Casting 1 - Copy

8/8/2019 Casting 1 - Copy

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Contents of Todays Lecture Introduction to Sand Casting process.

Importance of casting process. Casting terminology.

Pattern Materials and types of patterns.


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Casting Processes


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The Casting Process Material is given desired shape bymelting and subsequent solidification.

This process exploit the fluidity of aliquid as it assumes the shape of the

prepared container and solidifies after

cooling.

Sometimes the molten material is

modified in its chemical composition.


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Importance of Metal Casting

Process Very large sized components can be

produced by casting. (As it becomes

impossible to have work holding device or onany machine tool for such components).

When it becomes impossible to machinecomponents of very hard materials.

Parts having very complicated featuresdemand high operator skills, production ratebecomes slower and maintaining consistentquality becomes difficult in mass production.


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Salient Features of Casting

Process Components of intricate and odd shapes canbe produced.

Almost all metals and alloys can be casted.

Good mechanical and service properties. Large number of Castings can be produced

at one time.

Complicated sequence of operations are

involved. Accuracy not so high, requires further

machining.

Process control is difficult that may lead toserious casting defects.


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Classification of Casting Process The casting is conveniently classified by knowing the

molds, patterns and cores are permanent orexpendable.

Expendable mold Casting: In this the mold cavity isobtained by consolidating a refractory material aroundthe pattern.

-The mold has to be broken out to take out the casting.

-Sand casting, Shell casting and Investment castingcome under this category.

Permanent mold casting: In this category the mold isused repeatedly and is not destroyed after solidificationof casting.

-Non ferrous parts of small and medium mass can beconveniently casted by this method.

-The castings produced have good surface finish andaccuracy.

-Die casting and permanent casting fall under this

category.


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Applications of Casting.

Automobile engine blocks, cylinder blocksof such engines, pistons and piston ring

etc.

Machine tool beds, rollers for mills etc. Housings in power plants.

Piping in water supply and sewer lines.

Sanitary fittings.

Agricultural parts.


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Applications Fields of Castings

Transport: Automobiles, aerospace,railways andshipping.

Heavy Equipments: Construction, farming andmining.

Machine Tools: machining, casting, plastics,forming, extrusion etc.

Plant Machinery: Chemical, petroleum, sugar, textileand thermal plants.

Electrical Machines: motors, generators, pumps andcompressors.

Municipal castings: Pipes, joints, valves and fittings.

Household: Appliances, kitchen and gardeningequipments, furniture and fittings.

Art Objects: Sculptures, idols, lamp stands anddecorative objects.


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Applications of Casting.

Crank case

Cylinder heads

Handle Bar

Motor Casing


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Casting Applications


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The Sand Casting Process

Most widely used method of casting. Sand molds are used to prepare the

casting.

Sand molds are made by ramming the

sand in a metallic or wooden flask. The important sequence of operations are:

Pattern Making

Mold making.

Core making Melting and pouring of metal.

Cooling and solidification.

Cleaning of casting.


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Casting Terminology


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Casting Terminology

Pattern: An approximate duplicate of the finalcasting.

Mold: The cavity, made in suitable refractorymaterial into which molten metal can be poured.

Flask: The box that contains molding aggregate.

Cope: Top half of the pattern, flask, mold or corein a two part mold.

Drag: Bottom half of the pattern, flask, mold orcore in a two part mold.

Core: A sand shape that is inserted into mold toproduce internal features of the casting.

Core print: A region added to the pattern or moldthat is used to locate and support the core in the

mold cavity.


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Casting Terminology Mold cavity: The mold material and the core

combine to give the mold cavity. Riser: It is a hole cut in the cope to permit the

molten metal to rise above the highest point in thecasting. The riser facilitates feeding of molten metalinto main casting to compensate for the shrinkage

during solidification. Gating System: A network of channels used to

deliver the molten metal to the mold cavity.

Poring cup: The portion of gating system thatinitially receives the molten metal from pouring

vessel. Sprue: The vertical portion of the gating system

(below the poring cup) through which the moltenmetal flows down.

Runners: The horizontal channels through whichmolten metal flows.


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Sprue, Gates and Runners


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Sprue, Gates and Runners with

Riser and Casting


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Pattern In Casting The pattern is the model of casting. It

is used for making impression in the

sand.

The mold cavity is the replica of castingrequired.

This is made in the desired shape and

size with the help of a pattern. Thus pattern making is a very important

step in casting.


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Pattern Materials Wood: Widely used due to low cost, easy availability

and low weight.It also can be worked and shaped easily, joined atlesser costs.

