Permanent Mold Casting: The Production Process Explained

Permanent mold casting pours molten aluminum into a reusable steel die instead of a sand mold that is broken away after every pour. The die gives you the same geometry shot after shot, and because there is no mold to make for each part, the cost per casting falls once the tooling is paid for. This is the production process at Protocast, and it is the umbrella for two ways of filling the die that each have their own page in this hub. This guide covers how the process works, where it fits between sand casting and die casting, what to expect from a permanent mold program, and the questions buyers ask most.

How permanent mold casting works

The mold is machined from steel in two or more sections that close to form the cavity of the part, together with the gating that lets metal in and the risers that feed it as it shrinks. Holes and pockets that pull straight are formed by steel cores; passages that curve or undercut are formed by sand cores set in the die, which is called semi-permanent mold. The die mounts in a machine that clamps it, tilts it for the pour, opens it, and ejects the casting. A typical cycle looks like this:

1. Preheat and coat. The die is brought to operating temperature and sprayed with a refractory mold wash. The coating controls how fast heat leaves the metal, protects the steel, and helps the casting release.
2. Set cores and close. Any sand cores are placed in their prints, the die closes, and the machine clamps it.
3. Pour. In tilt pour, metal is ladled into a basin and the die rotates so the metal enters as a smooth front. In low pressure, metal is pushed up into the die from a furnace beneath it.
4. Solidify. The steel pulls heat quickly. The casting freezes from the walls inward and from the far end back toward the risers, which feed shrinkage as it happens.
5. Open and eject. The die opens and ejector pins push the casting out. Cycle time depends on part size and section thickness.
6. Trim, heat treat, machine, inspect. Gates and risers are removed, the casting is heat treated to T6, machined where the drawing calls for it, and inspected.

Permanent mold dies between runs at Protocast. Each die belongs to the customer who paid for it and is stored, maintained, and refurbished at the foundry.

Left: a tilt pour die opening after a shot. Right: the castings coming out, gating still attached.

Two ways to fill the die

“Permanent mold” describes the tooling. What changes underneath it is how the metal gets into the die, and each method has its own guide:

• Tilt pour, also called gravity pour. The die tilts as metal is poured so gravity fills the cavity in a controlled front. The simplest tooling and the most common method. The industry also calls this gravity die casting or gravity permanent mold.
• Low pressure. The alloy is pushed up into the die from a sealed furnace underneath. The pressure fills thin walls and heavy sections that gravity struggles with and keeps feeding the casting as it solidifies. It covers the same volume range as tilt pour; the part decides. Many buyers call this low pressure die casting; it is the same process.

Both use steel tooling. Neither is high pressure die casting, which injects metal at far higher pressure with different tooling and different economics. If you are comparing quotes, that distinction matters more than the name on the quote, and the tilt pour versus low pressure guide walks through choosing between the two.

Where permanent mold sits between sand casting and die casting

Every casting process is a trade between tooling cost and per-part cost. Sand casting has a very low tooling cost and a short path to a first part, but every casting needs a new mold, so the per-part price reaches a floor and stays there regardless of quantity. High pressure die casting has the lowest per-part price but the highest tooling cost, the largest minimum volumes, and castings that generally cannot be heat treated. Permanent mold is the middle: real steel tooling at a fraction of a high-pressure die, per-part prices that fall with volume, and mechanical properties and heat treatability that die casting cannot match.

Typical figures. The sand versus permanent mold guide walks through the crossover point with numbers.

What to expect from a permanent mold program

Figures are typical and depend on geometry, draft, radii, and current workload. Your quote is the number that counts.

Designing a part for permanent mold

A steel die does not forgive the way a sand mold does. The rules are not exotic, and most parts that cast well in sand need only small changes, but they need to be made before the die is cut. Keep walls uniform and no thinner than about 0.125 inch. Put draft on every face parallel to the pull, 2 degrees as a starting point and more on deep pockets and cores. Radius every inside corner, which protects the casting from cracking and the die from heat checking. Place the parting line so critical dimensions sit on one side of it. Leave machining stock on faces that need tighter tolerance than as-cast. The design guide covers each rule with numbers, and the DFM review on every quote applies them to your part.

Semi-permanent mold: sand cores in a steel die

When a part needs internal passages a steel core cannot pull out of, a sand core is set into the steel die before each pour. You keep the finish and repeatability of the steel mold on the outside and get the complex interior of a sand casting. It adds a core to every shot, so it costs a little more per part, and it is often the answer for manifolds and housings with internal galleries. The semi-permanent mold guide covers what it costs and when to use it.

Alloys

Most permanent mold castings at Protocast are poured in 356, an aluminum-silicon casting alloy that flows well, resists hot cracking, and heat treats to a strong, ductile T6 condition. 357 is the same family with more magnesium and tighter chemistry, and it is specified when the part is strength critical, which usually means aerospace, defense, or a highly loaded component. Zinc alloys are also poured for small, detailed parts that do not need heat treatment. Alloy chemistry is verified on an in-house spectrometer. The alloy guide has the published property tables and the trade-offs.

