Editing Metal casting (section)

===Shrinkage===
There are three types of shrinkage: ''shrinkage of the liquid'', ''solidification shrinkage'' and ''patternmaker's shrinkage''. The shrinkage of the liquid is rarely a problem because more material is flowing into the mold behind it. Solidification shrinkage occurs because metals are less dense as a liquid than a solid, so during solidification the metal density dramatically increases. Patternmaker's shrinkage refers to the shrinkage that occurs when the material is cooled from the solidification temperature to room temperature, which occurs due to [[thermal contraction]].<ref>{{Harvnb|Degarmo|Black|Kohser|2003|pp=285–286}}</ref>

====Solidification shrinkage====
{| class="wikitable" align="right"
|+ Solidification shrinkage of various metals<ref name="degarmo286"/><ref>{{harvnb|Stefanescu|2008|p=66}}.</ref>
! Metal !! Percentage
|-
| Aluminium || 6.6
|-
| Copper || 4.9
|-
| Magnesium || 4.0 or 4.2
|-
| Zinc || 3.7 or 6.5
|-
| Low carbon steel || 2.5–3.0
|-
| High carbon steel || 4.0
|-
| White cast iron || 4.0–5.5
|-
| Gray cast iron || −2.5–1.6
|-
| Ductile cast iron || −4.5–2.7
|}

Most materials shrink as they solidify, but, as the adjacent table shows, a few materials do not, such as [[gray cast iron]]. For the materials that do shrink upon solidification the type of shrinkage depends on how wide the freezing range is for the material. For materials with a narrow freezing range, less than {{convert|50|C|abbr=on}},<ref name="stefanescu67">{{harvnb|Stefanescu|2008|p=67}}.</ref> a cavity, known as a ''pipe'', forms in the center of the casting, because the outer shell freezes first and progressively solidifies to the center. Pure and eutectic metals usually have narrow solidification ranges. These materials tend to form a ''skin'' in open air molds, therefore they are known as ''skin forming alloys''.<ref name="stefanescu67"/> For materials with a wide freezing range, greater than {{convert|110|C|abbr=on}},<ref name="stefanescu67"/> much more of the casting occupies the ''mushy'' or ''slushy'' zone (the temperature range between the solidus and the liquidus), which leads to small pockets of liquid trapped throughout and ultimately porosity. These castings tend to have poor [[ductility]], [[toughness]], and [[fatigue (material)|fatigue]] resistance. Moreover, for these types of materials to be fluid-tight, a secondary operation is required to impregnate the casting with a lower melting point metal or resin.<ref name="degarmo286"/><ref>{{Citation | last1 = Porter | first1 = David A. | last2 = Easterling | first2 = K. E. | title = Phase transformations in metals and alloys | page = 236 | publisher = CRC Press | year = 2000 | edition = 2nd | url = https://books.google.com/books?id=eYR5Re5tZisC | isbn = 978-0-7487-5741-1}}.</ref>

For the materials that have narrow solidification ranges, pipes can be overcome by designing the casting to promote directional solidification, which means the casting freezes first at the point farthest from the gate, then progressively solidifies toward the gate. This allows a continuous feed of liquid material to be present at the point of solidification to compensate for the shrinkage. Note that there is still a shrinkage void where the final material solidifies, but if designed properly, this will be in the gating system or riser.<ref name="degarmo286">{{Harvnb|Degarmo|Black|Kohser|2003|p=286}}</ref>
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====Risers and riser aids====
[[File:Casting riser types.svg|class=skin-invert-image|thumb|right|400px|Different types of risers]]
{{Main article|Riser (casting)|chill (casting)}}

Risers, also known as ''feeders'', are the most common way of providing directional solidification. It supplies liquid metal to the solidifying casting to compensate for solidification shrinkage. For a riser to work properly the riser must solidify after the casting, otherwise it cannot supply liquid metal to shrinkage within the casting. Risers add cost to the casting because it lowers the ''yield'' of each casting; i.e. more metal is lost as scrap for each casting. Another way to promote directional solidification is by adding chills to the mold. A chill is any material which will conduct heat away from the casting more rapidly than the material used for molding.<ref>{{harvnb|Degarmo|Black|Kohser|2003|pp=286–288}}.</ref>

Risers are classified by three criteria. The first is if the riser is open to the atmosphere, if it is then it is called an ''open'' riser, otherwise it is known as a ''blind'' type. The second criterion is where the riser is located; if it is located on the casting then it is known as a ''top riser'' and if it is located next to the casting it is known as a ''side riser''. Finally, if the riser is located on the gating system so that it fills after the molding cavity, it is known as a ''live riser'' or ''hot riser'', but if the riser fills with materials that have already flowed through the molding cavity it is known as a ''dead riser'' or ''cold riser''.<ref name="degarmo287">{{Harvnb|Degarmo|Black|Kohser|2003|p=287}}</ref>

Riser aids are items used to assist risers in creating directional solidification or reducing the number of risers required. One of these items are ''chills'' which accelerate cooling in a certain part of the mold. There are two types: external and internal chills. External chills are masses of high-heat-capacity and high-thermal-conductivity material that are placed on an edge of the molding cavity. Internal chills are pieces of the same metal that is being poured, which are placed inside the mold cavity and become part of the casting. Insulating sleeves and toppings may also be installed around the riser cavity to slow the solidification of the riser. Heater coils may also be installed around or above the riser cavity to slow solidification.<ref name="degarmo288">{{Harvnb|Degarmo|Black|Kohser|2003|p=288}}</ref>
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====Patternmaker's shrink====
{| class="wikitable" align="right"
|+ Typical patternmaker's shrinkage of various metals<ref name="degarmo289"/>
! Metal !! Percentage !! in/ft
|-
| Aluminium || 1.0–1.3 || {{Frac|1|8}}–{{Frac|5|32}}
|-
| Brass || 1.5 || {{Frac|3|16}}
|-
| Magnesium || 1.0–1.3 || {{Frac|1|8}}–{{Frac|5|32}}
|-
| Cast iron || 0.8–1.0 || {{Frac|1|10}}–{{Frac|1|8}}
|-
| Steel || 1.5–2.0 || {{Frac|3|16}}–{{Frac|1|4}}
|}

Shrinkage after solidification can be dealt with by using an oversized pattern designed specifically for the alloy used. ''{{visible anchor|Contraction rule}}s'', or ''{{visible anchor|shrink rule}}s'', are used to make the patterns oversized to compensate for this type of shrinkage.<ref name="degarmo289">{{Harvnb|Degarmo|Black|Kohser|2003|p=289}}</ref> These rulers are up to 2.5% oversize, depending on the material being cast.<ref name="degarmo288"/>  These rulers are mainly referred to by their percentage change. A pattern made to match an existing part would be made as follows:  First, the existing part would be measured using a standard ruler, then when constructing the pattern, the pattern maker would use a contraction rule, ensuring that the casting would contract to the correct size.

Note that patternmaker's shrinkage does not take phase change transformations into account. For example, eutectic reactions, [[martensitic]] reactions, and [[graphitization]] can cause expansions or contractions.<ref name="degarmo289"/>