If the curves for the stress and the resistance of the filters (red and blue lines) overlap, then the filters will break. This could happen after a while, even though some earlier cast- ings were successfully produced. Because of that behaviour, the foundry engineer must carefully check his ingate system for at least the following conditions:
• Is the ingate system symmetrical to promote a bal- anced flow to each filter chamber and filter?
• Will the filter chamber be filled fast and first (be- fore metal runs into the mould cavity)?
• Could turbulence be reduced by the design of the ingate system?
• Is there direct pouring on a filter? • Do all filters have the same resistance to the flow or do they have different geometries (hole sizes/ porosities)?
Filter Positioning in Large Castings First, the task of a filter in an ingate system must be defined:
The filter should promote laminar flow and the separation of inclusions from the liquid metal.
The problems of turbulence and inclusions are linked, be- cause turbulences allow oxidation of the liquid metal by ex- posing more of it at the surface, which leads to a larger area in contact with atmospheric oxygen. On the other hand, tur- bulence requires high velocities, as the equation for Reyn- olds number shows.
Re = (c x dh)/ν Eqn. 2
The variable ν is the kinematic viscosity, (dh) is the hydrau- lic diameter and (c) the velocity. As the Reynolds number increases, the risk of turbulence increases. A critical veloc- ity of turbulent flow, which could lead to dross defects in ductile iron castings, is about 0.5 m/s. If the velocity is even higher, the risk of turbulent flow and oxidation is increased. In large castings, pouring heights of 2, 3 or 4 m are com- mon and in the downsprue tubes, the metal follows a free fall. Free fall means the iron starts its fall at the bottom of the pouring basin with a velocity of zero and then starts to accelerate.
c = √(2 x g x h) [m/s] Eqn. 3
It can be seen that when the height (h) increases, the velocity (c) also increases. For a critical velocity of 0.5 m/s, the equa- tion gives a height of free fall of around 1.3 cm. That means that a velocity in a vertical downsprue tube easily reaches a velocity of around 2 m/s and encourages the formation of some slag or dross.
Using a filter battery runs a higher risk of frozen or over- loaded filters, and the battery cannot prevent inclusions that occur after the battery due to turbulence. The filters have to be installed as close as possible to the ingates to be of
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most benefit, and this is usually recommended in green sand mould too. Because of the horizontal parting line and the large heights of the mould flasks in large castings, the ingate levels of these castings are somewhere below the main part- ing line. The rule of placing the filters as close as possible to the ingates would require either a “third” parting line or a filter chamber that could be connected to the ceramic ingate tubes during the moulding process. To create a third parting line is often too expensive, therefore this solution is not very cost-efficient for the foundries. Also, the use of filter cham- bers with a single filter increases the costs because each filter chamber needs an extra connection with the ingate system and the system becomes more complicated.
New Filter Material/New Opportunities
The biggest problem or challenge in ingate system design for large castings is to maintain symmetry and find enough space on the pattern or in the mould to place all required fil- ters. If a round filter with a diameter of 200 mm could handle a pouring weight of more than 2.5 tons per filter, the number of filters in an ingate system could be reduced dramatically and it would become much easier to design an ingate system for large castings.
A suitable ceramic material for pressed filters has been available for several years. With this material, it is possible to combine high temperature resistance with the high dimen- sional and mechanical strength of a pressed filter. In some tests, foundries have already poured more than 5 tons per filter. A “typical” pouring weight is 2.75 tons per filter.
In the first tests, foundry engineers replaced standard filters with the new filters in cases where they faced serious trouble with broken filters. Simple replacement did not bring the real benefits of these filters, because of their higher cost. Other solutions had to be found. As time went by, the number of filters was reduced but even then, without a change of the ingate designs, the results were not satisfactory.
Filter in Parting Line Between Core Print and Mould
To install something like a filter in an ingate system, engi- neers have to look for a parting line. One possibility is to use the main parting line where a second downsprue tube must be installed to achieve bottom filling, but this solution has another disadvantage mentioned earlier. The other possibil- ity is to install a ceramic filter chamber or a core that could contain the filters and give the required “parting line.” Usu- ally these filter cores were made to install a filter battery to reduce the amount of required space in the mould.
The following example shows a typical design used to install such cores with filters in an ingate system for a 30 ton ductile iron casting. Figure 7 shows an ingate system that has 12 fil- ter chamber cores with six filters in each chamber. A total of 72 filters sized 150x150x24 mm were used for this casting.
International Journal of Metalcasting/Volume 8, Issue 2, 2014
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