Metal Casting Lectures 2
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Transcript of Metal Casting Lectures 2
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Part Two:
Metal casting Processes
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SyllabusPart A(Sand casting)1. Patterns and cores
2. Molds and mold making3. The casting operations
Part B (Other expandable mold casting processes)1. S hell molding2. Vacuum molding3. Expanded polystyrene process4. Investment molding5. Plaster mold and ceramic mold casting
Part C(Permanent mold casting processes)1. The basic permanent mold process2. Variations of permanent mold casting3. Die casting4. Centrifugal casting
Part D(Foundry Practice)1. Furnaces2. Pouring, Cleaning and heat treatment
Part E(Casting quality)
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Part A(Sand casting)
1. Patterns and cores2. Molds and mold making3. The casting operations
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Two Categories of Casting Processes
1-Expendable mold processes
uses an expendable mold which must bedestroyed to remove castingMold materials
1. sand,
2. plaster,3. and similar materials, plus binders
2-Permanent mold processesuses a permanent mold which can be used over and over to produce many
castingsMade of metal (or, less commonly, a ceramic refractory material
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Advantages and Disadvantages
More intricate geometries are possible
with expendable mold processes
Part shapes in permanent mold processes
are limited by the need to open the mold
Permanent mold processes are more
economic in high production operations
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Types of casting
Casting
Conventional Methods Unconventional Methods
Green sand mouldDry sand mould
CO2 Moulding (Strong mould)Permanent (Metal mould)
Shell Moulding (Thin mould)
Investment casting (Precision)
Centrifugal ( without core)
Continuous Casting (Open)
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Casting
Casting is one of the oldest manufacturing process and eventoday is the first step in manufacturing most of the products
It is based on the property of a liquid to take up the shape of
vessel containing it.
Products ranging from a few mm to several m anda few grams to several tons
Foundry is the place where metal is melted
and casting are produced
Similar to that of making ice cubes in a refrigerator
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Sand Casting Sand casting is the process of pouring molten metal in to
a mould cavity of required shape & size and allowing forcooling and the breaking up the mold to remove the casting
Six Basic Steps of Casting1. Pattern making
2. Core making
3. Mould making
4. Melting
5. Pouring6. Cleaning and inspection
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Six Basic Steps of Casting
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1-The Pattern
A full-sized model of the part, slightly enlarged to account for shrinkageand machining allowances in the casting
Pattern materials:
Wood- common material because it is easy to work, but it warps
Metal- more expensive to make, but lasts much longer
Plastic- compromise between wood and metal
Waxprecision casting
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Types of Patterns(pattern geometry)
(a) Solid pattern ( single piece)
(b) Split pattern ( Two piece)
(c) Match plate pattern
(d) Cope and drag pattern
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Pattern Allowances
1. Shrinkage or contraction allowance
2. Draft or taper allowance
3. Machining or finish allowance 4. Distortion or camber allowance
5. Rapping allowance
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1-Shrinkage or Contraction Allowance
All most all cast metals shrink or contract volumetrically on cooling. The
metal shrinkage is of two types:
i. Liquid Shrinkage: it refers to the reduction involume when the metal changes from liquid state to
solid state at the solidustemperature. To account for this shrinkage; riser, which
feed the liquid metal to the casting, are provided in the
mold.
ii. Solid Shrinkage: it refers to the reduction involume caused when metal loses temperature in
solid state. To account for this, shrinkage allowance
is provided on the patterns
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2. Draft or taper allowance
By draft is meant the taper provided by the pattern
maker on all vertical surfaces of the pattern so that it can
be removed from the sand without tearing away the
sides of the sand mold and without excessive rapping by
the molder. Figure 3 (a) shows a pattern having no draftallowance being removed from the pattern. In this case,
till the pattern is completely lifted out, its sides will
remain in contact with the walls of the mold, thus tending
to break it. Figure 3 (b) is an illustration of a patternhaving proper draft allowance. Here, the moment the
pattern lifting
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Pattern Having Draft on Vertical Edges
Draft
Pattern Having No Draft on Vertical Edges
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3-Machining or finish allowance
The finish and accuracy achieved in sand
casting are generally poor and therefore when
the casting is functionally required to be of good
surface finish or dimensionally accurate, it isgenerally achieved by subsequent machining.
Machining or finish allowances are therefore
added in the pattern dimension.
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2-Core in Mold Acore consists of two portions: the body of the core and one or more extensions
(called prints)
Cores are used to create internal cavities. Core is a separate entity placed in a mould to produce a corresponding cavity hole
or undercut in the casting
Cores for sand casting are manufactured by packing specially prepared sand in coreboxes
Chaplets: is used to the support the core
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Major part of Moulding material in sand casting are
1. 85-90% silica sand (SiO2)
2. 3-7% bonding material e.g., clay cereal etc.
