Gas Porosity in Castings: Causes and Additive Control

Gas porosity — blowholes and pinholes — is one of the most common casting defects. Carbon additives can contribute if not controlled.

Sources of gas

• Moisture in sand
• Volatile matter from additives and binders
• Dissolved gases in the metal

Carbon additive role

Gilsonite releases gas by design. In the right amount, it helps the surface; in excess, gas can enter the metal.

Control measures

• Measure LOI and volatile content regularly
• Keep moisture and compactability on target
• Ensure adequate mould permeability and venting
• Avoid overdosing new additive

Distinguishing gas defects

Additive-related checks

• Compare LOI and volatiles with target.
• Check if new additions were recently increased.
• Review moisture and permeability.

See foundry grade gilsonite and loss on ignition.