The reason for the second problem is often much more com- plicated, but one explanation is the effective pouring height (Hp) is reduced at the end of the pouring process because of the rising iron level in our mould cavity, which is illustrated in the Figures 16 and 17.
Figure 16 shows the beginning of the pouring process, where the counter pressure is low and the velocity is high. Figure 17 shows the end of the pouring process when the velocity is reduced. If the viscosity is on the borderline of allowing the iron to flow through the filter and the dynamic pressure drops to a critical level because of the reduced velocity, the iron could stop running through the filter and the pouring process cease. Even in this situation, a filter with bigger holes or pores gives a solution, but sometimes even an 8 PPI (pores per inch) foam ceramic filter or a pressed filter with 3.9 mm diameter holes shows this effect.
In recent months, some investigations were done to find out the real reasons why this can happen and why some ingate systems with short pouring times were working without any problems, even for large castings. A high velocity is known to contain some risks of dross defects (turbulence) and some vacuum-related problems due to the Venturi principle. With that knowledge, some people always prefer long pouring times. After some calculations with real ingate systems, a possible reason was found.
For an explanation, a 45 ton ductile iron casting was de- signed with two different filter types:
Figure 16. Pouring height at the beginning of the pouring process.
• Ingate system with high resistant filters (Ø 200x35 mm, Fig. 18): Sixteen filters with a pouring rate of around 2.81 ton/filter and a pouring time of around 150 seconds. With this system, the velocity of the metal flow was around 48 cm/s at the ingate tubes (diameter 80 mm) which should be low enough to avoid too much turbulence (< 50 cm/s).
• Ingate system with standard filters (Ø 200x35 mm, Fig. 19): Forty filters with a pouring rate of around 1.13 ton/filter and a pouring time of around 80 sec- onds. With such a system, a velocity of around 50 cm/s occurred at the 80 mm ingate tubes.
Figure 14. A core with a filter chamber for two filters.
Figure 17. Pouring height at the end of the pouring process.
Figure 15. Frozen filter at the beginning of the pouring process.
International Journal of Metalcasting/Volume 8, Issue 2, 2014
Figure 18. Ingate system, 45 tons of ductile iron, 16 filters. 23
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