Shrinkage defects can be both open and closed. While open shrinkage is visible from the outside, closed shrinkage is within the casting and therefore needs X-ray to see it clearly.
Gas porosity is caused by the formation of bubbles within a casting as it changes from a liquid state to a solid state. This can cause weakness in the area around it.
Inclusions are the result of dross or slag being introduced into the casting during the pouring process. These can either come from the mold
Hot cracking, or hot tears, occur while the casting is cooling. This can be caused by inconsistent cooling across the casting.