Que signifie réellement “ 20 PPI ” dans le domaine des filtres en mousse céramique ?
PPI signifie Pores par pouce — littéralement, le nombre d'ouvertures de pores que l'on compterait sur une ligne d'un pouce de long à la surface du filtre. Une mousse filtrante de 20 PPI comporte environ 20 pores par pouce linéaire, ce qui correspond à un diamètre moyen des pores d'environ 1,0–1,2 mm.
Cela peut sembler être un détail insignifiant, mais cela détermine pratiquement tous les aspects des performances du filtre : la vitesse à laquelle le métal s'écoule, la taille des particules retenues et la hauteur de pression métallostatique nécessaire pour maintenir un débit régulier.
Voici une comparaison rapide pour mettre en perspective la valeur de 20 PPI :
| Catégorie PPI | Taille moyenne des pores (mm) | Application type | Résistance à l'écoulement |
|---|---|---|---|
| 10 PPI | 2.0–2.5 | Fonte, alliages non ferreux lourds | Très faible |
| 20 PPI | 1.0–1.2 | Moulage général de l'aluminium, moulage sous pression par gravité | Faible à modéré |
| 30 PPI | 0.7–0.9 | Composants de précision en aluminium, moyeux de roue | Modéré |
| 40 PPI | 0.5–0.6 | Pièces moulées en aluminium de qualité aérospatiale, à parois minces | Élevé |
Comme vous pouvez le constater, Filtres en mousse céramique 20 PPI se situent exactement au milieu — suffisamment polyvalentes pour permettre le traitement d’une vaste gamme d’alliages d’aluminium sans restreindre le débit ni nécessiter une pression de refoulement excessive.
Pourquoi la mousse filtrante de 20 PPI est-elle si largement utilisée dans les fonderies d'aluminium ?
Entrez dans n'importe quelle usine de moulage d'aluminium produisant des pièces automobiles, des dissipateurs thermiques ou des profilés structurels, et il y a de fortes chances que vous trouviez des filtres PPI empilés près du four. Cette popularité s'explique par des raisons pratiques.
Il élimine les inclusions qui posent réellement problème
L'aluminium fondu est réactif. Dès qu'il entre en contact avec l'air, un mince film d'oxyde d'alumine (Al₂O₃) se forme à sa surface. Lors de la coulée, les turbulences brisent ce film en fragments — généralement de 50 à 500 microns — qui sont entraînés dans la cavité du moule. Ces inclusions non métalliques agissent comme des concentrateurs de contraintes dans la pièce finie, ce qui entraîne l'apparition de fissures sous contrainte ou lors de l'usinage.
Une mousse filtrante de 20 PPI retient les particules d'une taille allant jusqu'à environ 80 à 100 microns grâce à une interception directe, et les inclusions encore plus fines (20 à 50 microns) sont piégées par filtration par gâteau — où la couche initiale de débris retenus devient elle-même un milieu filtrant secondaire. En pratique, un filtre de 20 PPI fonctionnant correctement réduit la teneur en inclusions de 60–80%, en fonction de la pureté de l'alliage et de la température de coulée.
Cela représente une amélioration considérable de la qualité de moulage, sans pour autant compliquer votre système d'alimentation.
Cela n'entrave pas votre débit de versement
C'est plus important qu'on ne le pense. Un filtre trop fin pour l'application génère une contre-pression, ralentit l'écoulement et peut entraîner défauts d'impression ou soudures à froid — en particulier dans le cas des pièces moulées à parois minces, où le métal doit se solidifier rapidement avant de perdre sa surchauffe.
Avec une ouverture moyenne des pores d'environ 1,1 mm, les filtres en mousse de 20 PPI permettent à l'aluminium de s'écouler à 1,0 à 2,5 kg/s à travers une tuile filtrante standard de 75 × 75 × 22 mm, ce qui est largement suffisant pour la plupart des installations de coulée par gravité et à basse pression.
