The liquid Metal has a Volume "A

It solidifies to solid with a new volume "B"

The solidified casting further contracts (shrinks) through the cooling process to Volume "C"

Three Stages of Contraction (Shrinkage)

DIRECTIONAL SOLIDIFICATION

LIKE A PRESSURISED SYSTEM

MOULDING BOARDFLASKSHOWELDRAW SPIKERIDDLESLICKRAMMERLIFTERSTRIKE-OFF BARTROWELS GATE CUTTER BELLOWSSPRUE PINS VENT ROD ..

MOULDERSTOOLS AND EQUIPMENT

Design of Risers and Feeding of Castings

A simplified diagram by putting in references to the equations (1, 2 & 4) there is no Equation 3, diagram not changed

EQ(1) - Freeze Point Ratio (FPR)

FPR=X X = (Casting Surface/Casting Volume) / (Riser Surface/Riser Volume)

EQ(2) - Volume Ratio (VR) (Y Axis)

VR=Y=Riser Vol/Casting Vol*Note: The riser volume is the actual poured volumeReferences - AFS Text Chapter 16; Chastain's Foundry manual Vol 2, Google EQ(4) - (Freeze Point Ratio) Steel

X=0.12/y-0.05 + 1.0**The constants are from experiments and are empirical

Volumes, Surface Areas, Castings and Risers...

There are relationships between all these items and values that will help in designing a complete mold that controls progressive solidification, and influences directional solidification to produce castings with minimal porosity and shrinkage defects. This is by ensuring that the riser(s) are the last to solidify.

Gating / Runner Design

Now a look at the flow characteristics of the metal as it enters the mold and how it fills the casting.

Of the flow characteristicsfluidity/viscosity plays a role. Also, velocity, gravitational acceleration & vortex, pressure zones, molten alloy aspiration from the mold and the momentum or kinetic energy of a fluid.

The demarcation point is

Re < 2000 is considered a Laminar Flow Re > 2000 is considered a Turbulent Flow

Objective is to maintain Re below 2000.

LAMINAR FLOW- REFERENCE

TURBULENT FLOW- REFERENCE

SEVERELY TURBULENT FLOW

TEST FOR FLUIDITY

USING A SPIRAL MOULD.

FLUIDITY INDEX IS THE LENGTH OF THE SOLIDIFIED METAL IN THE SPIRAL PASSAGE. GREATER THE LENGTH, GREATER THE FLUIDITY INDEX.

Basic Components of a Gating System

The basic components of a gating system are: Pouring Basin, Sprue, Runners and Gates that feed the casting.

The metal flows through the system in this order.

Some simple diagrams to be familiar with are:

The runner system is fed by the well and is the path that the gates are fed from.This path should be "Balanced" with the model of heating or AC ductwork serving as a good illustration. The Runner path should promote smooth laminar flow by a balanced volumetric flow, and avoiding sharp or abrupt changes in direction.The "Runner Extension" is a "Dead-End" that is placed after the last gate. The R-Ext acts as a cushion to absorb the forward momentum or kinetic energy of the fluid flow. The R-Ext also acts as a "Dross/Gas Trap" for any materials generated and picked-up along the flow of the runner.An Ideal Runner is also proportioned such that it maintains a constant volumetric flow through virtually any cross-sectional area. In the illustration, notice that the runner becomes proportionally shallower at the point where an in-gate creates an alternate path for the liquid flow.

The Runner System

DIRECTIONAL SOLIDIFICATION-

Formulae, Ratios and Design Equations

What is covered so far is comprehensive, and intuitive on a conceptual level, but the math below hopefully offers some insight into quick approximations for simple designs, and more in-depth calculations for complex systems. Computerized Flow Analysis programs are used extensively in large Foundry operations. From basic concepts, designing on a state of the art system shall be attempted:

Continuity Equation

This formula allows calculation of cross-sectional areas, relative to flow Velocity and Volumetric flow over unit time. This is with the assumption that the fluid flow is a liquid that does NOT compress (that applies to all molten metals).

Here, a flow passes through A1 (1" by 1", 1 sq") The passage narrows to a cross-sectional area A2 (.75" by .75", 0.5625 sq")

The passage expands to a cross-sectional area A3 (1" by 1", 1 sq").Q= Rate of Flow (Constant - uncompressible)V=Velocity of flowA=Area (Cross-section)If A1 and A2 are considered, the Area A2 is almost half of A1, thus the velocity at A2 has to be almost double of A1.

GATING RATIO is-

Areas of Choke : Runner : Gate(s)

The base of the Sprue and Choke are the same. The ratios between the cross-sectional Area can be grouped into either Pressurized or Unpressurized.

Pressurized: A system where the gate and runner cross-sectional areas are either equal or less than the choke cross-sectional area.

Areas A2 & A3 do not get added as they are positioned in line with each other and flow is successive between the points and not simultaneous.

While Areas A4 & A5 are added together as flow does pass through these points simultaneously.

This example would resolve to a pressurized flow of 1 : 0.75 : 0.66A1= Choke = 1 Sq InchA2 = 1st Runner c/s Area = 0.75 Sq InchA3 = 2nd Runner c/s Area = 0.66 Sq InchA4 = 1st Gate = 0.33 Sq inchA5 = 2nd Gate = 0.33 Sq Inch

An exception is noted in Chastain with a 1 : 8 : 6 ratio to promote dross capture in the runner system of Aero-Space castings.The Continuity Equation is simplified with the use of ratios as the velocity is inversely proportional between any 2 adjacent ratio values. ie H : L equates to an increase in velocity while a L : H equates to a drop in velocity.Laminar Flow is harder to control at a high velocity than a relatively lower velocity.Chastain's Vol 2 has much more mathematical expressions and calculations.

PURE METALS-

Have clearly defined melting/freezing point, solidifies at a constant temperature.

Eg: Al - 6600C, Fe - 15370C, and W- 34100C.

NITC

Solidified structures of metal - solidified in a square mould(a). Pure metal(b). Solid solution(c). When thermal gradient is absent within solidifying metal

Development of a preferred texture - at a cool mould wall. A chill zone close to the wall and then a columnar zone away from the mould.Three basic types of cast structures- (a). Columnar dendritic; (b). equiaxed dendritic; (c). equiaxed nondendritic

SOLIDIFICATION TIME

During solidification, thin solidified skin begins to form at the cool mould walls.

Thickness increases with time.For flat mould walls thickness time (time doubled, thickness by 1.414)

NITC

CHVORINOVS RULE solidification time (t) is a function of volume of the casting and its surface area t = C ( volume/ surface area )2C is a constant [depends on mould material, metal properties including latent heat, temperature]

A large sphere solidifies and cools at a much slower rate than a small diameter sphere. (Eg- potatoes, one big and other small) Volume cube of diameter of sphere, surface area square of diameterNITC

Solidification time for various shapes:Eg: Three pieces cast with the SAME volume, but different shapes. (i)Sphere, (ii)Cube, (iii)Cylinder with height = diameter. Which piece solidifies the fastest?Solution: Solidification time = C (volume/surface area)2Let volume = unity. As volume is same, t = C/ surface area2. Cylinder: V = r2h = 2 r3; ie, r = (1/2 ) 1/3 A = 2 r2 + 2rh = 6 r2 = 5.54. Then, t cube = 0.028C ; t cylinder = 0.033C ; t sphere= 0.043CMetal poured to cube shaped mould solidifies the fastest.Sphere: V= 4/3 ( r3); i.e. r = (3/4 )1/3A= 4 r2 = 4 (3/4 )1/3 = 4.84Cube: V = a3; ie a = 1; A = 6 a2 = 6.NITC

