Investment Casting Defects Buyers Should Understand Before Approval

Investment casting — also called lost wax casting — delivers near-net-shape parts in stainless steel, carbon steel, and alloy steel. That precision does not mean the castings are defect-free. Every production run carries some level of internal shrinkage, gas porosity, or surface irregularity. The question is not whether defects exist; it is what you agreed to accept at the approval stage. That agreement decides every batch that follows.

Defect control starts at melting and shell building. Waking’s investment casting foundry runs silica sol casting with in-house X-ray NDT and metallography.

The approved sample is the standard you will be held to

Foundries treat the approved sample as the quality reference. When production parts are challenged, the first comparison is always against what you approved. If the sample carried minor porosity you never checked, the foundry can reasonably argue that is the accepted level.

So the approval stage is the wrong time to be casual. This is where first article inspection comes in.

Waking performs first article inspection in-house before shipping. The first part from the first production run is measured and matched against your drawing. Sample approval is a formal step in the quality process, not a courtesy. Because Waking is a real factory rather than a trading company, the people who inspect the part are the same people who cast it. If something is wrong, there is no third party to shift the blame to.

Do not stop at the physical sample. Ask for the records that go with it.

The defects that actually show up in investment castings

Buyers who know the defect names get better answers from suppliers. Here are the ones you will meet.

Gas porosity. Gas trapped during pouring forms round voids, usually near the surface. In stainless steel grades like 304 or 316, it is one of the more common findings. Small, scattered porosity is often cosmetic. Dense porosity near a sealing face is not.

Shrinkage porosity. When a thick section solidifies last, the contracting metal leaves irregular cavities behind. This is the defect that ruins pressure tightness and weakens structural parts. Shrinkage tends to show up in bosses, ears, and heavy walls — exactly the areas that carry load or seal.

Inclusions. Non-metallic material trapped inside the casting acts as a stress raiser. In a valve body or impeller, an inclusion in the wrong place is a rejection.

Misruns and cold shuts. These happen when a thin wall does not fill completely, or when two streams of metal meet without fusing. Usually visible, but sometimes only on the inside of a complex geometry.

Hot tears. Cracks that form during solidification when the casting is restrained while it shrinks. These are geometry-driven. They repeat on the same features until the part or the gating changes.

Here is the practical point: not all defects are equal. A pinhole on a non-critical face is cosmetic. Shrinkage in a pressure-containing area is a functional failure. Buyers who treat every defect the same either overpay for inspection or approve parts that leak.

Internal defects hide where visual inspection cannot reach

You cannot see gas porosity or shrinkage by looking at the outside of a casting. That is where most buyer-supplier disputes start: the part looks fine, but it leaks, or it fails at the wrong place under load.

The practical answer is X-ray NDT. In this case, the foundry’s equipment list is evidence, not marketing.

Waking runs an XTH320 X-ray NDT unit in-house. It screens internal defects — gas porosity, shrinkage — before parts ship. That is a different level of control than a supplier that inspects visually and only sends castings to a lab when the customer complains.

The internal checks do not stop at X-ray. An inverted metallurgical microscope analyzes material structure. A pendulum impact tester checks mechanical properties, including low-temperature performance. Dimensional accuracy is verified on two CMM systems — a COORD3 and a ZEISS SPECTRUM — plus roundness and profile measuring instruments and hardness testers.

For parts that carry pressure, such as valve bodies in CF8 or CF8M, leak testing is the functional check. The part has to hold pressure, not just look like it will. So when you talk to a foundry, ask which inspection method covers each critical feature. “All parts are inspected” is not an answer. “Shrinkage is screened by X-ray on this boss and the sealing face is leak tested” is an answer.

The approval file: what to demand before signing off

Approval is not a moment. It is a file. Before you sign, make sure it contains:

• the first article inspection report, with dimensions checked against your drawing;
• the material certificate and inspection report for the batch;
• X-ray or other NDT records if internal soundness is part of your requirement;
• a written statement of acceptance criteria — which zones may show porosity, what size, what density, and which zones must stay clean;
• batch traceability, so if a defect turns up at your incoming inspection, the foundry can trace backward to the exact pour and forward to every affected part.

Waking issues material certificates and inspection reports together with batch traceability. Its quality system is built around ISO 9001 and IATF 16949. But the acceptance criteria are your job. A foundry can inspect against a rule you wrote down. It cannot guess a rule you kept in your head.

If the acceptance criteria are not written, a “defect” is just an argument. Write them down, and a defect becomes a decision.

Set the acceptance rules before quoting, not after casting

The cheapest time to define quality is before production starts. Here is what that looks like in practice.

First, mark your drawing. Indicate which surfaces are critical, which features will be machined, and which areas will be pressure-tested. A foundry cannot protect a zone it does not know about.

Second, state the NDT requirement in the inquiry. If you need X-ray screening on certain zones, say so in the RFQ. The foundry will quote and plan accordingly, and you will not be surprised later.

Third, use realistic tolerances. Investment casting is known for tight tolerances, but every feature has a practical limit based on geometry and material. The foundry’s engineering review should flag risky features before quoting. If it does not, ask why.

Fourth, consider a supplier that keeps casting, machining, and inspection in one place. Waking runs silica sol investment casting, low pressure and gravity aluminum casting, and CNC machining under one roof — a 70,000 m² plant in Qingdao with over 400 CNC machines. It has exported to buyers in 40+ regions, and it is a real factory, not a trading company. When a machining issue starts looking like a casting defect, one team resolves it. There is no finger-pointing between a foundry and an outside machine shop.

Approval is the last real control point you have before production runs. Use it to set the standard, document it, and make the inspection method explicit. A good foundry will welcome that. It is far easier to cast against a clear rule than to win an argument after the parts are already on a ship.

Frequently Asked Questions

What is the role of the approved sample in investment casting, and how does it affect later production lots?

The approved sample is the quality reference that foundries compare every production part against. If the sample carried minor porosity that was never checked, the foundry can reasonably argue that is the accepted level. Buyers should treat sample approval as a formal quality step supported by first article inspection records.

How do I decide whether gas porosity or shrinkage porosity should be accepted on a stainless steel casting?

Small, scattered porosity on a non-critical face is often cosmetic; dense porosity near a sealing face or in a pressure-containing area is a functional failure. Acceptance criteria should be written before production, specifying which zones may show porosity, what size and density, and which zones must stay clean.

Can visual inspection detect internal shrinkage porosity in investment cast parts?

No. Gas porosity and shrinkage hide below the surface, so outside visual checks cannot catch them. The practical answer is X-ray NDT; Waking runs an XTH320 X-ray unit in-house to screen internal defects before parts ship, and buyers should ask which inspection method covers each critical feature.

What documents should I demand before signing off on a first article from an investment casting foundry?

The approval file should include the first article inspection report with dimensions checked against your drawing, the material certificate and batch inspection report, X-ray or other NDT records if internal soundness is required, written acceptance criteria, and batch traceability. Waking issues material certificates and inspection reports together with batch traceability under ISO 9001 and IATF 16949.

Key Parameters at a Glance