Lecture 13 and Lecture 14 Manufacturing Technology [Compatibility Mode]

8/11/2019 Lecture 13 and Lecture 14 Manufacturing Technology [Compatibility Mode]

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Design of Casting

Lecture 13 and lecture 14


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Pouring temperature refers to the initial

temperature of the molten metal used for the

casting as it is poured into the mold. This

temperature will obviously be higher than the

solidification temperature of the metal. Thedifference between the solidification

temperature and the pouring temperature of the

metal is called the superheat.

Volumetric rate in which the liquid metal is introduced into the mold. Pouring

rate needs to be carefully controlled during the metal casting operation, since it

has certain effects on the manufacture of the part. If the pouring rate is too fast,

then turbulence can result. If it is too slow, the metal may begin to solidify before

filling the mold.

Turbulence is inconsistent and irregular variations in the speed and direction of

flow throughout the liquid metal as it travels though the casting. The random

impacts caused by turbulence, amplified by the high density of liquid metal, can

cause mold erosion. An undesirable effect in the manufacturing process ofmetal casting, mold erosion is the wearing away of the internal surface of the

mold. It is particularly detrimental if it occurs in the main cavity, since this will

change the shape of the casting itself. Turbulence is also bad because it can

increase the formation of metal oxides which may become entrapped, creating

porosity in the solid casting.


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Pouring is a key element in the manufacturing process of metal casting and the main

goal of pouring is to get metal to flow into all regions of the mold before solidifying. The

properties of the melt in a casting process are very important. The ability of a particular

casting melt to flow into a mold before freezing is crucial in the consideration of metal

casting techniques. This ability is termed the liquid metals fluidity.

In manufacturing practice, the relative fluidity of a certain metal casting melt can be

quantified by the use of a spiral mold. The geometry of the spiral mold acts to limit the

flow of liquid metal through the length of its spiral cavity. The more fluidity possessed by

the molten metal, the farther into the spiral it will be able to flow before hardening. The

maximum point the metal reaches upon the casting's solidification may be indexed as

that melts relative fluidity.

Figure Spiral Mold Test


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Shrinkages during solidification and cooling

Most materialsare less dense in their liquidstate than in their solid state,

and more dense at lower

temperatures in general. Due

to this nature, a metal casting

undergoing solidification will

tend to decrease in volume.

Gray cast iron

expands uponsolidification due to phase

changes


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Most materials are less dense in their liquid state than in their solid state, and

more dense at lower temperatures in general. Due to this nature, a metal casting

undergoing solidification will tend to decrease in volume. During the manufactureof a part by casting this decrease in volume is termed shrinkage. Shrinkage of

the casting metal occurs in three stages:

Figure:10

1. Decreased volume of the material as it goes down from Pouring Temperature to

freezing temperature .

Shrinkages during solidification and cooling


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2. Decreased volume of the material due to solidification.

3. Decreased volume of the material as it goes from freezing temperature to room

temperature.


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When designing a setup for manufacturing a part by metal casting, risers are almost

always employed. As the metal casting begins to experience shrinkage, the mold will

need additional material to compensate for the decrease in volume. This can be

accomplished by the employment of risers. Risers are an important component in the

casting's gating system. Risers, (sometimes called feeders), serve to contain additional

molten metal. During the metal's solidification process, these reservoirs feed extramaterial into the casting as shrinkage is occurring. Thus, supplying it with an adequate

amount of liquid metal. A successful riser will remain molten until after the metal casting

solidifies. In order to reduce premature solidification of sections within the riser, in many

manufacturing operations, the tops of open risers may be covered with an insulating

compound, (such as a refractory ceramic), or an exothermic mixture.


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Porosity:

One of the biggest problems caused by shrinkage, during the manufacture of a cast

part, is porosity. It happens at different sites within the material, when liquid metal can

not reach sections of the metal casting where solidification is occurring. As the isolated

liquid metal shrinks, a porous or vacant region develops.

Development of these regions can be prevented during the manufacturing operation,

by strategically planning the flow of the liquid metal into the casting through good

mold design, and by the employment ofdirectional solidification.


