Core (manufacturing)

Core (manufacturing)

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A core is a device used in casting
Casting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...

 and molding
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....

 processes to produce internal cavities and reentrant angles. The core is normally a disposable item that is destroyed to get it out of the piece. They are most commonly used in sand casting
Sand casting
Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material.It is relatively cheap and sufficiently refractory even for steel foundry use. A suitable bonding agent is mixed or occurs with the sand...

, but are also used in injection molding
Injection molding
Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity...

.

An intriguing example of the use of cores is in the casting of engine blocks. For example, one of the GM V-8 engines requires 5 dry-sand cores for every casting.

Advantages and disadvantages


Cores are useful for features that cannot tolerate draft or to provide detail that cannot otherwise be integrated into a core-less casting or mold.

The main disadvantage is the additional cost to incorporate cores.

Requirements


There are seven requirements for core:
  1. In the green condition there must be adequate strength for handling.
  2. In the hardened state it must be strong enough to handle the forces of casting; therefore the compression strength should be 100 to 300 psi (0.6894757293 to 2.1 MPa).
  3. Permeability
    Permeation
    Permeation, in physics and engineering, is the penetration of a permeate through a solid, and is related to a material's intrinsic permeability...

     must be very high to allow for the escape of gases.
  4. As the casting or molding cools the core must be weak enough to break down as the material shrinks. Moreover, they must be easy to remove during shakeout.
  5. Good refractoriness is required as the core is usually surrounded by hot metal during casting or molding.
  6. A smooth surface finish
    Surface finish
    Surface finish, also known as surface texture, is the characteristics of a surface. It has three components: lay, surface roughness, and waviness.-Lay:...

    .
  7. A minimum generation of gases during metal pouring.

Types


There are many types of cores available. The selection of the correct type of core depends on production quantity, production rate, required precision, required surface finish, and the type of metal being used. For example, certain metals are sensitive to gases that are given off by certain types of core sands; other metals have too low of a melting point to properly break down the binder for removal during the shakeout.

Green-sand cores


Green-sand cores are not a typical type of core in that it is part of the cope and drag
Cope and drag
In foundry work, the terms cope and drag refer respectively to the upper and lower parts of a two-part casting flask, used in sand casting. The flask is a wood or metal frame, which contains the molding sand, providing support to the sand as the metal is poured into the mold...

, but still form an internal feature. Their major disadvantage is their lack of strength, which makes casting long narrow features difficult or impossible. Even for long features that can be cast it still leave much material to be machined. A typical application is a through hole in a casting.

Dry-sand cores


Dry-sand cores overcome some of the disadvantages of the green-sand cores. They are formed independently of the mold and then inserted into the core prints in the mold, which hold the core in position. They are made by mixing sand with a binder
Binder
The reaper-binder, or binder, was a farm implement that improved upon the reaper. The binder was invented in 1872 by Charles Withington. In addition to cutting the small-grain crop, it would also tie the stems into small bundles, or sheaves...

 in a wooden or metal core box, which contains a cavity in the shape of the desired core.

The most simple way to make dry-sand cores is in a dump core box, in which sand is packed into the box and scraped level with the top. A wood or metal plate is then placed over the box, and then the two are flipped over and the core segment falls out of the core box. The core segment is then baked or hardened. Multiple core segments are then hot glue
Hot glue
Hot melt adhesive , also known as hot glue, is a form of thermoplastic adhesive that is commonly supplied in solid cylindrical sticks of various diameters, designed to be melted in an electric hot glue gun...

d together or attached by some other means. Any rough spots are filed
Filing (metalworking)
Filing is a material removal process in manufacturing. Similar, depending on use, to both sawing and grinding in effect, it is functionally versatile, but used mostly for finishing operations, namely in deburring operations. Filing operations can be used on a wide range of materials as a finishing...

 or sanded down. Finally, the core is lightly coated with graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

, silica, or mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

 to give a smoother surface finish
Surface finish
Surface finish, also known as surface texture, is the characteristics of a surface. It has three components: lay, surface roughness, and waviness.-Lay:...

 and greater resistance to heat.

