The most common casting defects are gas porosity, blow holes, shrinkage, sand inclusions, cold shuts and misruns. Most of them come from three sources: gas in the metal or mould, poor feeding as the metal shrinks, or loose sand and slag getting into the cavity.
Many defects are hidden under the skin and only appear when the part is machined or pressure tested. That makes them expensive to find late.
Gas defects
- Blow holes are smooth, rounded cavities near the surface, caused by gas from damp sand, cores or chills that cannot escape the mould.
- Gas porosity is a scatter of small round holes, often from hydrogen or nitrogen dissolved in the melt or from wet ladles.
- Pinholes are very small gas holes just below the surface, often revealed by the first machining cut.
Shrinkage defects
Metal shrinks as it solidifies. If the last area to freeze is not fed with liquid metal from a riser, a cavity forms. Shrinkage cavities are rough and jagged, unlike the smooth walls of a gas hole.
They usually sit in thick sections, at junctions between thick and thin walls, or under bosses. Steel and SG iron are more prone to shrinkage than grey iron, which partly compensates by expanding as graphite forms. Fixes include better riser design, chills, and design changes that even out wall thickness.
Mould and pouring defects
- Sand inclusions are lumps of mould sand trapped in the metal, usually from weak moulds or eroded gating.
- Slag inclusions are non-metallic material from the melt that was not skimmed or filtered out.
- Cold shuts are seams where two metal streams met but did not fuse, often from low pouring temperature.
- Misruns are incomplete sections where the metal froze before filling the mould.
- Mould shift and core shift move features out of position across the parting line or around cores.
- Hot tears are cracks formed while the metal was still weak and hot, usually where shrinkage was restrained.
How to catch them
Visual inspection finds surface defects. Machining trials on first samples reveal subsurface porosity. For internal defects, radiography and ultrasonic testing are used, as explained in radiography and ultrasonic testing for castings. Pressure-tight parts such as pump and valve bodies should be leak tested.
Agree acceptance criteria before production. Your drawing or order should say which areas are critical, what size and number of defects are acceptable, and whether repair welding is allowed. Without this, a foundry and a buyer can argue for weeks over the same part.
Defects as a supplier signal
A good foundry does not have zero defects, but it tracks them. Ask for its rejection records by defect type and what it changed after the last problem. A foundry that cannot answer is guessing. Defects that appear only after the first container are a familiar pattern, discussed in why quality drops after the first container.
Next step
We agree acceptance criteria with the foundry, check first samples, and inspect production before loading. To plan inspection for your castings, book a free sourcing review.