J'ai moi-même vu des fonderies passer de 30 PPI à 20 PPI dans le seul but d'éliminer les problèmes de temps de remplissage sur les pièces moulées de grande taille destinées aux boîtiers — et les taux d'inclusion sont restés largement dans les limites des spécifications.
Il fonctionne sur une large plage de températures
Les alliages d'aluminium sont généralement coulés entre 680 °C et 760 °C, selon la famille d'alliages (systèmes Al-Si hypoeutectiques, eutectiques ou hypereutectiques). Un filtre en mousse céramique d’alumine de qualité 20 PPI — présentant une teneur en Al₂O₃ comprise entre 80 et 85% ou supérieure — résiste à cette plage de températures sans subir de fissuration due au choc thermique ni de dégradation chimique.
Certains filtres de qualité inférieure utilisent des systèmes de liants moins coûteux qui commencent à se dégrader au-delà de 730 °C. Si vous utilisez de l’A356 ou de l’A319 avec une surchauffe plus élevée pour améliorer la fluidité, assurez-vous que votre fournisseur de filtres précise un température de fonctionnement d'au moins 1 100 °C dont la teneur en alumine est supérieure à 80%.
Comment fabrique-t-on un filtre en mousse céramique de 20 PPI ?
Comprendre le processus de fabrication permet d'expliquer pourquoi la qualité des filtres varie autant d'un fournisseur à l'autre. Voici ce qui se passe concrètement :
Étape 1 — Préparation du gabarit en mousse
Un bloc de mousse de polyuréthane à cellules ouvertes — déjà découpé aux dimensions requises et présentant l'indice PPI approprié — sert de structure porteuse.
Étape 2 — Imprégnation par suspension
The foam is dipped into a carefully formulated alumina-based ceramic slurry. The composition typically includes Al₂O₃ powder, SiO₂ as a sintering aid, and organic binders for green strength. Getting the slurry viscosity right is critical: too thin and the coating won’t stick; too thick and it blocks the pores.
Step 3 — Excess Slurry Removal
The coated foam passes through rollers or a centrifuge to squeeze out excess slurry, ensuring uniform wall thickness across the struts.
Step 4 — Drying & Sintering
The coated foam is dried, then fired in a kiln at 1500–1680°C. During firing, the polyurethane template burns out completely, leaving behind a rigid, interconnected ceramic skeleton — the open-cell structure that defines the filter’s performance.
Step 5 — Quality Inspection
Finished filters are checked for dimensional accuracy, pore uniformity, crush strength (typically ≥1.0 MPa for a 20 PPI filter), and surface defects. Any filter with blocked pores, cracks, or uneven coating gets rejected.
The entire process takes roughly 24–48 hours from raw foam to finished filter. Cheap filters often cut corners at the slurry stage or use lower sintering temperatures, which results in weak struts that fracture during priming and release ceramic particles directly into your melt — the exact opposite of what you want.
What Size 20 PPI Filter Do I Need for My Casting?
This is one of the most common questions, and honestly, a lot of foundries get it wrong. They either oversize the filter (wasting money) or undersize it (restricting flow or letting it clog mid-pour).
The general rule: filter area should be 4–6× the choke area of your gating system. But for practical purposes, here’s what works for most aluminum casting applications:
| Casting Weight (kg) | Recommended Filter Size (mm) | Filter Thickness (mm) | Remarques |
|---|---|---|---|
| 0.5–3 | 50 × 50 | 22 | Small parts, gravity permanent mold |
| 3–10 | 75 × 75 | 22 | Mid-size castings, tilt pour setups |
| 10–30 | 100 × 100 | 22 | Structural components, wheel blanks |
| 30+ | 150 × 150 or custom | 25–30 | Large plate castings, ingot filtration |
Round filters (Ø50, Ø75 mm) are also available and preferred in some automated pouring systems where the filter seat is cylindrical. If your gating system uses a round sprue, ask your supplier about round 20 PPI ceramic foam filter options — they seat more reliably and reduce the risk of bypass.
One thing worth noting: thickness matters more than most people think. A 22 mm thick filter gives the molten aluminum more contact time with the ceramic struts, improving both filtration efficiency and flow stabilization compared to a thinner 15 mm filter of the same PPI. If you’re experiencing marginal quality, try stepping up to 25 mm thickness before jumping to a finer PPI.
