Designing for Aluminium Gravity Die Casting: Guidelines for Engineers

1. Start from the die, not the part

In sand and investment casting the mould is broken up to get the part out, so it can wrap around almost any shape. In gravity die casting the mould survives, and Campbell identifies this as the central constraint of the process: the die has to be designed to come apart to release the casting, and that limits both the geometry of the part and the filling and feeding systems that can be built around it (section 15.1.1, p. 798). Everything else about the die follows. It opens along a parting plane, usually vertical, with one moving half that carries the casting and pushes it off with ejector pins as it opens (section 16.1.2, p. 824). Straight-draw features can be formed by metal cores that retract before ejection. Anything that cannot be drawn needs a loose piece, a collapsible core or a sand core, each of which adds cost, cycle time and variation.

The die is also hot and dry, which is why the process gives clean, dense metal: melt and mould are practically inert to each other (p. 824). But the die is still hundreds of degrees colder than the aluminium, so it is sprayed with a refractory die coat of the order of 0.5 to 2 mm thick that slows heat transfer enough for the cavity to fill without cold laps. Campbell notes that this coat thickness varies and eats into the accuracy of the casting (p. 824). Bear that in mind when you tolerance an as-cast face.

The useful habit is to sketch the parting plane and the direction of draw on your own model before you send it anywhere. If you cannot see how the part comes off a two-piece die, the foundry will be adding slides and cores, and the price will show it.

2. Draft and the parting line

Draft is not optional. The casting contracts onto the die as it freezes and has to be pulled off against friction and, on internal features, against the shrinkage grip of the metal on the core. Design values for aluminium: 1 to 2° on external walls, 2 to 3° on internal walls and on cored features, and more on deep pockets, on textured faces and on surfaces that carry a thick coat. A zero-draft face is possible only where it lies in the parting plane or is formed by a slide that pulls perpendicular to it.

Put the parting line where flash and mismatch do no harm. A flat parting plane with every feature drawing perpendicular to it gives the cheapest die and the best repeatability. Dimensions that cross the parting line carry the die's mismatch and its opening tolerance, so ISO 8062-3 adds a parting line allowance to them; keep critical dimensions within one die half. Ejector pins leave shallow marks and must push on a flat or a boss in the moving half, so give the foundry somewhere to put them that is not a sealing or cosmetic face. Say on the drawing which faces are cosmetic; the die designer will keep the split line, ejectors and gate away from them if told, and cannot if not.

3. Wall thickness and where the metal freezes

3.1 How thin

Minimum wall thickness for the Al-Si alloys is 3 to 4 mm as a general design value, with 2.5 mm possible over short distances in the eutectic AlSi12 alloys, and 4 to 5 mm advisable for Al-Mg and Al-Cu alloys. The limit is set by heat: metal loses temperature to the die from the moment it enters, and a thin wall far from the gate, or fed only through another thin wall, freezes before it fills. Thin walls fill best when they are short, close to the gate, and attached to a thicker section that keeps supplying metal. If the part needs 2 mm walls throughout, it is a pressure die casting, not a gravity die casting, and it will not be heat treatable.

3.2 Uniform walls, and the junction problem

Uniform walls are the standard advice and they are right as far as they go. The trap is where walls meet. Campbell summarises the classical junction studies: a thin rib on a wall acts as a cooling fin; a rib of about half the wall thickness is thermally neutral; a rib equal to the wall thickness creates a hot spot at the junction; and only when the appendage is about twice the wall thickness does the wall start to chill the junction instead (section 5.1, pp. 185 to 187). His pointed conclusion is that a casting of uniform wall thickness therefore contains 1:1 T-junctions that are all hot spots. Each one freezes after its surroundings and shrinks with nothing to feed it. In a metal die the effect is sharper than in sand because the die pulls heat out of the plain walls quickly and leaves the junction isolated.

The responses, in order of preference: take metal out of the junction with a cored dimple or recess on the far side so the local mass is no larger than the wall; use an internal radius but do not add a matching build-up outside; or accept the hot spot and place it where a riser or a die cooling insert can reach it. Cored dimples are cheap in a gravity die, since they are just a shaped boss on the die face, so use them.

3.3 Heavy sections, and why the die cares too

A thick boss, flange or lug is an isolated heavy section that freezes last and cannot be fed through the thin wall carrying it. In gravity die casting it has a second cost. Campbell links thermal fatigue failure of the die to the thick regions of the casting, a heavy boss being his example, because there the iron or steel of the die is cycled repeatedly through its transformation and cracks after relatively few cycles (section 16.1.2, p. 824). A heavy section on your part is therefore a hot spot in the casting and a short life for that region of the die. Ask whether the thickness is carrying load or is left over from a machined-from-solid design; core it from the back where function allows; and where it must stay, blend into it with a taper rather than a step.

