All metal castings contain porosity, which is inherent to the casting process. The sponge-like porosity is caused by internal shrinkage, gas cavitation, oxide films and inclusions. Porosity may go to the heart of a casting and infiltrate the entire part.
Metal Castings can have two general types of porosity: macro and micro. Macroporosity (large flaws) may be visible; microporosity (small voids) may be almost invisible. In powdered-metal (P/M) parts, the structure is similar to macroporosity in low-density parts or microporosity in high-density parts.
Impregnation: The Answer
Impregnating to fill the blind and continuous pores with an inert material, such as a thermosetting resin, can improve or eliminate painting, plating and other surface finishing problems. Impregnation can be done on large or small, ferrous and nonferrous die, sand, investment, rubber mold and pressure casings, forgings, weldments and P/M parts, Iron, Steel, Bronze, Aluminum, Zinc, Lead, white metal and Magnesium casting can be impregnated.
Impregnating permanently seals micro- or macroporosity voids for pressure tightness and blocks corrosion and fungus growth. Impregnation also can improve the machinability of P/M parts; the impregnating resins act as a lubricant to eliminate machining chatter and prolong tool life.
Impregnation is designed to fill micro- and macroporosity, but it cannot cure surface flaws or repair cracked or unsound castings. Also, the process is not intended to increase the strength of a casting. (non-structured)
