Call Girls Delhi {Jodhpur} 9711199012 high profile service
Castability
1. 1
Ain Shams University
Faculty of Engineering
Mechanical Department
2nd
Year – production
Castability
Submitted to:
Eng. AHMED WAGDY
Prepared by:
EBTESAM ABD-ELAZIZ ALI
OMNIA AHMED IBRAHIM
OMNIA SAEED EL-BADRY
AMIRA ABD EL-WANEES IBRAHIM
2. 2
Contents
Factors affecting castability: .................................................................................. 4
I. Metal properties:............................................................................................. 4
1. Viscosity........................................................................................................ 4
2. Surface tension............................................................................................. 5
3. Inclusions...................................................................................................... 5
4. Freezing zone................................................................................................ 6
5. Melting Point................................................................................................ 6
6. Slag formation .............................................................................................. 7
Fluidity test......................................................................................................... 7
II. Characteristics of Casting processes: ............................................................... 8
............................................................................................................................... 8
1. Pouring temperature.................................................................................... 8
2. Degree of super-heat.................................................................................... 9
3. Pouring rate.................................................................................................. 9
4. Heat transfer .............................................................................................. 10
5. Solidification time....................................................................................... 10
6. Mold material............................................................................................. 11
7. Mold design................................................................................................ 11
Casting properties of some metals:. ................................................................ 11
i. Ferrous alloys .......................................................................................... 11
ii. Non-ferrous alloys ................................................................................... 12
III. geometric consideration of mold design : .................................................. 12
1. mold geometry........................................................................................... 12
2.Gating system................................................................................................ 12
Geometric consideration :- ............................................................................ 13
3. 3
Gating system ( cup ,sprue ,gate and runner): - ............................................ 15
Cup design :-.................................................................................................. 15
Sprue design.................................................................................................. 16
Sprue well design........................................................................................... 16
Runners ......................................................................................................... 17
Gates design ( ingates ):-................................................................................... 19
3. Feeding system:.......................................................................................... 21
Feeding system function:............................................................................... 21
Factors affecting riser design:........................................................................ 22
Types of riser: ................................................................................................ 22
References:.......................................................................................................... 23
4. 4
Castability:
• Castability is the capability of the molten metal to flow filling the mould
cavity in a casting process without sudden, early solidification.
• Castability is concerned with the production of a casting with least cost,
defects & time.
• For a metal with higher castability, it’s easier to get, even a thin groove,
filled with molten metal in less time, without great casting defects such as
misruns or incomplete filling of the mould cavity.
• The castability of a metal depends on some of its properties, some
characteristics of the casting process itself, and geometry & design of the
mould and cavity.
Factors affecting castability:
I. Material properties
II. Casting Processes
III. geometric considerations:
1. mold geometry
2. gating system
3. feeding system
I. Metal properties:
1. Viscosity
It’s the shear forces between the liquid layers resisting its
flowing. For a viscous fluid, the layers are stuck to each Figure 1.viscosity effect
5. 5
other so that it has little flowability as layers can’t slip over each other to continue
moving.
Since viscosity is inversely proportional to temperature, heating the metal to
higher temperatures improves castability.
2. Surface tension
It’s caused by attraction between the particles at the surface & the liquid bulk.
Surface tension of liquid metals appears mostly in thin narrow cavities. It’s like the
capillary repulsion action. Since the mould is designed not to be wet by the
molten metal as the attraction among the particles of the liquid metal is more
than the attraction between the metal and the mould material. Also, it is
noticeable that formation of a very fine layer of oxide metal /molten increases the
surface tension. Surface tension
has adverse effects on castability.
3. Inclusions
Impurities decrease the fluidity of the molten metal. For insoluble granules of
impurities, the case is like water carrying sand
grains resisting the flow. This can also appear
in alloys. Insoluble granules increase the
viscosity of the liquid metal
& consequently decrease its
fluidity. Alloying a pure
metal with even a slight
amount of alloying elements
Figure 2.surface tension effect
Figure 4Figure 3
6. 6
reduces the fluidity by a factor of 2 to 16.
4. Freezing zone
It’s the range of temperature
where the solidification of liquid
metal starts & ends. Castability
decreases as the Freezing zone
increases. For pure metals &
eutectic alloys the freezing zone is
very short (nearly zero) and they
have the best castability.
5. Melting Point
Melting temperature is a property of the metal. Low melting points are desirable
in casting so that less energy can be consumed to melt the metal. For high melting
points, the mold must be more refractory
& so more expensive. Low meting points
are also essential for the durability of the
mould.
