Sump melting

is as a rule practiced in mains frequency induction furnaces, The optimum sump height corresponds to approx. 2/3 of the filling level, since in this case the whole coil is filled with molten metal, and the maximum electrical / inductive coupling is available. Medium-frequency furnaces are operated with a sump of approx. 15% of the total capacity. After tapping a medium-frequency furnace, the sump is still at a relatively high temperature. When switching on the output again after charging, the remaining sump would be very well coupled, and would be over-heated for a short time. This produces an “elephant’s foot” as is normal in mains frequency furnace operation. To avoid this, approx. 30% of the sump weight charged into the sump should consist of compact cooling scrap (risers, sprues and approx. 100 mm large pieces). In larger furnaces (from approx. 5 t), a “starter block” with a weight of approx. 30% of the sump weight can also be charged in. This simultaneously lowers the temperature, raises the bath level and reduces the wear to the crucible wall.