Inclusions (Ash or Dust)
Missing areas (small, round, irregular or angular craters or cavities) of metal in the casting
Indicates that there may be ash particles in the mold (on the top side of parts); while on the bottom side of parts, it may be dust particles in the mold (top and bottom are relative to the location of the part while they are treed up for casting)
These particles, present in the mold cavity or the metal, prevent the casting from fully filling the shell
Causes of this defect are evident in the wax and other portions of the process; to cure, R&R recommends taking the following actions:
Ash in wax.
Use lower ash wax, improve burnout.
Patterns are not oriented so the wax can drain out during dewaxing.
Orientate parts so that the wax can drain from the pattern or use extruded wax tied to the sprue to drain the wax.
Cover molds after dewax and use a filter.
Molds are being fired with the cup upright, entrapping dust or ash from residual wax residue.
Fire molds with the cup facing down. This eliminates the potential for foreign material to enter the mold cavity and become a metal inclusion later in the process.
Dust particles are trapped in the shell cavity.
If a double-firing cycle (firing/preheating) is occurring, blow the molds out prior to preheat. Consider covering the molds after firing to prevent foreign material from entering the mold cavity. Cleaning the top of the ladle or furnace prior to pouring metal may also eliminate the opportunity to introduce foreign material into the shell.