A reasonable part structure can prevent many casting defects. To ensure high-quality castings, the following aspects should be considered regarding the requirements for part structure:
**1. Minimum Wall Thickness of Castings**
Under specific casting conditions, the minimum thickness at which the casting alloy can completely fill the mold is referred to as the minimum wall thickness of the casting. To ensure the alloy's ability to fill the mold and avoid defects such as misruns or cold shuts, the wall thickness of the casting must not be less than the minimum thickness. The minimum wall thickness depends on factors such as the casting method, casting alloy, part structure, pouring temperature, and mold properties.
**2. Critical Wall Thickness of Castings**
While increasing the wall thickness of a casting benefits the alloy's ability to fill the mold, it can also lead to defects such as shrinkage cavities, porosity, coarse microstructures, and segregation, which reduce the mechanical properties of the casting. For various casting alloys, there exists a critical wall thickness. Beyond this critical wall thickness, the mechanical properties of the casting do not increase proportionally with thickness but instead decrease significantly. For malleable iron castings, the wall thickness must not be excessively large to ensure the production of white iron preforms. For ductile iron castings, excessively thick sections can lead to graphite floatation and nodularity degeneration, resulting in poor nodularity and a significant deterioration in mechanical properties. Therefore, the wall thickness of ductile iron castings should also be kept within reasonable limits.
The structural design of castings should scientifically determine wall thickness based on factors such as the casting alloy and method, avoiding excessively thick sections. By selecting appropriate cross-sectional shapes and using reinforcing ribs to reduce wall thickness, the wall thickness can be maintained between the minimum and critical values, thereby saving material and reducing the weight of the casting. The critical wall thickness is closely related to the casting alloy and method. For sand casting, the critical wall thickness for various casting alloys can generally be considered as three times the minimum wall thickness.
**3. Thickness of Inner Walls of Castings**
The heat dissipation conditions for inner walls of castings are poorer, resulting in slower cooling and solidification rates. At the junctions between inner and outer walls, thermal stresses can easily form, leading to cracks in the casting. For alloys with significant solidification shrinkage, inner walls are also prone to shrinkage cavities and porosity. Therefore, the inner walls of castings should be thinner than the outer walls to ensure uniform cooling and minimize thermal stresses, thereby avoiding defects such as cracks.
**4. Transitions and Connections of Casting Walls**
Casting wall thickness should be as uniform as possible to avoid thick sections and prevent the formation of hot spots. The structure of the casting should not create significant resistance to shrinkage. Attention should be paid to wall thickness transitions and fillets, using gradual transitions and larger fillet radii to connect sections and prevent cracks caused by stress concentration.
The types of wall connections in castings can generally be categorized into five forms: K-shaped, X-shaped, H-shaped, Y-shaped, and cross-shaped. At the junctions of walls, the increased thickness creates hot spots, leading to slower solidification and defects such as stress concentration, cracks, deformation, shrinkage cavities, and porosity. Therefore, L-shaped connections should be preferred to minimize and disperse hot spots and avoid cross connections.
For castings made of alloys with significant volumetric shrinkage, such as steel castings, which are prone to shrinkage defects, the part structure should be carefully reviewed to ensure the possibility of directional solidification.
**5. Preventing Warping and Deformation of Castings**
Thin-walled elongated castings, large flat castings, and elongated box-shaped parts with uneven wall thickness, such as machine tool beds, are prone to warping and deformation. For the first two types, deformation occurs due to insufficient structural rigidity and internal stresses caused by differences in cooling rates across various surfaces, leading to significant warping. For the latter, deformation results from large differences in wall thickness, which cause substantial internal stresses during cooling. Warping and deformation of castings can be addressed by improving the casting structure or using reverse deformation patterns.
**6. Avoiding Large Horizontal Surfaces**
During pouring, if a large horizontal surface appears in the mold cavity, the molten metal's rising speed decreases significantly when it reaches this area due to the sudden expansion of the cross-section. Prolonged exposure of the top mold surface to heat can easily lead to defects such as sand inclusion, burn-on, sand holes, and misruns. To mitigate this, horizontal walls can be redesigned as inclined walls. If structural changes are not possible, tilted pouring techniques should be adopted.
