Aluminium Casting Process
The aluminium casting process, stage by stage.
Every casting moves through the same disciplined sequence — core, cast, treat, finish — before it reaches inspection. This page covers all eight stages, plus the tolerances, surface finish, defects and costs that follow from them.
Sand Casting Process
How an aluminium sand casting comes together.
Eight stages, none of them subcontracted. A sand mould is formed around a pattern, cores are set to create internal passages, aluminium is melted and poured under gravity, and once solid the mould is broken away for finishing.
Core Making
High quality core making for accurate, repeatable casting geometry.
Melting & Pouring
Aluminium melted under controlled conditions and poured with precision.
Sand Casting
High quality sand casting engineered for strong, reliable components.
Heat Treatment
Thermal treatment improves strength, durability and performance.
Vacuum Impregnation
Porosity elimination for leak-proof, pressure-tight castings.
Sandblasting
Clean, uniform surface preparation ahead of finishing.
Grinding & Finishing
Precision grinding and fettling that ensure accurate dimensions and a flawless finish.
Machining
Precision machining tailored to meet complex specifications and stringent industry standards.
Core Making
Core-making methods, matched to the part.
The right core process depends on the casting's complexity, wall section and production volume.
CO₂ Core
- · CO₂ gas curing process
- · High strength and dimensional accuracy
- · Ideal for large production runs
No Bake Core
- · No bake (air-setting) process
- · Good strength and surface quality
- · Cost effective across applications
Cores are what make sand casting worth choosing. Packed sand forms the outside of a part, but internal cavities — hydraulic passages, oil galleries, hollow sections — are formed by cores set inside the mould and broken out once the metal has solidified. Our fully automatic cold-core machine produces cores up to 25 kg. For how core choice affects castability and cost, see our aluminium sand casting overview.
Grinding & Finishing
From raw aluminium casting to dispatch-ready part.
A casting leaves the mould with flash, feeder stubs and a rough surface. Six finishing operations take it from there to a part your assembly line can use.
Grinding
- Remove extra material
- Smooth surface
- Accurate dimensions
Fettling
- Remove flash & burrs
- Fine edge cleaning
- Better surface finish
Sand Blasting
- Clean, uniform surface
- Remove sand & residues
- Improve appearance
Belt Grinding
- Smooth finishing
- Remove sharp edges
- Final touch
Inspection & Finished Parts
- Dimensional & surface check
- Quality assurance
- Ready for assembly
Packing & Dispatch
- Secure packing
- Safe dispatch
- On-time delivery
Dimensional Tolerance
What tolerances aluminium sand casting holds.
Aluminium sand casting generally works to ISO 8062 casting tolerance grades CT8 to CT12, loosening as the casting grows and as more cores are involved. The figures below describe the process; achievable tolerance on your part is confirmed at quotation against your own drawing.
| Feature | As cast | Machined | Notes |
|---|---|---|---|
| Small castings, up to ~100 mm | ISO 8062 CT8 – CT10 | To drawing | Fewest cores, least thermal spread — the tightest as-cast band we see. |
| Medium castings, ~100–400 mm | ISO 8062 CT9 – CT11 | To drawing | Tolerance widens with the distance across which the casting contracts. |
| Large castings, above ~400 mm | ISO 8062 CT10 – CT12 | To drawing | Contraction and core movement both scale with size. |
| Across a parting line | Add mismatch allowance | To drawing | Any dimension spanning the joint carries the mould-half alignment on top of the base grade. |
| Cored internal features | One grade looser, typically | To drawing | Core print clearance and core float both add variation. |
| Bores, datums, sealing faces, threads | Not held as-cast | To drawing | Machined on our 3-axis and 4-axis VMC line, then 100% dimensionally inspected. |
Every critical feature is machined rather than relied on as-cast, which is why the as-cast grade matters less than it first appears. Bores, sealing faces, mounting datums and threads are cut on our 3-axis and 4-axis VMC line and then verified against drawing — 100% inspected, not sampled. See our quality control and inspection process.
Surface Finish
Surface finish at each stage of the process.
A sand casting reproduces the sand grain in its surface, so it leaves the mould matte and slightly granular. Each finishing operation brings it further down.
