Understand Shrinkage in Industrial Casting

Fabrication shops and foundries attempt to create metal castings that closely adhere to design specifications with as few deviations as possible. However, avoiding defects in a product run can be a challenging task since most metals shrink to some degree as they cool. When a component undergoes shrinkage, it can seriously undermine the integrity of the entire device and may eventually break under stress. To help reduce or eliminate the chances of faulty parts entering the marketplace, many facilities employ inspection equipment to detect both superficial and internal imperfections. Despite these efforts, metal casting shrinkage remains an important concern for many manufacturers.”\”casting” Shrinkage Porosity There are two main types of porosity problems in the metalworking industry: shrinkage porosity and gas porosity. Shrinkage is by far the most common type and can usually be detected on the surface of a cast part by what appear to be small holes or cracks. These holes may seem round, but are actually angular in shape and tend to form branching internal fractures. Thick multi-angled parts are most susceptible to such shrinkage, which occurs as the metal cools and solidifies in a non-uniform pattern. Types of Casting Shrinkage There are four types of shrinkage that can occur in metal castings: cavity, sponge, filamentary, and dendritic shrinkage. • Cavity shrinkage: This defect occurs when two different sources of molten material are joined to create a common front while solidification is already taking place. A lack of additional feed material to fill in the accumulating gaps can further exacerbate the cavity shrinkage problem. • Sponge shrinkage: This usually arises in the thicker mid-section of the casting product and causes a thin lattice texture similar to filament or dendrites to develop. • Filamentary shrinkage: This results in a network of continuous cracks of various dimensions and densities, usually under a thick section of the material. It can be difficult to detect, and the fracture lines tend to be interconnected. • Dendritic shrinkage: Dendritic fractures are narrow, randomly distributed lines or cavities that are often unconnected. They are typically thinner and less dense than filamentary cracks. How Temperature Affects Casting Shrinkage To reduce the potential for metal casting shrinkage, it is helpful to work within a delineated temperature range. Metal should be heated to achieve appropriate molten characteristics, but without reaching its full liquid state. This usually entails heating the material to slightly above its flow point, but well below its melting point. Preventing overheating can be just as important to effective casting as cultivating a molten flow. It is also useful to note that castings can cool at a rate of up to 100 degrees per minute once molten pouring is complete. Since shrinkage can be caused by working material while solidification is under way, it is important to have equipment prepared to treat the workpiece before it solidifies. Avoiding Common Shrinkage Defects The most common causes of shrinkage are related to the casting sprue, which is the passage through which molten metal is poured into a mold. In some areas, such as the heavy sections of the mold, the metal takes longer to contract and solidify, which reduces feed material availability and increases the likelihood of shrinkage, especially if the sprue is too small for the volume of flow. A properly sized sprue attached directly to the heavy section can fill the cavity and provide the feed material necessary to counteract shrinkage as cooling occurs. In addition, using a rounded, rather than a flat or square, gate on the sprue can further reduce the risk of forming defects. Using a narrow or tapered sprue can result in the molten metal being sprayed rather than poured into the cavity. When this happens, certain sections of the workpiece begin to solidify before the entire mold is filled. Molten flow into the cavity should be as uniform as possible, and a larger central sprue or a multiple-sprue arrangement can help achieve the even supply of material. Fabrication shops and foundries attempt to create metal castings that closely adhere to design specifications with as few deviations as possible. However, avoiding defects in a product run can be a challenging task since most metals shrink to some degree as they cool. When a component undergoes shrinkage, it can seriously undermine the integrity of the entire device and may eventually break under stress. To help reduce or eliminate the chances of faulty parts entering the marketplace, many facilities employ inspection equipment to detect both superficial and internal imperfections. Despite these efforts, metal casting shrinkage remains an important concern for many manufacturers. Shrinkage Porosity There are two main types of porosity problems in the metalworking industry: shrinkage porosity and gas porosity. Shrinkage is by far the most common type and can usually be detected on the surface of a cast part by what appear to be small holes or cracks. These holes may seem round, but are actually angular in shape and tend to form branching internal fractures. Thick multi-angled parts are most susceptible to such shrinkage, which occurs as the metal cools and solidifies in a non-uniform pattern. Types of Casting Shrinkage There are four types of shrinkage that can occur in metal castings: cavity, sponge, filamentary, and dendritic shrinkage. • Cavity shrinkage: This defect occurs when two different sources of molten material are joined to create a common front while solidification is already taking place. A lack of additional feed material to fill in the accumulating gaps can further exacerbate the cavity shrinkage problem. • Sponge shrinkage: This usually arises in the thicker mid-section of the casting product and causes a thin lattice texture similar to filament or dendrites to develop. • Filamentary shrinkage: This results in a network of continuous cracks of various dimensions and densities, usually under a thick section of the material. It can be difficult to detect, and the fracture lines tend to be interconnected. • Dendritic shrinkage: Dendritic fractures are narrow, randomly distributed lines or cavities that are often unconnected. They are typically thinner and less dense than filamentary cracks. How Temperature Affects Casting Shrinkage To reduce the potential for metal casting shrinkage, it is helpful to work within a delineated temperature range. Metal should be heated to achieve appropriate molten characteristics, but without reaching its full liquid state. This usually entails heating the material to slightly above its flow point, but well below its melting point. Preventing overheating can be just as important to effective casting as cultivating a molten flow. It is also useful to note that castings can cool at a rate of up to 100 degrees per minute once molten pouring is complete. Since shrinkage can be caused by working material while solidification is under way, it is important to have equipment prepared to treat the workpiece before it solidifies. Avoiding Common Shrinkage Defects The most common causes of shrinkage are related to the casting sprue, which is the passage through which molten metal is poured into a mold. In some areas, such as the heavy sections of the mold, the metal takes longer to contract and solidify, which reduces feed material availability and increases the likelihood of shrinkage, especially if the sprue is too small for the volume of flow. A properly sized sprue attached directly to the heavy section can fill the cavity and provide the feed material necessary to counteract shrinkage as cooling occurs. In addition, using a rounded, rather than a flat or square, gate on the sprue can further reduce the risk of forming defects. Using a narrow or tapered sprue can result in the molten metal being sprayed rather than poured into the cavity. When this happens, certain sections of the workpiece begin to solidify before the entire mold is filled. Molten flow into the cavity should be as uniform as possible, and a larger central sprue or a multiple-sprue arrangement can help achieve the even supply of material.

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