casting process. ©2010 john wiley & sons, inc. m p groover, fundamentals of modern...
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Casting process
Casting process
©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e
Classification of solidification processes
Polymer modified cement
Module 4a 5
Casting Principle of the process
Configuration / Structure
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
Module 4a
Principle of the Casting Process
Module 4a 7
Casting – Principle of the process
Casting is a process in which the molten metal
flows by gravity or other force into a mold and
solidifies in the shape of the mold cavity.
Any material can be cast, as any material
undergoes the liquid state and solid state.
Physics: 1. Fluid flow and interaction with solid
2. Thermal dynamics
Basic casting steps (very simplified):
1. Melt metal
2. Pour it into mold
3. Let it freeze
Furnace
Casting system
Module 4a 9
Casting Principle of the process
Configuration / Structure
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
Module 4a
Casting system
Module 4a 11
Basic components and their connections
Make a cavity
Casting System – (Generic) structure
Module 4a
Animation
http://nptel.ac.in/courses/112107144/metalcasting/lecture5.htm#
Module 4a 13
Casting Principle of the process
Configuration / Structure
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
Module 4a
Process modeling
1. Heat up metal to a certain temperature
2. Pour molten metal into mold cavity
3. Solidify molten metal
Module 4a 15
-Heat metal above the melting point
-Pour it into the mold cavity
Heat energy required for
(1)heat for raising the temperature to the melting point,
(2)heat of fusion to convert the solid to the liquid,
(3)heat for raising the molten metal to the metal at the
desired temperature ready to pour it into a cavity.
Heat up metal
Module 4a 16
Heat up metal
3.1 Heating Period Thermal Analysis
)}()({ 0 mplfms TTCHTTCVH
(1) heat for raising the temperature to the melting point
(2) heat of fusion to convert the solid to the liquid
(3) heat for raising the molten metal to the metal at the desired temperature ready to pour it into a cavity.
Given T0, determine the total energy H, and then set up the furnace property
Module 4a
Pour molten metal into mold cavity
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12
222
2
211
1 Fg
vPhF
g
vPh
Flow rate at the base of a sprue (energy balance:
ghv 2
Module 4a 18
Mass in = mass out
Q=v1 A1 = V2 A2
Assumption: Fluid is incompressible.
Pour molten metal into mold cavity
Mass balance
v2>v1, so A2<A1, so the sprue must be designed as tapered
Module 4a 19
Time to fully fill the model cavity
Q
VMFT
Assumption: not consider the transient process of molten metal in the cavity.
Pour molten metal into mold cavity
MFT is shorter than the actual time needed to fill up the mold cavity.
Module 4a
Process modeling
1. Heat up metal to a certain temperature
2. Pour molten metal into mold cavity
3. Solidify molten metal
Module 4a 21
Chvorinov’s rule: the time needed for the molten metal to completely solidify in the mold cavity
nm A
VCTST )(
TST: total solidification time, min
V: volume of the casting, in.3 (cm3)
A: surface area of the casting, in.2 (cm2)
N: exponent, 2
Cm: mold constant, min.in.2 (min/cm2)
Solidify molten metal
Module 4a 22
Cm is a function of
mold material
thermal properties of the cast metal
pouring temperature relative to the melting point of the metal.
Determination of Cm can be done through experiment.
The principle of such an experiment is to have a scenario that is the same as the casting and a known cavity geometry, to operate the process, and to record the Time TST.
Module 4a 23
Implication of Chvorinov’s Rule:
A casting with a higher volume-to-surface area ratio will cool and solidify more slowly than one with a lower ratio.
Module 4a 24
Casting Principle of the process
Configuration / Structure
Process modeling
Defects/quality control
Design For Manufacturing (DFM)
Process variation
Module 4a25
Defect 1: incomplete filling of cavity
Defect 2: gaps in casting
Defects
Incomplete filling is caused by too fast solidification and flow blockage. Solutions: multiple pouring cups, riser, etc.
Gaps in casting is caused by a phenomenon called shrinkage. Solutions: riser, pattern allowance, etc.
Module 4a 26
Shrinkage:
The source of problems in any process involving the heat transfer
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Liquid Contraction0.5%
Further Contraction
Lack of molten metal
Different metal thermal expansion coefficients
Shrinkage gaps and voids
Module 4a 28
Riser: overcoming the two defects
CastingRiser
Cup and Sprue to pour the molten metal
Riser = Reservoir
Module 4a 29
Summary of discussions so far
Principle of casting process (casting is liquid to fill up cavity)
Casting can be applied to any material
Three processes: heat up, pour, solidify
Defects (incomplete filling, gap)