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CRITERIA USED FOR SAND TESTING / PROPERTIES: Moisture content, green and dry sand permeabilities, compression, tension, transverse and shear strengths, deformation during compression tests, green and dry hardness, clay content, grain-size distribution, combustible content, pressure, volume of gases evolved, flowability, sintering point, resistance to spalling etc.

Continued…….

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SAND TESTING For maintaining consistent quality of moulding sand The sand or say blended sand is characterized by

Grain Size Grain Shape Surface smoothness Density Contaminants Moisture content Clay content Compactibility

Final Mould key properties are mould hardness, permeability and strength.

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GENERAL TEST PERFORMED TO JUDGE THE MOULDING AND CASTING CHARACTERISTIC ARE: Moisture Content Test Clay content Test Chemical Composition of sand Grain Shape and surface texture of sand Grain Size distribution of sand Specific Surface of sand grains. Water absorption capacity of sand Refractoriness of sand Strength Test Permeability Test Flowability Test Shatter Index Test Mould Harness Test.

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Grain fineness test: (Sieve Analysis Method)

A sample of dry silica sand 50g free from clay taken Place on topmost sieve bearing US series 6. A set of 11 screens bearing the standard mesh

number are 6,12,20,30,40,50,70,100,140,200,270 are mounted on mechanical shaker.

The series are place in order of fineness from top to bottom.

Shaking is done for 15 min. Weight of retained sand in each sieve is measured. The value of weight is multiply by 2 which gives %

weight. It will give the GFN number finally by simple

mathematics.

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Sand Sieve analysis Tester for GFN

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Moisture Content:

It is determined by special device that measures the electrical conductivity of small sample of compressed sand.

Direct method is to measure the weight loss by a 20g to 50g sample after it has been subjected to temperature of 100*C in oven for one hour time.

More speedily by instrument “ Speedy moisture teller”.When water and calcium carbide react , they form acetylene gas which can be measured and which can be directly proportional to moisture.

CaC2+2H2O = Ca(OH)2+C2H2

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Clay content:

It is determined by washing the clay from 50-g sample of moulding sand , using water that contains sodium hydroxide to make it alkaline .

Several cycles of washing and agitation required to fully remove the clay.

The remaining sand is dried and weighed to determine the amount of clay removed from sample.

Clay in moulding sand of 50g is defined as particles which when suspended in water, fail to settle at a rate of 1inch per min. Clay consist of particle less than 20micron , per 0.0008inch in diameter.

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Rapid Sand Washer

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Refractoriness Test:

It is found by heating the AFS specimen to very high temperature ranges depending upon the types.

Sand test piece are cooled at room temperature .

Examined under microscope for surface characteristics or by scratching it with steel needle.

Every time sand heated above 1000*C with a step interval of 50*C temp rise.

Again it is examined under microscope and needle scratch , till the sintering is not seen.

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Strength Test:The sand specimen is placed in the lugs and

the pressure is applied using the hand wheel till the specimen breaks.

The reading of the needle of high and low pressure manometer indicates the compressive strength in kgf/sq.cm.

Bending, shear, compressive and tensile test are performed.

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Permeability test:

It is conducted on compacted sand , using standard rammed specimen.

A amount of sand is first placed in a 2-inch (50.8mm) diameter steel tube.

A 14-lb (6.35kg) weight is dropped on it 3 times from the height of 2 inch (50.8mm).

A rammed specimen is subjected to an air pressure of 10g/sq.cm

Test is performed in permeability meter consisting of balanced tank, water tank, nozzle, adjusting lever, nose piece for fixing the sand rammed specimen and a manometer.

AFS number can be calculated directly from calibrated permeability meter.

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Permeability is the volume of air in cubic cm. passing through a sand specimen of 1sq.cm cross sectional area and 1cm height at a pressure difference of 1gm/sq.cm in one minute.

In generalP = vh / patP = Permeabilityv = Volume air passing through specimenh = Height of specimen in cm.p = Pressure of air in gm/sq.cma = cross sectional area of specimen in sq.cm.t = time in min.

