PE 459 LECTURE 2- natural gas basic concepts and properties
MANUFACTURING SCIENCE S5ME -NITC-2016-SUBTOPICS
1. Design of Risers and Feeding of
Castings• A simplified diagram by putting in
references to the equations (1, 2 & 4)
there is no Equation 3, diagram not changed
• EQ(1) - Freeze Point Ratio (FPR)
FPR=X
X = (Casting Surface/Casting Volume) /
(Riser Surface/Riser Volume)
• EQ(2) - Volume Ratio (VR) (Y Axis)
VR=Y=Riser Vol/Casting Vol*
Note: The riser volume is the actual poured
volume
References - AFS Text Chapter 16; Chastain's Foundry manual Vol 2, Google
• EQ(4) - (Freeze Point Ratio) Steel
X=0.12/y-0.05 + 1.0*
*The constants are from experiments and
are empirical
2. Volumes, Surface Areas, Castings and
Risers...
There are relationships between all these
items and values that will help in designing
a complete mold that controls progressive
solidification, and influences directional
solidification to produce castings with
minimal porosity and shrinkage defects.
This is by ensuring that the riser(s) are the
last to solidify.
3. 4 points about the Riser/Casting
Relationship
• 1 - Risers are attached to the
heaviest sections of the casting
• 2 - Risers are the last to solidify
• 3 - A casting that has more than
one heavy section requires at
least one riser per heavy section
• 4 - Occasionally the thermal
gradient is modified at the mold-
metal interface by the introduction
of a "Chill" that can better conduct
the heat away from the casting
and lower the solidification time
for that section.
4.
5. Gating / Runner Design
• Now a look at the flow characteristics of the
metal as it enters the mold and how it fills the
casting.
Of the flow characteristics
fluidity/viscosity plays a role. Also,
velocity,
gravitational acceleration & vortex,
pressure zones,
molten alloy aspiration from the mold and
the momentum or kinetic energy of a fluid.
6. The demarcation point is
Re < 2000 is considered a Laminar Flow
Re > 2000 is considered a Turbulent Flow
Objective is to maintain Re below 2000.
10. Basic Components of a Gating System
• The basic components of a gating system are:
Pouring Basin,
Sprue,
Runners and
Gates that feed the casting.
The metal flows through the system in this order.
Some simple diagrams to be familiar with are:
11. "Crucible-Mold Interface" is where the metal
from the crucible first contacts the mold
surface. This area is lower than where the
Mouth of the Sprue is located, by having a pool
of metal from the flow will be less chaotic than
pouring from the crucible down into the sprue.
"Dross-Dam" - to skim or hold back any dross
from the crucible or what accumulated through
the act of pouring.
As the lower portion fills and the metal is
skimmed, the clean(er) metal will rise up to
meet the opening of the sprue in a more
controlled fashion.
Pouring Basin - This is the "Crucible -Mold Interface", A pouring cup and
pouring basin are not equivalents, The pouring cup is simply a larger target
when pouring out of the crucible, a Pouring Basin has several components
that aid in creating a laminar flow of clean metal into the sprue.
The basin acts as a point for the liquid metal to enter the gating system in
a laminar fashion.
12. Sprue Placement and Parts
The sprue is the extension of the sprue
mouth into the mold
The choke or narrowest point in the
taper is the point that would sustain a
"Head" or pressure of molten metal.
To reduce turbulence and promote
Laminar Flow, from the Pouring Basin,
the flow begins a near vertical incline
that is acted upon by gravity and with
an accelerative gravity force
Fluids in free fall tend to distort from a
columnar shape at their start into an
intertwined series of flow lines that
have a rotational vector or vortex effect
(Clockwise in the northern hemi-
sphere, and counter clockwise in the
southern hemi-sphere)...
13.
14.
15.
16.
17.
18. Pressurized - is a system
where the gate and runner
cross-sectional areas are
either equal or less than
the choke cross-sectional
area;
A1= Choke = 1 unit
A2 = 1st Runner c/s
Area = 0.75 unit
A3 = 2nd Runner c/s
Area = 0.66 unit
A4 = 1st Gate = 0.33 unit
A5 = 2nd Gate = 0.33 unit
Unpressurized - The key
distinction is that the
Runner must have a c/s
area greater than the
Choke, and it would
appear that the Gate(s)
would equal or be larger
than the Runner(s).
Common Ratio's noted are;
1 : 2 : 4; 1 : 3 : 3
1 : 4 : 4; 1 : 4 : 6
19. • The rotational effect, though not a strong
force, is causing the cork-screwing effect
of the falling fluid. If allowed to act on the
fluid over a great enough duration or free
fall the centrifugal force will separate the
flow into droplets.
• None of the above promotes Laminar flow,
plus it aids the formation of dross and gas
pick-up in the stream that is going to feed
the casting.
20. Some dimensioning ratio's from
Chastain's Foundry Manual (no.2)
• 1- Choke or sprue base area is 1/5th the area of the well.
• 2- The well depth is twice the runner depth.