SHRINKAGE AND POROSITY

METALS SHRINK(CONTRACT) DURING SOLIDIFICATION

- CAUSES DIMENSIONAL CHANGES

LEADING TO CENTRE LINE SHRINKAGE, POROSITY, CRACKING TOO

NITC

TTime123NITCSHRINKAGE DUE TO: (1).CONTRACTION OF MOLTEN METAL AS IT COOLS PRIOR TO SOLIDIFICATION (2) CONTRACTION OF SOLIDIFYING METAL, LATENT HEAT OF FUSION (3) CONTRACTION OF SOLIDIFIED METAL DURING DROP TO AMBIENT TEMPOUT OF THESE, LARGEST SHRINKAGE DURING COOLING OF CASTING (ITEM 3) eg:pure metal

SOLIDIFICATION CONTRACTION FOR VARIOUS METALS

METAL Volumetric Solidification Contraction Al 6.6 Grey cast Iron Expansion 2.5 Carbon Steel 2.5 to 3 Copper 4.9Magnesium 4.2 Zinc 6.5NITC

POROSITY DUE TO SHRINKAGE OF GASES AND METAL TOO. RELATED TO DUCTILITY AND SURFACE FINISH(DUCTILITY V/S POROSITY CURVES FOR DIFFERENT METALS) ELIMINATION BY VARIOUS MEANS(ADEQUATE SUPPLY OF LIQUID METAL, USE OF CHILLS, NARROWING MUSHY ZONE- CASTING SUBJECTED TO ISOSTATIC PRESSING

NITC

POROSITY BY GASES

LIQUID METALS HAVE HIGH SOLUBILITY FOR GASESDISSOLVED GASES EXPELLED FROM SOLUTION DURING SOLIDIFICATION(Hydrogen, Nitrogen mainly)ACCUMULATE IN REGIONS OF EXISTING POROSITY ORCAUSE MICROPOROSITY IN CASTING- TO BE CONTROLLEDNITC

Effect of microporosity on the ductility of quenched and tempered cast steel Porosity affects the pressure tightness of cast pressure vesselDuctilityPorosity(%)ElongationReduction of area0 5 10 15NITC

Type of pattern depends on: Shape and size of casting, number of castings required, method of moulding employed, easiness or difficulties of the moulding operations, other factors peculiar to the casting. NIT CALICUT

NITC

Pattern, Finish Allowance, and Wall Thickness

MetalPattern Oversize Factor (each direction)Finish Allowance (smaller number for larger sizes)Min Wall mm Aluminum1.08 - 1.120.5 to 1.0 %4.75Copper alloys1.05 - 1.060.5 to 1.0 %2.3Gray Cast Iron1.100.4 to 1.6 %3.0Nickel alloys1.050.5 to 1.0 %N/ASteel1.05 - 1.100.5 to 2 %5Magnesium alloys1.07 - 1.100.5 to 1.0 %4.0Malleable Irons1.06 - 1.190.6 to 1.6 % 3.0

CHARACTERISTICS OF PATTERN MATERIALSCHARACTERISTIC RATING

WOOD AL STEEL PLASTIC CAST IRONMACHINABILITY E GFG G WEAR RESISTANCE PGEFESTRENGTH EGEGGWEIGHT EGPGPREPAIRABILITY EPGFGRESISTANCE TO:CORROSION(by water) EEPEPSWELLING PEEEE

E- Excellent; G- Good; F-fair, P- PoorNITC

Functions of pattern

Moulding the Gating system;Establishing a parting Line,Making Cores, Minimising casting Defects,Providing Economy in mouldingOthers, as needed

MOULDING SANDGranular particles from the breakdown of rocks by frost, wind, heat and water currents

Complex Composition in different places

At bottom and banks of rivers- mainly silica (86 to 90%); Alumina (4% to 8 %); Iron oxide (2 to 5%) with oxides of Ti, Mn, Ca. etc.

NITC

NATURAL SAND , called Green sand. Only water as binder; can maintain water for long time

SYNTHETIC SAND.- (1)GREEN and (2)DRY types (1) Artificial sand by mixing clay free sand, binder(water and bentonite) Contains New silica sand 25%; Old sand 70%; bentonite 1.5%;moisture 3% to 3.5%

(2) New 15%; Old 84%; bentonite and moisture 0.5 % eachNITC

DRY SAND- for moulding large castings. Moulds of green sand dried and baked with venting done. Add- cow dung, horse manure etc.LOAM SAND- mixture of clay and sand milled with water to thin plastic paste. Mould made on soft bricks. The mould dried very slowly before cast. For large regular shapes- drums, chemical pans etc.FACING SAND- used directly with surface of pattern; comes in contact with molten metal; must have high strength, refractoriness.Silica sand and clay without used sand- plumbago powder, Ceylon lead, or graphite used. Layer of 20 to 30 mm thick--- about 10% to 15% of whole mould sandNITC

BACKING SAND- old used moulding sand called floor sand black in colour. Used to fill mould at back of facing layer. Weak in bonding strengthSYSTEM SAND- used in machine moulding to fill whole of flask. Strength, premealibility and refractoriness highPARTING SAND- used for separating boxes from adhering, free from clayCORE SAND- for making cores. Silica sand with core oil (linseed oil, rosin, light mineral oil, binders etc)SPECIALISED SANDS - like CO2 sand, Shell sand, etc for special applicationsMould washers- slurry of fine ceramic grains applied on mould surface to minimize fusing NITC

About MOULDING SANDNATURAL SANDSYNTHETIC SAND.- GREEN and DRYDRY SANDLOAM SANDFACING SANDBACKING SANDSYSTEM SANDPARTING SANDCORE SAND SPECIALISED SANDS Mould washers

NITC

ADV - Acid Demand ValueDefined as the property of a sand or additive to affect the cure process as a function of the materials acidity or basicity on the pH scale.

MOULDING SAND- PROPERTIES

Green Strength- Adequate strength after mixing, and plasticity for handlingDry Strength- After pouring molten metal, adjacent surface loses water content. Dries. Dry sand must have enough strength to resist erosionHot Strength- Strength at elevated temperature after evaporation of moisture Permeability- Permeable or porous to permit gases to escape. Ability of sand moulds to allow the escape of gases

NITC

Thermal stability- Rapid expansion of sand surface at mould-metal interface. May crack. Results in defect called SCABRefractoriness- Ability of sand to withstand high temperatureFlowability- Ability to flow & fill narrow portions around patternSurface finish- Ability to produce good surface finish in castingCollapsibility- Allow easy removal of casting from mouldReclamation- Should be reusable and reclaimable

NITC

FURNACES Proper selection depends on:Composition and melting point of alloy to be castControl of atmospheric contaminationCapacity and rate of melting requiredEnvironmental considerations- noise, pollutionPower supply, availability, cost of fuelsEconomic considerations-initial cost, operating cost, maintenance cost etc.CUPOLAS (> 50 T, VERTICAL, HIGH RATES) ELECTRIC FURNACESINDUCTION FURNACESNITC

NITC

NITC

Casting DefectsMetal casters try to produce perfect castings. A few castings, however, are completely free of defects. Modern foundries have sophisticated inspection equipment which can detect small differences in size and a wide variety of external and even internal defects. For example, slight shrinkage on the back of a decorative wall plaque is acceptable whereas similar shrinkage on a position cannot be tolerated. No matter what the intended use, however, the goal of modern foundries is zero defects in all castings

Scrap castings cause much concern.

In industry, scrap results in smaller profits for the company and ultimately affects individual wages.

Scrap meetings are held daily. Managers of all the major departments attend these meetings. They gather castings that have been identified as scrap by inspector. The defect is circled with chalk. An effort is made to analyze the cause of the defect, and the manager whose department was responsible for it is directed to take corrective action to eliminate that specific defect in future castings.

There are so many variables in the production of a metal casting that the cause is often a combination of several factors rather than a single one.

All pertinent data related to the production of the casting (sand and core properties, pouring temperature) must be known in order to identify the defect correctly.

After the defect is identified attempt should be to eliminate the defect by taking appropriate corrective action.