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Consider V/A Ratios:

In casting manufacture, V/A ratio stands for volume to surface area or

mathematically (volume/surface area). When solidification of a casting begins a thin

skin of solid metal is first formed on the surface between the casting and the mold

wall. As solidification continues the thickness of this skin increases towards thecenter of the liquid mass. Sections in the casting with low volume to surface area

will solidify faster than sections with higher volume to surface area. When

manufacturing a part by metal casting consideration of the of V/A ratios is critical in

avoiding premature solidification of the casting and the formation of vacancies.


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Heat Masses:

Avoid large heat masses in locations distant to risers. Instead, locating sections of

the casting with low V/A ratios further away from the risers will insure a smooth

solidification of the casting.

Sections of the Casting:

The flow of material is very important to the manufacturing process. Do not feed

a heavy section through a lighter one.


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Prevent Planes of Weakness:

When metal castings solidify, columnar grain structures tend to develop, in the

material, pointing towards the center. Due to this nature, sharp corners in the

casting may develop a plane of weakness. By rounding the edges of sharp

corners this can be prevented.


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Reduce Turbulence:

When manufacturing a metal casting, turbulence is always a factor in our flow of molten

metal. Turbulence, as covered earlier in the pouring section, is bad because it can trap

gases in the casting material and cause mold erosion. Although not altogether

preventable in the manufacturing process, turbulence can be reduced by the design of agating system that promotes a more laminar flow of the liquid metal. Sharp corners and

abrupt changes in sections within the metal casting can be a leading cause of

turbulence. Their affect can be mitigated by the employment of radii.


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Connection Between Riser

and Casting Must Stay Open:

Riser design is very important in

metal casting manufacture. If

the passage linking the riser to

the metal casting solidifies

before the casting, the flow of

molten metal to the casting will

be blocked and the riser will

cease to serve its function. If

the connection has a larger

cross sectional area it will

decrease its time to freeze.

Good manufacturing design,

however, dictates that that we

minimize this cross section as

much as possible to reduce the

waste of material in the casting

process. By making thepassageway short we can keep

the metal in its liquid state

longer since it will be receiving

more heat transfer from both

the riser and the casting.


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Ingate Design:

The ingate is another aspect of manufacturing design that relates to the flow of metal

through the casting's system. The ingate, (Figure ) is basically where the casting

material enters the actual mold cavity. It is a crucial element, and all other factors of the

metal casting's mold design are dependent on it. In the location next to the sprue base

the cross sectional area of the ingate is reduced (choke area). The cross sectionalreduction must be carefully calculated. The flow rate of casting material into the mold

can be controlled accurately in this way. The flow rate of the casting metal must be high

enough to avoid any premature solidification. However, you want to be certain that the

flow of molten material into the mold does not exceed the rate of delivery into the

pouring basin and thus ensure that the casting's gating system stays full of metal

throughout the manufacturing process.


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Use of Chills:As mentioned earlier directional solidification is very important to the manufacture of a part during the

metal casting process, in order to ensure that no area of the casting is cut off from the flow of liquidmaterial before it solidifies. To achieve directional solidification within the metal casting, it is important to

control the flow of fluid material and the solidification rate of the different areas of the metal casting. Withrespect to the solidification of the metal casting's different sections, regulation of thermal gradients is

the key. Sometimes we may have an area of the metal casting that will need to solidify at a faster rate inorder to ensure that directional solidification occurs properly. Manufacture planning, and design of flowand section locations within the mold may not be sufficient. To accelerate the solidification of a section like

this in our casting, we may employ the use of chills. Chills act as heat sinks, increasing the cooling rate inthe vicinity where they are placed. Chills are solid geometric shapes of material, manufactured for this

purpose. They are placed inside the mold cavity before pouring. Chills are of two basic types. Internalchills are located inside the mold cavity and are usually made of the same material as the casting. Whenthe metal solidifies the internal chills are fused into the metal casting itself. External chills are located just

outside of the casting. External chills are made of a material that can remove heat from the metal castingfaster than the surrounding mold material. Possible materials for external chills include iron, copper, and

graphite. Figure demonstrates the use of the two types of chills to solve the hot spot problem in a + and Tjunction.


