Along with the development of industrial technology and production process of continuous development, as well as the domestic and foreign industrial market competition intensifies, especially since China's accession to the WTO, machine manufacturing and engineering circles on casting quality especially the casting surface quality becomes more and more important. Casting coatings are coated on the surface of mould cavity or core, in order to improve the surface fire resistance, chemical stability, resistance to metal erosion, sand adhesion resistant properties of a covering. Foundry coatings and surface quality of castings are closely related, this article from the foundry coating effect, performance and other aspects about casting coating on casting surface quality influence.
One, the action of coating
1 reduction of casting surface mechanical sand and chemical bonded sand mold and core has a lot of pore, in casting and solidification with static pressure and dynamic pressure of liquid metal infiltration pore formation, adhesion on the casting surface is difficult to clean the metal shell, called "mechanical sand ". Application of foundry coating can be closed for mold and core sand surface layer between the liquid metal infiltration jam pore, the channel, reduce the casting penetration. The liquid steel in the casting or lower temperature, the surface will continue to generate metal oxide, the metal oxide and silica sand chemical reactions occur, resulting in the " chemical bonded sand casting surface ". The use of foundry coating can make the liquid metal and mold or core surface to inhibit their isolation, chemical reaction between, reduce or eliminate the casting surface chemical sand.
2 reduction of surface of the casting sand and sand casting process, the high temperature molten metal on the mold and core surface has strong heat radiation, mold and core heat generated by the thermal stress and thermal wet tensile strength will lead to the cause of casting sand. Foundry coatings can reduce the mold or core of the radiation heating, thereby reducing or eliminating the sand inclusion defect. Have a certain bonding ability of coating, also can penetrate into the mold and core sand between surface, thereby strengthening the mold or core surface strength and ability to resist erosion, reducing casting sand flushing defect.
3 improve the casting surface properties and internal quality through added in the coating heat insulating material and chill material, to improve the mold cavity temperature distribution and control of solidification and crystallization process of casting surface, thereby reducing the cold cracking and heat crack. Added in the coating or some inoculant alloy elements can also have local breeds or surface alloying, improve casting microstructure of role.
In two, the performance of coatings
1 suspension stability of coatings in storage or in use process, the solid particles in suspension state should as far as possible, no delamination, no precipitation, not caking, to ensure that the paint uniform property and coating quality.
2 brushing brushing of good coating performance is: brush in the brush paint brush Shuanghua, non-stick, do not take up sand, after brushing paint can automatically leveling, without Shuahen, cavity vertical surface without paint loss.
3 penetration of high-quality coatings should have penetrated into the mold and core proper depth performance, general requirements into the depth of 2 ~ 3 times of sand diameter, coating to sand pore permeability can also enhance the affinity between the grains.
4 surface coating after curing strength by external force and scratching, brush grinding role after leaving traces, off powder ability is called paint surface strength, surface of sufficient intensity can be prevented in handling, core and case occurs when the mold damage. Good paint should also have good anti-stick sand, crack resistance, durability and a small amount of gas properties.
Three, casting coatings affect the surface quality of the casting principle
1 gas and carbon membrane separation graphite powder coating for iron castings, with excellent peeling resistance. Graphite in the casting, solidification and cooling period to generate carbon monoxide gas membrane and bright carbon membrane, prevents the metal liquid and the sand mold or core surface interactions, prevent sand. In the wet type cavity surface spraying waste oil and graphite powder mixing coatings made by casting, high temperature reduction of the resulting film and carbon membrane, can get abnormal surface smooth thin pieces of cast iron.
Isolation of 2 slag casting coating layer and the casting is formed at the interface of slag layer, it has high viscosity will refractory aggregate, stick together, the liquid metal infiltration barrier. Slag layer on the metal liquid silicate chemical reaction, and the liquid metal without wetting or basically not wetting effect best. Cooling, slag shell and casting metal line contraction coefficient of difference, in the slag shell and casting interface produced a larger shear stress, causing the coating shell can automatically stripping, the use of the slag layer to prevent the sticky sand theory called the " slag separation theory ". With easy reaction of metal and metal oxide compound coating by thermal reaction, generate a new refractory oxide, can prevent the liquid metal infiltration.
3 oxidation theory thinks, whether or not depends on the casting sand casting and coating layer between the existence of critical thickness of the oxide layer, the thickness of about 100μ M. In order to prevent chemical adhering sand, should increase the oxide thickness, when it exceeds the critical thickness, coating or molding sand and metal oxide sintered layer from peeling easy casting. Limestone sand in casting steel parts due to the following response: CaCO3 = CaO+CO2, leading to the mold cavity internal strong oxidizing atmosphere, exacerbated by the steel casting surface oxidation, and limestone sand has excellent anti-sticking effect of sand. In casting sand mold coating with good application effect in cast iron sand is easy to stick sand, even serious sand, which can be derived from the casting surface is easy to generate ferric oxide layer to be explained. The cast iron with high carbon content, carbon to iron oxide, thus casting cast iron when it is difficult to generate critical thickness of the iron oxide layer, thus " oxidation " theory is not applicable to cast iron sand coating.
