Metal casting

Classification of casting ProcessesCasting processes can be classified into following FOUR categories:g p g g

1. Conventional Molding Processes Green Sand Moldinga) Green Sand Molding b) Dry Sand Moldingb) Dry Sand Moldingc) Flask less Molding

2. Chemical Sand Molding Processesa) Shell Moldinga) Shell Moldingb) Sodium Silicate Moldingc) Investment castingd) No-Bake Molding

3. Permanent Mold Processesa) Gravity Die castingb) Low and High Pressure Die Casting

4. Special Casting Processesa) Lost Wax b) Ceramics Shell Moldingc) Evaporative Pattern Casting d) Vacuum Sealed Moldinge) Centrifugal Casting

Pattern

Functions

Obtaining a shaped mold cavity for casting

Molding the gating system

Establishing the parting line

Making core prints

Establishing locating points

Minimizing the casting defects attributable to the patternp

Providing economy in molding.

Characteristics Wood Aluminium Cast Iron Polyurethane

Machinability E G F GMachinability E G F G

Wear Resistance P G E E

Strength P G E FStrength P G E F

Repairability E F G E

Corrosion E E P ECorrosion Resistance

E E P E

E E ll t G G d F F i P PE - Excellent, G - Good, F - Fair, P - Poor

Patterns Loose pattern Loose pattern Match plate pattern Cope and drag Pattern Expendable patterns

1. Loose pattern

simplest least expensive type simplest, least expensive type

reusable pattern

also known as one-piece patterns or solid patterns

suited only for low-quantity production

generally used for experimental or prototypes

made either in one piece or in two pieces

gates and risers are cut by hand

2. Match plate pattern

pattern is split and the two portions are fixed on the opposite sides of a plate, called the match plate.

match plate confirms to the parting line of the mold match plate confirms to the parting line of the mold size of the match plate corresponds to the size of the molding flask used for moderate to high-volume production of small- and medium-

size castingssize castings considerable dimensional accuracy

3. Cope and Drag pattern

cope and drag portions of a split pattern are mounted or integrally

cast on separate plates

used for high-volume production or for the production of large

castings

Pattern cost is higher than for match plate patterns

t t l ldi t ti b l total molding cost per casting may be lower

cope and drag impression is simultaneously made

Pattern Allowances

Sh i k ll Shrinkage allowance

Machine finish allowance

Draft allowance Draft allowance

Distortion allowance

Sh i k llShrinkage allowance

• It is a correction for shrinkage of the metal• Shrinkage allowance is the amount the pattern be made larger than theShrinkage allowance is the amount the pattern be made larger than the

casting to provide for contraction

Liquid shrinkage

Solidification shrinkage

Solid shrinkage

Pattern maker’s shrinkage rule is a special scale which is generally used in foundries

Machine finish allowance

• The amount the dimensions on a casting is made oversize to• The amount the dimensions on a casting is made oversize to provide stock for machining

• This is influenced by the metal, casting design, method of casting and cleaning

Draft allowanceDraft allowance

• Draft is the taper allowed on the vertical faces of a pattern for easy removal from mold.y

• Taper is generally 1.5o provided for hand molding

• Machine drawn patterns requires 1o taper.

Distortion allowance

• Large casting (flat plates , dome  or U‐shaped casting) distort when d d f f t ttproduced from a perfect pattern.

• To take care of this the pattern may be intentionally distorted.

Sand molding

Drag half of mold made by hand. Drag is ready to be rolled over in preparation for making the cope

Cope mold rammed up.

Procedure Place the pattern on the molding board Place the pattern on the molding board.

The drag part of the molding flask is placed on the board

Dry facing sand or dust is sprinkled over the board andy g ppattern to provide a non sticky layer.

Molding sand is then riddled in to cover the pattern with thefi h h d i l l fill dfingers; then the drag is completely filled.

The sand is then compacted firmly in the drag by means ofhand rammers. The ramming must neither be too hard orhand rammers. The ramming must neither be too hard orsoft.

After the ramming is over, the excess sand is leveled off withh b k k da straight bar known as a strike rod.

With the help of vent rod, vent holes are made in the drag tothe full depth of the flask as well as to the patternthe full depth of the flask as well as to the patternto facilitate the removal of gases during pouring andsolidification.

