Classification of solidification processes
©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e
Casting
Principle of the process
Configuration / Structure
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
Module 4a
2
Principle of the Casting
Process
Module 4a
Casting – Principle and 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.
 Physics: 1. Fluid flow and interaction with solid
2. Thermal dynamics
 Any material can be cast, as any material
undergoes the liquid state and solid state.
Module 4a
4
Basic casting steps (very simplified):
1. Melt metal
Furnace
2. Pour it into mold
Casting
system
3. Let it freeze
Casting
Principle of the process
Configuration / Structure
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
Module 4a
6
Casting system
Module 4a
Casting System – (Generic) structure
Basic components
and their connections
Make a
cavity
Module 4a
8
Casting
Principle of the process
Configuration / Structure
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
Module 4a
9
Process modeling
1. Heat up metal to a certain temperature
2. Pour molten metal into mold cavity
3. Solidify molten metal
Module 4a
Heat up metal
-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.
Module 4a
11
Heat up metal
(2) heat of fusion to convert
the solid to the liquid
3.1 Heating Period Thermal Analysis
H  V{Cs (Tm  T0 )  H f  Cl (Tp  Tm )}
(1) heat for raising the temperature
to the melting point
(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 energyModule
H, and
4a then set up the furnace property
12
Pour molten metal into mold cavity
Flow rate at the base of a sprue (energy balance:
v12
P2 v22
h1  
 F1  h2  
 F2
 2g
 2g
P1
v  2gh
Module 4a
Pour molten metal into mold cavity
Mass balance
Mass in = mass out
Q=v1 A1 = V2 A2
Assumption: Fluid is incompressible.
Module 4a
14
Pour molten metal into mold cavity
Time to fully fill the model cavity
V
MFT 
Q
Assumption: not consider the transient process of
molten metal in the cavity.
MFT is shorter than the actual time needed to fill up
the mold cavity.
Module 4a
15
Process modeling
1. Heat up metal to a certain temperature
2. Pour molten metal into mold cavity
3. Solidify molten metal
Module 4a
Solidify molten metal
Chvorinov’s rule: the time needed for the molten metal to
completely solidify in the mold cavity
V n
TST  C m ( )
A
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)
Module 4a
17
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
18
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
19
Casting
Principle of the process
Configuration / Structure
Process modeling
Defects/quality control
Design For Manufacturing (DFM)
Process variation
Module 4a
20
Defects
Basic Problems in Casting Processes
- Incomplete filling of cavity
- Gaps in casting
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
21
Liquid
Contraction
0.5%
Further
Contraction
Lack of molten
metal
Different metal
thermal
expansion
coefficients
Shrinkage
handouts 3
22
Riser: conducive to overcoming defects 1 and 2
Riser
Casting
Cup and Sprue to pour
the molten metal
Module 4a
Riser = Reservoir
23
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,
Module 4a
)
24