Guide
What causes sand inclusions in castings, and what should you require from the foundry?
Sand inclusions are grains or clumps of molding or core sand that break loose from the mold and end up trapped in or just under the surface of a casting. They come from outside the metal: loose sand left in the cavity, mold walls eroded by the incoming metal, broken or crushed cores, and flakes of mold coating. Because sand is a hard ceramic, an inclusion that a cutter hits can chip or break the tool, and an inclusion that crosses a wall can open a leak path once a sealing face or bore is machined.
By Saola Manufacturing engineering team · Published · Updated
This guide is for the design, quality or sourcing engineer who has a broken tap, a chipped insert or a leaking housing and a casting with a gritty pocket in it. It covers how to tell sand from the other hard things that end up in castings (slag, oxide films and intermetallic “hard spots”), which causes to check first, how to confirm the cause and size the problem in a lot, which fixes belong to the foundry and which belong to the drawing, and what to write on the drawing and RFQ so the next lot is clean. The short version: sand inclusions are a foundry process problem, but a drawing that never says where they matter, how they are found or what the acceptance level is leaves the foundry nothing to hold to.
On this page
What does a sand inclusion look like, and how is it different from slag, oxide or a hard spot?
A sand inclusion is a gritty, granular pocket of rounded or angular grains, often light gray or tan, that shows on the as-cast surface or appears when a cut breaks into it. Slag and dross look glassy or spongy, oxide films look like thin folded skins, and an intermetallic hard spot is a bright metallic particle that does not crumble.
The difference matters because each one has a different owner and a different fix. In a technical review for Foundry Trade Journal (April/May 2023), Wolfram Stets of Foseco classes sand inclusions as exogenous, meaning they come from outside the melt, and traces them to loose sand, mold erosion and mold and core wash particles. Dross, by contrast, he describes as caused mainly by contact between the melt surface and the air. In practice, sand points to the mold, the cores and the gating; slag, dross and oxide films point to melting, melt treatment and pouring.
Aluminum castings add one more suspect. Iron, manganese and chromium can combine into coarse, very hard intermetallic crystals called sludge, which settle in melting and holding furnaces and are carried into the casting as hard spots. Zedan and co-authors cite a sludge hardness of 500 to 900 BHN from earlier studies, and in their drilling tests sludge hard spots cut drill life by about half compared with the base Al-Si alloy (Zedan et al., Materials, 2022). A broken tool in a die casting, where no sand mold is involved, is far more likely to be sludge or oxide than sand, unless the part used sand cores.
| Inclusion | What you see | Where it comes from | Usual owner of the fix |
|---|---|---|---|
| Sand inclusion | Gritty pocket of grains, crumbles under a pick; on film, irregular spots (ASTM E446 Category B in steel) | Loose sand, mold erosion, broken cores, coating flakes | Molding, coremaking and gating |
| Slag or dross | Glassy or spongy dark patches, often near the top surface (cope side) or in pockets | Melt reacting with air, poor skimming, turbulent pouring | Melting, treatment and pouring |
| Oxide film (aluminum) | Thin folded skins, seen on fracture faces or as leak paths | Turbulent filling, dirty ladles, poor melt cleaning | Melt treatment, filtration, gating |
| Sludge hard spot (aluminum) | Bright, hard metallic particle that chips tools and does not crumble | Iron, manganese and chromium in the alloy, cold holding furnace | Alloy chemistry and melt temperature control |
| Residual core sand | Loose sand or burnt binder inside passages and blind cavities | Incomplete core knockout and cleaning | Cleaning room, plus a cleanliness spec on the drawing |
What causes sand inclusions, ranked by how often they explain the defect?
Most sand inclusions come from four places: sand eroded from the mold or core by fast, turbulent metal, loose sand left in the cavity when the mold is closed, cores or mold edges that crack or crush, and a weak or poorly bonded sand. Gating design decides how hard the metal hits the sand; sand control decides how well the sand resists it.
- Mold and core erosion at the gates. Metal entering fast and turbulently scours sand from the runner, the ingate area and any core it strikes. Foseco engineers list poorly designed gating and mold erosion among the main causes and recommend gating and riser layouts that reduce turbulence (Foseco, 2024). A telltale sign is inclusions clustered downstream of the ingate, with a matching rough or washed spot on the casting near the gate.
- Loose sand in the cavity. Sand knocked off during pattern draw, core setting or mold closing, or dropped from the cope, sits in the cavity and is carried along by the first metal. These inclusions tend to be random in location from casting to casting.
- Cracked, crushed or weak cores and mold edges. A core that is crushed when the mold closes, or a thin sand section that breaks off, leaves a clump of sand in the metal and often a matching fin or bump nearby. Insufficient binder in resin-bonded molds and cores is named in the same Foseco guidance, which also warns that sand inclusions can stay hidden under the casting surface until machining.
- Poor sand control. Low or uneven compaction, the wrong moisture or binder level, and sand that has picked up fines or old binder all make the mold face easier to wash away. Sand properties such as compactability, strength and permeability are routine foundry checks for this reason.
