FILTERING A HEAVY-SECTION DI CASTING—NEW OPPORTUNITIES Thorsten Reuther
hofmann CERAMIC GmbH, Breitscheid, Germany Copyright © 2014 American Foundry Society
A version of this paper was previously published in the 2013 Keith Millis Symposium Proceedings Abstract
Because of the increasing requirements for the quality of heavy-section ductile iron castings, foundries that manufac- ture large castings have been using ceramic filters regularly since around 2000.
Standard ceramic materials were used in the past, which required a high number of filters per mould. Considering the high cost to manufacture a large iron casting, which in- cludes moulding, resin bonded sand preparation, finishing, transport and machining, the design of such an ingate sys- tem requires a lot of attention because of the use of filters. Large castings require a high number of filters. It is difficult to design an ingate system so that each filter will receive the same amount of iron and therefore avoid overloading a single filter. Furthermore, an ingate system with 60 to 100 filters requires a lot of space in the mould.
Introduction
The appearance of slag and dross defects or defects due to sand erosions is known to have something to do with pour- ing velocity and poor ingate system design. These will pro- mote turbulence and consequently oxidation of the metal. The quality of a casting could be improved in an effective way with the help of ceramic filters.
The filtering effect is based on the rules of fluid mechanics, which means separation of the inclusions by physical sepa- ration in front of a filter and the promotion of laminar flow after the filter (reduced turbulence). These mechanisms are illustrated and shown in Fig. 1 and 2.
Zone (1) is an area of turbulent flow due to a high velocity (Reynolds number) in the downsprue area where we have a free fall of the liquid metal. All oxides that appear in this tur- bulent area and all exogenous inclusions, such as sand and pieces of refractories, have a lower density (approximately 2.3 g/cm³) than the liquid iron (7 g/cm³). When the molten metal reaches the filter, a tailback (zone 2) forms and the flow velocity is slowed so the inclusions are separated (zone 3) by floating up to the surface of the metal due to density differ- ences to create a conglomeration in front of the filter. After the filter, we find an area of laminar flow (zone 4), which also
International Journal of Metalcasting/Volume 8, Issue 2, 2014
Even filter materials developed for steel foundries with higher temperature resistance, mainly ZrO or graphite, have a maximum capacity of 800 kg to 1.2 ton per 8 in. diameter filter. Around 100 filters are used in a 100 ton casting.
This paper compares the cost to use current filter mate- rials to the cost of substituting with a high-temperature and high-strength filter in the mould. It includes a short explanation of the risks of filter breakages and also some new ideas of simple ingate designs. These new designs will avoid the risk of overloading single filters due to asymmet- ric ingate systems.
Keywords: gating, ingate system, filter, filtering, dross de- fects, metal flow
helps to avoid oxidation of the metal. This effect is also very important, because any inclusion that appears after the filter will reach the mould cavity and could create problems.
With this in mind, many simulations using water and/or oil were done to illustrate these effects, as shown in Fig. 2. To demonstrate the flow, some coloured particles with the same density as water to avoid an upfloating effect were added. Here the turbulence in front of a filter and the laminar flow after a filter are easy to see.
Recently, students at Penn State University investigated these effects under real pouring conditions. To do this, green sand moulds were prepared with quartz glass windows so
Figure 1. Flow in front of and after a filter. 17
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