In metal casting, one small specification on a filter datasheet can make or break your casting quality. The PPI rating of a ceramic foam filter — pores per inch — determines what gets filtered out of your melt, and how fast the metal flows through. Get it right, and you see cleaner castings, fewer defects, fewer scrapped parts. Get it wrong, and the problems are hard to trace.
This guide cuts through the confusion around PPI selection. Whether you are dealing with oxide inclusions in aluminum, slag in ductile iron, or flow restrictions in large steel castings, the right PPI choice follows a clear logic — one that most filter guides don't fully explain. As casting quality standards tighten across automotive, aerospace, and industrial sectors, PPI selection is becoming a more deliberate engineering decision, not an afterthought. This guide is written for:
- Foundry engineers responsible for casting quality and defect reduction
- Technical procurement teams sourcing ceramic foam filters
- Process engineers optimizing gating systems and filtration setups
- Quality managers evaluating filter specifications and supplier documentation
Here you will find everything from what PPI actually means, how it compares to pore size in microns, how each rating performs in real casting conditions, and how to verify filter quality on arrival — all in one place. Choosing the right PPI is not guesswork. Read on to make it a decision you can defend.
Table of Contents
- What PPI Actually Means — and Why It Matters in Casting
- PPI vs. Pore Size in Microns: Are They the Same Thing?
- 10, 20, and 30 PPI — What's the Real Difference?
- How to Choose the Right PPI for Your Application
- The Mistake Engineers Make: When Higher PPI Works Against You
- PPI and Your Gating System: A Connection Most Guides Skip
- How to Verify PPI Quality When Receiving a Filter
- Quick Reference — PPI Selection by Alloy and Casting Type
- FAQ
What PPI Actually Means — and Why It Matters in Casting
PPI stands for Pores Per Inch. It measures how many open pores appear along one linear inch of a ceramic foam filter.
Simple to count. Powerful in effect.
How to Picture It
Hold a 10 PPI filter up to the light. You'll see large, clearly visible holes — like a coarse kitchen sponge.
Now do the same with a 30 PPI filter. The structure looks almost solid. The pores are there, but far finer and more tightly packed.
Think of it like window screens:
A 10-mesh screen lets in airflow — and the occasional fly.
A 30-mesh screen catches almost everything — but also slows the breeze.
PPI works the same way. More pores per inch = finer filtration = more flow resistance.
Most ceramic foam filters used in casting fall into three standard ratings. The table below gives you a quick feel for how they differ.
| PPI Rating | Visual Character | Approx. Pore Size | What It Looks Like |
|---|---|---|---|
| 10 PPI | Open, coarse | 1,500 – 2,000 μm | Pores clearly visible to the naked eye |
| 20 PPI | Medium, balanced | 700 – 1,000 μm | Pores visible but noticeably finer |
| 30 PPI | Dense, fine | 300 – 500 μm | Looks almost solid without close inspection |
These aren't just visual differences — each rating behaves differently once molten metal starts flowing through it.
Why It Matters More Than Most Specs on the Datasheet
PPI controls two things every casting engineer cares about.
First: what gets caught.
Every melt carries unwanted passengers — oxide films, slag particles, refractory debris. PPI determines which ones your filter stops, and which ones reach the mold.
A 10 PPI filter catches large inclusions. A 30 PPI filter catches much finer particles — the kind that cause subtle porosity you only discover after machining.
A real example:
A foundry producing aluminum automotive brackets kept seeing fine porosity in machined surfaces. The melt looked clean. The gating looked right.
The filter was 10 PPI — adequate for iron, insufficient for aluminum oxide films.
Switching to 30 PPI eliminated the defect within the first trial pour.
Second: how fast metal moves.
A finer filter slows the metal down. Smaller pores create more resistance.
For a thin-wall casting, slower flow helps — it reduces turbulence and gas entrapment.
For a large, heavy-section casting, slow flow is a problem. Metal cools before the mold fills. You get cold shuts. Misruns.
PPI sits at the center of that tension — filtration vs. flow. Get it right, and both improve. Get it wrong, and you're trading one defect type for another.