• 3- the Runner is positioned above the midpoint of the
well's depth
•By creating a sprue with a taper, the fluid is constrained to
retain it's shape, reducing excessive surface area development
(dross-forming property) and gas pick-up.
•The area below the sprue is the "Well". The well reduces the
velocity of the fluid flow and acts as a reservoir for the runners
and gates as they fill.
21. • The runner system is fed by the well
and is the path that the gates are fed
from.
• This path should be "Balanced" with the
model of heating or AC ductwork
serving as a good illustration. The
Runner path should promote smooth
laminar flow by a balanced volumetric
flow, and avoiding sharp or abrupt
changes in direction.
• The "Runner Extension" is a "Dead-
End" that is placed after the last gate.
The R-Ext acts as a cushion to absorb
the forward momentum or kinetic
energy of the fluid flow. The R-Ext also
acts as a "Dross/Gas Trap" for any
materials generated and picked-up
along the flow of the runner.
• An Ideal Runner is also proportioned
such that it maintains a constant
volumetric flow through virtually any
cross-sectional area. In the illustration,
notice that the runner becomes
proportionally shallower at the point
where an in-gate creates an alternate
path for the liquid flow.
The Runner System
22. The Gating System
• The Gates (in this case)
accommodate a directional
change in the fluid flow and
deliver the metal to the
Casting cavity.
• Again, the design objective
is to promote laminar flow,
the primary causes of
turbulence are sharp
corners, or un-proportioned
gate/runner sizes.
• The 2 (two) dashed blue
areas when added together
form a relationship to the
dashed blue area of the
Runner, which forms a
relationship to the Choke or
base of the Sprue Area.
23. • The issue of sharp corners (both inner
and outer) create turbulence, low & high
pressure zones that promote aspiration of
mold gases into the flow, and can draw
mold material (sand) into the flow. None
of this is good... By providing curved
radius changes in direction the above
effects are still at play but at a reduced
level. Sharp angles impact the
solidification process and may inhibit
"Directional Solidification" with cross-
sectional freezing...
• The image to the right is just too good a
representation to pass-up..
• By proportioning the gating system, a
more uniform flow is promoted with near
equal volumes of metal entering the mold
from all points. In an un-proportioned
system the furthest gates would feed the
most metal, while the gates closest to the
sprue would feed the least.
(this is counter to what one initially thinks).
25. Formulae, Ratios and Design Equations
• What is covered so far is comprehensive, and intuitive on a
conceptual level, but the math below hopefully offers some insight
into quick approximations for simple designs, and more in-depth
calculations for complex systems.
• Computerized Flow Analysis programs are used extensively in large
Foundry operations.
• From basic concepts, designing on a state of the art system shall be
attempted:
• Continuity Equation –
• This formula allows calculation of cross-sectional areas, relative to
flow Velocity and Volumetric flow over unit time. This is with the
assumption that the fluid flow is a liquid that does NOT
compress (that applies to all molten metals).
26. Here, a flow passes through A1
(1" by 1", 1 sq")
The passage narrows to a cross-
sectional area A2
(.75" by .75", 0.5625 sq")
The passage expands to a cross-
sectional area A3
(1" by 1", 1 sq").
Q= Rate of Flow
(Constant - uncompressible)
V=Velocity of flow
A=Area (Cross-section)
If A1 and A2 are considered, the Area A2 is almost half of
A1, thus the velocity at A2 has to be almost double of A1.
27. GATING RATIO is-
Areas of Choke : Runner : Gate(s)
• The base of the Sprue and Choke are the
same.
• The ratios between the cross-sectional Area can
be grouped into either Pressurized or
Unpressurized.
• Pressurized: A system where the gate
and runner cross-sectional areas are either
equal or less than the choke cross-sectional
area.
28. • Areas A2 & A3 do not get
added as they are
positioned in line with
each other and flow is
successive between the
points and not
simultaneous.
• While Areas A4 & A5 are
added together as flow
does pass through these
points simultaneously.
• This example would
resolve to a pressurized
flow of 1 : 0.75 : 0.66
A1= Choke = 1 Sq Inch
A2 = 1st
Runner c/s Area = 0.75 Sq Inch
A3 = 2nd
Runner c/s Area = 0.66 Sq Inch
A4 = 1st
Gate = 0.33 Sq inch
A5 = 2nd
Gate = 0.33 Sq Inch
29. Unpressurized:
• The key distinction is that the Runner must have
a cross sectional area greater than the Choke,
and it would appear that the Gate(s) would equal
or be larger than the Runner(s).
• Common Ratio's noted in Chastian's Vol 2 are:
• 1 : 2 : 4
• 1 : 3 : 3
• 1 : 4 : 4
• 1 : 4 : 6
30. • An exception is noted in Chastain with a 1 : 8 : 6
ratio to promote dross capture in the runner
system of Aero-Space castings.
• The Continuity Equation is simplified with the
use of ratios as the velocity is inversely
proportional between any 2 adjacent ratio
values. ie H : L equates to an increase in
velocity while a L : H equates to a drop in
velocity.
• Laminar Flow is harder to control at a high
velocity than a relatively lower velocity.
• Chastain's Vol 2 has much more mathematical
expressions and calculations.
