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Wikipedia About Casting (MetalWorking)

2014-12-22 9:25:47      Click£º

Expendable mold casting

Sand casting

Plaster mold casting

Plaster casting is similar to sand casting except that plaster of paris is substituted for sand as a

mold material. Generally, the form takes less than a week to prepare, after which a production

rate of 1¨C10 units/hr¡¤mold is achieved, with items as massive as 45 kg (99 lb) and as small as 30 g

(1 oz) with very good surface finish and close tolerances.[4] Plaster casting is an inexpensive

alternative to other molding processes for complex parts due to the low cost of the plaster and its

ability to produce near net shape castings.

The biggest disadvantage is that it can only be used with low melting point non-ferrous materials,

such as aluminium, copper, magnesium, and zinc.

 Shell molding

Shell molding is similar to sand casting, but the molding cavity is formed by a hardened "shell" of

sand instead of a flask filled with sand.

The sand used is finer than sand casting sand and is mixed with a resin so that it can be heated

by the pattern and hardened into a shell around the pattern. Because of the resin and finer sand,

it gives a much finer surface finish. The process is easily automated and more precise than sand

casting. Common metals that are cast include cast iron, aluminium, magnesium, and copper alloys.

This process is ideal for complex items that are small to medium sized.

Investment casting

Investment casting (known as lost-wax casting in art) is a process that has been practiced for

thousands of years, with the lost-wax process being one of the oldest known metal forming techniques.

From 5000 years ago, when beeswax formed the pattern, to today¡¯s high technology waxes,

refractory materials and specialist alloys, the castings ensure high-quality components are p

roduced with the key benefits of accuracy, repeatability, versatility and integrity

Investment casting derives its name from the fact that the pattern is invested, or surrounded,

with a refractory material. The wax patterns require extreme care for they are not strong enough

to withstand forces encountered during the mold making. One advantage of investment casting is

that the wax can be reused.

The process is suitable for repeatable production of net shape components from a variety of

different metals and high performance alloys.

Although generally used for small castings, this process has been used to produce complete aircraft

door frames, with steel castings of up to 300 kg and aluminium castings of up to 30 kg. Compared

to other casting processes such as die casting or sand casting, it can be an expensive process,

however the components that can be produced using investment casting can incorporate intricate

contours, and in most cases the components are cast near net shape, so require little or no rework

once cast.

Waste molding of plaster

A durable plaster intermediate is often used as a stage toward the production of a bronze sculpture

or as a pointing guide for the creation of a carved stone. With the completion of a plaster, the work

is more durable (if stored indoors) than a clay original which must be kept moist to avoid cracking.

With the low cost plaster at hand, the expensive work of bronze casting or stone carving may be

deferred until a patron is found, and as such work is considered to be a technical, rather than

artistic process, it may even be deferred beyond the lifetime of the artist.

In waste molding a simple and thin plaster mold, reinforced by sisal or burlap, is cast over the

original clay mixture. When cured, it is then removed from the damp clay, incidentally destroying

the fine details in undercuts present in the clay, but which are now captured in the mold. The

mold may then at any l ater time (but only once) be used to cast a plaster positive image, identical

to the original clay.

The surface of this plaster may be further refined and may be painted and waxed to resemble a

finished bronze casting.

Non-expendable mold casting  

Non-expendable mold casting differs from expendable processes in that the mold need not be

reformed after each production cycle. This technique includes at least four different methods:

permanent, die, centrifugal, and continuous casting. This form of casting also results in improved

repeatability in parts produced and delivers Near Net Shape results.

 

Permanent mold casting

Main articles: Permanent mold casting, low-pressure permanent mold casting and vacuum

permanent mold casting

Permanent mold casting is a metal casting process that employs reusable molds ("permanent molds"),

usually made from metal. The most common process uses gravity to fill the mold, however gas

pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush

casting, produces hollow castings. Common casting metals are aluminum, magnesium, and copper

alloys. Other materials include tin, zinc, and lead alloys and iron and steel are also cast in graphite

molds. Permanent molds, while lasting more than one casting still have a limited life before

wearing out.

Die casting

Main article: Die casting

The die casting process forces molten metal under high pressure into mold cavities

 (which are machined into dies). Most die castings are made from nonferrous metals,

specifically zinc, copper, and aluminium based alloys, but ferrous metal die castings are possible.

The die casting method is especially suited for applications where many small to medium sized parts

are needed with good detail, a fine surface quality and dimensional consistency.

 

Semi-solid metal casting

Main article: Semi-solid metal casting

Semi-solid metal (SSM) casting is a modified die casting process that reduces or eliminates the

residual porosity present in most die castings. Rather than using liquid metal as the feed

material, SSM casting uses a higher viscosity feed material that is partially solid and partially l

iquid. A modified die casting machine is used to inject the semi-solid slurry into re-usable

hardened steel dies. The high viscosity of the semi-solid metal, along with the use of controlled

die filling conditions, ensures that the semi-solid metal fills the die in a non-turbulent manner so

that harmful porosity can be essentially eliminated.

Used commercially mainly for aluminium and magnesium alloys, SSM castings can be heat treated

to the T4, T5 or T6 tempers. The combination of heat treatment, fast cooling rates

(from using un-coated steel dies) and minimal porosity provides excellent combinations of strength

and ductility. Other advantages of SSM casting include the ability to produce complex shaped parts

net shape, pressure tightness, tight dimensional tolerances and the ability to cast thin walls.

Centrifugal casting

Main article: Centrifugal casting (silversmithing)

In this process molten metal is poured in the mold and allowed to solidify while the mold is rotating.

Metal is poured into the center of the mold at its axis of rotation. Due to centrifugal force the liquid

metal is thrown out towards the periphery.

Centrifugal casting is both gravity- and pressure-independent since it creates its own force feed

using a temporary sand mold held in a spinning chamber at up to 900 N. Lead time varies with

the application. Semi- and true-centrifugal processing permit 30¨C50 pieces/hr-mold to be produced,

with a practical limit for batch processing of approximately 9000 kg total mass with a typical

per-item limit of 2.3¨C4.5 kg.

Industrially, the centrifugal casting of railway wheels was an early application of the method

developed by the German industrial company Krupp and this capability enabled the rapid

growth of the enterprise.

Small art pieces such as jewelry are often cast by this method using the lost wax process, as the

forces enable the rather viscous liquid metals to flow through very small passages and into fine

details such as leaves and petals. This effect is similar to the benefits from vacuum casting,

also applied to jewelry casting.

Continuous casting

Main article: Continuous casting

Continuous casting is a refinement of the casting process for the continuous, high-volume

production of metal sections with a constant cross-section. Molten metal is poured into an

open-ended, water-cooled mold, which allows a 'skin' of solid metal to form over the still-liquid

centre, gradually solidifying the metal from the outside in. After solidification, the strand,

as it is sometimes called, is continuously withdrawn from the mold. Predetermined lengths

of the strand can be cut off by either mechanical shears or traveling oxyacetylene torches and

transferred to further forming processes, or to a stockpile. Cast sizes can range from strip

(a few millimeters thick by about five meters wide) to billets (90 to 160 mm square) to slabs

 (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process

before being cut.

Continuous casting is used due to the lower costs associated with continuous production of a

standard product, and also increased quality of the final product. Metals such as steel, copper,

aluminum and lead are continuously cast, with steel being the metal with the greatest tonnages

cast using this method.