The higher melting point is, the higher the
pouring point is & the longer the cast takes
to solidify
This affects castability 2nd
definition “the
least cost, least time & the least defects”
Figure 5
Figure 6.diagram showing different melting temp of different material
7. 7
6. Slag formation
It’s a defect that can’t be avoided. Slags are formed by all metals because of
oxides & some other gas impurities. It’s,
also, independent on the type of casting
process or on the mold type or shape. The
amount of slag differs only according to
the material itself.
There are two types of slags:
1. High viscosity slags: this type forms
during solidification.
2. Low viscosity slags: form after
solidification. They rise much faster
to the surface.
Even though Slag
defects can be
detected by naked
eye we use
electron
microscope to be
specified & sure.
Fluidity test:
Although there are many tests to quantify fluidity, none of them is universally
accepted. They can’t give accurate results and they still depend on the casting
Figure 7.high viscosity slag and low viscosity slag
8. 8
parameters. These tests are used to have an approximate indication of the metal
behaviour without direct relation with the application of real casting.
One of the most popular tests is the spiral mould test. The molten metal is made
to flow along a channel at room temperature. The distance of metal flow before it
solidifies and stops is a measure of its fluidity. Obviously this length is a function
of the thermal properties of the metal and the mould, as well as the design of the
channel. Such tests are useful and simulate casting situations to a reasonable
degree.
II. Characteristics of casting processes:
1. Pouring temperature
The molten metal must be super-heated above its melting point. This
temperature is restricted by the properties of the mold such as its thermal
conductivity & design of the cavity. For more fluidity, more super-heat is required.
Figure 8.f
Figure 9.schematic pic showing effect of
increasing temp
9. 9
2. Degree of super-heat
The temperature difference between
the pouring & the melting
temperatures is called 'super-heat’.
Increasing the superheat & the
temperature of the flowing metal
increases the fluidity of the molten &
increases its ability to fill narrow
cavities as it delays the solidification.
However, increasing the super heat
increases the probability of formation
of saturate gases and the formation of oxides. It, also, increases the melts ability
to penetrate into the surface of the mold.
3. Pouring rate
The rate of introducing the melt to the mold is very important. It needs to be
carefully controlled. If it was too fast, it might cause turbulence which, in turn,
can cause mold erosion and increase formation of oxides causing porosity in the
casting. On the other hand, if the rate was too slow, the molten metal might
solidify before completely filling the mold creating defective casting.
Figure 10.effect of shrinkage in the volume
10. 10
4. Heat transfer
Considering the heat transfer is important in casting process from pouring to
solidification then to cooling to room temperature. It depends on the casting
material and the mold and the process parameters.
• Decreasing the volume of the liquid as it cools before solidification
• Decreasing the vol. due to solidification
• Decreasing the volume due to cooling the solid to room temp.
5. Solidification time
Solidification starts with a thin layer
beside the walls, then it thickens. The
core is the last region to solidify. The
solidification rate depends on the
volume of the casting and its surface
area. This relation is determined by
Chvorinov's rule:
Solidification time = C (volume/surface area) 2
Figure 11.effect of solidification on
volume
Figure 12.steps of solidification
11. 11
Where ‘C’ is a constant that depends on the material & thickness of the mould,
the metal properties and the temperature.
6. Mold material
The material of the mold should have low thermal conductivity to keep the
molten metal hot, so flowing. Higher thermal conductivities lower the castability.
7. Mold design
The mold design and dimensions have great influence on the castability,
beginning from the dimensions of the cavity, & ending with the design and
dimensions of the sprue, runner & riser of the gating system
Casting properties of some metals:
i. Ferrous alloys
• Cast iron alloys
The most important metal in casting. It can form different compositions as grey,
white, nodular, malleable cast iron. This variety can be made by varying the
solidification & cooling rate.
• Cast steel
It has high pouring temperature (~1650°C) so it has limitations on the mold
material. It rapidly oxidizes. It has poor fluidity. Its alloys have long freezing zone
so they have bad castability.
12. 12
ii. Non-ferrous alloys
• Aluminum alloys
They have good castability with low pouring temperature (Tm~660°C). They have
good thermal conductivity and light weight.
• Zinc alloys
They are highly castable with low pouring temperature (Tm~420°C). They have
good fluidity.
III. Geometric consideration of mold design:
1. Mold geometry
2. Gating system
3. Feeding system
As castability refer to how ease the molten metal flow and fill all cavities and thin
sections without misruns (and this depend on molten metal properties that we
talked about in the previous topic), it also refers generally to how ease we can
cast a product without defects and with high quality, low cost and time, and this
depends on casting design or properties of casting process.
If we talk about casting properties mould design comes first, as it is very
important factor affects castability and the quality of the product. In mould design
we should avoid many complexities which will cause defects in product and
decrease castability.
One of these complexities is geometric consideration for simplifying.
