Formula of EPC coating:
1, 90% bauxite, 10% quartz powder, 2% phenolic resin, 2% white latex, 3% calcium bentonite, 0. 1% washing powder, 0. 1% isopropanol, and a proper amount of water;
2. The ratio of silica sand powder to bauxite is 2: 3, (refractory 100%), GM- 1 resin 2%, silica sol 3%, lithium bentonite 1%, CMC 1.5%, leveling agent 0.3% and water reducer 0.
3.60% bauxite, 40% flake graphite, 4% silica sol, 4% binder, 3% bentonite 1% iron oxide, 3% carboxymethyl cellulose, nitrogen and 0.5% nitrogen additive;
The sum of the above formulas is 100%.
? Inorganic binder (high temperature binder): hydrophilic: clay, bentonite, water glass, silica sol, phosphate, sulfate, polyaluminum chloride.
Organic bentonite organic binder (low temperature binder): hydrophilic: syrup, pulp waste liquid, dextrin, starch, polyvinyl alcohol (PVA), polyvinyl acetate emulsion, water-soluble phenolic resin, natural rubber. Hydrophobic:? Asphalt, coal tar, rosin, phenolic resin, dry vegetable oil, synthetic oil, ethyl silicate, polyvinyl butyral (PVB)3. Carrier: EPC coating can be divided into water-based coating and alcohol-based coating, but water-based coating is used more in production.
4. Suspending agent: The commonly used suspending agents for waterborne coatings include bentonite, attapulgite, sodium carboxymethyl cellulose, polyacrylamide, sodium alginate, etc. Let's first look at its chemical composition: Al2O3 content in bauxite, sintering temperature and impurities such as Ti02 and FeO3 have great influence on the coating. Therefore, it is still the key step to select bauxite with high Al2O3 content, calcine it at 1450℃- 1500℃, and grind the selected clinker into fine powder, and acid cleaning treatment. Titanium dioxide and ferric oxide are removed from bauxite without pickling. Dust and mechanical iron mixed in the process of calcination and grinding increase. (At present, bauxite fine powder is mainly processed in raymond mill, and wear-resistant parts are mostly high-strength steel castings, so it is difficult to remove mixed mechanical iron by magnetic separation. ) The fire resistance of the coating is affected, and the increase of floating objects and deposits affects the brushing, suspension and storage time of the coating.
Prevent sand sticking defects
Sand sticking of EPC castings can be divided into mechanical sand sticking and chemical sand sticking. The essence of mechanical sand adhesion is the filling and infiltration of molten metal into the pores between sand particles. This situation is because the infiltration pressure of molten metal exceeds the additional pressure caused by the gas back pressure and the surface tension of molten metal in the sand gap, and the molten metal penetrates into the sand gap. If a dense coating is coated on the outer surface of the foam mold, the penetration of molten metal can be prevented, thus effectively preventing the mechanical sand sticking of the casting. Chemical sand sticking is mostly caused by low refractoriness and low melting temperature of molding materials. When high-temperature molten metal is poured, it is easy to be melted by molten metal, forming defects such as knots. Therefore, it is very beneficial to isolate a layer of coating with high fire resistance between molten metal and molding materials to prevent chemical sand sticking.
Coating and hanging method
The coating of EPC should be coated on the surface of EPC pattern, while the coating of traditional sand casting is coated on the surface of prepared mold. The coating methods of the two are very different, and the technical requirements are not the same. The coating methods of EPC coating include brush coating, dip coating, spray coating, spray coating or dip coating. Generally speaking, the surface roughness and dimensional accuracy of EPC castings depend on the quality of EPC pattern itself, but the thickness of coating does not affect the casting size. The coating with rough surface not only does not affect the surface roughness of castings, but is beneficial to improve air permeability.
brush drawing/painting
Brush method is to brush by hand with a brush. This method requires less strict coating performance than spraying or dipping, but it requires higher skills and experience of operators.
When brushing, the thickness uniformity of the coating is not as good as that of spraying or dipping, but the effect is satisfactory. For some EPC with complex structure and shape, it is best to choose brushing. Because when spraying, some "dead corners" are often not sprayed; However, it is often difficult to discharge the accumulated paint in some grooves by dip coating.
