basic of metal casting

MEL120: Manufacturing Practices 1


Clay Moulding


Metal Casting

Sunil Jha Room No. 351, Block III [email protected] Ph. 1125 web.iitd.ac.in/~suniljha


Steps in Casting

Pattern and Mould Melting and Pouring Solidification and Cooling Removal, Cleaning, Finishing and Inspection


Casting Process Flow

Casting Process Overview


Sand Mould

Casting

Ladle

Parting Line

Molten Metal


Pattern and Mould A pattern is

a replica of the final product and is used for preparing mould cavity

made of wood or metal

Pattern Cope Pattern

Drag Pattern


Pattern and Mould Mould cavity

which contains molten metal is essentially a negative of the final product Mould material

should posses refractory characteristics and withstand the pouring temperature

When the mold is used for single casting, it made of sand and

known as expendable mold

When the mold is used repeatedly for number of castings and is made of metal or graphite are called permanent mould

For making holes or hollow cavities inside a casting, cores made of sand are used.


Pattern and Mould

Melting and Pouring



Melting and Pouring Several types of furnaces are available for melting

metals Furnace selection depends on the type of metal, the maximum temperature required and the rate and the mode of molten metal delivery.

Before pouring, provisions are made for the escape of dissolved gases.

The gating system should be designed to minimize the turbulent flow and erosion of mould cavity.

The other important factors are the pouring temperature and the pouring rate.


Solidification & Cooling The properties of the casting significantly depends

on the solidification time cooing rate. Shrinkage of casting, during cooling of solidified

metal should not be restrained by the mould material, otherwise internal stresses may develop and form cracks in casting.

Proper care should be taken at the design stage of casting so that shrinkage can occur without casting defects.


Removal, Cleaning, Finishing and Inspection

After the casting is removed from the mould it is thoroughly cleaned and the excess material usually along the parting line

and the place where the molten metal was poured, is removed using a potable grinder.

White light inspection, pressure test, magnetic particle inspection, radiographic test, ultrasonic inspection etc. are used

Removal, Cleaning, Finishing



Pattern & Mould

Open and Closed Mould

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Sand Casting

Expendable Mould Permanent Pattern

Sand Casting



Chaplets: To avoid Core Shifting

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Patterns Choice of pattern depends on: Configuration of casting Number of casting required

Pattern Types Single-piece pattern Split pattern Follow board pattern Cope and drag pattern Match plate pattern Loose-piece pattern Sweep pattern Skeleton pattern

Pattern

Cope Pattern

Drag Pattern

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Patterns a) Split pattern,

b) Follow-board,

c) Match Plate,

d) Loose-piece,

e) Sweep,

f) Skeleton pattern


Pattern Geometry

a) Solid Pattern b) Split Pattern c) Match-Plate Pattern b) Cope and Drag Pattern

Solid Pattern (Single Piece)


Pattern

Split Pattern


Cope Pattern

Drag Pattern

Match Plate Pattern


Cope and Drag Pattern



Pattern Allowances Pattern always made larger than final job Excess dimensions – Pattern Allowance Shrinkage allowance

Contraction of casting Liquid – Pouring Temp to Freezing Temp Change of phase – Liquid to Solid Solid casting – Freezing Temp to Room temp

Draft allowance To withdraw pattern from mould

Machining allowance For final shape


Mould Material Major part of Moulding material in sand casting are

1. 70-85% silica sand (SiO2) 2. 10-12% bonding material e.g., clay etc. 3. 3-6% water Requirements of molding sand are: (a) Refractoriness – ability to remain solid at high temp (b) Cohesiveness – bonding (c) Permeability – gas flow through mould (d) Collapsibility – ability to permit metal to shrink after solidification The performance of mould depends on following factors: (a) Permeability (b) Green strength (c) Dry strength


Desirable properties of a Sand based Molding material Inexpensive in bulk qty Retain properties through transportation and storage Uniformly fills a flask or container Compacted or set by simple methods Sufficient elasticity to remain undamaged during

pattern withdrawal Withstand high temperatures and maintain its

dimensions until metal solidifies Sufficient permeable to allow the escape of gases


Desirable properties of a Sand based Molding material Sufficiently dense to prevent metal penetration Sufficiently cohesive to prevent washout of mold

material into the pour stream Chemically inert to metal being cast Good collapsibility to permit easy removal and

separation of casting Can be recycled


Effect of moisture, grain size and shape on mould quality


Melting and Pouring The quality of casting depends on the method of melting. Molten metal is prevented from oxidation by covering the molten

metal with fluxes The two main consideration during pouring are the temperature

and pouring rate

Fluidity: Capability of molten metal to fill mold cavities Characteristics of molten metal Casting parameters

