Metallic and non-metallic impurities as well as hydrogen and oxides may negatively affect the quality of an aluminium melt.
By measuring the density index, it is possible to define, control and optimise the process of cleaning the melt, thereby improving the quality. In this way, the consistently high quality of the melt is assured and the reject rate effectively reduced.
In order to determine the density index, two samples of the melt are required. One of the samples is made under a defined vacuum of 80 mbar and the other at atmospheric pressure. The vacuum allows hydrogen pores to form in the melt sample, which means that this sample is less dense than the one made at atmospheric pressure.
After solidification, the samples are cooled in water, weighed and their exact densities determined.
The density index represents the percentage difference in density between the two samples.
The lower the density index the cleaner the aluminium melt and the fewer pores formed.