Ceramic Foam Filter for Aluminum: PPI, Priming and Filter Box Practice

A ceramic foam filter is the most common filter type in aluminum casting, used to strip oxides and other inclusions just before the metal freezes. It only works when the grade, the priming and the seating are right.

This guide explains how the filter captures inclusions and how to choose pore size for your application. It also covers how to prime and seat the plate, and when fiberglass mesh is the better tool.

How a ceramic foam filter removes inclusions

The filter is a rigid open-cell ceramic sponge. It is made by coating a polymer foam with ceramic slurry, then drying and firing it so the foam burns out. A summary of filtration practice on Aluminium Guide, based on the ASM handbook, describes the result as roughly 75% porous and used as flat plates.

Inclusions are captured in two ways:

• Cake filtration. Larger particles are trapped at the inlet face. The trapped layer then helps catch more particles. The same source says this mode mostly captures particles larger than 30 micrometers.
• Depth filtration. Smaller particles travel into the pores and attach to the ceramic walls along the winding path. This mode can remove particles much smaller than 30 micrometers.

Attachment depends on surface energy, inclusion size and flow. Those are the factors studied by Zhang and co-authors in a TMS Light Metals paper on inclusion removal through filtration. In their tests, both uncoated and slurry-coated alumina filters filtered well, and the uncoated filter performed better.

Flow rate matters as much as the filter. Lower metal flow rates and larger filter area both raise efficiency. So a ceramic foam filter that is undersized for the pour rate will pass more inclusions, even in a fine grade.

Choosing PPI by application

Grades are named by pores per inch (PPI). A higher number means smaller pores and windows. A US patent on filter priming notes that 30 PPI has a very open structure. At 80 PPI, pore and window size are much smaller. Filters of 60 PPI or more have typical window sizes of 150 to 500 microns.

Finer is not automatically better. A finer grade captures smaller inclusions but needs more metal head to prime. It also carries less metal before it blinds. Use these questions to narrow the choice:

1. What does the customer reject? Thin-gauge foil, can stock and critical extrusions need cleaner metal than general ingot.
2. How dirty is the incoming melt? Heavy scrap content loads the filter faster. Fix fluxing and degassing upstream before going finer.
3. What metal head is available? The filter box and launder set the limit on how fine you can go.
4. How much metal per cast? Long drops need a grade and size that will not blind before the end.

When in doubt, trial two adjacent grades and compare results with your own inclusion measurement. That data beats any generic table.

Priming a ceramic foam filter

Priming means filling the open pores with metal so the whole plate works. The patent stresses that the open porosity must be completely filled with liquid metal. The difficulty is poor wetting: liquid aluminum does not readily enter the ceramic.

Incomplete priming has a cost. According to the same patent, it creates locally higher velocities in the active parts of the filter. Pressure drop rises, and collection efficiency falls.

Metal head

In a conventional box, metal builds up above the ceramic foam filter and gravity forces it through. The patent cites a typical range of priming heads of about 20 to 80 cm. Higher values go with finer grades and are often impractical in existing casthouses.

Watch for signs of a poor prime. Metal level in the box keeps rising at the start, flow through the plate stays slow, and the spent plate shows dry patches. Any of these means part of the filter area did no work.

Preheating

Filters are normally preheated to help metal flow into the media. Preheat the plate in place, with the box, so both are hot when metal arrives. A cold box chills the first metal and makes priming harder.

Research continues on other priming methods. A KTH doctoral thesis studied 30, 50 and 80 PPI alumina filters. It notes that priming with electromagnetic forces has recently been shown to be possible. It also found that stacks of filters needed about three times the pressure gradient of a single plate.

Filter box design and seating

The best grade is wasted if metal flows around it. The patent notes that space must be allowed for gasket material to prevent leakage around the filter. Bypass flow carries unfiltered metal straight into the casting.

Checklist for the filter box:

• Seat each ceramic foam filter on a clean, flat ledge with a gasket around the full perimeter.
• Check plate dimensions and squareness on receipt. An undersized plate cannot seal.
• Size the plate to the pour rate, not only to the box opening.
• Keep the launder flow calm, so the filter is not hit by turbulent metal.
• Drain the box after casting and remove the spent plate while it is still workable.
• Log the metal level before and after the filter. A growing difference shows the plate is loading up.
• Never reuse a plate between casts.

The box itself is part of the system. Well-built launders and ladles keep flow steady from the furnace to the box.

Fiberglass mesh and other alternatives

Ceramic foam is not the only option. The right tool depends on where in the process you filter.

The Aluminium Guide summary describes fiberglass filters as moderate, aimed mainly at larger inclusions above 100 micrometers. Woven fiberglass filters and mesh are easy to cut and place in molds and sprues. For in-line casthouse filtration, ceramic foam filters remain the usual choice.

Filtration also works best on metal that was treated first. Degassing with a well-maintained rotor floats out part of the oxides. Our article on graphite rotors for aluminum degassing covers that step.

Ceramic foam filter FAQ

Why does the filter fail to prime?

Usually low metal head, a cold box or filter, or a grade too fine for the setup. Check all three before changing supplier.

Can we use a filter for more than one cast?

No. A spent plate is loaded with inclusions and frozen metal. Reuse risks bypass and contamination.

What should we send for a quote?

Plate dimensions and bevel, PPI, ceramic type, alloy, pour rate and monthly consumption.

Looking for a ceramic foam filter or fiberglass mesh to your specification? AL-LINX quotes each item to your drawing, and several items can share one request. Request a quote for ceramic foam filters and fiberglass mesh.