Selecting the right pore density—measured in PPI (pores per inch)—is one of the most common decisions aluminum foundries face when specifying ceramic foam filters. Choose too coarse, and fine inclusions pass through. Choose too fine, and the filter chokes the flow, causing cold shuts and incomplete fills. This guide breaks down the practical selection logic for alumina (Al₂O₃) ceramic foam filters in aluminum casting, based on real foundry experience.
Why PPI Matters in Aluminum Filtration
Alumina ceramic foam filters work by depth filtration: molten aluminum flows through a three-dimensional network of ceramic walls, and inclusions smaller than the pore diameter are deposited on the filter walls as the metal passes through. The pore density directly determines two competing outcomes:
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Filtration efficiency: finer pores capture smaller inclusions
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Flow capacity: finer pores restrict metal flow, requiring higher pouring temperatures or larger filter cross-sections
Finding the right balance depends on your alloy, casting geometry, inclusion load, and pouring system design.
10 PPI: Coarse Filtration for Large Castings
10 PPI alumina filters are best suited for:
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Large aluminum castings with thick sections (automotive wheels, large housings)
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High pouring volumes where flow rate is critical
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Melts with heavy dross or large oxide film contamination
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Gating systems with limited filter cross-sectional area
The coarse pore structure allows high flow rates with minimal pressure drop. However, 10 PPI filters will not capture fine inclusions below approximately 200–300 microns. If surface finish or machinability is critical, a finer grade is required.
20 PPI: The General-Purpose Workhorse
20 PPI is the most widely used pore density in aluminum foundries for good reason:
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Captures a broad range of inclusion sizes (approximately 100–200 microns)
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Maintains adequate flow for most gravity and low-pressure casting setups
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Works well across a wide range of aluminum alloys (A356, A360, ADC12, etc.)
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Provides a practical balance between filtration efficiency and flow restriction
For general-purpose aluminum casting—engine blocks, transmission housings, structural components—20 PPI alumina ceramic foam filters are the standard recommendation. They align with the standard practice for typical oxide inclusion removal in aluminum foundries.
30 PPI: Fine Filtration for Precision Castings
30 PPI alumina filters are specified when casting quality demands are highest:
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Thin-wall castings and complex geometries where surface finish matters
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Aerospace and high-integrity structural components
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Alloys with stringent inclusion specifications (e.g., A356-T6 for safety-critical parts)
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Applications where machining defects from hard inclusions are unacceptable
The trade-off: 30 PPI filters require higher pouring temperatures (typically 720–760°C) and larger filter surface areas to maintain fill rates. They also clog faster and need more frequent replacement.
Selection Quick Reference
| PPI | Best For | Approx. Inclusion Capture | Flow Restriction | Typical Alloys |
|---|---|---|---|---|
| 10 PPI | Large castings, high flow | >200 μm | Low | General aluminum |
| 20 PPI | General purpose | 100–200 μm | Moderate | A356, ADC12, A360 |
| 30 PPI | Precision, thin-wall | 50–100 μm | High | Aerospace, safety-critical |
Practical Tips from the Foundry Floor
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Preheat the filter to 300–400°C before use. Cold filters cause thermal shock cracking and incomplete metal flow.
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Size the filter cross-section at 2–4× the runner area. The filter’s flow resistance means a larger surface area is needed to maintain fill rates.
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Place the filter as close to the ingate as practical. The closer to the mold cavity, the better the filtration effectiveness.
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Match PPI to alloy cleanliness. If you’re using high-quality ingot with in-line degassing and filtration at the smelter, a coarser filter may suffice. Recycled or lower-grade charge stock benefits from finer filtration.
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Monitor pressure drop. If pouring time increases significantly with a new filter batch, the PPI or filter area may need adjustment.
When to Consider SiC Instead
Alumina (Al₂O₃) filters are the standard choice for aluminum because they resist attack from molten aluminum and match the chemistry of typical oxide inclusions. Silicon carbide (SiC) filters are sometimes used in aluminum for high-temperature applications, but SiC can react slowly with molten aluminum at casting temperatures and is generally specified for iron and copper alloys. For the vast majority of aluminum casting operations, alumina ceramic foam filters are the correct material choice.
Need Help Selecting the Right Filter?
SF-Foundry manufactures alumina ceramic foam filters in 10, 20, and 30 PPI, available in standard and custom sizes. If you’re unsure which PPI fits your application, send us your alloy, casting weight, pouring temperature, and gating system details—our technical team will recommend the right specification.
Email: info@sf-foundry.com
WhatsApp: +86 13785803358