As cast
Ra 6.3 – 25 µm
Matte, slightly granular — the sand grain is reproduced in the surface.
Fettled and ground
Ra 6.3 – 12.5 µm
Flash, ingates and risers removed; edges dressed back to the casting line.
Sand blasted
Ra 3.2 – 12.5 µm
Uniform matte finish, clean and paint-ready. Our blasting plant runs at 110 kg/hour.
Machined
Ra 0.8 – 3.2 µm
VMC-machined faces and bores. Verified with a surface roughness tester.
Defects & Prevention
Aluminium casting defects, and how we prevent them.
Casting defects are not bad luck — each one has a known cause and a known control. These are the eight that matter in an aluminium sand foundry, and what we do about each.
Gas porosity
Cause. Hydrogen dissolved in the melt comes out of solution as the metal freezes, leaving rounded pores through the section.
Control. Controlled melt temperature and degassing before pouring, plus dry, properly stored charge material. Residual porosity in pressure-tight parts is sealed by vacuum impregnation.
Shrinkage porosity
Cause. Aluminium contracts as it solidifies. Where a heavy section freezes last with no feed metal available, it pulls a jagged internal void.
Control. Feeding the casting correctly — risers and gating sized and placed so the heavy sections stay liquid longest and draw metal from the riser rather than from themselves.
Inclusions and dross
Cause. Oxide skin and furnace dross carried into the mould with the metal stream, appearing as dark films or hard spots in the casting.
Control. Careful skimming, a quiet non-turbulent pour, and gating designed so the first metal through is trapped rather than delivered into the cavity.
Cold shut and misrun
Cause. Two metal fronts meet without fusing, or the metal freezes before it fills the cavity — usually thin sections, low pour temperature or slow pouring.
Control. Correct pouring temperature, an unhesitating pour, and reviewing wall sections for castability at the quotation stage before the pattern is ever cut.
Sand inclusion and erosion
Cause. Loose or washed sand from the mould or core ends up embedded in the casting surface.
Control. Correct sand bonding and ramming, clean mould cavities, and gating that does not fire the metal stream directly at a sand face.
Hot tearing
Cause. The casting contracts while still weak but the mould or core resists it, tearing the metal — typically at a section change or a sharp internal corner.
Control. Generous fillet radii, adequate core collapsibility, and pattern design that lets the casting contract rather than fighting it.
Mismatch and core shift
Cause. The two mould halves close out of alignment, or a core moves under the buoyancy of the metal, so the wall is thick on one side and thin on the other.
Control. Well-maintained pattern plates and dowels, correctly fitted core prints, and first-off dimensional checks at the start of every run.
Veining and scabs
Cause. The sand or core surface cracks under thermal shock and metal fills the crack, leaving raised fins on the casting.
Control. Correct core binder levels and sand additives, controlled pouring temperature, and avoiding unnecessarily hot, heavy sections against thin cores.
Lead Time
How long an aluminium casting takes.
Four stages from drawing to delivered part. Exact timing is confirmed with your quotation — these are the durations we work to.
Drawing review and quotation
Usually same working day
We review geometry, wall sections, draft and tolerance for castability, and flag anything that would drive avoidable cost.
Pattern preparation
Days to a few weeks
Depends on part size and complexity. If you already hold a pattern, this stage disappears entirely.
First article casting
Around a week
Cored, poured, heat treated, finished and dimensionally inspected against drawing for your approval.
Production casting cycle
Around a week per batch
The full eight-stage route from core making to final machined inspection.
Cost Drivers
What actually drives the price of a casting.
Seven levers, roughly in order of impact. If your target price is tight, these are the places to look — and we will tell you which one is driving yours.
Piece weight and alloy
Metal is the base cost. Weight drives it directly, and the grade on the drawing sets the ingot price.
Core count and complexity
Every core is a separate part to make, store, set and break out. Internal passages are powerful, but they are the single biggest lever on casting cost.
Machining content
Which features actually need machining, and how many setups they take. Consolidating machined faces onto one setup is usually the cheapest change a designer can make.
Heat treatment
A full T6 solution and age cycle adds furnace time. Worth it where the part is structural; wasted cost where it is not.
