Shrinkage cavity and porosity are the most common defects in castings. Shrinkage cavity is produced by poor feeding during solidification of casting. Porosity is caused by the insufficient permeability of the mold (core) and the large amount of gas produced during pouring, which can not be discharged in time.
1) Identification of shrinkage cavity in sand casting
The characteristics of shrinkage cavity in casting are rough wall and irregular shape, which often appear in the thick part of the last solidification or the joint of the casting wall. The main methods to identify shrinkage are as follows:
(1) Observe the surface shape of casting defects, such as the holes with uneven surface, very rough, dark gray and irregular shape, which are called shrinkage holes.
(2) If the location of the hole is at the fat part of the final solidification of the casting, or at the hot spot where the two walls intersect, and it is in the middle or upper part of its section, it is a shrinkage cavity.
(3) Generally, the concentrated hole defects on the thick section of steel castings are shrinkage or air shrinkage.
2) Identification of stomata in sand casting
There is a big difference between porosity and shrinkage cavity in casting, and its characteristics are as follows.
(1) The pore wall is smooth, and the inner surface is bright white or slightly oxidized.
(2) The stomata were round, strip or irregular.
(3) The size of the pores varies greatly from a few centimeters to a fraction of a millimeter.
(4) Porosity often exists on the surface of casting or near the sand core, chill, core support, pouring or riser in a single, several or honeycomb shape, sometimes covering the whole section.