casting process
AIM - Study of production consideration for casting.Theory
Casting - Castingis amanufacturingprocess by which a liquid
material is usually poured into a mold, which contains a hollow
cavity of the desired shape, and then allowed to solidify. The
solidified part is also known as a casting, which is ejected or
broken out of the mold to complete the process.1. Solidification of
MeltsWhen a melt is poured into a colder mold, metal in contact
with the mold solidifies in the form of roughly equiaxed fine
grains, because cooling rates are high, and the wall induces
heterogeneous nucleation.Solidification proceeds by the growth of a
few favorably oriented nuclei, in the direction of heat extraction.
This leads to be observed columnar structure. Because of the
preferred growth direction of the large grains, the casting will
have very anisotropic properties.Since most metals shrink on
solidification, the liquid meniscus gradually drops and a shrinkage
cavity (pipe) remains.2. Casting MaterialsAlthough some non-metals
are cast, the process is primary importance in the production of
metal products. The metals most frequently cast are iron, steel,
aluminium, brass, bronze, magnesium, and certain zinc alloys.3.
Casting ProcedureIn all casting processes six basic factors are
involved. These are as follows:1. A mold cavity, having the desired
shape and size and with due allowance for shrinkage of the
solidifying metal, must be produced. Any complexity of shape
desired in the finished casting must exist in the cavity.
Consequently, the mold material must such as to reproduce the
desired detail and also have a refractory character so that it will
not be significantly affected by the molten metal that it contains.
Either a new mold must be prepared for each casting, or it must be
made from a material that can withstand being used for repeated
castings, the latter being called permanent molds.2. A suitable
means must be available for melting the metal that is to be cast,
providing not only adequate temperature, but also satisfactory
quality and quantity at low cost.3. The molten metal must be
introduced into the mold in such a manner that all air or gases in
the mold, prior to pouring or generated by the action of the hot
metal upon the mold, will escape, and the mold will be completely
filled. A quality casting must be dense and free from defects such
as air holes.4. Provision must be made so that the mold will not
cause too much restraint to the shrinkage that accompanies cooling
after the metal has solidified. Otherwise, the casting will crack
while its strength is low. In addition, the design of the casting
must be such that solidification and solidification shrinkage can
occur without producing cracks and internal porosity or voids.5. It
must be possible to remove the casting from the mold so a permanent
mold must be made in two or more sections.6. After removal from the
mold, finishing operations may need to be performed to remove
extraneous material that is attached to the casting as the result
of the method of introducing the metal into the cavity, or is
picked up from the mold through contact with the metal.4. CASTING
PROCESSESMuch of the development that has taken place in the
foundry industry has been directed toward meeting these six
objectives with greater economy. Six major casting processes
currently are used. These are:1. Sand casting2. Plaster-mold
casting3. Investment casting4. Centrifugal casting5. Permanent-mold
casting6. Die casting7. Squeeze casting
6.1. SAND CASTINGSand casting is a flexible, inexpensive
process. Sand is used as the mold material. The sand grains, mixed
with small amounts of other materials to improve the mold ability
and cohesive strength, are packed around a pattern that has the
shape of the desired casting. Products covering a wide range of
sizes and detail can be made by this method. A new mold must be
made for each casting, and gravity usually is employed to cause the
metal to flow into the mold. The process is not so accurate as die
casting or investment casting.6.2. PLASTER-MOLD CASTINGPlaster-mold
casting is somewhat similar to sand casting in that only one
casting is made and then the mold is destroyed, in this case the
mold is made out of a specially formulated plaster. 70 to 80%
gypsum and 20 to 30% fibrous strengtheners. Water is added to make
a creamy s1urry. This process is limited to non-ferrous metals,
because ferrous metals react with sulphur in gypsum. The core boxes
are usually made form brass, plastics, or aluminium. 6.3.
INVESTMENT CASTINGCasting processes in which the pattern is used
only once are variously referred to as "lost-wax" or
"precision-casting" processes. In any case they involve making a
pattern of the desired form out of wax or plastics (usually
polystyrene). A metal flask is placed around the assembled patterns
and refractory mold slurry is poured to support the patterns and
form the cavities. A vibrating table equipped with a vacuum pump is
used to eliminate all the air from the mold. After the mold
material has set and dried, the pattern material is melted and
allowed to run out of the mold.When the metal is cooled, the
investment material is removed by means of vibrating hammers or by
tumbling. As with other castings, the gates and risers are cut off
and ground down.6.4. CENTRIFUGAL CASTINGCentrifugal casting
consists of having sand, metal, or ceramic mold that is rotated at
high speeds. When the molten metal is poured into the mold it is
thrown against the mold wall, where it remains until it cools and
solidifies. The process is being increasingly used for such
products as cast-iron pipes, cylinder liners, gun barrels, pressure
vessels, brake drums gears, and flywheels. The metals used include
almost all castable alloys.Because of the relatively fast cooling
time, centrifugal castings have a fine gram size. There is a
tendency for the lighter non-metallic inclusions slag particles,
and dross to segregate toward the inner radius of the casting where
it can be easily removed by machining. Due to the high purity of
the outer skin, centrifugally cast pipes have a high resistance to
atmospheric corrosion.6.5. PERMANENT-MOLD CASTING PROCESSESThe
process utilizes a metal casting die in conjunction with metal or
sand cores. Molten metal is introduced at the top of the mold that
has two or more parts, using only the force of gravity. After
solidification, the mold is opened and the casting ejected. The
mold is reassembled and the cyc1e is repeated. The molds are either
metal or graphite and, consequently, most permanent-mold castings
are restricted to lower melting point nonferrous metals and
alloys.6.6. DIE CASTINGDie casting differs from ordinary
permanent-mold casting in that the molten metal is forced into the
molds by pressure and held under pressure during solidification.