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For AFS standard permeability meter , 2000cc of air is passed through 5.08cm height specimen and 20.268sq.cm c/s area at a pressure of 10gms/sq.cm and total time measured is 10sec = 1/6 min.

P = 300.66 Approximately.

Permeability Meter (AFS)

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Check that the open orifice is in position in the center post.

Prepare a standard specimen of sand. Before stripping from tube, place in position on the center post and seal by rotating the knurled ring anti-clockwise.

Turn air valve to vent and raise air drum until it is above the water level.

Turn valve to close and allow the drum to descend slowly into the water. Turn the air valve gradually towards vent and allow the air drum to descend until the „X‟ mark on the drum is level with the top edge of the tank. Turn valve to „closed‟

Allow the air drum to descend by turning the air valve to a position midway between closed and vent.

Time the descent of the air drum between the zero and the 2000 ml mark with a stop watch and record the pressure indicated on the manometer during the descent of the drum.

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Block Diagram of Permeability Meter

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Green Compressive StrengthIt is determined by removing the rammed

from compacting tube and place in mechanical testing device.

A compressive load is applied until specimen breaks. Which usually occur in range of 10 to 30 psi.

Hardness:It is determined by resistance of the sand to

penetration of a 5.08mm diameter spring loaded steel ball.

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Flowability:Put the sand in the steel cylinder.By movement of rammer plunger between

the 4th and 5th dropIt is indicated in percentage.This can be directly taken on the dial of the

flow indicator.Then the stem of this indicator again put on

the top of the plunger of hammer .It records the actual movement of plunger

between 4th and 5th drops.

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Shatter Index Test:Put the sand in the steel cylinder.AFS standard specimen is rammed by 10

blowsAllowed to fall on a 1/2 inch mesh sieve

from height of 6 feet.The weight of sand retained in sieve is

measured.It is expressed as percentage of total weight

of the specimen which is a measure of shatter index.

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Mould Hardness test:Performed by mould hardness tester.Principle is based on brinell hardness testing

machine.A ½ inch diameter steel hemi-spherical ball

is loaded with spring load of 980gm.The ball is made to penetrate the mould

sand and core sand surface.The dial indicates the penetration in

thousands of inch, which directly reads the hardness value.

The penetration of the probe into the mould surface is registered on the dial and is read direct as Green Hardness „B‟ Scale.

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Dial gauge

Sand Rammer

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CORE

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Compact mass of core sand prepared separately that when placed in mould cavity at required location with proper alignment does not allow the molten metal to occupy the space for solidification in that portion and hence help to produce hollowness in casting.

Hollowness – Holes, recesses, Projections , undercut, internal cavities etc.

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CORE must meet following functions/objectives:

Core produce hollowness in casting in the form of internal cavities.

It must be sufficiently permeable to allow the easy escape of gases during pouring and solidification.

It may form a part of green sand mould. It may be deployed to improve mould surface. It may provide external undercut feature in casting. It may be inserted to achieve deep recesses in

casting. It may be used to strengthen the mould. It may be used to form gating system of large size

mould.

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CORE Sand:Special Kind of moulding sand. Consideration in selection of

Core sand are

Core sand should be High in refractory nature. Permeability should be high compare to moulding sand. It should not possesses any material which produce gas ,

while /during contact with molten metal. It must be collapsible in nature.

Constituents are : SILICA SAND (COARSE GRAINS) + BINDER (ORGANIC)

Binders : Cereal , Protein, Thermosetting resin, Sulphite, Dextrin, Pitch,

Molasses, Core oil.

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Cereal Binder : It develops Green strength and collapsibility. Amount vary from 0.2 To 2%.

Protein Binder : Used to increase collapsibility property of core.

Thermo setting resin : It imparts high strength, collapsibility to core.

