SUMMARY
Casting speed is of paramount importance in centrifugal casting processes. In fact, in the event that the cylinders have not reached their optimal mechanical properties during the calcination phase, there is a risk of flaking or even breakage by the plaster during the casting phase. For this reason, the quality of the plaster and the careful control of all the steps of its preparation represent a factor of primary importance in the final quality of the objects.
PROBLEM
In the case in question, an object that had a superficial macroporosity was analyzed. This defect was particularly widespread in all areas that were in contact with the mold outwards during casting. Through the analyses carried out in the laboratory, it was possible to ascertain that the shape and arrangement of the defect were due to the detachment of the plaster from the mold, a typical problem of centrifugal casting processes.
The speed given to the metal produced by a centrifugal machine is far greater than that produced by a static machine: the metal therefore fills the mold with an extremely turbulent motion. In this way, there are erosions of the plaster or even flaking and breaking of parts of the plaster. The metal therefore drags the plaster with it which, when it is removed during cleaning, leaves pores similar to those depicted in the following images.
SOLUTION
To solve the problem of surface macroporosity due to gypsum detachment, it is necessary to make sure that the gypsum itself has the best possible mechanical characteristics. For this reason, it is important to:
- Make sure that the water-to-gypsum ratio is correct, following the instructions provided by the gypsum manufacturer
- check that an adequate temperature of about 750°C is reached in the calcination cycles, and be careful that this is maintained for a sufficiently long time so that there is a homogeneous heating of the cylinders (therefore for at least four hours).