Gravity Casting vs. High Pressure Die Casting: How to Choose the Right Process for Your Part

Two of the most common ways to turn molten aluminum into a finished component are gravity casting and high pressure die casting. They sound similar, and both use a permanent metal mould, but they produce different parts, at different costs, for different volumes. Pick the wrong one and you either overpay for tooling you will never amortize, or you end up with a part that leaks, cracks under heat treatment, or fails a pressure test.

At Align Manufacturing we run both processes through our partner foundries in Vietnam, Thailand and India, so we see the same question from customers every week: which one should this part be? This is the answer we give them.

What actually happens in each process

Gravity casting (also called permanent mould or gravity die casting) fills a steel or iron mould using nothing but the weight of the metal. The mould is preheated, coated with a refractory wash, and the aluminum is poured in through a riser system by hand or by a tilting machine. The metal flows slowly, in a controlled, non-turbulent stream, and solidifies from the thin sections toward the risers, which feed the shrinkage. Cycle times run from one to several minutes. Sand cores can be placed in the mould to make internal passages.

High pressure die casting (HPDC) injects the metal. A shot of molten aluminum is ladled into a cold chamber, and a hydraulic plunger forces it into a hardened steel die at speeds of 30 to 100 metres per second, then holds it under pressures of 50 to 120 MPa while it freezes. The whole shot takes a fraction of a second. The die is water cooled, sprayed with release agent, and opens again in 30 to 90 seconds. Only metal cores (slides) can be used, because sand would be blown apart by the injection.

That one difference, gravity versus injection, drives everything else.

Materials

Both processes run aluminum, and that is where most of the overlap ends.

Gravity casting works well with heat treatable aluminum alloys. A356 and A357 (AlSi7Mg) are the workhorses, and they can be solution treated and aged to T6 for mechanical properties close to a forging. Gravity foundries also pour copper alloys (brass and bronze for valves and fittings), zinc, and some magnesium. Because the fill is quiet and the mould is fed by risers, the castings are dense and low in gas porosity, which is why they survive heat treatment and welding.

HPDC alloys are chosen for castability, not for heat treatment. ADC12, A380 and A383 (roughly AlSi11Cu3 to AlSi9Cu3) dominate. They have high silicon and copper content so they flow well and resist die soldering. Standard HPDC parts trap air during the high speed fill, which means they cannot be solution treated (the trapped gas blisters the surface) and cannot be reliably welded. Vacuum assisted HPDC and the newer low-iron structural alloys (Silafont 36, Castasil, Aural) have changed that for automotive body parts, but they need a vacuum die, tighter melt control and a foundry with that specific capability. Zinc alloys (Zamak 3 and 5) are the other big HPDC family, and magnesium AZ91 for lightweight housings.

The practical rule: if the drawing calls for a T6 temper, a weld, or a pressure tight section thicker than 6 to 8 mm, you are probably looking at gravity casting.

Tolerances and surface finish

HPDC wins on precision. Under DIN 1688 or ISO 8062, a pressure die casting can hold roughly ±0.1 mm over the first 25 mm and ±0.05 mm per additional 25 mm, with draft angles of 1 to 2 degrees and wall thicknesses down to 1.5 to 2 mm on aluminum and under 1 mm on zinc. Surface finish is typically Ra 1.6 to 3.2 µm straight out of the die. Holes, threads bosses, and fine lettering can be cast in, and many parts ship with only a trim and a light deburr.

Gravity castings are coarser. Expect ±0.3 to ±0.5 mm on the first 25 mm, draft of 2 to 3 degrees, and a minimum wall closer to 3 to 4 mm. Surface finish depends on the mould coating and lands around Ra 3.2 to 6.3 µm. Almost every gravity casting gets machined on its functional faces, and the drawing is usually written that way, with cast dimensions on the body and machined dimensions where it matters.

If a customer sends us an HPDC drawing with ±0.05 mm on an as-cast feature, we ask about it. If they send a gravity drawing with the same call-out, we tell them it will be machined.

Tooling and quantities

This is where the decision usually gets made.

An HPDC die is a serious piece of engineering. The cavity inserts are H13 or SKD61 tool steel, heat treated, machined and polished, mounted in a steel base with cooling circuits, ejector pins, slides and overflows. For a part the size of a gearbox shift fork, a four cavity die runs USD 30,000 to 45,000 from a Vietnamese or Chinese toolmaker, and that is before a machining fixture. Lead time is 10 to 14 weeks to first samples. In return, the die will produce 80,000 to 150,000 shots on aluminum before major refurbishment, and each shot may deliver two, four or eight parts in under a minute.