CASTING DEFECTS

SURFACE

METALLIC PROJECTION (4)DEFECTIVE SURFACE (11)CHANGE IN DIMENSION- WARPINCOMPLETE CASTING MISRUN, RUNOUTCAVITY- BLOWHOLES, SHRINKAGE PINHOLESDISCONTINUITY HOT CRACKCOLD SHUT, COLD CRACKSUBSURFACE

SUBSURFACE CAVITYINCLUSIONSDISCONTINUITY NITC

CASTING DEFECTS

SURFACE METALLIC PROJECTION Swell, Crush, Mould Drop, Fillet Vein DEFECTIVE SURFACE Erosion Scab, Fusion, Expansion Scab, Rat tails, Buckle, Seams, Gas Runs, Fillet Scab, Rough Surface, Slag Inclusion, Elephant Skin CHANGE IN DIMENSION- Warped castingINCOMPLETE CASTING- Misrun, Run outCAVITY- Blow Holes, Shrinkage cavity, PinholesDISCONTINUITY-Hot6 Cracking, Cold Shut, Cold CrackingSUBSURFACESUBSURFACE CAVITY- Blow Holes, Pin Holes, ShrinkagePorosity, Internal Shrinkage, SevereRoughness

INCLUSIONS- Gas Inclusions, Slag, Blow Holes

DISCONTINUITY- Cold ShutsNITC

Repairability

FINS OR FLASH ON CASTINGS -AsMetallic Projections

Joint flash or fins. Flat projection of irregular thickness, often with lacy edges, perpendicular to one of the faces of the casting. It occurs along the joint or parting line of the mold, at a core print, or wherever two elements of the mold intersect. Possible CausesClearance between two elements of the mold or between mold and core; Poorly fit mold joint. RemediesCare in pattern making, molding and core making; Control of their dimensions; Care in core setting and mold assembly; Sealing of joints where possible.

Flask was disturbed while investment was setting. Base was removed too soon. Flask was allowed to partially dry before dewaxing. Incorrect dewaxing or a furnace malfunction. Flask burned out and allowed to cool below (500oF (260oC) before casting reheating, flask allowed to cool between dewax and placement in preheated oven. Flask was improperly handled or dropped. Speed was set too high on centrifugal casting machine. Patterns were placed on one plane. The should be staggered on top rack. Incorrect water powder ratio was used. Not enough investment was placed over the patterns. Flask was placed too close to heat source in burnout oven. Flasks were not held at low burnout temperature long enough.

DEFECTS IN CASTINGS- CAN BE ELIMINATED/MINIMISED BY PROPER DESIGN, MOLD PREPARATION, PROPER POURING.NITC

DEFECTS IN CASTINGS- AS HOT TEARS - DUE TO CONSTRAINTS IN LOCATIONS, CASTINGS CANNOT SHRINK FREELYNITC

Cavities

Blowholes, pinholes. Smooth-walled cavities, essentially spherical, often not contacting the external casting surface (blowholes). The largest cavities are most often isolated; the smallest (pinholes) appear in groups of varying dimensions. The interior walls of blowholes and pinholes can be shiny, more or less oxidized or, in the case of cast iron, can be covered with a thin layer of graphite. The defect can appear in all regions of the casting.

Possible Causes

Because of gas entrapped in the metal during the course of solidification: Excessive gas content in metal bath (charge materials, melting method, atmosphere, etc.); Dissolved gases are released during solidification.

In steel and cast irons: formation of carbon monoxide by the reaction of carbon and oxygen, presents as a gas or in oxide form. Blowholes from carbon monoxide may increase in size by diffusion of hydrogen or, less often, nitrogen.

Excessive moisture in molds or cores.Core binders which liberate large amounts of gas.Excessive amounts of additives containing hydrocarbons.Blacking and washes which tend to liberate too much gas. Insufficient evacuation of air and gas from the mold cavity; -insufficient mold and core permeability. Entrainment of air due to turbulence in the runner system.

RemediesMake adequate provision for evacuation of air and gas from the mold cavityIncrease permeability of mold and cores Avoid improper gating systemsAssure adequate baking of dry sand moldsControl moisture levels in green sand molding

Reduce amounts of binders and additives used or change to other types; -use blackings and washes, which provide a reducing atmosphere; -keep the spree filled and reduce pouring height

Increase static pressure by enlarging runner height.

Discontinuities

Hot cracking. A crack often scarcely visible because the casting in general has not separated into fragments. The fracture surfaces may be discolored because of oxidation. The design of the casting is such that the crack would not be expected to result from constraints during cooling.

Possible CausesDamage to the casting while hot due to rough handling or excessive temperature at shakeout.

RemediesCare in shakeout and in handling the casting while it is still hot; Sufficient cooling of the casting in the mold; For metallic molds; delay knockout, assure mold alignment, use ejector pins

Defective Surface

Flow marks. On the surfaces of otherwise sound castings, the defect appears as lines which trace the flow of the streams of liquid metal. Possible CausesOxide films which lodge at the surface, partially marking the paths of metal flow through the mold.

RemediesIncrease mold temperature; Lower the pouring temperature; Modify gate size and location (for permanent molding by gravity or low pressure); Tilt the mold during pouring; In die casting: vapor blast or sand blast mold surfaces which are perpendicular, or nearly perpendicular, to the mold parting line.

Incomplete Casting

Poured short. The upper portion of the casting is missing. The edges adjacent to the missing section are slightly rounded, all other contours conform to the pattern. The spree, risers and lateral vents are filled only to the same height above the parting line, as is the casting (contrary to what is observed in the case of defect).

Possible CausesInsufficient quantity of liquid metal in the ladle; Premature interruption of pouring due to workmans error.

RemediesHave sufficient metal in the ladle to fill the mold; Check the gating system; Instruct pouring crew and supervise pouring practice.

Incorrect Dimensions or ShapeDistorted casting. Inadequate thickness, extending over large areas of the cope or drag surfaces at the time the mold is rammed.

Possible CausesRigidity of the pattern or pattern plate is not sufficient to withstand the ramming pressure applied to the sand. The result is an elastic deformation of the pattern and a corresponding, permanent deformation of the mold cavity. In diagnosing the condition, the compare the surfaces of the pattern with those of the mold itself.

RemedyAssure adequate rigidity of patterns and pattern plates, especially when squeeze pressures are being increased.

Inclusions or Structural Anomalies

Metallic Inclusions. Metallic or intermetallic inclusions of various sizes which are distinctly different in structure and color from the base material, and most especially different in properties. These defects most often appear after machining.

Possible CausesCombinations formed as intermetallics between the melt and metallic impurities (foreign impurities); Charge materials or alloy additions which have not completely dissolved in the melt; Exposed core wires or rods; During solidification, insoluble intermetallic compounds form and segregate, concentrating in the residual liquid.

RemediesAssure that charge materials are clean; eliminate foreign metals; Use small pieces of alloying material and master alloys in making up the charge; Be sure that the bath is hot enough when making the additions; Do not make addition too near to the time of pouring; For nonferrous alloys, protect cast iron crucibles with a suitable wash coating

INCLUSIONS (FOREIGN PARTICLES) IN CASTINGS

Patterns were improperly sprued to wax base or tree or not filleted, causing investment to break at sharp corners during casting. Flask was not sufficiently cured before placing into burnout oven. Improper dewaxing cycle was used. Flask was not cleaned from prior cast. Loose investment in sprue hole. Molten metal contains excess flux or foreign oxides. Crucible disintegrating or poorly fluxed. Improperly dried graphite crucible. Investment was not mixed properly or long enough. Contaminants in wax pattern. Flask was not held at low burnout temperature long enough. Flask was placed too close to heat source in burnout oven.

POROSITYPattern is improperly sprued. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage porosity. Not enough metal reservoir to eliminate shrinkage porosity. Metal contains gas. Mold is too hot. Too much moisture in the flux. Too much remelt being used. Always use at least 50% new metal. Metal is overheated. Poor mold burnout.

