Aluminium sand casting (also spelled aluminum) is a versatile foundry method that pours molten aluminium into a compacted sand mold shaped by a pattern. After the metal solidifies, the mold is broken to release the part. Because tooling is simple and mold sizes are almost unlimited, sand casting is the fastest and most economical way to make aluminium parts from a handful of prototypes to steady production.
Aluminium sand casting step-by-step process
1) Pick the alloy and mechanical target
Choose from common cast alloys such as A356/356 (Al-Si-Mg), 319 (Al-Si-Cu), 413 (Al-Si), and 535 Almag (Al-Mg). Decide the final condition (as-cast, T5, or T6) and which properties matter most: strength, ductility, pressure-tightness, or corrosion resistance.
2) Choose the sand system and binder
- Green sand: silica/olivine sand with clay and water. Best for broad part sizes and economical runs.
- Dry (chemical) sand: resin-bound systems such as phenolic urethane or furan. Delivers stronger molds, finer surface, and tighter tolerance—ideal for thin ribs, tall cores, and large, flat faces.
3) Design and build the pattern set
Add shrink allowance (≈1.0–1.3% for aluminium), draft (1–3° outer faces; 2–5° inside pockets), and fillets (≥1.5–3 mm) to avoid hot spots. Provide core prints if the part needs internal cavities. Patterns can be wood (fast/low cost), aluminium (durable), or 3D-printed for complex shapes.
4) Ram the mold and set the cores
Pack sand around the pattern to make the drag (bottom half). Place cores, add vents, and close with the cope (top half). Cut gates/runners to deliver metal and risers to feed shrinkage. Coat the cavity with mold-wash where a smoother skin or higher heat resistance is needed.
5) Melt, treat, and pour
Degas and flux the melt to control hydrogen and inclusions. Typical pour for Al-Si alloys is 690–740 °C (process dependent). Fill steadily to prevent erosion, and keep risers hot so they feed the last-to-freeze zones.
6) Solidify, shake-out, and fettle
After solidification, break the mold, remove gates/risers, tumble or shot-blast, and trim flash. Remove or dissolve cores.
7) Heat-treat and straighten if required
For A356/356 and 319, T6 (solution ~540 °C → quench → age ~155–180 °C) raises strength and fatigue. Straighten critical geometry before or after age as your spec requires.
8) Inspect, machine, and finish
Use dimensional checks and NDT (dye penetrant/X-ray) where needed. Machine datums, bores, and threads. Apply the specified finishing (anodize, conversion coat, e-coat, powder, or paint).
Reference design rules for aluminium sand casting
| Topic | Practical guidance |
|---|---|
| Minimum wall | 3–6 mm for general work; use bonded sands + chills for localized thinner features |
| Draft | 1–3° external; 2–5° internal or deep cores |
| Fillets & radii | ≥ 1.5–3 mm to reduce stress and improve feeding |
| Linear tolerance | About ±0.8–1.6 mm up to 100 mm length; then add ±0.02% of feature size (process/size dependent) |
| Surface roughness | Ra 6.3–25 µm (resin-bound sands are smoother than green sand) |
| Parting line | Keep simple/planar; place on the largest section when possible |
| Cores | Provide stable prints, adequate clearance, and venting |
| Feeding & solidification | Use risers on hot spots, add chills where thick-to-thin transitions risk shrinkage |
Treat these as working ranges for RFQs. Final capability depends on pattern quality, alloy, section size, binder system, and foundry controls.
Which aluminium casting alloy should you choose?
| Alloy | Where it shines | Heat-treat options | Typical behavior at room temperature* |
|---|---|---|---|
| A356 / 356 (Al-Si-Mg) | Best all-round for structural parts; weldable; good fatigue | As-cast, T5, T6 | High strength in T6 with fair ductility |
| 319 (Al-Si-Cu) | Excellent fluidity for complex shapes; stable at moderately elevated temperature | As-cast, T5/T6 | Moderate strength; good machinability |
| 413 (Al-Si) | Pressure-tightness and thin walls; high fluidity | Usually as-cast | Moderate strength; very good castability |
| 535 Almag (Al-Mg) | Marine service and high corrosion resistance with good elongation | Non-heat-treatable (work-harden) | Medium strength; high ductility; excellent corrosion |
*Exact numbers vary with melt quality and section thickness. Use the table to shortlist and ask suppliers for data sheets or test bars.
When to use green sand and when to use dry (chemical) sand
Use green sand when:
- you need the lowest tooling cost and fastest start,
- the design tolerates Ra ~12.5–25 µm surfaces and moderate dimensional accuracy,
- parts are robust in wall thickness and do not demand very tall, slender cores.
Use dry/chemical sand when:
- you need strong molds and cores for thin ribs, deep pockets, and tall features,
- you want smoother skin (Ra ~6.3–12.5 µm) and tighter tolerance,
- the part is large and flat where green sand might relax or warp,
- you must minimize veining, scabs, and erosion on sharp geometry.
In practice, many programs combine both: green sand for cost-sensitive items and resin-bound molds or cores for features that push accuracy.
Defects to watch in aluminium sand casting and how to fix them
Gas porosity (pinholes, “spangle”)
- Why it happens: melt hydrogen, wet returns, excessive superheat, turbulent gating.