13. 13
Geometric consideration:-
1.We should avoid difference in walls thicknesses during pouring and
solidification of molten metal as thinner walls solidify more rapidly than thicker
wall which in turn create large grains in in thicker walls and small ( fine grains ) in
thin walls , so grains in between two different structures will see different
arrangements and this will cause residual stresses in these grains , and as we
know residual stresses are considered as a product defect because it is a source
of crack propagation .
Another product defect can appear due to difference in thicknesses is a cavities or
voids which appear in thick walls as thick walls act as feeding system that feed
excess metal which needed for compensating shrinkage in thin walls which
solidify first and cause cavities in hot thick spot .
We can solve this problem by making uniform sections as possible as we can, if
we cannot make all sections to be uniform, we can improve gating system design
so all thick sections compensate its shrinkage and feeding from risers.
2. The second complexity we should avoid is right
angles and sharp corners lead to:
A.sharp corners represent place of stress
concentrations and source of crack propagation.
B. sharp corners cause turbulence during filling
process which in turn cause trapping of gases and
oxides in the molten metal in addition that
Figure 13.effect of changing thickness
14. 14
turbulence cause the molten metal to penetrate the mould (mould erosion)
C. sharp corners sometimes cause hot spots.
We can avoid this problem by decreasing number of fillets and
right angels in the pattern and sharp corners should be replaced
by round corners and junctions.
3.The third problem is that the inner surfaces of hollow casting
cool slower than the outer surfaces and this variation in cooling
rate will cause variation in microstructure and material strength
(slow cooling leads to coarse grain when rapid cooling leads to fine grains).
We can overcome that by one of two following methods:-
a. Using internal padding or chills to be stick to the inner surface of the
casting to increase heat extraction from it
b. Increasing inner areas relative to the external area to ensure directional
cooling rather than progressive cooling .if we cannot make any of these
solutions it is better to produce solid part and make inner surfaces by
machining.
Figure 15 directional solidification by using chills
Figure 14.turbulance due to
sharp corners and right angel
15. 15
4. Another problem that decreases castability is sudden change in section
thicknesses which always results in sharp edges and variation in cooling rate and
cause hot spots effect, stress concentration and turbulence.
To avoid that we can make fillets between different areas to ensure gradual
change in area.
Factor affect castability is design of gating system and feeding system.
Gating system(cup, sprue, gate and runner): -
The main objective of gating system is to deliver molten metal to mould cavity but
its design controls and affects castability and quality of the product as we aim to
ensure filling all cavities rapidly before molten metal solidification ,eliminate
turbulent flow and eddies to avoid mould erosion , remove molten metal slags
and inclusions as they reduce metal fluidity , control flow velocity and pressure in
different stages during filling and another goal should be taken in consideration
that all gating system is considered as a scrap (not a part of a product) so we
should minimizing its weight as possible as we can to increase casting yield .
Cup design:-
In design of cup it should be large with wide surface area to capture more and
more slags that float up to the cup surface and prevent it to enter mould cavity ,
also its large volume prevent overflowing during pouring rapidly so it make it
easy for pouring from ladle ,in small castings pouring cup can be dug into cope
itself ,but in huge castings it’s difficult to cut a very big basin in cope and it will be
a time – consuming process ,so we can prepare an external basin by dry sand (hot
box or cold box) then it can be welded on the upper surface of the cope .
16. 16
The basin has a concave shape to collect the poured metal and decreasing its
velocity then it will redirect the metal to flow up against gravity to decrease its
energy before flowing down again to the mold.
Sprue design
Sprue is the following passageway after cup, it has a tapered shape to increase
the velocity of the molten metal which in turn reduces the pressure of the metal
to prevent mould erosion according to high pressure flow, and we also need rapid
filling of metal to ensure molten state of metal until complete filling of cavity.
as molten metal fall down from cup through sprue its kinetic energy increase by
nature so the molten metal cross section decrease gradually as a result so if we
use straight cylindrical sprue a space between sprue wall and molten flow will
exist and it will be easy chance for air and hot gases to be trapped in this space, so
taper shape also help to overcome this space and prevent gases and air.
Sprue well design
Sprue well or well is the third part after sprue, it considered as a relatively large
reservoir that collect molten metal from sprue and redirect it to the runners in
Figure 16.pouring basin with concave welL
17. 17
different directions - dependant on number of runners - after decreasing its
velocity or momentum before entering the mould. We should avoid any sharp
corners in design of gate to avoid turbulence and capturing oxides to the cavity.
Sprue well always be under parting line (in drag) so the molten metal will need to
flow up against gravity to get into runners and this will decrease molten metal
energies to avoid erosion of mould .
Runners
Runners are the following passageways after sprue well , as the sprue located
vertically to ensure rapid filling by increasing velocity , runners located
horizontally to avoid increase in velocity before entering mould to avoid
erosion , we design runners to be located in cope to force molten metal to
flow up from sprue well (at drag) to runners (at cope) against gravity .