ROUGH CASTINGSA poor quality pattern Flask was not sufficiently cured before placing into burnout oven. Flask was held in steam dewax too long. Metal, flask or both were too hot. Patterns were improperly sprued. Flask was placed too close to heat source in burnout oven.

BUBBLES OR NODULES ON CASTINGSVacuum pump is leaking air. Vacuum pump has water in the oil. Vacuum pump is low on oil. Investment not mixed properly or long enough. Invested flasks were not vibrated during vacuum cycle. Vacuum extended past working time.

SPALLING (an area of the mold wall flakes into the mold cavity)Flask was placed into a furnace at low temperature (below 150oC) for an extended period. Flask was placed too close to the source of heat. Sharp corners are struck by metal at high centrifugal velocities. Improper burnout cycle was used.

NON-FILL OR INCOMPLETE CASTINGSMetal was too cold when cast. Mold was too cold when cast. The burnout was not complete. Pattern was improperly sprued, creating turbulence when casting in a centrifugal casting machine. Centrifugal casting machine had too high revolution per minute.

GROWTH-LIKE ROUGH CASTING THAT RESISTS REMOVAL IN PICKLING SOLUTIONBurnout temperature too high. Mold temperature was too high when casting. Metal temperature was too high when casting.

SHINY CASTINGSCarbon residue was left in the mold, creating a reducing condition on the surface.

DESIGN CONSIDERATIONSCAREFUL CONTROL OF LARGE NUMBER OF VARIABLES NEEDED- CHARACTERISTICS OF METALS & ALLOYS CASTMETHOD OF CASTINGMOULD AND DIE MATERIALSMOULD DESIGNPROCESS PARAMETERS- POURING, TEMPERATURE, GATING SYSTEMRATE OF COOLING Etc.Etc.

NITC

Poor casting practices, lack of control of process variables- DEFECTIVE CASTINGS

TO AVOID DEFECTS-Basic economic factors relevant to casting operations to be studied.General guidelines applied for all types of castings to be studied.

DESIGN CONSIDERATIONS

NITC

CORNERS, ANGLES AND SECTION THICKNESSSharp corners, angles, fillets to be avoided Cause cracking and tearing during solidificationFillet radii selection to ensure proper liquid metal flow- 3mm to 25 mm. Too large- volume large & rate of cooling lessLocation with largest circle inscribed critical. Cooling rate less shrinkage cavities & porosities result- Called HOT SPOTS

NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- AVOID SHARP CORNERS MAINTAIN UNIFORM CROSS SECTIONSAVOID SHRINKAGE CAVITIESUSE CHILLS TO INCREASE THE RATE OF COOLINGSTAGGER INTERSECTING REGIONS FOR UNIFORM CROSS SECTIONSREDESIGN BY MAKING PARTING LINE STRAIGHTAVOID THE USE OF CORES, IF POSSIBLEMAINTAIN SECTION THICKNESS UNIFORMITY BY REDESIGNING (in die cast products)

NITC

LARGE FLAT AREAS TO BE AVOIDED- WARPING DUE TO TEMPERATURE GRADIENTSALLOWANCES FOR SHRINKAGE TO BE PROVIDEDPARTING LINE TO BE ALONG A FLAT PLANE- GOOD AT CORNERS OR EDGES OF CASTINGDRAFT TO BE PROVIDEDPERMISSIBLE TOLERANCES TO BE USEDMACHINING ALLOWANCES TO BE MADERESIDUAL STRESSES TO BE AVOIDED

ALL THESE FOR EXPENDABLE MOULD CASTINGS.NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- AVOID SHARP CORNERS TO REDUCE STRESS CONCENTRATIONSNITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- MAINTAIN UNIFORM CROSS SECTIONS TO AVOID HOT SPOTS AND SHRINKAGE CAVITIESNITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- GOOD DESIGN PRACTICE NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- STAGGERING OF INTERSECTING REGIONS NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- SECTION THICKNESS UNIFORMITY MAINTAINED THROUGHOUT PARTNITC

DESIGN MODIFICATIONS TO AVOID DEFECTSNITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- USE OF METAL PADDING (CHILLS) TO INCREASE RATE OF COOLING NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- MAKING PARTING LINE STRAIGHT NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS-IN DESIGNNITC

CARBON-DI OXIDE PROCESS(SILICATE BONDED SAND PROCESS)FIRST IN 1950sMIXTURE OF SAND AND 1.5% TO 6 % SODIUM SILICATE (AS BINDER)MIXTURE PACKED AROUND THE PATTERN, HARDENED BY BLOWING CO2DEVELOPED FURTHER BY ADDDING OTHER CHEMICALS AS BINDERSMAINLY TO MAKE CORES-AS USE IS IN ELEVATED TEMPERATURE APPLICATION

Na2O SiO2 + H2O +CO2 Na2CO3 + (SiO2 +H2O) (Silica Gel)Formation of Silica Gel gives strength to the moulds

+ Points:Drying not necessaryImmediately ready for pouringVery high strength achievedDimensional accuracy very goodPointsCollapsibility poor, can be improved by additivesNa2O SiO2 attacks and spoils wooden pattern

CO2FunnelCO2 Moulding Mould

DIE CASTING

GRAVITY SEMI PERMANENT MOULDOR PERMANENT MOULD COLD CHAMBER HOT CHAMBER (HEATING CHAMBER) OUTSIDE THE MACHINE INTEGRAL WITH THE MACHINE

PERMANENT MOULD OR GRAVITY DIE CASTING*METALLIC MOULDS USED *TWO HALVES OF DIES- ONE FIXED, ONE MOVABLEVERY CLOSE TOLERANCE CASTINGS, MORE STRENGTH, LESS POROUS-BETTER SURFACE FINISH COMPARED TO SAND CASTING-SURFACE FREE FROM SAND & DENSITY HEAVYONLY FOR SMALL AND MEDIUM SIZE CASTINGSFOR NON FERROUS, MAINLYLARGE QUANTITY, BUT IDENTICAL PIECES ONLY

PERMANENT MOULD OR GRAVITY DIE CASTING*METALLIC MOULDS USED - MOULD TO WITHSTAND TEMPERATURE

*NO EXTERNAL PRESSURE APPLIED,

*HYDROSTATIC PRESSURE BY RISERING

*LAMP BLACK/CORE OIL APPLIED TO DIE SURFACES FOR EASY REMOVAL

*FAST CONDUCTION, RAPID COOLING

*TWO HALVES OF DIES- ONE FIXED, ONE MOVABLE

NITC

+POINTS- VERY CLOSE TOLERANCE CASTINGS, MORE STRENGTH, LESS POROUS- BETTER SURFACE FINISH COMPARED TO SAND CASTING- SURFACE FREE FROM SAND- DENSITY HEAVY- MORE DIMENSIONAL ACCURACY - 0.06 TO 0.3 MM- DIES LESS COSTLY THAN PRESSURE DIE CASTING DIES- GOOD FOR PRESSURE TIGHT VESSELS- LESS COOLING CRACKS- LESS SKILL - GOOD FOR LARGE QUANTITIES

NITC

-POINTS

ONLY FOR SMALL AND MEDIUM SIZE CASTINGS FOR NON FERROUS, MAINLY LARGE QUANTITY, BUT IDENTICAL PIECES ONLY POOR ELONGATION STRESS AND SURFACE HARDNESS DEFECTS OBSERVED CASTING TO BE WITHDRAWN CAREFULLY FROM DIES

