You designed the gating systems. You picked the right filter material. You even calculated the mesh size. And still—slag inclusions. Cold shuts. Scrap.
Here’s the uncomfortable truth: a filter is only as good as its placement.
The main practical problem in foundry filtration isn’t the filter itself—it’s the way it’s placed inside the gating system. Improper placement can reduce or completely eliminate the filter’s positive influence.
After reviewing hundreds of foundry setups and real-world failure cases, here are the three critical mistakes iron foundries make when placing mesh filters in their gating systems—and exactly how to fix each one.
Mistake #1: Placing the Filter in the Wrong Location
This is the most common—and most destructive—mistake.
What Foundries Do Wrong
Foundries place filters in one of three common positions:
| Position | What Foundries Think | What Actually Happens |
|---|---|---|
| Under the sprue / pouring cup | “Easy to install, catches everything early” | High-velocity metal impacts the filter directly—cracks it, clogs it, or washes slag straight through |
| At the bottom of the sprue | “Catches slag before it spreads” | Turbulent flow from the sprue bottom negates the filter’s effect—and the filter often breaks under pressure |
| Too far from the casting | “Anywhere in the runner is fine” | Metal re-oxidizes after filtration, creating new inclusions before it reaches the cavity– |
The data backs this up. A study found that in some parting-plane casting operations, filters were placed up to 60 cm away from the pouring basin—far too distant to be effective–.
Why It Matters
When you place a filter under the sprue or at the sprue bottom, the metal hasn’t stabilized yet. The filter becomes a sacrificial barrier rather than a filtration device. It catches some slag, sure—but the high-velocity impact also:
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Breaks the filter (especially ceramic foam filters)
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Pushes slag through the filter under pressure
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Clogs the filter prematurely, slowing the pour and causing cold shuts
One foundry engineer reported that placing a filter at the sprue bottom in ductile iron casting actually caused more defects than running without a filter—because the broken filter fragments became inclusions themselves–.
How to Fix It
Place the filter in the runner—after the sprue, before the ingate.
This is the most common and practical location. The runner allows the metal to:
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Stabilize after falling through the sprue
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Slow down to a controlled velocity
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Flow evenly across the entire filter face
If you must place a filter near the sprue, use a rigid cap-style filter rather than flat mesh—it provides more structural stability under impact.
Rule of thumb: Position the filter in the runner, more than 150mm from the cavity, but as close to the casting as practical. The closer the filter sits to the mold cavity, the better it works.
Mistake #2: Poor Sealing – Gaps That Let Metal Bypass the Filter
You installed the filter. You poured the metal. But your castings still have slag inclusions.
Here’s why: the metal went around the filter, not through it.
What Foundries Do Wrong
Foundries often cut filters to size and drop them into a seat without checking the fit. If there’s a gap—even 1–2 mm—molten metal will take the path of least resistance.
Common sealing failures:
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Filter too small for the seat—gaps on one or more sides
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Wrinkled filter—creates bypass channels along the folds
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No sealing material around the edges
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Filter not flush with the runner floor—metal flows underneath
Why It Matters
When metal bypasses the filter, filtration doesn’t happen. You’re pouring unfiltered metal into your cavity—defeating the entire purpose of the filter investment.
Even worse: bypassing metal often carries eroded sand from the gap itself, introducing new inclusions that wouldn’t have existed without the filter.
In ductile iron, where dross and slag are particularly problematic, a bypass path can allow dross to travel straight into the cavity—causing the very defects you were trying to prevent.
How to Fix It
Seal the filter completely.
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Cut the filter slightly oversized—1–2 mm larger than the seat on each side—so it fits snugly
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Design a recessed filter seat with a flat bottom and a depth equal to the filter thickness
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Use a sealing material around the edges—ceramic fiber gasket or refractory paste
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Press the filter flush with the sand surface before closing the mold
If you’re using fiberglass mesh, make sure the filter sits flat—no wrinkles, no folds-2. A crooked filter spreads flow unevenly across its face, causing one section to clog first and creating pressure differentials that pull metal around the edges.
For ceramic foam filters, the gap should be no more than 1–2 mm per side-1.
Mistake #3: Insufficient Filter Area – Starving the Cavity
You used a filter. The casting came out cold—or incomplete. The filter choked the flow.
What Foundries Do Wrong
Foundries often treat filter area as an afterthought. They pick a filter that fits the gating system—not one that feeds the gating system.
The result: the filter becomes the choke point in the system.
Why It Matters
When the filter area is too small, the metal flow slows down dramatically. The mold doesn’t fill fast enough. The metal cools before reaching the extremities of the cavity. You get:
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Cold shuts—where two metal fronts fail to fuse
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Misruns—incomplete fill
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Premature clogging—slag blocks the entire filter face
One foundry engineer reported that adding a filter reduced flow rate by half—and they had to redesign their gating system to compensate.
The problem gets worse in ductile iron. The amount of metal a filter can pass before blocking depends heavily on the foundry’s practice, filter location, and metal chemistry. If you place the filter too early in the system, it blocks faster.
How to Fix It
Calculate your filter area correctly.
A common rule: filter area should be at least 3–4 times the choke area for iron.
Step-by-step:
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Calculate your choke area—the narrowest point in your gating system
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Multiply by 3–4—that‘s your minimum filter area
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Choose a filter that meets or exceeds that area
If you can’t fit a single filter large enough, use multiple filters or increase the filter area by placing filters in multiple runners.
Also, consider the mesh size. If you‘re seeing premature clogging, your mesh might be too fine. For gray and ductile iron, a mesh opening of 1.5–2.5 mm is typical. If you’re using 2×2 mm mesh and experiencing clogging, try 2.5 mm. The filter still catches slag—but lets more metal through.
Quick Reference: The Three Mistakes & How to Fix Them
| Mistake | Symptom | Root Cause | Fix |
|---|---|---|---|
| Wrong location | Slag inclusions despite using a filter | Filter under sprue or too far from cavity | Move filter to runner—after sprue, before ingate |
| Poor sealing | Metal bypass + unfiltered castings | Gaps around filter edges | Cut oversized; use sealant; ensure flat seat |
| Insufficient area | Cold shuts, misruns, slow fill | Filter area <3–4× choke area | Increase filter area; use coarser mesh; add multiple filters |
The Bottom Line
The filter is only half the solution. Placement is the other half.
Get these three things right:
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Put the filter in the runner—not under the sprue, not at the sprue bottom
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Seal it completely—no gaps, no wrinkles, no bypass
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Size it adequately—3–4 times the choke area, with the right mesh opening
Do that, and your filters will do their job. Get any of these wrong, and you‘re pouring unfiltered metal—wasting time, money, and castings.
Seeing a specific defect pattern you can’t solve? We’ve probably seen it before. Send us a photo of your gating system and we’ll help you troubleshoot the placement.
Email: info@sf-foundry.com
WhatsApp: +8618636913699