Editing Sand casting (section)

===Components===

====Patterns====
From the design, provided by a designer, a skilled ''pattern maker'' builds a ''pattern'' of the object to be produced, using wood, metal, or a plastic such as expanded polystyrene. Sand can be ground, swept or [[Wiktionary:strickle|strickle]]d into shape. The metal to be cast will contract during solidification, and this may be non-uniform due to uneven cooling. Therefore, the pattern must be slightly larger than the finished product, a difference known as ''contraction allowance''. Different scaled rules are used for different metals, because each metal and alloy contracts by an amount distinct from all others. Patterns also have core prints that create registers within the molds into which are placed sand [[core (manufacturing)|cores]]. Such cores, sometimes reinforced by wires, are used to create under-cut profiles and cavities which cannot be molded with the cope and drag, such as the interior passages of valves or cooling passages in engine blocks.

Paths for the entrance of metal into the mold cavity constitute the runner system and include the [[sprue (casting)|sprue]], various feeders which maintain a good metal 'feed', and in-gates which attach the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via [[Riser (casting)|risers]],{{#tag:ref|'Riser' (UK) is a term for an up-runner, in which the poured metal ''rises'' from the casting. In US practice, a riser is another term for a feeder to the top of a casting.<ref>{{Cite book
  |title=Castings
  |last=Campbell  |first=John
  |year=1993
  |publisher=[[Butterworth-Heinemann]]
  |isbn=0-7506-1696-2
  |page=49
}}</ref>|group=note}} which are added either in the pattern itself, or as separate pieces.

====Tools====
In addition to patterns, the sand molder could also use tools to create the holes.
[[File:Sand molding tools.jpg|thumb|Sand molding tools and books used in Auckland and Nelson, New Zealand, between approximately 1946 and 1960]]

====Molding box and materials====
A multi-part molding box (known as a [[casting flask]], the top and bottom halves of which are known respectively as the cope and drag) is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object&mdash;flat on one side&mdash;the lower portion of the box, closed at the bottom, will be filled with a molding sand. The sand is packed in through a vibratory process called ramming, and in this case, periodically [[Screed|screeded]] level. The surface of the sand may then be stabilized with a sizing compound. The pattern is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again. This forms a "green" mold which must be dried to receive the hot metal. If the mold is not sufficiently dried a steam explosion can occur that can throw molten metal about. In some cases, the sand may be oiled instead of moistened, which makes casting possible without waiting for the sand to dry. Sand may also be bonded by chemical binders, such as furane resins or amine-hardened resins.

{{Anchor|Additive_manufacturing_of_sand_molds}} [[3D printing|Additive manufacturing]] (AM) can be used in the sand mold preparation, so that instead of the sand mold being formed via packing sand around a pattern, it is 3D-printed. This can reduce lead times for casting by obviating patternmaking.<ref name="Donaldson_2017-11-01">{{Citation |last=Donaldson |first=Brent |date=2017-11-01 |title=Foundry Says Robotic Sand Printing a "Game Changer" for Metal Casting |journal=Additive Manufacturing |url=https://www.additivemanufacturing.media/articles/foundry-says-robotic-sand-printing-a-game-changer-for-metal-casting |access-date=2017-11-14 |postscript=.}}</ref> Besides replacing older methods, additive can also complement them in hybrid models, such as making a variety of AM-printed cores for a cavity derived from a traditional pattern.<ref name="Donaldson_2017-11-01"/>

====Chills====
To control the solidification structure of the metal, it is possible to place metal plates, ''[[Chill (foundry)|chills]]'', in the mold. The associated rapid local cooling will form a finer-grained structure and may form a somewhat harder metal at these locations. In ferrous castings, the effect is similar to [[quench]]ing metals in [[forge]] work. The inner diameter of an engine cylinder is made hard by a chilling core. In other metals, chills may be used to promote [[directional solidification]] of the casting. In controlling the way a casting freezes, it is possible to prevent internal voids or porosity inside castings.

====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.

====Design requirements====
The part to be made and its pattern must be designed to accommodate each stage of the process, as it must be possible to remove the pattern without disturbing the molding sand and to have proper locations to receive and position the cores. A slight taper, known as [[draft angle|draft]], must be used on surfaces perpendicular to the parting line, in order to be able to remove the pattern from the mold. This requirement also applies to cores, as they must be removed from the core box in which they are formed. The sprue and risers must be arranged to allow a proper flow of metal and gasses within the mold in order to avoid an incomplete casting. Should a piece of core or mold become dislodged it may be embedded in the final casting, forming a ''sand pit'', which may render the casting unusable. Gas pockets can cause internal voids. These may be immediately visible or may only be revealed after extensive machining has been performed. For critical applications, or where the cost of wasted effort is a factor, non-destructive testing methods may be applied before further work is performed.