Cores and core techniques: how holes and channels are created in castings

What you can't see from the outside is often what makes the item

Introduction

Many items from the foundry need not just an outer appearance — they must also have cavities inside: a channel, a through hole or a complex internal geometry. You can't manage that with the form alone. The solution is a core: a separate, often sand-based mold piece placed inside the mold cavity before the metal is poured in.

The core as a form in the form

A core is typically made in a core box — a mold solely for the core — and is then placed precisely inside the casting mold. When the metal has been poured and solidified, the core is broken into pieces and removed through the cavity it created. This requires that the core can both hold its shape during casting and be easily removed afterwards.

Two families of binders

Type	Characteristic
Cold-box (organic binder)	The sand is bound with an organic resin and hardens quickly when blown through with a catalyst gas. Fast process suitable for mass production and gives a fine surface — good for complex precise pieces
Inorganic core box (e.g. water glass).	The sand is bound with an inorganic binder and hardens by heat or chemical reaction without organic catalysts. Fewer harmful fumes and good high-temperature resistance making it suitable for pieces with high melting temperature

Assembly and inspection before casting

01The core must fit exactly in the mold's core bearing so it does not move when the metal flows in
02The surface must be free from loose sand and moisture before the mold is closed
03If there are multiple cores in the same component, they must fit together without colliding with the rest of the form's geometry
04An incorrectly placed or skewed core typically gives an uneven wall thickness in the finished part — and in the worst case a part that must be rejected

Core technique is one of the areas where precision in detail has great significance for the final result. A workpiece that looks simple on the outside can easily hide complicated internal geometry — and it is often the core work that determines whether the workpiece meets the drawing.

“What you can't see on the finished workpiece is often what took the most craftsmanship to get right.”

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Type

Characteristic

Cold-box (organic binder)

The sand is bound with an organic resin and hardens quickly when blown through with a catalyst gas. Fast process suitable for mass production and gives a fine surface — good for complex precise pieces

Inorganic core box (e.g. water glass).

The sand is bound with an inorganic binder and hardens by heat or chemical reaction without organic catalysts. Fewer harmful fumes and good high-temperature resistance making it suitable for pieces with high melting temperature