Casting production is a complex, multi-step process, and the skill level required for each process varies significantly. Worker skills, quality management, testing methods, and equipment all influence the final quality of castings. If you want to understand the types of casting defects and their prevention methods, this guide will provide an overview of all casting defects.
Porosity defects occur when gases become trapped in the metal during casting and solidification. The most common gases that form pores are nitrogen, oxygen, or hydrogen. Although molds allow gases to escape, many factors can prevent this, resulting in common surface defects in investment casting. There are three common casting defects related to porosity:
Pinhole casting defects are another term for small gas pores. Pinholes are the most common defects in metal castings and can be caused by any malfunction in the operating process. Pinholes—either near the casting surface or in clusters—are often visible to the naked eye.
Using sand with high moisture content.
Pouring molten steel from a damp ladle or insufficient degassing.
Molten metal absorbing hydrogen or carbon monoxide gas.
Reduce the moisture content of the sand.
Improve the permeability of the sand.
Use high-quality fluxes and proper melting methods.
Achieve rapid solidification of the metal.
Larger cavities than pinholes are called blowholes in casting defects. They are usually located inside the casting. Before machining, they are difficult to identify and require harmonic, ultrasonic, magnetic particle, or X-ray inspection.
Excessive moisture in the sand.
Low permeability of the sand.
Too fine sand grains.
Over-compacted sand.
Insufficient venting.
Control the moisture content of the sand and maintain it at the required level.
Use sand with high permeability.
Use sand with appropriately sized grains.
Avoid over-compacting the sand.
Provide adequate venting in the mold.
Blowholes in casting defects are harder to detect than open pores that appear on the casting surface. In most cases, cracked pores form on the casting surface. These defects result from air being trapped in the mold when pouring molten metal.
Poor venting of the mold and core.
Inadequate drying of the mold and core.
Use dry, permeable sand to avoid open pores.
Select coarse sand (coarser sand has better permeability).
Ensure the mold and core are fully dried.
Metals shrink because their density in liquid form is lower than in solid form. Shrinkage issues are common during cooling. While solidification shrinkage is typical for alloys, uneven shrinkage can cause surface defects in castings.
During solidification, shrinkage leads to depressions in the casting. Shrinkage cavities have sharp edges, unlike the smooth surfaces of gas pores. Shrinkage pores often occur alongside dendritic fractures or cracks. Large shrinkage cavities can compromise the integrity of the casting, causing it to break under pressure. Shrinkage can cause casting defects in two ways:
Open shrinkage (also called "pipes") creates depressions (also known as "concaves") or holes on the casting surface. When the metal shrinks unevenly, air is drawn into the mold at that location, resulting in such defects.
Closed shrinkage defects—also called shrinkage porosity—are defects that create closed cavities in the casting. Micro-shrinkage pores, however, are invisible to the naked eye.
Closed shrinkage defects most often occur near "hot spots" or the top of isolated molten metal pools.
Shrinkage defects occur when the temperature of the molten metal in the mold is inconsistent. They can also form if the metal is poured into the mold at too high a temperature or solidifies unevenly. Uneven solidification is common if the mold design ignores the directional solidification pattern.
Heat-treat ferrous alloys to reduce residual stress, which is particularly helpful for preventing warpage. Straightening castings between quenching and aging processes can prevent warpage in aluminum alloys. Additionally, warpage can be suppressed by ensuring critical parts have more ribbed shapes (e.g., corrugations) to promote uniform cooling.
Design a gating system with risers to ensure continuous flow of molten metal.
Insert internal coolers, heat sinks, or cooling coils to enhance local heat dissipation.
Lower the pouring temperature to minimize overall volume deficiency.
Scabs and erosion occur when molten metal erodes part of the mold during pouring, and the sand lacks sufficient thermal strength to resist this erosion. On the final casting surface, scabs and erosion appear as low protrusions sloping toward one end—where molten metal flows with greater force.
Excessive metal flow through the gate due to the high velocity of the molten metal leads to scabs and erosion.
