Casting
Investment casting: what goes wrong and where to inspect
Lost wax casting produces near net shape parts with fine detail and good surface finish: valve trim, pump impellers, medical and aerospace components.
01How the process works
Wax patterns are injected in a metal die, assembled into a tree and dipped repeatedly in ceramic slurry and stucco to build a shell. The wax is melted out, the shell is fired and metal is poured into the hot shell. After knock-out the parts are cut off, finished and heat treated.
Dimensional accuracy depends on wax and shell shrinkage control. Typical problems are ceramic inclusions from shell cracking, shrinkage in isolated sections and surface defects from shell build-up. Because the process has many steps, process audits and first article layouts are essential.
- Wax injection. Patterns injected and assembled into trees.
- Shell building. Repeated slurry and stucco coats, controlled drying.
- Dewax and firing. Wax removed in autoclave; shell fired.
- Pouring. Metal poured into the hot shell, often under vacuum.
- Finishing. Knock-out, cut-off, grinding, heat treatment, inspection.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Ceramic inclusions | Shell cracking, poor shell strength | Visual, X-ray, machining |
| Shrinkage porosity | Isolated thick sections, poor gating | RT, sectioning |
| Dimensional deviation | Wax or shell shrinkage not compensated | CMM layout of first articles |
| Surface roughness and metal flash | Shell defects, cracks | Visual, roughness |
| Misrun | Thin walls, low temperature | Visual, PT |
| Decarburisation | Atmosphere during heat treatment | Hardness, metallography |
03Where an inspector should look
Before production
- Die and wax pattern approval
- Shell system and drying parameters
- Alloy and heat treatment requirements
During production
- Wax pattern dimensions
- Shell drying humidity and time
- Pour temperature, vacuum records
After production
- Full dimensional FAI
- PT and RT per class
- Chemical and mechanical certificates
04Standards and references
ISO 8062 (casting tolerances), ASTM A743 and A351 (stainless castings), AMS specifications for aerospace alloys, customer FAI to AS9102.
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.