Sand inclusion is one of the most frequent causes of casting rejection.
It is often difficult to diagnose, as these defects generally
occur at widely varying positions and are therefore very difficult
to attribute to a local cause. Areas of sand are often torn away by
the metal stream and then float to the surface of the casting
because they cannot be wetted by the molten metal. Sand inclusions
frequently appear in association with CO blowholes and slag
particles. Sand inclusions can also be trapped under the casting
surface in combination with metal oxides and slags, and only
become visible during machining. If a loose section of sand is
washed away from one part of the mould, metallic protuberances
will occur here and have to be removed.