3. 3-6% water
Requirements of molding sand are:
(a) Refractoriness
(b) Cohesiveness
(c) Permeability
(d) Collapsibility
The cavityin the sand mold is formed by packing sand around a pattern,
then separating the mold into two halves and removing the pattern
3-Moulds and mould making
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Mould Material
Major part of Moulding material in sand castingare
1. 70-85% silica sand (SiO2)
2. 10-12% bonding material e.g., clay etc.
3. 3-6% water
The performance of mould depends on following
factors:
(a) Permeability
(b) Green strength
(c) Dry strength
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Requirements of molding
sand
1. Refractoriness(strength)
2. Permeability
3. Collapsibility4. Thermal stability
5. Reusability
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Requirements of molding sand
(1) Refractoriness(strength)
Is the ability to remain solid at high temp and resist
erosion caused by flow of molten metal
(2) Permeability
Is the ability gas flowthrough mould
(3) Collapsibility
Is theability of the mold to give a way and allow the castingto shrink without cracking the casting,i,e to permit metal
to shrink after solidification
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Requirements of molding sand
(4)Thermal stability
Is the ability of sand at the surface of the mold cavity to
resist cracking and buckling upon contact with the
molten metal.
(5)Reusability
Is the ability of sand to be used for making other mold
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4-Melting (next)
Furnaces most commonly used in foundries:
Cupolas
Direct fuel-fired furnaces
Crucible furnaces Electric-arc furnaces
Induction furnaces
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5-Pouring the Molten Metal(next)
For this step to be successful, metal must flow into
all regions of the mold, most importantly the main
cavity, before solidifying
Factors that determine success:
Pouring temperature
Pouring rate
Turbulence
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Sections of foundry
Metal
Melting
Handling
molten metal
Sand
Sand mixing & preparation
Moulding
Pouring
Shaking out
Finishing
Heat treatment
Inspection & Testing
Melting Section Moulding Section
Sand additives
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General layout of foundry
Foundry store
Melting
Sand
Moulds ready for pouring
Furnaces
Inspection Bench Core
making
Grinding
Muller
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Preparation of sand mould (Sand Mold Making Procedure)(
Before any casting can take place a wooden pattern is made precisely. This iscalled pattern making and in industry this is a very skilful job. Any inaccuracy
at this stage will result in the final cast being wrong or even failing.
Drag is placed inverted on the mouldfloor and pattern is placed at the center
of the box
SAND CASTING
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Special casting sand will soon bepacked around the pattern for easy
removal of pattern from
parting powder is sprinkled over and
around it. It stops the casting sand
sticking to the pattern and pulling away
with it when the pattern is finallyremoved from the sand.
Casting sand is then shaken through a sieve (called
riddled sand) so that only fine particles fall around
the pattern. This is called facing sand and it must befine so that detail on the pattern shows up on the
final casting.
Preparation of sand mould
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The drag is then packed with more casting sand
and then ram it down firmly using a ramming tool.The tool has two ends, one is cylindrical and is
used for general packing down of the sand. The
other end is quite pointed and this can be used for
packing sand close up to the pattern.
When the drag is packed fully it is levelled
off (called strickled off ) using a straightsteel bar.
Preparation of sand mould
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The entire drag and its contents are then turned over so that the base of
the pattern can be seen
A top box called a cope is then placed on top of the drag and locating pins
are put in position so that the casting boxes cannot move sideways.
Preparation of sand mould
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Sprue pins are positioned. One usually on
the back of the pattern and the other to the side.
These will eventually provide an entrance and
exit for the molten aluminium when it is poured
into the sand.
The sand is packed/rammed into the cope in the same way as the drag.
Preparation of sand mould
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The top box (the cope) is then removed and if all is well the cope with the
sand inside should lift off the drag (bottom box) without the sand falling out.A small gate is cut below the position of one of the sprue pins. This will
help the molten metal to flow into the cavity left by the mould. Small
tools are available or can easily be made to dig a variety of shapes in the
casting sand. They are similar to small trowels
Preparation of sand mould
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The pattern is removed using a spike. Before
removing the pattern it is a good idea to gentlytap the spike so that it loosens the pattern from
the sand. It can then be lifted away from the
casting box (drag).
The cope (top casting box) is placed back on top of the drag and thelocating pins put in position. Before this is done vents can be created using a
thin piece of welding rod, pushing it through the sand . This allows gases to
escape once the molten metal is poured.
Preparation of sand mould
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The molten metal is poured with great care. The molten metal is poured
down the hole left by the first sprue pin (now called the runner). As it runs
down the runner it flows through the gate cut by the trowel, into the cavity
left by the pattern and up the riser (the hole left by the second sprue pin).
The casting should be left for at least an hour before removal from the sand
When removed from thesand, the runner and riser are
cut away and the casting is
ready for machining
Preparation of sand mould
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