It has a lower life and suitable for very largecastings.

Metal: Closer dimensional tolerances and largequantity of production.

Longer life and do not absorb moisture.

Aluminum is the widely used material.

Plastic: Low weight, easier formability, smoothsurfaces and durability.Do not absorb moisture and dimensionally stable.

Good corrosion resistance.

Polystyrene: Disposable patterns can be made fromPolystyrene.

It changes to gaseous state on heating.


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Characteristics of pattern

Materials

Characteristics Wood Al Steel Plastic CIMachinability E G F G G

Wear

Resistance

P G E F E

Strength F G E G G

Weight E G P G P

Reparability E P G F G

Resistance toCorrosion

E E P E P

Resistance to

Swelling

P E E E E

E=Excellent, G=Good, F=Fair; P=Poor,


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Some patterns


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Types of Patterns Solid pattern.

Split Pattern.

Loose Piece pattern.

Gated pattern.

Match plate Pattern.


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Solid or One Piece Pattern

This type ofpattern ismade in a single piece

and hence is least

expensive.

It has no partings and

loose pieces.

It is generally used for

casting of simpleshapes.


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Split Pattern


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Split Pattern showing two sections

Together

Separated


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Acase study

Split Patterns Cavity made in Drag


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Final Pulley


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Split Pattern This pattern is divided into two segments

along a single parting line.

The bottom segment of the pattern is

positioned in the flask and lowerportion ofthe mold ( Drag) is formed.

This flask is then inverted and uppersegment of the pattern is attached.

The two parts are aligned by tapered pinscalled as Dowel pins.


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ALoose Piece Pattern

Loose Pieces


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Loose Piece Pattern This pattern is used when one piece or loose piece

pattern couldnt be removed from the moldingsand.

This type of pattern is used when casting has theprojections, undercuts and other suchconfigurations.

The portion of the pattern that may causeobstruction is made in loose piece.

After ramming is over, the main part is removedfollowed by the loose pieces through the gapgenerated by main pattern.

It is highly skillful and expensive job.


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A Loose Piece PatternLoose Pieces held by pins


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Gated pattern Small sized casting can be produced inone mold using gated patterns.

Herein a number of small patterns of thedesired casting are attached to a single

runner.

Generally they are made of metal

considering the strength criterion.


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Gated patternPattern

Desired casting


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Match Plate Pattern


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Match Plate pattern A match plate is one on which cope and dragsegments of split patterns are mounted.

Using the holes that align with the pins on the

flask, the match plate can be placed betweenthe upper and the lower flask.

Match plate patterns speed up the production

and maintain the uniformity in size and shape

of casting.


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Pattern Allowances Pattern allowance is the difference between

the dimensions of the casting and the pattern.

These are classified as:

Shrinkage Allowance.

Machining Allowance.

Draft or Taper Allowance.

Rapping or Shake Allowance.

Distortion Allowance.


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Shrinkage Allowance

This allowance accounts for the shrinkage of

metal on solidification and contraction on

further cooling to room temperature.

Providing this allowance, the lineardimensions of the pattern are increased in

respect to the finished casting to be obtained.

It is expressed as mm/m.


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Typical Shrinkage Allowances

Metal Shrinkage

Allowance

CI 10mm/m

Brass 15mm/m

Steel 20mm/m

Zinc lead 25mm/m


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Machining Allowances

This represents the extra material added tocertain parts of the casting to to enable theirmachining or finishing to the required size,

accuracy and surface finish. This allowance is left on the surfaces whichhave to undergo further machining liketurning or shaping.

It depends upon the material used for casting,its size, machining process used, degree ofaccuracy and surface finish desired.


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Draft or Taper Allowance


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Draft or taper Allowance


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Draft or taper Allowance This allowance is provided in terms of a

taper on the pattern in a direction

parallel to its removal.

This allows the removal of pattern

without damaging the mold cavity.

It is generally kept between 10 to 30 .


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Distortion Allowance The unequal rate of cooling in different parts

of the casting may tend to distort the casting.

The pattern is distorted suitably to yield acorrect casting..

This allowance is provided in such a way thaton distortion the casting assumes the correct

shape. Generally this is required in case of flat, long,U and V shaped castings.


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Rapping or Shake Allowance.

To remove the pattern out from the mold

cavity, slight shaking of the pattern is

done.

This results in the increase in the mold

cavity .

To account for this, the pattern is made

slightly smaller.

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Casting 1 - Copy