It also evolves minimum amount of core and mould gases, so less

defects in casting. ( Penol formaldehyde , urea formaldehyde)

Sulphite Binder : used with certain amount of clay. Dextrin: For increasing bake strength and collapsibility. Pitch : Increase the hot strength of the core. Molasses : Used as secondary binder to increase the

hardness on baking. It is in the form of molasses liquid and sprayed on the core before baking.

Core Oil: It is in liquid state when mixed with core. Forms Coherent Solid Films holding the grains together when baked.

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Core Types

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CORE MAKING

It is carried out in four steps :Core Sand Preparation

Using Roller mills and core sand mixer.Core Making:

It is mechanized using core blowing, core ramming and core drawing machines.

Small bench blowers , Large floor blowers.Squeezing, jolting, slinging.Core drawing for deep draws is used. (rapping action on vibrator).

Core BakingTo remove moisture and harden the binder.To provide strength to core.Core oven and di-electric baker is used as dryer.

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Core Finishing - Rubbing or filing of the surface to remove projections.

Core are coated with refractory and protective material using brushing, Deeping, spraying to improve the refractoriness and surface finish.

Bars , Wires and arbors used to reinforce core from inside. Also lifting rings are provided for heavy core.

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CASTING DEFECTS

Defects generated due to : Improper Pattern Design

Improper Mould Construction

Improper Melting Practice

Improper Pouring Practice

Also from Moulding and core making materials.

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Common Defects in casting: Blow Holes Misrun or Short run Cold Shut Mismatch Flashes Metal Penetration Drop Runout Cut or Wash Scars and Blisters Hard Spots Pinhole Porosity Shrinkage Cavities Hot Tears Sponginess Scab Swell Buckle Rat-Tail

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CLASSIFICATION OF CASTING DEFECTSSurface Defects

Internal Defects

Visible Defects

Blow Blow Holes Wash

Scars Porosity Rat Tail

Blisters Pin Holes Swell

Drop Inclusions Misrun

Scab Dross Cold Shut

Penetration

Hot Tear

Buckle Shrinkage / Shit

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CLASSIFICATION OF DEFECT IN CASTING (IITB)

SHAPE DEFECTSFlash , Mismatch, Core, Distortion

FILLING DEFECTSBlow hole, Cold shut, Gas porosity, Inclusions

SOLIDIFICATION DEFECTSShrinkage, Sink mark, Hot Tear , Fettling

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BLOW HOLES: Well rounded cavities with clean and smooth surface. Appear on casting surface or in body of casting

(cavities). Reason – Excessive evolved gas entrapped in mould. It collects into a bubble at high point of mould cavity

and prevents the liquid metal to fill that space. Occur due to:

Excessive moisture content , organic content of sand moisture on chills, chaplets or metal inserts Less permeability of moulding sand Poor venting of mould Insufficient drying of mould and cores Cores not properly vented High gas content of molten metal Low pouring temperature Incorrect feeding of casting.

Usually occur on “COPE” of casting.

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BLOW HOLES DEFECTS EXAMPLES

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MIS RUN OR SHORT RUN:We call it as “Incomplete Filling of cavity”.Reason can be :

Inadequate metal supplyToo Low mould or melt temperature Improperly designed gatesLength to thickness ratio of casting is too large.

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COLD SHUT:It is interface between casting , where two

metal streams meet without complete fusion.

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MISMATCH: It is the shift of individual parts of a casting

with respect to each other.This may occur due to mould shift or core

shift.Reasons:

In expert assembling of the two halves of the mould.

From wear of pin bushes and pinsDimensional discrepancy between the core prints

of the pattern and core prints of the core.

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FLASHES: Other name is Fins .It appear along the mould joints.Reasons:

Mould halves do not fit together properlyMuch wear of the two halves of the flasks Improper fastening of cope and drag portion.

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METAL PENETRATION (BURNT ON SAND): It is strong crust of fused sand on the

surface of the casting.Reasons:

Insufficient refractoriness of moulding materials.

Large content of impurities Inadequate mould packingPoor quality of mould wash.

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DROP ( or CRUSH):These are irregularly projections on the

cope surface of the casting.Reasons:

Break away a part of mould sand as a result of weak packing of mould.