Most specs on a filter datasheet describe what the filter is. PPI describes what it does. It's the single number that ties your filter choice directly to your casting outcome.
PPI vs. Pore Size in Microns: Are They the Same Thing?
Short answer: no. But they're closely related — and confusing the two is a common and costly mistake.
PPI is a count. It tells you how many pores fit in one inch.
Microns (μm) measure actual pore diameter. It's a physical size, not a frequency.
Higher PPI generally means smaller pores — but the relationship isn't a fixed formula. Two filters with the same PPI rating can have different pore diameters depending on the manufacturer's production process, slurry coating thickness, and sintering conditions.
Why this matters in practice:
A supplier quotes you "20 PPI." Another quotes "800 μm."
Are they the same filter? Maybe. Maybe not.
Always ask for both specs — especially when switching suppliers or comparing quotes.
Here's a general reference range used across the industry. Treat it as a guide, not a guarantee.
| PPI Rating | Typical Pore Size (μm) | Filtration Level |
|---|---|---|
| 10 PPI | 1,500 – 2,000 μm | Coarse |
| 20 PPI | 700 – 1,000 μm | Medium |
| 30 PPI | 300 – 500 μm | Fine |
| 40–50 PPI | 100 – 300 μm | Ultra-fine |
For most foundry applications, PPI is the working language. Micron values come into play when you need precise inclusion capture targets — particularly for aerospace or hydraulic components where cleanliness specs are defined in writing.
10, 20, and 30 PPI — What's the Real Difference?
On paper, it's just a number. In the foundry, it's the difference between a clean casting and a scrapped one.
10 PPI — Built for Flow
This is the coarsest standard rating. Pores are large, flow resistance is low, and metal moves through quickly.
It catches coarse slag, large refractory fragments, and visible inclusions. Fine oxides and submillimeter particles? Most pass straight through.
Use it when volume matters more than cleanliness — large steel or iron castings where you need fast mold filling and the metal is relatively clean to begin with.
Where it fits: Heavy-section gray iron castings, large steel ingots, applications where pouring speed is critical and fine inclusion control is secondary.
20 PPI — The Workhorse
This is the most commonly specified PPI in foundry practice — and for good reason.
It strikes a workable balance between filtration efficiency and flow rate. Fine enough to catch medium-sized inclusions and oxide clusters. Open enough to handle most standard pouring conditions without premature clogging.
If you're uncertain where to start, most ductile iron and general steel castings land here.
Where it fits: Ductile iron components, general-purpose steel castings, medium-weight aluminum parts where a balanced approach is preferred over extreme filtration.
30 PPI — Built for Cleanliness
This is where filtration becomes serious. Pores are fine, surface area is high, and even small oxide films and non-metallic inclusions get caught before they enter the mold.
The trade-off is real. Flow slows down. Pressure drop increases. If your gating system isn't designed for it, the mold won't fill properly — and you'll blame the metal when the filter was the culprit.
30 PPI requires a cleaner melt, a properly sized filter area, and a gating system with enough metallostatic head to push metal through.
Where it fits: Aluminum alloy castings, thin-wall components, safety-critical parts in automotive or hydraulic systems where internal cleanliness is non-negotiable.
The table below puts all three side by side for a quick comparison.
| PPI | Flow Rate | Filtration Efficiency | Clogging Risk | Best For |
|---|---|---|---|---|
| 10 PPI | High | Low (coarse only) | Low | Large iron / steel castings |
| 20 PPI | Moderate | Medium | Moderate | General purpose, ductile iron |
| 30 PPI | Lower | High (fine inclusions) | Higher | Aluminum alloys, critical components |
None of these ratings is universally "best." The right one depends on your specific application — which is exactly what the next section covers.
How to Choose the Right PPI for Your Application
There's no universal answer. But there is a clear logic — four variables that, taken together, point you to the right PPI almost every time.
Variable 1: Alloy Type
Different metals carry different contamination profiles. That shapes everything.
Aluminum alloys are highly sensitive to oxide films. Even a thin layer of Al₂O₃ causes porosity and surface defects in finished parts. You need finer filtration — typically 20 to 30 PPI.
