A thicker filter has a higher mechanical strength and the softening effect is delayed (under the same pouring condi- tions) because of the larger amount of ceramic material. The higher temperature-resistant ceramic material also delays the softening effect. The disadvantage of the more resistant ceramic material is the high cost compared to the standard ceramic filter. Therefore both materials are used in filtering large castings. Both solutions still require a higher number of filters and a properly designed ingate system to avoid overloading them.
Effects On Ingate Designs
All these conditions have a tremendous effect on the design of an ingate system. Pouring weights of more than 1 ton of liquid ductile iron may require more filters. To reduce the amount of work involved in such systems, foundry engi- neers often use a filter battery to install the filters in an easy and simple way. The filter battery keeps costs down, but it also hides a high risk of filter breakage due to overloading, which is illustrated in theory in Fig. 4 and shown in reality in Fig. 5. Filter batteries are still in use and some work with- out problem. Each filter battery requires careful design and calculation to reduce an overloading effect.
In Fig. 5, the pouring weight was around 2 tons of liquid ductile iron, and the foundry engineers installed four filters (150x150x24 mm) to handle that amount of liquid iron (ap- proximately 500 kg per filter). Unfortunately, some castings
later exhibited filter pieces after machining, and the castings had to be rejected. This can happen even after a number of successful trials. This phenomenon is explained in Figure 6. In this practical example, the pouring height of the cope or the “downsprue” was around 2 m, so the velocity at the beginning of the pouring process was approximately 160 cm/s. The high velocity creates a hard impact when the iron “bombs” into the filter battery, especially in this design where the volume above the filters was slightly too small to accommodate a slowdown of the velocity. But why did it work for a while?
Figure 6 explains why an ingate system could work for a while (even under unfavourable conditions) until problems occur. This illustration is based on investigations over the last ten years of ingate systems that were fine most of the time but would occasionally fail. In case of a failed system, all available foundry parameters like pouring temperature, pouring height, pouring weight, etc., were compared with all available parameters from the filter manufacturing. Here the affected filters and the retained samples underwent sev- eral tests such as dilatometer tests and pouring tests to get an idea about the filters real strength. Figure 6 shows the overlap of the strength curve and the stress curve. Where the filter strength drops below the pouring stress in any in- dividual casting, it runs a risk of failure. The green line illustrates the minimum strength or resistance of a filter, and the irregular blue curve shows the actual resistance of a filter with more than the minimum strength. This mini- mum strength is recommended in the filter manufacturer’s brochures as a maximum strength for the filter application (max. pouring rate).
The red line indicates an actual pouring condition that is over the maximum recommended loading for a single filter in a filter battery (to stay with the example shown in Fig. 4 and 5). This line is also an irregular curve because of the variations in pouring.
The actual strengths of the filters are shown with the blue curve, and the variations of the manufacturing process are considered.
Figure 4. Risk of overloaded filters in a filter battery.
Figure 5. Overloaded and broken filters in a filter battery. International Journal of Metalcasting/Volume 8, Issue 2, 2014
Figure 6. Stress conditions for filter breakages. 19
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