Production of ceramic foam filters for molten metal filtration using expanded polystyrene

15 Dec.,2022


Ceramic Foam Filter

The presence of nonmetallic inclusions is considered to be one of the most widespread causes of defects encountered in the casting process.1 Nonmetallic inclusions are particles that are present in virtually all metals and depending on their shape, size, number and distribution, can influence the deformation properties of that metal and subsequent quality of the finished product.2 The use of conventional gating systems even with generously dimensioned runner bars is not sufficient to retain enough slag and suspended reaction to meet the high quality standards of today's castings. These demands can be achieved with the help of filters.3

Liquid metal filtration during the casting process is now a very common technology in both ferrous and nonferrous alloy casting. Exogenous and indigeneous inclusions in the melts can be removed using filtration.4

Filtration is the process of seperating solid particles from the melt, with the solid particles being captured on the filter and the liquid phase passing through the filter.5 Filtration improves the surface finish and pressure tightness of castings and improves mechanical properties of the castings. It reduces the rework on castings, increases machinability and improves casting yield.4

There are several established filter technologies presently on the market. These include strainer cores, woven cloth or mesh, and ceramic tile filters. Ceramic tile filters are generally considered to be the most effective. Pressed cellular, extruded cellular and foam filters are ceramic tile filters. Pressed cellular are characterized by round cells, extruded filters have square cells, foam filters have a random dodecahedron type structure.6

Metal filtration using foam ceramic filter media has been found to be an effective means of controlling the level and particle size of inclusions.7 Reticulated ceramic foam filters have been used commercially in the foundry industry since 1977. The first application was in the United States in the production of premium quality aluminium castings for aerospace applications.8

The foam filter was recognised to have a unique, tortuous path through its body,which trapped inclusions and allowed clean, smooth-flowing metal to exit into the mold cavity.9 Their main advantages are: the best filtration effectiveness and turbulence reduction, also their refractoriness and erosion reduction for the most demanded casting.10

Ceramic foam filters have open pore reticulated structure with a very high volume of porosity and very high surface area to trap inclusions.11 They are open foam structures composed of ceramic material, such as alumina, mullite or silica. Ceramic foam filters operate in a mode of deep bed filtration where inclusions smaller than the pore openings are retained throughout the cross-section of the filter.12

Inclusion capture in deep bed filtration is considered to be a result of two sequential events: transport of an inclusion to capture sites on the filter media. Attachment of the particles to these sites.13

Ceramic foam filters are produced by impregnating reticulated polyurethane foam with a ceramic slip, removing the excess slip by squeezing the foam, then drying and firing the body.14

Correct choice of refractory material is essential for clean foundry practice. Poor quality refractories tend to be eroded very quickly causing particulate to be washed into the mold cavity.15

Refractory filter material must be able to withstand the initial metallostatic priming head and thermal shock without of spalling of filter fragments or breaking down.16 Filter material properties necessary to attain temperature capability, chemical inertness, mechanical, physical properties and thermal properties.17

Proper pore size also is important, and size selection often involves a trade-off between product quality and size of the filter.12 Finer pore size filters offer higher filtration efficiency and improved casting cleanliness,but larger areas are required to offset their higher flow resistance.16

In this work; it is aimed to produce a new filter that has larger surface area for deep bed filtration than the conventional one. The dimension of the molds can be changed so the size and amount of the filter porosity can be easily controlled.