Gray and ductile iron carry larger slag and graphite inclusions. A 20 PPI filter handles most of it without restricting flow.
Steel castings run at very high temperatures. Flow rate is critical — a filter that slows the metal too much risks misruns. Start at 10 to 20 PPI and adjust based on cleanliness requirements.
| Alloy | Recommended PPI | Primary Concern |
|---|---|---|
| Aluminum alloys | 20 – 30 PPI | Oxide film removal |
| Gray iron | 10 – 20 PPI | Slag and large inclusions |
| Ductile iron | 20 PPI | Balanced inclusion control |
| Steel castings | 10 – 20 PPI | Flow rate at high temperature |
| Copper alloys | 10 – 20 PPI | Depends on casting complexity |
Variable 2: Casting Size and Section Thickness
Larger castings need more metal to flow through the filter, faster. Fine PPI works against you here.
A 500 kg valve body poured through a 30 PPI filter is an experiment in frustration — slow fill, cold metal, likely misruns. The same filter on a 2 kg aluminum bracket works perfectly.
Thin-wall castings have the opposite problem. Metal freezes quickly. You need controlled, smooth flow — which finer PPI actually helps deliver by reducing turbulence at the gate.
| Casting Weight | Recommended PPI |
|---|---|
| Large (> 500 kg) | 10 PPI |
| Medium (50 – 500 kg) | 20 PPI |
| Small / thin-wall (< 50 kg) | 20 – 30 PPI |
Variable 3: Quality Requirements
Not every casting needs the same level of cleanliness. And chasing maximum filtration when it isn't required just adds cost and risk.
Standard commercial castings — agricultural equipment, general machinery — can tolerate moderate inclusion levels. 10 to 20 PPI is sufficient.
Safety-critical components — hydraulic valve bodies, automotive brake parts, aerospace brackets — demand higher internal cleanliness. 30 PPI, paired with proper gating design, is the right call.
A useful rule of thumb:
If the casting will be pressure-tested, X-rayed, or machined to tight tolerances — go finer.
If it's a structural or non-critical component — match PPI to your alloy and size, and don't over-engineer it.
Variable 4: Melt Cleanliness
This one is often overlooked. PPI selection doesn't happen in isolation — it depends on what the metal looks like before it hits the filter.
A clean melt, from good charge materials and careful slag removal, allows you to use finer PPI with a much lower clogging risk.
A dirty melt pushed through a 30 PPI filter will block within seconds. You'll get an incomplete fill and no idea why.
If your melt practice is inconsistent, start coarser. Fix the upstream process first, then refine the PPI.
Bottom line:
PPI is not a substitute for good melting practice. It's the last line of defense — not the first.
The Mistake Engineers Make: When Higher PPI Works Against You
More filtration sounds better. In casting, that instinct gets people into trouble.
Higher PPI doesn't automatically mean better castings. In the wrong conditions, it creates exactly the defects you were trying to prevent.
The Slow Fill Problem
Every time you increase PPI, you increase flow resistance. Metal moves slower. For large or thick-section castings, that slowdown is fatal.
Metal temperature drops during filling. If it drops too far before the mold is full, you get:
- Cold shuts — two streams of metal that meet but don't fuse
- Misruns — sections of the mold that never fill at all
- Surface laps — visible fold lines on the casting surface
A real scenario:
A foundry switched from 20 PPI to 30 PPI on a 380 kg ductile iron pump housing — chasing cleaner metal after a run of porosity complaints.
The porosity didn't improve. Instead, they started seeing cold shuts in the thinner wall sections.
The filter was blocking flow before the mold could fill properly.
They went back to 20 PPI, addressed the melt practice upstream, and the porosity resolved.
The Premature Clogging Trap
Fine PPI filters have a smaller margin for error. If the melt carries more inclusions than expected — from a dirty charge, inconsistent slag removal, or refractory erosion — a 30 PPI filter can block within the first few seconds of pouring.
The result looks like a gating problem. Or a temperature problem. The filter is rarely the first suspect.