Most die castings are made from nonferrous metals and alloys, but
substantial quantities of ferrous die castings now are being
produced. Because of the combination of metal molds or dies, and
pressure, fine sections and excellent detail can be achieved,
together with tong mold life. Special zinc-, copper-, and
aluminium-base alloys suitable for die casting have been developed
which have excellent properties, thereby contributing to the very
extensive use of the process.Because die-casting dies usually are
made from hardened tool steel, they are expensive to make. In
addition, the die sections must contain knockout pins, which eject
the casting.6.7. SQUEEZE CASTINGSqueeze casting, also known as
liquid-metal forging, is a process by which molten metal solidifies
under pressure within c1osed dies positioned between the plates of
a hydraulic press. Squeeze casting consists of metering liquid
metal into a preheated, lubricated die and forging the metal while
it so1idifies. The load is applied shortly after the metal begins
to freeze and is maintained until the entire casting has
solidified. Casting ejection and handling are done in much the same
way as in closed die forging.The applied pressure and the instant
contact of the molten metal with the die surface produce a rapid
heat transfer condition that yields a pore-free fine-grain casting
with mechanical properties approaching those of a wrought
product.The squeeze casting process is easily automated to produce
near-net to net shape high-quality components.The process was
introduced in the United States in 1960 and has since gained
widespread acceptance.7. MELTING AND CASTINGDuring melting process
chemical content of the raw material is determined. Alloying is
achieved at this stage where raw materials are in liquid phase.
Most known furnace types to prepare the liquid metal are:a.
Crucible Type Furnacesb. Coupol Type Furnacesc. Electric-Arc
Furnacesd. Converterse. Siemens-Martins Furnaces
8. Design of CastingsWhen designing casting the most important
consideration is the effect of shrinkage during cooling. Other
important factors include metal flow, and porosity.Some general
rules are,- Avoid sharp corners - they can lead to hot tearing
during cooling.- Use fillets cautiously - they lead to stresses as
they shrink a radius of 1/8" to 1" are acceptable.- Avoid large
masses - they will cool more slowly, and can lead to pores and
cavities in the final part. Cores can be used to hollow out these
large volumes. Metal padding `chills' can also be placed inside the
mold near large masses to help increase cooling rates.- Use uniform
cross sections -this will keep the cooling rate relatively uniform
and avoid stresses.- Avoid large flats - large flat areas tend to
warp.- Allow some give as the part cools - by allowing the
shrinkage of one part to deform another slightly, the internal
stresses will be reduced. Figures of 1-2% shrinkage are common.-
Put parting lines near corners - this will hide the flash.-
Straight Parting Lines - where possible a straight parting line
will allow easier mold making.- Use a Draft angle - A small angle
of 0.5-2 on the vertical walls will make the pattern easier to
remove.- Machining Allowances - allow excess material for later
machining of critical dimensions- Wide Tolerances - because
shrinkage occurs as the part cools it will be very hard to keep
tight tolerances.- Stress Relieve When Needed - Stress relief can
reduce the effects of non-uniform cooling.- Avoid thin sections -
These will be very hard to fill, and will tend to harden
quickly.-Avoid internal features - These will require extra steps
in mold making, and may create metal flow problems.Advantages of
Casting:1. On basis of size of object to be manufactured: Size of
cast objects varies over large range. An object from 5gm to
200tonn, anything can be cast.2. On basis of complexity: Casting
can be effectively used for complex shaped objects. It can work
where general machining processes can not be used, as in
complicated inner and outer shapes of object.3. Weight saving:
Component made with casting process is lighter than the component
made with other machining processes.4. Control over the process:
Casting provides versatility. Wide range of properties can be
attained by adjusting percentage of alloying elements.5. Accuracy:
Casting can be made with hair like precision provided proper
molding and casting technique is employed.6. Fibrous structure:
Only casting has this advantage. Casting leaves component with its
solid fibrous structure which inherit great compressive strength.
So, component subjected to compressive strength are made with
casting ex. IC engine cylinder.7. Control over grain size: Grain
size of cast component can be easily controlled by controlling
cooling rate which in turn can be used to modify the properties.8.
Low cost: Casing is one of cheapest method for mass
production.Disadvantages of Casting:1. Though casting is cheapest
formassProduction, it becomes non economical in case
ofjobproduction.2. Sand casting leaves rough surface which needs
machining in most of cases. It adds up the cost in production.3.
Again in sand casting, poor dimensional accuracy is achieved.4.
Cast products are superior for compressive loads but they are very
poor in tensile or shock loads. (They are brittle).