Single-piece cores do not need to be assembled because they are made in a split core box. A split core box, like it sounds, is made of two halves and has at least one hole for sand to be introduced. For simple cores that have constant cross-sections they can be created on special core-producing extruders. The extrusions are then just cut to the proper length and hardened. More complex single-piece cores can be made in a manner similar to injection molding
Injection molding
Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity...

s and die casting
Die casting
Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process...

s.
Types of core boxes
  • half core box
  • dump core box
  • split core box
  • left and right core box
  • gang core box
  • strickle core box
  • loose piece core box
  • Ghayoor

Lost cores


Core are used for complex injection molding
Injection molding
Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity...

s in the fusible core injection molding
Fusible core injection molding
Fusible core injection molding, also known as lost core injection molding, is a specialized plastic injection molding process used to mold internal cavities or undercuts that are not possible to mold with demoldable cores...

 process. First, a core is made from a fusible alloy
Fusible alloy
A fusible alloy is a metal alloy capable of being easily fused, i.e. easily meltable, at relatively low temperatures. Fusible alloys are commonly, but not necessarily, eutectic alloys....

 or low melting temperature polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

. It is then placed inside the injection mold's dies and the plastic is shot into the mold. The molding is then removed from the mold with the core still in it. Finally, the core is melted or washed out of the molding in a hot bath.

Binders


Special binders are introduced into core sands to add strength. The oldest binder was vegetable oil, however now synthetic oil
Synthetic oil
Synthetic oil is a lubricant consisting of chemical compounds that are artificially made . Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials...

 is used, in conjunction with cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...

 or clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

. The core is then baked in a convection oven
Convection oven
Although the word convection is usually used to describe the natural circulation of gas or liquid caused by temperature differences, the convection in "convection oven" has a more general definition: the transfer of heat via movement of gas or liquid...

 between 200 and 250 °C (392 and 482 F). The heat causes the binder to cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...

 or polymerize. While this process is simple, the dimensional accuracy is low.

Another type of binder process is called the hot-box process, which uses a thermoset and catalyst for a binder. The sand with the binder is packed into a core box that is heated to approximately 230 °C (446 °F) (which is where the name originated from). The binder that touches the hot surface of the core box begins to cure
Cure
A cure is a completely effective treatment for a disease.The Cure is an English rock band.Cure, or similar, may also refer to:-Film and television:* The Cure , a short film starring Charlie Chaplin...

 within 10 to 30 seconds. Depending on the type of binder it may require further baking to fully cure.

In a similar vein, the cold-box process uses a binder that is hardened through the use of special gases. The binder coated sand is packed into a core box and then sealed so that a curing gas can be introduced. These gases are often toxic (i.e. amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

 gas) or odorous (i.e. SO2
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

), so special handling systems must be used. However, because high temperatures are not required the core box can be made from metal, wood, or plastic. An added benefit is that hollow core can be formed if the gas is introduced via holes in the core surface which cause only the surface of the core to harden; the remaining sand is then just dumped out to be used again. For example, a cold-box sand casting core binder is sodium silicate
Sodium silicate
Sodium silicate is the common name for a compound sodium metasilicate, Na2SiO3, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles...

 which hardens on exposure to carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

.

Special binders are used in air-set sands to produce core at room temperature. These sands do not require a gas catalyst because organic binders and a curing catalyst are mixed together in the sand which initiates the curing process. The only disadvantage with this is that after the catalyst is mixed in there is a short time to use the sand. A third way to produce room temperature cores is by shell molding
Shell molding
Shell molding, also known as shell-mold casting, is an expendable mold casting process that uses a resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labor requirements...

.

The term no-bake sands can refer to either the cold-box process or air-set process.

Other considerations


To increase the strength of cores internal wires and rods can be added. To enhance collapsibility straw can be added to the middle of the core or a hollow core can be used. This attribute is especially important for steel casting because a large amount of shrinkage is present.

Except for very small cores, all cores require vent holes to release gases. These are usually formed by using small wires to create holes from the surface of the mold to the core. When this is not feasible cinder
Cinder
A cinder is a pyroclastic material. Cinders are extrusive igneous rocks. Cinders are similar to pumice, which has so many cavities and is such low-density that it can float on water...

 and coke
Coke (fuel)
Coke is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made.- History :...

 can be added to the core to increase permeability.
Chaplets
As mentioned earlier, cores are usually supported by two core prints in the mold. However, there are situations where a core only uses one core print so other means are required to support the cantilevered end. These are usually supplied in the form of chaplets. These are small metal supports that bridge the gap between the mold surface and the core, but because of this become part of the casting. As such, the chaplets must be of the same or similar material as the metal being cast. Moreover, their design must be optimized because if they are too small they will completely melt and allow the core to move, but if they are too big then their whole surface cannot melt and fuse with the poured metal. Their use should also be minimized because they can cause casting defects or create a weak spot in the casting. It is usually more critical to ensure the upper chaplets are stronger than the lower ones because the core will want to float up in the molten metal.
Cheeks
When casting a reentrant angle instead of using a core a cheek can be used instead. This is a third segment in the flask, in addition to the cope and drag. This allows the entire mold to be made from green sand and from removable patterns. The disadvantage of this is more mold-making operations are required, but it is usually advantageous when the quantities are low. However, if large quantities of casting are required, it is usually more cost effective to simply use a core.