Resin sand process, the sand core is coated sand core, and there are succulent defects at the casting sand core. How to eliminate them?

In the resin sand process, when using coated sand cores, the casting exhibits "succulent" defects at the core position. What is the reason for this defect?

1、 Possible factors due to defects

1. Deformation or displacement of sand core: Insufficient strength of sand core: The strength of coated sand core is lower than the requirement. The reasons may include: improper curing process: low heating temperature, insufficient heating time, or uneven heating, resulting in incomplete solidification of the sand core ("undercooking"), crispy core, and low strength. Quality issues with laminated sand: poor quality of raw sand, low resin content, poor resin quality (such as low thermal strength), uneven sand mixing, improper storage leading to performance degradation (such as moisture, clumping, and resin pre curing due to prolonged storage time). Sand core design issues: The sand core is too slender, thin-walled, or structurally complex, and its own support stiffness is insufficient. Core head gap/positioning issue: Excessive core head gap: The sand core is loose in the core seat, and it is prone to shaking or displacement during the box closing or pouring process. The gap between the core heads is too small: when inserting the core or closing the box, it is forcibly inserted, causing local deformation or fracture of the sand core due to compression. Positioning pin/groove size deviation or wear: The sand core cannot be accurately positioned or shakes during operation. Improper core removal operation: During the core removal process, bumps or improper operation may cause damage or slight deformation of the sand core. Improper box closing operation: Excessive pressure and unstable operation during box closing can cause partial or overall displacement, deformation, or even breakage of the compacted sand core. Metal liquid impact: During pouring, the high-speed metal liquid flow directly impacts the weak parts of the sand core (such as the core head or cantilever part), causing it to bend, deform, or partially break and fall off. Unreasonable design of the pouring system (such as direct sand core) will exacerbate this problem. Lifting the box (running fire): After pouring, if the clamping force of the upper and lower molds is insufficient (loose or insufficient force of the pressure iron/fixture), or the static pressure of the metal liquid is too high, it may cause the upper mold to be lifted (lifting the box, running fire). At this time, the sand core may float or move as a whole, resulting in an increase in the size of the casting or the appearance of succulents. Although this is a change in the entire cavity, the characteristics of the sand core position will be more pronounced.

2. Thermal expansion of sand cores (a unique problem of coated sand): "Shell effect" expansion: This is a significant characteristic of coated sand cores. When the high-temperature metal liquid contacts the surface of the sand core, the surface sand quickly heats up and solidifies (forming a hard "shell"), while the core sand heats up slowly and continues to expand. This hard shell hinders the internal expansion, causing the internal expansion force to be released outward. This may result in the outward compression of the molding sand: if the molding sand (resin sand) around the sand core has sufficient strength and poor yielding (resin sand has high strength at room temperature but some thermoplasticity at high temperature), the expansion of the sand core will compress towards the direction of the casting cavity, causing the cavity size to decrease. But in specific situations (such as uneven yielding, complex structure, and local obstruction), it may also manifest as distortion or local bulging of the sand core itself, indirectly leading to succulent castings. Squeezing castings inward (mainstream explanation): When the sand core is completely surrounded by molten metal, the expansion force of the core will squeeze the metal melt pool that has not yet fully solidified towards the center, causing the shrinkage of the casting cavity to be obstructed and the casting size to develop towards a "bulging" direction (the cavity size becomes smaller). This is exactly the opposite of succulents. Extreme expansion leading to cracking or displacement (which may result in succulent): If the sand core expands too much or is severely obstructed, internal stress may cause the sand core to crack, or gaps may form at the joint between the sand core and the mold wall (causing burrs), or unpredictable overall deformation may occur under specific constraints, all of which may lead to abnormal changes in casting wall thickness or succulent protrusions. Excessive pouring temperature: Excessive pouring temperature exacerbates the thermal expansion phenomenon of the sand core. The influence of cooling process: After the metal solidifies and shrinks, the sand core collapses. If the sand core expands and collapses poorly or unevenly during the cooling process, it may also affect the final shape of the casting.

3. Foreign objects or locally high compactness in the cavity: scattered sand core blocks or debris: during the core insertion process, the sand core blocks or fractures locally, or scattered sand or debris falls into the cavity before closing the box, ultimately occupying the cavity space and being wrapped by metal, forming succulents. Coating accumulation or detachment: Local coating accumulation on the sand core or cavity is too thick or falls off in patches, occupying space and forming succulents. Uneven compaction of molding sand: During molding, there is excessive compaction in certain areas (although resin sand is formed by hardening, the sand pounding operation may also affect the local compaction), resulting in excessive expansion of this area under heat or metal pressure, causing the extrusion cavity to form succulents.

2、 How to troubleshoot and resolve:

1. Observe the location and morphology of defects: Is succulent regular (appearing in the same position every time) or random? The shape of succulent protrusions (are they regular protrusions, burrs, or irregular bulges?)

2. Check the sand core: Size measurement: Focus on measuring the key dimensions (especially assembly dimensions) of the sand core after solidification and core extraction, and compare them with the theoretical dimensions of the mold. Check for any signs of deformation or collision. Visual inspection: Check the surface for cracks, looseness, and chipping. Fracture test strength (is the center crispy?). Traceability of curing process: Check the temperature curve records, temperature uniformity, and insulation time of the baking oven to ensure they meet the requirements of coated sand and sand core wall thickness.

3. Check the core head and core holder: Measure the size of the core head of the core box and the size of the core holder of the lower core model, and check whether the gap is reasonable and consistent. Check the size and wear of the positioning pin/groove.

4. Check the operation process: Observe whether the core and box closing operations are standardized and stable? Is there any collision or hard squeezing phenomenon? Check if the weight and fastening of the pressure iron/fixture are sufficient and reliable? Is the pouring process smooth? Is the molten metal flowing directly into the sand core?

5. Check material and process parameters: Film coated sand: Check batch, storage time, and whether there are any damp lumps? If necessary, test its hot strength and tensile strength. Pouring temperature: Is it too high? Attempt to reduce appropriately (depending on the material) while ensuring liquidity. Sand core design: Evaluate whether the size of the core head, sand consumption, and reinforcement arrangement are reasonable? Have you considered the thermal expansion characteristics of coated sand? Increase process subsidies (anti deformation amount) in key areas. Pouring system: Avoid aligning the sprue directly with weak areas of the sand core. Exhaust: Ensure smooth exhaust of the sand core to avoid gas blockage that may cause local metal liquid pressure to be too high and push the sand core.

6. Simulation analysis: When conditions permit, use casting simulation software to analyze the filling and solidification process of the molten metal, observe the deformation, displacement, and stress of the sand core.

Summary of key points: The primary suspected factor for succulent defects caused by coated sand cores is deformation or displacement of the sand cores during manufacturing, operation (core insertion/box closing), and pouring processes. Insufficient strength, core gap issues, operational collisions, metal liquid impact, and box lifting caused by poor solidification (under burning) are direct and common reasons. The unique thermal expansion problem of laminated sand may lead to insufficient shrinkage of the inner cavity (increase in casting wall thickness or decrease in inner cavity), but in extreme cases (such as severe obstruction causing abnormal deformation or cracking of the sand core), it may also indirectly or directly manifest as succulent. Systematically investigating deformation/displacement factors (strength, clearance, operation, lifting) is usually the breakthrough point for solving problems.