molds and cores are made.

Molding materials are divided into the main ones - sands, clays and auxiliary ones, which include binders used for the preparation of core mixtures, non-stick materials (coal, graphite, paints, chromium iron ore, zircon, etc.), as well as glue, putties, powders and etc.

foundry sands

Molding sands are supplied in natural and enriched states. According to GOST 2138-74, sands, depending on the content of the clay component (the so-called grains with a diameter of less than 0.022 mm), silica and harmful impurities, are divided into classes, and depending on the size of the grains of the main fraction - into groups.

To determine the group of sand, it must be sieved through a standard set of sieves and find out on which three adjacent sieves the largest amount of residues (in mass units), called the main fraction, remained. Knowing on which sieves the main fraction of sand is located, it can be attributed to the group, which is determined by the average sieve number.

molding clays

Molding clays used in foundries as mineral binders in molding and core sands are classified according to their mineralogical composition, tensile strength in wet and dry states, content of harmful impurities, and some other properties.

According to the mineralogical composition, molding clays are divided into types, according to the compressive strength in the wet state - into groups, in the dry state - into subgroups. Depending on the content of harmful impurities, molding clays are divided into groups.

The main difference between molding clays is that they have different crystal lattices, and therefore water films of different thicknesses can form on the surface. The smallest amount of water can be retained on the surface of kaolinite grains, and the largest - on the surface of montmorillonite grains. From this it follows that montmorillonite (bentonite) clays should be used when molding on a wet basis. The use of these clays makes it possible to reduce the content of the clay additive in mixtures by a factor of 2–3, increase their gas permeability, in some cases replace dry molding with wet molding, improve the surface of castings, etc. Any type of clay can be used in dry molding.

When preparing molding and core sands, all components, with the exception of water and liquid binders, are loaded into mixers in ground or loose form. Since the process of obtaining clay powder is associated with abundant dust emission, in the manufacture of raw iron castings, clay or clay-coal suspensions are used instead.

3. Bonding materials

Core mixtures in which molding clay is a binder, as a rule, do not provide such qualities of cores as strength, gas permeability, knockout. As a result, clay has to be replaced by materials that have a high binding capacity and give the rods significant strength while maintaining good knockout and gas permeability.

Binders are divided into organic and inorganic and into three classes:
A - organic non-aqueous, B - organic water and C - inorganic water.

Class A combines binders that have a binding capacity and do not require the addition of water. They do not dissolve in water, do not mix with it and are not wetted by it (oils, drying oils, pitches, bitumens, rosin). Class B includes binders that dissolve in water, after which they acquire the ability to bind sand (dextrin, sulphite-alcohol stillage and mash). Class C includes all inorganic binders (molding clay, cement, liquid glass), which, like class B binders, have their effect only after adding water to them.

For ease of use, binders of each class are divided into three groups. Each of the three groups includes binders with approximately the same physical, mechanical and technological properties. The main sign of assigning a binder to one or another group is the strength (tensile strength, in kgf/cm2, of a test sample in a dry state) per 1% of the binder introduced into the mixture.

Binders are evaluated according to a technological sample in laboratory conditions. Samples are made from the obtained mixture with a binder for testing wet compressive strength and dry tensile strength, as well as gas permeability. Drying of the samples is carried out in accordance with the specifications for this binder.

In most cases, binders are by-products obtained from the processing of oil, oil shale, wood, cottonseed oil, etc.

4. Non-stick and other auxiliary materials

As a result of the chemical and mechanical interaction of a mold or a rod with a liquid alloy, insufficient refractoriness and increased porosity of the mixtures, as well as high pouring temperatures, burns form on the castings. To combat it, special non-stick materials are used.

Coal. When molding on a raw basis, coal additives are introduced into the mixture in a crushed state of the following composition, in%: volatile substances - not less than 30, sulfur - not more than 2 and ash - not more than 11, moisture - not more than 12. Coal can be replaced Estonian slate in powder form.

When the mold is heated with a liquid alloy, particles of coal or shale dust emit volatile substances and burn with the formation of carbon monoxide, while a gas layer forms between the alloy and the mold, which eliminates the possibility of wetting the sand grains with the alloy and the formation of burning.

Pulverized quartz. There are two types of this material: natural and artificial. The greatest application is artificial pulverized quartz, which is obtained by grinding quartz sand.

Pulverized quartz is used in the production of steel castings as an additive in facing mixtures. This reduces the porosity of the working layer of the mold or core, resulting in reduced mechanical sticking.

When powdered quartz is introduced into the composition of the paint for coating the mold and the core, a highly refractory layer is formed on the surfaces, which protects them from the influence of the high temperature of the poured alloy.

Zircon. When enriching titanium-zircon ores, a material called zircon is obtained. The industry produces zircon concentrate for the preparation of facing molding and core sands and zircon powder for paints.

Zircon is a highly refractory material (its melting point is 2190 °C), it does not enter into a chemical combination with iron and alloying elements and is a good non-stick material.

Chrome iron. The product of grinding chromite ore - chromium iron ore is characterized by high refractoriness. Its melting point is about 1850 ° C. The lack of affinity for iron oxides and the constancy of volume when heated provide high-quality castings.

Apply facing molding and core mixtures of the following composition, in : chrome iron ore (sieved through a sieve with cells of 1.5 × 1.5 mm) -100 and over 100 sulfite-alcohol bard - 2-3.

Physical and mechanical properties of the mixture: compressive strength in the raw state - 0.5-0.7 kgf/mm2; humidity - 5-6%.

The thickness of the facing layer should be 10-30 mm, and the sublayer of sand-clay mixture - 40-60 mm. The rest of the flask is filled with the usual filler mixture, and the rods - core sawdust mixture.

Graphite. Graphite, widely used in iron foundries, is a highly refractory material. There are crystalline graphite - in the form of silvery flakes and cryptocrystalline (amorphous) - in the form of a black powder.

Sprays and paints. When molding on a wet surface, the molds are covered with various powders (silvery graphite, slate, cement, etc.). To improve the surface strength of the mold, along with dusting, spraying of surfaces with sulphite-alcohol stillage (density 1.1) or molasses (density 1.28) is used.

Paints and rubbing are used to cover molds and rods dry. They include non-stick materials (amorphous graphite, powdered quartz, talc, ground coke, etc.) and binders (bentonite clay, sulfite bard, molasses, etc.). Formalin is introduced into them to protect paints from fermentation.

Rubbing pastes, putty and glue. Rubbing pastes are used in cases where the cavities formed by the rods are not subsequently subjected to mechanical processing and require high dimensional accuracy and surface cleanliness. For especially critical rods for iron castings, pastes of the following composition are used: graphite silver - 1 part; amorphous graphite - 1 part; sulfite-alcohol bard - until a homogeneous paste in the form of thick sour cream is obtained.

Rod adhesives are used for gluing and repairing rods. Sulphite glue consists of 5 parts of sulphite-alcohol stillage, 5 parts of molding clay and 2 parts of water. The glue is applied in an even layer on the surfaces of the halves of the rods to be glued.

When pairing large and medium-sized rods, the seams are sealed with special putties, which include, in%:
fine quartz sand - 60, black graphite - 25 and molding clay - 15.

