Over the years, we’ve talked with hundreds of iron foundries about silicon carbide (SiC) ceramic foam filters. Some questions come up again and again. Here are the answers to the ten we hear most often – practical, no‑nonsense, and based on real foundry experience.
1. Why should I use a SiC foam filter instead of fiberglass mesh for iron casting?
Fiberglass mesh works like a strainer – it catches large particles on its surface but lets finer dross and oxides slip through. For small, non‑critical gray iron castings, that may be enough. For ductile iron or any casting where quality matters, you need depth filtration. SiC foam traps inclusions throughout its three‑dimensional structure, capturing the fine, sticky dross that mesh misses.
Bottom line: Mesh for low‑cost, low‑quality gray iron. SiC foam for everything else.
2. What PPI should I use for ductile iron vs. gray iron?
This is the most common question – and getting it wrong causes most filter problems.
| Alloy | Recommended PPI | Why |
|---|---|---|
| Ductile iron (standard) | 10–15 PPI | Sticky, abundant dross needs larger pores to avoid clogging |
| Ductile iron (high quality, thin‑wall) | 15–20 PPI | Finer filtration; must increase filter area to maintain flow |
| Gray iron | 15–20 PPI | Gray iron has less slag; can use finer pores |
Golden rule: When in doubt, start with 10–15 PPI for ductile iron and 15–20 PPI for gray iron. Then adjust based on your scrap results.
3. Do SiC filters need to be preheated? If so, to what temperature?
Yes. For iron casting, preheating is mandatory. A cold SiC filter hit by 1400°C iron will experience extreme thermal shock and can crack – sometimes invisibly.
Preheat to 300–500°C for 30–60 minutes. The higher end (400–500°C) is recommended for ductile iron, which pours hotter. Use a dedicated preheat oven, not just a torch.
4. How do I know what size filter (area) I need for my casting?
A common rule of thumb: filter area = 3–5 × choke area (the smallest cross‑section in your gating system). Use the higher multiplier for ductile iron (more dross) and for finer PPI filters (20 PPI).
Example: If your choke area is 10 cm² and you’re pouring ductile iron with a 15 PPI filter, aim for at least 40–50 cm² of filter area (e.g., a 70×70 mm square filter).
If you’re unsure, contact your filter supplier with your casting weight and gating layout – they can help calculate the right size.
5. Can I use the same SiC filter for both gray and ductile iron?
Technically yes – SiC is the correct material for both. But the ideal PPI differs (see question 2). If you standardise on one PPI for both alloys, you’ll compromise either flow (if you choose 10 PPI for gray iron) or dross capture (if you choose 20 PPI for ductile iron).
Best practice: Stock two grades – 10–15 PPI for ductile, 15–20 PPI for gray. Your scrap rate will thank you.
6. Why does my filter sometimes clog before the pour finishes?
Three common causes:
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PPI too fine for your alloy – Ductile iron with 20 PPI will often clog. Switch to 10–15 PPI.
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Filter area too small – Increase size or use multiple filters in parallel.
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Melt extremely dirty – Improve skimming, ladle practice, or charge cleanliness.
Check the used filter. If it’s completely covered in dross, the filter worked – but you need more area or coarser PPI. If it’s clean and the pour failed, the metal may have bypassed (poor sealing).
7. What’s the best way to seal a SiC foam filter in a sand mold?
Metal will always take the path of least resistance. If there’s a gap around the filter, it will flow around – zero filtration.
Best method: Use a ceramic fiber gasket. Place the gasket in the filter seat, then press the filter on top. When the mold closes, the cope compresses the gasket, creating an airtight seal.
Alternative: Apply refractory paste around the edge of the seat before placing the filter. This works well but can be messier.
Do not rely on sand alone. It will wash away.
8. Can I reuse a SiC foam filter?
No. SiC filters are single‑use. After one pour, the filter is full of trapped dross and inclusions, and the ceramic structure has been thermally stressed. Reusing it will likely result in breakage or bypass.
Some foundries try to “clean” and reuse filters – don’t. The small cost savings are not worth the risk of scrapping a whole casting.
9. Where should I place the filter in the gating system?
Place it in the runner, as close to the casting cavity as possible. This minimises reoxidation after filtration.
Do not place it directly under the sprue. The high‑velocity metal stream can erode the filter face, cause premature clogging, or push the filter out of position.
Correct order: Sprue → Sprue well → Runner → [Filter] → Gate → Casting cavity.
For large castings with tall metal heads, consider a horizontal placement with full bottom support.
10. How do I know if my SiC filter is actually working?
Inspect the used filter after shakeout:
| What you see | What it means |
|---|---|
| Heavy, dark, covered in dross | Good – the filter captured many inclusions. |
| Clean and light | Either your melt is exceptionally clean, or metal bypassed (check sealing). |
| Cracked | Thermal shock – preheat more next time. |
| Broken edges | Mechanical stress – check support and handling. |
Also track your scrap rates. If inclusion‑related defects drop significantly after introducing SiC filters, they’re working – even if the used filter doesn’t look dramatic.
Need More Help?
Every foundry is different. If you have a question that’s not on this list – or if you need help sizing a filter for a specific casting – just ask.
Contact SF-Foundry Technical Support:
Email: info@sf-foundry.com
Phone / WhatsApp: +86 18636913699
Website: www.sf-foundry.com