4. Feeding: heavy at the top, thin at the bottom

Shrinkage porosity in a gravity die casting sits under the heaviest section and at the neck of the riser. The riser can only do its job if it is above the section it feeds and freezes last. Campbell's sixth rule for good castings puts it plainly: no feeding uphill in larger sections, because the pressure gradient is adverse and convection interferes (Rule 6 summary, p. 536). The foundry therefore wants to orient the die so that the heavy sections are at the top, next to the riser, and the thin walls are at the bottom, near the gate, so that the casting freezes progressively from the thin walls up towards the feed metal. The classical check is to inscribe circles in the section and confirm that their diameters increase steadily towards the feeder, refined in three dimensions as a progressive increase in modulus, volume over cooling surface area (section 10.6, p. 584).

Your design decides whether that orientation exists. A part with a thick flange at one end and a thin wall at the other can be oriented flange-up. A part with heavy sections at both ends of a thin span cannot, and will need two risers, or a die cooling insert on one of them, or a change. A closed shape with its heaviest section in the middle of a thin wall has nowhere for a riser to sit at all. Look at your part and ask which face could carry a riser 20 to 40 mm across and where the metal would come in from below; if there is no answer, the foundry will find one you did not intend.

5. Corners, radii, ribs and hot tears

Sharp internal corners concentrate stress in the casting while it is still partly liquid and weak, and they concentrate thermal fatigue in the die. The design rules for avoiding hot tears that Campbell condenses from Kearney and Raffin read as a specification for the process: no sharp re-entrant corners; no straight member joining two potential hot spots, curve it instead; curved gates so that contraction can be accommodated; angled or offset stiffeners and ribs; and, specifically for gravity die casting, rapid withdrawal of any internal steel core to reduce constraint (section 8.1.10.2, p. 436). The last point matters because a metal core does not yield the way a sand core does. A rigid steel core inside a contracting aluminium ring will tear the casting unless it is pulled early, and Campbell notes that die makers routinely design cores to withdraw or collapse inwards as soon as possible after pouring, with aluminium pistons made on complex collapsible five-piece cores (section 8.1.10.4, p. 439). He also warns that timing that removal is difficult, and that passive design measures are the more reliable route. Passive measures are the ones you control: radii, rib layout and section symmetry.

Design values: internal radii 2 to 3 mm, external radii 1 to 2 mm, larger on heavy sections. A grid of ribs meeting at right angles locks the casting against its own contraction inside a rigid die; stagger them or let them meet the wall at an angle, and size them at about half the wall thickness so they stiffen without creating a hot spot (section 3.2). The Al-Mg and Al-Cu alloys are far more tear-sensitive than AlSi7Mg or AlSi10Mg; with those alloys the rules above are mandatory. Campbell adds that in his experience these geometrical dangers rarely give trouble if the melt is clean and the filling is quiet (section 8.1.10.2, p. 438), which is a reason to buy from a foundry that takes filling seriously, not a reason to draw sharp corners.

6. Holes, undercuts and cores

A hole in a gravity die casting is formed one of four ways, in rising order of cost. A straight-draw metal core pin forms holes down to about 6 mm diameter at a length-to-diameter ratio of around 4:1, with larger cores tapered to ease withdrawal. A retractable slide or loose piece forms an external undercut. A sand core forms an internal passage that cannot be drawn: a water jacket, an oil gallery, a manifold runner. A salt core does the same job with a better finish and is dissolved out in water; Campbell describes salt cores as the standard way to make oil galleries in gravity cast pistons and as a technique that deserves wider use (section 15.1.2, pp. 799 to 800).

Sand cores are the option to challenge hardest. They cost more per piece, give an internal surface of Ra 12.5 µm or coarser, and bring binder gas into a hot die. Campbell reports that vents behind core prints block with condensed tars after 15 to 25 castings, that cleaning them out is what usually sets the campaign length of the die, and that neglected vents turn core blows into a major source of scrap (section 16.1.2, p. 825). A design that needs a sand core is therefore a design with a shorter run between die cleans and a higher scrap allowance. If the passage can be split into two straight-draw bores that meet, or made as a drilled and plugged hole, or formed by a cover plate, do that instead. Where a cored passage is unavoidable, give the core generous prints (the seats that hold it in the die), keep its section large enough to survive the pour, and make sure the passage has an exit big enough to shake the sand out and to inspect through.