5. Basic properties of molding materials and mixtures

Molding materials and mixtures from which casting molds and cores are made must have certain properties that ensure the production of high-quality molds, cores and castings.

Humidity affects all properties of sands and mainly on gas permeability, strength and fluidity. Reduced humidity increases the crumbling of the mixture and makes molding difficult, and increased humidity reduces the wet strength, increases the adhesion of the mixture to the model and reduces gas permeability, resulting in the risk of formation of casting boil.

Gas permeability is a very important property of molding materials and mixtures. The low gas permeability of mixtures can be the reason for the formation of gas pockets in castings. Gas permeability depends on the shape of the grains, the homogeneity of the grain components of the mixture, on the content of clay substances in it, and a number of other reasons. To increase the gas permeability of fine sand, it must be mixed with 50-60% of coarse sand.

Strength. Insufficient strength of molding sands leads to deformation of molds and cores, distortion of castings, causes gaps and collapses. The strength depends on the moisture content of the mixture, the amount of clay component, the grain size of the sand and the degree of compaction. It is regulated by the dosage of clay.

Dry strength of molding sands increases with increasing clay and moisture content. Higher strength can be achieved by using special bonding materials.

The strength of core mixtures depends on the type and amount of binder used and must be within certain limits.

Hardness characterizes the degree and uniformity of compaction of molding sands. Overconsolidation, as well as insufficient compaction of the mixture, causes casting defects: gaps, boiling, gas and earth sinks, burns, etc.

The determination of these and other properties of molding materials and mixtures is carried out in workshop laboratories.

6. Sands

In the foundry, sand-clay mixtures are most widely used, which are classified according to the method of molding and the type of alloy poured into the molds.

Mixtures are divided into uniform facing and filling. A single mixture is called a mixture used to stuff the entire mold (mainly in machine molding). Facing mixtures make out only that part of the mold that is in contact with the liquid alloy. The filler mixture is applied to the facing layer, and the rest of the mold is filled with it.

According to the state of the mold before pouring, mixtures for molding are distinguished for wet and dry molding. According to the type of alloy poured into molds, molding sands for iron, steel and non-ferrous castings are distinguished.

The composition of the mixture for iron casting depends on the mass of the casting, the wall thickness and the mold manufacturing technology.

For steel castings, molding sands must have higher refractoriness and gas permeability than sands for iron castings.

For non-ferrous casting molds, mixtures with significantly lower refractoriness than those for iron and steel casting mixtures can be used.

To improve the surface cleanliness of castings from copper-based alloys, clay sands of class P are introduced into the composition of the molding sand. It can be replaced with boric acid or sulfuric acid.

7. Fast curing, chemically curing and self-curing plastic and liquid mixtures

Along with the usual sandy-clayey sands, molding sands with special properties developed in our country have become widespread.

Fast setting mixes.

The binding material in them is also liquid glass. However, the curing process is carried out not by purging with carbon dioxide, but under the action of an additive mixture of hardener - ferrochromium production slag. The survivability of the plastic mixture is usually 20-25 minutes, so it is prepared in two stages: the main liquid-glass mixture is produced in the mixture preparation department, and the introduction of slag into it, sifted through a sieve with 0.5 mm cells, is carried out directly at the molding section with stirring in a screw mixer.

The facing mixture is applied to the model with a layer of 50 mm or more, depending on the dimensions and wall thickness of the casting. The rest of the flask is filled with recycled mixture. The exposure time for large molds is at least 1 hour. After removing the model, the mold is painted with self-drying refractory or ordinary water-based paint. In the latter case, surface drying is used.

Liquid self-hardening mixtures (ZhSS) differ from plastic ones in that surfactants (surfactants) are introduced into their composition, which, when the mixture is mixed, form foam at the grain boundaries. The bubbles of this foam reduce the friction forces between the grains of sand, which makes the mixture fluid (fluidity). As a surfactant, the Soviet refined detergent (DS-RAS) is most often used.

ZhSS is used in the manufacture of large castings and cores, and, unlike all mixtures, they are “poured” into flasks and core boxes. The flow time of the mixture is usually 9-10 minutes, during which it must be used. The installation for the preparation of the JSS is placed directly on the molding or core sections. Plant productivity - up to 30 t/h.

8. Core mixtures

9. Technology for the preparation of molding core sands

The technological process for the preparation of molding and core sands consists of three stages: the preparation of fresh materials, the preparation of spent sands and the production of sands.

The preparation of fresh materials consists in their drying, crushing and screening.

Drying of sand and clay is carried out in drum dryers with a capacity of 3.2 to 29.2 t / h for sand and 0.9-8 t / h for clay, as well as in installations for drying and cooling sand in a fluidized bed with a capacity of 3- 10 t/h

For crushing and grinding clods of sand and dry clay, coal, clods of waste mixture, dry defective rods, grinding runners, roller crushers, ball mills for wet grinding of coal are used.

Screening of molding materials before use is carried out in mobile dredgers, as well as in vibrating and polygonal sieves with a capacity of 5 to 125 t/h and through flat sieves with a capacity of 50 t/h.

The preparation of the spent mixture consists in its magnetic separation for the extraction of metal inclusions. Mixtures used in sandblasting are subjected to double separation.

Preparation of mixtures. The technological process of preparing molding sands consists of dosing dry components and loading them into runners in the following sequence: recycled sand + sand + clay in powder or in the form of an emulsion - coal (for cast iron castings molded in wet) or sawdust (for molding in dry ); after preliminary mixing, liquid components are added.

To mix the components, batch runners with vertically rotating rollers or centrifugal rollers with horizontally rotating rollers are used.

In the foundries of serial and mass production, central mixing departments are being created, equipped with high-performance modern equipment and an extensive transport system. In some of them, the management of all operations for the preparation of mixtures is comprehensively mechanized and automated.

10. Regeneration of spent molding and core sands

The widespread introduction in the foundry of special mixtures prepared from fresh quartz sands, as well as the annual increase in the production of castings, leads to a systematic increase in the consumption of quartz sands, whose natural resources are not unlimited. In order to reduce consumption, they must be partially replaced with regenerated (restored) sands from waste mixtures currently dumped.

Rice. 1. Installation for the regeneration of waste mixtures.

The five-year experience of the installation has shown that the resulting regenerate is a full-fledged substitute for fresh quartz sand and can be used to prepare molding and core sands.

Consider the three most famous of them:

1. Casting according to patterns in earthen molds.
2. Investment casting.
3. Casting on burnt models.

Molding tools and fixtures for mold casting

Forming tools (tools used to stuff the mold and remove the model or template from it): spatula, sieve, tamper, ruler, special straightener, ventilation needle, mallet, trowel, spatula, brushes.

Form finishing tools: trowels, finishing and scoring lancets.

Tooling for mold casting

Flask - a frame (a box without a bottom) with molding earth for pouring metal; wooden or metal.

Rice. 2. Forming tool: 1 - ventilation needle; 2 - molding rammer; 3 - right bar

Rice. 3. Lancet

Under-model board - a wooden or metal plate with a smooth surface.

