How can defects like sand pimples on the top surface of gray cast iron parts produced by resin sand casting be resolved?

Sand nodules appear on the surface of gray cast iron machine castings produced by resin sand, which is a typical problem that affects the surface quality of castings. Sand nodules are usually manifested as hard lumps of metal and sand particles adhering to the surface of castings, which are difficult to clean and can also affect dimensional accuracy and appearance.

1、 The main reason for the formation of sand nodules is the insufficient local strength of the resin sand mold/core surface. Under the thermal and mechanical action of the molten metal, the surface sand blocks peel off and are sucked into the molten metal, but are not completely washed away or remelted, and are ultimately wrapped around the surface of the casting. The specific reasons can be analyzed from the following dimensions:

1. Uneven compactness due to molding/core making process: Insufficient compactness in some areas (especially corners, grooves, and deep cavities), loose sand mold surface, low strength, and easy to be washed off. Improper mold removal/repair operation: Damage to the sand mold during mold removal or incomplete repair during mold repair, leaving potential loose sand blocks. Coating quality and brushing process: The coating layer is too thin or not covered, and it fails to effectively isolate the high-temperature metal liquid from the sand mold surface. Insufficient penetration depth of the coating: failure to form a solid "sintered layer" with the sand mold, resulting in poor adhesion. Insufficient drying of coatings: residual moisture or solvents, gas generated when encountering high-temperature metal liquids, can impact the coating and cause it to crack and peel off. Poor exhaust of sand mold/core: Excessive local gas pressure may cause the coating layer or weak surface layer to "collapse".

2. Performance reasons for raw sand and resin sand: Raw sand particle shape and size: The surface of the sand particles is smooth, with poor roundness (such as a large angular coefficient), weak mechanical meshing between sand particles, and low surface strength of the sand mold. Insufficient resin addition or uneven mixing: resulting in low local resin content and insufficient strength after curing. Uneven curing: Especially for thick sand cores, the interior may not be fully cured or the surface may become brittle due to excessive blowing (furan resin). Long storage time of sand molds or high environmental humidity: Resin sand molds absorb moisture, reduce strength, and increase gas generation.

3. The reason for the design of the pouring system is that the metal liquid is excessively washed away: the sprue is facing the mold wall or sand core, forming a high-speed metal liquid flow that directly impacts. The design of the pouring system is unreasonable, and the filling speed is too fast, which causes severe erosion and wear on the surface of the sand mold/core.

4. The pouring temperature of the molten metal itself is too high: excessive heat load will accelerate the thermal decomposition of the resin on the surface of the sand mold and the sintering of the sand particles, causing the peeling layer to thicken. At the same time, the fluidity of the molten iron is too good, and the scouring force is enhanced. Severe oxidation of molten iron: High content of oxides (such as FeO) in molten iron can react chemically with SiO ₂ sand particles, exacerbating the erosion and sand sticking tendency of the metal liquid on the sand mold.

2、 A systematic solution to solve the problem of sand nodules requires starting from prevention and control, conducting systematic investigations and process optimization.

1. Optimize the shape and core making process to ensure uniform compaction: Focus on ensuring the compaction of complex parts such as corners, protrusions, and deep cavities, and use specialized tools to assist in compaction. Standardized molding and repair: Carefully operate and use specialized repair paste or coating to repair damaged areas. Application of reinforced coatings: Choose high-quality alcohol based zircon powder or corundum powder coatings, which have high fire resistance and strong corrosion resistance. Ensure that the coating has sufficient concentration and viscosity, using dip coating, flow coating, or spray coating to ensure uniform coating thickness (generally 2-4 layers, dry film thickness 0.3-0.8mm). Fully dry: After igniting (alcohol based coatings) or drying, ensure that the coating is completely dry and free of moisture spots. Ensure smooth exhaust: Set sufficient number and size of exhaust holes at appropriate positions in the sand mold/core.

2. Strictly control the quality of resin sand and select high-quality raw sand: priority should be given to using silica sand with round particle shape and smooth surface (such as Inner Mongolia Dalin sand), with an angular coefficient controlled within 1.2-1.3, which is conducive to improving fluidity and sand compaction and strength. Optimize resin ratio and sand mixing process: Adjust the amount of resin and curing agent added according to the season and environmental temperature and humidity (usually 0.8% -1.5% of the original sand weight) to ensure uniform mixing. Control curing effect: Avoid over blowing or insufficient curing. For thick and large sand cores, hollow or coke block filling processes can be used. Control storage and usage environment: Try to shorten the storage time of sand molds/cores and avoid moisture absorption. It is recommended to control the humidity in the workshop below 60%.

3. Improve the design of the pouring system to follow the principle of "smooth, fast, and orderly" filling: adopt open, bottom injection, or stepped pouring systems. Avoid direct flushing: The sprue should not be directly facing the weak parts of the mold wall or sand core. Measures such as introducing tangent lines, setting up sprue pits, and using filter screens can be used to buffer, disperse, and purify the flow of molten metal. Calculate the appropriate pouring time: avoid filling too quickly.

4. Optimize the melting and pouring process to control the appropriate pouring temperature: while ensuring the integrity of the filling and avoiding cold insulation, try to use a lower pouring temperature. For medium wall thickness machine tool castings, it is generally controlled at 1350-1420 ℃ (optical temperature measurement). Purification of molten iron: Strengthen furnace pre-treatment (such as using slag removal agents) to prevent excessive oxidation of molten iron and ensure its purity. Smooth pouring: Avoid interruption and splashing, and keep the pouring cup full.

5. On site management and inspection of first piece inspection: For the first piece after new molds or process changes, carefully inspect the parts that are prone to sand nodules. Establish process control points: monitor and record key processes such as compaction, coating, and pouring temperature. Analyze the location of sand nodules: Based on the regular position of sand nodules, trace back whether it is a problem with the shape, coating, or pouring system.

Summary and Suggestions: The solution to the problem of sand nodules in the production of gray cast iron machine castings using resin sand is to "enhance the surface strength of the sand mold" and "reduce the erosion/thermal effect of the metal liquid on the mold wall". Suggest following these steps to troubleshoot:

Firstly, check the coating: this is the most direct and common reason. Confirm the quality of the coating, coating thickness, and drying condition.

2. Next, check the compactness and molding operation: check whether the sand mold at the corresponding position of the sand nodule is loose.

3. Then analyze the pouring system: observe whether sand lumps appear directly opposite the sprue or at the location where the metal flow is most affected.

4. Finally verify the process parameters: check whether the resin sand ratio, raw sand quality, and pouring temperature are within the controlled range. Through this systematic analysis and step-by-step improvement, sand dune defects can be effectively controlled and eliminated, significantly improving the surface quality and dimensional accuracy of machine tool castings.