When Should You Choose 20 PPI Over 30 PPI or 10 PPI?
This is where real-world experience beats catalog specs every time.
Choose 20 PPI when:
- You’re casting general-purpose aluminum components (A356, A380, A319, 6061)
- Your inclusion defect rate is moderate — say 2–5% scrap from porosity or hard spots
- You need a balance between filtration and flow, especially on gravity die casting et sand casting setups
- You want a single filter grade that works across multiple part numbers without re-engineering the gating
Step up to 30 PPI when:
- You’re producing safety-critical or pressure-tight parts (brake calipers, hydraulic manifolds)
- The end customer specifies X-ray or CT inspection with strict inclusion size limits
- Your melt treatment (degassing, fluxing) is already excellent, and the filter is your final polishing step
Drop down to 10 PPI when:
- You’re filtering iron or copper-based alloys with higher viscosity
- The primary goal is flow stabilization rather than inclusion removal
- You’re dealing with very large volume pours where back-pressure is a serious concern
I’ve talked with foundry engineers who run 30 PPI on everything “just to be safe,” only to find that the higher flow resistance causes turbulence at the filter exit — reintroducing oxide films downstream of the filter. More isn’t always better. For the vast majority of aluminum casting, 20 PPI gives you the best results with the fewest headaches.
Does Filter Placement Affect 20 PPI Performance?
Absolutely — and this is something that often gets overlooked during gating design.
Horizontal vs. Vertical Placement
La plupart filtres en mousse céramique place the filter horizontally in a filter print (a recessed pocket in the runner). This is the standard approach and works well for gravity-fed systems. The full face of the filter contacts the metal, and the weight of the metallostatic head drives flow through evenly.
Disposition verticale is sometimes used in tilt-pour or low-pressure systems. It can work, but you need to ensure the filter is fully primed before the cavity starts filling — otherwise you get partial bypass and unfiltered metal entering the mold.
How Far From the Casting?
Place the filter as close to the mold cavity as practical — ideally in the runner, just before the ingates. If the filter is too far upstream (e.g., right below the pouring cup), any turbulence or oxide generation that happens between the filter and the cavity won’t be caught.
That said, you need enough runner length after the filter to let the flow re-stabilize. A good target is 2–3× the runner width between filter exit and the first ingate.
Key Material Properties of a Quality 20 PPI Filter Foam
Not all 20 PPI ceramic foam filters are created equal. The physical and chemical properties of the filter material directly impact durability, thermal stability, and whether the filter itself introduces contaminants into your melt.
Here’s what to look for on a supplier’s datasheet:
| Propriété | Target Value | Pourquoi est-ce important ? |
|---|---|---|
| Teneur en Al₂O₃ | ≥ 80% (ideally 85%+) | Chemical resistance to molten aluminum |
| Porosity (open) | 80–90% | Ensures adequate flow-through |
| Résistance à la compression | ≥ 1,0 MPa | Prevents filter breakage during priming |
| Température de fonctionnement | ≥ 1100°C | Safety margin above aluminum pour temps |
| Résistance aux chocs thermiques | Survives ΔT > 800°C | Filter contacts 700°C+ metal from cold start |
| Volume Density | 0.3–0.5 g/cm³ | Indicates proper strut coating thickness |
If a supplier can’t provide these numbers, that’s a red flag. Reputable manufacturers — particularly those with ISO 9001 certification and dedicated foundry-grade production lines — will have full test reports available.
One detail that separates good filters from cheap ones: strut uniformity. Under magnification, a well-made 20 PPI alumina filter shows consistent wall thickness on every strut and clean, open pore windows. A poor-quality filter will have thin spots (weak points) and partially blocked cells (dead zones that reduce effective filtration area).
