Casting defects: how they arise and how to avoid them
Porosity, cold connections and inclusions — know the enemy
Introduction
Not all castings become perfect. Technically, casting defects are typically divided into several main categories: gas porosity, shrinkage defects, moulding defects, pouring metal defects and metallurgical defects. Being able to recognise a defect type and explain why it has occurred is a core competence in the trade — it is often the way to fix the problem before the next casting.
Gas porosity and voids
Small gas bubbles in the solidified metal are called porosity; if the bubbles are larger, they are called blowholes or blisters. The defect occurs when air is introduced into the melt, or when steam and smoke from the molding sand or other sources cannot escape from the mold while the metal solidifies.
Cold joints (cold shuts)
A cold joint occurs when two fronts of molten metal meet in the mould cavity but do not fuse properly together. The result is a weak line in the workpiece where the material is not homogeneous. The defect is often linked to the metal having cooled too much before it fills the mould completely, or to a poorly designed gate.
Inclusions
| Type of enclosure | What it is |
|---|---|
| Metallic inclusions | Pieces of metal — for example leftovers or fragments — that by mistake end up in the mould cavity during casting |
| Non-metallic inclusions | Slag or remnants of refractory material that did not flow out of the melt before it was poured |
Why it is a professional task
- 01The error type often points directly to the cause — and thus where in the process adjustment is needed
- 02Some defects are cosmetic and can be finished in post-processing, others make the item unsuitable for use
- 03Quality control (see the article on NDT) is key to discovering errors that are not visible from the outside
- 04Documenting errors systematically helps the entire foundry correct production, not just the individual item
An experienced foundry technician uses knowledge of defect types proactively: already when the mold is designed and the metal chosen thought is given to how to avoid the classic pitfalls. That is the difference between reacting to errors and preventing them.
“An error you can name and explain is an error you can correct. This is why error types are part of the professional toolkit.”