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Filtering basics: Who, what, where, why & how.

Fram, a national manufacturer of automotive oil filters, has relied on a slogan, "You can pay me now or pay me later," to demonstrate the importance of its oil filters. The idea is that spending money on oil filters in the present eliminates spending more money in the future to fix larger problems stemming from not using a filter.

This slogan, though created for cars, is relevant for the foundry industry. Filters are often seen as a type of insurance policy for foundries and can be used to reduce many of the inclusion defects commonly resulting from today's melting and pouring processes.

What Are Filters?

Ceramic foam filters (CFF) are open foam structures composed of ceramic material, such as alumina, mullite or silica (Another filter type is cellular, but it will not be discussed in this article.). CFF operate in a mode of deep bed filtration where inclusions smaller than the pore openings are retained throughout the cross-section of the filter. They differ from strainers, which often only retain inclusions larger than the strainer holes. Deep bed filtration forces the molten metal to flow through a torturous path, which allows more opportunities for inclusions to come in contact with and be retained by filter filaments. The inclusions become retained on the filter surface throughout the pores as the metal stream continues through the filter into the mold.

CFF also can reduce turbulence in the gating system by removing eddies and smoothing the flow. By reducing turbulence, opportunities for reoxidation of the molten metal are reduced, and, as a result, slag formation in the gating system is reduced. Filtration helps provide a smooth, laminar metal flow through the gating system of the casting.

Filter Benefits

CFF remove inclusion material from the metal, making a cleaner casting. This benefits the foundry through cost savings due to reduction in scrap related to inclusions, improved machining characteristics of filtered castings, and improved mechanical properties in the finished components.

CFF increase productivity in the machine shop by limiting sub-surface/surface inclusions in a casting, which lengthens machine tool life because the tool wear related to inclusions will decrease. With an increase in tool life and less downtime due to tool breakage, an increase in the production rate of machined castings can occur.

Using a CFF also can increase casting mechanical properties such as ductility because inclusions can cause discontinuities or act as crack initiators. A filter also can improve the pressure tightness of a final casting by eliminating any "leaker" defects and enhance the final surface finish of the casting.

Primer on Inclusions

Inclusions come in several types, all of which cause problems in castings of any metal. Slag inclusions, blow hole defects and eroded molding media and refractory material are all inclusions that can be minimized through the use of filters.

Gray Iron--Slag inclusions--liquid, solid or a combination of both--commonly are found in gray iron. The primary source for these inclusions is oxidizing reactions in molten iron between the major alloying elements, including iron, manganese, silicon and carbon.

The product of some of these reactions is an acid that oxidizes elements of the casting. Silicon carbide CFF help remove these inclusions by trapping the oxidized element particles in the filter filaments.

Another type of defect often detected in gray iron castings is the slag blow hole. This defect typically is found on the cope surface of the casting or on the internal surfaces adjacent to the core of the casting. Blow holes can result from a chemical reaction producing carbon monoxide.

Low pouring temperatures, manganese sulfide inclusions and slag from the ladle or from turbulent metal flow also cause slag blow hole defects.

To reduce this defect, filters retain the slag, which keeps it from becoming part of the casting, and smooth the flow of the metal, reducing the turbulence and creating a smoother casting.

Figure 1 shows a liquid slag inclusion where sand grains have been embedded in the slag material, creating a liquid sand-slag agglomerate.

Ductile Iron--The major type of inclusions found in ductile iron are dross inclusions. These inclusions originate from the addition of magnesium as a "spheroidizing" agent to the ductile iron melt.

Chemical microanalysis has determined that dross commonly found in ductile iron applications contains magnesium oxide (MgO), silica (Si[O.sub.2]), magnesium sulfide (MgS), forsterite ([Mg.sub.2]Si[O.sub.4]), enstatite (MgSi[O.sub.3]), alumina ([Al.sub.2][O.sub.3]), fayalite ([Fe.sub.2]Si[O.sub.4]), silicides and amorphous silicates. Forsterite, also known as magnesium silicate, is the most common type of dross found in ductile iron.

Filters prevent dross from entering the casting. Reduction in turbulence is also important in ductile applications because reoxidation reactions are minimized.

Steel--Plain carbon and low alloy steels often develop oxide macro-inclusions because of molten metal reoxidation. CFF can modify flow to reduce turbulence during casting, which can diminish the amount of reoxidation and the formation of macro-oxide inclusions in steel castings.

Aluminum--Aluminum alloys experience oxide inclusions because both aluminum and magnesium react with oxygen in the atmosphere to form aluminum oxide and magnesium oxide, respectively.

If the molten aluminum is held at elevated temperatures for long periods of time, the reaction of aluminum oxide and magnesium oxide will occur, resulting in the formation of magnesium aluminate spinels (Mg[Al.sub.2][O.sub.4]) (Fig. 2). These dense inclusions can cause extensive problems in the machining of castings.

