1) Investment casting
2) Permanent mold casting
3) Centrifugal casting
4) Continuous casting
5) Sand casting
Investment casting
Investment
casting (known as lost-wax casting in art) is a process that has been
practiced for thousands of years, with lost wax process being one of the
oldest known metal forming techniques. From 5000 years ago, when bees
wax formed the pattern, to today’s high technology waxes, refractory
materials and specialist alloys, the castings ensure high quality
components are produced 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
it 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 aluminum 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 requiring little or no rework
once cast.
Permanent mold casting
Permanent
mold casting (typically for non-ferrous metals) requires a set-up time
on the order of weeks to prepare a steel tool, after which production
rates of 5-50 pieces/hr-mold are achieved with an upper mass limit of 9
kg per iron alloy item (cf., up to 135 kg for many nonferrous metal
parts) and a lower limit of about 0.1 kg. Steel cavities are coated with
a refractory wash of acetylene soot before processing to allow easy
removal of the workpiece and promote longer tool life. Permanent molds
have a limited life before wearing out. Worn molds require either
refinishing or replacement. Cast parts from a permanent mold generally
show 20% increase in tensile strength and 30% increase in elongation as
compared to the products of sand casting.
The
only necessary input is the coating applied regularly. Typically,
permanent mold casting is used in forming iron, aluminum, magnesium, and
copper based alloys. The process is highly automated.
Sub-types of permanent mold casting
1. Gravity Die Casting.
2. Low pressure die casting.(LPDC)
3. High pressure die casting.(PDC)
Centrifugal casting
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 (90 g).
Lead time varies with the application. Semi- and true-centrifugal
processing permit 30-50 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 2.3-4.5 kg.
Industrially,
the centrifugal casting of railway wheels was an early application of
the method developed by German industrial company Krupp and this
capability enabled the rapid growth of the enterprise.
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 copper mold,
which allows a 'skin' of solid metal to form over the still-liquid
centre. The strand, as it is now called, is withdrawn from the mold and
passed into a chamber of rollers and water sprays; the rollers support
the thin skin of the strand while the sprays remove heat from the
strand, gradually solidifying the strand from the outside in. After
solidification, predetermined lengths of the strand are cut off by
either mechanical shears or travelling 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 metres 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 increases the quality of the
final product. Metals such as steel, copper and aluminium are
continuously cast, with steel being the metal with the greatest tonnages
cast using this method.
Sand casting
Sand
casting is one of the most popular and simplest types of casting that
has been used for centuries. Sand casting allows for smaller batches to
be made compared to permanent mold casting and a very reasonable cost.
Not only does this method allow for manufacturers to create products for
a good cost there are other benefits to sand casting such as there are
very little size operations. From castings that fit in the palm of your
hand to train beds (one casting can create the entire bed for one rail
car) it can be done with sand casting. Sand casting also allows for most
metals to be cast depending in the the type of sand used for the molds.
Sand
casting requires a lead time of days for production at high output
rates (1-20 pieces/hr-mold), and is unsurpassed for large-part
production. Green (moist) sand has almost no part weight limit, whereas
dry sand has a practical part mass limit of 2300-2700 kg. Minimum part
weight ranges from 0.075-0.1 kg. The sand is bonded together using clays
(as in green sand) or chemical binders, or polymerized oils (such as
motor oil.) Sand in most operations can be recycled many times and
requires little additional input.
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