You’ve just machined a casting and you see dark spots or gritty patches. Your first thought: inclusions. But what kind? Slag? Sand? The answer matters – because the solutions are completely different.
If you treat a slag inclusion like a sand problem, you’ll waste time changing your sand system when the real issue is in your melt. If you treat sand like slag, you’ll adjust your furnace practice while sand keeps eroding from your runner.
Quick Visual Identification
| Feature | Slag Inclusion | Sand Inclusion |
|---|---|---|
| Colour | Dark gray, black, sometimes greenish | Light tan, brown, off‑white |
| Hardness | Soft to moderate (can be scratched) | Very hard – dulls cutting tools |
| Surface | Smooth, often flaky or layered | Gritty, irregular, angular |
| Location in casting | Top surfaces, under sprue, near gates | Near gates, runner entry, changes in section |
| Magnetic? | Usually no | No (unless sand contains iron oxide) |
| Reaction to acid | Minimal | None |
Simple shop‑floor test: Try to scratch the defect with a steel pick or a small file. If it scratches easily, it’s likely slag. If it’s hard and resists scratching, it’s probably sand.
Slag Inclusions – Source and Cause
What Is Slag?
Slag is a non‑metallic by‑product of melting. It consists of oxides, flux residues, and other compounds that float on top of molten metal. When you skim the furnace or ladle, most slag is removed – but some can be carried into the mold, especially during turbulent pouring.
Common Causes
| Cause | Why It Happens |
|---|---|
| Incomplete skimming | Slag left on the melt surface gets pulled into the stream. |
| Turbulent gating | Metal splashing folds slag into the flow. |
| No filter or filter bypass | Slag travels straight to the casting. |
| Poor pouring practice | Vortex in the sprue pulls surface slag down. |
Appearance in Castings
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Dark, irregular patches on machined surfaces.
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Often found near the top of the casting (because slag is lighter than metal).
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Can appear as thin, flaky layers – sometimes called dross in ductile iron.
Fixes for Slag Inclusions
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Use a ceramic foam filter – placed in the runner, sealed properly.
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Skim thoroughly – just before pouring, not an hour earlier.
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Improve gating – use a pouring basin to keep sprue full, avoid vortexing.
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Reduce turbulence – round sprue bottom, enlarge runner.
Sand Inclusions – Source and Cause
What Is a Sand Inclusion?
A sand inclusion is a grain or cluster of molding sand that has eroded from the mold or core and become trapped in the casting. Unlike slag, sand comes from the mold itself, not the melt.
Common Causes
| Cause | Why It Happens |
|---|---|
| Low mold strength | Sand erodes under metal flow. |
| High metal velocity | Fast metal scours the runner and cavity walls. |
| Poor gating design | Sharp corners or sprue bottom erosion release sand. |
| No coating or inadequate coating | Sand directly exposed to metal. |
| Friable cores | Core sand breaks loose during pouring. |
Appearance in Castings
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Light‑coloured, gritty spots – often visible after machining.
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Hard – damages cutting tools.
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Often located near gates, runner junctions, or areas of high turbulence.
Fixes for Sand Inclusions
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Improve mold strength – increase compaction, adjust binder.
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Use paper casting runners – replaces sand runner with paper (for iron/steel).
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Apply mold coating – zircon or graphite wash.
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Reduce metal velocity – increase runner cross‑section.
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Use a filter – captures sand that has already eroded.
Side‑by‑Side Comparison Table
| Aspect | Slag Inclusion | Sand Inclusion |
|---|---|---|
| Origin | Melt (furnace/ladle) | Mold or core |
| Typical size | Variable – thin films to large clumps | Grain‑size to clusters |
| Effect on tooling | Minor (soft) | Major (dulls tools quickly) |
| Detection | Visual after machining, often dark | Visual, often light‑coloured |
| Primary fix | Filtration + clean melting | Mold strength + gating design |
| Filter effectiveness | High (ceramic foam) | High (any filter type) |
Diagnostic Flowchart
Defect found on machined surface
│
▼
Is it hard? (scratch test)
│ │
Yes No
│ │
▼ ▼
Sand likely Slag likely
│ │
▼ ▼
Check mold: Check melt:
• Low strength? • Skimmed?
• High velocity? • Filter used?
• Coating? • Turbulence?
│ │
▼ ▼
Fix gating Add filter
and mold improve pouring
Can You Have Both? Yes – And Here’s How to Know
It’s possible to have both slag and sand inclusions in the same casting. For example:
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A poorly sealed filter allows slag to bypass.
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Eroded sand enters the runner downstream of the filter.
In that case, you’ll see both dark slag patches and light sand spots. Fix the most likely cause first (often filter bypass or sand erosion), then re‑evaluate.
Preventing Both – A Checklist
| Preventive Action | Stops Slag? | Stops Sand? |
|---|---|---|
| Ceramic foam filter (sealed) | ✓ | ✓ |
| Proper skimming | ✓ | ✗ |
| Paper runner | ✗ | ✓ |
| Mold coating | ✗ | ✓ |
| Low‑turbulence gating | ✓ | ✓ |
| Full sprue (pouring basin) | ✓ | ✓ |
The single most effective step for both is a properly placed and sealed ceramic foam filter – it catches slag from the melt and sand that has already eroded.
Conclusion
Slag inclusions are dark, soft, and come from the melt. Fix them with filtration, clean melting, and smooth pouring.
Sand inclusions are light, hard, and come from the mold. Fix them with better mold strength, lower velocities, and paper runners.
Use the scratch test to tell them apart in seconds. Then fix the real cause – not the one you assume.
At SF-Foundry, we supply ceramic foam filters that catch both types, as well as paper runners for sand‑free gating. Need help diagnosing a recurring defect? Send us a photo of the defect and your gating layout – we’ll give you a straight answer.
Contact SF-Foundry Technical Support:
Email: info@sf-foundry.com
Phone / WhatsApp: +86 18636913699
Website: www.sf-foundry.com