Fluidity of molten metal is more at higher temperature but it

results into more amount of dissolved gases and high temperature also damage the mould walls and results into poor surface quality of the casting


Fluidity Characteristics of molten metal Viscosity Surface tension Inclusions Solidification pattern of the alloy

Casting Parameters Mold design Mold material and surface characteristics Degree of superheat Rate of pouring Heat transfer


Gating system 1. Minimize turbulent flow to reduce

• absorption of gases, • oxidation of metal and • erosion of mould surfaces

2. Regulate the entry of molten metal into the mould cavity

3. Ensure complete filling of mould cavity, and 4. Promote a temperature gradient within the casting

so that all sections irrespective of size and shape could solidify properly


Gating system

A: pouring basin B: Weir C: Sprue D: Sprue well E: Runner F: Ingates G: Runner break up H: Blind J: Riser

MEL120: Manufacturing Practices

38

Cooling and Solidification

Pure metal


Mechanism of Solidification Pure metals solidifies at a constant temp. equal to its

freezing point, which same as its melting point. The change form liquid to solid does not occur all at

once. The process of solidification starts with nucleation, the formation of stable solid particles within the liquid metal.

Nuclei of solid phase, generally a few hundred atom in size, start appearing at a temperature below the freezing temperature.

A nuclei, more than a certain critical size grows, and causes solidification.


Mechanism of Solidification By adding, certain foreign materials (nucleating agents)

the undercooling temp. is reduced which causes enhanced nucleation.

In case of pure metals fine equi-axed grains are formed near the wall of the mold and columnar grain growth takes place upto the centre of the ingot.

In typical alloy, the columnar grains do not extend upto the center of casting but are interrupted by an inner zone of equiaxed graines.

By adding typical nucleating agents like sodium, magnesium or bismuth the inner zone of equiaxed grained can be extended in whole casting.


Crystal structure in Castings


Dendrite formation

• In alloys, such as Fe-C, freezing and solidification occurs over a wide range of temp. There is no fine line of demarcation exists between the solid and liquid metal.

• Here, ‘start of freezing’ implies that grain formation while progressing towards the center does not solidify the metal completely but leaves behind the islands of liquid metals in between grains which freeze later and there is multidirectional tree like growth.


Riser Risers are added reservoirs designed to feed

liquid metal to the solidifying casting as a means for compensating for solidification shrinkage.

Riser must solidify after casting. Riser should be located so that directional

solidification occurs from the extremities of mold cavity back toward the riser.

Thickest part of casting – last to freeze, Riser should feed directly to these regions.


Why Risers? The shrinkage occurs in three stages,

1. When temperature of liquid metal drops from Pouring to Freezing temperature

2. When the metal changes from liquid to solid state, and 3. When the temperature of solid phase drops from

freezing to room temperature The shrinkage for stage 3 is compensated by

providing shrinkage allowance on pattern, while the shrinkage during stages 1 and 2 are compensated by providing risers.


Riser – Location & Types

• Top riser

•Riser located on the casting

• Side Riser

• Riser located next to the casting

• Blind risers

• Open Riser


Cleaning and Finishing 1. Casting is taken out of the mould by Shaking and the

Moulding sand is recycled often with suitable additions. 2. The remaining sand, some of which may be embedded

in the casting, is removed by means of Shot blasting. 3. The excess material in the form of sprue, runners,

gates etc., along with the flashes formed due to flow of molten metal into the gaps is broken manually in case of brittle casting or removed by sawing and grinding in case of ductile material.

4. The entire casting is then cleaned by either shot blasting or chemical pickling.

5. Sometimes castings are heat treated to achieve better mechanical properties.


Casting Defects

Defects may occur due to one or more of the following reasons: Fault in design of casting pattern Fault in design on mold and core Fault in design of gating system and riser Improper choice of molding sand Improper metal composition Inadequate melting temperature and rate of

pouring


Classification of Casting Defects

Surface Defects Blow, Scar, Blister, Drop, Scab, Penetration,

Buckle Internal Defects Blow holes, Porosity, Pin holes, Inclusions, Dross

Visible Defects Wash, Rat tail, Swell, Mis run, Cold shut, Hot tear,

Shrinkage/Shift


Surface Defects Blow is relatively large cavity produced by gases which displace molten metal from convex surface. Scar is shallow blow generally occurring on a flat surface. A scar covered with a thin layer of metal is called blister. These are due to improper permeability or venting. Sometimes excessive gas forming constituents in moulding sand. Drop is an irregularly-shaped projection on the cope surface caused by dropping of sand.


Surface Defects A scab when an up heaved sand gets separated

from the mould surface and the molten metal flows between the displaced sand and the mold.