A gravity mould is simpler and cheaper. Cast iron or lower grade steel, often single cavity, no hydraulic slides, sand cores made in a separate core box. Tooling for a comparable part is typically USD 5,000 to 15,000 with samples in 6 to 8 weeks. The mould life is also long, but the cycle is slow and manual, so the labor content per part is higher.

Run the arithmetic and the crossover appears. At 500 to 5,000 pieces a year, the gravity mould pays for itself and the higher piece price does not matter much. At 20,000 pieces a year and above, the HPDC die amortizes to a few cents per part and the fast cycle drives the unit cost well below gravity. Between 5,000 and 20,000, the answer depends on the part: complexity, machining content, and how many years the program will run.

Where each process lives

Gravity casting dominates where integrity matters more than cosmetics. Oil and gas valve bodies and pump housings, hydraulic manifolds, marine fittings, railway brake components, cookware, lighting housings and any part that will see a pressure test, a T6 heat treatment or a weld. Wheels for the aftermarket are gravity cast for the same reason. Volumes are moderate, the parts are often thick walled, and machining is expected.

HPDC owns the high volume, thin walled world. Automotive is the biggest customer: transmission housings, shift forks, brackets, oil pans, steering components and electronic housings. After that, power tools, appliances, telecom enclosures, motor housings and consumer hardware. Zinc HPDC covers locks, hardware and decorative trim. The common thread is annual volume in the tens or hundreds of thousands and a design that can live with the as-cast material properties.

Why each is right for its own job

Gravity casting is the right answer when the part must be sound all the way through. The slow fill keeps oxide skins out of the metal, the risers feed the shrinkage, and the result can be X-rayed, machined into a sealing face, pressure tested to 30 bar and heat treated without surprises. It also accepts sand cores, so complex internal galleries that no metal slide could ever pull are routine. The price you pay is a rougher part, more machining, a slower cycle and a wider tolerance band.

HPDC is the right answer when the part is thin, detailed, high volume and cosmetic. The high pressure fill packs a 2 mm wall with a near net shape finish, holds tolerances that let you skip machining on most features, and does it every 45 seconds. The price you pay is the tooling investment, the porosity inherent in the process, and a material that cannot be strengthened or welded afterwards. Design for HPDC and you also design for uniform walls, generous draft and no undercuts, because the die has to open.

How to choose

Ask five questions in this order.

First, what happens to the part after casting? Heat treatment, welding or a pressure test on a section over 6 mm pushes you to gravity. If the answer is "trim, deburr, ship," HPDC stays in play.

Second, what is the wall thickness? Under 3 mm, HPDC. Over 6 mm with a thick hub or boss, gravity, because HPDC will leave a shrinkage void in the middle of anything that thick.

Third, are there internal passages a metal slide cannot pull? If yes, gravity with sand cores.

Fourth, what is the volume, and for how many years? Multiply annual quantity by program life and divide the tooling difference by that number. If HPDC tooling costs USD 25,000 more and you will make 200,000 parts, that is 12 cents a part, which the faster cycle will recover several times over. If you will make 3,000 parts, it is USD 8 a part and gravity wins before you even look at piece price.

Fifth, what tolerances are as-cast and which are machined? If the drawing needs ±0.1 mm on features that cannot be machined, HPDC. If everything tight is going to be machined anyway, the casting process only has to deliver stock, and gravity is fine.

There are parts that sit on the line: a 10,000 per year pump bracket with 4 mm walls and no heat treatment could go either way. In those cases we quote both, and the customer decides with real tooling and piece prices in front of them rather than a rule of thumb.

A worked example

A customer recently sent us three gearbox shift forks: 130 grams each, 2 to 4 mm walls, an 18 mm bore and two reamed pin holes, 220,000 pieces a year per part for six years. No heat treatment, no welding, a machined bore and pad faces. That is a textbook HPDC part. The four cavity dies cost more than a gravity mould would, but at 1.3 million parts per die over the program they amortize to about 3 cents apiece, and the as-cast tolerances mean the only machining is the bore, the pin holes and the pad faces.

The same week we quoted a 4140 alloy component for a defense contractor at 490 pieces. That is not a casting job at all, it is a forging, and the customer was right to ask. Knowing which process a part belongs to is half the value a contract manufacturer brings, and it is the first thing we work out before a factory ever sees the drawing.

If you have a drawing and are not sure which way it should go, send it to us. We will tell you which process fits, what the tooling looks like, and what the part will cost at your volume, in Vietnam, Thailand or India.