Lost-wax investment casting process is a process in which one-off, precision, one-piece ceramic shell molds are used to produce a casting, made from one-off patterns using liquid sands. Before pouring the melt, the model is removed from the ceramic mold by melting, burning, dissolving, or evaporation. To remove the remnants of the model and harden the form, it is heated to high temperatures.
By calcining the ceramic mold before pouring, the almost complete exclusion of its gas content is achieved, and the fallibility of the mold with the melt improves.
A model or a link of model 2 is made in a detachable mold 1, the working cavity of which has the configuration and dimensions of an investment wax pattern with allowances for shrinkage of the model composition and casting material, as well as cutting.
The model is made from materials that either have a low melting point (wax, stearin, paraffin) or are able to dissolve (carbamide) or burn without the formation of solid residues (polystyrene).
Finished Model Molten metal Wax pattern
Finished models or links of models are assembled into block 3, which has models of the elements of the gating system from the same material as the investment wax pattern model. The block of models consists of links, the central part of which forms models of feeders and a riser.
Models of the bowl and the lower part of the riser are made separately and installed in the block during its assembly.
Wax Patterns with Metal Casting
To obtain shell molds, the resulting block of models is immersed in a container with liquid molding sand – a suspension consisting of dust-like refractory material, for example, powder-like quartz or electrocorundum, and a binder. As a result, a layer of suspension 4 with a thickness of less than 1 mm is formed on the surface of the model.
To strengthen this layer and increase its thickness, layers of refractory granular material 5 (fine quartz sand, electrocorundum, granular fireclay) are applied to it. The operations of applying the suspension and sprinkling are repeated until the shell of the required thickness is obtained on the model (3–10 layers). In this case, each coating layer is dried in air or in ammonia vapor 6, which depends on the binder.
After drying the shell form, the model is removed from it by melting, dissolving, burning or evaporation.
In order to strengthen the mold before pouring, it is placed in a metal container and covered with refractory material 8.
Wax Pattern and Invest Material
To remove the remnants of models from the mold and strengthen the binder, the container with the shell mold is placed in oven 9 for calcination. The calcination of the mold is carried out at a temperature of 900 – 1100 ° C, then the calcined mold 10 is removed from the furnace and poured with a melt.
After the investment wax pattern has solidified and cooled to a predetermined temperature, the mold is knocked out, the castings are cleaned of ceramic residues, and sprues are cut off from them. In many cases, shells are calcined in a furnace until they are filled with refractory material, and then they are filled with preheated refractory material to strengthen them.
This makes it possible to reduce the duration of mold annealing before pouring and reduce energy consumption. So, for example, a technological process was organized on automatic lines for the mass production of investment wax patterns.
Dimensional Accuracy in Lost Wax Process
The low surface roughness of the mold with a sufficiently high refractoriness and chemical inertness of the material makes it possible to obtain an investment wax pattern with a high-quality surface. After cleaning from the remains of the shell mold, the surface roughness of the castings ranges from Rz = 20 µm to Ra – 1.25 µm.
The absence of a mold split, the use of materials for the manufacture of models that make it possible not to disassemble the mold for their removal, the high fire resistance of the mold materials, as well as heating it to high temperatures before pouring, help to improve fallibility.
The ceramic shell and sand casting make it possible to obtain investment wax patterns of the most complex configuration, as close as possible or corresponding to the configuration of the finished part, from almost all known alloys.
Produce Wax Patterns and its manufacturing process
The achievable coefficient of the accuracy of the investment casting process by weight (KTM = 0.85 – 0.95) contributes to a sharp reduction in the volume of cutting and metal waste into chips.
The accuracy of the investment casting process can correspond to accuracy classes 2, machining allowances for castings up to 50 mm in size usually do not exceed 1 mm, and for investment casting process up to 500 mm in size – about 3 mm.
Features of the formation of the investment casting process and their quality
Obtaining castings in a shell mold is associated with some features, in particular, before molten metal is poured; the mold cavity is heated to relatively high temperatures. This determines the following technological moments.
Small thermal conductivity, heat capacity, the density of shell-shaped materials, and elevated mold temperature reduce the rate of heat removal from the melt, which helps to improve the fallibility of the mold.
Ceramic Slurry with Wax Patterns
Due to this, it is possible to obtain a complex steel investment casting process with a wall thickness of 0.8 – 2 mm with a significant surface area. Improving the fallibility of the mold is also facilitated by the low roughness of its walls, the possibility of using external influences on the melt, such as the field of centrifugal or electromagnetic forces, pouring using a vacuum, etc.
The low intensity of melt cooling in a heated shell mold leads to a decrease in the rate of solidification of castings, an enlargement of the crystalline structure, the possibility of the appearance of massive nodes and thick (6–8 mm) walls of shrinkage defects in the central part – shells and porosity. Thin walls (1.5 – 3 mm) harden rather quickly, and axial porosity does not form in them.
Multiple Wax Patterns
To reduce shrinkage defects, it is necessary to create conditions for directional solidification and feeding of the investment casting process. Heat treatment is used to improve the crystal structure of castings.
