In sand casting production, casting coating is an essential auxiliary material to guarantee casting quality, which directly affects the surface finish and yield rate of castings. This article systematically introduces the functions of coatings and the features of each component.

Ⅰ. Core Functions of Casting Coatings

1. Prevent Metal Penetration and Burn-on

• The refractory aggregate in coatings has much finer particles than molding sand. After coating, it seals the pores on the surface of molds and cores to block molten metal from penetrating into the sand mold.

• Refractory fillers feature good thermal conductivity and high heat storage capacity. They shorten the contact time between the casting mold and molten metal and reduce the penetration depth of molten metal into mold pores.

• Refractory fillers possess high refractoriness and excellent thermochemical stability. A stable barrier forms at the interface under high temperature, preventing chemical reactions between castings and molds that generate low-melting substances and stopping chemical burn-on.

• Coatings follow the principle of easily peelable sintered layers. Refractory fillers sinter moderately under the heat of molten metal, and the coating can peel off automatically after cooling to eliminate burn-on defects.

2. Reduce Surface Roughness of Castings

Refractory fillers inside coatings have fine particles, forming smooth and dense coatings after application. Finer filler particles enable tighter packing and smaller pores to resist molten metal penetration. The coating is not wetted by molten metal and will not chemically react with molten metal or metal oxides at high temperatures. It avoids rough casting surfaces caused by chemical erosion and delivers castings with smooth surfaces.

3. Reinforce Mold Surfaces and Reduce Sand Washes During Pouring

Coatings have relatively high strength. When applied, they penetrate the surface layer of sand molds and cores and fill gaps between sand grains, improving bonding strength among sand grains. They greatly increase the surface strength of molds and cores, resist scouring by molten metal and reduce sand wash defects during pouring.

4. Adjust Surface Structure and Modify Casting Surfaces

Special coatings containing tellurium or bismuth vaporize under high temperature to change the critical cooling rate of molten iron and accelerate crystallization. They can form a white iron structure locally on castings to improve surface hardness and wear resistance, reduce local shrinkage porosity, and realize surface alloying and grain refinement of castings.

Ⅱ. Five Major Components of Casting Coatings

Casting coatings mainly consist of five components: refractory filler, carrier, suspending agent, binder and additives. These components work together to ensure brushing performance and high-temperature service performance of coatings.

(1) Refractory Filler

As the core component of casting coatings, refractory filler plays a decisive role in preventing burn-on. It resists penetration and erosion of molten metal and insulates high-temperature reactions between molds and molten metal. Common refractory aggregates in the casting industry include graphite powder, zircon powder, corundum powder, bauxite powder, quartz powder, magnesium oxide, silicon carbide, mullite, boron nitride and olivine. Different refractory fillers vary in physicochemical properties and shall be selected according to the casting metal type:

• Steel castings (pouring temperature＞1500℃): Zircon powder coating, quartz powder coating, magnesia powder coating and high-alumina coating are available. For high manganese steel with alkaline molten metal, magnesium oxide coating is preferred, and zircon coating is also applicable. Zirconia features high refractoriness, low thermal expansion coefficient and excellent erosion resistance. It will not react with iron oxide under high temperature and delivers outstanding anti-burn-on performance.

• Iron castings (pouring temperature＞1300℃): Zircon powder coating, aluminosilicate coating, magnesium silicate coating and composite graphite powder coating are optional. Graphite is a neutral refractory filler that does not react with molten metal and has strong chilling capacity for smooth castings. Flake graphite has a refractoriness up to 2000℃, while amorphous graphite is prone to oxidation. Graphite coatings are not suitable for steel castings as they cause carburization. Light-colored iron casting coatings adopt aggregates such as quartz and aluminosilicate to further improve resistance to molten iron erosion.

• Aluminum alloy castings (pouring temperature＞550℃): Zircon powder coating, aluminosilicate coating, magnesium silicate coating, composite graphite coating, boron nitride coating and special coatings with tellurium and bismuth powder are available.

  
  

（2) Carrier (Solvent)

Carriers disperse and suspend refractory fillers to form coatable slurry, divided into water-based carriers and alcohol-based carriers.

• Water-based carrier: Widely used with low cost. Water-based coatings have good levelling property and form dense coatings after drying, producing castings with superior surface finish. They are mostly applied in resin sand mold scenarios but require supporting drying ovens.

• Alcohol-based carrier: Including methanol, ethanol, isopropanol, butanol, etc. Isopropanol burns gently and sufficiently without damaging the sand mold bonding system, delivering stable coating quality and good brushing performance. The downside is higher raw material cost.

(3) Suspending Agent

Suspending agent stabilizes coating slurry, prevents settlement of refractory fillers, keeps uniform components after long-term storage and facilitates on-site coating operations.

(4) Binder

Binders bond refractory aggregate particles to each other and firmly attach coatings onto sand molds and cores. They improve both ambient and high-temperature strength of coatings to avoid coating peeling and shedding during pouring.

 (5) Additives

Additives optimize comprehensive coating performance, enhance storage stability, and improve on-site performance such as brushing, levelling and drying to meet production requirements of different casting processes. Coatings without additives tend to delaminate and agglomerate. Additives endow coatings with good fluidity and significantly improve brushing performance.

XINDA Casting Technology focuses on the R&D and production of casting coatings. Our full product portfolio covers alcohol-based coatings, water-based coatings, lost foam coatings, V-process coatings, chilling coatings and metal-specific coating solutions. Developed specially for mold and core working conditions, our coatings feature excellent surface modification, heat insulation and anti-penetration performance. They effectively suppress casting defects such as veining, sand burn-on and metal penetration. Applicable to brushing, dipping and spraying, our products suit steel, iron and non-ferrous alloy castings to stabilize casting quality and improve overall production efficiency.