Causes of Porosity in Casting and Corresponding Solutions

Causes of Porosity

1. Internal shrinkage cavities

2. Cracking at the junction of thick and thin sections caused by porosity

3. Porosity at fractured gating or riser sections

Solutions

1. Prevention of internal porosity
Porosity most commonly occurs in thick-walled castings. Although the casting may appear flawless on the surface, internal porosity is not visible. To prevent this defect, a properly designed gating and riser system must be established before applying refractory coating to the foam pattern, ensuring adequate feeding during solidification and avoiding porosity formation.

2. Prevention of cracking at thick–thin section junctions
Cracks caused by porosity usually occur at the transition between thick and thin walls. To prevent cracking, internal chills can be installed at thicker sections according to wall thickness.
These internal chills can be made from iron nails or thin round steel rods inserted into the foam pattern, with approximately 10 mm exposed outside the pattern so that the surrounding sand can hold them in place, preventing displacement by molten metal. The internal chills accelerate solidification at thick sections, enabling more uniform solidification and preventing porosity.

3. Prevention of porosity at fractured gating or riser sections
Porosity at broken gating or riser areas is caused by improper gating size—either too large or too small—which leads to the gating system feeding the casting or the casting feeding the gating system incorrectly. This occurs when the gating design does not match the wall thickness requirements, preventing simultaneous solidification.
Similarly, riser size must be properly matched to the gating system. Risers that are too large or too small are also major contributors to porosity.