The design of feeding system feeding calculations

MSE3231
Lecture 10
The Design of FeedingSystem
2. Feeding calculations
1: Optimizing size offeeder
2: Optimizing shape and placement offeeder
Ref:
1 P.Beeley, Foundry Technology, Butterworth-Heinemann, 2001
2 J. Campbell, Castings, Butterworth-Heinemann, 2001
3 Heine, Loper, Rosenthal, Principles of Metal Casting, Tata McGraw-Hill, 1976
Courtesy: AKMB Rashid, Dept. of MME, BUET.

1. Introduction
2. Optimizing size of feeder
3. Optimizing shape of feeder
4. Optimizing placement of feeder
5. Increasing the efficiency of feeder
Topics to discuss....

 The key criteria that should be considered in feeder design:
1. Feedersize andshape
2. Feedernumberand feederdimensions
3. Locationoffeeder
4. Feederconnectionsto the casting
5. Increasein efficiency offeeder
6. Specialconditionsarising from joining sections(junction problem)
1. Introduction

Caine’sMethod
 If a cylindrical casting is fed by a top feeder, the diameter of the feeder should be
at least equal to the diameter of the casting.
 One the other hand, a plate casting of the same volume and thickness smaller
than the diameter of the cylinder need not require a feeder of the same volume,
because it will not have to remain molten as long as the feeder on the cylinder.
 So, obviously then, the A/V ratio of feeder can be related to the A/V ratio of the
casting.
 Caine developed an equation for steel which expressed the relative freezing time
of feeder and casting in terms of the relative volumes of feeder and casting:
X =
a
Y – b
+ c
Y = Vfeeder / Vcasting
(A/V)casting
X = freezing ratio, or relative freezing time=
(A/V)feeder
2.1 Determination of feeder size

sound
casting
unsound
casting
Provides the basic understanding
of feedingprinciples
• requires trial-and-error calculation
to obtaindesiredfeedersize
• nature of shape of shrinkage
cavitygeneratedin feederaffects
feedersize
For identical freezing rates of feeder
andcasting,feedervolumerequirement
becomesinfinite
When casting freezes increasingly
rapidly relative to the feeder, the feeder
volumerequirementdecreasestowards
a minimum.
The values of the constants (a, b, c) vary
with additional conditions imposed (e.g.,
use of exothermic sleeves, etc.) on the
feeder during casting

Bishop’s Method
With increasing values of S (thin
casting),the feederheaddiminishes
towards the limiting level at which
the controlling influence is no
longer the shape factor but the
volumeof feed metal required
Simple modification of Caine’s method
that considers the shape of casting instead of freezing ratio
Shapefactor, S = L + W
T

Wlodawer’s ModulusMethod
Based on Chvorinov’smethod
Deduction of the feeder head requirement
1. Determine the cooling modulus of the casting (cooling surfaces are included only)
2. Feeder head is then selected on the principle that it should have a modulus value
1.2 times that for the casting or section concerned.
 Extended to include systematic consideration of exothermic materials, padding,
chills and other aids to directional solidification
 Since the feeder head requirement for a slender, extensive cast shape is governed
not by its modulus but by the volume of feed metal, a further check is therefore
necessary to verify that the feed volume from the proposed head will be adequate
in the particular circumstances.
volume of casting/segment(V)
cooled surface area(CSA)
M =

Modulusof somecommonshapes
2.2 Modulus determination

 more complicated shapes should be
broken down into simpleshapes
 moduli of the individual simple shapes
should bedetermined
 the section having the highest modulus
should be considered as the significant
section as sensitive for porosity formation
M =
a .b
2 (a +b)
perimeter
M =
 for more complex shapes, a general
formula of the following can be used
cross-sectionalarea
Example: For simple rectangular shape
 if any of the sections contain directly non-
cooling surface, its dimension (c) should
be excluded from the perimeter
Example: For simple rectangular section
M =
a .b
2 (a + b) -c