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Prediction of Solidif ication Time: Chvorinov's

Rule.

Theamount of heat that must be removed from a casting to cause it tosolidify is directly proportional to the amount of superheating and the

amount of metal in the casting, or the casting volume. Conversely, the

ability to remove heat from a casting is directly related to the amount of

exposed surface area through which the heat can be extracted and the

insulating value of the mould. These observations are reflected in

Chvorinov's rule, which states that ts

, the total solidification t ime, can be

computed by:

ts = B (V/A)n where n = 1.5 to 2.0

The total solidification time is the time from pouring to the completion of

solidification; V is the volume of the casting;A is the surface area; and B is

the mould constant, which depends on the characteristics of the metal

being cast (its density, heat capacity, and heat of fusion), the mouldmaterial (its density, thermal conductivity, and heat capacity), the mould

thickness, and the amount of superheat.


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The minimum size of a riser can be calculated from Chvorinov's rule by

setting the total solidification time for the riser to be greater than the total

solidification time for the casting. Since both will receive the same metal and

are in the same mould, the mould constant, B, will be the same for both

regions. Assuming n = 2, and a safe difference in solidification time of 25% (theriser takes 25% longer to solidify than the casting), we can write this condition

as:

triser= 1.25 tcasting ( V/ A)2riser= 1.25 (V/ A)

2casting

Calculation of the riser size then requires the selection of a riser geometry

(generally cylindrical) and specification of a height-to-diameter ratio, so that the

riser side of the equation will have only one unknown. For a cylinder of diameter

D and height H:

V = D2H / 4

A = DH + 2 ( D2 / 4)

Specifying the riser height as a function of the diameter enables the V/ A ratio to

be written as a simple expression with one unknown, namely, D. The VIA ratio

for the casting can be calculated from its particular geometry.


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CALCULATION OF RISER SIZE

The riser must satisfy both of the following two requirements:

Heat transfer criteria

MC: MN: MR= 1 : 1.1 : 1.2 (1)

where MR, MCand MNare modulus of riser, modulus of casting, and modulus of the

neck of riser at the junction of casting respectively.

Feed volume criterion

VR VC/ ( - ) (2)where is the efficiency of the feeder, is the solidification shrinkage, and VRand

VCare volume of Riser and casting respectively.

Metal utilisation of risers of various forms moulded in sand. The (a) cylindr ical and (b) hemospherical

heads have been treated with normal feeding compounds; (c) the efficiency of the reverse tapered

heads depends on detailed geometry; (d) shows an exothermic sleeve.


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In normal conditions, there is a

limit to how far a Riser can feed

along a flow path. Up to this

distance from riser, the casting

will be sound. Beyond this

distance the casting will exhibit

porosity.

FEEDING DISTANCE


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Mould Filling PhenomenonThe fluidity as defined by the foundry community is different that defined by

physicists (as the reciprocal of viscosity). The casting fluidity is driven by

metallostatic pressure and hindered by: viscosity and surface tension of moltenmetal, heat diffusivity of mould, back pressure of air in mould cavity and friction

between the metal-mould pair.

Metallostatic head:The metallostatic pressure is given by g h where is themetaldensity and h is the height of liquid metal column above the filling point. A higher

metallostatic pressure gives higher velocity of molten metal, and thereby higher fluidity.

Viscosity: Viscosity depends on the metal family, composition and the instantaneous

temperature. For most metals, the viscosity at the pouring temperature is close to that

of water (1 centistoke); aluminum: 1.2 and iron: 0.9 centistokes. In comparison, the

viscosity of typical mineral oils is about 600.

Surface tension:For a flat plate of thickness t, the relation between head, thickness

andsurface tension is given by: g h = / t, where is the surfacetension. At the pouringtemperature, the surface tension of aluminum and iron is 0.5

and 0.9 N/m respectively; similar to mercury at room temperature (0.46 N/m), but

higher than water (0.07 N/m).


