Lost wax casting is a fairly popular casting method. The method is notable for the complexity of the technological process and high labor costs for preparatory processes. Therefore, it is used where it is necessary to precisely comply with the dimensions and ensure high quality surface parts. This is how turbine blades and high-performance tools are molded, dentures and decorations, as well as sculptures of complex configuration. The essence of investment casting is that the casting mold is one-piece, the model from fusible materials during molding is not removed, but smelted. This ensures that size and terrain are meticulously maintained. In the remaining cavity from the model, metal is poured. Upon completion of cooling, the mold is destroyed and the product is removed. When casting large batches, the cost of the product is reduced.
Method Advantages
The main plus of investment casting is the thoroughness of mold transfer and low surface roughness. In addition, other advantages are available:
- Available production of parts from alloys that are slightly susceptible to machining.
- The need for further machining is reduced.
- Products are cast that by other methods would have to be manufactured in parts and assembled together.
- With large series, a reduction in specific labor intensity (per one product) and its cost is achieved.
- The possibility of mechanization and partial automation of preparatory operations of the casting itself.
These advantages make the method one of the most popular and used in today's metallurgy, especially in combination with modern progressive casting methods.
Lost Wax Castings
The undoubted advantages of the method, it would seem, were to ensure its dominance among other methods. However, despite the popularity of the investment casting method, the drawbacks hinder its widespread use. The main disadvantage is the complexity of the multi-stage process. It requires quite complex and expensive technological equipment for the preparatory stages. For simple products manufactured in small batches, this method has a higher cost.
For the cost-effective use of investment casting, the advantages and disadvantages of the method are compared, the decision on its choice is made on the basis of an assessment of the price / quality ratio. Therefore, it is used mainly for the most demanding and expensive products that are difficult to obtain in another way, such as turbine blades, sculptures, high-speed tools, etc. Another area of application is large-scale castings, on which the economies of scale can achieve a significant reduction in cost.
Technology
Lost wax casting technology is a multi-stage production process, which is characterized by a relatively high complexity. At the first stage, a master model is performed, it will become the standard for the manufacture of working models and after passing through all stages of the final product. For the production of master models, both special model compositions and traditional ones are used - gypsum or wood. The material of the master model must combine strength and ease of processing.
Further, investment casting technology involves the creation of a mold into which all working models will be cast. Molds are made of gypsum, rubber, silicone, less commonly metal. Structurally, it must necessarily be detachable and designed for repeated use. The mold is filled with a model composition, after hardening it is disassembled and the next working model is removed.
In the production of unique parts or small runs, the stages of creating a master layout and mold are skipped, and the layout (or several) is done by manually molding the material.
The next stage of the investment casting process is the manufacture of a casting mold around a mock (or mock block). These matrices are structurally non-separable and disposable, which allows to achieve thorough compliance with the dimensions and roughness of the product. In modern industry, two types of molds are used - traditional sand-clay molds for casting in the earth and shell molds - for the production of precise and expensive parts.
After completion of the form, the model is smelted from it by heating or blowing with superheated steam. Shell forms are additionally strengthened by heating to 1000 ° C.
The final stage of the process includes the actual filling of the product, its cooling in natural conditions or by a special technique in a thermostat, breaking the mold and cleaning the product. The method allows to obtain high-quality castings weighing from several grams to tens of kilograms.
Model compositions
Material for the production of the layout must have certain properties. It should have properties such as:
- Plasticity in the solid phase. It is necessary for the exact repetition of the shape of the future product and its correction if necessary.
- Strength. The model must withstand without deformation the process of form formation around it.
- Fusibility. Warming up the model should not require much time and energy.
- Melted fluidity. The composition should easily penetrate into all recesses and relief details, accurately repeating the outlines of the future part.
- Profitability. It is especially important for the production of large series.
For model formulations, a mixture of stearin and paraffin is usually used. These materials successfully complement each other's parameters, compensating for the insufficient melting point of paraffin and the excessive viscosity of stearin.
Brown wax formulations are no less popular in the industry. Its main properties are moisture resistance, strength and the ability to form very smooth coatings, which is especially valuable for product modeling.
Compounds consisting of a mixture of brown coal wax, paraffin and stearin are also used.
Mold making
For the production of unique products, the layout is prepared by cutting from a piece of model material manually or according to patterns. Models in the form of bodies of revolution are also made on lathes. Recently, the method of 3D-printing models is becoming more widespread. It is suitable for both single layouts and small series.
