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The essence of the method of casting by melting models (LVM) is that the model is made of a material that can be melted or dissolved without destroying the mold in order to obtain an integral mold. This ensures high geometric accuracy and surface quality of the castings. Most often, an easily melted waxy mass is used to make the model.
One-time casting models are made in molds by pouring or pressing the model mass with a syringe. Then the models are assembled into blocks. A layer of a liquid mixture – suspension consisting of a pulverized refractory material and a binder is applied to the surface of the model block by dipping. Ethyl silicate (ETS) and liquid glass are most often used as a binder. The resulting layer of the mixture is sprinkled with granular refractory material and dried. Then a new layer of suspension is applied and sprinkled with granular refractory material again. As a result, a 4-6-layer shell is obtained on the surface of the model block. After that, by melting or dissolving in hot water, the model is removed from the shell. The resulting multilayer shell form is molded in a loose refractory filler in flasks made of heat-resistant steel. To remove moisture and residues of the model mass from the shell form, it is calcined in furnaces at a temperature of 800-1000 ° C. The calcined shell form in the heated state is filled with melt.
In foundries at various machine-building enterprises, ethyl silicate (ETS-32, ETS–40) is used as a binder - an organosilicon liquid, which is a mixture of polysilicon acid esters. The ETS itself is not a binder. To obtain a binder, it is necessary to carry out hydrolysis. ETS is hydrolyzed with water in the presence of organic solvents (alcohols, acetone and their mixtures), and a catalyst (hydrochloric and sulfuric acid) is used to accelerate the reaction. The essence of the hydrolysis reaction is that during the chemical reaction, ethoxyl groups are replaced by hydroxyl groups. As a result of the hydrolysis reaction, colloidal solutions of silicic acids (sols) are obtained, which, when drying and calcining the molds, combine the grains of the refractory base of the suspension. [ 1, 2 ]
One of the ways to solve the problem of reducing the duration of the manufacturing process of shell molds is the use of new binding materials.
Work was carried out on the application of the technological process for the manufacture of shell molds using the "Sialit 20" grade silica as a binder (manufacturer of CJSC "Sialit", Yelabuga) in the concern "Tractor Plants" Cheboksary.
"Sialit20" is five times cheaper than ethyl silicate, an environmentally friendly material, convenient for the preparation of a suspension, since the hydrolysis operation is excluded, it retains its properties unchanged for a long time. It belongs to the group of silicate binders – silica salts with a low content of alkaline compounds and has a high silicate modulus (25 units), which ensures high temperature resistance and fire resistance of the shells. When using this binder, castings are obtained with a clean surface, and ceramics are easily separated. [ 3 ]
"Sialite 20" is an aqueous dispersion of very small SiO2 particles with a size of 8-12 nm, which are connected to each other and are capable of forming bonds with all refractory oxides used in castings. "Sialit 20" is a slightly yellow or gray liquid without visible mechanical impurities and inclusions. The mass fraction of SiO2 is 20-22%; density is 1.133 - 1.135g/ cm3; pH = 9.5-10.5; kinematic viscosity is 10 mm2/s; the wetting edge angle is 90 degrees, the foam drop time is 10-15s.
Sialit 20 particles do not accumulate and do not precipitate, since each particle has a negative charge. Due to its low viscosity (similar to the viscosity of water), "Sialite 20" is a good binder in which the filler of refractory oxides is suspended. After the water evaporates, the binder particles bind to the ceramic particles and to each other, forming a "glue" that holds the ceramic mold together. Once formed, this bundle becomes waterproof, which allows you to apply subsequent layers. The bundle is preserved and even strengthened after calcining the ceramic mold before pouring the melt.
According to a number of properties, "Sialit 20" surpasses ETS, provides increased gas permeability of the ceramic mold, less tendency to cracking, and is also non-flammable and non-toxic.
Testing of the technology of manufacturing shell molds using "Sialit 20" showed that the molds can be made in various ways: single – from suspensions based on the binder "Sialit 20"; combined – the first layer of suspension based on "Sialit 20" and subsequent layers based on a liquid-glass suspension or alumomagnesium borate phosphate binder.
A refractory suspension based on a modern binder "Sialit 20" is manufactured using the following technology. [ 4 ]
1. Components are poured into the hydrolyzer tank in the following ratio, mass%:
"Sialit 20" - no more than 80;
ETS-32 (non–hydrolyzed) - no more than 10;
OP-7 (liquid soap) – about 1% over 100%.
2. The components are mixed for 1-2 minutes.
3. The calculated amount of refractory filler – marshalite is filled in.
4. The suspension is stirred for 20-30 minutes.
5. The viscosity of the suspension is measured (50-60 s).
6. If necessary, a refractory filler is added to increase the viscosity of the suspension.
7. If it is necessary to reduce the viscosity of the suspension, binders are added at the rate of: "Sialit 20" - 88%, ETS-32 – 12%.
8. The finished suspension is kept in a hydrolyser for 10-15 minutes to remove air.
9. A refractory suspension is applied to cover the first layer of the surface of the fusible model.
In the manufacture of shells from a suspension based on "Sialit 20", the cost of castings is significantly reduced, the process of preparing a refractory suspension is simplified, its stability in quality is ensured, and the survivability period of the refractory suspension is increased to 4-5 days. The strength and thermomechanical properties of the shells are satisfactory. The quality of the surface of the castings increases, there is no prigar.
The disadvantage of a silica–based suspension is the inadmissibility of its storage at negative temperatures due to the loss of its original properties by the binder.
However, despite this drawback, the use of a binder material "Sialit 20" for the manufacture of shell molds during casting on smelted models is a promising direction.