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Heat diffusivity: Moulds with high heat diffusivity transfer heat faster from the molten

metal, causing it to freeze earlier and stop flowing. It is given by (Km m Cm),where Km is thermal conductivity, m is density and Cm is specific heat of the mouldmaterial.

Back Pressure:As molten metal advances in the mould, the back pressure of air that is

being compressed in the cavity ahead effectively reduces the metallostatic pressure,

and thus hinders filling. The back pressure depends on the cavity volume, mould

permeability and the velocity of the advancing front. Venting helps.

Friction: The rough surface of sand mould hinders metal flow. Thus mould coating

(usually water based, containing silica flour and graphite) reduces the friction between

the metal and mould, contributing to higher fluidity. In general, fluidity of pure metals is

higher than alloys.


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Turbulence implies irregular, fluctuating flow w ith disturbances.

It is observed when: (1) inertia forces (which make the fluid continue in the same

direction), are much higher than the drag forces (which tend to stop the fluid motion),

and (2) there are obstructions in the path of flow, such as a sharp corner or a changeof section thickness. The drag forces include those caused by viscosity and surface

tension. The viscous forces mainly operate in the bulk of the liquid metal, whereas

surface tension forces operate near the mould wall. Thus we have two types of

turbulence: bulk and surface.

Bulk turbulenceis quantified by Reynolds numberRe, which is the ratio of inertia to

viscous pressure in a fluid. It is given by V d /where is the density,is theviscosityand V is the velocity of the liquid; d is a characteristic dimension of the flow

path. If Re ismore than 20000, then the flow is usually turbulent.

Surfaceturbulence is quantified by the Weber numberWe, which is the ratio of inertia

to surface tension pressure in a fluid. It is given by V2

r / where r is theradius ofcurvature of the free liquid surface. ForWe is less than 1, surface turbulenceis absent.When it is 100 or more, surface turbulence is prominent, leading to violent

mixing of surface layers with the bulk of the molten metal.


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Gating System and TypesThe main objective of a gating system is to lead clean molten metal poured from ladle to

the casting cavity, ensuring smooth, uniform and complete filling. Clean metal implies

preventing the entry of slag and inclusions into the mould cavity, and minimizing surfaceturbulence. Smooth filling implies minimizing bulk turbulence. Uniform filling implies that

all portions of the casting fill in a controlled manner, usually at the same time. Complete

filling implies leading molten metal to thin and end sections with minimum resistance.

Major elements of a gating system


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Optimal Filling TimeThe higher limit of filling time (slowest filling) is governed by the need to avoid

premature freezing in thin sections before complete filling. The lower limit of the filling

time (fastest filling) is governed by the onset of surface turbulence. The correct fillingtime lies somewhere in between, and is a function of cast metal, weight, minimum

section thickness and pouring temperature.

Several empirical equations for determining the correct filling time for major metals

have been developed by casting researchers, based on experimental investigations.

The filling time f is expressed as a function of casting weight W in kg, section

thickness t in mmand fluidity length Lf in mm. A generalized equation forfilling time can be written as:

f= K0 (KfLf / 1000 ) ( Ks + Kt t / 20 ) ( Kw W )PThere are five coefficients: K0 is an overall coefficient, and Kf, Ks , Kt , Kware thecoefficients for fluidity, size, thickness and weight, respectively. For

grey iron the following values may be used: K0 = 1.0, Kf = 1.0, Ks = 1.1 (for

castings of size 100-1000mm), Kt = 1.4 (for wall thickness up to 10 mm),

Kw = 1 and P = 0.4. Based on individual experience, an expert casting

engineer can set the values of the coefficients for each metal-process

combination. These form a valuable part of the knowledge base of a

foundry specializing in specific castings.


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Gating Ratio:

It is given byAs:Ar:Ag where As , Ar , Ag are the cross-

sectional areas of sprue exit, runner(s) and ingate(s). Ifmultiple runners and ingates are present, the total area (of all

runners, or all ingates, respectively) must be considered. A

converging diverging system, where the ingate area is more

than the sprue exit area, is to be preferred. This ensures that

the metal slows down (thereby reducing turbulence-relatedproblems). Examples of such gating ratios include: 1:2:1.5 for

ferrous and 1:4:4 for nonferrous metals. Higher values of

ingate area may be used (such as 1:4:8) to further reduce the

velocity of molten metal through the ingates to within the

recommended range, as long as flow separation (and thereby

air aspiration) is avoided.