How to Get the Best Results From Your 20 PPI Filter Foam
After working with various foundries and testing different configurations, here are the practical tips that consistently make the biggest difference:
1. Always preheat the filter before use.
A dry ceramic foam filter dropped into 720°C aluminum will experience a thermal shock of 700°C+ in under a second. Quality filters survive this, but preheating to 200–300°C dramatically reduces the risk of hairline fractures that release ceramic debris. Some automated lines use radiant preheaters built into the filter print — if you have the option, use it.
2. Don’t reuse filters.
This should be obvious, but I’ve seen it happen. Once a filter has been primed with molten aluminum, the pores are partially filled with captured inclusions and solidified metal. Reheating a used filter releases trapped oxides back into the melt. One filter per pour — no exceptions.
3. Monitor your scrap data after switching PPI grades.
When you move from one PPI to another — say from 30 to 20 — track your defect rates for at least 50 pours before drawing conclusions. A single bad pour doesn’t tell you anything; statistical trends do.
4. Match the filter to your melt quality, not the other way around.
A 20 PPI filter isn’t a substitute for proper degassing and dross removal. If your incoming metal is excessively dirty, the filter will clog prematurely, slow the pour, and potentially crack under pressure buildup. Clean your metal first, then let the filter handle the residual inclusions — that’s where it shines.
5. Ensure a proper filter seat.
The filter print (the recessed pocket in the runner) should hold the filter snugly with no gaps around the edges. Any gap allows unfiltered metal to bypass the filter entirely. A clearance of 0.5–1.0 mm per side is typical; anything more and you’re losing filtration effectiveness.
Wrapping This Up
There’s a reason 20 PPI filter foam has become the industry default for aluminum casting filtration. It delivers meaningful inclusion removal — typically capturing particles down to 80 microns and achieving 60–80% cleanliness improvement — while maintaining the flow rates that keep your pour times on target and your fill patterns stable.
It’s not the finest filter available, and it’s not meant to be. It’s the one that works reliably, day after day, across the widest range of aluminum alloys and casting methods. For most foundries producing automotive, industrial, or structural aluminum castings, 20 PPI is where you start — and more often than not, where you stay.
If you’re currently running without filtration, or using a lower-grade filter and seeing persistent inclusion-related scrap, switching to a properly made ceramic foam filter for aluminum in 20 PPI is one of the highest-ROI changes you can make on the shop floor. The cost per filter is negligible compared to the cost of a single scrapped casting — let alone a batch.
Choose a reputable supplier, match the filter size to your pour weight, place it correctly in the gating system, and let the ceramic do its job. That’s really all there is to it.
FAQ
1. What is a 20 PPI filter foam?
A 20 PPI filter foam is a ceramic foam filter with 20 pores per inch, commonly used to clean molten aluminum during casting.
2. What is 20 PPI filter foam used for?
It is mainly used to remove inclusions, reduce turbulence, and improve casting quality in aluminum foundries.
3. Is 20 PPI good for aluminum casting?
Yes. 20 PPI is one of the most widely used grades for aluminum casting because it offers a good balance between filtration efficiency and metal flow.
4. How does a 20 PPI ceramic foam filter work?
It traps oxide films, slag, and other non-metallic inclusions while allowing molten aluminum to flow smoothly through the filter.
5. What size 20 PPI filter foam should I choose?
The right size depends on casting weight, gating design, and flow rate. Common sizes include 50×50 mm, 75×75 mm, and 100×100 mm.
6. What material is 20 PPI filter foam made of?
For aluminum casting, it is usually made of high-purity alumina (Al2O3) for strong thermal shock resistance and chemical stability.
7. Can 20 PPI filter foam improve casting quality?
Yes. It helps reduce inclusions, improves surface finish, and lowers the risk of porosity and machining defects.
8. What is the difference between 10 PPI and 20 PPI filter foam?
A 20 PPI filter has smaller pores than 10 PPI, so it provides finer filtration but slightly higher flow resistance.
9. Can 20 PPI filter foam be reused?
No. Ceramic foam filters are designed for single use only.
10. Why is 20 PPI filter foam so popular?
Because it is versatile, cost-effective, and suitable for many aluminum casting applications without causing excessive flow restriction.