If you’ve ever shopped for ceramic foam filters for your aluminum foundry, you’ve seen the term PPI – pores per inch. It’s one of the first specifications suppliers ask for. But what does it actually mean for your castings?
Choosing the wrong PPI can lead to problems:
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Too coarse – inclusions pass right through, and you still have defects.
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Too fine – filter clogs quickly, flow slows down, and you risk misruns or cold shuts.
This article focuses on aluminum casting. For iron and steel, similar principles apply, but the recommended PPI ranges may differ.
What Is PPI?
PPI stands for pores per inch. It’s a measurement of the number of open cells in a straight line of one inch of filter material.
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A 10 PPI foam filter has about 10 pores per inch → larger openings
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A 20 PPI foam filter has about 20 pores per inch → medium openings
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A 30 PPI foam filter has about 30 pores per inch → smaller openings
Important: PPI is not the same as pore diameter in microns, but generally, higher PPI means smaller individual pore openings.
Three PPI Grades Compared
10 PPI – Coarse Filtration
| Property | Typical Value |
|---|---|
| Average pore opening | ~2.0 – 3.0 mm |
| Flow resistance | Low |
| Inclusion capture | Large particles only (slag, some sand) |
| Typical applications | Large aluminum castings (>100 kg), very high flow rate requirements |
When to consider 10 PPI:
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Very large castings where flow rate is critical
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Extremely clean melt – you only need to catch occasional large slag
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When using a single filter for a very high throughput
Limitation: Does not capture fine oxide films or small dross particles.
20 PPI – Medium Filtration (Most Common)
| Property | Typical Value |
|---|---|
| Average pore opening | ~1.0 – 1.8 mm |
| Flow resistance | Moderate |
| Inclusion capture | Balance of large and medium inclusions, some fine oxides |
| Typical applications | Most general aluminum castings (10 – 100 kg) |
Why 20 PPI is the “workhorse”:
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Provides good inclusion removal without excessive flow restriction
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Works well for engine blocks, wheels, transmission housings
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Acceptable for both gravity and low-pressure die casting
When to choose 20 PPI: For the majority of your aluminum castings, start here. It’s the safest default choice.
30 PPI – Fine Filtration
| Property | Typical Value |
|---|---|
| Average pore opening | ~0.5 – 1.0 mm |
| Flow resistance | Higher |
| Inclusion capture | Fine oxides, small dross, most inclusions |
| Typical applications | Thin‑wall castings, high‑integrity components (aerospace, critical automotive) |
When to consider 30 PPI:
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Small, thin‑wall castings where oxide films are a major defect risk
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High‑purity alloys (e.g., 99.99% purity requirements)
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Components with fine internal passages (e.g., oil galleries)
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You already have very clean melt and just need the final polish
Limitation: Requires clean melt – if your melt is dirty, 30 PPI will clog quickly. Also requires higher metal head pressure to maintain flow.
How PPI Affects Filtration Efficiency
Filtration efficiency is the percentage of inclusions removed from the melt. It depends on both the filter and the inclusion size.
General Relationship
| Inclusion Size | 10 PPI | 20 PPI | 30 PPI |
|---|---|---|---|
| > 1 mm | High | High | High |
| 0.5 – 1 mm | Low‑Medium | High | High |
| 0.1 – 0.5 mm | Very Low | Medium | High |
| < 0.1 mm (fine oxides) | Negligible | Low | Medium‑High |
Key takeaway: Finer PPI captures smaller inclusions, but at the cost of flow.
The Flow Rate Trade‑off
Pressure drop across the filter increases as PPI increases. For the same filter area and same metal head:
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10 PPI: Lowest pressure drop, fastest flow
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20 PPI: Moderate pressure drop, acceptable for most gating systems
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30 PPI: Higher pressure drop – requires larger filter area or higher metal head to maintain same flow rate
If you switch from 20 to 30 PPI, increase filter area by about 25‑30% to maintain the same flow.