Improve the design of the gating system.
Strengthen the mold and core.
Add binders to the facing sand and core sand.
Burn-on refers to sand grains fusing with molten metal. On the casting surface, burn-on appears as a glassy crust—a thin, brittle, glass-like layer tightly adhering to the casting. This occurs when some sand in the mold melts and fuses with the casting.
Clay or sand with low refractoriness.
Pouring molten metal at an excessively high temperature.
The ability of molding materials to resist melting and fusing with metal at liquid temperatures is called "refractoriness." Silica sand has the highest refractoriness. To prevent burn-on, improve the refractoriness of the molding material or lower the pouring temperature of the molten metal.
Metal penetration occurs when molten metal seeps out of the mold, resulting in incomplete or missing sections of the casting.
Using sand with low strength and high permeability.
Using coarse or oversized sand grains (coarser sand increases metal penetration).
Incomplete mold cleaning.
Impact on soft sand.
Ensure the casting mold is properly designed to avoid such defects. Inspect and replace any damaged molds before casting.
High temperatures cause excessive mold wear; use high-quality raw materials that can withstand high temperatures for the mold.
Swelling defects occur when the weight of molten metal pulls the sand outward because the mold is too soft. Swelling usually appears as small, smooth humps on the vertical surfaces of the casting.
Swelling defects are usually caused by improper mold use or low mold strength.
Ensure the casting mold is properly designed. Inspect and replace any damaged molds before casting.
High temperatures cause excessive mold wear; use high-quality raw materials that can withstand high temperatures for the mold.
Drops occur when undried sand falls into the molten metal casting. Drops appear as uneven lumps on the top of the casting. This happens because the mold is not strong enough, allowing sand to enter the molten metal. Drops may contaminate the metal surface.
Sand with low strength.
Inadequate ramming of the sand.
Insufficient fluxing of the molten metal.
Protruding sand grains on the mold cope with inadequate reinforcement.
Use sand with higher tensile strength.
Ensure thorough ramming of the sand.
Adequate fluxing removes impurities from the molten metal.
Reinforce protruding sand sections with nails or clamps.
Rat tails (or "veins") appear as uneven lines or cracks on the surface of the mold sand. They are most common on the mold drag (bottom surface), as this area is often covered by molten material. "Sand buckling" is a more severe form of rat tails.
Sand with poor expandability.
Excessively high pouring temperature.
Improper mold design.
Ensure the mold has sufficient ramming to contain the molten metal. Additionally, ensure the pouring temperature of the metal is not too high. If these methods are ineffective, the procedures mentioned above can be used to resolve the issue.
Add combustible additives to the sand.
Lower the pouring temperature of the metal.
Avoid over-compacting the mold.
Excessive cooling of the metal when it reaches the interior of the mold can cause various defects in the poured metal. These defects also occur if the metal is poured into a mold that is not hot enough. Even if the metal is poured at the correct temperature, flawed pouring methods can cause it to cool too quickly before entering the mold, or excessively thin mold sections may lead to defects. Due to their small size, tiny components may harden and block other parts of the mold.
Some cracks are visible to the naked eye, while others require magnification. Cracks appear after the casting cools and solidifies.
Hot cracks occur if the strength of the solidifying metal is insufficient to resist tensile stresses during solidification.
Improve the mold design.
Modify the mold to enhance collapsibility.
Hard spots are another term for hot spots. These defects occur when a specific section of the casting cools faster than the surrounding areas. This leads to the formation of harder microstructures in that section. Such defects cause tool wear and hinder machining operations.
Hot spots result from improper cooling.
Adopt proper cooling practices.
Consider adjusting the chemical composition of the metal.
Excessive cooling of the metal when it reaches the mold interior can cause various defects in the poured metal. Even if the metal is initially poured at the correct temperature, pouring it into a mold that is not hot enough can lead to these issues. Later, malfunctions in the pouring equipment may cause the metal to cool excessively before entering the mold. Additionally, overly thin mold parts can result in defects.