Pressure tightness
Vacuum impregnation adds a process step, and is only needed where the part has to hold a fluid or gas under pressure.
Batch size
Moulding setup is amortised across the batch. Ordering a year's requirement in two batches rather than eight measurably reduces the piece price.
Inspection and documentation
100% dimensional inspection is standard on every job. Additional certification or reporting for a vendor onboarding pack is quoted on request.
Frequently Asked Questions
The aluminium casting process — frequently asked questions.
Process, tolerance, finish, defect and cost questions, answered in full.
What is the aluminium casting process?
Aluminium casting is the process of forming a part by pouring molten aluminium into a mould and letting it solidify. At Metline Casting the route is aluminium sand casting in eight controlled stages: core making, melting and pouring, casting, heat treatment, vacuum impregnation, sand blasting, grinding and fettling, and finally VMC or CNC machining. A bonded sand mould is packed around a pattern, cores are set to form internal cavities, aluminium is melted and poured under gravity, and once solid the expendable mould is broken away for finishing.
How long does it take to produce an aluminium casting?
Quotation is usually same working day. Pattern preparation takes days to a few weeks depending on size and complexity, or no time at all if you already hold a pattern. A first article casting takes around a week to pour, heat treat, finish and inspect. Production batches then run on roughly a week-long cycle through the full eight-stage route. Exact timing is confirmed with your quotation.
What tolerances can an aluminium sand casting hold?
As a process, aluminium sand casting generally works to ISO 8062 casting tolerance grades CT8 to CT12, loosening as the casting gets larger and as more cores are involved. Metline Casting confirms achievable tolerance against your specific drawing rather than quoting a blanket figure. Critical features — bores, sealing faces, mounting datums and threads — are machined to drawing tolerance after casting on our 3-axis and 4-axis VMC line, and every dimension is verified before packing.
What surface finish does aluminium sand casting produce?
An as-cast sand casting surface is typically around Ra 6.3–25 µm, matte and slightly granular because the sand grain is reproduced in the metal. Grinding and fettling bring that to roughly Ra 6.3–12.5 µm, sand blasting evens it to around Ra 3.2–12.5 µm and leaves it paint-ready, and machined faces reach Ra 0.8–3.2 µm. We verify finish with a surface roughness tester.
What are the most common aluminium casting defects and how are they prevented?
The common ones are gas porosity, shrinkage porosity, inclusions and dross, cold shut and misrun, sand inclusion, hot tearing, core shift and veining. Each has a specific control: degassing and controlled melt temperature for gas porosity, correct riser and gate design for shrinkage, careful skimming and quiet pouring for inclusions, correct pour temperature for cold shuts, generous fillet radii for hot tearing, and maintained pattern plates with first-off checks for core shift. Residual porosity in pressure-tight parts is sealed by vacuum impregnation.
What drives the cost of an aluminium casting?
Piece weight and alloy set the metal cost. After that the biggest lever is core count — every internal passage is a separate core to make, set and break out. Machining content and the number of setups come next, then heat treatment, then vacuum impregnation where pressure tightness is required. Batch size matters too, because moulding setup is amortised across the batch. Send the drawing and we will tell you which of these is driving your price and whether the design offers a cheaper route.
Do you heat treat aluminium castings?
Yes, in-house. Heat treatment develops mechanical strength and dimensional stability. LM25 responds particularly well: from roughly 130 N/mm² as-cast to a 230 N/mm² minimum tensile in sand-cast T6 condition. Because our castings are gravity poured rather than pressure die cast, they carry no entrapped gas and take a full solution and age cycle without blistering.
What is vacuum impregnation and does my part need it?
Vacuum impregnation draws a sealant into the fine porosity that remains in a casting, so the part holds pressure without weeping. Your part needs it if it contains a fluid or gas under pressure — hydraulic manifolds, pump bodies, valve housings, fire fighting fittings. A structural bracket or a lighting housing does not. We run the process in-house, so it adds a step rather than a subcontract handoff.
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Where to go next.
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Send your drawing or sample — we’ll cast it right.
From core making to finished, inspected components, Metline Casting runs the full aluminium casting process in-house.