NITC

A TYPICAL DIE ECCENTRIC CLAMPINGMOULD / DIE HALFVENT HOLEPOURING BASIN

MOULD CAVITY

SEMIPERMANENT DIECASTING

DIE PRESSURE AT 20 TO 20,000 ATM

PRESSURE FILL SOLIDIFICATION

FOR NONFERROUS METALS

FOR INTRICATE SHAPES

CLOSE TOLERANCES POSSIBLE

FOR MASS PRODUCTION, >10,000

NITC

FOR SEMI AND PRESSURE DIE CASTING SET UPS, THE FOLLOWING FACTORS A MUST

1. A GOOD DIE SET MECHANISM

2. MEANS FOR FORCING METAL

3. DEVICE TO KEEP DIE HALFS PRESSED

4. ARRANGEMENT FOR AUTOMATIC REMOVAL OF CORES- IF ANY

5. EJECTOR PINS

NITC

TWO TYPES OF PRESSURE DIE CASTING COLD CHAMBER- HEATING CHAMBER OUTSIDE THE MACHINE - FOR Al, Mg, Cu, AND HIGH MELTING ALLOYS HOT CHAMBER- HEATING INTEGRAL WITH THE HANDLING GOOSE NECK MECHANISMS WIDELY USED FOR LOW MELTING ALLOYS- Zn, Pb, Etc.ALSO VACUUM DIE CASTING MACHINES- SPACE BETWEEN THE DIES AND PASSAGE VACUUMISED BEFOR POURING- SUBMERGED PLUNGE TYPE, DIRECT AIR DIE CASTING MACHINES

231123

D.A.D.C. MACHINE showing two positions of pot

ANOTHER TYPE OF D.A.D.C. MACHINE

SQUEEZE CASTINGDEVELOPED IN 1960S (also called liquid forging)SOLIDIFICATION OF MOLTEN METAL UNDER HIGH PRESSURE (pressure application when liquid partially solidifies 70 to 140 MPa)A COMBINATION OF CASTING & FORGINGDIE, PUNCH, EJECTOR PIN PUNCH KEEPS ENTRAPPED GASES IN SOLUTION, RAPID COOLING DUE TO HIGH PRESSURE DIE- METAL INTERFACEPARTS OF NEAR-NET SHAPE MADE, COMPLEX AND FINE SURFACE DETAILS OBTAINED. No riser needed FOR FERROUS & NON FERROUSAUTOMOTIVE WHEELS, SHORT BARRELED CANNONS ETC.

VACUUM DIE CASTING MACHINESSOME AIR ENTRAPPED IN ORDINARY DIE CASTING MACHINESTHIS PRODUCES BLOW HOLESIN VACUUM DIE CASTING TYPE, VACUUM PUMP CREATES VACUUM IN DIE CAVITY, A SEAL CUTS OFF THE PIPE CONNECTION AFTER EVACUATINGTHIS PREVENTS FLOW OF METAL FROM DIE TO VACUUM PIPEFLOW OF MOLTEN QUICK AND AUTOMATIC

FINISHES:ALL DIE CASTINGS SUSCEPTIBLE TO CORROSION, HENCE SUBJECTED TO FINISHING OPERATIONS OR PLATING

NITC

DESIGN CONSIDERATIONSUSE OF RIBS, HUBS, BOSSES MUST BE TO REDUCE WEIGHT, STRENGTHEN THE PART, IMPROVE THE APPEARANCETHICK SECTIONS MAKE DIE HOTTER AND THUS LESSEN DIE LIFELARGE SECTIONS TO BE COOLED MAY CAUSE POROSITYEXCESSIVE SECTIONAL CHANGES TO BE AVOIDEDAVOID UNDERCUTSFILLETS DESIRABLE OVER SHARP EDGESDRAFTS NEEDED ON ALL CASTINGSEJECTOR PINS AT BACK TO AVOID VISIBILITY OF MARKSFLASH NECESSARY , TO BE REMOVED LATER BY TRIMMING

NITC

DIE MATERIALS NITC

DIE CASTING ALLOYSMAINLY NON-FERROUS CASTINGS WITH PROPERTIES COMPARABLE WITH FORGINGS

ZINC ALLOYS:- WIDELY USED ( 70%)- Al 4.1%; Cu MAX 1%, Mg 0.4%; BALANCE ZINC-- PERMITS LONGER DIE LIFE, SINCE TEMP. IS LOWGOOD STRENGTH, Tensile Strength: 300 Kg/cm2VERY GOOD FLUIDITY, THUS THIN SECTIONS POSSIBLEUSES: AUTOMOBILES, OIL BURNERS, FRIDGES, RADIO, TV COMPONENTS, MACHINE TOOLS, OFFICE MACHINERIESNITC

ALUMINIUM ALLOYS:

BY COLD CHAMBER PROCESS-Cu 3 to 3.5%, Si 5 to 11 %, BALANCE Al.LIGHTEST ALLOYS, GOOD CORROSION RESISTANCE, FINE GRAINED STRUCTURE DUE TO CHILLING EFFECTTensile Strength: 1250 to 2500 Kg/cm2GOOD MACHINABILITY, SURFACE FINISHUSES: MACHINE PARTS, AUTOMOTIVE, HOUSE HOLD APPLIANCES ETC.NITC

COPPER BASED ALLOYS:

Cu 57 to 81%;Zn 15 to 40%; SMALL QUANTITIES OF Si, Pb, SnVERY HIGH TENSILE STRENGTH: 3700 to 6700Kg/cm2;GOOD CORROSION RESISTANCE; WEAR RESISTANCELOW FLUIDITY, HENCE REDUCED DIE LIFEUSES; ELECTRICAL MACHINERY PARTS, SMALLGEARS, MARINE, AUTOMOTIVE AND AIR CRAFT FITTINGS, HARDWARES

NITC

MAGNESIUM BASED ALLOYS:LIGHTEST IN DIE CASTING, PRODUCTION COST SLIGHTLY HIGH, Al: 9%; Zn: 0.5%; Mn: 0.5%; Si: 0.5%, Cu:0.3%; REMAINING Mg.USES: IN AIRCRAFT INDUSTRY, MOTOR & ISTRUMENT PARTS, PORTABLE TOOLS, HOUSE HOLD APPLIANCES

LEAD & TIN BASED ALLOYS; Lead base: 80% Pb & ; Tin base 75% tin, antimony, copper LIMITED APPLICATIONS. LIGHT DUTY BEARINGS, BATTERY PARTS, X-RAY SHIELDS, LOW COST JEWELLERY, NON-CORROSIVE APPLICATIONSNITC

V-Process

1. Pattern (with vent holes) is placed on hollow carrier plate.2. A heater softens the .003" to .007" plastic film. Plastic has good elasticity and high plastic deformation ratio.3. Softened film drapes over the pattern with 300 to 600 mm Hg vacuum acting through the pattern vents to draw it tightly around pattern.4. Flask is placed on the film-coated pattern. Flask walls are also a vacuum chamber with outlet shown.5. Flask is filled with fine, dry unbonded sand. Slight vibration compacts sand to maximum bulk density.6. Sprue cup is formed and the mold surface leveled. The back of the mold is covered with unheated plastic film.7. Vacuum is applied to flask. Atmospheric pressure then hardens the sand. When the vacuum is released on the pattern carrier plate, the mold strips easily.8. Cope and drag assembly form a plastic-lined cavity. During pouring, molds are kept under vacuum.9. After cooling, the vacuum is released and free-flowing sand drops away leaving a clean casting, with no sand lumps. Sand is cooled for reuse. NITC

Benefits Of Using The V-Process:

Very Smooth Surface Finish 125-150 RMS is the norm. Cast surface of 200 or better, based on The Aluminum Association of America STD AA-C5-E18. Excellent Dimensional Accuracy Typically +/-.010 up to 1 inch plus +/-.002 per additional inch. Certain details can be held closer. +/-.010 across the parting line. Cored areas may require additional tolerances. Zero Draft Eliminates the need for machining off draft to provide clearance for mating parts and assembly. Provides consistent wall thickness for weight reduction and aesthetic appeal. Allows for simple fixturing for machining and inspection.

