Sand sticking to gray iron castings is a common surface defect in the casting process. It is mainly manifested as the difficult-to-remove molding sand or sintering layer attached to the surface of the casting, which affects the surface quality and dimensional accuracy of the casting. Its causes involve many factors such as molding sand performance, process parameters, and metal characteristics.

1. 1. Types and mechanisms of sand sticking for Gray iron castings

1.1 Mechanical sand sticking
Sand grain gap penetration: High-temperature molten iron (especially gray iron with good fluidity) penetrates into the gap between sand grains under static pressure, and forms a mechanical bond between metal and sand grains after solidification.

Insufficient compactness of sand mold: When the compactness of molding sand is low, the gaps between sand grains are large, and molten metal is easy to invade.

1.2 Chemical sand sticking
Interface reaction: Fe, C, Si and other elements in molten iron react chemically with SiO₂ and binder (such as bentonite) in molding sand to generate low-melting point compounds (such as fayalite Fe₂SiO₄) and form a sintering layer.

2. 2. Specific reasons

2.1 Problems with molding sand performance

Insufficient refractoriness:
Quartz sand (SiO₂) has low purity (such as high content of impurities Al₂O₃ and Na₂O), and is easy to react with molten iron at high temperature. Excessive accumulation of ash in regenerated sand reduces the refractoriness of molding sand.

Irrational particle size distribution:
Sand particles are too coarse (such as AFS fineness <50), resulting in large gaps between sand particles and high risk of penetration; Sand particles are too fine (such as AFS fineness >100), with poor air permeability, and gas pressure promotes metal liquid penetration.
Excessive binder: When the amount of bentonite added is >8%~10%, the high-temperature strength of the sand mold decreases, and it is easy to crack and form a penetration channel.

2.2 Coating and coating process
The coating is not effectively covered: the coating layer is too thin (<0.3mm) or missed, and the metal liquid and the sand mold cannot be isolated; the alcohol-based coating is not fully ignited, and the residual solvent reduces the density of the coating.
Poor fire resistance of coating: Graphite powder or zircon powder coating has high impurity content and is easy to sinter and fail at high temperature.

2.3 Characteristics of molten metal
Carbon equivalent (CE) is too high: When gray iron CE>4.3%, the fluidity is excellent and the penetration tendency is enhanced.
Pouring temperature is too high: When the temperature is>1450℃, the surface of the sand mold is heated violently, the quartz sand phase changes and expands (such as β→α quartz), and the bonding force between sand particles decreases; high temperature intensifies the chemical reaction between molten iron and sand mold. Severe metal oxidation: Molten iron contains oxides such as FeO and MnO, which react with molding sand to form low-melting silicates.

2.4 Process parameters and design

Improper design of the pouring system: The inner gate is facing the weak area of the sand mold, and local scouring causes sand particles to fall off; the filling speed is too fast, the kinetic energy of the molten metal is large, and the erosion of the sand mold is aggravated.
Uneven compactness of the sand mold/sand core: Low local compactness during manual molding forms a penetration channel.
Excessive static pressure of metal: The static pressure of molten iron of thick and large castings (such as machine tool base) is high, and the risk of penetration is significantly increased.

2.5 Operational factors
Poor repair of sand mold before assembly: insufficient compactness of repaired parts or no repainting.
Sand mold storage time is too long: evaporation of moisture in wet sand causes surface powdering and reduced strength.
Unpacking too early after pouring: The casting is in contact with the sand mold for too long at high temperature, which aggravates chemical sticking.

3. 3. Prevention and improvement measures of Gray iron castings

3.1 Optimization of molding sand
Select high-purity quartz sand: SiO₂ content> 98% to reduce the risk of impurity reaction.
Control particle size distribution: AFS fineness 60~80, coarse sand (40/70 mesh) and fine sand (70/140 mesh) are mixed.
Adjust the proportion of bentonite: the bentonite content in wet sand is controlled at 6%~8%, and coal powder (2%~3%) is added to form a reducing atmosphere.

3.2 Coating reinforcement
Improve coating quality: Use zircon powder or chromite powder coating, spray thickness ≥ 0.5mm to ensure complete coverage.
Pre-baking sand mold: The water-based coating sand mold is dried at low temperature (150~200℃) before assembling to reduce the water vaporization pressure.

3.3 Process control
Pouring temperature management: The suitable pouring temperature for gray iron is 1380~1420℃, with the upper limit for thin-walled parts and the lower limit for thick-walled parts.
Reducing metal static pressure: A stepped pouring system is used for tall castings to disperse the impact of molten metal.
Increasing the compactness of sand molds: High-pressure molding machines ensure the hardness of molding sand (85~90 units).

3.4 Adjustment of metal composition
Control CE value: CE=3.9%~4.2%, taking into account both fluidity and reducing penetration tendency.
Add trace alloys: Add 0.1%~0.3% Cr or 0.05% Ti to increase the surface tension of molten iron and inhibit penetration.

3.5 Operation Specifications
Shorten the time from box assembly to pouring: complete pouring within 4 hours after wet sand molding to prevent moisture in the sand mold.
Reasonable unpacking: cool the casting to below 600℃ before unpacking to avoid high temperature oxidation and sand sticking.

4. 4. Identification of sand sticking defects
      Mechanical sand sticking: sand grains are interlaced with metal, and metallic luster can be seen after peeling, mostly appearing in thick and large parts or near the inner gate

Chemical sand sticking: glassy sintered materials exist at the interface between the sand layer and the metal, which are difficult to remove mechanically and require pickling or sandblasting. Observe the interface composition through scanning electron microscopy (SEM). If Fe-Si-O compounds are found, it can be determined as chemical sand sticking; if the gaps between the sand grains are filled with metal, it is mechanical sand sticking.