There are many types of casting defects. The following mainly introduces the causes and preventive measures of common ones.
Blowholes and Pinholes
Blowholes can be classified into three types according to their formation mechanisms: entrapped blowholes, precipitated blowholes, and reaction blowholes. Gas porosity defects may arise in both sand casting and investment casting; therefore, corresponding preventive measures must be implemented.
Causes:
When gases are dissolved in molten metal, if the pouring temperature is too low, the precipitated gases may not have enough time to escape; furnace charge may be damp, rusty, or oily, containing gas-generating inclusions; the runner and ladle are not properly dried; the moisture content of molding sand exceeds the specified range, or its permeability is poor; the coating contains excessive gas-evolving materials; the cores are not dried or cured, or have absorbed moisture after long storage; local parts of the green sand mold are rammed too tightly, leading to poor venting; the gating and riser system is improperly designed, with insufficient head pressure or poor venting; and during pouring, interruptions or air entrapment may occur.
Preventive Measures:
Dry, derust, and degrease the furnace charge; ensure the coke quality (moderate size, high fixed carbon, low sulfur, and low ash content) to increase molten metal tapping temperature; dry the inoculant, nodulizer, and tools before use; avoid excessive oxidation of molten metal during smelting; reduce the sulfur content of base iron for ductile iron; mix molding sand evenly and strictly control its moisture content; minimize clay addition while maintaining strength to improve permeability; reduce the content of gas-evolving substances in the sand; control the drying degree of molds and cores; ram molds uniformly and ensure adequate venting; design a reasonable gating system, increase the height of the sprue to enhance metallostatic pressure; and set risers or vents at the highest parts of the cavity and areas where gas is difficult to escape.
Shrinkage Cavities, Shrinkage Holes, and Porosity
Causes:
When the liquid and solidification shrinkage of the alloy is greater than its solid-state shrinkage, and the molten metal cannot be adequately fed during solidification, shrinkage defects occur. Excessively high pouring temperature tends to cause concentrated shrinkage cavities, while too low temperature leads to dispersed porosity. Improper connection between gating system, risers, and castings may cause large hot spots. If the mold rigidity is insufficient, expansion of mold walls under metallostatic pressure and graphite precipitation may also occur.
Preventive Measures:
Design the ingates, risers, and chills correctly to ensure continuous feeding of molten metal during solidification; improve the casting structure to facilitate feeding; ensure adequate mold rigidity; and use dry molds for large castings to prevent mold wall expansion.
Cold Cracks
Causes:
Large differences in wall thickness without smooth transitions lead to uneven cooling and inconsistent contraction, resulting in localized stress concentration; high phosphorus content increases brittleness; large residual stresses within the casting may cause cracking under mechanical load.
Preventive Measures:
Keep wall thickness uniform and promote even cooling; minimize constraints on contraction; improve the melting quality of the alloy by reducing harmful elements and non-metallic inclusions; improve the quality and collapsibility of mold and core sand; extend the mold shakeout time to allow slow cooling in the mold; and perform aging treatment to relieve residual stresses.
Hot Cracks
Causes:
Sudden changes in wall thickness cause stress concentration during solidification; high sulfur content in molten metal leads to hot brittleness; the gating system obstructs contraction; poor collapsibility of molds and cores, improper core reinforcement structure, or insufficient sand thickness around the core may also contribute.
Preventive Measures:
Avoid abrupt changes in wall thickness in casting design; use proper fillets at corners; apply chills or process ribs at locations prone to tensile stress or delayed solidification; use multiple small ingates rather than a single large one, and avoid forming hot spots where ingates connect to the casting; provide appropriate fillets between risers and castings, and ensure the gating and riser design does not hinder contraction; add a suitable amount of sawdust or use organic binders in clay sand to improve collapsibility; avoid excessive ramming of molds and cores; use core reinforcement of proper stiffness with sufficient sand thickness around it.
Post time: Oct-24-2025