Ceramic Foam Filters in Metal Casting: A Simple Guide

Many casting defects start with dirty metal. Oxide films, bits of slag, sand and refractory get carried into the mould and end up inside the part as inclusions (small foreign particles trapped in the metal). They weaken the casting, cause leaks, and make machining harder. A ceramic foam filter is a simple, low-cost way to stop most of them.

What a ceramic foam filter is

It is a block of ceramic with an open, sponge-like structure. It is made by coating a plastic foam with a ceramic slurry, then firing it. The plastic burns away and leaves a ceramic copy of the foam: a network of connected holes.

When molten metal flows through, dirt is caught in three ways:

1. Sieving: large particles cannot pass through the holes.
2. Sticking: small particles touch the ceramic walls and stay there.
3. Cake filtration: caught particles form a layer that then catches even finer ones.

The filter also calms the flow. Metal that leaves the filter moves more smoothly, so it makes fewer new oxides as it fills the mould.

Choosing the material

The filter must survive the metal’s temperature and not react with it.

Filter material	Metals	Notes
Alumina	Aluminium and its alloys	The most common type
Silicon carbide	Grey iron, ductile iron, copper alloys	Strong at iron pouring temperatures
Zirconia	Steel, stainless steel, superalloys	For the highest temperatures

Choosing the pore size (PPI)

PPI means pores per inch: how many holes you count along one inch (25 mm) of the filter.

• 10 PPI: big holes, fast flow. Used for large castings and dirty metal.
• 20 PPI: a balance between flow and cleaning. A common starting point.
• 30 PPI and up: small holes, fine cleaning, slower flow. Used for thin-walled or high-quality parts.

If the filter is too fine for the amount of metal, the flow slows too much or the filter blocks. If it is too coarse, small inclusions pass through.

Size and placement

• Size: the filter area must be large enough for the weight of metal and the pouring time. Too small an area blocks early.
• Placement: put the filter in the gating system (the channels that lead metal into the mould), usually in a filter print after the sprue. The metal should fill the space before the filter, then pass through it evenly.
• Seating: the filter must sit tight in its print, so no metal can flow around the edges.
• Priming: a filter needs some metal pressure (head) to start flowing. Make sure the design gives enough.

Common mistakes

• Storing filters in damp places: moisture can cause gas defects.
• Using an iron filter for steel, or an aluminium filter for iron.
• Breaking filter edges when placing them in the mould.
• Choosing a fine PPI for a heavy pour and blocking the filter.

Results you can expect

Foundries that start filtering usually see fewer rejects from inclusions, better machined surfaces and more consistent castings. The filter is a small cost compared with one scrapped part.

How we can help

Tell us the metal, the pouring temperature, the casting weight and the filter size you use now. We will suggest the material, size and PPI. See our ceramic foam filters or contact us.