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Chapter 11: Metal Casting Processes and Equipment - Coggle Diagram
Chapter 11: Metal Casting Processes and Equipment
Expendable Mold,
Permanent
pattern:
Sand casting :star:
Uses compacted sand (silica) for mold
sand could be naturally bonded (bank sand) and synthetic (lake sand)
fine-grained sand enhances mold strength because smaller particles pack more closely, but it reduces permeability by creating smaller, less connected pore spaces.
Types of Sand Molds:
green sand mold
: sand in the mold is damp/moist while metal is poored
cold box
: organic and inorganic binders that are blended into the sand to bond the grains chemically
no bake
: synthetic liquid resin is mixed with the sand and allow to harden at room tempreture.
Major Features of Sand Casting:
flask:
supports the mold itself, two parts:
cope
on top and
drag
on bottom.
sprue
: through which the molten metal flows downward
riser
: supply additional molten metal to the casting as it shrinks during solidification
gate
: entry to the mold cavity from sprue
pouring cup/basin
vent
: placed in the molds to carry off gases
core
: inserts made of sand, place in the mold to form hollow regions
Patterns
are used to mold the sand mixture into the shape of the casting
pattern material selection depends on
dimensional accuracy
quantity of casting required
molding process
size and shape of casting
vertical flask less molding
: the halves of the pattern form a vertical chamber wall against which sand is blown and compacted
Shell casting :star:
produce many types of castings with close dimensional tolerances and a good surface finish at low cost
Applications are small mechanical parts that require precision. Complex shapes can be produced with less labor because it is automated easily
Shell sand has lower permeability
than sand used for green-sand molding
Plaster-mold Casting :star:
the mold is made of plaster of paris with the addition of talc and silica flour to improve strength
can only be used for metals with lower melting points
Casting-mold Casting :star:
similar to the plaster-mold process but uses refractory mold materials suitable for high-temperature applications
Expendable Mold,
Expendable
pattern:
Evaporative-pattern casting :tada:
(Lost-foam Process)
polystyrene pattern, which evaporates upon contact with molten metal to form a cavity for the casting
Used for ferrous and nonferrous metals therefor it is applicable in automotive
advantages:
simple
inexpensive flasks and polystyrene
Minimal finishing and cleaning operations
Process can be automated
Investment casting / lost wax process :tada:
Pattern is invested (surrounded) with the
refractory material to form a shell then pattern is melted out and replaced with metal.
Ceramic shell investment casting
Process is economical and is used for the precision casting of steels and high-temperature alloys
Permanent-mold Casting
permanent-mold casting :pen:
2 halves of a mold are made from materials with
high resistance to erosion and thermal fatigue
In order to increase the life of permanent molds, the surfaces of the mold cavity are coated with a refractory slurry or sprayed with graphite
Equipment costs is high but labor costs are kept low through automation so its not economical for small production runs
Vacuum Casting :pen:
Basic vacuum casting is an alternative to other casting and is suitable for thin-walled complex shapes with uniform properties
A mixture of fine sand and urethane is molded
over metal dies and cured with amine vapor
Automated and production costs are similar
green-sand casting
Die casting :pen:
two types:
hot chamber process
: use a piston to forces a certain volume of metal into the die cavity through a gooseneck and nozzle
cold chamber process:
is where molten metal is poured into the injection cylinder (shot chamber)
Die casting is able to produce strong and highquality parts with complex shapes
produces good dimensional accuracy and
surface details
Strength-to-weight ratio of die-cast parts
increases with decreasing wall thickness
die casting can be
Single cavity
Multiple cavity (several identical cavities) :
Combination cavity (several different cavities)
Unit dies
Centrifugal Casting :pen:
utilizes inertial forces to distribute the molten metal into the mold cavities
3 types:
True Centrifugal Casting
: fot hollow cylinder parts like pipes and gun barrels. horizontal axis of rotation.
Semicentrifugal Casting:
Used to cast parts with rotational symmetry, such as a wheel with spokes.vertical axis of rotation.
Centrifuging
: Mold cavities of any shape are placed at a certain distance from the axis of rotation
Squeeze Casting :pen:
Involves the solidification of molten metal under high pressure
Products made are automotive components and
mortar bodies
Composite-mold operations :pen:
are made of two or more
different materials
for casting compplex shapes like the impellers for turbines
they
increase the strength of the mold
Improve the dimensional accuracy and surface
finish
Help reduce overall costs and processing time
Casting Techniques for
Single-crystal Components
Conventional Casting of Turbine Blades
Molten metal poured into ceramic mold
Grain structure developed is polycrystalline
Grain boundaries cause structure to creep and
cracking along the boundaries
Directionally Solidified Blades
Metal is poured into the
preheated
ceramic mold and column crystals are allowed to grow from a chill plate to the mold
Single Crystal Blades
mold has constriction in shape of corkscrew or helic, with cross-section so small it allows only one crystal to fit through
Only the most favourably oriented crystals are
able to grow
More expensive, but absence of grain boundaries makes them resistant to creep and thermal shock
Longer and more
reliable service life
single crystal
growing
:
type 1:Czochralski (CZ) process
Seed crystal dipped into molten metal and then pulled up slowly while being rotated.
.
Liquid metal begins to solidify on the seed and the crystal structure of the seed continues throughout
type 2:Floating-zone method
a rod of polycrystalline silicon resting on a
single crystal
.
induction coil heats these two pieces while the coil moves slowly upward
,
single crystal grows upward while maintaining its orientation
Rapid solidification:
cooling molten metal quickly so that it does not have time to crystalize to making amorphous alloys
melt spinning: the alloy is melted by induction in a ceramic crucible and propelled by high gas pressure against a rotating copper disk