Porosity
Porosity
In spite of careful production techniques, castings made of grey iron, copper, aluminium and magnesium alloys often turn out porous and thus unsuitable for their intended use, e.g. as motor parts, oil pans, brake cylinders, pump casings, filter parts or valves. Furthermore, cast parts that were originally pressure tight may start leaking after post impregnation machining as enclosed pores are cut.
How does porosity appear?
Porosity mainly appears because the outer contours solidify first when cooling after casting. When further cooling of the castings takes place, the metal shrinks and porosity can form inside the castings. In spite of advances in processing, porosity cannot always be avoided. Other factors like melting temperature, alloy composition and mould design additionally influence casting quality.
Three forms of porosity in the casting
1 The encapsulated or enclosed pore:
It is only a problem if opened by a machining process.
2 One side open or blind porosity:
Leads to blooming after galvanic treatment, bubbles or craters form in the paint after stove enamelling. This type of porosity also poses the risk of inner corrosion.
3 Through porosity:
Liquids and gases penetrate the cast wall. A single pore is sufficient to make the part leak and thus useless.
Impregnation according to the Maldaner process fills the open pores.
Cast parts that are machined after impregnation can leak again if unfilled porosities are cut open.
Therefore, we recommend impregnation after the final machining.
Impregnation according to the Maldaner process fills the open pores.
Cast parts that are machined after impregnation can leak again if unfilled porosities are cut open.
Therefore, we recommend impregnation after the final machining.