The main disadvantages of brush coating are high labor intensity and low production efficiency, and it is only suitable for touch-up brush operation or large-volume single-piece production that cannot be dipped or sprayed.
spray
The spraying method is to spray paint on the lost foam pattern with a spray gun. Spraying is more efficient than brushing and the labor intensity is much lower.
Because of the heavy proportion and high viscosity of refractory coatings, it is necessary to spray coatings in mist; In addition, the object of painting is the easily damaged foam mold, and the impact of painting on the foam mold is required to be as small as possible.
The spraying method has the following problems:
① High technical requirements for coatings. When the coating concentration is too high, it is often impossible to spray or uneven; If the concentration is too small, it will drip or stick easily. In addition, the large particles and foreign matter mixed in the coating are easy to block the nozzle and interrupt the production;
(2) The pattern of EPC is not easy to be clamped, which often causes the pattern of EPC to be deformed or damaged during spraying;
(3) Waste of paint: a considerable part of the misty paint sprayed by the spray gun cannot fall on the pattern of the lost foam, which not only wastes paint but also pollutes the environment;
④ Poor quality control: some dead corners can't be sprayed, and it is easy to destroy the first coating when spraying for the second time.
Dip coating method
Dip coating method has the characteristics of high production efficiency, saving coating, uniform coating, low requirements for coating, simple operation and no need for special equipment. Because the density of foam pattern is small (several times different from that of coating) and its strength is not high, attention should be paid to prevent the pattern from deformation and damage due to the high density of coating.
The coating thickness of dip coating can be controlled by coating concentration, dip coating times and controlling dip coating operation. Generally, two dip coating operations are adopted: the first dip coating with a small concentration to obtain a uniform thin coating and improve the coating performance of the two operations; In the second dip coating, the uniform coating thickness should be obtained by adjusting the coating concentration. If the coating concentration is unchanged, it is required to control the coating thickness, which can only be adjusted by the number of jitter pattern losses after dip coating. It is generally not difficult for a skilled painter to meet this requirement.
Matters needing attention in dip coating operation:
( 1)? Before dip coating, the paint must be stirred reasonably, and it is best to keep stirring slowly during dip coating to prevent air from being involved;
(2)? When dipping, the direction and position of the pattern of EPC should be selected to prevent the pattern from being deformed;
(3)? The coating should be uniform, and there should be no whitening phenomenon on the coating;
(4)? The lost foam after dip coating should be prevented from deformation when it is taken out of the container, transported and placed.
(5)? After dip coating, the appearance of EPC should be shaken in time to make the coating uniform and remove the excess coating.
In order to overcome the deformation of EPC pattern caused by buoyancy of dip coating method, the EPC pattern can be fixed by suitable fixture and then immersed in paint container. In mass production, special mechanized dip coating equipment can be used. First, the pattern of the EPC is fixed with a fixture, then the paint is pressed in from the bottom, and then it is taken out after the pattern of the EPC is submerged, and the paint will automatically fall down to complete the dipping process.
Dip coating method is suitable for producing small castings. As long as the volume of the coating barrel allows, the larger EPC pattern can also be dipped in sections for many times.
Paint coating thickness
The thickness of EPC coating affects its permeability and fire resistance. As far as fire resistance is concerned, enterprises hope that the thicker the coating, the better, because it is a very effective process to solve the sand sticking defect of castings. However, if the coating is too thick, it will affect the permeability of the coating and the coating operation is also very troublesome. Considering the permeability of the coating, the thinner the coating, the better. However, too thin coating will lead to needle sticking defects on the surface of castings, even sand sticking defects, which will affect the surface quality of castings. Casting weight (kg)? Coating thickness (mm)? Use? path
& lt 100? 0.5~ 1? Aluminum castings and copper castings, small iron castings
100~500? 1.0~ 1.5? Medium-sized iron castings, small and medium-sized steel castings
& gt500? 1.5~2.0? Large cast iron and steel castings
Wow, Bian Xiao introduced so much professional knowledge to all readers at once. I don't know if you understand? We are bound to encounter all kinds of problems in our daily life, hehe, we have benefited a lot! So is Bian Xiao! Lost foam coating is also a kind of coating that often appears in our lives. We should use it effectively while understanding it, so as to apply what we have learned. The above is the content of EPC coating formula, I hope it will help everyone!
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