Editing Sand casting (section)

== Mold materials ==
There are four main components for making a sand casting mold: ''base sand'', a ''binder'', ''additives'', and a ''parting compound''.

===Molding sands===
''Molding sands'', also known as ''foundry sands'', are defined by eight characteristics: refractoriness, chemical inertness, permeability, surface finish, cohesiveness, flowability, collapsibility, and availability/cost.<ref name="rao18">{{harvnb|Rao|2003|p=18}}.</ref>

'''Refractoriness''' — This refers to the sand's ability to withstand the temperature of the liquid metal being cast without breaking down. For example, some sands only need to withstand {{convert|650|C|F}} if casting aluminum alloys, whereas steel needs a sand that will withstand {{convert|1500|C|F}}. Sand with too low refractoriness will melt and fuse to the casting.<ref name="rao18"/>

'''Chemical inertness''' — The sand must not react with the metal being cast. This is especially important with highly reactive metals, such as [[magnesium]] and [[titanium]].<ref name="rao18"/>

'''Permeability''' — This refers to the sand's ability to exhaust gases. This is important because during the pouring process many gases are produced, such as [[hydrogen gas|hydrogen]], [[nitrogen gas|nitrogen]], [[carbon dioxide]], and [[steam]], which must leave the mold otherwise [[casting defects]], such as blow holes and gas holes, occur in the casting. Note that for each cubic centimeter (cc) of water added to the mold 1600 cc of steam is produced.<ref name="rao18"/>

'''Surface finish''' — The size and shape of the sand particles defines the best surface finish achievable, with finer particles producing a better finish. However, as the particles become finer (and surface finish improves) the permeability becomes worse.<ref name="rao18"/>

'''Cohesiveness''' (or '''bond''') — This is the ability of the sand to retain a given shape after the pattern is removed.<ref>{{harvnb|Degarmo|Black|Kohser|2003|p=300}}.</ref>

'''Flowability''' – The ability for the sand to flow into intricate details and tight corners without special processes or equipment.<ref name="rao19">{{harvnb|Rao|2003|p=19}}.</ref>

'''Collapsibility''' — This is the ability of the sand to be easily stripped off the casting after it has solidified. Sands with poor collapsibility will adhere strongly to the casting. When casting metals that contract a lot during cooling or with long freezing temperature ranges a sand with poor collapsibility will cause cracking and [[hot tear]]s in the casting. Special additives can be used to improve collapsibility.<ref name="rao19"/>

'''Availability/cost''' — The availability and cost of the sand is very important because for every ton of metal poured, three to six tons of sand is required.<ref name="rao19"/> Although sand can be screened and reused, the particles eventually become too fine and require periodic replacement with fresh sand.<ref>{{cite web |url=http://www.cwc.org/industry/ibp951fs.pdf |title=Beneficial Reuse Of Spent Foundry Sand |year=1996}}</ref>

In large castings it is economical to use two different sands, because the majority of the sand will not be in contact with the casting, so it does not need any special properties. The sand that is in contact with the casting is called ''facing sand'', and is designed for the casting on hand. This sand will be built up around the pattern to a thickness of {{convert|30|to|100|mm|in|abbr=on}}. The sand that fills in around the facing sand is called ''backing sand''. This sand is simply silica sand with only a small amount of binder and no special additives.<ref name="rao22">{{harvnb|Rao|2003|p=22}}.</ref>

====Types of base sands====
''Base sand'' is the type used to make the mold or core without any binder. Because it does not have a binder it will not bond together and is not usable in this state.<ref name="rao19"/>

=====Silica sand=====
[[File:Silica Sand.jpg|thumb|150px|Silica sand at [[Panavally]]]]
[[Silica]] (SiO<sub>2</sub>) sand is the sand found on a beach and is also the most commonly used sand. It is either made by crushing [[sandstone]] or taken from natural occurring locations, such as beaches and river beds. The [[fusion point]] of pure silica is {{convert|1760|C|F}}, however the sands used have a lower melting point due to impurities. For high melting point casting, such as steels, a minimum of 98% pure silica sand must be used; however for lower melting point metals, such as [[cast iron]] and [[non-ferrous]] metals, a lower purity sand can be used (between 94 and 98% pure).<ref name="rao19"/>

