{"id":85271,"date":"2026-04-28T05:51:46","date_gmt":"2026-04-28T05:51:46","guid":{"rendered":"https:\/\/www.alalloycasting.com\/?p=85271"},"modified":"2026-04-28T05:51:46","modified_gmt":"2026-04-28T05:51:46","slug":"20-ppi-filter-foam-for-clean-aluminum-castings","status":"publish","type":"post","link":"https:\/\/www.alalloycasting.com\/ar\/20-ppi-filter-foam-for-clean-aluminum-castings\/","title":{"rendered":"20 PPI Filter Foam: The Go-To Choice for Clean Aluminum Castings"},"content":{"rendered":"

What Does “20 PPI” Actually Mean in Ceramic Foam Filters?<\/h2>\n

PPI stands for\u00a0Pores Per Inch<\/strong>\u00a0\u2014 literally, the number of pore openings you’d count along a one-inch line across the filter’s surface. A 20 PPI filter foam has roughly 20 pores per linear inch, which translates to an average pore diameter of approximately\u00a01.0\u20131.2 mm<\/strong>.<\/p>\n

That might sound like a small detail, but it determines almost everything about filter performance: how fast metal flows through, what size particles get captured, and how much metallostatic head you need to maintain a steady pour.<\/p>\n

Here’s a quick comparison to put 20 PPI in context:<\/p>\n

\n\n\n\n\n\n\n\n\n
PPI Grade<\/th>\nAvg. Pore Size (mm)<\/th>\nTypical Application<\/th>\nFlow Resistance<\/th>\n<\/tr>\n<\/thead>\n
10 PPI<\/td>\n2.0\u20132.5<\/td>\nIron casting, heavy non-ferrous alloys<\/td>\n\u0645\u0646\u062e\u0641\u0636 \u062c\u062f\u064b\u0627<\/td>\n<\/tr>\n
20 \u0646\u0642\u0637\u0629 \u0641\u064a \u0627\u0644\u0628\u0648\u0635\u0629<\/td>\n1.0\u20131.2<\/td>\nGeneral aluminum casting, gravity die casting<\/td>\nLow\u2013Moderate<\/td>\n<\/tr>\n
30 \u0646\u0642\u0637\u0629 \u0644\u0643\u0644 \u0628\u0648\u0635\u0629<\/td>\n0.7\u20130.9<\/td>\nPrecision aluminum components, wheel hubs<\/td>\nModerate<\/td>\n<\/tr>\n
40 \u0646\u0642\u0637\u0629 \u0641\u064a \u0627\u0644\u0628\u0648\u0635\u0629<\/td>\n0.5\u20130.6<\/td>\nAerospace-grade aluminum, thin-wall castings<\/td>\n\u0645\u0631\u062a\u0641\u0639<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

As you can see,  20 PPI ceramic foam filters <\/span><\/strong><\/em><\/a>\u00a0sit right in the middle \u2014 versatile enough for a huge range of aluminum alloy work without choking flow or demanding excessive head pressure.<\/p>\n

\"\u0645\u0631\u0634\u062d
\u062a\u062a\u064a\u062d \u0627\u0644\u0628\u0646\u064a\u0629 \u0630\u0627\u062a \u0627\u0644\u062e\u0644\u0627\u064a\u0627 \u0627\u0644\u0645\u0641\u062a\u0648\u062d\u0629 \u0627\u0644\u0645\u062a\u0631\u0627\u0628\u0637\u0629 \u0644\u0645\u0631\u0634\u062d \u0627\u0644\u0631\u063a\u0648\u0629 \u0627\u0644\u062e\u0632\u0641\u064a\u0629 \u0625\u062c\u0631\u0627\u0621 \u062a\u0631\u0634\u064a\u062d \u0639\u0645\u064a\u0642 \u0648\u0627\u062d\u062a\u062c\u0627\u0632 \u0627\u0644\u062c\u0633\u064a\u0645\u0627\u062a \u0628\u0643\u0641\u0627\u0621\u0629.<\/em><\/figcaption><\/figure>\n

Why Is 20 PPI Filter Foam So Widely Used in Aluminum Foundries?<\/h2>\n

Walk into any aluminum casting facility producing automotive parts, heat sinks, or structural profiles, and chances are you’ll find 20 PPI filters stacked by the furnace. There are practical reasons for that popularity.<\/p>\n

It Removes the Inclusions That Actually Cause Problems<\/h3>\n

Molten aluminum is reactive. The moment it contacts air, a thin Al\u2082O\u2083 oxide film forms on the surface. During pouring, turbulence breaks that film into fragments \u2014 typically 50\u2013500 microns \u2014 that get carried into the mold cavity. These non-metallic inclusions act as stress concentrators in the finished part, leading to cracks under load or during machining.<\/p>\n

A 20 PPI filter foam catches particles down to roughly\u00a080\u2013100 microns<\/strong>\u00a0through direct interception, and even finer inclusions (20\u201350 microns) get trapped via\u00a0cake filtration<\/strong>\u00a0\u2014 where the initial layer of captured debris itself becomes a secondary filter medium. In practice, a well-functioning 20 PPI filter reduces inclusion content by\u00a060\u201380%<\/strong>, depending on alloy cleanliness and pouring temperature.<\/p>\n

