[China Aluminum Industry Network] With the continuous development of industrial technology and production technology, as well as the intensification of competition in large industrial markets at home and abroad, especially since China's accession to the WTO, the requirements of casting quality, especially the surface quality of castings, have been demanded by machine and engineering industries. Higher and higher. Cast coating refers to a coating applied to the surface of a mold cavity or core to improve its surface refractoriness, chemical stability, resistance to metal erosion, resistance to sticky sand, and the like. Casting coating has close relationship with the surface quality of the casting. This article talks about the effect of casting coating on the surface quality of the casting from the aspects of the role and performance of the casting coating.
First, the role of paint 1. To reduce the casting surface of the mechanical viscous sand and chemical clay mold and core there are many holes in the casting and solidification process with static pressure and dynamic pressure of the metal liquid will infiltrate the pores, forming adhesion Metal shells that are difficult to clean on the surface of castings are called "mechanical sands". The application of a casting coating can close the pores between the sand grains of the surface layer of the casting mold and the core, block the passage of the molten metal, and reduce the mechanical adhesion of the casting. Molten steel at the casting or lower temperature, the surface will continue to generate metal oxide film, this metal oxide film can react with the silica sand, resulting in the surface of the casting "chemical adhesion." The use of foundry coatings can isolate the molten metal from the surface of the mold or core, inhibit chemical reactions between them, and reduce or eliminate chemically adhered sand on the casting surface.
2. To reduce the casting sand and sand casting process of the casting surface, the high-temperature metal liquid has a strong heat radiation effect on the surface of the mold and the core, and the heat pressure and the hot-wet tensile strength generated after the mold and the core are heated Causes the casting to produce sand. Cast coatings can reduce the radiation heat of the mold or core, thereby reducing or eliminating the generation of sand inclusion defects. Coatings with a certain binding capacity can also be infiltrated between the casting mold and the core surface sand, thereby strengthening the surface strength and erosion resistance of the casting mold or core, and reducing the sandblasting defects of the casting.
3. Improve the surface performance and internal quality of castings By adding thermal insulation materials and chilled materials to the coating, it can improve the mold cavity temperature distribution and control the alloy solidification and crystallization process, thereby reducing the cold cracking and hot cracking of the casting surface. The production. Adding some inoculants or alloying elements to the coating can also play a role in local inoculation or surface alloying to improve the microstructure properties of the casting.
Second, the performance of the paint 1. Suspension stability coating In the process of storage or use, solid particles should be suspended as possible, no delamination, no precipitation, no agglomeration, to ensure uniform coating performance and coating quality.
2. The brushing brushing performance is good: the brush is smooth, non-sticky, and does not bring sand when brushing the scum paint. After the brushing, the coating can automatically level, no brush mark, cavity Loss of paint does not occur on the vertical surface.
3. Permeable quality paint should have the property of penetrating into molds and cores with proper depth. Generally, the depth of penetration should be 2 to 3 times the diameter of sand. The penetration of paint into sand pores can also increase the affinity between sand grains. 4.Surface Strength The ability of the coating to be scratched by external force after being cured, without leaving any traces after the brushing action, and without powder particles, is called the surface strength of the coating. Sufficient surface strength can prevent the handling, the core and the box from occurring. Casting damage. Good paint should also have good anti-adhesive properties, crack resistance, preservation and a small amount of gas performance.
Third, the principle of casting coating on the surface quality of the casting 1. Insulating and carbon membrane segregation Graphite powder coating has good peelability when used in iron castings. Graphite produces a carbon monoxide film and a bright carbon film during casting, solidification and cooling, preventing the interaction of the molten metal with the sand or core surface and preventing sticking. The surface of the wet cavity is coated with a mixture of a waste oil and a graphite powder, and a thin-walled cast iron piece with an abnormally smooth surface is obtained by casting a reducing gas film and a carbon film generated at a high temperature.
2. The slag separation layer forms a slag layer on the interface between the coating layer and the casting, which has a high viscosity to stick together the refractory aggregates and block the penetration of the molten metal. The slag layer has a good effect when it is chemically reacted with the molten metal free of silicate and is not wetted or substantially not wetted with the molten metal. When cooling, the shrinkage coefficient of the slag shell layer and the metal wire of the casting has a large difference, and a large shear stress is generated at the interface between the slag shell layer and the casting, resulting in the automatic peeling of the coating shell. This type of slag layer prevents sticking of sand. The theory is called "slag segregation theory." Coatings made of easily reacted metals and metal oxides undergo thermal reactions to form a new refractory oxide that can prevent the penetration of molten metal.
3. Oxidation Theory It is believed that whether or not the casting is bonded depends on the presence or absence of a critical thickness of the iron oxide layer between the casting and the coating. The thickness is about 100 μm. In order to prevent chemical adhesion, the thickness of the oxide should be increased. When it exceeds the critical thickness, the sinter layer of the coating or the molding sand and the metal oxide is easily peeled from the casting. Due to the following reaction during the casting of the limestone, the limestone sand type CaCO3=CaO+CO2 leads to a strong oxidizing atmosphere inside the mold cavity and exacerbates the oxidation of the surface of the cast steel part. Therefore, the limestone sand has an excellent anti-adhesion effect.
The use of good castings for cast steel sand castings on sand castings of cast iron sands, but even severe sanding, can be explained by the fact that the iron oxide layer is easily formed on the surface of cast steel. Cast iron has a high carbon content, and carbon is oxidized prior to iron. Therefore, it is difficult to cast an iron oxide layer having a critical thickness when casting cast iron. Therefore, the “oxidation†theory is not applicable to anti-stick sand coating for cast iron parts.
Metal Polishing Wax Paste Compounds
Metal Polishing Wax Paste Compound ,Yellow Buffing Wax,Solid Wax For Mirror Polishing,White Car Wax
NINGBO HENGHUA METAL SURFACE TREATMENT TECHNOLOGY CO.,LTD. , https://www.henghuametal.com