Upload
parimal-bhambare
View
223
Download
0
Embed Size (px)
Citation preview
7/29/2019 Casting Lec2
1/20
CASTING PROCESSES I
Lecture Notes
by
Zulkepli Muhamad
Solidification Processes
Starting work material is either a liquid or is in ahighly plastic condition, and a part is createdthrough solidification of the material
Solidification processes can be classified
according to engineering material processed:z Metals
z Ceramics, specifically glasses
z Polymers and polymer matrix composites(PMCs)
7/29/2019 Casting Lec2
2/20
Classification of solidification processes
Classification of casting processes
Metal casting processes Foundries
Expandable mold Permanent mold Single-crystal growing
Composite
moldSandShell
Expandable pattern
Plaster
Ceramic
investment
Slush
Pressure
Die
Centrifugal
Squeeze
semisolid
Single-crystal for microelectronics
Single-crystal turbine blade
Directional solidification
7/29/2019 Casting Lec2
3/20
Casting Processes
1. Expendable mold processes uses an expendable
moldwhich must be destroyed to remove casting
Mold materials: sand, plaster, and similar
materials, plus binders
2. Permanent mold processes uses apermanent
moldwhich can be used many times to produce
many castings
Made of metal (or, less commonly, a ceramic
refractory material
Overview of Casting Technology
Castingis usually performed in a foundry
Foundry = factory equipped for making
molds, melting and handling molten metal,
performing the casting process, andcleaning the finished casting
Workers who perform casting are called
foundrymen
7/29/2019 Casting Lec2
4/20
Casting
Process in which molten metal flows by gravity or otherforce into a mold where it solidifies in the shape ofthe mold cavity
The term castingalso applies to the part made in theprocess
Steps in casting seem simple:1. Prepare mould/die
2. Melt the metal
3. Pouring melt into a mould/die4. Let it solidify
5. Finishing processes
Metal casting components
7/29/2019 Casting Lec2
5/20
Metal casting components
Metal casting components
7/29/2019 Casting Lec2
6/20
Production steps
Mold
Molding
Sand
Melting Pouring Casting Heat treatment Finishing
Pattern makingCore Making
Gating system
Furnaces Solidification ShakeoutRemoval of risers
and gates
Cleaning &
Inspection-additioanl heat
treatment
-defects
-dimensions
Why Castings ?
7/29/2019 Casting Lec2
7/20
Capabilities and Advantages
of Casting
Can create complex part geometries
Can create both external and internalshapes
Some casting processes are net shape;others are near net shape
Can produce very large parts
Some casting methods are suited tomass production
Parts Made by Casting
Big parts: engine blocks and heads for
automotive vehicles, wood burning stoves,
machine frames, railway wheels, pipes,
church bells, big statues, and pumphousings
Small parts: dental crowns, jewelry, small
statues, and frying pans
All varieties of metals can be cast, ferrous
and nonferrous
7/29/2019 Casting Lec2
8/20
Sand Casting process
680 Kg Compressor Frame
7/29/2019 Casting Lec2
9/20
7/29/2019 Casting Lec2
10/20
Sand mould
Sand mould
7/29/2019 Casting Lec2
11/20
Sand Moulding Processes
Sand Moulding Processes (cont.)
7/29/2019 Casting Lec2
12/20
Sand Moulding Processes (cont.)
Two forms of mold: (a) open mold, simply a
container in the shape of the desired part;
and (b) closed mold, in which the mold
geometry is more complex and requires a
gating system (passageway) leading into the
cavity
7/29/2019 Casting Lec2
13/20
The Pattern
A full-sized model of the part, slightly
enlarged to account for shrinkage and
machining allowances in the casting
Pattern materials:
z Wood - common material because it is easy to
work, but it warps
z Metal - more expensive to make, but lasts much
longerz Plastic - compromise between wood and metal
Types of patterns used in sand casting:
(a) solid pattern
(b) split pattern
(c) match-plate pattern
(d) cope and drag pattern
7/29/2019 Casting Lec2
14/20
7/29/2019 Casting Lec2
15/20
The Core
Full-scale model of interior surfaces of part
It is inserted into the mold cavity prior topouring
The molten metal flows and solidifiesbetween the mold cavity and the core to formthe casting's external and internal surfaces
May require supports to hold it in position in
the mold cavity during pouring, calledchaplets
Core held in place in the mold cavity by
chaplets
(b) possible chaplet design
(c) casting with internal cavity
7/29/2019 Casting Lec2
16/20
Sand Casting - Process variations
z Green sand casting: the most common and
the cheapest. Associated problems arethat the mould has low strength and high
moisture content.
z Dry sand: core boxes are used instead of
patterns. Expensive and time consuming.
z Skin-dried sand: the mould is dried to a
certain depth. Used in the casting of steels.
Sand Casting - Economicconsiderations
z Production rates of 1-60 pieces/hour, but
dependent on size.
z Lead time ranges from days to several weeks
depending on complexity and size of casting.
z Material utilization is low to moderate - 20-50% of
material lost in runners and risers.
z Both mould material and runners and risers may
he recycled.
z Patterns are easy to make and set, and are
reusable.
7/29/2019 Casting Lec2
17/20
Sand Casting-Economic
considerations
z Pattern material dependent on the number of
castings required.z Easy to change design during production.
Economical for low production runs. Can be used
for one-offs.
z Tooling costs are low.
z Equipment costs are low. Direct labour costs are
moderate to high. Can be labour intensive.
z Finishing costs can be high. Cleaning and fettling
are important before secondary processing.
Process Parameters
7/29/2019 Casting Lec2
18/20
Pouring
Desirable Mold Properties andCharacteristics
Strength - to maintain shape and resist
erosion
Permeability- to allow hot air and gases to
pass through voids in sand
Thermal stability- to resist cracking on
contact with molten metal
Collapsibility- ability to give way and allow
casting to shrink without cracking the casting
Reusability- can sand from broken mold be
reused to make other molds?
7/29/2019 Casting Lec2
19/20
Sands
The sands used may be one of the following
categories;naturally bonded (bank) - less expensive
synthetic (lake) - this sand can have avariety of controlled compositions.
Types of sand include,
Zircon (ZrSiO4) - low thermal expansion
Olivine (Mg2SiO4) - low thermal expansion
Iron Silicate (Fe2SiO4) - low thermal
expansionChromite (FeCr2O4) - high heat transfer
Foundry Sands
Silica (SiO2) or silica mixed with other minerals
Good refractory properties - capacity toendure high temperatures
Small grain size yields better surface finish onthe cast part
Large grain size is more permeable, to allowescape of gases during pouring
Irregular grain shapes tend to strengthenmolds due to interlocking, compared to roundgrainsz Disadvantage: interlocking tends to reduce
permeability
7/29/2019 Casting Lec2
20/20
Types of Sand Mold
Green-sand molds - mixture of sand, clay,
and water;
z Green" means mold contains moisture at time of
pouring
Dry-sand mold- organic binders rather than
clay and mold is baked to improve strength
Skin-dried mold- drying mold cavity surface
of a green-sand mold to a depth of 10 to 25
mm, using torches or heating lamps
Other Expendable Mold
Casting Processes
Shell Molding
Vacuum Molding
Expanded Polystyrene Process
Investment Casting
Plaster Mold and Ceramic Mold Casting