Example:
 Heavy truck wheel hub casting
 Weight = 68kg
 Very high scrap rate due to shrinkage
defect “A” in segment 3
M =
a .b
2 (a + b) -c
M1
=
5 * 2.5
= 1.0
2 (5 + 2.5) –2.5
2
M =
5 * 3
= 1.5
2 (5 + 3) – 3 –3
3
M =
5 * 4
= 1.8
2 (5 + 4) – 4 –3
Significant modulus, MS = 1.8

3. Optimizing FeederShape
 The shape of the feeder plays an important part in keeping the metal it contains in
liquid form. So the feeder should be shaped so that it promotes a slow cooling rate.
surfacearea of the casting relativeto volume is important in determiningthe rate of heat transfer
from thecasting.
 According to Chvorinov's rule, solidification time of a given shape is proportional to
its modulus , i.e., square of its (V/A) ratio
for a feederof a given size to have a maximum solidification time,
it must havethe smallestsurfacearea
 Amongst all shapes, a sphere has the smallest surface area
a sphericalfeeder headwill remain liquid for the longestperiod of time amongstall shapes
of unitvolume

 Spherical feederheads
 difficult tomould
 causefeeding problems
the last metal to solidify would be near the centre of the sphere,
where it could not be used to feed a casting
 Practicalities dictate the use of cylinders for most
feeders, although hemispherical feeders or a
hemispherical base of cylindrical feeder are
oftenused.
 Besides maximizing freezing time, other factors influencing the shape of a
feeder headinclude
1. the timing of the demand for feed metal, affecting the shape of the shrinkage cavity
in the head, and
2. the permissible area of junction with the casting: this should be as small as possible
to minimize fettlingcosts

 Now the diameter of a cylindrical feeder depends on too many factors.
But what about its height?
 Most authorities and researchers agree that the minimum height of a riser
should be no less than one-half times its diameter and the maximum height should be
no more than one and a half times its diameter.
H = 50 – 150mm
D = 100 mm 
This is becauseof the V/A ratio.
Dimensionsoutside those limits would make the V/A ratio lower than it is within them.
 These limitations apply to both top and side feeders.
3.1 Optimising feeder dimensions

 The top and bottom surfaces of the cylindrical feeders
can be made spherical to stay liquid longer.
That bulb, as it’s called, will keep the feeder liquid.
D
r
It has the same diameter as the feeder (i.e., D = 2r)
 When we have blind feeder, we can use the same
technique on top and bottom of it for the same
purpose.
Which of these two feeders should stay liquid longer?
1000
cc
1000
cc
(a) (b)
 The dome, as we called the top of the blind feeder, has the same dimensions
as the bulb i.e., the radius of the dome is one-half the diameter of the feeder.
The answer is(a).

 Like feeder, the neck should stay liquid as long as possible.
That means that the cross-sectional shape of the neck should be circular.
For some castings,a round neckmay be impossible.
In that cases,we have to use a neckwith a square,or perhapsrectangular,cross-section.
 Even though the neck has a circular section, the longer it is for a given diameter,
the more area it will have and the more rapidly it will solidify.
The feederneckshouldneverexceeds one-halfthe diameterof the feeder.

4. Optimizing Placement of Feeder
 Feeder heads are normally placed in direct contact with the heavier sections
of a casting, since this enables directional solidification to be maintained
throughoutfreezing.
 In complex castings, the shape is divided into a number of natural zones of feeding,
each centred on a heavy section separated from the remainder of the casting by
more constrictednumbers.
 Each zone is then fed by a separately calculated feeder
 In normal conditions, there will be a limit to how far feed liquid can be provided
along a flowpath.
 Up to this distance from the feeder, the casting will be sound.
 Beyond this distance, the casting will exhibit porosity.

