Overview of Metal Casting

Definition:
Casting is a manufacturing process whereby the desired material is
heated to the liquid state, then introduced into a previously prepared
mold cavity of proper design, and allowed to solidify in the mold before
being extracted, trimmed and cleaned. (Joseph Datsko, 1966)

 Casting uses the idea that a liquid metal can take the shape of any
vessel containing it.
 When the metal cools it has taken the shape of its container
 Casting is one of the most versatile manufacturing processes
 Advantages of Casting

 Low cost and quick

 Easier to manufacture
 Can produce intricate shapes and internal openings
 Can produce parts in one piece
 Best suited for composite components
 What does a casting
customer need to know?

 Characteristics of castings
 Alternative metals
 Alternative process capabilities & limitations
 Casting design concerns
 When are Metal Castings required

 To allow the shaping of difficult materials

 To produce certain material properties
 To produce efficient stress distributions
 To produce very complex shapes with internal cavities or
hollow sections
 To produce very large parts ( 1 oz to 200 tons )
 To produce work-piece materials that are difficult or
uneconomical to process by other means
 casting is competitive with other processes.
 Typical application
 Automotive
Engine block, pistons, rods, cam, crankshaft, manifolds, valve lifters,
brake drum, brake cylinder, transmission case, differential case, wheels

 Plumbing
Valve bodies, pipe fittings, pipe, control handles

 Aerospace
Avionics cases, seat parts, window bezels, turbine parts, handles,
levers

 Machine Tools
Base, carriage, ways, tailstock, knee
 Difficult shape

 Complicated
 Hollow
 Contoured surfaces
 Large holes
 Much metal removal required if made from billet
 Much welding required if fabricated
 Some economic quantity required
 Fundamental of Casting Process

 Casting process:

 Creating a mold from a pattern

 Pouring molten metal into the mold
 Allowing part to cool
 Removing the metal part from the mold
 Heat treating and finish machining
 Factors that affect the overall properties
of the cast part
 Type of metal
 Thermal properties of the metal
 Thermal properties of the mold
 Geometric relationship between volume and surface
area of the casting
 Shape of the mold
 Porosity of the mold
 Metal pour rate
 The important considerations in casting
process
 Flow of Molten Metal
 Solidification and Cooling
 Type of Mold Material

 Solidification of Metals
 Involves liquid metal turning back in to solid metal
 The process is different for Pure metals and alloys
 Can be divided into two steps:
 Formation of stable nuclei
 Growth of crystals
 Pure Metals
 Have a clearly defined melting point
 Temperature remains constant during freezing
 Solidifies from the walls of the mold toward the center of
the part
 Grain Structure for Pure Metals
 Two types of grains are formed for a pure metal
 Fine equiaxed grains
 Columnar
 Rapid cooling at the walls produces fine equiaxed
grains
 Columnar grains grow opposite of the heat transfer
throughout the mold following the chill zone
 Equiaxed Grains
 If crystals can grow approximately equally in all
directions – equiaxed grains will grow.
 Large amounts of under cooling is needed near the wall
of the mold.
 Alloys
 Solidification in alloys begins when the temperature
drops below the liquidus TL and is complete when it
reaches the solidus, TS.
Fluid Flow
Sprue – is a vertical
channel though which the
molten metal flows
downward in the mold

Runners – channels that

carry the molten metal
from the sprue to the mold
cavity

Gate – is the portion of the

runner though which the
molten metal enters the
mold cavity

Risers – serve as
reservoirs to supply the
molten metal necessary to
prevent shrinkage
 Principles of fluid flow
 Turbulence is an important consideration in gating systems.
 Reynolds Number is used to quantify this aspect
 0 < Re < 2000 => laminar flow
 2000 < Re < 20 000 =>mixture of laminar and turbulent
flow
 Re > 20 000 => severe turbulence
 Techniques for minimizing
turbulence
 Avoid sudden changes in flow direction
 Dross or slag can be eliminated by vacuum casting
 Use of filters eliminates turbulent flow in the runner system
Fluidity of Molten Metal

Characteristics of molten metal

 Viscosity
 Surface tension
 Inclusions
 Solidification pattern of the alloy

 Casting parameters
 Mold design
 Mold material and its surface characteristics
 Degree of superheat
 Rate of pouring
 Heat transfer
Castability
Describes the case with which a metal can be cast to
obtain a part with good quality.
Metal Casting Processes

 Types of casting
 Expendable molds-
molds made of sand, plaster, ceramics, or other
materials capable of withstanding high temperatures
which are broken up and discarded after a single use.
 Permanent Molds-
molds usually made of metal which maintain strength at
high temperatures and are used for making a large
number of repeated castings.
 Composite Molds-
molds which use elements of both expendable and
permanent molds.
 Expendable mold methods
 Sand
 Shell
 Expendable Pattern or (Lost Foam)
 Plaster
 Ceramic
 Investment
 Permanent mold methods
 Gravity Feed Permanent
 Slush
 Low-Pressure
 Die
 Centrifugal
 Squeeze
 Semisolid
 Sand Casting