Low strength of moulding sand.Malfunctioning of moulding equipment.Strong jolts and strikes at flask during

assembling the mould.

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RUNOUT:In this metal is leaking out of a mould

during pouring.Reason:

Faulty moulding procedure or process.Faulty flask equipment.

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CUT or WASH:A low projection on drag face of a casting

that extends along the surface.It is decreasing in height as it extends from

one side of the casting to the other end.Reason:

It occurs in the casting having bottom gating system , having insufficient hot strength

When more amount of metal allowed to flow through one gate into the mould cavity.

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SCARS & BLISTERS:Scar is a shallow blow.Generally occurs on flat surface.Whereas blow occurs on convex casting

surface.

Blister is a shallow blow like a scar with a thin metal layer covering it.

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HARD SPOTS:Only occur with certain metals – like C.I with

insufficient silicon.Metal becomes hardened by the chilling

effect of moulding sand.Hard spots creates a difficulty while

machining of casting.

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PINHOLE POROSITY:Small gas holes on or below the surface.They occur in large numbers and fairly

uniformly distributed over the surface.Reason:

Due to dissolved gas in alloyAlloy not properly degassed.

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Gas porosity / pin hole porosity

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SHRINKAGE CAVITIES: Internal void or depression in casting due to

volume contraction on solidification. It occurs at hot spot. Open shrinkage defects are open to

the atmosphere, therefore as the shrinkage cavity forms air compensates.

open air defects: pipes and caved surfaces. Pipes form at the surface of the casting and

burrow into the casting, while caved surfaces are shallow cavities that form across the surface of the casting.

Closed shrinkage defects, also known as shrinkage porosity, are defects that form within the casting.

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HOT TEARS:Hot cracks appear in the form of irregular

crevices with a dark oxidized fracture surface.Is a crack on casting surface usually near

a thick (hot, weak) section, caused by resistance of a hard mold or core to casting contraction.

Reason:When metal does not have sufficient strength to

resist tensile forces during solidification.Excessively high temperature of cast metal. Increased metal contraction. Incorrect design of gating system and casting as a

whole.Poor deformability / Collapsibility of the core. Internal stresses due to non-uniform cooling.

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SPONGINESS (honeycombing):Large number of small cavities in close

proximity.Apparently occur on surface.Reason:

Suspension of dirt or inclusion in molten metal. Imperfect skimming of slag in the ladle. Incorrect gating system design.

Impurities being lighter comes up with evolved gases in the cavity.

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SCAB:It occurs when a portion of face of a mould

lifts or breaks down and the gap/recess is filled by metal.

Reason:Very Fine Sand particles.Low permeability of sand.High moisture content of sand.Uneven mould ramming.

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SWELL:It is a smooth , slight/small bulge found on

the vertical faces of casting.It results from the pressure of liquid metal.Reason:

Low strength of mould.Mould with too high water content. Improper / insufficient mould ramming.

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BUCKLE:Long , shallow , V-shaped depression on

surface of flat casting.It extends in straight line on entire flat

surface.Reason:

Sand Expansion due to heat of metal.Poor casting design – Too much large flat area/

surface in mould cavity.Prevention:

Adding cereal or wood flour to sand .On burning / oxidation by molten metal a void

are created in mould which give space for expansion of sand particles and reduce compressive stresses.

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SINK MARK:is a smooth depression seen on the top of a

thick section caused by volumetric contraction occurring inside the section pulling the top surface.

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RAT TAIL: (or Veins Defect)Long , Shallow , Angular depression in the

surface of flat casting.Reason are same as buckle.

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REFERENCES

http://nptel.ac.in/courses/112107144/metalcasting/lecture1.htm

Core preparing - https://www.youtube.com/watch?v=3GtaL679lrw

http://www.iron-foundry.com/casting-defects-pictures.html

http://efoundry.iitb.ac.in http://www.foundryinfo-india.org/ https://www.youtube.com/user/TheEfoundry