NITC

Pattern construction becomes more accurate and efficient. Total tolerance range becomes more accurate and efficient. Geometry/tolerance of part is at its simplest form. Draft does not use up tolerance. Design/drafting is less complex. Calculations and depictions related to draft are eliminated. Thin Wall Sections Walls as low as .100 in some applications are possible. Excellent Reproduction Of Details Very small features and lettering are possible. Consistent Quality All molding is semi-automatic. Variable "human factor" has been reduced. Superior Machining Sound metal and no hidden sand in the castings means fewer setups, reduced scrap and longer tool life. Low Tooling Costs

NITC

All patterns are made from epoxy, machined plastics, SLA or LDM. There is no need to retool for production quantities. Unlimited Pattern Life Patterns are protected by plastic film during each sand molding cycle. Easy Revisions To Patterns No metal tooling to weld or mill. Great for prototypes. Short-Run Production Capability Excellent for short-run production while waiting for hard tooling. The V-PROCESS method can outproduce traditional prototype methods such as plaster or investment castings. Fast Turnaround From placement of order to sample casting in as little as two to four weeks.

NITC

Known for several hundred years.

But its evolution into a sophisticated production method for other than simple shapes has taken place only in this century.

Today, very high quality castings of considerable complexity are produced using this technique. CENTRIFUGAL CASTING

AN OVERVIEWNITC

To make a centrifugal casting, molten metal is poured into a spinning mold.

The mold may be oriented horizontally or vertically, depending on the casting's aspect ratio.

Short, square products are cast vertically while long tubular shapes are cast horizontally. In either case, centrifugal force holds the molten metal against the mold wall until it solidifies.

Carefully weighed charges ensure that just enough metal freezes in the mold to yield the desired wall thickness.

In some cases, dissimilar alloys can be cast sequentially to produce a composite structure. NITC

CENTRIFUGAL CASTINGTRUE- C.I. PIPES, LINERS, BUSHES, CYLINDER BARRELS ETC.SEMI- CENTRE CORE FOR INNER SURFACE- SHAPE BY MOULD AND CORE, MAINLY NOT BY CENRTRIFUGAL ACTION- Eg:FLYWHEELSPRESSURE OR CENTRIFUGAL CASTING- ALSO TERMED AS CENTRIFUGINGFOR NON SYMMETRICAL SHAPES MOULD WITH ANY SHAPE PLACED AT CERTAIN DISTANCE FROM AXIS

SEMI- CENTRE CORE FOR INNER SURFACE- SHAPE BY MOULD AND CORE, MAINLY NOT BY CENRTRIFUGAL ACTION- Eg:FLYWHEELSSPEED OF ROTATION- 60 TO 70 TIMES GRAVITY FOR HORIZONTAL AND INCLINED TYPES ABOVE 100 FOR VERTICAL TYPES.

NITC

CENTRIFUGING PROPERTIES OF CASTING DEPEND ON DISTANCE FROM AXIS

SQUEEZE CASTINGDIE, PUNCH, EJECTOR PINPARTS OF NEAR-NET SHAPE MADE, COMPLEX AND FINE SURFACE DETAILS OBTAINEDFOR FERROUS & NON FERROUS

CENTRIFUGAL CASTING

+ points:Denser structure, cleaner, foreign elements segregated (inner surface)Mass production with less rejectionRunners, risers, cores avoidedImproved mechanical propertiesCloser dimensions possible, less machiningThinner sections possibleAny metal can be cast

NITC

points:

Only for cylindrical and annular parts with limited range of sizesHigh initial costSkilled labour neededToo high speed leads to surface cracks- (high stresses in the mould )NITC

For copper alloy castings, moulds are usually made from carbon steel coated with a suitable refractory mold wash.

Molds can be costly if ordered to custom dimensions, but the larger centrifugal foundries maintain sizeable stocks of molds in diameters ranging from a few centimetres to several metres. The inherent quality of centrifugal castings is based on the fact that most nonmetallic impurities in castings are less dense than the metal itself. Centrifugal force causes impurities (dross, oxides) to concentrate at the casting's inner surface. This is usually machined away, leaving only clean metal in the finished product.

Because freezing is rapid and completely directional, centrifugal castings are inherently sound and pressure tight.

Mechanical properties can be somewhat higher than those of statically cast products. NITC

Centrifugal castings are made in sizes ranging from approximately 50 mm to 4 m in diameter and from a few inches to many yards in length.

Size limitations, if any, are likely as not based on the foundry's melt shop capacity.

Simple-shaped centrifugal castings are used for items such as pipe flanges and valve components, while complex shapes can be cast by using cores and shaped molds.

Pressure-retaining centrifugal castings have been found to be mechanically equivalent to more costly forgings and extrusions.

NITC

NITC

INVESTMENT CASTINGAlso called LOST WAX PROCESS- used during 4000-3000 BC

Die for casting wax pattern made with allowances for wax and metal. Pattern and gating systems made of wax (bee wax, aera wax, paraffin) or plastic (polystyrene) by injecting -in molten condition - into the metal diePRECOATING- The pattern dipped in a slurry of refractory material (fine 325 mesh silica &binders, water, ethyl silicate, acids), and sprinkled with silica sandThis pattern with initial coating dried, coated repeatedly to increase thickness

The one piece mould is driedDEWAXING- Inverted and heated to 900C -1750 C for 12 hoursWax melts. Can be reclaimed and reused.Mould fired to 6500C-10500C for about 4 hoursPOURING- Metal poured, allowed to solidifyMould broken, casting taken out

INVESTMENT CASTING- SEQUENCES

Plus and Minus pointsVery good dimensional accuracyNo or very little finishing Intricate and thin shapes possibleAbout 40 kg parts castBoth for ferrous and nonferrous alloysSuited for mechanization

Careful handling needed,as the patterns are not strong.Close control of process neededLabour and material costs high, but high melting point alloys cast with good surface finish & close tolerances. Eg: gears, cams, valves, ratchets, turbine blades, electrical & electronic components etc.

INTRODUCTION Investment casting, often called lost wax casting, is regarded as a precision casting process to fabricate near-net-shaped metal parts from almost any alloy. Although its history lies to a great extent in the production of art, the most common use of investment casting in more recent history has been the production of components requiring complex, often thin-wall castings. A complete description of the process is complex. But, the sequential steps of the investment casting process are as below, with emphasis on casting from rapid prototyping patterns. NITC

Fig: 1- Investment casting process NITC

The investment casting process begins with fabrication of a sacrificial pattern with the same basic geometrical shape as the finished cast part Patterns are normally made of investment casting wax that is injected into a metal wax injection die. Fabricating the injection die is a costlier process and can require several months of lead time.

Once a wax pattern is produced, it is assembled with other wax components to form a metal delivery system, called the gate and runner system. The entire wax assembly is then dipped in a ceramic slurry, covered with a sand stucco, and allowed to dry. The dipping and stuccoing process is repeated until a shell of ~6-8 mm (1/4-3/8 in) is applied. NITC

Fig. 2- Investment casting process - dewaxing NITC

Once the ceramic has dried, the entire assembly is placed in a steam autoclave to remove most of the wax. After autoclaving, the remaining amount of wax that soaked into the ceramic shell is burned out in a furnace. At this point, all of the residual pattern and gating material is removed, and the ceramic mold remains.

The mold is then preheated to a specific temperature and filled with molten metal, creating the metal casting. Once the casting has cooled sufficiently, the mold shell is chipped away from the casting.

Next, the gates and runners are cut from the casting, and final post-processing (sandblasting, machining) is done to finish the casting.