Clogging mid-pour means incomplete fill. It means scrap. And it means a diagnosis that's hard to trace after the fact.
When to Resist Going Finer
Ask these questions before increasing PPI:
- Is my casting weight above 200 kg? Fine PPI may restrict fill too much.
- Is my melt practice consistent? Inconsistent melts clog fine filters fast.
- Is my gating system designed for higher pressure drop? If not, increasing PPI without adjusting the system moves the problem, not solves it.
The principle:
Use the coarsest PPI that meets your cleanliness requirement.
Don't use the finest PPI your filter catalog offers.
PPI and Your Gating System: A Connection Most Guides Skip
Choosing the right PPI is only half the job. If your gating system isn't designed around it, even the right filter will underperform.
Pressure Drop and Metallostatic Head
Every filter creates resistance. The metal needs pressure to push through — and that pressure comes from metallostatic head, the height of liquid metal above the filter.
As PPI increases, so does pressure drop. A 30 PPI filter demands significantly more head pressure than a 10 PPI filter to maintain the same flow rate.
If your sprue height doesn't provide enough head, the metal stalls at the filter. The mold fills slowly, or not at all.
Practical implication:
When you upgrade from 20 PPI to 30 PPI, don't just swap the filter.
Review your sprue height and runner cross-sections at the same time.
Filter Area Sizing — the Rule Most Foundries Ignore
Filter area needs to increase as PPI increases. A common industry reference gives this guidance:
| PPI Rating | Minimum Filter Area | Basis |
|---|---|---|
| 10 PPI | 3× choke area | Low resistance, smaller area sufficient |
| 20 PPI | 4× choke area | Moderate resistance, larger area needed |
| 30 PPI | 5× choke area | High resistance, maximum area recommended |
Undersizing filter area is one of the most common reasons a correctly specified PPI still produces defects. The filter is right — the seat is too small.
Where You Place the Filter Matters Too
Position affects both filtration efficiency and flow behavior.
In the runner: The most common placement. Metal flows horizontally through the filter, inclusions are trapped, and turbulence is dampened before the metal enters the gate.
At the base of the sprue: Effective for catching large early-pour inclusions, but the filter sees the highest velocity metal — clogging risk increases with fine PPI.
In the pouring basin: Less common, used when very early contamination control is needed.
General guidance:
For most applications, runner placement with the filter seated flat and fully supported on all edges gives the most consistent results.
An unsupported filter edge allows metal bypass — and bypassed metal carries every inclusion the filter was meant to stop.
How to Verify PPI Quality When Receiving a Filter
A filter labeled "20 PPI" isn't always 20 PPI. Batch variation exists. So does misrepresentation. Knowing how to verify what you've received protects your process.
Step 1: Visual Inspection
Start with your eyes. It costs nothing and catches the most obvious problems.
Hold the filter up to a light source and look through it. You should see:
- Uniform pore distribution — no clusters of large pores in one area and fine pores in another
- Open cell structure — no pores blocked by excess ceramic slurry from coating
- Clean edges — no loose ceramic particles, no crumbling corners
- Consistent color — uneven sintering shows up as color variation across the surface
Quick field test:
Blow lightly through the filter by hand. Air should pass through with moderate resistance — not freely like open air, not blocked like a wall.
A filter that's almost impossible to blow through may have blocked pores from over-coating.
One that offers no resistance at all may be under-sintered and fragile.
Step 2: Dimensional Check
Measure the filter against your specification. Dimensional drift causes seating problems — gaps between filter and seat allow metal bypass.
Acceptable tolerances for a quality filter:
| Dimension | Acceptable Tolerance |
|---|---|
| Length / Width | ± 1.0 mm |
| Thickness | ± 0.5 mm |
| Flatness | No visible warping when placed on a flat surface |
If filters from a new supplier fall outside these ranges, raise it immediately — before they go into production.
Step 3: PPI Count Verification
For critical applications, verify PPI directly. It's straightforward.
Place a ruler along the filter surface. Count the number of open pores in one linear inch. Do this in three locations — center, one edge, and one corner. Average the counts.
