Common casting defects: causes and solutions of pores and sand holes

Pores and sand holes are the most common appearance and internal defects in casting production, which directly affect the density, strength, and sealing of castings. They can increase the cost of subsequent repairs in mild cases and lead to the scrapping of castings in severe cases. Although the defect forms of the two are different, the causes are mostly related to the molding, melting, and pouring processes. Targeted control of process details is the core of solving the problem. The specific causes and corresponding solutions of the two types of defects are analyzed below.

1、 Causes and Solutions of Pores

Pores are circular or elliptical holes formed inside or on the surface of castings, often caused by gas not being discharged from the mold in a timely manner. The main causes can be divided into three categories: first, poor permeability of molding sand and core sand, excessive addition of water and binder during molding, and inability to quickly discharge the water vapor generated during pouring, resulting in the formation of bubbles in the molten metal; The second issue is insufficient deoxidation of the molten metal during the smelting process, resulting in a large amount of residual gases such as hydrogen and nitrogen in the liquid; The third reason is improper pouring operation and unreasonable gating design, which leads to rapid filling of the molten metal, causing it to be sucked into the air and unable to float and escape.

The solution needs to start from three aspects: exhaust, deoxidation, and pouring: optimizing the molding sand formula, controlling the moisture content, and reasonably setting the gas outlet needle and gas outlet riser to improve the permeability of the mold; Add sufficient deoxidizer during melting, stir thoroughly to remove gas, and let the metal liquid stand until the gas rises before pouring; Improve the structure of the sprue and adopt bottom pouring method to reduce the pouring speed and avoid metal liquid entrapment.

2、 Causes and solutions of sand holes

Sand hole is an irregular pit defect formed by embedding sand and core sand particles on the surface or inside of a casting. Essentially, it is caused by the detachment of mold sand particles into the molten metal and their retention. The core causes include: insufficient strength of molding sand and core sand, uneven compaction, and easy detachment due to high-temperature metal liquid erosion during pouring; Improper operation during the molding and fitting process, sand falling off the surface of the mold, sand inclusion in the fitting gap, or unstable support of the core bone leading to damage to the core body; The design of the pouring system is unreasonable, causing the metal liquid to flow directly into the mold wall, resulting in sand flushing.

The solution focuses on mold strength and casting protection: adjust the proportion of sand binder to improve the compressive and erosion resistance of the sand, and ensure uniform compactness of the mold during molding; Increase the support of the core structure during core production, handle with care during molding, seal the mold to prevent sand inclusion, and lay refractory sand pads on areas prone to sand flushing; Optimize the pouring system, set up sand barriers and buffer tanks, change the flow direction of the molten metal, and avoid direct impact on the high casting wall.

3、 General prevention and control points for two types of defects

The prevention and control of pores and sand holes require standardized process control: in terms of raw materials, the quality of molding sand and metal materials should be strictly controlled to avoid impurities from entering; During the production process, strengthen the inspection of various processes such as molding, fitting, melting, and pouring, and promptly identify problems such as mold damage and insufficient deoxidation; After pouring, sand and clean according to specifications, analyze process issues through defect tracing, and continuously optimize parameters. At the same time, professional training should be provided to operators to standardize operating procedures and reduce defects caused by human factors.