Casting Processes: Investment, Sand & Permanent Mold

Investment casting, sand casting, and permanent mold casting all deliver a finished cast metal part, for example in aluminum. Tolerance, surface finish, and unit cost differ substantially between them, though. The choice of process can therefore decide whether a project succeeds. Leave that detail out of the inquiry, and the difference often only shows up at acceptance. In the worst case, it only becomes visible once the part reaches your own production line.

In brief: Investment casting, sand casting, and permanent mold casting differ mainly in the mold. Investment and sand casting destroy the mold after every cast, while permanent mold casting reuses a metal mold. Investment casting achieves the highest precision, with tolerances around ±0.6% of nominal dimension. Permanent mold casting only pays off at medium to large series. Sand casting is the most flexible process, covering single units through mass production.

What fundamentally distinguishes the common casting processes?

The three processes differ mainly in whether the foundry destroys the mold after casting or reuses it. Investment casting and sand casting both use so-called lost molds, which have to be destroyed to remove the part. Permanent mold casting, by contrast, uses a durable metal mold that gets reused across many casting cycles.

That basic choice largely determines the economics. Lost molds cost roughly the same per part, regardless of quantity. A permanent mold's high tooling cost, by contrast, only pays off once spread across a sufficiently large series. The same basic principle applies to material selection here as with any other manufacturing process. The material has to match the part's actual load, regardless of the chosen casting process. Which of the three casting processes ends up more economical can never be answered in general. It depends on batch size, tolerance requirement, and part geometry in each specific case.

How does investment casting work?

The investment casting process runs in four steps:

• A wax model of the part takes shape and gets coated with a refractory ceramic shell.

• After drying, the foundry melts the wax out of the ceramic shell.

• It fires the remaining shell, then fills it with molten metal.

• Once the metal solidifies, the foundry breaks away the shell to retrieve the finished part.

The process achieves tolerances around ±0.6% of nominal dimension. Surface roughness comes in at Ra 1.6 to 6.3 micrometers, noticeably finer than the other two processes. Investment casting suits parts from under a gram to over 100 kilograms. It also handles complex geometries that would be hard to produce economically with other casting processes.

In our sourcing projects, investment casting often gets underestimated because it looks more elaborate than sand casting on paper. For complex stainless steel or aluminum parts with tight tolerance requirements, though, it's frequently the more economical solution overall. It needs correspondingly less machining afterward.

How does sand casting work?

With sand casting, the mold is made of special sand compacted with binding agents. After casting, this so-called lost mold has to be destroyed to remove the part. A new sand mold gets made for every subsequent part.

Of the three casting processes, sand casting is the most flexible across industries. It suits nearly all metals and covers batch sizes from a single unit through mass production. The achievable dimensional accuracy and surface finish sit below investment casting's. Modern molding materials and compaction techniques improve them noticeably, though. For large-format parts like machine beds or housings, a permanent mold often doesn't pay off economically. Sand casting is often the only practical option in those cases.

How does permanent mold casting work?

The permanent mold casting process uses a durable, indestructible mold made of steel or cast iron. It runs in four recurring steps:

• The foundry prepares the mold and, if needed, coats it with a release layer.

• It fills the mold with molten metal.

• Once the metal solidifies, it opens the mold and removes the part.

• It reuses the same mold for the next casting cycle.

That's what only makes this casting process economical at medium to large series. The mold's high tooling cost has to be spread across many casting cycles.

The faster cooling inside the metal tool produces a finer-grained, denser structure with greater uniformity than sand or investment casting. General tolerances don't automatically apply to permanent mold casting, though. You should always coordinate dimensions directly with the foundry.

In our sourcing projects, we repeatedly see sand casting requested first for hydraulic components rated above 250 bar of operating pressure. After talking to the manufacturer, permanent mold casting in ductile iron often turns out to be the better choice. That holds true both for leak-tightness and for repeatability.

How do tolerance and surface finish differ by casting process?

Achievable accuracy ties directly to the mold material. The finer and more dimensionally stable the mold material, the tighter the achievable tolerance and the smoother the finished part's surface. For machined parts, form and position tolerances cover this with general standard values. Casting processes have no comparably unified table, since the three processes differ too fundamentally in mold material for that.

Which casting process fits which batch size and part complexity?

The economical batch size depends mainly on the mold type. Lost molds, as in investment and sand casting, allow single-unit production. A permanent mold only pays off from medium to large series. On part complexity, investment casting leads, since the wax model can capture nearly any geometry.

What applies if the drawing doesn't specify a casting process?

If the casting process is missing from the drawing, the manufacturer in practice usually decides based on their own equipment. The process that actually fits the part often plays no role in that decision. General tolerances per ISO 2768 apply automatically without a special note. Choosing a casting process itself works differently: there's no default case buyers can silently assume.

Specifying the desired casting process and the critical tolerances alongside the material at the inquiry stage avoids rework or rejection. That matters even more the tighter the tolerance requirement on the part in question.

What materials get used with each casting process?

Which casting process fits also depends on the material chosen. Not every alloy processes equally well in every mold type:

Frequently Asked Questions

What is a permanent mold?

A permanent mold is a durable, reusable casting mold made of metal, usually steel or cast iron. It gets opened after every cast and reused for further casting cycles, unlike the lost molds used in investment and sand casting.

What's the difference between investment casting and sand casting?

Both use lost molds destroyed after casting, but differ in mold material and achievable precision. Investment casting uses a ceramic shell around a wax model and reaches noticeably tighter tolerances. Sand casting uses compacted sand and is more flexible on part size and material.

When does permanent mold casting pay off?

As a casting process, permanent mold casting only pays off economically at medium to large series. The metal mold's high cost has to be spread across many casting cycles. For single parts or small batches, investment or sand casting is usually the more economical choice.

Which casting process is cheapest for small batches?

For small batches and single parts, sand casting and investment casting are usually cheaper than permanent mold casting. No expensive permanent mold needs to be procured. The choice between the two then mainly depends on the required tolerance and part complexity.

Conclusion: the casting process determines tolerance, cost, and batch size

Investment casting, sand casting, and permanent mold casting solve the same basic problem: shaping a part. Each casting process does that in its own way, with its own strengths in precision, batch size, and material. Fixing the right process at the inquiry stage, rather than leaving it to the manufacturer, avoids surprises later on. That applies to tolerance just as much as to cost.

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