How to Choose the Right PPI for Your Casting
Step 1 – Consider Your Casting Size and Section Thickness
| Casting Type | Recommended PPI | Reason |
|---|---|---|
| Large, heavy sections (>50 kg) | 10 or 20 | Flow priority |
| Medium, general (5–50 kg) | 20 | Balanced |
| Small, thin‑wall (<5 kg) | 20 or 30 | Inclusion control priority |
Step 2 – Consider Your Melt Quality
| Melt Condition | Recommended PPI |
|---|---|
| Very clean (degassed, skimmed, filtered before transfer) | 20 or 30 |
| Average (typical foundry practice) | 20 |
| Dirty (visible slag, heavy dross) | 10 (or clean the melt first) |
If your melt is dirty, a 30 PPI filter will clog almost immediately. Always deal with melt cleanliness before choosing a fine filter.
Step 3 – Consider Your Gating System
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Low metal head (<100 mm): Use coarser PPI (10 or 20) to avoid slow filling.
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High metal head (>200 mm): Finer PPI (20 or 30) can work with adequate pressure.
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Large filter area: Allows finer PPI without restricting flow.
Common Mistakes
| Mistake | Consequence | Correction |
|---|---|---|
| Using 30 PPI for everything | Frequent clogging, slow fills | Match PPI to casting needs |
| Using 10 PPI for critical castings | Inclusions still present | Go to 20 or 30 PPI |
| Changing PPI without adjusting filter area | Flow problems | Increase area when using finer PPI |
| Assuming higher PPI = always better | Not true – trade‑off exists | Consider melt quality and gating |
Practical Advice from the Foundry Floor
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Start with 20 PPI for new castings. It works for most aluminum applications. Then adjust based on results.
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If you see oxide films after machining, try 30 PPI on a trial batch – but also check your gating for turbulence.
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If the filter clogs before the mold fills, either use 20 PPI (coarser) or increase filter area.
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For low‑pressure die casting (LPDC), 20 PPI is common. 30 PPI can work with good melt cleanliness and proper gating design.
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For gravity casting, 20 PPI is the standard starting point.
Frequently Asked Questions
Q1: Can I mix different PPI filters in the same gating system?
A: Yes. Some foundries use a coarser filter (10 PPI) upstream to catch large slag, followed by a finer filter (20 or 30 PPI) closer to the casting. This is more expensive but can extend fine filter life.
Q2: How do I know if my PPI is wrong?
A: Two signs: (1) Inclusions remain → PPI probably too coarse. (2) Slow filling or clogging → PPI too fine or filter area too small.
Q3: Does filter thickness matter with PPI?
A: Yes. A thicker filter has more depth, which improves inclusion capture, especially for finer PPI. But thickness also increases pressure drop.
Q4: What PPI do most aluminum wheel casters use?
A: 20 PPI is the most common for wheels. Some use 30 PPI for very high‑quality wheels, but they also maintain very clean melt.
Q5: Can I use 10 PPI for aluminum?
A: Yes, but only for very large, non‑critical castings where flow is the main concern. For quality‑sensitive parts, 10 PPI is usually too coarse.
Conclusion
Choosing the right PPI is a balance between filtration efficiency and flow rate.
| If you need… | Choose PPI |
|---|---|
| Maximum flow, less critical quality | 10 PPI |
| Best balance for most castings | 20 PPI |
| Maximum inclusion capture, fine oxides | 30 PPI (with adequate filter area) |
For the majority of aluminum foundries, 20 PPI is the safest starting point. Adjust finer or coarser based on your specific casting size, melt cleanliness, and defect history.
At SF-Foundry, we supply high‑quality alumina ceramic foam filters in 10, 20, and 30 PPI, as well as custom sizes and shapes. Our technical team can help you select the right PPI for your casting application.
Contact us:
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Email: info@sf-foundry.com
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Technical Support: +8618636913699
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Website: www.sf-foundry.com
For application‑specific recommendations, please consult with our technical team.