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Gating Element DesignThe gating system can be designed to fill a given casting in a predetermined time, by

keeping a constant level of liquid metal in the pouring basin during pouring, to achieve

a controlled rate of flow through the choke. The choke is the smallest cross-section in

thegating system that controls the flow rate of molten metal. The element (sprue exit,runners or ingates) with the smallest value in the gating ratio is considered the choke.

The choke areaAc is given by: Ac = W / (cfVc )Where, W is the total casting weight (including risers and gating channels), c is themetal density, f is the total filling time and Vc is the choke velocity.

The choke velocity is given by: Vc = Vp + cf (2 g H)where H is the metallostatic pressure head, given by the vertical distance between theliquid level in pouring cup and the centerline of the choke. The value of pouring velocity

Vp is non-zero, if poured from a height or if bottom pouring ladles are used. The friction

factor cf within the gating system depends on its geometry and surface finish, and

rangesbetween 0.6-0.9.

During actual filling, the metallostatic pressure head gradually decreases after themolten metal starts rising above the level of choke. Thus the average value of actual

choke velocity is less than the one used above, leading to slower filling. This can be

compensated by estimating the actual filling time and then correcting the choke area.


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The cross-sectional area of sprue exit, runners and ingates, is initially determined

based on the choke area, gating ratio and the number of individual elements. Then

the sectional area of individual elements, as well as their shape and dimensions are

determined as follows:

Sprue:It usually has a circular cross-section, which minimizes turbulence and heat

loss. The cross-sectional area at the sprue exit or bottom is calculated from the

choke area and gating ratio. The area of the sprue top should be calculated using

mass and energy balance equations, to prevent flow separation in

the sprue.

Essentially, A1H

1= A

2H

2Where, H1 and H2 are the metallostatic pressure head at the top andbottom of the sprue, respectively;A1 and A2 being the respective

cross-sectional areas. The ideal sprue must be larger at the top and

smaller at the bottom. Since this leads to an undercut, such a sprue

can not be created by the pattern during moulding operations, and

must be formed by a core. If this is not economical, then the choke

can be created in the beginning of runner.


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Sprue well:It arrests the free fall of molten metal through the sprue and turns it by a right angle

towards the runner. It must be designed to minimize turbulence and air aspiration. The

recommended shape of a sprue well is cylindrical, with diameter twice that of sprue exit and depth

twice that of runner. A fillet between the well and runner will facilitate smooth transfer of molten

metal.

Runner:The main function of the runner is to slow down the molten metal, which speeds up duringits free fall through the sprue, and take it to all the ingates. This implies that the total cross-sectional

area of runner(s) must be greater than the sprue exit. In general, a ratio of 1:2 is recommended. A

much higher ratio (such as 1:4) may lead to flow separation in the runner. The second implication is

that the runner must fill completely before letting the molten metal enter the ingates. Finally, in

casting where more than one ingate is present, the runner cross-section area must be reducedafter each ingate connection (by an amount equal to the area of that ingate), to ensure uniform flow.

Ingate:The ingate leads the molten metal from the gating system to the mould cavity. A number of

conflicting requirements apply to the design of ingates, as listed below.

1. Ingate section must be designed to reduce the metal velocity below the critical limit. This implies

that in general, the ingate area must be more than the sprue exit (choke).

2. Ingate must be easy to fettle. This implies a smaller cross-section, preferably a flat section

(against a square one), is preferred.3. Ingate must not lead to a local hot spot. This implies that the ingate modulus (ratio of volume to

cooling surface area) must be smaller than that of the connected section.4. Flow of molten metal through an ingate (and therefore its cross-sectional area) must be

proportional to the volume of the connected casting region.

The number, shape (aspect ratio) and dimensions of ingates must be carefully designed to optimize

the above requirements.

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Lecture 13 and Lecture 14 Manufacturing Technology [Compatibility Mode]

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