The finished drag flask is now rolled over to the bottom board exposing the patternexposing the pattern. 

Cope half of the pattern is then placed over the drag pattern with the help of locating pins. The cope flask on the drag is located aligning again with the help of pins 

The dry parting sand is sprinkled. A sprue pin for making the sprue passage is located at a small p p g p p g

distance from the pattern. Also, riser pin, if required, is placed at an appropriate place. 

Repeat the operations like filling ramming and venting Repeat the operations like filling, ramming and venting. The sprue and riser pins are removed first and a pouring basin 

is scooped out at the top to pour the liquid metal.  h f h d d d d f Then pattern from the cope and drag is removed and facing 

sand in the form of paste is applied all over the mold cavity and runners. 

The mold is assembled and is ready for pouring 

Shell molding processShell molding process

Advantages of shell molding Much superior finish dimensional accuracy and consistency in Much superior finish, dimensional accuracy and consistency in

producing quality casting compared with even mechanizedgreen sand moulding. Shell molding process can accuratelyg g g p yreplicate even the fine pattern details and dimensions andhence can be used for precision casting

Capital outlay on sand preparation plant is not essential. Good utilization of space Low sand to metal ratio Low sand to metal ratio The mould is separated from the pattern without the need to

enlarge the cavity.g y Only a small quantity of sand is used hence cheaper process. Costly Zircon sand, which has better cooling characteristics

and lower thermal expansion than silica sand, can also be usedsince the sand consumption is less.

Mould coating are unnecessary Lightweight moulds are produced which are readily handled Lightweight moulds are produced which are readily handled

and have good storage characteristics Skilled labor is not requiredq Shells have excellent breakdown at the knockout stage Lower cleaning at fettling costs

Disadvantages The raw material are relatively expensive High cost of The raw material are relatively expensive. High cost of

pattern equipment and resin Relative inflexibility in gating and risering as these have toy g g g

be provided in the shell itself. The size and weight range of casting is limited h i f hi h b The process generates noxious fumes which must be

effectively extract

Investment casting

Investment casting Process

1. Master pattern 

2 Mould making2. Mould making 

3. Producing wax patterns  

4. Wax pattern assembly 

5. Investment 

6. Dewax

7. Burnout & preheating

8. Pouring

9 Removal9.Removal

Wax pattern

Finished Turbocharger Rotor

Investment casting

PLASTER MOULDING

The conventional plaster mould casting process. The foamed plaster process. The Antioch process The Antioch process

Conventional Plaster mold casting

Plaster of paris is mixed with water to produce a slurry. Slurry is poured over a permanent pattern placed inside a moulding

box and allowed to set to form a rigid mold. The pattern is stripped, and dried at an elevated temperature to

remove free and chemically combined waterremove free and chemically combined water. Molten metal is poured in to the mold and allowed to solidify.

The method produces a strong, dense, but inherently impermeablemould..

Foam Plaster molding process

Plaster of paris is mixed with water to produce a slurry. Air is incorporated in to the slurry during the mixing stage.p y g g g The Slurry with air bubbles is poured on the permanent pattern

and allowed to set. It is possible to produce a mould in which 50% of the volume

consists of air bubbles. This increases the permeability from 1 or 2 units to between

15-30 units, as measured by the AFS permeability test. The process provides the more economical use of the plaster The process provides the more economical use of the plaster.

However, the inherent strength of the mould is reduced and ifcomplex shapes are to be produced than flexible patternscomplex shapes are to be produced than flexible patternstraditionally rubber may be required.

Advantages of plaster molding• The ability to produce complex shapes• The ability to produce complex shapes.• The ability to produce thin section castings.• The excellent replication of pattern detail.• The ability to produce castings which are dimensionally accurate.• The ability to produce castings with good surface finish.• The minimization of residual stresses and distortion in castings.g

Disadvantages• Poor productivity due to lengthy processing problems.• The need for multiple patterns to improve moulding productivity• The need for multiple patterns to improve moulding productivity.• The requirement for close control production process.• The need for special procedure to overcome the problems of poor

mould permeability.• The possibility of impaired mechanical properties arising as a

result of slow cooling of the casting.g g• The mould materials are not reclaimable.