After the casting: heat treat, machining, finishing, inspection

A permanent mold casting rarely ships as cast. Aluminum castings reach their published strength only after T6 heat treatment, which Protocast runs in house on a bottom-drop furnace that handles parts up to 48 by 48 by 60 inches and can take most castings to T6 in 2 to 3 days, often overnight. Machining happens in the same building on eight CNC machines that also cut the permanent mold tooling: tapped holes, sealing faces for gaskets, and any feature a raw casting cannot hold, using fixtures designed around the casting. Before machining, every permanent mold casting goes through the grinding department, where the runner and gate are cut off on a band saw and the gate is ground back by hand; that hand work is part of why permanent mold tolerances sit between sand and high pressure die casting. Anodizing, chem film, and powder coating are available. Dimensional inspection runs on an in-house CMM, and pressure and leak testing, radiographic inspection, fluorescent penetrant inspection, and tensile testing are available when a program requires them.

Aluminum castings out of heat treat, headed for the machine shop.

Applications and industries

Medical cart handleTilt pour. About 20 lb, 1,000 a year.

Pressure relief valveLow pressure. About 4 lb, 30,000 a year.

Aluminum ground screwTilt pour. About 2 lb, 15,000 a year.

PVC pipe fusion equipmentLow pressure. About 50 lb, 1,000 a year.

Permanent mold suits parts that need a good surface, consistent dimensions, and real mechanical properties at volumes from a few hundred to tens of thousands a year: the chunkier castings that high pressure die casters in North America usually pass on because the quantities are under 25,000, 50,000, or 100,000 a year. That is the niche Protocast is built around. Typical parts include housings and enclosures, pump and valve bodies, manifolds, brackets and structural supports, covers and end caps, heat sinks, and machine components. Protocast has served aerospace, defense, medical, industrial equipment, automotive, agriculture, and consumer product customers from Commerce City since 1995, and its permanent mold work is ITAR registered and ISO 9001:2015 certified for programs that require it. The aerospace and defense guide covers what those programs ask for.

Common defects and how they are prevented

A well-designed permanent mold program produces very consistent castings, and most problems trace to a handful of causes that are addressed before the die is cut.

What drives the cost

• The die. $35,000 to $40,000 for small, simple parts, up to $200,000 for large parts with slides, most under $100,000. Judged as die cost divided by lifetime volume; see the tooling guide.
• Metal weight. The casting plus its gating, part of which is remelted. Lighter, better-designed parts cost less.
• Cycle time. Heavy sections take longer to freeze; sand cores add setting time.
• Secondary operations. Heat treatment, machining content, finishing, and testing, each quoted as its own line.
• Volume and release pattern. Steady releases run more efficiently than sporadic ones.

Why Protocast is building around permanent mold

Protocast started in 1995 as a prototype foundry, and prototype work has a rhythm to it: every job is small, every customer needs finding again, and the shop is either buried or looking for work. Permanent mold changes that. A production program on blanket order keeps the foundry steady, and a part that starts as a sand prototype here can move to a die here without changing suppliers. Protocast has been investing in permanent mold since 2019 and has kept at it: two tilt pour machines run today with a third ready to come online, and one low pressure machine runs daily with a second being brought into service. The model is simple. Sand casting and printed sand cover new product development and prototypes; a low-production sand tool feeds parts while the permanent mold die is cut over 12 to 20 weeks; then the program moves to the die and stays there.

The reason it works as a niche is that permanent mold foundries are scarce compared with high pressure die casters, and most North American die casters do not want programs under 25,000 pieces a year, or in many cases 50,000 or 100,000. Those programs are exactly what Protocast wants, and for many of them the casting is better for it: a slower, gravity or low pressure fill leaves less porosity and a more uniform grain than high pressure injection, so the part is stronger.

Terms you will see on a quote

EAU

Estimated annual usage: the number of pieces per year. It decides whether a part belongs in sand or permanent mold; geometry then decides tilt pour or low pressure.

Tilt pour / gravity pour

Filling the die by rotating it so metal enters under gravity as a controlled front.

Low pressure (LPDC)

Filling the die from below with a small amount of air pressure from a sealed furnace.

Semi-permanent mold

A permanent mold with sand cores for internal geometry.

Mold wash

The refractory coating applied to the die surface between shots.

Riser

A reservoir in the mold that feeds metal to the casting as it shrinks.

Draft

The taper on faces parallel to the die opening so the casting releases.

Parting line

Where the die halves meet; visible as a fine line on the casting.

T6

Solution heat treated, quenched, and artificially aged; the standard temper for 356 and 357.

DFM

Design for manufacturability: the review of your part for how it will cast, machine, and inspect.

First article

The first production-representative parts, inspected against the drawing for your approval.

How Protocast fits in

Nearly half of the staff hold engineering degrees, and the same people who review the prototype design the die. Permanent mold dies are cut in Protocast’s own tool room whenever possible, and the customer owns them. The die room, the heat treat furnace, the machine shop, and the CMM are in one 55,000 square foot building, so a permanent mold part never leaves the property between the pour and the shipping dock. Tilt pour and low pressure both run in house, so the fill method is chosen for the part rather than for the machine that happens to be free.

Frequently asked questions about permanent mold casting