Production Technology
J.BABU
AssociateProfessorDepartment of Mechanical Engineering
Sreenidhi Institute of Science and Technology
Gating System Design
Lecture Objectives
Pouring Time
Choke Area
Sprue
Pouring Basin
Sprue Base Well
Gating Ratios
Ingate Design
Slag Trap Systems
- Runner Extension
- Whirl Gate
Pouring time Objective for gating system design
to fill the mould in smallest time Pouring time: Time required to completely fill the mold Too long pouring time higher pouring temperature Too less pouring time turbulent flow in the mold Aim Optimum Pouring Time for any given casting Pouring time depends on
- casting material
- complexity of casting
- section thickness ratio of surface area to volume is imp. rather than total mass of the casting
- size of casting Relations are experimentally obtained rather than theoretical
formulations.
Standards methods to calculate pouring time for different materials:
1) Grey CI (mass < 450kg)
2) Grey CI (mass > 450kg)
kg casting, the of mass W
mm thickness, section averageT 40
inches in iron ofFluidity K where
s W15.59
T1.41Kttime, Pouring
s W15.65
T1.236Kttime, Pouring 3
Pouring time cont..
Casting Mass Pouring time (s)
20 kg 6 to 10
100 kg 15 to 30
100, 000 kg 60 to 180
3) Grey CI (mass < 450kg)
4) Shell moulded ductile iron (vertical pouring)
5) Copper alloy castings
s W Wlog 0.3953 -2.4335Kttime, Pouring
sections heavier for 2.970
thick mm 25 to 10 sections for 2.670
sections thinner for 2.080 K where
s W Kttime, Pouring
1
1
Pouring time cont..
2.80 Bronze Tin
1.90 Brass
1.80 gating Bottom
1.30 gating Top
by given constant a is K
s W Kttime, Pouring
2
32
6) Intricately shaped thin walled castings with Grey CI mass <450kg
where W` = mass of the casting with gates and risers, kg
K3 = a constant as given below
7) For casting above 450 kg and upto 1000 kg
where K4 = a constant as given below
s W Kttime, Pouring 3 '3
Pouring time cont..
T (mm) K3
1.5 to 2.5 1.62
2.5 to 3.5 1.68
3.5 to 8.0 1.85
8.0 to 15.0 2.20
s T W Kttime, Pouring 3 '4
T (mm) K3
Up to 10 1.00
10 to 20 1.35
20 to 40 1.50
Above 40 1.70
Choke Area The main control area which meters the metal flow into the mould cavity
- so that that mould is filled within the calculated pouring time Located at the bottom of the sprue
used system gating the of function a is whichfactor efficiencyC
mm height), (sprue head metal effectiveH
mm/s gravity, to due onacceleratig
kg/mm metal, molten the ofdensity massd
stime, pouringt
kgmass, castingW
mm area, choke Awhere
H g 2C t d
W A Area,Choke
3
2
3
2
Choke Area
Different Gating Systems
Choke Area cont… The effective height ,H of a mould depends on the casting dimensions
and the type of gating used.
cavity mould of height totalc
cope incavity mould of heightp
sprue of heighth where2c
p-hH gate, Parting
2
c-hH gate, Bottom
hH gate, Top
2
Efficiency of the Gating System Efficiency coefficient of the gating system depends on the various
sections that are normally used in the gating system. Elements circular in cross-section
lower surface area to volume ratio reduce heat loss less friction
Streamlining various elements of gating system causes increase in volumetric efficiency of the gating system allow smaller size gates and runners increase in casting yield.
Overall efficiency can be calculated by considering the loss in metal head when a runner changes direction or joins with another runner or gate.
area choke the is A
changes from stream down areas ....are A,A
area or direction in changes at occuring tscoefficien loss ....areK ,K where
AA
KAA
K1
1C factor, Efficiency
21
21
22
2
221
2
1
Sprue
Should be tapered down to take into account the gain in velocity of the metal at it flows down reducing the air aspiration.
