How to improve the strength and anti sticking ability of coated sand in sand casting?

1. Key measures to enhance strength

a. Optimization of resin and curing system

Resin selection:

Choosing high degree of polymerization phenolic resin (such as linear phenolic resin), which has longer molecular chains and higher residual carbon content at high temperatures, can improve the high-temperature strength of sand molds; The resin dosage is controlled at 1.8%~2.2% (weight ratio of raw sand), and can be increased to 2.2%~2.5% for bottom sand molds or thick walled castings.

By using modified resins (such as adding a small amount of epoxy resin or silane coupling agent) to enhance the interfacial bonding between the resin and sand particles, the tensile strength at room temperature can be increased by 10% to 15%.

Curing agent adjustment:

Urotropin (hexamethylenetetramine) is selected as the curing agent, with a dosage of 12% to 15% of the resin content, and 0.5% to 1% of calcium stearate is added to improve the uniformity of resin coating and avoid strength fluctuations caused by insufficient "bridging" between sand particles.

b. Grading control of raw sand and sand particles

Raw sand selection:

Using quartz sand with good roundness and smooth surface (roundness coefficient>0.8) can reduce the angular gaps between sand particles, improve the packing density after compaction, and increase the strength at room temperature by 5% to 8%; Avoid using raw sand with a mud content greater than 0.2% to prevent clay impurities from weakening the resin bonding effect.

Grain size grading:

Using dual or multi particle mixed sand (such as mixing 50/100 mesh and 70/140 mesh in a ratio of 7:3) to fill the gaps between sand particles, the compactness is increased to 90%~95%, and the strength is correspondingly improved.

c. Process assisted enhancement

Film coating process:

Control the coating temperature at 180-200 ℃ and the resin coating time at 3-5 minutes to ensure a uniform and continuous resin film (thickness of 5-8 μ m) is formed on the surface of the sand particles, avoiding local thinning or accumulation.

Tightness control:

Adopting the sand blasting or vibration compaction+compaction composite compaction process, the compactness of the bottom sand mold is ≥ 95%, and the compactness of the upper sand mold is ≥ 90%, to avoid looseness and insufficient strength.

2. The core method to enhance the ability to resist sand adhesion

a. Improve fire resistance and barrier properties

High refractory raw sand and additives:

Use zircon sand (with a fire resistance of 1850 ℃) or chromite sand (1800 ℃) instead of quartz sand in areas prone to sticking sand (such as the bottom and thick walls), or add 3% to 5% magnesium sand powder (MgO) and bauxite powder to the mix, forming a high melting point isolation layer at high temperatures to prevent the reaction between molten iron and sand particles to produce low melting point compounds (such as FeO · SiO ₂).

Inert powder addition:

Add 2% to 4% flake like graphite powder or molybdenum disulfide (MoS ₂) to form a lubricating carbon film at high temperatures, reducing the infiltration of molten iron into the sand mold. At the same time, the thermal conductivity of graphite can accelerate local heat dissipation and shorten the high-temperature residence time of molten iron.

b. Optimize interface reaction suppression

Coating reinforcement:

Brush zircon powder coating (concentration 40%~50%) or graphene based coating on the surface of the sand mold, with a coating thickness of 0.3~0.5mm, forming a physical barrier; 1%~2% boric acid can be added to the coating to generate a glass phase at high temperatures, filling the gaps between sand particles and further blocking the penetration of molten iron.

Anti adhesive sand additive:

Add 1% to 2% calcium carbonate (CaCO) or magnesium carbonate (MgCO) to the ingredients, which will decompose at high temperatures to produce CO ₂ gas, forming a gas film on the surface of the sand mold and hindering the mechanical adhesion of the molten iron to the sand; The CaO and MgO generated by simultaneous decomposition can react with FeO in the molten iron, reducing chemical sand adhesion.

c. Control gas generation and sand mold stability

Low emission formula:

Dry the raw sand at 200-250 ℃ for 2 hours before use to remove moisture and organic matter; The resin is selected as low gas release phenolic resin, with a gas release rate of less than 20mL/g, to avoid local softening of the sand mold and infiltration of molten iron caused by gas escape at high temperatures.

Fragmentation and Strength Balance:

Add 0.5%~1% barium sulfate (BaSOx) to the resin, which decomposes slightly at high temperatures to weaken the strength of the resin film, making the sand mold prone to collapse after solidification and preventing sand sticking residue; At the same time, ensure high temperature strength (tensile strength>0.8MPa at 800 ℃) to avoid premature softening of the sand mold.

3. Collaborative optimization strategy (balancing strength and sand resistance)

Formula coupling adjustment:

For example, a mixture of high refractory zircon sand (60%) and quartz sand (40%), combined with 2.2% modified phenolic resin, 15% Urotropin, 3% magnesia sand powder, and 2% graphite powder, ensures the high-temperature strength of the sand mold while suppressing sand sticking through the composite effect of magnesia sand and graphite.

Process validation and iteration:

Compare castings with different formulations during trial production:

Strength testing: The target tensile strength at room temperature is 1.2-1.5MPa, and the hot strength at 800 ℃ is greater than 0.8MPa;

Anti sand adhesion effect: Dissect the casting and observe the thickness of the sand adhesion layer. The qualified standard is<0.5mm, and the surface roughness Ra is ≤ 25 μ m.

Summary:

The strength and anti sand adhesion ability need to be achieved through the synergy of "resin reinforced bonding, refractory material barrier, and interface reaction inhibition". In actual production, resin modification and high refractory sand can be used to improve the basic performance, and then combined with coatings and additives to optimize the interface anti sticking sand ability. At the same time, the gas generation and collapsibility can be controlled to avoid exacerbating sand sticking due to insufficient collapsibility caused by high strength.