Rubber mold - a device made of rubber, two polished steel plates and a vulcanizer (in a private workshop, an automobile, 12 V through a transformer, is quite suitable).

Press syringe - homemade syringe for filling under pressure the model composition into the mold.

Manual centrifuge - a device for centrifugal casting in an individual workshop; with the help of such a device, liquid metal fills the mold under pressure.

Materials for molding Molding earth - moist mixture of clay (up to 25% content) and sand.

• Graphite.
• Gypsum.
• Pumice.
• Quartz.
• Glucose (as a moderator).
• Alkalis (as a separator).
• Limestone (slate).
• Kaolin.

Materials for making models

1. Plasticine, plaster, plastic, wood. 2. Wax, paraffin, stearin; technical gelatin, wood glue. 3. Polystyrene (polystyrene) - cellular plastic.

Model casting in earth molds

This is the simplest way to obtain castings. In short, the technology is as follows: according to the desired template (model), a mold is made from molding earth for pouring molten metal. A mold made according to one pattern or another is disposable: when the casting is removed, it is destroyed, since it is created from a mixture of sand and clay (25% clay mixture content, 75% sand). But the mixture itself to obtain a mold can be used repeatedly, updating only the inner facing layer. The template can be made of any material - plasticine, gypsum (the most acceptable and convenient materials), wood, plastic, metal. The part itself can serve as a model; if you need to make the same (restore its original appearance), then the plasticine builds up the missing parts on the restored or restored part according to the initial sample.

If for some reason it is impossible to use a plasticine copy of the original as a model, there is still a way out: you can make a plaster cast from the original (albeit a more time-consuming and troublesome method).

The process of obtaining a plaster model of the product is as follows: the original is placed face up on a flat slab in a frame made of wood or other material, while the sides of the frame must be higher than the copied product and smeared with soap suds from the inside.

Gypsum is dissolved in an abundant amount of water to the state of a liquid creamy mass. At a fast pace, the original is carefully covered with a layer of liquid gypsum, applied with a wide paint brush, and then the frame is filled with gypsum mortar to the brim. You can speed up or slow down the setting of gypsum: in the first case, you need to add 4% sodium chloride solution, in the second - 1% acetic acid solution. Next, the plaster mold (cast) is dried at a temperature not exceeding 50 "C, processed in a counter-relief, the relief is increased as necessary, the protrusions are smoothed, the shells are sealed. Before the direct production of the model, the cast is covered with a 3% solution of lye, and even more simply - with well-whipped soap foam, which will create a separating layer, and pour liquid plaster.Thus, the template is ready, and you can start molding it.

The process of forming a template and obtaining a finished casting

The flask is placed on a fake board, on which a template or original is also placed. The board is sprinkled with graphite so that the facing mixture does not stick, which is poured through a sieve to completely cover the model. The flask is tightly filled to the brim, laying the earth in layers and compacting with a rammer, and the excess earth is smoothed with a special bar or even plank, passing along the edges of the flask, and turning it over; a second flask is placed on top, in which cone bars are molded - models of the sprue and sprue. Then, having removed the upper flask, the bars are removed, and the template is removed from the lower flask, after which narrow connecting channels are cut from the template cavity to the holes left from the models of the sprue and the upstream. The flasks are combined in the same position and liquid metal is poured through the sprue, which flows into the mold cavity, and air is displaced from the mold through another channel directed to the upstream, the mold is evenly and completely filled with metal. The target casting has been received.

Rice. 4. Technology for obtaining a casting in the simplest way: 1 - model; 2 - under-model plate; 3 - flask; 4 - extrusion; 5 sprue

Investment casting technology

The investment casting process is based on the use of fusible materials: the casting model and its gating system are made of wax, paraffin or stearin. Any of these fusible materials is hot poured into a mold, and after hardening, a wax model is obtained and coated with a special composition. After drying, a refractory shell is formed on the model - a ceramic mold, from which the model composition is smelted and a thin-walled casting mold is obtained, which, after calcination, is poured with molten metal.

To obtain several identical wax models, an elastic form is used, using wood glue or technical gelatin for its manufacture. The second material is more preferable both in terms of quality and preparation time. If the gelatin swells in half an hour (150 g of gelatin per 15 mg of water with regular stirring), the wood glue is soaked in water for a day. Gelatin swells after some addition of water, but when heated, it regains its former volume. The gelatin mass is boiled until it is homogeneous, resembling thick sour cream in appearance, 708 ml of hot water with a plasticizer (3-4 g of glycerin) are added and mixed thoroughly. In order to protect the resulting mass from mold during storage, half a gram of an antiseptic - formalin or phenol - is poured into it. After that, the mass is cooled to 50ºС and the sample is poured into it. So that the elastic form does not deform after solidification, it is additionally reinforced with plaster from the back. When molding a plaster model in the form of glue, it is degreased by wiping with talcum powder and tanned twice with a 20% solution of aluminum alum.

In order to replicate wax models for casting identical parts, for example, cast ornaments for a manor fence, a rubber mold is made.

Molds are divided into split and split. Detachable ones are equipped with bearing balls, which serve as locks-retainers of the parts of the mold, and they are placed on the bottom of the rubber mold so that they do not interfere with the extraction of the wax model.

In a split mold, there is no need for bearing balls. Sheets of raw rubber are cut to the size of metal clamping plates, washed with gasoline and folded into stacks, which are layered depending on the size of the model. The mold itself consists of two halves, between which a metal model is placed, the rubber around which is rubbed with talcum powder. After that, the bag is placed on a talcated clamping plate, covered with a second plate and clamped in the clamp of the vulcanizer for 40-50 minutes at a temperature of 140-150ºС. After vulcanization, the released package, together with the plates, is cooled under water. If there was no sprue on the sample, then it is cut out directly in the mold.

Rice. 5. Making a rubber mold: 1 - vulcanizer; 2 - steel plates; 3 - raw rubber; 4-lock (steel balls); 5 - sample

A rubber mold is very convenient when making a large number of identical parts - chain links, bracelets, elements of detachable ornaments and other decorative items, since many wax models are required to cast them.

There are fusible and refractory compositions for creating models. The former are more pliable, they are made on a paraffin and stearin basis (see Table 1).

Table 1. Compositions for creating models

prescription number	Components, minimum %
	Paraffin	Stearin	Wax	Remelted
1	50	50	-	-
2	25	25	50	-
3	12	8	-	80
4	17	17	-	66

The model composition is pressed into the mold under the pressure of a press syringe, which the caster can easily make himself. This requires a piece of pipe, 2 fittings, a piston, an aluminum tube.

This is how it is made. On the one hand, the pipe is welded or soldered. A piston is cut out of aluminum along the pipe hole, which must be equipped with a handle (the rod is equal to the length of the pipe). A hole is drilled in the embedded part of the pipe, into which a fitting is soldered for a rubber hose, the other end of which is provided with a fitting fitting corresponding to the diameter of the mold sprue.

Filled with a model composition, the press syringe is dipped into boiling water until the melt is ready, which is thoroughly mixed and cooled to a paste-like state at a temperature of 55-60ºС and pressed into a talcum mold.