(The CAD solid model, the shell, and the pattern produced in the QuickCast process is schematically shown) NITC

Fig. 3. Investment casting process Preheating and pouring NITC

SHELL MOULDING-DEVELOPED IN 1940sTHERMOSETTING RESINS USED AS BINDERSPHENOL FORMALDEHYDE(3% BY WT.OF SAND) 15% HEXAMETHYLENE TETRAMINE ADDED TO GIVE THERMOSETTING PROPERTYRESIN SETS AT ABOUT 2500 C (1750 C- 3700 C)SHELL OF 4 to 9 MM FORMS SHELL MOULDING MACHINES USEDPATTERN MADE OF METALMOUNTED ON MATCH PLATES WITH GUIDE PINS

PATTERN HEATED TO 2500 CCLEANED WITH COMPRESSED AIR, PETROLEUM SPIRIT APPLIEDPATTERN INVERTED, PLACED IN DUMP BOX CONTAINING SAND MIX , LOCKEDDUMP BOX INVERTED, KEPT FOR A FEW MINUTES, (1-3 MINS) SHELL FORMS RE-INVERTED, SHELL FORMED IS TRIMMED, REMOVED USING GUIDE PIN EJECTION,ANOTHER HALF ASSEMBLED, READY FOR POURING

SHELL MOULDING - SEQUENCES

+ AND - POINTSAdvantagesLeast moulding material (about 5%)Quick operating cycle (

V-Process

1. Pattern (with vent holes) is placed on hollow carrier plate.2. A heater softens the .003" to .007" plastic film. Plastic has good elasticity and high plastic deformation ratio.3. Softened film drapes over the pattern with 300 to 600 mm Hg vacuum acting through the pattern vents to draw it tightly around pattern.4. Flask is placed on the film-coated pattern. Flask walls are also a vacuum chamber with outlet shown.5. Flask is filled with fine, dry unbonded sand. Slight vibration compacts sand to maximum bulk density.6. Sprue cup is formed and the mold surface leveled. The back of the mold is covered with unheated plastic film.7. Vacuum is applied to flask. Atmospheric pressure then hardens the sand. When the vacuum is released on the pattern carrier plate, the mold strips easily.8. Cope and drag assembly form a plastic-lined cavity. During pouring, molds are kept under vacuum.9. After cooling, the vacuum is released and free-flowing sand drops away leaving a clean casting, with no sand lumps. Sand is cooled for reuse. NITC

Benefits Of Using The V-Process:

Very Smooth Surface Finish 125-150 RMS is the norm. Cast surface of 200 or better, based on The Aluminum Association of America STD AA-C5-E18. Excellent Dimensional Accuracy Typically +/-.010 up to 1 inch plus +/-.002 per additional inch. Certain details can be held closer. +/-.010 across the parting line. Cored areas may require additional tolerances. Zero Draft Eliminates the need for machining off draft to provide clearance for mating parts and assembly. Provides consistent wall thickness for weight reduction and aesthetic appeal. Allows for simple fixturing for machining and inspection.

NITC

With wall thickness to 0.12 in., this casting requires moderate strength, good stability and resistance to stress-corrosion cracking to 600F (316C). This casting exhibits mechanical properties at room temperature of 32-ksi tensile strength, 24-ksi yield strength and 1.5% elongation, while maintaining a 16-ksi tensile strength and 4% elongation at 600F. The component's as-cast surface finish meets the customer's requirements, and the invest casting process reduced the customer's finishing and machining costs.

SEMI-PERMANENT MOLD CASTINGSemi-permanent mold is a casting process - producing Aluminum alloy castings - using re-usable metal molds and sand cores to form internal passages within the casting.Molds are typically arranged in two halves - the sand cores being put into place before the two halves are placed together. The molten metal flows into the mold cavity and surrounds the sand core while filling the mold cavity.When the casting is removed from the mold the sand core is removed from the casting leaving an internal passage in the casting.

The re-usable metal molds are used time and again, but the sand cores have to be replaced each time the product is cast, hence the term semi-permanent molding.Semi-permanent molding affords a very high precision quality to the casting at a reduced price compared to the sand casting processes.

NO BAKE CASTING The No-Bake Sand Casting process consists of sand molds created using a wood, metal or plastic pattern. Sand is mixed with a urethane binder and deposited into a box containing the pattern (and all necessary formers and inserts) for pouring. Filling a wood mold with sand

PRODUCTS 15. TENSIONER PULLEY Material: Gray iron Process: Nobake sand Casting Supplier: Wellsville Foundry, Wellsville, Ohio

This 175-lb component is used as a brake that puts tension on a 4 ft. wide roll of rubber feeding into a tire press. Converted from a steel fabrication (two ring burn-outs with spokes), the foundry provided the end-user with a 50% cost savings.

Previously made from two steel stampings welded together with two tube sections and subsequently tin-plated for corrosion resistance (r), this bronze cast component (l) now is a one-piece permanent mold casting. The cast component (l) exhibits good corrosion resistance (without plating or painting), 50 ksi yield strength and 95 ksi tensile strength. By converting this part to a copper-based permanent mold casting, the

CASTING TECHNIQUES FOR SINGLE CRYSTAL GROWING (S.C.G.)POLYCRYSTALLINE- ANISOTROPYSINGLE CRYSTAL- PROPERTIES SAME IN ALL DIRECTIONSCASTING OF GAS TURBINE BLADES BY S.C.G.

CASTING TECHNIQUES FOR SINGLE CRYSTAL GROWING (S.C.G.)CONVENTIONAL USE OF CERAMIC MOULDGRAINS WITH THE ABSENCE OF THERMAL GRADIENTDIRECTIONAL SOLIDIFICATION PROCESS CERAMIC MOULD PREHEATED. MOULD SUPPORTED BY WATER COOLED CHILL PLATES.AFTER POURING, ASSEMBLY LOWERED CRYSTALS GROW AT CHILL PLATE SURFACE UPWARD. COLUMNAR GRAINS FORM

CONVENTIONAL

USE OF CERAMIC MOULDGRAINS- AS WITH THE ABSENCE OF THERMAL GRADIENTPRESENCE OF GRAIN BOUNDARIES- MAKES STRUCTURE SUSCEPTIBLE TO CREEP AND CRACKING ALONG BOUNDARIES

DIRECTIONAL SOLIDIFICATION PROCESS, (1960s) CERAMIC MOULD PREHEATED. MOULD SUPPORTED BY WATER COOLED CHILL PLATES.AFTER POURING, ASSEMBLY LOWERED CRYSTALS GROW AT CHILL PLATE SURFACE UPWARD. COLUMNAR GRAINS FORMBLADE DIRECTIONALLY SOLIDIFIED WITH LONGITUDINAL- NOT TRANSVERSE- GRAIN BOUNDARIES. THUS STRONGER

SINGLE CRYSTAL BLADES, (1967),MOULD HAS CONSTRICTION IN THE SHAPE OF CORK SCREWTHIS CROSS SECTION ALLOWS ONLY ONE CRYSTAL TO FIT THROUGHWITH THE LOWERING, SINGLE CRYSTAL GROWS UPWARD THROUGH CONSTRICTIONSTRICT CONTROL OF MOVEMENT NEEDEDTHERE IS LACK OF GRAIN BOUNDARIES, MAKES RESISTANT TO CREEP AND THERMAL SHOCK.--EXPENSIVE

SINGLE CRYSTAL GROWING (S.C.G.)FOR SEMICONDUCTOR INDUSTRYCRYSTAL PULLING METHOD- CZOCHRALSKI PROCESSSEED CRYSTAL DIPPED INTO THE MOLTEN METAL, PULLED SLOWLY, (AT 10 m/ s), WITH ROTATION

LIQUID METAL SOLIDIFIES ON THE SEED AND CRYSTAL STRUCTURE CONTINUED THROUGHOUT

FLOATING ZONE METHODPOLYCRYSTALLINE ROD (SILICON)- ALLOWED TO REST ON A SINGLE CRYSTALINDUCTION COIL HEATS THE PIECESCOIL MOVED UPWARD SLOWLY (20 m/ s) SINGLE CRYSTAL GROWS UPWARD WITH ORIENTATION MAINTAINEDTHIN WAFERS CUT FROM ROD, CLEANED, POLISHEDUSE IN MICROELECTRONIC DEVICES

PLASTER MOULD CASTINGFor casting silver, gold, Al, Mg, Cu, and alloys of brass and bronze.Plaster of Paris (Gypsum) (CaSo4.nH2O) used for cope and drag mouldingA Slurry of 100 parts metal casting plaster and 160 parts water used. Plaster added to water and not water to plaster. To prevent cracks, 20-30% talc added to plaster. Lime and cement to control expansionStirred slowly to form cream Poured carefully over a match plate pattern (of metal)Mould vibrated to allow plaster to fill all cavities. Initial setting at room temperature(setting time reduced by either heating or by use of terra-alba/ magnesium oxide)Pattern removedCope and drag dried in ovens at 200- 425 C(about 20 hours)Mould sections assembled

+ points Dimensional accuracy 0.008 t0 0.01 mm per mmExcellent surface finish as no sand used.. No further machining or grindingNon ferrous thin sectioned intricate castings made. - pointsLimited to non ferrous castings.(sulphur in gypsum reacts with ferrous metals at high temperatures)Very low permeability as metal moulds used. Moulds not permanent, destroyed when castings removed.

