Casting
Gravity and low pressure die casting: what goes wrong and where to inspect
Permanent metal moulds filled by gravity or low pressure give denser castings than high pressure die casting and allow T6 heat treatment, used for wheels, cylinder heads and structural parts.
01How the process works
In gravity die casting, metal is poured into a preheated steel or iron mould, often with sand cores. In low pressure die casting, the mould sits above a sealed furnace and metal is pushed up through a riser tube by gas pressure, filling the mould calmly from below.
Calm filling and directional solidification give sound castings, but mould temperature, coating and pressure curves must be controlled. Parts are commonly solution treated and aged (T6), so heat treatment control and mechanical testing are part of every inspection plan.
- Mould preparation. Mould coated and preheated; cores set.
- Filling. Gravity pour or low pressure fill from below.
- Solidification. Directional cooling under controlled pressure.
- Heat treatment. Solution treatment, quench and ageing (T6).
- Finishing. Machining, X-ray, leak and dimensional checks.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Shrinkage in thick sections | Mould temperature, cooling design | RT, sectioning |
| Oxide films | Turbulence, dirty melt | Fracture tests, RT |
| Low mechanical properties | Poor T6 treatment, coarse structure | Tensile and hardness tests |
| Distortion after quench | Quench practice, fixturing | Dimensional check |
| Coating marks | Worn mould coating | Visual |
| Core gas defects | Core binder, venting | RT, visual |
03Where an inspector should look
Before production
- Mould and core approval
- Alloy, temper and property requirements
- X-ray zones and acceptance levels
During production
- Pressure curves and mould temperature
- Melt treatment (degassing, grain refinement, modification)
- Furnace charts for T6
After production
- X-ray of critical zones
- Hardness and separately cast or cut test bars
- Dimensional and balance or runout checks
04Standards and references
EN 1706 (aluminium casting alloys), ASTM E155 reference radiographs, customer specifications for wheels and structural castings.
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.