- Fix it: rotary degassing or tablets, dry charge materials, optimized pour temperature, calmer gates and properly placed vents.
Shrinkage cavities and micro-shrink
- Why it happens: last-to-freeze hot spots starve for liquid metal.
- Fix it: hotter risers/feeder sleeves, directional solidification using chills, blended fillets on thick-to-thin transitions, feed-path analysis (Niyama/modulus).
Inclusions and erosion (sand, oxide skins)
- Why it happens: high velocity on sharp turns, poor skimming, inadequate wash.
- Fix it: gate for laminar flow, install filters, improve skimming and ladle practice, upgrade mold wash and sand hardness.
Misrun and cold-shut
- Why it happens: metal loses heat before streams knit.
- Fix it: raise superheat within spec, widen runners, shorten flow length, reduce section where feasible, improve venting.
Veining, scabs, fins
- Why it happens: thermal expansion of sand and inadequate mold strength.
- Fix it: higher-strength/resin sands, lower thermal expansion media (olivine/chromite), proper wash and baking.
Hot tears and warpage
- Why it happens: restraint during contraction; severe quench from T6.
- Fix it: generous fillets, balanced section changes, relieve constraints, straighten before/after age as your spec allows; reduce quench severity or use polymer quench if properties permit.
For leak-critical parts, vacuum impregnation is a proven safeguard even when gating/feeding are excellent.
Surface finishes for aluminium sand castings
As-cast / shot-blast / tumble
Economical; hides small blemishes; typical Ra 12.5–25 µm (finer with resin-bound sands).
Machined surfaces
Use for sealing faces, datums, and tight fits; achievable tolerance is driven by your machining spec, not by casting.
Anodizing
- Type II (sulfuric) for decorative corrosion protection and dye colors.
- Type III (hard anodize) for wear; specify thickness and masking of threaded/grounded areas; remember Type III reduces conductivity.
Chromate conversion coating
Conductive and paint-ready finish (Alodine-type); ideal for electronics and bonding.
Powder coating / e-coat / liquid paint
Durable color and corrosion barrier. Provide masking for bores, gasket lands, and ground points.
Post-treats for leak-tightness
Impregnation with polymer resins seals micro-porosity for hydraulic or cooling parts.
What aluminium sand casting really costs
What drives unit price
- Part mass vs. yield (gating/risers), alloy price, sand/binder choice, heat treat, NDT level (X-ray/leak), machining minutes, finishing steps, and order size.
What tooling costs look like
Patterns and coreboxes are modest versus die casting. 3D-printed pattern sets can reduce up-front cost and lead time for complex parts or short runs.
A simple cost picture (illustrative)
- Low-mass bracket (~0.8 kg) in A356 green sand, light machining, no heat treat: lowest cost tier.
- Housing with deep cores in bonded sand, A356-T6, machined datums/bores, anodized and leak-tested: mid-to-high cost tier due to cores, T6, machining, finishing, and QA.
Ask suppliers to quote breaks (e.g., 50/200/1,000 pcs) because sand casting scales well with lot size.
A buyer checklist you can paste into your RFQ
- 2D/3D files with draft directions, critical walls, and machining datums.
- Alloy + temper (e.g., A356-T6) and property targets; test method and sample location (separately cast bars or on-casting coupons).
- Required NDT (X-ray level, dye penetrant) and leak rate if applicable.
- Finish plan (as-cast / machined / anodized / coated) and masking map.
- Annual volume & EAU, shipment cadence, PPAP/FAI requirements.
- Packaging: racking, VCI, labels, barcodes.
Typical capability ranges you can expect
| Capability | Working range |
|---|---|
| As-cast tolerance | ±0.8–1.6 mm up to 100 mm; add ±0.02% beyond |
| Surface roughness | Ra 6.3–25 µm (process dependent) |
| Minimum wall | 3–6 mm typical; ribs/pins thinner with bonded sands + chills |
| Pressure tightness | Achievable with suitable alloys (e.g., 413/319) and process; impregnation optional |
| Heat treatments | T5/T6 common for A356/356 and 319; 535 is non-heat-treatable |
Where aluminium sand castings are commonly used
Automotive brackets and housings, industrial pump bodies and manifolds, marine hardware in 535 Almag, heat-sinks and electronics enclosures with chromate or anodize, and non-flight-critical aerospace/medical fixtures.
FAQs built from real search behavior
Can aluminium sand castings be used for structural brackets?
Yes. A356-T6 is widely specified for structural housings and brackets thanks to its strength-to-weight and fatigue performance. For safety-critical use, specify tensile tests and NDT.
How thin can I go on wall sections?
Plan 3–6 mm for general areas. With bonded sand and chills, local features can be thinner, but global thin-wall designs are better suited to die casting.
Which is better for accuracy—green sand or chemical-bonded sand?
Chemical-bonded sand provides stronger molds/cores, smoother surfaces, and tighter tolerance. Green sand wins on tooling cost and speed.
Can sand castings be leak-tight without impregnation?
Yes—especially with 413 or 319 and well-designed gating/feeding. Impregnation is a safety net for hydraulic parts or complex, thin-walled housings.
Why work with us on aluminium sand casting
You get fast DFM feedback (draft, feeds, chills, and core consolidation), dependable melting and heat-treat for A356/356, 319, 413, 535, in-house machining and finishing, and QA that matches your industry standard.
