the area around the filter could be observed. Many test pours with grey and ductile iron were performed, and the new the- ories of filtration were clearly proven (Fig. 3).
With this knowledge, it would seem easy to design an ingate system by placing a filter in an ingate system and pouring the liquid metal through the filter to get a clean casting. Unfortu- nately it is not that easy because liquid metal, especially iron or steel is hotter and much heavier than the water often used for flow simulations. The knowledge derived from these simulations helped the industry understand the separation of the clean, heavy metal from the much lighter inclusions (up- floating effect), but brings with it the danger of overloading the ceramic filter material.
Conditions for a Ceramic Filter
Ceramic materials do not have an exact melting point but a fusing range that changes under different conditions and depends on the pouring temperature and pouring time. The pouring height and velocity also play important roles. The higher the velocity (short pouring time, great pouring height) the higher the dynamic pressure on the filter.
Pdyn = (δ x c²)/2 [N/m²] Eqn. 1
By doubling the velocity, force will increase four times due to dynamic pressure on the filters. As shown in the equation, the density of molten metal also has a great influence on the dynamic pressure. Furthermore, the density of iron is around seven times greater than the density of water. When the iron comes running in an ingate system, it also creates a high stress
factor.The other critical parameter for a ceramic filter is the temperature. The ceramic material has to resist a tempera- ture of between 1,300°C (2,372°F) and 1,500°C (2,732°F) in iron castings. How can this possibly work? Higher pour- ing temperatures are used mainly in smaller castings with thin wall thicknesses to avoid problems with cold runs, and lower temperatures are used mainly in heavy-section castings. The main differences between both applications are shorter pouring times and lower pouring heights in smaller castings and longer pouring times in combination with lower pouring temperatures in bigger castings. So the short pouring times and lower pouring heights in smaller castings balance the higher temperature, and the lower pouring temperature in big- ger castings balances the longer pouring times and the greater pouring height. By the time the filter ceramic is too hot in a small casting, the pouring time has already expired. Because of these circumstances, a filter breaks for two main reasons:
1. Thermal shock in combination with a hard impact. 2. Overloading of a filter due to too long a period of “hydrodynamic” loading under hot conditions.
Both situations usually provide different indicators on a casting that has been affected by a filter breakage. In the first case, bigger pieces of filter will be found with almost no
18 Figure 3. Real flow, ductile iron. International Journal of Metalcasting/Volume 8, Issue 2, 2014
deformation. In this scenario, the filter breaks immediately at the beginning of the pouring process and the undeformed filter pieces are washed into the mould cavity. In the second case, deformed filter pieces will occur due to a softening of the ceramic material when it was loaded for too long in its fusing range.
Effects On Filter Ceramics & Designs
As a result of these findings, filter manufacturers reacted in two different ways to find a solution for foundry engineers:
• The invention of higher-resistance ceramic materials. • The increase of filter sizes and thicknesses.
When the first large castings were manufactured with filters, the maximum filter size was around 133x133x22 mm or 150 x150x25 mm. The filters had a maximum capacity of approx- imately 550 kg of liquid ductile iron and were made either from a mullite ceramic in a pressed version or from SiC in foam ceramic filters. Later, the foam ceramic manufacturers introduced the Ø 200x35 mm round filter which was manu- factured in ZrO on the basis of a graphite material. This big filter size could not be manufactured in a pressing process at the time, and the foam ceramic filters had the highest flow rate of a single filter. They are used to a capacity of up to 1.2 tons of liquid ductile iron. By increasing the thickness and improv- ing the material, the risk of a filter breakage for the two main reasons outlined above can be avoided.
Figure 2. Flow simulation with water.
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