Sand inclusion, Sand hole. Sand inclusion is also called as sand hole, which is a kind of metal casting defect. It looks like small or middle holes with sand grain in the internal or on the surface of castings. There are 3 main reasons caused the sand inclusion defect. 1. sand crush, sand drop. The partial sand blocks drop. Blow holes, gas holes or gas cavities are well rounded cavities having a clean and smooth surface. They appear either on the casting surface or in the body of a casting. These defects occur when an excessive evolved gas is not able to flow through the mould. So, it collects into a bubble at the high points of a mould cavity . is very tiny hole. Subsurface blowhole only can be seen after machining. Gases entrapped by solidifying metal on the surface of the casting, which results in a rounded or oval blowhole as a cavity. Frequently associated with slag's or oxides. The defects are nearly always located in the cope part of the mould in poorly vented.