Penetration occurs when the molten metal flows between the sand particles in the mould. These defects are due to inadequate strength of the mold and high temperature of the molten metal adds on it.

Buckle is a v-shaped depression on the surface of a flat casting caused by expansion of a thin layer of sand at the mould face.


Internal Defects The internal defects found in the castings are

mainly due to trapped gases and dirty metal.

Gases get trapped due to hard ramming or improper venting.

These defects also occur when excessive moisture or excessive gas forming materials are used for mould making.


Internal Defects Blow holes are large spherical shaped gas

bubbles

Porosity indicates a large number of uniformly distributed tiny holes.

Pin holes are tiny blow holes appearing just below the casting surface.

Inclusions are the non-metallic particles in the metal matrix,

Lighter impurities appearing the casting surface are dross.


Visible Defects

Insufficient mould strength, insufficient metal, low pouring temperature, and bad design of casting are some of the common causes.

Wash is a low projection near the gate caused by erosion of sand by the flowing metal.

Rat tail is a long, shallow, angular depression caused by expansion of the sand.

Swell is the deformation of vertical mould surface due to hydrostatic pressure caused by moisture in the sand.


Visible Defects

Misrun and cold shut are caused by insufficient superheat provided to the liquid metal.

Hot tear is the crack in the casting caused by high residual stresses.

Shrinkage is essentially solidification contraction and occurs due to improper use of Riser.

Shift is due to misalignment of two parts of the mould or incorrect core location.


Shell Molding 1. A match plate or cope-drag metal pattern is heated and

placed over a box containing sand mixed with thermosetting resin.

2. Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell.

3. Box is repositioned so that loose, uncured particles drop away.

4. Sand shell is heated in oven for several minutes for complete curing.

5. Shell mold is stripped from the pattern 6. Two halves of the shell mold are assembled, supported by

sand or metal shot in a box, and pouring is accomplished. The finished casting with sprue is removed.

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Shell Molding


Shell Molding


Shell Molding Advantages & Limitations

Shell thickness typically 9 mm is used Surface of shell mold cavity is smoother than sand mold. Easy flow of molten metal, good surface quality Finish is of the order of 2.5 micrometer. Good dimensional accuracy Can be mechanized for mass production and is very

economical Gears, valve bodies, bushings, and cam shafts are typical

products Expensive metal pattern as compared to sand casting Difficult to justify for small quantities manufacturing Possible on small to medium size parts Suitable for steel castings less than 10 kg.


Investment Casting


Investment Casting Process Steps

Pattern creation - The wax patterns are typically injection

molded into a metal die and are formed as one piece.

Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly.

The gating system forms the channels through which the molten metal will flow to the mold cavity.



Mold creation - This "pattern tree" is dipped into a

slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system.

This process is repeated until the shell is thick enough to withstand the molten metal it will encounter.

The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting.



Pouring - The mold is preheated in a furnace

to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity.

Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used.



Cooling - After the mold has been filled, the molten metal is

allowed to cool and solidify into the shape of the final casting.

Cooling time depends on the thickness of the part, thickness of the mold, and the material used.

Casting removal - After the molten metal has cooled, the mold can be

broken and the casting removed. The ceramic mold is typically broken using water

jets, but several other methods exist. Once removed, the parts are separated from the

gating system by either sawing or cold breaking (using liquid nitrogen).



Finishing - Often times, finishing operations such as grinding or sandblasting

are used to smooth the part at the gates. Heat treatment is also sometimes used to harden the final part.


Investment Casting Advantages & Limitations

Parts of greater complexity and intricacy can be cast Close dimensional control ±0.075mm Good surface finish The lost wax can be reused Additional machining is not required in normal course Al, Cu, Ni, Carbon and alloy steels, tool steels etc. are the

common materials Preferred for casting weight less than 5 kg, maximum

dimension less than 300 mm, Thickness is usually restricted to 15mm


Die Casting In Die casting the molten metal is forced to flow

into a permanent metallic mold under moderate to high pressures, and held under pressure during solidification

This high pressure forces the metal into intricate details, produces smooth surface and excellent dimensional accuracy

High pressure causes turbulence and air entrapment. In order to minimize this larger in-gates are used and in the beginning, pressure is kept low and is increased gradually


Die Casting


Die Casting Machine


Hot Chamber Casting


Cold Chamber Casting


Centrifugal Casting

A permanent mold made of metal or ceramic is rotated at high speed (300 to 3000 rpm).

The molten metal is then poured into the mold cavity and due to centrifugal action the molten metal conform to the cavity provided in the mould.


Centrifugal Casting

Castings are known for their higher densities in the outer most regions.

The process gives good surface finish Applications: pipes, bushings, gears, flywheels etc.


Comparison of Casting Processes

basic of metal casting

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