Rice. 6 Manual centrifuge

Also, under pressure, molten metal is fed into the mold.

Also, the caster can independently make one more device necessary for work - a manual centrifuge.

A steel rod with a diameter of 7 mm must be passed into a wooden handle, an earring must be fixedly attached to it (while the handle must rotate freely on the rod). A steel cylinder will serve as a support for the flask, the bottom of which is no more than 100 mm in diameter. A bracket with a ring in the middle is welded to the stand, which is connected to an earring with a rocker arm (40 cm) made of strong wire with reliable rings at the ends. The flask should fit freely in the stand and duplicate it in shape - the same cylinder, but without a bottom.

The model is formed in this way. With molten wax, steel needles are attached to the model - sprue pins, which must intersect at one point, where they are also fastened with wax. Based on the dimensions of the model, the flask is chosen so high that there is a gap of at least a centimeter between its bottom and the model, and at the top in the molding mass it would be possible to cut a gating cup for melting metal.

The composition of the molding mass of the proposed recipes (see table.2).

Table 2. Compositions of molding composition

The finished molding mass is filled into a flask on a refractory sheet (asbestos). Taking the model by the pin, it is immersed in the uncured molding mass, slightly shaking from side to side so that air does not enter. After the mass has hardened (in the presence of a moderator - not earlier than in an hour), a gating cup is cut out in the upper part of the flask and the pins are pulled out. Sprue channels should be in the center of the bowl.

The operation of melting (removing) the wax model is as follows: the flask is placed in a lit oven of a gas stove and gradually, in order not to damage the shape, the temperature is raised to 350 ° C for about two hours; then the flask is taken out and placed alternately with one or the other side on the burner, having previously placed asbestos tiles, and the wax is finally melted.

Receiving a casting

As soon as the sides of the flask are red-hot, it is placed in a manual centrifuge, and the gating bowl is loaded with metal with the addition of an appropriate flux and melted on a burner flame. After complete melting, the centrifuge is rotated, as a result of which the liquid metal rushes into the mold cavity, filling it and crystallizing in approximately 20 centrifuge revolutions. The process ends with cooling in water and the removal of the finished casting, that is, artistic casting products.

The most advanced investment casting method is considered to be a process in which the original is preserved and hollow products are obtained, the original serving as a model. Technologically, this method consists of two parts: first, a hollow model is made according to the original, and then a casting mold is made according to this model.

The process of obtaining castings on burnt models

To trace the technology of this method, consider a specific example - the manufacture of a complex figured vase or goblet.

When casting a goblet, the upper part of the model of a simple geometric shape is made of any material, the lower, more complex, is cut out of foam. After that, having laid the upper part of the model on the under-model plate, they begin molding in the flask. When the molding earth is compared with the level of the model, the second (foam) part is attached to it and molded to the end. Next, the flask is turned over, a second flask is installed on it and final molding is carried out, while making a gating system. After the flask, the upper part of the model is unclenched and removed, and the lower (foam) part is left molded into the ground.

When using such combined methods, one-piece, fairly high-quality castings of complex shape are obtained. However, at the moment of forming the model element, they can move relative to each other. To prevent this from happening, sewing needles or pins are inserted into the joints of the plaster mold and the foam plastic elements, on which the elements are pricked. Multiple needles can be used to prevent axial rotations.

For the manufacture of a hollow model, the flask is installed on a model plate and half of the original product is molded into it with earth - the so-called false flask is performed.

Rice. 7. Molding of the combined model: 1 - foam part of the model; 2 - plaster part of the model

The surface of the original, which is small in size, is smeared with soap suds and covered with a layer of plasticine up to 1 cm thick. Larger products are covered with a layer of clay. In order for the clay not to stick to the original, paper is used as a separating layer. A second flask is placed on top of the fake flask with the original on top and filled with plaster. Gating channels are made in gypsum, which reach the plasticine or clay layers. After the gypsum has hardened, the flasks are turned over. The false flask that is on top is removed along with the ground and a new one is installed.

Rice. 8. Making a model: 1 - flask; 2 - model board; 3 - molding sand; 4 - sprue; 5 - extrusion; 6 - additional holes; 7 - model

A layer of plasticine or clay is also laid on the second half of the original, previously in a false flask. Having lubricated the lower flask, filled with gypsum, with soapy foam, the upper flask is poured with gypsum, leaving sprue holes. When the plaster hardens, the top flask is removed and the plasticine or clay layer is removed, making sure that nothing remains on the original. Then the flask is set in place.

After removal of the lining layer between the gypsum poured into the flask and the original, a free space was formed corresponding to the thickness of the lining layer. A solution based on carpentry glue or technical gelatin is poured into the resulting cavity through the sprue channels left in the gypsum layer.

The flasks are turned over after the adhesive solution has cooled, the separating layer is removed from the second flask and filled with the adhesive solution. Then the flasks are separated, and the original product is removed from the resulting form. Due to the elasticity of the adhesive solution, it is possible to mold a product with a complex surface shape (patterns, ornaments, fonts, etc.), as well as having sinuses, which is difficult to perform with the usual molding method. In addition, the adhesive mass is the protection of the original. The inner surface of the adhesive shirt is varnished, and after drying, a layer of wax is applied with a brush.

The mold is assembled and molten rosin is poured into its cavity through a previously left hole, which is immediately poured out of the mold until it has cooled down, but part of it remains on the walls. This operation is repeated until the required product thickness is reached. Do not overheat the rosin melt, as small elements of the adhesive form may melt.

After the rosin layer has hardened, the flasks are carefully separated and the resulting model is removed, which is a hollow thin-walled copy of the original, which will serve as an investment model.

The molding of hollow products begins with the manufacture of the core. The core is the part of the sand that is filled into the mold cavity. The basis of the rod can be a metal frame made of wire, the diameter of which depends on the size of the model. The basis of the frame is a thicker rod, the end of which comes out of the model. After the frame is made, it is inserted into the cavity of the model and filled with molding mass. As a core, as well as molding mass for small products cast from metals with a low melting point, you can use a mass based on gypsum and talc or gypsum and quartz. When using masses based on gypsum, it should be remembered that there is practically no gas permeability in these masses, therefore, during the molding process, it is necessary to make additional holes for the release of gases formed at the time of melting the model.

If casting is performed from bronze, brass or other metals with a high melting point, quartz, quartz sand with the addition of office silicate glue are used as the core mass. The sand is calcined at a temperature of 750-900 ° C in a cast-iron container, for example, in a frying pan, so that iron oxides do not get into it. Liquid glass in the mixture should be contained within 30%, the rest is sand.

When casting large products, 1-2% of technical borax or boric acid is added to the molding sand, which, having their own melting point of 741 ° C and 575 ° C, respectively, melt at the moment of annealing the mold and, enveloping the grains of the filler, fasten the molding mass.

The smelted model with the core is molded into a flask in the usual way. The rosin model is smelted in an oven, gradually raising the temperature. The flask is placed with the gating system down. Molten rosin will come out through it, so a container must be placed under the outlet of the gating system. In this case, the mold walls will be reinforced with molten rosin particles. When the rosin is completely drained, the mold is calcined in a muffle furnace. If it is not available, then this can be done in a gas stove oven at a temperature of 350 ° C, since the rosin begins to char at a temperature of 310 ° C. The resulting soot from the burnt rosin covers the walls of the mold, which improves the quality of the casting.

