15-02-2013, 03:50 PM
Metal Casting Methods
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Introduction
Metal casting is one of the direct methods of manufacturing the
desired geometry of component. The method is also called as near net shape
process. It is one of the primary processes for several years and one of important
process even today in the21st century. Early applications of casting are in
making jewellery items and golden idols. Today, casting applications include
automotive components, spacecraft components and many industrial & domestic
components, apart from the art and jewellery items.
The principle of manufacturing a casting involves creating a cavity
inside a sand mould then pouring the molten metal directly into the mould.
Casting is a very versatile process and capable of being used in mass
production. The size of components is varied from very large to small, with
intricate designs. Out of the several steps involved in the casting process,
moulding and melting processes are the most important stages. Improper control
at these stages results in defective castings, which reduces the productivity of a
foundry industry. Generally, foundry industry suffers from poor quality and
productivity due to the large number of process parameters, combined with lower
penetration of manufacturing automation and shortage of skilled workers
compared to other industries. Also, Global buyers demand defect-free castings
and strict delivery schedule, which foundries are finding it very difficult to meet.
History
Sand casting, also known as sand moulded casting, is a
metalcasting process characterized by using sand as the mould material. The
term "sand casting" can also refer to an object produced via the sand casting
process. Sand castings are produced in specialized factories called foundries.
Over 70% of all metal castings are produced via a sand casting process.
Sand casting is relatively cheap and sufficiently refractory even for
steel foundry use. In addition to the sand, a suitable bonding agent (usually clay)
is mixed or occurs with the sand. The mixture is moistened, typically with water,
but sometimes with other substances, to develop strength and plasticity of the
clay and to make the aggregate suitable for moulding. The sand is typically
contained in a system of frames or moldboxes known a flask. The moldcavities
and gatesystem are created by compacting the sand around models, or patterns,
or carved directly into the sand.
Die casting
Die casting is a metal casting process that is characterized
by forcing molten metal under high pressure into a mould 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. Most die
castings are made from non-ferrous metals, specifically zinc, copper, aluminum,
magnesium, lead, pewter and tin based alloys. Depending on the type of metal
being cast, a hot- or cold-chamber machine is used. The casting equipment and
the metal dies represent large capital costs and this tends to limit the process to
high volume production. Manufacture of parts using die casting is relatively
simple, involving only four main steps, which keeps the incremental cost per item
low.
Continuous casting
Continuous casting, also called strand casting, is the
process where by molten metal is solidified into a "semi finished" billet, bloom, or
slab for subsequent rolling in the finishing mills. Prior to the introduction of
continuous casting in the 1950, steel was poured into stationary molds to form
ingots. Since then, "continuous casting" has evolved to achieve improved yield,
quality, productivity and cost efficiency. It allows lower-cost production of metal
sections with better quality, due to the inherently lower costs of continuous,
standardized production of a product, as well as providing increased control over
the process through automation.
Sand casting
Sand casting, also known as sand molded casting, is a
metal casting process characterized by using sand as the mold material. The
term "sand casting" can also refer to an object produced via the sand casting
process. Sand castings are produced in specialized factories called foundries.
Over 70% of all metal castings are produced via a sand casting process.
Sand casting is relatively cheap and sufficiently refractory
even for steel foundry use. In addition to the sand, a suitable bonding agent
(usually clay) is mixed or occurs with the sand. The mixture is moistened,
typically with water, but sometimes with other substances, to develop strength
and plasticity of the clay and to make the aggregate suitable for molding. The
sand is typically contained in a system of frames or mold boxes known a flask.
The mold cavities and gate system are created by compacting the sand around
models, or patterns, or carved directly into the sand.
Plaster Casting
A plaster cast is a copy made in plaster of another 3-
dimensional form. The original from which the cast is taken may be a sculpture,
building, a face, a fossil or other remains such as fresh or fossilized footprints –
particularly in paleontology Sometimes a blank block of plaster itself was carved
to produce mock-ups or first drafts of sculptures (usually relief sculptures) that
would ultimately be sculpted in stone, by measuring exactly from the cast, for
example by using a pointing machine. These are still described as plaster casts.
Examples of these by John Flaxman may be found in the central rotunda of the
library at University College London, and elsewhere in the University's
collections. It may also describe a finished original sculpture made out of plaster,
though these are rarer.
Semi solid casting
Semi-solid metal casting is a near net shape variant of
casting. The process is used with non-ferrous metals, such as aluminum, copper,
and magnesium. The process combines the advantages of casting and forging.
The process is named after the fluid property thixotropy, which is the
phenomenon that allows this process to work. Simply, thyrotrophic fluids shear
when the material flows, but thicken when standing. The potential for this type of
process was first recognized in the early 1970s. There are four different
processes: thixocasting, thixomolding, and SIMA. SSM is done at a temperature
that puts the metal between its liquids and solidus temperature. Ideally, the metal
should be 30 to 65% solid. The metal must have a low viscosity to be usable, and
to reach this low viscosity the material needs a globular primary surrounded by
the liquid phase.[2] The temperature range possible depends on the material and
for aluminum alloys is 5–10 °C, but for narrow melting range copper alloys can
be only several tenths of a degree.
Shell mold casting
Shell moulding, also known as shell-mould casting, is an
expendable mould casting process that uses a resin covered sand to form the
mould. As compared to sand casting, this process has better dimensional
accuracy, a higher productivity rate, and lower labour requirements. It is used for
small to medium parts that require high precision. Shell mould casting is a metal
casting process similar to sand casting, in that molten metal is poured into an
expendable mould. However, in shell mould casting, the mould is a thin-walled
shell created from applying a sand-resin mixture around a pattern. The pattern, a
metal piece in the shape of the desired part, is reused to form multiple shell
moulds. A reusable pattern allows for higher production rates, while the
disposable moulds enable complex geometries to be cast. Shell mould casting
requires the use of a metal pattern, oven, sand-resin mixture, dump box, and
molten metal.
Slush Casting
Slush casting is a variant of permanent moulding casting to
create a hollow casting or hollow cast. In the process the material is poured into
the mould and allowed to cool until a shell of material forms in the mould. The
remaining liquid is then poured out to leave a hollow shell. The resulting casting
has good surface detail but the wall thickness can vary. The process is usually
used to cast ornamental products, such as candlesticks, lamp bases, and
statuary, from low-melting-point materials. A similar technique is used to make
hollow chocolate figures for Easter and Christmas. The method was developed
by William Britain in 1893 for the production of lead toy soldiers. It uses less
material than solid casting, and results in a lighter and less expensive product.
Hollow cast figures generally have a small hole where the excess liquid was
poured out.