You’re standing in front of a rack of filters. Both are silicon carbide foam filter. Both look similar. One says 10 PPI. One says 20 PPI. Which one goes into your mold?
Pick wrong, and you’ll either have inclusions ruining your machined surfaces or a filter that clogs halfway through the pour, leaving you with a misrun and a pile of scrap.
This guide cuts through the confusion. Here’s exactly when to grab the 10 PPI filter and when to reach for the 20 PPI.
What 10 PPI and 20 PPI Mean (In Plain English)
PPI stands for pores per inch – literally how many open cells fit in one linear inch of filter material. A 10 PPI filter has about 10 pores per inch, making the openings wide and open. A 20 PPI filter packs in 20 pores per inch, so the openings are smaller and finer.
Think of it like gravel vs. sand:
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10 PPI is like coarse gravel – big gaps, moves fast, catches only big debris.
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20 PPI is like fine gravel – still gets good flow but snags smaller particles that slip through coarse filter.
The 10 PPI filter gives you speed. It flows fast, handles high metal head, and resists clogging. But if you’re trying to catch fine dross, it’ll miss a lot.
The 20 PPI filter cleans better. It catches medium and fine inclusions that a coarse filter would let through. But it restricts flow more and clogs faster on dirty melts.
10 PPI vs. 20 PPI Foam Filter – Side by Side
| Factor | 10 PPI | 20 PPI |
|---|---|---|
| Pore size | ~2.5 mm (wide) | ~1.2 mm (medium) |
| Flow rate | High – fast fill, minimal pressure drop | Moderate – slower fill, noticeable restriction |
| Filtration fineness | Coarse – catches large slag, sand, big dross | Medium – traps medium inclusions, some fine dross |
| Clogging tendency | Low – resists blocking even with dirty melt | Moderate – can clog if melt has heavy dross |
| Best for | Ductile iron, large gray iron, steel | Small gray iron, malleable iron |
| Filter capacity | ~2 kg/cm² for ductile iron | ~4 kg/cm² for gray iron |
The Real Choice – Ductile Iron vs. Gray Iron
Here’s where most foundries make costly mistakes. The same filter does not work for both alloys.
For Ductile Iron – Choose 10 PPI
Ductile iron contains magnesium. That magnesium reacts with oxygen and sulfur to create magnesium oxide, magnesium sulfide, and other reaction products. These form sticky, abundant dross. If you try to filter it through a 20 PPI filter, the dross will build up on the face almost immediately. The filter clogs. Flow stops. The pour fails.
That’s why industry practice recommends 10 PPI for ductile iron. The larger openings give the sticky dross room to pass through while still catching the big, damaging inclusions. One source puts it plainly: “Ductile iron castings use 10~15PPI filters – larger apertures are needed because ductile iron melts often contain more slag; smaller pores would clog easily.”
For Gray Iron – Choose 20 PPI
Gray iron is different. It has less slag and the inclusions are finer. A 10 PPI filter would let many of those finer particles pass right through, leaving you with inclusion defects on your machined surface. A 20 PPI filter catches those medium-size inclusions without restricting flow too much.
“Gray iron castings use 10~20 PPI filters – smaller apertures are suitable here since gray iron melts typically have less slag, allowing for finer filtration.”
Multiple sources confirm this distinction: gray iron works with 10, 15, or 20 PPI, while ductile iron performs optimally with 10 or 15 PPI, and you should generally avoid 20 PPI. One summary puts it bluntly: “General gray iron uses 20ppi, ductile iron uses 10ppi.”
Don’t Forget Size – The Filter Capacity Rule
Choosing PPI is only half the decision. You also need enough filter area.
For the same casting weight, a 10 PPI filter can handle roughly double the metal of a 20 PPI filter. That’s not speculation – it’s a standard industry spec: gray iron filter capacity is about 4 kg/cm², ductile iron is about 2 kg/cm².
Here’s the calculation:
| Alloy | Capacity per cm² | Example (100 kg casting) |
|---|---|---|
| Gray iron (20 PPI) | 4 kg/cm² | Need at least 25 cm² filter area |
| Ductile iron (10 PPI) | 2 kg/cm² | Need at least 50 cm² filter area |
Do not just drop a 10 PPI filter into your ductile iron pour expecting the same performance as your 20 PPI filter on gray iron. Adjust the size. Calculate the area. Then test.
Installation – What Actually Matters for Iron
Filter material: Silicon carbide (SiC). Not alumina. SiC has high thermal conductivity, good abrasion resistance, and stands up to iron’s aggressive melt.
Preheat: For iron, preheat to 300-400°C. Skip this step and you’ll likely crack the filter when hot metal hits it.
Filter thickness: Cast iron filters should be 15-22mm thick. This gives enough depth to trap inclusions without over-restricting flow.
Placement: Put the filter in the runner, as close to the casting cavity as possible, and avoid the metal stream hitting it directly. When pouring iron, follow the 4 kg/cm² rule for gray iron and 2 kg/cm² rule for ductile iron, staying within or below these limits to avoid overloading the filter.
Common Mistakes That Kill Your Castings
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Using 20 PPI on ductile iron. That’s the biggest one. The filter clogs, you get a misrun, and you blame the filter instead of your PPI selection.
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Using 10 PPI on small gray iron castings. You’ll see inclusions on your machined surface and wonder why. Your filter was too coarse.
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Using the same filter size for both alloys. If you switch from 20 PPI gray iron to 10 PPI ductile iron without increasing filter area, you’ll overload the filter. Clogs. Fail. Scrap.
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Skipping preheat. Cold SiC plus 1400°C iron equals cracked filter. Every time.
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Placing filter directly under the sprue. The metal hits it like a hammer. Place it in the runner.
Quick Decision Guide
Grab the 10 PPI filter if:
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You’re casting ductile iron (standard recommendation)
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You’re casting large gray iron parts
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Your melt has heavy dross or slag
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You need maximum flow rate
Grab the 20 PPI filter if:
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You’re casting gray iron (standard recommendation)
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You’re casting small gray iron parts
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Your melt is relatively clean
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You need finer inclusion removal
Still not sure? Start with 10 PPI for ductile iron, 20 PPI for gray iron, test one casting line, and adjust based on your scrap numbers. That’s how you get it right.
Need Help Sizing Your Filter?
Not sure which PPI to use for your specific casting – or how much filter area you need? We can help.
Contact SF-Foundry Technical Support:
Email: info@sf-foundry.com
Phone / WhatsApp: +86 18636913699
Website: www.sf-foundry.com