Classification and standard of casting coatings

Casting coating is one of the indispensable raw and auxiliary materials in the casting process, which has a significant impact on the quality of castings. When the metal liquid is poured into the mold cavity and contacted with the sand mold and sand core, there will be various physical and chemical effects between the sand and the metal liquid interface. As a result of these effects, some casting defects may appear on the casting. In casting practice, casting coating is often used to prevent these casting defects.

As early as more than 3000 years ago, China’s foundry craftsmen have prepared and used coatings, making contributions to the development of foundry technology. But until the end of World War II, the casting industry did not regard coating as an important process, and the preparation and operation of coating were relatively random. In the 1950s, with the use of chemical binders, the traditional coating and new molding material technology can not adapt to each other. On the basis of the coating prepared by each foundry, it entered a new stage of coating research and professional production.

Casting coating refers to a layer of casting auxiliary material with specific functions, which is used to attach to the mold and core and to isolate the metal liquid during the casting process. General casting coating is composed of refractory filler, liquid carrier, binder, suspension stabilizer, other additives and other materials.

Generally, the casting coating has the following specific functions: to prevent the casting from adhering to sand; to improve the surface finish of the casting; to strengthen the surface of the sand core; to shield or isolate (s, N and other gases); to prevent the casting defects (veins, sintering, pores and sand holes); to reduce the surface metallization and grain refinement of the casting; to adjust the solidification temperature field of the casting (enthalpy change casting coating, heat preservation casting coating); Reduce the amount of casting sand and cleaning labor, save labor time.

Generally, the casting coating should have the following characteristics: it can withstand the high temperature fire resistance of liquid metal; it has good adhesion to the core; it is not easy to produce bubbles, cracks and peeling when drying; it has good rheological property, good suspension stability, good storage and good covering ability.

In order to facilitate the foundry engineers and operators to better understand and apply the casting coating, based on a large number of practical experience in the factory, this paper briefly summarizes the key technical characteristics and precautions involved in the production and application of the casting coating.

Casting coating can be divided into different methods from different perspectives, such as:

According to solvent: water-based casting coating, alcohol based organic solvent casting coating;

According to application: casting coating for cast steel, casting coating for cast iron, casting coating for non-ferrous metal, flow casting coating, EPC coating and die casting coating.

In order to embody the practicability, advanced nature and development of technology, and fully absorb the latest technological achievements at home and abroad, it reflects the interrelation and guidance of casting practice. The original standard JB / t9226-1999 “coating for sand casting” and JB / T 5107-1991 “coating for sand casting – Test Method” were incorporated into the new standard JB / T 9226-2008 “coating for sand casting” which was approved on March 12, 2008 and formally implemented on August 11, 2008. In this standard, the type of coating used for sand casting is determined, the technical requirements and indexes of coating used for sand casting are determined, and the testing method of coating performance for sand casting is improved [2].

According to the standard JB / T 9226-2008, casting coatings are classified into 9 categories according to different refractories. They are: graphite powder casting coating, talcum powder casting coating, refined quartz powder casting coating, high alumina bauxite casting coating, zircon powder casting coating, brown corundum powder casting coating, forsterite powder casting coating, magnesia powder casting coating and other casting coating.

Due to the wide range of casting coating, JB / T 9226-2008 is only for the specification of sand casting coating, but also due to the different casting coating formulations used by various suppliers, the functions of casting coating provided are different, and the standards, specifications and technical indicators of casting coating are different, so the specific standards, specifications and technical indicators of casting coating, It shall be carried out in strict accordance with the instructions of the product provided by the supplier, or in accordance with the recommendations of the supplier.

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