Cold shuts are surface defects. You will find lines or cracks with rounded edges on the casting surface. Cold shuts occur when two streams of molten metal meet at a temperature below the melting point—they fail to fuse and solidify separately, resulting in a fracture-like appearance with rounded edges. This defect is visible to the naked eye.
Imperfect gating system.
Low melting temperature.
Poor fluidity of the molten metal.
Optimize the gating system.
Maintain the proper pouring temperature.
Improve the permeability of the mold.
Misruns occur when molten metal hardens before filling the mold cavity completely, leaving gaps in the casting. The mold cavity is not fully filled with liquid metal.
Low fluidity of the molten metal.
Reduced fluidity of the molten metal at low temperatures.
Inadequate gating system with overly thin cross-sections.
Optimize the mold design and gating system design.
Increase the metal flow rate.
Avoid overly thin cross-sections in the mold.
During the pouring of liquid metal, splashing can cause the formation of solid particles. These small globules become trapped in the casting during solidification. Cold shots (slag inclusions) are usually spherical, teardrop-shaped, or pearl-shaped, scattered throughout the metal. Slag inclusions and cold shots are closely related and often occur together. Typically, removing the surface layer reveals underlying slag inclusions.
Slag inclusions form when molten metal containing particles is poured into the mold cavity and solidifies.
Melt the metal with fluxes in a vacuum or inert atmosphere.
Use a dedicated ladle to pour metal from the bottom (to avoid stirring up slag).
Add ceramic filters to the gating system.
Shape defects in castings lead to misalignment or flash. These defects are the easiest to repair, as they usually only require readjusting the existing mold.
Misalignment (also called "shift") occurs when separate casting sections are misaligned—usually horizontally. The top and bottom of the mold are not aligned, causing mold displacement. Horizontal displacement is the most common form of mold shift. If vertical displacement occurs, it is the core (not the mold) that is misaligned.
Failure to properly align the upper and lower mold components during mold preparation.
Misalignment of the flask.
Inattention to detail when mounting the cope (top mold section) to the drag (bottom mold section), leading to misalignment.
If a shift occurs, check the installation and alignment of the matching plate patterns. Ensure the correct molding boxes and closing pins are used.
Flash is one of the most common casting defects. Casting fins, burrs, and flash are all general terms for this defect. Flash appears as excess material adhering to the casting—usually in sheet form at the joints of mold sections. Scrap material that is remelted to produce slag is called flash.
Cracks or holes on the core surface can cause flash on the casting surface. Gaps may result from insufficient mold weight or poor flask clamping.
This issue can be resolved by reassembling the mold and core. The top of the mold should have sufficient weight to ensure the two sections fit tightly together.
Flash varies from minor to severe. For minor cases, manufacturers can remove flash by breaking it off with a hammer or pliers, then filing along the parting line. However, this can be a costly process.
Warpage is a casting defect that occurs during or after solidification.
Warpage is often caused by uneven solidification rates in different areas, which creates stress in adjacent walls. Large, flat sections are more prone to warpage.
Residual stress in cast iron parts can be eliminated through normalizing heat treatment. Aluminum castings may require straightening between quenching and aging processes.
This is an external defect characterized by multiple small cavities adjacent to each other on the metal casting.
This defect is caused by mechanically trapped debris and scale in the molten metal suspension.
Improper skimming of the ladle is also a contributing factor.
Prevent dust and scale from entering the molten metal.
Avoid sand erosion (sand washing into the molten metal).
Perform proper skimming in the ladle to remove slag from the molten metal.
Quality management in casting requires a comprehensive understanding of casting defects and their causes. To help suppliers understand your quality standards, you should establish specific defect tolerances and quality expectations before production.
Defect tolerances vary depending on the product and the type of casting issue. Defining your tolerances for these casting defects will help suppliers understand your expectations and avoid future misunderstandings and quality problems.
Professional technical research and development team to provide customers with the best solution.
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