 When a long bar or a plate is cast without a feeder, a certain length of the casting
from each end of the bar/plat is sound.
this results from the directional solidification that developed at the ends because of faster
coolingrate.
This is called the end effect.
 When a long bar/plate is cast using a feeder at the centre of the casting,
a certain distance from the feeder (in any direction) the bar/plat is sound.
This is called the feeder effect.
 The use of chill provides a powerful influence in extending the feeding range
of heads when placed at intermediate positions of feeders.
the spacingbetweenfeederheadscan in this casebe more than doubled,
which also greatly increasesthe castingyield.

Fig. 6.9 (b): Feeding distances for steel plates
cast ingreensand.
Fig. 3.26: Feeding distance relationships for bars
FE EE
>>>>>>>>
Fd = D + 2FE +2EE

Summary:
Feeder contribution:
Edge contribution:
Chillcontribution:
Ld = 2.0 T
Ld = 2.5T
Ld = 50mm
Appliesfor heavysectionsonly
rangingfrom 50 – 200 mm
Fig. 3.27: Feeding distance relationships
for dual and multiple sections
Work of Bishop et. al, (1969):
When plates joined at their edges to form
steppedmembers(wherethe conjunction
of two plates differing in thickness by a
factor exceeding1.4),
the feeding distance in the thinner or
‘parasite’ plate is increased, when
compared withthat when the latter is cast
alone.

5. Increasing Efficiency of Feeder
 The efficiency of a feeder head may be defined as the amount of feed metal supplied
to the casting in relation to the total weight of metal initially present in the head.
Efficiency, U = x100
I – F
I
I = initial volume of metal in head
F = final volume of metal in head
 The efficiency of a plain feeder head is very low, since solidification proceeds in the
head at the same time as in the casting.

 A feeder can be made more efficient by
some artificial means to keep the top
molten so that the liquid beneath can be
exposed to atmospheric pressure.
 Theseinclude:
• use of insulatingmaterials around the
feederhead
• use of exothermic compounds
use of an insulating compound or exothermic mixture in the
feeder reduces the piping tendency and decrease the amount
of metal required in the feeder
 The net effect of using these artificial means is to reduce the size of feeder.
 This also means a higher casting yield.

The design of feeding system feeding calculations

Insulation
 The use of insulating materials to retard radiation losses from the exposed metal
surface is a long established practice.
 More thorough insulation is now frequently sought by lining the heads with
moulded sleeves to reduce conductive heat loss through the mould.
 The materials used derive their low heat diffusivity from porous or granular structures.
• Top coverings materials include: dry sand, powdered slag, chopped straw (which first char and
then burn away to leave a bulky ash), proprietary anti-piping compounds (contain a certain amount
of exothermic material; those used of steel contains carbonaceous matter which becomes
absorbed by the metal and locally reduces its freezing temperature)
• pre-formed sleeves for lining materials include: foamed gypsum plaster (to develop high porosity)
(for non-ferrous castings), diatomaceoussilica and vermiculite (for steel castings).

Exothermicmaterials
 Considerable heat is generated by exothermic reaction; in some cases molten metal
is also produced.
 Common materials used: thermit mixture (fine mixture of aluminium and iron oxide),
powdered charcoal or graphite, rice or oat husk, and refractory powder.
Thermitreactions:
2Al + Fe2O3 = Al2O3 +2Fe;
8Al + 3Fe3O4 = 4Al2O3 +9Fe;
H298 = -853 kJ
H298 = -3347 kJ
 Added in twoways:
1. Material is added on top of feeder to control feeding. Composition must be compatible with
that of the casting.
2. Materialis mixed with bondingmaterial and water to mould sleevefor lining feederhead.
The added substance helps delaying exothermic reacting and extend the period during which
heat is generated. This diminishes the danger of contamination, since the exothermic reaction
is confinedto the mould wall. The period during which heat is generated iscontrolled.

NextClass
MSE 3231, Lecture11
The Design of FeedingSystem
3. Case study in design of a feeding system

The design of feeding system feeding calculations

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