It is advisable to use a flask with a bottom, mold the model with the usual molding sand, and make the top layer not in contact with the model from a mixture of quartz sand or fireclay chips with liquid glass. At the time of melting the model, it will hold the entire mold in the flask. The metal fed into the mold through the gating system will fill it due to the pressure of its own weight.

If a hollow model has one hole through which the reinforcement of the rod comes out, after smelting its rod loses its support and settles inside the mold.

To fix it in the desired position when making large castings or when casting products that have invisible places (for example, vases), the main rod with the reinforcement fixed to it is passed through the model through and supported with both ends on the edges of the flask, giving it a strictly fixed position .

The holes remaining after the casting of the product and the removal of the reinforcement are closed up or one or more holes are drilled in the place of the model that is located below and, as it were, rests on the molding sand. Then corks are made from the metal from which the product will be cast. The size must correspond to the diameter of the holes in the thickness of the model. Plugs are inserted into the hole of the model and molded.

Having the same thickness as the model, the metal plug remains in the mold after melting the model and fixes the distance between the rod and its edge. After pouring, the plugs are fused with the base metal, and no traces remain of them.

The cross-sectional area of ​​the plugs must be such that they can support the weight of the core and not be pressed into the sand. It should be noted that when melting the model, the mold is turned over, so plugs must also be placed in its upper part. Steel rods can also be used as clamps, which are passed through the entire mold (pattern and sand). After casting, the rods are removed, and threads are cut in the holes formed and screw plugs are screwed in. Sometimes the holes are countersinked and plugged with rivets made of the same metal - metal studs. Then these places are carefully cleaned or minted.

The originals of artistic products with a relatively flat surface (medallions, bas-reliefs) are usually made of soft materials - plasticine, clay, wax. For molding, plaster models are removed from them, while the reverse side of the model turns out to be flat and does not repeat the shape of the front surface from the inside. A casting made according to this model has a significant mass, which is impractical, since a large amount of metal is consumed. To avoid this, the method of molding on a plaster model with a frame is used. In this case, a casting is obtained, in which the internal relief repeats the shape of the front surface, and the wall thickness corresponds to the thickness of the frame and is the same over the entire surface of the product. Molding with a frame is used in the manufacture of casting molds for gypsum models of small height and with sloping walls.

If the gypsum model has high vertical walls with a small slope, then this method is undesirable, because during molding the vertical walls are much thinner than the top one, and the metal during pouring may not fill the entire mold, but only its upper part.

When molding with a frame, the model must be fixed on a model plate, which can be used as a piece of chipboard with several drilled holes. Through them, the model is fixed with screws, and holes are also made in the plate for fixing pins of the lower flask.

Having strengthened the model on the slab and installed a flask on it with a frame placed under the edges, they begin to fill it with molding sand, carefully ramming it. The thickness of the frame will correspond to the wall thickness of the future casting. The molded flask is turned over together with the under-model plate and, gently tapping on the surface of the slab, it is carefully removed from the flask along with the frame.

After removing the frame above the flask, a protrusion from the molding sand is formed, which must be cut off over the entire surface of the flask to the level of its edge. In this way, an imprint of a platform of a smaller height of the model is obtained for the thickness of the frame placed under the flask, and corresponding to the wall thickness of the future casting. Then, a second one is installed on the molded flask, and the upper half-mould with a sprue channel and a blowout is stuffed according to the imprint in the lower part.

The upper flask is molded more carefully and accurately, since the fragile surface of the sand model can be easily damaged when the mixture is compacted with a rammer.

Having removed the sprue, the upper flask is removed and, if necessary, the shape is corrected. The molded lower flask with the frame, which served as a model for the upper half of the mold, is knocked out and, with the help of fixing pins, is installed again on the under-model plate in the same position in which it was originally. Then they fill it with molding sand, but without a frame. After the molding is completed, the flask is turned over, the under-model tile with the model is removed, and both halves of the mold are assembled. In this way, a cavity corresponding to the thickness of the frame is obtained.

Rice. 9. Clay casting mold: 1 - sprue; 2 - clamps; 3 - form; 4 - bulging

In addition to the main methods of casting into the ground and using lost wax models, in the old days, craftsmen used casting in solid collapsible molds. Jewelry, buttons, decorative overlays for weapons were cast in this way. Clay and soft rocks of limestone served as the material for the molds. Hand-made clay molds consisted of 2 halves with recesses for fixing them relative to each other. The mold cavity was made by hand or molded from raw clay, then dried and fired.

Rice. 10. Clay mold screed: 1 - tightening screw; 2 - clamp; 3 - shape

For the manufacture of such forms, refractory chamotte clay or crucible mass can be used. Fireclay filler for these masses when making molds must be finely ground. It must be remembered that fireclay clay, when dried, shrinks significantly - from 7 to 14%. The clay mold is fired in a muffle furnace at a temperature of 900°C, and then the two halves of the mold are fastened together with clamps made of steel strip and connected with screws and nuts.

The principle of making molds from limestone is the same as from clay. The only difference is that the mold cavity is stuffed with incisors. Using for casting molds one of the varieties of limestone - slate, which has a dense structure and can be easily processed, ancient masters used engraving to perform complex forms and receive highly artistic works. As a material for such forms, plates made of crucible graphite or graphite electrodes for electric melting furnaces can be used, since graphite lends itself well to cutting. In the prepared plates of the required size, the adjacent surfaces are cleaned with fine sandpaper, and then rubbed one against the other. Through holes are drilled at two points of the plates, through which they are tightened with bolts and nuts. Holes are drilled in those places where they will not interfere with the manufacture of molds and sprues. After the preparatory operations, they proceed directly to the manufacture (cutting and engraving) of the casting mold and the gating system.

Before pouring the metal, the graphite mold must be covered from the inside with a thin layer of kaolin or chalk, diluted in water and with the addition of wood glue to protect it from fading.

After removing the casting from the mold, it usually has an ugly appearance - with burnt particles of the molding sand, all kinds of tint colors, etc. In this case, mechanical impurities are removed with a steel brush, and then the product is bleached in acids and alkalis.

Copper, bronze, brass and cupronickel are usually processed in two stages: first, preliminary etching is performed, and then final or glossy. The composition of the pre-etching solution is as follows: nitric and sulfuric acids - 250 ml each, sodium chloride - 0.5 g. Processing time - 4-5 seconds, solution temperature - 20-25°C. For the final etching, the following solution is used: nitric and sulfuric acids - 250 ml each, hydrochloric acid - 5 ml, Dutch soot - 1-1.5 g. Products are immersed in this solution for 6-8 seconds, then quickly washed in water.

Lead is poisoned with 5-10% nitric acid, zinc and cadmium - with 5-20% hydrochloric acid, and aluminum - with 10-20% sodium hydroxide solution.

In the given compositions of solutions, concentrated acids are used. It should be remembered that working with them requires special care, they must be cooked under the hood or on the street.