- Coating problems. Mold and core coatings protect the sand face, but a coating applied too thick, not dried, or flaking off becomes an inclusion source of its own; the review in Foundry Trade Journal lists mold and core wash particles alongside loose sand.
Sand expansion defects such as scabs, buckles and rat tails also start at the sand face, but they show as thin raised metal crusts or line indentations on the surface rather than sand trapped in the metal. Metal penetration (sand burned into the surface) is different again; it roughens the surface but rarely hides under a machined face.
How do you confirm it is sand, and find out how much of the lot is affected?
Pick the inclusion out or section it, look at it under magnification, and if the answer is not obvious, have a lab identify it by SEM with EDS: silica-rich grains mean sand, while iron-manganese-chromium particles mean sludge. Then map where the inclusions sit and use radiography or machining trials to size the lot.
- Keep the evidence. Save the broken tool, the chips and the casting. The pocket the tool hit is the best sample you will get.
- Look and probe. Under a loupe or stereo microscope, sand is granular and crumbles under a steel pick; a hard spot is metallic and does not. Note whether a dark, glassy or skin-like phase is present, which points to slag or oxide.
- Identify the chemistry if it matters. Scanning electron microscopy with energy-dispersive X-ray analysis (SEM/EDS) gives the composition of the particle. The Foundry Trade Journal review describes a method for aluminum that scans defects with an automated SEM stage and EDX detector, and notes that inclusions in steel generally need a metallographic sample, since non-destructive methods cannot identify them.
- Map the location. Mark each inclusion on a drawing across several castings. A cluster near the ingate or a core print points to erosion or core damage; scattered single grains point to loose sand. A cluster on the top surface of the casting as poured points to slag or dross, which floats.
- Size the lot. Internal inclusions are found by radiography; per ASTM E446 sand and slag inclusions in steel castings are Category B, graded in five severity levels, and ASTM E155 gives reference radiographs for aluminum and magnesium castings. If the critical faces are known, a machining trial on a sample of castings can be faster than film.
- Contain the rest of the lot. Hold the castings not yet machined. Where the routing allows, machine the sealing faces and tapped holes first, so a casting with an opened inclusion is rejected before more work goes into it. Keep the pattern, core box or cavity number and pour date with each reject; if the rejects share one, the cause is narrowed to that tool or that day.
- Check the leak path. For a leaking part, leak test it and mark the leak site, then section through it. An inclusion that bridges a wall, or a pocket opened by a sealing face, confirms the path.
- Ask for the process record. Pattern or core changes, a new sand batch, a different coating or a gating change that coincides with the first defects is the fastest route to the cause.
Which fixes stop sand inclusions, and who owns each one?
Most of the fixes sit with the foundry: calmer gating, filters, stronger and better-compacted sand, sound cores and coatings, and a clean cavity before closing. The drawing owner controls where inclusions are tolerated, how much machining stock covers the as-cast skin, and whether the part needs a sand mold at all.
The Foundry Trade Journal review (2023) recommends filtering the melt during pouring to remove inclusions and minimize turbulence, and reducing turbulence during mold filling to avoid reoxidation. Foseco’s sand inclusion guidance adds adequate compaction, mold inspection before pouring, correct binder levels, sand temperature control and a suitable refractory coating. Match the fix to what the diagnosis found.
| Diagnosis | Foundry fix | Drawing or buyer action |
|---|---|---|
| Cluster downstream of the ingate | Lower ingate velocity, larger or more ingates, add a filter, harden or coat the eroded area | Let the foundry choose gate locations; agree any gate moves before first article |
| Random single grains | Blow out the cavity before closing, protect the mold from sand drop, check pattern draw | Set a zone-based acceptance level so the foundry knows where it matters |
| Clumps next to a core print or thin sand section | Stronger cores, correct core print clearance, check closing pressure and core handling | Avoid long, thin cores where possible; allow core prints and vents where the foundry needs them |
| Coating flakes | Correct coating type and thickness, full drying before closing | None |
| Inclusions just under machined faces | Gate away from critical faces, place those faces in the drag (bottom) where practical | Mark critical faces; review machining stock so the cut clears the as-cast skin |
| Sludge hard spots (aluminum) | Control iron, manganese and chromium; keep holding temperature above the sludge-forming range | Name the alloy by standard designation; ask for chemistry certs per heat |
| Sand left in internal passages | Better core knockout, shot blast, flushing, inspection of passages | Add a cleanliness requirement for passages and a check method |
One design choice is worth a second look. If the part is a high-volume aluminum housing that only uses sand because of an internal passage, a die-cast or permanent-mold version with the passage machined or redesigned may remove sand from the process entirely. The casting process comparison covers that trade-off.
What should you require from the foundry on the drawing, RFQ and PO?
Require an acceptance level by zone, the inspection method that proves it, a cleanliness rule for internal passages, a repair rule, and notification before any process change. “Free of sand and inclusions” on its own cannot be measured, so neither side can prove it was met.