The exact tapering can be obtained by
AtVt = AcVc
At = Ac(Vc/Vt)
Since, velocities are proportional to the square of the potential heads,
At = Ac (hc/ht)
Equation indicates that the profile of the sprue should be parabolic as per the above equation.
As it is difficult to make a parabolic a straight taper is made. Straight taper
- reduces air aspiration
- increase the flow rate compared to a parallel sprue.
Sprue cont… Should be tapered down to take into account the gain in velocity of
the metal at it flows down reducing the air aspiration. The exact tapering can be obtained by
AtVt = AcVc
At = Ac(Vc/Vt)
Since, velocities are proportional to the square of the potential heads,
At = Ac (hc/ht)
H= actual sprue heightHt
= h + H
Pouring Basin
The main function of the pouring basin is to reduce the momentum
of the liquid flowing into the mould.
To prevent the turbulence of the molten liquid the pouring basin
should be deep and the entrance into the sprue should be a
smooth radius of atleast 25 mm.
To prevent vortex formation the pouring basin should be kept full.
Pouring Basin cont… Constant flow conditions should be maintained by using a delay
screen or strainer core.
The metal should be poured steadily into the pouring basin keeping the lip of the ladle as close as possible.
Pouring basins are preferable with castings in alloys which form troublesome oxide skins (aluminium, aluminium bronze, etc..)
Sprue Base Well
It is provided at the bottom of the sprue. It helps to reduce the velocity of the incoming metal and mold
erosion. General guideline,
- area of sprue base well = 5 * sprue choke area.
- well depth = runner depth
Refers to the proportion of the cross-sectional area between the
sprue, runner and ingates Denoted as sprue area : runner area : ingate area It is selected depending on the characteristics of molten metal
being cast
Factors that are considered are
- fluidity
- slag or dross forming tendency
- pouring temperature
- mould material characteristics like resistance to erosion,
scabbing tendency, green sand, CO2, dry sand, shell molded, .
Gating Ratios
Some gating ratios used in practice
Metal Gating Ratio
Aluminium
1 : 2 : 1
1 : 1.2 : 2
1 : 2 : 4
1 : 3 : 3
1 : 4 : 4
1 : 6 : 6
Aluminium bronze 1 : 2.88 : 4.8
Brass
1 : 1 : 1
1 : 1 : 3
1.6 : 1.3 : 1
Copper2 : 8 : 1
3 : 9 : 1
Ductile iron
1.15 : 1.1 : 1
1.25 : 1.13 : 1
1.33 : 2.67 : 1
Some gating ratios used in practice cont…Metal Gating Ratio
Grey cast iron
1 : 1.3 : 1.1
1 : 4 : 4
1.4 : 1.2 : 1
2 : 1.5 : 1
2 : 1.8 : 1
2 : 3 : 1
4 : 3 : 1
Magnesium1 : 2 : 2
1 : 4 : 4
Malleable iron
1 : 2 : 9.5
1.5 : 1 : 2.5
2 : 1 : 4.9
Steels
1 : 1 : 7
1 : 2 : 1
1 : 2 : 1.5
1 : 2 : 2
1 : 2 : 2
1 : 3 : 3
1.6 : 1.3 : 1
Ingate
It can be considered as a weir with no reduction in cross-section of the stream at the gate.
The rate of flow of molten metal through the gates depend on
- the free height of the metal in the runner and gate area
- the velocity with which metal is flowing in the runner.
where Q = metal flow rate, mm3/s, b = gate width, mm
V = metal velocity in runner, mm/s, g = acceleration due to gravity (mm/s2)
Height of the gate, h1 = h -5 mm
mm 2g
V
gb
Q 1.6hheight, Free
2
2
2
Ingate cont..The points to be remembered while choosing the positioning of the ingates
1. Ingates should not be located near a protruding part of the mould to avoid the striking of vertical mould walls by the molten metal stream.
2. Ingates should be preferably be placed along the longitudinal axis of the mould wall.
3. Ingates should not be places near a core print or a chill.
4. Ingates cross-sectional area should preferably be smaller than the smallest thickness of the casting.
Small castings one ingate Multiple castings multiple ingates In case of multiple ingates,
- they should have uniform area.