In conclusion of the section on artistic casting in the conditions of an individual workshop, it will be useful to introduce our reader to a specific person, a true master of his craft, a caster-artist Sergei Popov and his technologies and practical advice.

A native of the city of Borisoglebsk, Voronezh region, after graduating from school, he went to the Moscow region, where he studied at the Abramtsevo Art and Industrial School named after Vasnetsov and taught there in the specialty "Artistic stone processing".

He was engaged in forging, he was drawn to foundry works.

Drilling machine

Sharpening machine 2-sided

Grinding and polishing machine.

Rice. 19. Vase

• Drill
• Table for manual processing of models
• soldering iron
• Sandblasting machine

Wax molding

The composition of the model mixture. A mixture of wax and paraffin, heated to 60ºС, is beaten with a drill to saturate it with air, then it is pumped into detachable plaster molds using a special syringe. After cooling, the form is disassembled and the model is removed from it. The model is then processed. The flash is removed, the feeder is soldered with a soldering iron and the model is coated.

Coating

For coating, a suspension is used, made from ethyl silicate, water and marshalite by long-term mixing of the components. A model is dipped into the prepared suspension, which is then sprinkled with fireclay sand.

After drying, 5-6 layers of coating are applied with an interval of 2-3 hours.

For the first or second coating, finer sand is used - 0.5 mm grain, for subsequent coatings - 1-1.5 mm.

After coating with 5-6 layers and sufficient drying, the model is melted in a melting bath at a temperature of 130ºС.

fill

The melted crusts are calcined to a temperature of 400-500ºС and metal (brass, bronze) is poured into the hot crusts. After crystallization of the bronze, the crust is carefully beaten off.

Feeders are cut off. The cast product is cleaned from the sticky crust with a jet of sand.

Locksmith processing

It is carried out using abrasives of various grain sizes. After removing the surface layer and the remains of the sprues, you can start grinding, which is done using rubber wheels (parapit).

For polishing, felt and rag circles and GOI paste are used.

When machining products with complex relief, in which the radius of the stone does not allow access to many parts of the product, a conventional dental drill and metal and carbide burs, as well as fine * abrasives are used.

Casting in the crust has limited possibilities in size, depending on the mass, thickness of the model. Therefore, large or voluminous works have to be divided into small fragments, for example, a candlestick can consist of 15-17 parts (stand, arms, etc.). All this is mounted on the base with the help of a central rod.

In other works, rivets, twists, various fasteners can be used. In some cases, gas or argon welding is used.

Casting in the crust has some features, for example, it is limited in size, which, in turn, is determined by the possibility of the model.

Before pouring, bronze must be deoxidized, a phosphorus-containing alloy must be added. Brass is poured without additives.

The model mass must be saturated with air, i.e. contain air bubbles, otherwise the paraffin model will break the crust during rendering due to expansion.

Lost wax casting (LWM) is an industrial process also called wax casting or destructible mold casting. The mold is destroyed when the product is removed. Lost wax models are widely used in both engineering and art casting.

Application area

Features of the process technology allow the LVM method to be applied in a wide range: from large enterprises to small workshops. Lost wax casting is also possible for home, personal and commercial purposes for the manufacture of detailed figurines, souvenirs, toys, structural parts, jewelry. Almost all metals can be used as a filler:

• steels (alloyed and carbon);
• non-ferrous alloys;
• cast iron;
• alloys that cannot be machined.

However, the technology is universal - it is quite possible to produce relatively large structures of complex shapes. To facilitate the technical process, specialized investment casting equipment and 3D modeling using specialized programs are used.

Casting in ceramic molds

Depending on the requirements for products, various, most suitable technologies are used. Precision investment casting (TLVM) allows you to get the most complex configuration castings with high precision, with a minimum wall thickness and surface roughness. For TLVM, the wax model is immersed in a ceramic-based liquid mixture. The ceramic mixture dries and forms the mold shell. This process is repeated until the desired thickness is reached. The wax is then removed in an autoclave. However, this method is characterized by high cost, duration of the technological process, the release of harmful substances in the production area and environmental pollution with the remains of ceramic molds.

Casting in molds from XTS

In many cases, in the manufacture of crafts at home, castings of complex configuration are not required to have low roughness, and for a number of artistic castings, a surface with a uniform roughness is not only acceptable, but is a design decision. In this case, it is advisable to use investment casting.

The technology developed for products that do not require smooth surfaces is quite simple. Such a surface can be obtained by casting into molds from cold-hardening mixtures (CTS). This process is much simpler, cheaper and environmentally friendly.

However, this method of investment casting does not allow complex castings to be obtained using investment patterns. This is explained by the fact that during the rendering of figures, a significant part of the model composition remains in the mold cavity and can only be removed by calcination. Calcination, that is, heating to the ignition temperature, of the model composition leads to the destruction of the resin binder CTS. When metal is poured into a mold with the remains of the model composition, they are burned, leading to metal emissions from the mold.

The use of liquid glass mixtures

To level the shortcomings of the CTS technology in the manufacture of some types of castings allows investment casting in liquid-glass mixtures with a liquid catalyst (LCG). These mixtures containing liquid glass in the amount of 3-3.5% and a catalyst of about 0.3% by weight of the sand base began to be used abroad in the early 80s and are still used. According to research, these mixtures, in contrast to the first generation JSS, are distinguished by environmental cleanliness, good knockout and slight burn marks on the castings.

Investment casting: technology

The LVM process includes the operations of preparing model compositions, manufacturing models of castings and gating systems, finishing and controlling the dimensions of models, and further assembly into blocks. Models, as a rule, are made from materials that are multicomponent compositions, combinations of waxes (paraffin-stearin mixture, natural hard waxes, etc.).

In the manufacture of model compositions, up to 90% of the return collected during the melting of wax models from molds is used. The return of the model composition should not only be refreshed, but also periodically regenerated.

Model making consists of six steps:

• mold preparation;
• introducing a model composition into its cavity;
• holding the model until hardening;
• disassembly of the mold and extraction of the model;
• cooling it down to room temperature.

Process Features

The essence of LVM lies in the fact that a silicone or wax model is melted from a workpiece by heating, and the vacated space is filled with metal (alloy). The process has a number of features:

• In the manufacture of molding sand, suspensions are widely used, consisting of refractory fine-grained materials held together by a binder solution.
• For pouring metals (alloys), one-piece molds are used, obtained by applying a refractory coating to the model, drying it, followed by melting the model and annealing the mold.
• For castings, disposable models are used, as they are destroyed during the mold making process.
• Thanks to fine-grained refractory dust-like materials, a sufficiently high surface quality of castings is ensured.

Advantages of LVM

The advantages of investment casting are clear:

• Versatility. You can use any metals and alloys for casting products.
• Getting configurations of any complexity.
• High surface finish and manufacturing precision. This allows 80-100% to reduce the subsequent costly metalworking.

Disadvantages of LVM

Despite the convenience, versatility and decent quality of products, it is not always advisable to use investment casting. The disadvantages are mainly related to the following factors:

• The duration and complexity of the casting production process.
• Overpriced molding material.
• Big burden on the environment.