- Zones. Mark sealing faces, threaded bores and load-bearing sections as Zone A. Allow more freedom elsewhere, so the foundry spends its effort where it counts.
- Internal quality. “Zone A: radiographic quality per ASTM E446, Category B, severity level [n] max” for steel. For aluminum or magnesium, cite ASTM E155 and name the reference plate (discontinuity type) and grade you accept. Name the sampling: first article, then [plan].
- Surface quality. For steel castings, as-cast surfaces per ASTM A802, which grades nonmetallic inclusions as its own category, with the acceptance level you need on each zone.
- Machined faces. “No inclusions or voids open on sealing faces after machining”, plus a pressure or leak test where the part holds fluid; see the leak testing guide.
- Passage cleanliness. “Internal passages free of loose sand and binder residue, verified by [visual with borescope / flushing and particle count].” Automotive buyers often reference ISO 16232 for particle cleanliness; name the method and the limit, not just the standard.
- Alloy and chemistry. Standard alloy designation and a chemistry certificate per heat, which also shows the iron, manganese and chromium that drive sludge in aluminum. Tillová and co-authors (2019) traced higher tool wear after a change of alloy supplier to sludge phases, and use a sludge factor that combines the three elements to predict the risk.
- Repair rule. Whether inclusions may be ground out and weld repaired, who approves it, and whether repaired castings are re-inspected. If weld repair is not allowed on sealing or pressure-retaining zones, say so in writing.
- Change control. “Gating, filtration and core changes after first article approval require notification.”
On the RFQ for a resourcing or corrective action, include photos of the inclusions, the location map and any SEM/EDS result. It lets the foundry quote the fix rather than the old process.
Who does what on a Saola order when castings have sand inclusions?
Sand castings and die castings are made by partner plants we manage, which are not necessarily in Vietnam, and the castings are machined at our lead CNC plant, T.K Viet Nam Precision Company Limited. Each quote names the production site and the country of origin for every step.
The casting partner owns the process fixes: gating, filtration, sand control, cores, coatings and cleaning. The drawing and the acceptance levels stay with you, and we pass the partner’s comments on gating, cores and critical faces back to you as a manufacturability note before tooling. When castings reach the lead CNC plant for CNC milling or CNC turning, an inclusion opened by a cut, or a tool broken on one, is recorded with photos and its location and reported back, so the cause can be fixed at the casting step rather than sorted at machining. See sand casting and die casting for the partner-plant programs. If your drawing calls for radiography, SEM/EDS or a passage cleanliness check, name the standard on the RFQ and we confirm before quoting whether and where it can be arranged. ISO 9001 certification is in progress, not held. Send the drawing, the photos and the location map through the quote form.
Frequently asked
Can a sand inclusion cause a leak even if the casting passed X-ray?
Yes. Radiography is graded against reference levels, so small inclusions can be accepted, and an inclusion just under the skin may only become a path when a sealing face or bore is machined into it. That is why parts that hold fluid need a leak test after machining.
Will vacuum impregnation seal a leak caused by a sand inclusion?
Impregnation is meant for fine, interconnected porosity. It may seal a small path, but it does not remove the inclusion or restore strength, so agree in writing whether it is allowed. The vacuum impregnation guide covers when it is accepted.
Why do inclusions break taps more often than milling cutters?
A tap is slender, cuts on every flank at once and cannot deflect around a hard particle, so one grain can stall and snap it. A milling insert usually chips instead. Tap breakage in cast bores is a strong hint to check for inclusions or hard spots near the hole.
Do die castings get sand inclusions?
Only where sand cores are used, which is uncommon in high-pressure die casting. Hard inclusions in die castings are usually oxides or sludge, so identify the particle before asking the die caster for a fix.
Is “free of sand inclusions” an acceptable drawing note?
Not on its own. It has no method, no level and no zone, so neither side can prove compliance. Pair it with a radiographic or visual standard, a severity level by zone and a sampling plan.
Sources
- Non-metallic inclusions in ductile cast iron, steel and aluminum castings (British spelling in the original), W. Stets, Foseco, Foundry Trade Journal, April/May 2023, foundrytradejournal.com
- Preventing sand inclusions and metal penetration in casting, Foseco, June 2024
- Effect of Intermetallics and Drill Materials on the Machinability of Al-Si Cast Alloys, Materials 15(3), Y. Zedan et al., 2022
- Sludge phases as cause of higher wear of cutting tools during machining of secondary AlSi12Cu1 castings, Manufacturing Technology 19(5), E. Tillová et al., 2019
- ASTM E446-20, Standard Reference Radiographs for Steel Castings Up to 2 in. in Thickness (scope), ASTM International, 2020
- ASTM E155-20, Standard Reference Radiographs for Inspection of Aluminum and Magnesium Castings (scope), ASTM International, 2020
- ASTM A802-19, Standard Practice for Steel Castings, Surface Acceptance Standards, Visual Examination (scope), ASTM International, 2019