The cost of a modern industrial 3D printer is still high, but due to the ease of reconfiguration from one product to another, it can become an effective tool for making models in the case of a large number of heterogeneous orders of small series.
In order to produce a large number of identical layouts, a matrix is made of gypsum, rubber, silicone or metal. Working models are produced, in turn, by casting into a matrix. By design, the mold must be collapsible to ensure the possibility of manufacturing a given number of models. The selected material should also provide such an opportunity, therefore, it is presented with such requirements as strength, density, low roughness, chemical inertness with respect to the layout. The substance of the mold should also have minimal adhesion to the layout to ensure ease of extraction of the finished layouts and compliance with the dimensions. An important property of the mold is its strength and wear resistance, especially with large series.
Making models and blocks
A widespread method for manufacturing investment castings is casting them under low pressure into molds. The injection of the liquid mixture is carried out both manually, using piston syringes, and mechanical, hydraulic or pneumatic superchargers. In the case of using brown coal wax, it is required to heat the supply pipelines of the composition due to its high viscosity. Mock-ups of foamed polystyrene are made by extrusion on automated molding units.
To increase economic efficiency and reduce the complexity in the case of mass production of small castings, their models are combined into blocks. Gating systems are formed above the blocks, attaching individual models to the gates by means of a hand soldering iron. In the case of single castings or small series, models are made manually.
When forming gate systems, it is necessary to ensure non-turbulent melt flow, uniform filling of all matrix elements. When stuffing a mold from ASG, it is also necessary to monitor the uniform filling of all openings between the gates and to prevent their damage.
Mold making
In this investment casting method, there are two main types of molds:
- Sand and clay mixtures (ASG).
- Shell.
Lost wax casting molds from PGS are mainly used in the production of small series of products that do not require very high precision. The process of their manufacture is rather laborious and requires high, and often - unique skills of modelers and molders. Partial mechanization lends itself to only certain operations, such as the preparation and filling of the molding sand, its ramming.
Shell forms, by contrast, are used to produce parts that require particular manufacturing accuracy. The manufacturing process is more complex and lengthy, but better amenable to mechanization.
Earth casting
This is the earliest method of metal processing mastered by mankind. It was mastered by our ancestors simultaneously with the beginning of the use of metal products as weapons, tools or utensils, that is, about 5 thousand years ago. Molten metal is cast into a prepared matrix from a mixture of sand and clay. The earliest places of metal processing just arose where metal deposits in the form of nuggets and placers were located nearby. A typical example is the Kaslinsky plant in the Urals, world-famous for its cast iron lace castings.
Lost wax casting method is used for the manufacture of metal products - both black and non-ferrous. And only for metals that exhibit an increased tendency to react in the liquid phase (such as titanium), it is necessary to make matrices from other compositions.
The production process of casting in ASG consists of the following phases:
- model making;
- preparation of the flask;
- filling and compaction of the mixture in the flask;
- metal casting;
- extraction and cleaning of the casting.
ASG form - single use. To get the finished product, it will have to be broken. At the same time, most of the mixture is available for recycling.
Compositions of predominantly quartz sand of various grain sizes and plastic clays, the content of which varies from 3 to 45 percent, are used as materials for ASG. So, for example, art castings are produced using a mixture with 10-20% clay content, for especially large castings the clay content is brought up to 25%.
Apply two subspecies:
- Facing mixtures. Located on the inner surface of the mold and interact with molten metal. They must be heat-resistant, capable of not being destroyed by temperature differences and resulting stresses. Such mixtures have fine grains to thoroughly convey surface details. The ability of the mixture to gas transmission is also very significant.
- Filling mixtures. They are used for filling between the facing layer and the walls of the flask. They must withstand the weight of the poured metal, maintain the shape of the product and facilitate the timely and complete removal of gases. They are made from cheaper grades of sand and are subject to reuse.
If the injection gases do not exit through the masses of the moldable mixture, but through the gating system, defects in the casting lead to marriage.
The traditional technology of casting in the ground is illustrated in detail in the tape by A. Tarkovsky “Andrei Rublev”. In the novel “The Bell”, the young man Boriska, the son of a deceased master, heads the foundry artel and casts the church bell.