An example of making a product at home: the preparatory stage

Lost wax casting at home does not require deep knowledge in metallurgy. First, let's prepare a model that we want to repeat in metal. The finished product will come down as a layout. Also, the figurine can be made independently from clay, sculptural plasticine, wood, plastic and other dense plastic materials.

We install the model inside a collapsible container fastened with clamps or a casing. It is convenient to use a transparent plastic box or a special mold. To fill the mold, we will use silicone: it will provide excellent detail, penetrating into the smallest cracks, holes, depressions and forms a very smooth surface.

Second step: filling with silicone

If precision investment casting is required, liquid rubber is indispensable for making a mold. Silicone is prepared according to the instructions by mixing different components (usually two) and then heating. To remove the smallest air bubbles, it is advisable to place a container with liquid rubber for 3-4 minutes in a special portable vacuum apparatus.

Pour the finished liquid rubber into the container with the model and re-vacuumize. It will take time for the subsequent hardening of silicone (according to the instructions). The translucent materials used (containers and silicone itself) allow you to personally observe the process of mold formation.

We remove the seized rubber with the model inside from the container. To do this, we release the clamps (casing) and separate the two halves of the box - the silicone easily moves away from the smooth walls. It will take 40-60 minutes for the liquid rubber to completely harden.

Third stage: making a wax model

Investment casting involves the melting of fusible material and the replacement of the resulting space with molten metal. Since wax melts easily, we use it. That is, the next task is to make a wax copy of the original model used. This required the creation of a rubber mold.

Carefully cut the silicone blank lengthwise and take out the model. There is a little secret here: in order to subsequently accurately connect the shape, it is recommended to make the cut not smooth, but zigzag. Attached parts of the form will not move along the plane.

We fill the resulting space in the silicone mold with liquid wax. If the product is being prepared for itself and does not require high precision in mating parts, you can pour wax separately into each half, and then, after hardening, connect the two parts. If it is necessary to accurately repeat the silhouette of the model, the rubber halves are connected, fixed, and hot wax is pumped into the resulting void using an injector. When it fills all the space and hardens, we disassemble the silicone mold, take out the wax model and correct the flaws. It will serve as a prototype for the finished metal product.

Fourth stage: molding

Now it is necessary to form a heat-resistant durable layer from the outer surface of the wax figure, which, after melting the wax, will become a mold for the metal alloy. Let us choose the investment casting method using a cristobalite mixture (quartz modification).

We form a model in a metal cylindrical flask (a device that holds the molding sand during its compaction). We install a soldered model with a gating system in the flask and fill in the mixture based on cristobalite. To force out air pockets, we place them in a vibro-vacuum apparatus.

Final stage

When the mixture thickens, it remains to melt the wax and pour the metal into the vacant space. The investment casting process at home is best done using alloys that melt at relatively low temperatures. Foundry silumin (silicon + aluminum) is perfect. The material is wear-resistant and hard, but it is brittle.

After pouring the molten silumin, we wait for it to harden. Then we remove the product from the trench, remove the sprue and clean it from the remnants of the molding sand. Before us is an almost finished part (a toy, a souvenir). Additionally, it can be sanded and polished. If the remnants of the foundry are firmly stuck in the grooves, they must be removed with a drill or other tool.

Investment casting: production

LVM is carried out a little differently for the manufacture of critical parts that have a complex shape and (or) thin walls. It can take from a week to a month to cast a finished metal product.

The first step is to fill the mold with wax. For this, enterprises often use an aluminum mold (an analogue of the silicone mold discussed above) - a cavity that has the shape of a part. The output is a wax model slightly larger than the final part.

Next, the model will serve as the basis for the ceramic mold. It should also be slightly larger than the final part, as the metal will shrink after cooling. Then, using a hot soldering iron, a special gating system (also made of wax) is soldered to the wax model, along which the hot metal will pour into the mold cavity.

Making a ceramic mold

Next, the wax structure is dipped into a liquid ceramic solution called slip. This is done by hand in order to avoid defects in the casting. For the strength of the slip, the ceramic layer is strengthened by spraying with fine zirconium sand. Only after that, the workpiece is “trusted” to automation: special mechanisms continue the phased process of spraying coarser sand. Work continues until the ceramic-sand durable layer reaches the specified thickness (usually 7 mm). In automated production, this takes 5 days.

Casting

Now the workpiece is ready for melting wax from the mold. It is placed for 10 minutes in an autoclave filled with hot steam. The wax melts and completely flows out of the shell. At the output, we get a ceramic mold that completely repeats the shape of the part.

When the ceramic-sand mold hardens, investment casting is carried out. Preliminarily, the form is heated for 2-3 hours in an oven so that it does not crack when pouring metals (alloys) heated to 1200 ˚C.

Molten metal enters the mold cavity, which is then left to cool and harden gradually, at room temperature. It takes 2 hours for aluminum and its alloys to cool, for steels (cast iron) - 4-5 hours.

Finishing

Actually investment casting ends here. After the metal hardens, the workpiece is placed in a special vibrator. From gentle vibration, the ceramic base cracks and crumbles, while the metal product does not change its shape. Next comes the final processing of the metal blank. First, the metal pouring system is sawn off, and the place of its contact with the main part is carefully polished.

Finally, the inspectors check that the dimensions of the product correspond to those specified in the drawing. Aluminum parts are measured cold (at room temperature), steel parts are preheated in an oven. Specialists use various tools for control and measurement work: from simple templates to complex electronic and optical systems. If a discrepancy is detected with the parameters, the part is either sent for revision (correctable marriage) or for remelting (irremovable marriage).

Gating system

The design of the gating-feeding system plays a leading role in the LVM. This is due to the fact that it performs three functions:

• In the manufacture of shells of molds and a block of models, gating systems are load-bearing structures that hold the shell and models on themselves.
• Through the sprue channel system, liquid metal is supplied to the casting during pouring.
• During solidification, the system performs the function of profit (feeding element that compensates for metal shrinkage).

casting shell

In the LVM process, the key is creating the layers of the shell of the shape. The shell manufacturing process is as follows. On the surface of the block of models, most often by dipping, a continuous thin film of suspension is applied, which is then sprinkled with sand. The suspension, adhering to the surface of the model, accurately reproduces its shape, and the sand of the sand is introduced into the suspension, wetted by it and fixes the composition in the form of a thin facing (first or working) layer. The non-working rough surface of the shell formed by quartz sand contributes to good adhesion of subsequent layers of the suspension to the previous ones.

Important indicators that determine the strength of the mold are the viscosity and fluidity of the suspension. Viscosity can be adjusted by introducing a certain amount of filler (fullness). At the same time, with an increase in the fullness of the composition, the thickness of the interlayers of the binder solution between the powder particles decreases, shrinkage and the negative effects caused by it decrease, and the strength properties of the mold shell increase.

Materials used

Shell materials are divided into the following groups: base materials, binders, solvents and additives. The former include dusty, used for the preparation of suspensions, and sands intended for sprinkling. They are quartz, chamotte, zircon, magnesite, high-alumina chamotte, electrocorundum, chromomagnesite and others. Quartz is widely used. Some shell base materials are prepared ready-to-use, while others are pre-dried, calcined, ground, sieved. A significant disadvantage of quartz is its polymorphic transformations, which occur with a change in temperature and are accompanied by a sharp change in volume, eventually leading to cracking and destruction of the shell.

