Types of Casting
And manufacturing process
China precision machining Casting is a manufacturing process in which molten metal is poured or
injected a nd allowed to solidify in a suitably
shaped mold cavity. During or after
cooling, the cast part is removed from the mold and then processed for
delivery.
Casting processes and cast-material technologies vary from simple to
highly complex. Material and process selection
depends on the part’s complexity and function, the product’s quality
specifications, and the projected cost
level.
Castings are parts that are made close to their final dimensions by a
casting process. With a history dating back 6,000 years, the various casting processes are in a state of continuous refinement and
evolution as technological advances are being made.
A:Sand Casting
Sand casting is used to make large parts (typically iron, but also
bronze, brass, aluminum). Molten metal is poured into a mold cavity formed out
of sand (natural or synthetic).
The processes of sand casting are discussed in this section, including
patterns, sprues and runners, design considerations, and casting allowance.
China precision machining The
cavity in the sand is formed by using a pattern (an approximate duplicate of
the real part), which are typically made out of wood, sometimes metal. The
cavity is contained in an aggregate housed in a box called the flask.
Core is a sand shape inserted into the mold to produce the internal
features of the part such as holes or internal passages. Cores are placed in
the cavity to form holes of the desired shapes. Core print is the region added
to the pattern, core, or mold that is used to locate and support the core
within the mold.
A
riser is an extra void created in the mold to contain excessive molten
material. The purpose of this is to feed the molten metal to the mold cavity as
the molten metal solidifies and shrinks, and thereby prevents voids in the main casting.
In a two-part mold, which is typical of sand castings, the upper half, including the top
half of the pattern, flask, and core is called cope and the lower half is called drag, The parting line or the parting surface is
line or surface that separates the cope and drag.
The
drag is first filled partially with sand, and the core print, the cores, and the gating system
are placed near the parting line. The
cope is then assembled to the drag, and the sand is poured on the cope half,
covering the pattern, core and the gating system.
The
sand is compacted by vibration and mechanical means. Next, the cope is removed
from the drag, and the pattern is carefully removed. The object is to remove the
pattern without breaking the mold cavity.
This
is facilitated by designing a draft, a slight angular offset from the vertical
to the vertical surfaces of the pattern. This is usually a minimum of 1.5mm(0.060in.),
whichever is greater. The rougher the surface of the pattern, the more the
draft to be provided.
The
molten material is poured into the pouring cup, which is part of the gating
system that supplies the molten material to the mold cavity.
The
vertical part of the gating system connected to the pouring cup is the sprue,
and the horizontal portion is called the runners and finally to the multiple
points where it is introduced to the mold
cavity called the gates.
Additionally
there are extensions to the gating system called vents that provide the path
for the built-up gases and the displaced air to vent to the atmosphere.
The
cavity is usually made oversize to allow for the metal contraction as it cools
down to room temperature. This is achieved by making the pattern oversize. To account
for shrinking, the pattern must be made oversize by these factors on the
average. These are linear factors and apply in each direction.
These
shrinkage allowances are only approximate, because the exact allowance is
determined by the shape and size of the casting. In addition, different parts of the casting might require different
shrinkage allowances.
Sand castings generally have a rough
surface sometimes with surface impurities, and surface variations. A machining
(finish) allowance is made for this type of defect.
In general, typical stages of sand casting
operation include :
1.
Patterns are made. These will be the shape used to form the cavity in the sand.
2.
Cores may also be made at this time. These cores are made of bonded sand that
will be broken out of the cast part after it is complete.
3.
Sand is mulled (mixed) thoroughly with
additives such as bentonite to increase
bonding and overall strength.
4.
Sand is formed about the patterns, and gates, runners, risers, vents and
pouring cups are added as needed. A compaction stage is typically used to
ensure good coverage and solid molds.
Cores
may also be added to make concave or internal features for the cast part.
Alignment pins may also be used for mating the molds later. Chills may be added
to cool large masses faster.
5.
The patterns are removed, and the molds may be put through a baking stage to
increase strength.
6.
Mold halves are mated and prepared for pouring metal.
7. Metal is preheated in a furnace or crucible
until is above the liquidus temperature in a suitable range (we don’t want the
metal solidifying before the pour is complete). The exact temperature may be
closely controlled depending upon the application.
Degassing,
and other treatment processes may be
done at this time, such as removal of
impurities (i.e. slag). Some portion of
this metal may be remelted scrap from previously cast parts—10% is reasonable.
8.
The metal is poured slowly, but continuously into the mold until the mold is full.
9. As the molten metal cools (minutes to days), the metal will shrink and the volume will
decrease. During this time molten metal may backflow from the molten risers to
feed the part and maintain the same shape.
10. Once the part starts to solidify small
dendrites of solid material form in the part. During this time metal properties
are being determined, and internal stresses are being generated. If a part is
allowed to cool slowly enough at a constant rate then the final part will be
relatively homogenous and stress free.
11. Once the part has completely solidified
below the eutectic point it may be removed with no concern for final metal
properties. At this point the sand is simply broken up, and the part removed.
At this point the surface will have a quantity of sand adhering to the surface,
and solid cores inside.
12. A bulk of the remaining sand and cores
can be removed by mechanically striking the part. Other options are to use a vibrating table, sand/shot blaster,
hand
labor, etc.
13. The final part is cut off the runner
gate system, and is near final shape using cutters, torches, etc. Grinding
operations are used to remove any
remaining bulk.
