Meaning
Solidification layers form on the inner walls of a delivery tube when the local temperature of the molten metal falls below its liquidus point. Development of melt tube skulling creates a self lining surface that can protect the tube material from chemical attack by the melt. However, excessive growth of this layer restricts the flow of the liquid and can eventually lead to a total blockage.
The thickness of the layer is governed by the balance between the heat supplied by the melt and the heat lost to the environment.
Thermal Equilibrium
Steady state operation of the casting system requires a precise balance of energy inputs and outputs. Melt tube skulling stabilizes when the rate of heat conduction through the solidified layer matches the convective heat transfer from the moving liquid. Adjusting the external insulation or the flow rate of the molten metal changes the thickness of the skull.
Continuous monitoring of the tube temperature allows for real time adjustments to the heating system. This equilibrium is sensitive to even minor fluctuations in the melt temperature or the gas flow rate.
Flow Obstruction
Reduction in the effective diameter of the delivery tube increases the pressure drop and slows the production rate. Melt tube skulling can lead to turbulent flow patterns that introduce gas bubbles or impurities into the metal stream. Irregular skull growth causes the metal jet to deviate from its intended path.
Cleaning procedures or thermal pulses are used to remove excessive deposits without damaging the ceramic tube.
Yield Impact
Decreased throughput and increased downtime for maintenance directly affect the profitability of the atomization process. Melt tube skulling reduces the overall recovery of the metal by trapping material inside the delivery system. Efficient management of the skull layer maximizes the service life of the equipment.
