Editing Sand casting (section)

===Binders===
''Binders'' are added to a base sand to bond the sand particles together (i.e. it is the glue that holds the mold together).

====Clay and water====
A mixture of [[clay]] and water is the most commonly used binder. There are two types of clay commonly used: [[bentonite]] and [[kaolinite]], with the former being the most common.<ref>{{harvnb|Rao|2003|p=23}}.</ref>

====Oil====
Oils, such as [[linseed oil]], other [[vegetable oil]]s and [[marine oil]]s, used to be used as a binder, however due to their increasing cost, they have been mostly phased out. The oil also required careful baking at {{convert|100|to|200|C|F}} to cure (if overheated, the oil becomes brittle, wasting the mold).<ref name="rao24">{{harvnb|Rao|2003|p=24}}.</ref>

====Resin====
Resin binders are natural or synthetic high melting point [[Natural gum|gum]]s. The two common types used are [[urea formaldehyde]] (UF) and [[phenol formaldehyde]] (PF) resins. PF resins have a higher heat resistance than UF resins and cost less. There are also cold-set resins, which use a [[catalyst]] instead of a heat to cure the binder. Resin binders are quite popular because different properties can be achieved by mixing with various additives. Other advantages include good collapsibility, low gassing, and they leave a good surface finish on the casting.<ref name="rao24"/>

MDI (methylene diphenyl diisocyanate) is also a commonly used binder resin in the foundry core process.

====Sodium silicate====<!-- Use {{anchor|Sodium silicate}} if you rename this subsection as it's linked from other articles. -->

Water glass ( [[sodium silicate]] [Na<sub>2</sub>SiO<sub>3</sub> or (Na<sub>2</sub>O)(SiO<sub>2</sub>)] ) is a high strength binder used with silica molding sand both for cores and molds.<ref name=":0">{{Cite book |last=Asthana |first=R. |url=https://www.worldcat.org/oclc/85814321 |title=Materials processing and manufacturing science |date=2006 |publisher=Boston |others=A. Kumar, Narendra B. Dahotre |isbn=978-0-08-046488-6 |location=Amsterdam |oclc=85814321}}</ref>{{Rp|pages=69-70}} To cure a mixture of finely ground [[sand]] (e.g. by using a sand muller) and 3 to 4% of sodium silicate the binder, [[carbon dioxide]] (CO<sub>2</sub>) gas is used.<ref name=":0" />{{Rp|pages=69-70}} The mixture is exposed to the gas at ambient temperature reacting as following:<ref name=":0" />{{Rp|pages=69-70}}

:<chem>{Na2O(SiO2)} + CO2 <=> {Na2CO3} + {2SiO2} + Heat</chem>

The advantage to this binder is that it can be used at room temperature and is fast. The disadvantage is that its high strength leads to shakeout difficulties and possibly hot tears (probably due to [[quartz inversion]]{{Cn|date=April 2023}}) in the casting.<ref name="rao24" /><ref name=":0" />{{Rp|pages=|page=70}} The mixed sodium silicate and sand may also be heated by a [[heat gun]] to achieve better rigideness.