Shell casting
The casting method in shell molds on investment casting is characterized by the best transfer of product dimensions and low surface roughness. The model is made of fusible compounds, such as brown coal wax. In foundries, the composition of paraffin-stearin in equal proportions is also widely used. In the case of large castings, salts that protect the layout from deformation are included in the model material. By immersion in a solution, the model is covered in 6-10 layers with a high-temperature suspension.
Hydrolyzed silicates act as a binder; crystals of electrocorundum or quartz are taken as heat-resistant dusting. Materials for the production of shell molds are characterized by high strength, low hygroscopicity and excellent gas permeability.
The layout is dried in an atmosphere of gaseous ammonia. At the next stage, the mold is heated to 120 ° C in order to remove the paraffin model. The remaining mixture is removed by superheated steam under high pressure. Further, the mold is calcined at a temperature of up to 1000 ° C, which leads to its final fixing and removal of substances that can be released in the form of gases during the casting process.
The shell is placed in a kind of flask, which is covered with steel shot. This helps maintain the configuration when filling the mold with the melt and at the same time improves the cooling conditions of the casting. The melt is poured in pre-heated to 1000 ° C forms. After cooling the product according to a special program in a thermostat, the mold is destroyed, the casting is removed and cleaned.
The main advantage of this casting method is the high accuracy of transferring product dimensions and low surface roughness.
Additional advantages of the method:
- Casting parts from alloys that are difficult to machine.
- The casting of products that otherwise would have to be cast in parts and then assembled together.
The disadvantages of this method of investment casting are the low metal utilization rate and increased labor intensity.
Precision casting
Lost wax casting - this is the name of the technology and the end product itself. High precision casting is ensured by the fact that in the process of preparing the mold there is no need to remove the product model from it. Using the traditional method, the production of a die for casting is a complex and very time-consuming multi-stage process. This is especially true in the case of casting parts of complex configuration, with recesses, depressions and internal cavities.
For example, when casting a cast-iron or copper vase with a variable surface curvature, many tricks have to be applied. So, first fill the lower half of the flask, then the model is removed, turned over and rammed the upper half. The model has to be made composite, the vase handles are made of two elements, they are pulled out through the model cavity in two stages - first the lower element, then the upper one. All these numerous turning and dragging can not positively affect the integrity of the surface of the mold and ultimately on the accuracy of compliance with the dimensions of the casting and the quality of its surface. In addition, the problem remains of the exact combination of the flask parts and their reliable fastening to each other.
Lost wax casting manufacturing is devoid of these drawbacks; it does not require such high qualification of modelers and significantly reduces the complexity of preparatory casting operations. This is especially pronounced with large runs of castings.
The method allows to achieve the 2-5th accuracy class according to GOST 26645-85. This allows you to cast precision products such as turbine blades, cutting tools, including high-performance milling cutters and drills, critical high-load brackets, small high-load parts of vehicles, machine tools and other complex mechanisms.
High dimensional accuracy and a high surface class minimize the need for further machining of the casting, which saves metal and reduces production costs.
Equipment
Lost wax casting equipment is diverse and complex. Enterprises combine them into a single and harmoniously working complex, organized as a site, workshop or separate production.
The composition of the complex depends on the scale of production, size, configuration and circulation of castings.
So, in the production of dentures and jewelry, the equipment will include:
- model table;
- muffle furnace with thermostat;
- a set of scalpels and spatulas for correcting the shape of the model;
- molding board;
- flask;
- containers for storage and preparation of the molding sand;
- set of tools for tamping the molding sand;
- crucible for melting metal;
- forceps;
- hammer for breaking the mold.
This production complex can easily fit on one table and in one cabinet. If it is planned to mass production, for example, aluminum castings - parts of any device, then equipment will be required for:
- molding and pouring ceramic molds;
- drying forms;
- smelting model material and applying a heat-resistant layer;
- cleaning castings from molding material.
And finally, the equipment of the foundry complex itself, designed to receive the melt and pour it into the mold. This may be casting equipment:
- under low pressure;
- centrifugal;
- in the usual gravitational way.
Installations for injection molding and centrifugal casting are a separate highly mechanized and automated production complex, isolated from the atmosphere of the workshop. They minimized manual labor and finding a person in harmful conditions. The sealed chambers in which the complexes are located provide for the complete capture and purification of exhaust gases, which significantly increases the environmental friendliness of the enterprise.
Lost wax casting has a high enough potential for development, especially in combination with progressive methods for manufacturing molds and pouring methods.