Magnesium aluminum spinels and other inclusions in aluminum can be retained by GFF because of the deep bed filtration action. The reduction of turbulence also minimizes formation of some oxides in molten aluminum.

Copper-Base--Copper castings experience dross or slag inclusions with a chemistry of complex oxides. The complex oxides consist of zinc oxide (ZnO), cupric oxide (GuO), iron oxide (FeO), tin oxide (Sn[O.sub.2]), silica (Si[O.sub.2]), calcia (GaO) and phosphates. One possible source for the formation of these inclusions is oxidation of the melt or byproducts from melt treatment practices.

Another source of inclusions is improper skimming or inadequate settling of the melt.

Filters retain these inclusions in the gating system, keeping castings clean.

Selecting the Right Filter

Understanding the cause of inclusions and how filters prevent them from entering the casting is just the beginning. Proper filter performance depends on installing the filter correctly, choosing the right pore size for the job and sizing the filter to the casting based on the metal flow rate.

Installation--Proper filter installation and sizing achieves maximum filtration efficiency. One of the best ways to ensure the cleanest casting is to place the CFF as close to the casting as possible.

The available space in the gating system design often requires filters to be placed in the runner system either vertically or horizontally (Fig. 3).

Pore size--Proper pore size also is important, and size selection often involves a trade-off between product quality and size of the filter. Three main pore sizes can be used: #10 (coarse), #15 (medium), and #25 (fine) (Fig. 4). The #25 filters provide the cleanest filtered metal, but they must occupy a larger surface area to offset their higher flow resistance. Pore size selection is slightly different for each type of metal cast, but the main factors to consider before choosing a pore size include:

* melt cleanliness prior to filtration;

* casting cleanliness requirements;

* available space on pattern equipment;

* metal flow rate requirements.

* For most gray iron foundry applications, #15 pore filters are adequate. However, for gray iron products such as compressor housings that require excellent pressure tightness, #25 pore filters are more appropriate. The finer pore size filters are more effective in removing fine slag inclusions that can be responsible for leaker defects in pressure tight castings.

* As a result of metal treatment, ductile iron foundries tend to have significant amounts of dross in their melts. For that reason, #10 or #15 pore filters satisfy most ductile iron casting applications. Some ductile iron foundries use #25 filters to gain higher filtration efficiency. Because of the finer pore sizes, filter blockage from the high retention of dross inclusions may be likely. In order to minimize or prevent filter blockage entirely, the filter size must be larger to provide more surface area.

* Filtering steel is similar to iron. For aluminum deoxidized carbon steels, #10 filters are used to allow the filter to prime with more ease. In most steel investment casting applications, #15 filters are used because of the high surface quality requirements for these castings. For aerospace and turbine applications, #25 and finer filters are used because these applications require the cleanest metal possible.

* In aluminum foundries, all three pore sizes can be used depending on quality requirements, metal cleanliness of each aluminum foundry, and the type of castings produced.

* In copper foundries, #10 and #15 filters are sufficient. For some high quality applications, #25 filters can be used to retain very fine inclusion material.

Filter Sizing--There are two main factors to consider when determining the optimum filter size. The first factor is the metal flow rate. Most castings have a critical pouring time and an optimum pouring rate. The pouring rate is the total metal pour weight divided by the pour time of the casting. The optimum pouring rate for each foundry and application will be different and is usually derived empirically or from observations of many casting samples.

The desired pouring rate for each foundry should be determined before a filter size is selected based on the gating ratio of the mold. The gating ratio is the ratio of the sprue base area: total runner area: ingate area. The smallest of the three areas is referred to as the choke area since this region of the gating system controls the metal flow rate.

All GFF restrict flow, but selecting the proper filter size will minimize the flow restriction. In order to do this, the total filter area is recommended to be at least 5-6 times the choke area of the casting.

The second factor in determining the correct filter size is the amount of metal flow through a filter before it becomes blocked. All filters have a finite quantity of metal they can filter before they become plugged. Once they are plugged, metal flow will decrease or, in extreme cases, stop completely. Blockage occurs as the pores of the filter become filled with inclusions captured from the metal. The amount of metal that can flow through a filter before blockage varies significantly based on the metal and alloy being cast, the cleanliness of the metal, the pouring temperature, filter pore size and gating system.

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For More Information

To see tables of recommended pouring flow rates when using filters, visit

About the Authors

Kyle Adams and Erin Williams, applications engineers, and Sudesh Kannan, technical service manager, work at Selee Corp. and provide technical support for inclusion reduction to ferrous and nonferrous foundries worldwide.
COPYRIGHT 2002 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Title Annotation:filters for foundries
Comment:Filtering basics: Who, what, where, why & how.(filters for foundries)
Author:Kannan, Sudesh
Publication:Modern Casting
Geographic Code:1USA
Date:Mar 1, 2002
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