FROZEN MERCURY MOULDING (MERCAST PROCESS)Frozen Mercury used for producing precision castingsMetal mould prepared to the shape with gates and sprue holesPlaced in cold bath and filled with acetone (to act as lubricant)Mercury poured into it, freezes at 20 C, after a few minutes (10mins)Mercury Pattern removed and dipped in cold ceramic slurry bath. A shell of 3 mm is built up. Mercury is melted and removed at room temperature.Shell dried and heated at high temperature to form hard permeable shape.Shell placed in flask- surrounded by sand-, preheated and filled with metal. Solidified castings removed.

For both ferrous and non ferrous castings.(melting temperature upto 16500C) Very accurate details obtained in intricate shapesExcellent surface finish, machining and cleaning costs minimum.Accuracy of 0.002 mm per mm obtained.

But, casting process costly.Casting cost high.

DESIGN CONSIDERATIONSCAREFUL CONTROL OF LARGE NUMBER OF VARIABLES NEEDED- CHARACTERISTICS OF METALS & ALLOYS CASTMETHOD OF CASTINGMOULD AND DIE MATERIALSMOULD DESIGNPROCESS PARAMETERS- POURING, TEMPERATURE, GATING SYSTEMRATE OF COOLING Etc.Etc.

NITC

Poor casting practices, lack of control of process variables- DEFECTIVE CASTINGS

TO AVOID DEFECTS-Basic economic factors relevant to casting operations to be studied.General guidelines applied for all types of castings to be studied.

NITC

CORNERS, ANGLES AND SECTION THICKNESSSharp corners, angles, fillets to be avoided Cause cracking and tearing during solidificationFillet radii selection to ensure proper liquid metal flow- 3mm to 25 mm. Too large- volume large & rate of cooling lessLocation with largest circle inscribed critical. Cooling rate less shrinkage cavities & porosities result- Called HOT SPOTS

NITC

LARGE FLAT AREAS TO BE AVOIDED- WARPING DUE TO TEMPERATURE GRADIENTSALLOWANCES FOR SHRINKAGE TO BE PROVIDEDPARTING LINE TO BE ALONG A FLAT PLANE- GOOD AT CORNERS OR EDGES OF CASTINGDRAFT TO BE PROVIDEDPERMISSIBLE TOLERANCES TO BE USEDMACHINING ALLOWANCES TO BE MADERESIDUAL STRESSES TO BE AVOIDED

ALL THESE FOR EXPENDABLE MOULD CASTINGS.NITC

DESIGN MODIFICATIONS TO AVOID DEFECTS- AVOID SHARP CORNERS MAINTAIN UNIFORM CROSS SECTIONSAVOID SHRINKAGE CAVITIESUSE CHILLS TO INCREASE THE RATE OF COOLINGSTAGGER INTERSECTING REGIONS FOR UNIFORM CROSS SECTIONSREDESIGN BY MAKING PARTING LINE STRAIGHTAVOID THE USE OF CORES, IF POSSIBLEMAINTAIN SECTION THICKNESS UNIFORMITY BY REDESIGNING (in die cast products)

NITC

PROPERTIES AND TYPICAL APPLICATIONS OF CAST IRONS, NON FERROUS ALLOYS etc. Tables shall be suppliedNITC

General Cost Characteristics of Casting ProcessesNITC

PROCESSCOSTPRODUCTION RATE (pc/hr)DIEEQUIPMENTLABOURSANDLLL-M

THIXOTROPIC DIE CASTING

Some of the die-cast joints used in the Insight's aluminum body are made using a newly developed casting technology invented by Honda engineers, called Thixotropic Die Casting.

Thixotropic Die Casting uses aluminum alloy that has been heated to a semi-solid condition, instead of the molten, liquid state normally used in die casting.

Pieces made with molten aluminum must be more highly processed and refined before casting.

NITC

However, Thixotropic Die Casting requires less energy for smelting (an important consideration since aluminum is more expensive than steel), and owes much of its strength to the controlled formation of discrete aluminum crystals within the metal casting.

Thixotropic casting involves vibratory casting of highly thixotropic slips of very high solids loadings that are fluid only under vibration, using porous or nonporous molds.

It is quite different from other conventional and new methods for solid casting ceramics, including vibroforming, vibraforming, in situ flocculation, direct coagulation casting, and gel casting. This is demonstrated in Table 1. NITC

Table 1. Thixotropic casting in comparison with the alternatives.

NITC

Thixotropic casting is a little-known derivative of solid slip casting, having reportedly been used in the refractories industry in the early 1970's. Since then, the refractories industry has since largely embraced low-cement and ultra-low-cement castables. It is also a suitable process for forming ceramic matrix composites and metal-ceramic functionally gradient materials. Thixotropic casting involves vibratory casting of highly thixotropic slips of very high solids loadings that are fluid only under vibration, using porous or nonporous molds. It is quite different from other conventional and new methods for solid casting ceramics, including vibroforming, vibraforming, in situ flocculation, direct coagulation casting, and gel casting. (This is demonstrated in Table 1)

NITC

INSPECTION OF CASTINGSSEVERAL METHODSVISUALOPTICAL- FOR SURFACE DEFECTSSUBSURFACE AND INTERNAL DEFECTS THROUGH NDTs & DTsPRESSURE TIGHTNESS OF VALVES BY SEALING THE OPENING AND PRESSURISING WITH WATER

NITC

NDTsMethods of testing Destructive-

Non destructive-RadiagraphicUltrasonic NITC

Non Destructive Testing with Ultrasonics for flaw Detection in Castings,Weldments, Rails, Forged Components etc.

NITC

INSPECTION OF CASTINGSSEVERAL METHODSVISUALOPTICAL- FOR SURFACE DEFECTSSUBSURFACE AND INTERNAL DEFECTS THROUGH NDTs & DTs

PRESSURE TIGHTNESS OF VALVES BY SEALING THE OPENING AND PRESSURISING WITH WATER

EXERCISE

PROCESS FLOW CHARTRECEIPT OF ORDER(REVIEW)ARE THE TERMS ACCEPTED? NO COMMUNICATE- NEGOTIATEYESPREPARE WORK ORDERWORK ORDER TO Q.C, INSPECTION, PLANNING, METHODS, PRODUCTION AND DESPATCH

PRODUCTION PLANMETHOD DRAWING, QA DATA, PATTERN PLAN MOULDINGWORK ORDER, CORE MAKING, HEAT CONFORMATION MELTING AND POURING FOR THESE, LAB TEST REPORTSKNOCK OUTSTAGE ISPECTION- NOT OK, REJECT OK, SHOT BLASTING, GAS CUTTING/ARC CUTTING ASTM STANDARDSHEAT TREATMENTROUGH FETTLING, FINISH FETTLING, INSPECTION

NDT- CUSTOMER REPORT, NOT OK, WELDING & RECTIFICATION WELDING LOG SHEET RE-INSPECTION, NOT OK- REJECTMACHINE - IF REQUIREDSTRESS RELIEFHYDRAULIC TESTS Etc.TEST CERTIFICATE DESPATCH DOCUMENTS, PACKING, Etc. Etc.