14. The part is taken down to final shape
using machining operations. And cleaning operations may be used to remove
oxides, etc.
B.Investment casting
Investment casting is also known as the lost wax process. This process
is one of the oldest manufacturing processes. The Egyptians used it in the time
of the Pharaohs to make gold jewelry (hence the name Investment) some 5,000
years ago.
Intricate shapes can be made with high accuracy. In addition, metals
that are hard to machine or fabricate
are good candidates for this process. It
can be used to make parts that cannot be produced by normal manufacturing
techniques, such as turbine blades that have complex shapes, or airplane parts
that have to withstand high temperatures.
The
mold is made by making a pattern using wax or some other material that can be
melted away. This wax pattern is dipped in refractory slurry, which coats the
wax pattern and forms a skin. This is dried and the process of dipping in the
slurry and drying is repeated until a robust thickness is achieved.
After
this, the entire pattern is placed in an
oven and the wax is melted away. This leads to a mold that can be filled with
the molten metal. Because the mold is formed around a one-piece pattern (which
does not have to be pulled out from the mold as
in a traditional sand casting process), very intricate parts and
undercuts can be made.
The
wax pattern itself is made by duplicating using a stereo lithography or similar
model—which has been fabricated using a computer solid model master.
The materials used for the slurry are a
mixture of plaster, a binder and powdered silica, a refractory, for low
temperature melts. For higher temperature melts, sillimanite or
alumina-silicate is used as a refractory, and silica is used as a binder.
Depending
on the fineness of the finish desired additional coatings of sillimanite and
ethyl silicate may be applied. The mold thus produced can be used directly for light castings, or be reinforced by placing it in a larger container and
reinforcing it more slurry.
Just
before the pour, the mold is pre-heated to about 1,000℃(1,832℉) to remove any
residues of wax, harden the binder. The
pour in the pre-heated mold also ensures
that the mold will fill completely.
Pouring
can be done using gravity, pressure or vacuum conditions. Attention must be paid to
mold permeability when using pressure,
to allow the air to escape as the pour
is done.
Tolerances
of 0.5% of length are routinely possible, and as low as 0.15% is possible for
small dimensions. Castings can weigh
from a few grams to 35kg (0.1oz to 80lb), although the normal size ranges from
200g to about 8kg(7oz to 15 lb). Normal minimum wall thicknesses are about 1mm
to about 0.5mm(0.040~ 0.020 in.) for
alloys that can be cast easily.
The types of materials that can be cast
are aluminum alloys, bronzes, tool steels, stainless steels, stellite,
hastelloys, and precious metals. Parts
made with investment castings often do
not require any further machining,
because of the close tolerances that can
be achieved.
C
.Centrifugal Casting
Centrifugal casting as a category includes centrifugal casting, semi-centrifugal casting and centrifuging. In
centrifugal casting, a permanent mold is rotated about its axis at high speeds
(300 to 3,000rpm) as the molten metal is poured.
The
molten metal is centrifugally thrown towards the inside mold wall, where it
solidifies after cooling. The casting is usually a fine grain casting with a very
fine-grained outer diameter, which is resistant to atmospheric corrosion, a
typical situation with pipes. The inside diameter has more impurities and
inclusions, which can be machined away.
Only
cylindrical shapes can be produced with this process. Size limits are up to 3m(10feet) diameter and 15m(50 feet)
length. Wall thickness can be 2.5mm to 125mm(0.1~5.0in.). The tolerances that
can be held on the OD can be as good as 2.5mm (0.1in.) and on the ID can be
3.8mm(0.15in.). The surface finish ranges from 2.5mm to 12.5mm(0.1~0.5in.)
rms(root-mean-square).nless steels, and
alloys of aluminum, copper and nickel. Two materials can be cast by introducing
a second material during the process.
Typical parts made by this process are pipes, boilers, pressure vessels, flywheels, cylinder liners and other
parts that are axis-symmetric.
Semi-centrifugal
casting. The molds used can be permanent or expendable, can be stacked as
necessary. The rotational speeds are lower than those used in centrifugal
casting.
The
center axis of the part has inclusion defects as well as porosity and thus is
suitable only for parts where this can be machined away. This
process is used for making
wheels, nozzles and similar parts where the axis of the part is removed
by subsequent machining.
Centrifuging.
Centrifuging is used for forcing metal from a central axis of the equipment
into individual mold cavities that are placed on the circumference.
This provides a means of increasing the
filling pressure within each mold and allows for reproduction of intricate
details. This method is often used for the pouring of investment casting
pattern.
Full-mold
casting is a technique similar to investment casting, but instead of wax as the
expendable material, polystyrene foam is used as the pattern. The foam pattern
is coated with a refractory material. The pattern is encased in a one-piece
sand mold. As the metal is poured, the
foam vaporizes, and the metal takes its place.
China precision machining This can make complex shaped castings without
any draft or flash. However, the pattern cost can be high due to the expendable
nature of the pattern. Minimum wall thicknesses are 2.5mm, tolerances can be
held to 0.3% on dimensions. Surface finish can be held from 2.5μm to
25μm(0.1μin. to 1.0μin.) rms(root-mean-square). Size limits are from 400g(1lb) to several
tons. No draft allowance is required. Typical materials that can be cast with
this process are aluminum, iron, steel, nickel alloys, copper alloys. Types of parts that can be made using
these processes are pump housings, manifolds, and auto brake components.