The increased temperature of the mold during pouring contributes to the development of physical and chemical processes on the surface of contact between the investment casting process and the mold.
This can result in both a desirable change in the structure of the surface layer of the investment casting process and an undesirable one, i.e., leading to the appearance of surface defects.
Liquid Investment Material
For example, on carbon steel castings, a characteristic defect is an oxidized and decarburized surface layer up to 0.5 mm deep. The reason for the oxidation and decarburization of the investment casting process is the interaction of atmospheric oxygen with the metal of the investment wax pattern during its solidification and cooling.
The main factors influencing the decarburization process are the composition of the gaseous medium surrounding the investment casting process, the temperature of the casting and mold, and the carbon content in the investment casting process.
Also, this technology is used in the casting of heat-resistant hard-to-cut alloys (turbine blades), corrosion-resistant steels, and carbon steels in mass production (automotive industry).
Lost-wax investment casting process
Lost-wax investment casting process is the process of obtaining castings from molten metal in molds, the working cavity of which is formed due to the removal (outflow) of low-melting material of the model during its preheating.
Investment Casting Advantages:
The lost wax investment casting process provides accurate and complex castings from various alloys weighing from 0.02 to 15 kg with a wall thickness of 0.5 to 5 mm.
Technological process of investment casting process:
Investment models are made in metal molds from model compositions, including paraffin, wax, stearin, and fatty acids. The composition fills the cavity of the mold well, and gives a clear imprint. After the model composition has hardened, the ceramic mold opens and the model is pushed into cold water.
Then the models are assembled into model blocks with a common gating system by soldering, gluing, or mechanical fastening. From 2 to 100 models are combined into one block.
The model block is immersed in a special liquid refractory mixture, followed by sprinkling with quartz sand. Then the model blocks are dried in air or in ammonia. Usually, 3…5 layers of refractory coating are applied, followed by drying of each layer.
Investment material Is Removed
Patterns are removed from molds by immersing the mold in hot water or using heated steam. After removal of the model composition, thin-walled investment casting process molds are placed in a flask, covered with quartz sand, and then calcined in a furnace for 6…8 hours at a temperature of 850…9500 C to remove residual model composition, water evaporation.
Poured into the Mold
Pouring of ceramic molds according to investment models is carried out immediately after calcination in a heated state. Filling can be free, under the action of centrifugal forces, in a vacuum, etc.
After solidification of the poured metal and cooling of the investment casting process, the ceramic mold is destroyed, the castings are separated from the sprues by mechanical methods, sent for chemical cleaning, washed, and subjected to heat treatment.
Production efficiency and scope
Based on production experience, the following advantages of the investment casting process method can be distinguished:
- the ability to manufacture castings of complex configuration from almost any alloy, thin-walled, with low surface roughness, high accuracy by weight, minimal allowances for machining, with a sharp reduction in metal waste into chips;
- the ability to create complex structures that combine several parts into one unit, which simplifies the technology of manufacturing machines and devices;
- the possibility of cost-effective implementation of the process in single (pilot) and mass production, which is important when creating new machines and devices;
- reducing the consumption of molding materials for the manufacture of the investment casting process, reducing the material consumption of production;
- improving working conditions and reducing the harmful effects of the foundry process on the environment.
Along with the advantages, this method has the following disadvantages:
- the ceramic mold manufacturing process is multi-operational, labor-intensive and lengthy;
- a large number of technological factors affecting the quality of the ceramic mold and investment casting process, and, accordingly, the complexity of their quality control associated with this;
- a large range of materials used to obtain a form (materials for models, suspensions, block sprinkling, support materials);
- the complexity of manipulatory operations for the manufacture of models and forms, the complexity of automating these operations;
- increased metal consumption for sprues and therefore low technological yield (TVG).
These advantages and disadvantages determine the effective scope of investment casting into shell molds, namely:
Production of investment casting processes that are as close as possible in configuration to the finished part, to reduce the laboriousness of machining hard-to-cut metals and alloys by cutting, reduce the use of pressure treatment of hard-to-deform metals and alloys, replace labor-intensive welding or soldering operations to increase rigidity, tightness, and reliability of parts and assemblies structures;
production of thin-walled large-sized investment casting process of increased accuracy to reduce the weight of the structure while increasing its strength, tightness, and other operational properties;
Production of the high-precision investment casting process from alloys with special properties and structure.
Precision Casting with Complex Shapes
The production of investment castings is widely used in various branches of mechanical engineering and in instrument making. The use of casting in shell molds to obtain blanks for machine parts instead of making them from forged blanks or rolled products leads to an average reduction in metal waste into chips by 34–90%.
At the same time, the labor intensity of machining is reduced by 25 – 85% and the cost of manufacturing parts – by 20 – 80%. However, it should be taken into account that the economic efficiency significantly depends on the choice of the nomenclature of castings produced by this method.
Only with the right choice of the nomenclature of parts, the high economic efficiency of this production is achieved.