Holes below about 5 mm, all threaded holes, and any hole with a positional tolerance tighter than the process are drilled after casting; leave them out of the die or cast them as dimples that locate the drill.

7. Leave the metal a quiet way in

Most engineers assume the foundry pours the metal into the top of the die and it runs down. Some do, and Campbell devotes a chapter to why that damages aluminium: after a fall of only a few millimetres the melt exceeds the critical velocity at which its surface breaks up and folds its own oxide film into the metal, and the velocity keeps rising with the fall, so the metal arrives fastest at the start of filling when it should be slowest and slowest at the end when the last thin walls need it most (section 16.1.6, p. 827). Those entrained oxide films, which he calls bifilms, are the cracks that later show up as leaks after machining and as scatter in fatigue life. The good foundries therefore fill from the bottom through a runner that keeps the velocity down, or tilt the die so that the metal rolls into the cavity in its own skin without a fall; in a controlled tilt process the filling rate is set by the rotation, and the channels that fill the cavity can afterwards be sized to feed it (section 16.2.2, p. 829).

What this asks of the design is modest. Leave an accessible face low in the die orientation where an ingate 8 to 15 mm thick can attach and be cut off, and do not make it a sealing face. Avoid geometry that forces metal to fall inside the cavity, for example a tall thin box open only at the top of the die. Keep the thinnest and most remote walls as close to the ingate as the layout allows. And do not insist on gate-free cosmetic faces on every side of the part; a witness has to go somewhere, and it costs least where it can be blended on a plain, non-functional surface.

8. Heat treatment: design for the quench

Most structural gravity die castings are supplied T6: solution treated near 535 °C, quenched, and artificially aged. The solution treatment removes the stress that casting put in, and Campbell notes that gravity die castings carry more of that stress and less reproducibly than sand castings because die temperature and ejection time vary (section 10.9.2, p. 617). The quench then puts stress back, and this time it stays: the surface cools and stiffens first, the interior cools later and is held in tension, and ageing is too cool to relieve it (section 10.9.3, p. 617). For aluminium the quench strain works out at roughly 1%, around ten times the yield strain (p. 618). Campbell's ninth rule is blunt: no water quench, cold or hot, after solution treatment of light alloys; polymer or forced air quench where the resulting properties can be shown acceptable (Rule 9 summary, p. 536).

The geometries he flags as most at risk are the ones designers like: hollow parts with limited access for the quenchant inside, and with internal dividing walls and ribs (p. 618). For the engineer this gives three rules and a conversation. Keep sections symmetric about their neutral plane, so that the quench bends the part less. Avoid closed or nearly closed boxes with internal ribs; open them up or rib them outside. Put flatness and position tolerances on machined faces and state that they apply after heat treatment. Then talk to the foundry about the quench. EN 1706 and most customer specifications are written around water quenching because that gives the highest test-bar properties; if your part is large, hollow or asymmetric, a polymer or hot water quench that gives up a few percent of strength may give a straighter and more reliable component, and the specification can be worded to allow it. Where full T6 strength is not needed, T5 or T64 avoids the quench distortion altogether.

9. Datums, location points and tolerances

9.1 Datums that exist on the casting

Campbell's example of a badly dimensioned casting is a gravity die cast aluminium sump, which makes it directly relevant here. The length of such a part varies with die temperature and ejection time; the drawing put the datum at one end, the only critical feature, a dipstick boss, at the far end, and defined the datum on a row of machined holes that did not exist when the casting was first inspected. Moving the datum to the boss made the boss impossible to misplace and let everything else float harmlessly (section 10.10.1, pp. 631 to 633). His rules, partly after Swing: three orthogonal datum planes; parallel to the machine tool axes; fixed on real cast features such as a boss edge or a wall face, chosen because their position is critical and because they sit close to the middle of the part (p. 633).

9.2 Location points

Datums are planes; the casting is located on location points, small as-cast pads that the toolmaker, the foundry and the machinist all use, in a 3-2-1 arrangement: three points for the primary plane spread over a wide triangle, two for the secondary, one for the tertiary (section 10.10.2, pp. 633 to 634). Campbell is emphatic that they must be cast-on features, never a bore centreline, never a surface that gets flash dressed off it, and never machined, since a machined location point raises the question of what located the casting to machine it. In a gravity die this is easy to arrange: ask for six small pads on the die in places the ejectors and gates do not touch, dimension from them, and agree them with the machinist before the die is cut.

