Casting Defect ID: Gas Holes, Shrinkage, Sand Inclusions
When a casting is rejected, the QC inspector first looks at the hole: round? irregular? smooth or sandy? Different features point to different causes.
Gas Holes
Features: round/elliptical, smooth shiny walls (oxidized), single or stringed, at surface or subsurface.
Causes: gas trapped before solidification — air entrained, mold gas, dissolved gas precipitation (H, N, O).
Fixes: better venting, permeability; steel settling; controlled pour speed; temperature control to avoid overheat gas pickup.
Shrinkage
Features: irregular shape, rough dendritic walls, at heavy sections (hot spots) or last-freezing regions. Fine dispersed = shrinkage porosity; big hole = shrinkage cavity.
Causes: no feeding during solidification — insufficient riser, blocked channel, wrong pouring temperature.
Fixes: bigger/better risers, chills to control solidification order, adjusted pouring temperature, reduced hot spots.
Sand Inclusions
Features: holes or pits with sand grains embedded, near bottoms and ingates.
Causes: mold erosion by flowing metal; low sand strength, poor compaction.
Fixes: stronger sand, mold coating, slower pour, optimized ingate position.
Quick ID Table
| Check | Gas | Shrinkage | Sand |
|---|---|---|---|
| Shape | Round | Irregular | Irregular |
| Wall | Smooth shiny | Rough dendritic | Sand grains |
| Position | Surface | Hot spot | Bottom/ingate |
| Cause | Gas | Unfed shrink | Sand entrainment |
Summary
Reading holes is a foundry QC basic skill. Round-shiny = gas; rough-jumbled = shrinkage; sand-stuck = inclusions. Match features to causes and scrap rate drops step by step.