Silica sand is the most commonly used sand because of its great abundance, and, thus, low cost (therein being its greatest advantage). Its disadvantages are high [[thermal expansion]], which can cause casting defects with high melting point metals, and low [[thermal conductivity]], which can lead to unsound casting. It also cannot be used with certain [[base (chemistry)|basic]] metals because it will chemically interact with the metal, forming surface defects. Finally, it releases silica particulates during the pour, risking [[silicosis]] in foundry workers.<ref name="rao20">{{harvnb|Rao|2003|p=20}}.</ref>

=====Olivine sand=====
[[Olivine]] is a mixture of [[orthosilicate]]s of iron and magnesium from the mineral [[dunite]]. Its main advantage is that it is free from silica, therefore it can be used with basic metals, such as manganese steels. Other advantages include a low thermal expansion, high thermal conductivity, and high fusion point. Finally, it is safer to use than silica, therefore it is popular in Europe.<ref name="rao20"/>

=====Chromite sand=====
[[Chromite]] sand is a [[solid solution]] of [[spinel]]s. Its advantages are a low percentage of silica, a very high fusion point ({{convert|1850|C|F}}), and a very high thermal conductivity. Its disadvantage is its costliness, therefore it is only used with expensive [[alloy steel]] casting and to make cores.<ref name="rao20"/>

=====Zircon sand=====
[[Zircon]] sand is a compound of approximately two-thirds [[zirconium oxide]] (ZrO<sub>2</sub>) and one-third silica. It has the highest fusion point of all the base sands at {{convert|2600|C|F}}, a very low thermal expansion, and a high thermal conductivity. Because of these good properties it is commonly used when casting alloy steels and other expensive alloys. It is also used as a [[mold wash]] (a coating applied to the molding cavity) to improve surface finish. However, it is expensive and not readily available.<ref name="rao20"/>

=====Chamotte sand=====
[[Chamotte]] is made by [[calcining]] [[fire clay]] (Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>) above {{convert|1100|C|F}}. Its fusion point is {{convert|1750|C|F}} and has low thermal expansion. It is the second cheapest sand, however it is still twice as expensive as silica. Its disadvantages are very coarse grains, which result in a poor surface finish, and it is limited to [[#dry sand molding|dry sand molding]]. Mold washes are used to overcome the surface finish problems. This sand is usually used when casting large steel workpieces.<ref name="rao20"/><ref>{{harvnb|Rao|2003|p=21}}.</ref>

===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.

===Additives===
Additives are added to the molding components to improve: surface finish, dry strength, refractoriness, and "cushioning properties".

Up to 5% of ''reducing agents'', such as coal powder, [[Pitch (resin)|pitch]], [[creosote]], and [[fuel oil]], may be added to the molding material to prevent wetting (prevention of liquid metal sticking to sand particles, thus leaving them on the casting surface), improve surface finish, decrease metal penetration, and [[burn-on defect]]s. These additives achieve this by creating gases at the surface of the mold cavity, which prevent the liquid metal from adhering to the sand. Reducing agents are not used with steel casting, because they can [[carburize]] the metal during casting.<ref name="rao25">{{harvnb|Rao|2003|p=25}}.</ref>

Up to 3% of "cushioning material", such as wood flour, [[sawdust]], powdered [[husk]]s, [[peat]], and [[straw]], can be added to reduce [[scabbing]], [[hot tear]], and [[hot cracking|hot crack]] casting defects when casting high temperature metals. These materials are beneficial because burn-off when the metal is poured creates tiny voids in the mold, allowing the sand particles to expand. They also increase collapsibility and reduce shakeout time.<ref name="rao25"/>

Up to 2% of ''cereal binders'', such as [[dextrin]], [[starch]], [[sulphite]] [[lye]], and [[molasses]], can be used to increase dry strength (the strength of the mold after curing) and improve surface finish. Cereal binders also improve collapsibility and reduce shakeout time because they burn off when the metal is poured. The disadvantage to cereal binders is that they are expensive.<ref name="rao25"/>

Up to 2% of [[iron oxide]] powder can be used to prevent mold cracking and metal penetration, essentially improving refractoriness. Silica flour (fine silica) and zircon flour also improve refractoriness, especially in ferrous castings. The disadvantages to these additives is that they greatly reduce permeability.<ref name="rao25"/>

===Parting compounds===
To get the pattern out of the mold, prior to casting, a parting compound is applied to the pattern to ease removal. They can be a liquid or a fine powder (particle diameters between {{convert|75|and|150|um}}). Common powders include [[talc]], [[graphite]], and dry silica; common liquids include [[mineral oil]] and water-based silicon solutions. The latter are more commonly used with metal and large wooden patterns.<ref name="rao26">{{harvnb|Rao|2003|p=26}}.</ref>