Figure 17.types of runners
18. 18
As we know removing slags
and preventing it from
entering mould cavity
increases castability as it
gives high quality product
without defects so we
should ensure that the
runner is full -filled to
compact slags against runner ceiling (become stick to ceiling ) and preventing
it to enter mould . runner cross section may be circular
,trapezoidal or rectangular , the best of all is the circular one as circular shape
has no corners that make large amount of
molten metal to stay on it and freeze so
large amount of molten metal are lost as
a scrap in runners and which decrease
casting yield ,circular shape help to save
flowing metal at molten state as possible
and better than other shapes , but we
need to cut the upper half of circle in
cope and the lower half in drag so we will
need extra care to ensure centring of two
parts and this will be difficult and high cost .
So, we use trapezoidal shape as it can be cut in one half only let’s say in cope
and we prefer trapezoidal than rectangular as it has wide ceiling so it permits
flow of large amount of metal and at the same time it has a small base so it
helps to avoid high friction to facilitate metal flow to cavity, so we should
design optimum runner geometry to increase castability.
Figure 18.different shapes of runners
19. 19
Gates design (ingrates):-
They are openings from which the molten metal fills mould cavity after getting
out from runners, it has different types top gate, bottom gate, parting gate and
step we should be accurate in our choice to increase castability.
In top gate the mould cavity is filled from its top point, it cause high velocity of
molten metal flow and mould erosion and may catch hot gases and air, but it has
best temperature gradient as the bottom point solidify before top point so we can
fill all mould and the bottom point will compensate for its shrinkage from top
point and we compensate corresponding shrinkage in top by pouring extra metal
in top.
In bottom gate , molten metal fills the cavity from bottom point ,it cause slow
filling of mould as the flow is against gravity so we avoid mould erosion ,but it is
difficult to be cut in mould ,and it produce bad temperature gradient as bottom
point may solidify before top point and obstruct metal from filling top mould in
addition that bottom layer may compensate for shrinkage from top layer which
cause voids in top and we will be not able to compensate it as the solidified layer
in bottom obstruct metal flow .
20. 20
In parting gate we fill the mould from point in the plan of parting line of mould it
can be a top gate ( if the whole cavity located in drag) , and it may represent
bottom gauge ( if the whole
cavity located in cope) .
The last kind is step gate , it is
used for very large product to ensure rapid and full filling of cavity as we fill from
many points on cavity (from bottom to top) .
In the bottom one we need relatively high pressure as the molten get into the
vacuum mould so we decrease its cross section, on the other hand the in top gate
we need lower pressure as the molten metal will enter semi-filled mould so we
increase its cross
Figure 19.main parts of gating system
Figure 20.types of gates
21. 21
3. Feeding system:
In sand casting it’s the riser
It is a passage for the molten to fill after the mold cavity is filled and solidifies
after the cast does
Risers are not required in casting materials with large freezing zone because
directional solidification is impossible, but multiple risers for varies sections may
be used.
Casting yield is greatly affected by the risers design so it must be carefully
designed
Feeding system function:
1. Provide the cast with extra molten during solidification to compensate the
shrinkage in order for the feeding system to perform its function it has to
be the latest part to solidify.
2. It also acts as vent through which gases and air escapes during filling the
mold.
3. It acts as indicator for the end of the filling process.
4. It helps in the directional solidification.
* Note:
Without the riser the shrinkage defect / cavities takes place at the hot spots
because they are molten for longer time than the rest of the cast ,so it acts as a
provider for the rest of the cast
22. 22
Factors affecting riser design:
1. The cast metal & its shrinkage ability.
2. The complexity, size & thickness of the cast.
*This will affect the size, shape & location of the riser
3. Pouring temperature.
4. Pouring rate.
Types of riser:
If the riser is filled with hot molten
before the mold cavity it is called liver
riser & if it’s filled with molten that has
already passed through the mold cavity
it’s called dead riser
Top riser = open riser is mostly
dead riser
Side riser=blind riser is mostly liver
riser
• Open riser:
Its top surface is exposed to the atmosphere
Sometimes insulating layer is applied at the riser’s surface to reduce solidification
rate
Example of This layer: refractory ceramic
Figure 21. insulating layer is applied at the riser’s surface to reduce
solidification rate
Figure 22.open riser
23. 23
Also heaters may be used to keep the metal in the riser
molten for longer time
• Blind riser:
It’s totally enclosed in the mold.
Blind risers are less preferred than the Open ones
because open risers are located at the top of the cast &
they don’t take much of space, they also provide shorter
feeding distance.
Figure 23.using heaters in casting process
Figure 24.blind riser