A quality filter should read within ± 2 PPI of its stated rating. Variation beyond that suggests inconsistent production — and inconsistent performance in your mold.
Step 4: Ask for Batch Documentation
A reliable supplier provides more than a box of filters. Request:
- PPI verification report for the production batch
- Dimensional inspection records
- Material certificate confirming filter composition (alumina, SiC, or zirconia content)
- Compressive strength test results if the application involves high metallostatic pressure
Suppliers who push back on documentation requests are telling you something important about their process control.
Quick Reference — PPI Selection by Alloy and Casting Type
Use this table as a starting point. Final selection should always account for melt cleanliness, gating design, and quality requirements specific to your operation.
| Alloy | Casting Type / Size | Quality Level | Recommended PPI | Key Reason |
|---|---|---|---|---|
| Gray Iron | Large, heavy section | Standard commercial | 10 PPI | Flow rate priority, coarse slag removal |
| Gray Iron | Medium, general purpose | Standard commercial | 20 PPI | Balanced filtration and flow |
| Ductile Iron | Medium, valve / pump bodies | Moderate to high | 20 PPI | Inclusion control without restricting fill |
| Steel | Large, mining / heavy equipment | Standard | 10 PPI | High temperature, fast fill required |
| Steel | Medium, general investment casting | Moderate | 20 PPI | Cleaner output, manageable pressure drop |
| Aluminum Alloy | Small to medium, general | Standard | 20 PPI | Oxide film removal, smooth flow |
| Aluminum Alloy | Thin-wall, automotive / aerospace | High / critical | 30 PPI | Fine oxide capture, turbulence reduction |
| Copper Alloy | Medium, general | Standard | 10 – 20 PPI | Depends on section thickness and complexity |
How to read this table:
Start with your alloy row. Then adjust one step finer if your quality requirements are critical, or one step coarser if your casting is significantly larger or your melt carries a high inclusion load.
When in doubt, 20 PPI is the safest starting point for most foundry applications.
FAQ
Q1: What does PPI mean in ceramic foam filters?
PPI stands for Pores Per Inch. It measures how many open pores appear along one linear inch of the filter surface. Higher PPI means smaller, more tightly packed pores — finer filtration, but more flow resistance.
Q2: Which PPI is most commonly used in foundries?
20 PPI is the most widely used rating across foundry applications. It offers a practical balance between filtration efficiency and flow rate, making it suitable for ductile iron, general steel, and standard aluminum castings.
Q3: Is higher PPI always better for casting quality?
No. Higher PPI improves inclusion capture but restricts metal flow. For large or heavy-section castings, a filter that's too fine can cause slow filling, cold shuts, or misruns — creating defects rather than preventing them. The right PPI depends on your alloy, casting size, and quality requirements together.
Q4: What's the difference between PPI and pore size in microns?
PPI is a count — how many pores per inch. Microns measure the actual diameter of each pore. They're related but not interchangeable. Two filters with the same PPI can have different pore diameters depending on the manufacturer's process. For critical applications, always request both specifications.
Q5: Why is 30 PPI recommended for aluminum castings?
Aluminum melts oxidize easily, forming thin Al₂O₃ films that cause porosity and surface defects in finished parts. These oxide films are too fine for 10 or 20 PPI filters to catch reliably. 30 PPI provides the finer pore structure needed to capture them — provided the gating system is designed to accommodate the higher flow resistance.
Q6: What causes a ceramic foam filter to clog prematurely?
Premature clogging usually comes from one of three sources: a PPI rating that's too fine for the inclusion load in the melt, a melt that's dirtier than expected, or a filter area that's undersized for the flow rate required. Address melt cleanliness upstream before increasing PPI — the filter is the last line of defense, not the first.
Q7: How do I verify the PPI of a filter I've received?
Place a ruler along the filter surface and count the open pores in one linear inch. Repeat in three locations — center, edge, and corner — and average the results. A quality filter should read within ± 2 PPI of its stated rating. Significant variation across the surface indicates inconsistent production and unpredictable performance.
