Editing Sand casting (section)

====Cores====
{{Main|Core (manufacturing)}}

Cores are apparatus used to generate hollow cavities or internal features which cannot be formed using pattern alone in molding, cores are usually made using sand, but some processes also use permanent cores made of metal.

To produce cavities within the casting&mdash;such as for liquid cooling in [[internal combustion engine|engine]] blocks and [[cylinder head]]s&mdash;negative forms are used to produce ''cores''. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Whenever possible, designs are made that avoid the use of cores, due to the additional set-up time, mass and thus greater cost.

With a completed mold at the appropriate moisture content, the box containing the sand mold is then positioned for filling with molten metal&mdash;typically [[iron]], [[steel]], [[bronze]], [[brass]], [[aluminium]], [[magnesium]] alloys, or various [[pot metal]] alloys, which often include [[lead]], [[tin]], and [[zinc]]. After being filled with liquid metal the box is set aside until the metal is sufficiently cool to be strong. The sand is then removed, revealing a rough casting that, in the case of iron or steel, may still be glowing red. In the case of metals that are significantly heavier than the casting sand, such as iron or lead, the casting flask is often covered with a heavy plate to prevent a problem known as ''floating the mold.'' Floating the mold occurs when the pressure of the metal pushes the sand above the mold cavity out of shape, causing the casting to fail.

[[File:CoreBoxPatternCoreCasting.jpg|thumb|300px|Left: Corebox, with resulting (wire reinforced) cores directly below. Right:- Pattern (used with the core) and the resulting casting below (the wires are from the remains of the core)]]

After casting, the cores are broken up by rods or shot and removed from the casting. The metal from the [[Sprue (manufacturing)|sprue]] and risers is cut from the rough casting. Various [[heat treatment]]s may be applied to relieve stresses from the initial cooling and to add hardness&mdash;in the case of steel or iron, by quenching in water or oil. The casting may be further strengthened by surface compression treatment&mdash;like [[shot peening]]&mdash;that adds resistance to tensile cracking and smooths the rough surface. And when high precision is required, various machining operations (such as milling or boring) are made to finish critical areas of the casting. Examples of this would include the boring of cylinders and milling of the deck on a cast engine block.