- they should be located at constant intervals.
- the runner area should be progressively reduced.
Slag Trap Systems
Runner extension Metal which moves first into the gating system contains slag and
dross. To prevent these from entering the mould cavity the runner is
extended beyond the ingate- the momentum will carry it past the gates into a blind alley
width of runner extension = 2 runner width
Slag Trap Systems cont..
• Whirl gate
Depending on the gating ratio two types of gating systems depending on the choke area:
1. Non-pressurized
2. Pressurized
Sprue
Runner
Ingate
Types of Gating Systems
Non-pressurized gating system Total runner area and ingate areas higher than sprue area No pressure existing in the metal flow system – low turbulence Useful for casting – drossy metals and alloys Gating ratio - 1:4:4
Drawbacks Low casting yield-due to large metal in runners and gates The metal flow should be full in all elements – else - air
aspiration
Pressurized gating system Ingate area is the smallest – more backpressure in the system Turbulent metal – flows full in the system Not used for light alloys, suitable for ferrous castings Provides higher casting yield Gating ratio – 1:2:1
Two basic principles of fluid flow are relevant to gating design:
Bernoulli’s theorem and the Law of mass continuity
Bernoulli’s theorem
Based on the principle of conservation of energy and relates pressure, velocity and elevation of the fluid at any location in the system, and the frictional losses in a system that is full of liquid
2ph+ constant
g 2
v
g h - elevation above a certain
reference lane
p -pressure at that elevation
v - velocity of the liquid
- density of liquid
g -gravitational constant
f - frictional loses
2 21 1 2 2
1 2
p ph h
g 2 g 2
v vf
g g
For two different elevations of a liquid Bernoulli’s equation is :
Law of mass continuity
For incompressible liquids and in a system with impermeable walls, the rate of flow is constant.
Q = AV=constant
For two different locations
Q = A1V1 = A2 V2
Q - Rate of flow m3/sec
A - cross-sectional area of liquid stream
V - average velocity of the liquid
Sprue The sprue should be tapered to take into account the gain in
velocity and thus reduce aspiration
The exact shape can be obtained by applying Bernoulli’s equation and continuity equation
1
2
3
hc
ht
h2
Atmospheric pressure
Open to Atmosphere
mould
sprue
2
3
Actual
Ideal
hc
ht
Ideal and actual shapes of sprue
Casting Yield
• Higher the casting yield higher is the economics of the foundry practice.• Hence the casting yield should be maximized during the design stage itself.
New material Metal melted Scrap metal
Melting losses Metal cast Scrap castings
Fettling losses Actual castingRunners &
Risers
Utilization of the metal in the foundry
mould the into poured metal the of mass - w
mass casting actual - Wwhere
100w
W yieldCasting
Casting Yield cont…
Casting Yields for different metals
Casting description Yield range
Simple shape and massive 0.85 to 0.95
Steels
simple shape 0.75 to 0.85
heavy machinery parts 0.65 to 0.75
Small pieces 0.35 to 0.45
Cast iron
heavy machinery parts 0.65 to 0.75
Small pieces 0.45 to 0.55
Aluminium 0.25 to 0.45
Casting of an Aluminum Piston
Aluminum piston for an internal combustion engine: (a) as-cast and (b) after machining.
Simulation of mold filling and solidification
(a) 3.7 seconds after start of pour. Note that the mushy zone has been established before the mold is filled completely. (b) Using a vent in the mold for removal of entrapped air, 5 seconds after pour.
Types of Internal and External Chills used in Casting
Various types of (a) internal and (b) external chills (dark areas at corners) used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, as shown in (c).
Design Rules for Casting
Suggested design modifications to avoid defects in castings
Elimination of Hot Spots
Examples of designs showing the importance of maintaining uniform cross-sections in castings to avoid hot spots and shrinkage cavities.
Examples of Good and Poor Designs
Examples of undesirable (poor) and desirable (good) casting designs. Source: Courtesy of American Die Casting Institute.