Smooth heating of forms in order to reduce the likelihood of cracking, which is carried out in the support filler, contributes to an increase in the duration of the technological process and additional energy costs. One of the options for reducing cracking during calcination is the replacement of pulverized quartz sand as a filler with dispersed quartz sand of polyfractional composition. At the same time, the rheological properties of the suspension are improved, the crack resistance of the molds is increased, and rejects due to blockages and breakdown of the shells are reduced.

Conclusion

The LVM method has received the widest distribution. It is used to obtain complex parts in mechanical engineering, in the manufacture of weapons, plumbing, and souvenirs. For the manufacture of jewelry from precious metals, investment casting is used.

For the manufacture of castings of various parts and their elements, modern foundries use semi-permanent and one-time casting molds. In accordance with the conditions of the technology of the foundry process, for the manufacture of such casting molds, special casting mixtures are used, which are a combination of highly refractory substances (asbestos, fireclay) with sandy-clay components. The components included in the compositions for casting can be both natural and artificial origin (synthetic). As a result of mixing the constituent molding sands in certain proportions, the finished compositions can have predetermined properties and have the desired compliance, refractoriness, strength, formability, gas permeability, and so on.

Types of mixtures

Molding sands for casting, depending on the nature of use, are divided into several main categories:

• Coating mixtures. This type of molding sand is intended for the manufacture of the working layer of the mold. High physical and mechanical properties of such mixtures are provided by an increased percentage of raw materials for molding (sand and clay);
• Filling mixtures for casting. These casting compounds are used to fill the mold after the veneer has been applied to the model. To prepare such a mixture, the initial molding materials (clay and sand) are processed together with the remnants of the recycled mixture;
• Single molding sand for casting. A mixture of this type is a molding material that combines the properties of both a filling and a facing mixture. Single mixtures are used on automatic lines in serial and mass production with machine molding. The durability of such mixtures is ensured by the presence in the composition of clays with a high binding capacity and the most refractory types of sands.

The composition of the sand for casting

The chemical composition that a casting sand can have depends on a combination of the following factors:

• From the type of alloy used and the dimensions of the casting;
• From the method of molding and the type of casting (non-ferrous casting, steel or cast iron);
• From the nature of production and the technological means available to production.

Also, the composition that the foundry sand has for casting may vary depending on the state in which it must be before pouring. Molding sands for dry molds contain an increased amount of water and clay. In addition, burnable additives such as peat or sawdust can be added to the composition of such mixtures. In the composition of molding sands for raw molds, the percentage of recycled sand is reduced. Molding compositions for casting metals into dried molds are distinguished by the simultaneous presence of both circulating components, and fresh materials (clay and sand), and fasteners.

molding mixtures. For the manufacture of molds and cores, various molding and core mixtures are used, the composition of which depends on the method of molding, the type of alloy, the nature of production, the type of casting, and the technological means and materials available to the production.

Depending on usage sand-clay molding mixtures are classified as follows:

• on application during molding (facing, filling and uniform);
• according to the condition of the mold before pouring (for wet, dry, dried and chemically hardening molds);
• according to the type of alloy poured into the mold (for cast iron, steel and non-ferrous castings).

Facing mixture used for facing the working surface of molds. The thickness of the facing layer depends on the composition of the facing mixture and on the dimensions of the casting (from 20 to 100 mm and more). On top of the facing mixture, a filling mixture is poured into the flasks, which is made from recycled earth with the addition of 5-10% fresh materials (sand, clay).

Single blend serves for stuffing the entire volume of the mold and is used for the manufacture of small and medium-sized castings in serial and mass production. A single mixture differs from a filling mixture by a high content of fresh materials and a certain amount of special additives (ground coal, peat pitch, etc.).

Mixes for dry molds differ from raw mold mixes a lower content of the recycled mixture and an increased percentage of clay and water. Often the forms that are subjected to drying are made from facing and filling mixtures, and burnable additives (sawdust, peat, etc.) are added to the mixture to increase their compliance.

Dry mold mixes have in their composition a recycled mixture, fresh materials (sand and clay) and fasteners (SP, SB). As facing mixtures, they are widely used in the manufacture of cast iron medium and large critical castings. Depending on the weight of the casting for which the mold is made, the drying time is 20-60 minutes. At the Moscow iron foundry Stankolit, to obtain castings weighing up to 1000 kg, mixtures dried for 30 minutes are used.

The composition of the mixture dried for 30 minutes(in % by volume)

Sand Lukhovitsky 1K315A (GOST2138-56) 88-89

Molding clay FV-1 1-2

sawdust 5

Asbestos crumb 5

SB fixer (over 100%) 1.5

Sulfite-alcohol stillage (over 100%) 2-3

When molds are dried, a strong, hard layer is formed on the working surfaces, which affects the production of a clean surface and increased accuracy in castings.

Mixtures for chemically hardening molds made from quartz sand with the addition of 4.5-6.5% liquid glass and 1.5% sodium hydroxide with a concentration of 10-20%. Adding caustic soda to the mixture (see page 25) allows you to keep the technological properties for a longer time, as well as increase the strength of the mixture after chemical hardening. For cast iron castings weighing from 1000 to 5000 kg at the Stankolit plant, a chemically hardening mixture of the following composition is used.

The composition of the chemically hardening mixture(in % by volume)

Sand Lukhovitsky 1K315A (GOST 2138-56) 88-89

Molding clay FV-1 3-4

Ground coal GK 8

Liquid glass (over 100%) with a module equal to 2.6-2.7 6

15% sodium hydroxide solution (density 1300 kg / m 3) 075-1.0

Liquid glass mixtures harden when blown with carbon dioxide (CO 2). When this occurs, the decomposition of sodium silicate and the formation of sodium carbonate and silica. Silica combines with water to form a chemical called silicic acid gel.

The silicic acid gel, enveloping sand grains in the mixture, has the ability to harden when part of the attached water is lost. Because of this, the gel films, being between the grains of sand, after a short period of time without heat supply, bind them into a strong and dry mass. When a liquid-glass mixture is blown with carbon dioxide, a long thermal cycle of moisture evaporation and mixture solidification is replaced by an accelerated process of chemical binding of water with the constituent elements of liquid glass.

Currently, self-hardening facing mixtures are widely used. The scope of these mixtures is the production of medium and large castings.

The finished self-hardening mixture is poured onto the model. In the manufacture of molds for large castings, the model is lined with a mixture and partially compacted.

After backfilling the filling mixture, its machine compaction is carried out. The filling mixture in the manufacture of large molds is compacted with a sand thrower with possible subsequent pre-pressing with rammers. After stuffing, the forms "self-harden" on the parade ground or on the conveyor.

The facing layer of the mold from the self-hardening mixture has high strength and gas permeability, which ensures the production of high quality castings.

Paint such forms with self-drying non-stick paints.

In table. 7 shows typical compositions of molding sands.