The produced models are made from a mixture or alloys of low-melting materials, most often of organic origin. Paraffin, stearin, ceresin, brown coal, peat bitumen, rosin, ethylcellulose, fatty acids, ozocerite, etc. are used as starting materials. Model compositions should have certain properties:
• The melting point is 60-81.6 ° С, the softening start temperature is higher than the working room temperature by 35-45 ° С;
• minimum and stable linear shrinkage, minimum volumetric and linear expansion;
• good fluidity;
• sufficient strength and hardness to protect against damage to the surface of models;
• Minimal ash content, unacceptability to the surface of the molds, tools and hands of the worker;
• chemical inertness in relation to the materials of molds and refractory coatings;
• absence of harmful vapors during heating and combustion;
• the ability to reuse;
• good wettability by the facing composition;
• Low cost of raw materials.
The 3 casting 1 is produced by filling a metal mold 2 with a liquid or paste-like model composition Figure 7.27, a). Examples of molds are shown in Fig. 7.28.
Fig. 7.28. Molds for winning models
In the first case, the mold is filled with a free fill or under pressure, in the second - by pressing the solid-liquid composition mixed with 8-20% of the air.
In molds, the model composition hardens and cools. Then the casting model is extracted and connected to a separately made melting model of the sprue-feeding system 4 in the block (Fig. 7.27, b). Examples of models are shown in Fig. 7.29.
Fig. 7.29. The appearance of the models being fished
The models obtained in single molds are soldered with a soldering iron 5 (see Fig. 7.27, b). In multi-seat molds, links are obtained from several models, which are connected with each other by a common bushing. When assembling a model of the sprue funnel and links of the models are mounted on a metal frame. In the block of models, the sleeves, which are put on the frame, form a riser.
To obtain the shell shape, the model block is dipped into a refractory slurry (Figure 7.27, in ) and sprinkled with a powder inert at high temperatures, for example from quartz sand, corundum or graphite in a fluidized bed (Fig. , r ) or sand. The resulting refractory coating is cured by air drying (Figure 7.27, d ) or in ammonia vapor. The second and subsequent layers are then applied to the block in the same way. The first layer is sprinkled with powder, the particle size of which is 0.2-0.315 mm; subsequent - coarse-grained powder. Usually, the shell mold is obtained by four or six times applying a refractory coating. When making large castings on the model block, more than 12 layers are formed (Figure 7.30).
Fig. 7.30. Refractory coating of the model after drying:
a - from corundum; b - quartz sand; in - graphite
After drying the last layer, the model is melted in hot water or a melt of the model mass (Fig. 7.27, e ). The shell mold is then dried in air (Figure 7.27, x ). The dried form is placed vertically in a heat-resistant flask and covered with dry quartz sand (Fig. 7.27, s ), which is compacted. The opoku will be placed in the electric furnace to remove the remnants of the model mass, moisture, products of incomplete hydrolysis, and also to harden the shell by sintering. The piercing lasts for 2 hours at a temperature of 900-950 ° C (Figures 7.27, and ).
In mass production shells are molded with hot support filler after their preliminary roasting. Thick-walled shell molds are poured without supporting filler.
Melting and pouring metal into shapes
Liquid metal is poured into a hot or cooled shell mold (Figure 7.27, to ). Its temperature when pouring steel and cast iron is 800-900 ° C, nickel-based alloys are 900-1816 ° C, titanium is 1470 ° C, copper is 600-700 ° C, aluminum and magnesium are 200-250 ° C./p>
Knockout of castings from molds and cleaning
After cooling, the castings are knocked out of the mold on the grilles. The supporting filler wakes up through the grate, and the casting block (Fig. 7.27, l) after further cooling is pre-cleaned on vibrating plants. In the manufacture of small castings, sprue systems are simultaneously separated. Cleaning of castings from refractory coating is a very laborious operation. Several methods of cleaning are used: vibration, sandblasting (Figure 7.31), hydro-sandblasting, chemical-thermal in alkali and acid solutions, as well as in molten salts, and other combined methods.
Fig. 7.31. Sandblast cleaning
In addition, conventional methods are used, with the difference that a very small fraction (no larger than 0.3 mm) should be used, so as not to distort the configuration of the castings.
Areas of Application
Casting for investment models is widely used for the production of small complex castings in aircraft, automobile, tractor and other engineering industries.
A model that can be made up of individual parts by soldering or gluing allows the production of complex solid parts that can not be produced by other metal processing methods. This way, parts of turbine blades made of heat-resistant alloys are produced (Fig. 7.32), which are poorly processed by cutting, wheels of pumps from corrosion-resistant alloys, parts of turbomachines, permanent magnets with oriented crystal structure, etc.
Fig. 7.32. Examples of blanks of complicated shape parts from heat-resistant alloys obtained by investment casting
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