Permanent mold castingPermanent mold casting

Low pressure Permanent mold  (LPPM) process

Simple split  permanent mold process

True centrifugal casting

Semi‐centrifugal casting

High pressure die‐casting

The dies prepared and assembled in the die casting machine.

Lubricant is sprayed on the mold cavity This is for easyremoval of the casting from the die as well as for controllingremoval of the casting from the die as well as for controllingthe die temperature. The dies are then closed.

Molten metal is injected into the dies (till the mold cavity isfilled) under high pressure ( in the range 10 and 175 MPa).The pressure injection leads to a quick filling of the die sothat the entire cavity fills before any part of the castingthat the entire cavity fills before any part of the castingsolidifies The pressure is maintained until the castingsolidifies.

The dies are then opened and the shot solidified metal isejected by the ejector pins.

The casting is trimmed or sawed or ground to remove the The casting is trimmed or sawed or ground to remove thescrap portion (like gate, runners, sprue, etc).

Ad t f hi h di tiAdvantages of high pressure die casting

Excellent dimensional accuracy.Smooth cast surfaces with Ra values in the range 1–2.5 μm).Thinner walls of the order of even 0.75 mm thick can be cast ascompared to sand and permanent mold casting.compared to sand and permanent mold casting.Inserts such as threaded inserts, heating elements, high strengthbearing surfaces, etc. can be cast-in.R d li i t d hi i tiReduces or eliminates secondary machining operations.Rapid production rates.Due to pressure application, high solidification rate and absence ofgas defects, high strengths of the order of 400 MPa can be obtainedfor castings.Low fluidity metals can also be cast..Low fluidity metals can also be cast..

Hot chamber die-casting

Initially the piston of the machine is retracted. This allows the molten metaly pto flow and fill the cylinder up to the goose neck through the intake port ,which allows the molten metal to fill the "gooseneck".

The externally powered piston then forces this metal out of the gooseneckinto the die and allow to solidify under pressure.

Cycle time is very fast (approximately 10-15cycles a minute).

Disadvantage high-melting point metals cannot be

utilized. Aluminium cannot be used because it

picks up some of the iron while in the molten pool.

Used primarily for zinc, tin, and lead based alloys

Hot chamber die‐casting

Cold chamber high pressure die casting

In cold chamber die casting the molten charge is ladled from a furnaceIn cold chamber die casting, the molten charge is ladled from a furnacein to the shot sleeve of the machine. Normally more material than thatrequired for filling the cavity is taken. This is to supplement shrinkageduring solidification A hydraulically or pneumatically operated plungerduring solidification. A hydraulically or pneumatically operated plungerpushes the metal in to the die under a pressure of up to 70 MPa tillsolidification is complete.

• High volume cost-effective production with consistent quality• Eliminating costly post-casting operations. Hence reduced cost• Easily manufacture complex Net shaped parts with tight tolerances• Easily manufacture complex Net shaped parts with tight tolerances• Lower tool cost and longer tool life• Lowest total acquisition cost

Cold Chamber die‐castingCold Chamber die casting

Continuous casting process

Continuous casting process

Molten metal from the furnace is transferred in to ladle. The ladle is transported to the top of the continuous casting machine. Hot metal is transferred from ladle to a Tundish. Molten metal from the tundish is fed continuously from the top into a

water cooled copper mold where it solidifies while in contact with the mold (primary solidification).mold (primary solidification).

A thin shell of molten metal in contact with the mold solidifies. Solidified metal is pulled out of the mold at a constant pull velocity. The mold oscillation is provided to prevent the metal sticking to the mold The mold oscillation is provided to prevent the metal sticking to the mold. The partially solidified metal is allowed to solidify while moving through a

series of water sprays. The uninterrupted supply of the molten metal at the mold inlet and mold The uninterrupted supply of the molten metal at the mold inlet and mold

exit leads to a continuous production of slabs or billets. After complete solidification , the strand passes through straightening

rolls and withdrawal rollsrolls and withdrawal rolls. Finally, the strand is cut into predetermined lengths by mechanical

shears or by travelling oxyacetylene torches.