 Sand casting is one of the older techniques. In this form a

mold is made from sand, and the part is cast into it. When
the metal has hardened and cooled the part is removed,
and the sand removed
 Most commonly used method of casting
 Inexpensive tooling
 Can be used for most metals
 No limit to size, shape, or weight of cast parts
 Coarse finish may required extra machining
 Limited control of tolerances.
 Parts of a sand casting system
 Flask--the frame and support
 Cope-- the upper half of sand mold
 Drag -- the lower half of the sand mold
 Pouring basin--orifice where molten metal is poured into the
mold
 Sprue--vertical channel through which molten metal flows
downward into mold
 Runners--channels that carry molten metal from sprue to mold
 Riser--reservoirs to supply the molten metal needed to make
up shrinkage losses during solidification.
 Gate-- portion of runner through which the molten metal enters
the mold cavity, used to trap contaminants
 Vents--openings used to carry off gases given off by the metal
and exhaust air
 Cores--insets used to produce interior cavities
 Chills--used to increase the rate of solidification by controlling
thermal heat transfer
 Pattern-- shape used to mold the shape of the casting
Mold
Molds are made by compacting sand around the shape of
the pattern
 Pattern
 Patterns are made of wood, metal and plastics - the material
must be stronger if a large number of molds are to be made.
 A parting agent can be used on a pattern to allow easy
removal after the mold is made
 Design of the patterns should include consideration of
shrinkage
 Slight taper should be added to the sides all patterns this will
make them easy to remove from the completed mold. i.e. a
cone is easier to remove than a cylinder
 Pattern types
 One piece patterns (loose or solid patterns) - low quantity
simple shapes
 Split patterns - for complex shapes made in two patterns for
each half of the part.
 Common Types of sand molds
 Green molding sand
 Mixture of sand, clay, and water.
 Least expensive
 Used for large parts
 Require drying
 Cold box sands
 Blends in inorganic and organic binders to chemically
bond sand grains
 Maintains more accurate tolerances of casting
 More expensive than green sand
 No-bake molds
 Liquid resin is mixed with sand which hardens at room
temperature
Typical stages of making mold
 Core
 Cores are typically used for more complex
shapes
 Cores allow features that could not be easily
formed into a sand core.
 Cores are made with techniques similar to those
for making sand molds.
 The cores may need structural support in the
mold - these metal supports are called chaplets.
 The cores are added when the cavity are made,
and they act as part of the mold during casting,
but they are rigid enough to allow internal
features on parts.
Sands
 The sands used tend to fall into the following
categories
 Naturally bonded (bank) - less expensive
 Synthetic (lake) - this sand can have a variety of
controlled composition

 Types of sand include

 Zircon (ZrSiO4) - low thermal expansion
 Olivine (Mg2SiO4) - low thermal expansion
 Iron Silicate (Fe2SiO4) - low thermal expansion
 Chromite (FeCr2O4) - high heat transfer
The sand effects the
following aspects of the
casting
 Granule shape - smaller and rounder grains
produce a better casting surface.
 Granule size - a coarse grained sand will be
porous and allow gases to escape during
casting. a fine grained sand leads to a
stronger mold.
 Collapsibility - if the sand can shift during
cooling of the part it will reduce stress tears
and cracks
 Green sand molding refers to a slightly wet
condition of the sand (much like `green
wood'). At the right level of humidity the
moisture will increase sand binding. But in
excess this moisture expand when heated
during pouring and blow metal back out of
the mold (i.e. explosion). This is one of the
least expensive molding techniques).
Shell Mold Casting
 Shell Mold Casting Process
 The basic process for these molds is,
 Create two mating patterns of desired shape.
 Coat the molds with a shell (sand and binders, such as
a resin) until desired thickness and other properties are
obtained.
 Cure the molds and remove the patterns.
 The mold halves are mated and held firm while metal is
poured.
 The final part(s) is removed
 Process Advantages Limitations
Sand Almost any metal cast; no limit to size, shape, Some finishing required; somewhat
or weight; low tooling cost coarse finish; wide tolerances

Shell mold Good dimensional accuracy and surface Part size limited; expensive
finish; high production rate patterns and equipment required
Expendable Most metals cast with no limit to size; complex Patterns have low strength and can
pattern be costly for low quantities
Shapes

Plaster Intricate shapes; good dimensional accuracy Limited to nonferrous metals;

mold and finish; low porosity. limited size and volume of
production; mold making time
relatively long.

Ceramic Intricate shapes; close tolerance parts; good Limited size.

mold surface finish
Investment Intricate shapes; excellent surface finish and Part size limited; expensive
casting accuracy; almost any metal cast patterns, molds, and labor.
Permanent Good surface finish and dimensional High mold cost; limited shape and
mold accuracy; low porosity; high production rate intricacy; not suitable for high-
melting-point metals.

Die casting Excellent dimensional accuracy and surface Die cost is high; part size limited;
finish; high production rate. usually limited to nonferrous
metals; long lead time.

Centrifugal Large cylindrical parts with good quality; high Equipment is expensive; part
production rate shape is limited.
 Casting Defect

 Definition
 Those characteristic that create a deficiency or imperfection
contrary to the quality specifications imposed by the design and
the service requirement
 Categories
 Major defects which cannot be rectified, resulting in rejection of
the casting and total loss
 Defects that can be remedied but whose cost of repair may not
be justify the salvage attempt
 Minor defects, which clearly allow the casting to be
economically salvaged and thereby leave a reasonable margin
for profit.
Factors Induced defect in casting
 Unsuitable or unsatisfactory raw material used in molding,
core making or casting
 The application of unsatisfactory molding or casting
practice by the individual worker or incorrect advise by the
supervisor
 The use of improper tools, equipment, appliances, or
pattern
 Un professional management policies relating to the fixing
of incentive plans and setting up of production procedure,
faulty organization and poor work discipline, or lack of
training.