As a common casting surface defect, sand inclusion casting defect often occurs in wet casting thick plate castings. Typical sand inclusion casting defects usually have three forms: sand inclusion, rat tail and scarring. Among them, sand inclusion casting defects and scab casting defects mostly appear on the upper surface of castings, while thick wall castings and castings with plane at the casting position are prone to serious scab casting defects.
There were many controversies about the causes of sand inclusion casting defects in the early years, but at present, through the study of the high-temperature performance of sand mold and the phenomenon of water migration, it is recognized that the causes are: the rapid expansion of the volume of the surface of sand mold due to high temperature produces thermal stress, and the water on the surface migrates inward, resulting in the decrease of the strength of the inner molding sand and the arching of the surface under the action of thermal stress, When it is serious, it will break and form sand inclusion casting defects. Therefore, the expansion of sand mold during heating and the thermal stress caused by different expansion of inner and outer sand layers due to temperature difference directly affect the formation of sand inclusion casting defects.
Sand inclusion is a common defect in sand casting, where sand particles from the mold are embedded in the cast metal. This can affect the integrity, appearance, and performance of the finished product. Preventing and controlling sand inclusion involves several strategies:
- Proper Mold Material and Preparation:
- Use high-quality, refractory sand with appropriate grain size and shape to minimize sand erosion.
- Ensure the sand mixture has the right moisture content and binder proportion to enhance its strength and cohesiveness.
- Employ mechanical or chemical coatings on the mold surface to improve its resistance to metal penetration and erosion.
- Mold Design and Gating System Optimization:
- Design molds with smooth surface finishes and rounded corners to reduce stress concentrations and turbulence during pouring.
- Use a well-designed gating system that minimizes the velocity of the molten metal as it enters the mold, reducing erosion.
- Implement risers and chills to control solidification patterns and reduce the formation of hot spots that can cause mold erosion.
- Control of Pouring Process:
- Regulate the pouring temperature and rate to prevent excessive turbulence and erosion of the mold.
- Utilize skimmers or filters to prevent slag and other impurities from entering the mold cavity.
- Implement controlled and steady pouring techniques to reduce the impact of the metal on the mold.
- Vibration and Compaction:
- Ensure adequate compaction of the sand mold to increase its density and uniformity, which can reduce sand erosion.
- Employ vibration techniques during mold making to achieve better packing and uniformity of the sand grains.
- Use of Additives:
- Introduce additives to the sand mixture to enhance its refractoriness and binding properties, reducing the likelihood of sand erosion.
- Regular Maintenance and Quality Control:
- Conduct regular inspections and maintenance of molding machines and tools to ensure consistent mold quality.
- Implement quality control measures, such as testing mold strength and monitoring sand properties, to detect and correct issues promptly.
- Training and Best Practices:
- Provide thorough training for foundry workers on best practices in mold making and casting.
- Encourage a culture of continuous improvement and adherence to standard operating procedures.
By combining these methods, manufacturers can significantly reduce the occurrence of sand inclusion defects in their casting processes. Each foundry may require a unique combination of these strategies, depending on the specific casting materials, product designs, and production conditions.
There are two ways to prevent sand inclusion casting defects: one is to reduce the expansion stress of molding sand, and the other is to improve the hot and wet tensile strength of sand mold. In order to achieve the above two points, considering the composition and properties of molding sand, the specific measures are as follows: select raw sand with less SiO2; Control the quality and quantity of clay in raw sand; Additives such as pulverized coal and residual oil liquid are added into the molding sand to prevent mechanical sand sticking. From the perspective of reasonable design of molding and pouring process, first of all, it is necessary to ensure reasonable sand mold compactness. According to production experience, the general suitable hardness range is 65 ~ 75 for the upper mold and 75 ~ 85 for the lower mold; If necessary, insert nails in some places where sand inclusion casting defects are easy to occur to improve the strength of the sand mold. In the design of gating system, ensure the stable inflow of molten iron and cover the lower profile as soon as possible; In addition, the shorter the pouring time is, the more favorable it is to prevent sand inclusion casting defects.