9.3 Tolerances and machining allowance

Specify a general tolerance to ISO 8062-3. Aluminium gravity die castings from a machined die typically hold DCTG 7 to 9, roughly ±0.3 to ±0.5 mm on a 100 mm dimension within one die half, with DCTG 6 achievable on small, well-controlled parts. Add the parting line allowance for dimensions across the split and the core allowance for sand-cored features. Tolerance the few features that matter tightly and everything else to the general grade; a drawing with ±0.1 mm everywhere is quoted as a machined part or not at all. Any feature tighter than process capability, any sealing face and any bore that must be round is shown as machined with an allowance of 1 to 2 mm depending on size, and a little more on faces that were at the top of the casting, where oxide and dross gather. As-cast surfaces from a coated die are typically Ra 3.2 to 6.3 µm and reproduce the coat texture, so agree a reference sample if appearance matters.

10. Where the money goes

Gravity die tooling is cheap by die casting standards and expensive by sand casting standards, so the process pays from a few hundred pieces a year. The cost of the casting itself is driven, in rough order, by metal weight including the runner and riser that are re-melted but not free; cycle time, which is set by the heaviest section; sand cores and the cleaning they impose; slides and loose pieces; heat treatment and straightening; machining; and the inspection level, especially leak testing on housings. Cast-in identification (alloy, part number, cavity number) costs nothing once it is in the die and should sit on a surface that is neither machined nor a gate face; raised characters cast better than recessed. Steel or brass inserts can be cast in if they are clean, preheated and positively located, but every insert adds handling to every cycle.

The strongest case for the process is consolidation with properties: a welded fabrication or a machined-from-solid housing becomes one heat-treatable, leak-tight part. Apply the rules above to the new geometry, not to the parts it replaces. A fabrication redrawn as a casting has just acquired a set of 1:1 junctions and a rib pattern chosen for a welding jig.

11. Checklist before releasing the die

For alloy selection, melt quality, heat treatment, defects and inspection standards, see our companion article, Aluminium Gravity Die Casting: An Engineer's Reference.

Frequently asked questions

How much draft does an aluminium gravity die casting need?

As a design value, 1 to 2° on external walls and 2 to 3° on internal walls and cored features, increasing for deep pockets. Unlike sand or investment casting the die is rigid and reusable, so the casting must be pulled off it; zero draft is not an option on any face that lies along the die opening direction.

What is the minimum wall thickness for gravity die casting?

About 3 to 4 mm as a general design value in the Al-Si alloys, with 2.5 mm possible over short distances near the gate in the eutectic AlSi12 alloys. The limit is set by the metal freezing against a die that is far colder than the melt; the die coat slows the heat loss but cannot remove it.

Why does the foundry want the heavy sections at the top of the die?

Because the riser that feeds a heavy section must sit above it and be the last thing to freeze. Feeding uphill is unreliable. A design that puts a thick flange at the bottom of the die and a thin wall above it cannot be fed properly, so it is either re-oriented, gated from the flange face, or changed.

Will my aluminium casting distort after T6 heat treatment?

Some movement is normal. The water quench after solution treatment cools the surface before the interior and locks in stress; hollow, ribbed or asymmetric parts move most. Keep sections symmetric, avoid closed boxes with internal ribs, tolerance flatness after machining, and discuss a polymer or hot water quench with the foundry where the specification allows it.

What tolerances should I put on a gravity die casting drawing?

A general tolerance to ISO 8062-3, typically DCTG 7 to 9 for aluminium from a machined die, with parting line and core allowances added for dimensions that cross the die split or are formed by sand cores. Show any feature that needs better than that as machined with a 1 to 2 mm allowance, and dimension everything from three datums that exist on the as-cast part.

Sources and further reading

Campbell, J., Complete Casting Handbook: Metal Casting Processes, Metallurgy, Techniques and Design, 2nd edition, Butterworth-Heinemann, 2020. Sections cited: 5.1 (junctions and hot spots), 8.1.10 (hot tearing: casting design, reduced constraint and metal cores), 10.6 (directional solidification and modulus), Rule 6 and Rule 9 summaries and 10.9 (feeding direction, residual stress and quenching), 10.10 (datums and location points, including the gravity die cast sump example), 15.1.1 and 15.1.2 (permanent moulds and salt cores), 16.1.2 and 16.1.6 (gravity dies, die coat, die life, vents and the limits of gravity pouring), 16.2.2 (controlled tilt casting). Statements attributed to Campbell are paraphrased; readers needing the underlying data and arguments should consult the handbook. The junction and directional solidification principles are general; the sump, die life, venting and quench examples are drawn from gravity die casting of aluminium.
Standards referenced: ISO 8062-3 (dimensional and geometrical tolerances for castings); EN 1706 (aluminium alloy castings, chemical composition and mechanical properties); EN 10204 (inspection documents). Standard numbers are given for identification; always work from the current edition.

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