That’s a significant jump in casting quality without adding complexity to your gating system.<\/p>\n

It Doesn’t Strangle Your Pour Rate<\/h3>\n

This matters more than people realize. A filter that’s too fine for the application creates back-pressure, slows the pour, and can cause\u00a0misruns or cold shuts<\/strong>\u00a0\u2014 especially in thin-wall castings where the metal needs to fill fast before it loses superheat.<\/p>\n

With an average pore opening around 1.1 mm, 20 PPI foam filters allow aluminum to flow at\u00a01.0\u20132.5 kg\/s<\/strong>\u00a0through a standard 75\u00d775\u00d722 mm filter tile, which is more than adequate for most gravity-fed and low-pressure casting setups.<\/p>\n

I’ve personally seen foundries switch from 30 PPI down to 20 PPI solely to eliminate fill-time issues on large housing castings \u2014 and the inclusion levels stayed well within spec.<\/p>\n

It Works Across a Wide Temperature Window<\/h3>\n

Aluminum alloys are typically poured between\u00a0680\u00b0C and 760\u00b0C<\/strong>, depending on the alloy family (hypoeutectic, eutectic, or hypereutectic Al-Si systems). A quality 20 PPI alumina ceramic foam filter \u2014 with an Al\u2082O\u2083 content of 80\u201385% or higher \u2014 handles this range without thermal shock cracking or chemical degradation.<\/p>\n

Some lower-grade filters use cheaper binder systems that start breaking down above 730\u00b0C. If you’re running A356 or A319 at higher superheat for fluidity, make sure your filter supplier specifies a\u00a0working temperature of at least 1100\u00b0C<\/strong>\u00a0with alumina content above 80%.<\/p>\n

\"Why
Why Is 20 PPI Filter Foam So Widely Used in Aluminum Foundries?<\/em><\/figcaption><\/figure>\n

How Is 20 PPI Ceramic Foam Filter Made?<\/h2>\n

Understanding the manufacturing process helps explain why filter quality varies so much between suppliers. Here’s what actually happens:<\/p>\n

Step 1 \u2014 Foam Template Preparation<\/strong>
\nA block of open-cell polyurethane foam \u2014 already cut to the right PPI count and dimensions \u2014 serves as the structural scaffold.<\/p>\n

Step 2 \u2014 Slurry Impregnation<\/strong>
\nThe foam is dipped into a carefully formulated alumina-based ceramic slurry. The composition typically includes Al\u2082O\u2083 powder, SiO\u2082 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.<\/p>\n

Step 3 \u2014 Excess Slurry Removal<\/strong>
\nThe coated foam passes through rollers or a centrifuge to squeeze out excess slurry, ensuring uniform wall thickness across the struts.<\/p>\n

Step 4 \u2014 Drying & Sintering<\/strong>
\nThe coated foam is dried, then fired in a kiln at\u00a01500\u20131680\u00b0C<\/strong>. During firing, the polyurethane template burns out completely, leaving behind a rigid, interconnected ceramic skeleton \u2014 the open-cell structure that defines the filter’s performance.<\/p>\n

Step 5 \u2014 Quality Inspection<\/strong>
\nFinished filters are checked for dimensional accuracy, pore uniformity, crush strength (typically \u22651.0 MPa for a 20 PPI filter), and surface defects. Any filter with blocked pores, cracks, or uneven coating gets rejected.<\/p>\n

The entire process takes roughly 24\u201348 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 \u2014 the exact opposite of what you want.<\/p>\n

What Size 20 PPI Filter Do I Need for My Casting?<\/h2>\n

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).<\/p>\n

The general rule:\u00a0filter area should be 4\u20136\u00d7 the choke area of your gating system.<\/strong>\u00a0But for practical purposes, here’s what works for most aluminum casting applications:<\/p>\n

\n\n\n\n\n\n\n\n\n
Casting Weight (kg)<\/th>\nRecommended Filter Size (mm)<\/th>\nFilter Thickness (mm)<\/th>\n\u0645\u0644\u0627\u062d\u0638\u0627\u062a<\/th>\n<\/tr>\n<\/thead>\n
0.5\u20133<\/td>\n50 \u00d7 50<\/td>\n22<\/td>\nSmall parts, gravity permanent mold<\/td>\n<\/tr>\n
3\u201310<\/td>\n75 \u00d7 75<\/td>\n22<\/td>\nMid-size castings, tilt pour setups<\/td>\n<\/tr>\n
10\u201330<\/td>\n100 \u00d7 100<\/td>\n22<\/td>\nStructural components, wheel blanks<\/td>\n<\/tr>\n
30+<\/td>\n150 \u00d7 150 or custom<\/td>\n25\u201330<\/td>\nLarge plate castings, ingot filtration<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

Round filters (\u00d850, \u00d875 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 \u2014 they seat more reliably and reduce the risk of bypass.<\/p>\n

One thing worth noting:\u00a0thickness matters more than most people think.<\/strong>\u00a0A 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.<\/p>\n

When Should You Choose 20 PPI Over 30 PPI or 10 PPI?<\/h2>\n

This is where real-world experience beats catalog specs every time.<\/p>\n

Choose 20 PPI when:<\/strong><\/p>\n