Module 3 Part II & Module 4 Part I

8/3/2019 Module 3 Part II & Module 4 Part I

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MouldingAccording to the method of removal of CastingExpendable moulding

Multiuse moulding

According to the material usedSand moulding

Investment or Precision mouldingCeramic moulding

Plaster moulding

Metallic mouling

Loam moulding

According to the method usedHand moulding

Machine moulding



Expendable mold Mold is sacrificed to remove part

Advantage: more complex shapes are possible

Disadvantage: production rates often limited by

time to make mold rather than casting itself

Multiuse Mold Mold is made of metal and can be used to make many

castings

Advantage: Higher production rates

Disadvantage: Geometries limited by need to open

mold



Sand Molding

The cavity in the sand mold is formed by packing sand

around a pattern, then separating the mold into two

halves and removing the patternThe mold must also contain gating and riser system

If casting is to have internal surfaces, a core must be

included in mold

A new sand mold must be made for each part produced



Sand Moulding



Types of Sand Mold

• Green-sand molds

Mixture of sand, clay, and water;

– “Green" means mold contains moisture at time of pouring

• Dry-sand molds

Organic binders rather than clay – And mold is baked to improve strength

• Skin-dried

Mold Drying mold cavity surface of a green-sandmold to a depth of 10 to 25 mm, using torches or

heating lamps



Types of Sand Mold

• Core-Sand molds

Mixture of sand, binders, water, and additives.

• Shell molds

Moulding process in which the mold is a thin shell of sand held

together by thermosetting resin binder

• Miscellaneous Molds Pit & Floor molds

Loam molds

Hot box molds CO2 process mold

Vacuum molds



Sand Casting

cope: top half

drag: bottom half

core: for internal cavities

pattern: positive

funnel sprue

runners gate

cavity

{risers, vents}



Shell Molding

Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder

STEP 1 A match-plate or cope-and-drag metal pattern isheated and placed over a box containing sand mixed with

thermosetting resin.

Steps in Shell Molding



Shell Molding

STEP 2 : Box is inverted so thatsand and resin fall onto the hot

pattern, causing a layer of the

mixture to partially cure on the

surface to form a hard shell

STEP 3 box is repositioned so

that loose uncured particles

drop away



Shell Molding

; Step 4: Sand shell is heated inoven for several minutes to

complete curing

STEP 5: Shell mold is stripped from the pattern;



Shell Molding

STEP 7: The finished casting with sprue removed.

STEP 6: Two halves of the

shell mold are assembled,

supported by sand or metalshot in a box, and pouring is

accomplished.



Shell molding is also known as croning process



Shell Moulding – Sand Mixture

Sand Mixture = Fine grained Quartz sand (92%)

+ Thermosetting resin (8%)

Coating Method

Fine grained Quartz sand (92%)+

Bakelite (5%)+Ethyl aldehyde (3%)

Thermosetting Resin

Phenolformaldehydes, Urea formaldehydes,

Ethyl aldehyde, etc.

Mold release agent

Silicone solutions



Advantages and Disadvantages

Advantages of shell molding:

Smoother cavity surface permits easier flow of molten

metal and better surface finish

Good dimensional accuracy - machining often notrequired

Mold collapsibility minimizes cracks in casting

Can be mechanized for mass production

Disadvantages:

More expensive metal pattern

Difficult to justify for small quantities

Shell Molding



Investment Moulding

A pattern made of wax is coated with a refractorymaterial to make mold, after which wax is melted

away prior to pouring molten metal

"Investment" comes from a less familiar definition of "invest" - "to cover completely," which refers tocoating of refractory material around wax pattern

It is a precision casting process - capable of producingcastings of high accuracy and intricate detail



Investment Casting

Steps in investment casting:STEP 1: Wax patterns are produced,

STEP 2: Several patterns are attached to a sprue to form apattern tree

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STEP 3: The pattern tree is coated with a thin layer of refractory material

STEP 4: The full mold is formed by covering the coated tree

with sufficient refractory material to make it rigid

Investment Casting



STEP 5: The mold is held in an inverted position andheated to melt the wax and permit it to drip out of thecavity,

STEP 6: The mold is preheated to a high temperature,

the molten metal is poured, and it solidifies

Investment Casting



STEP 7: The mold is broken away from the finished casting

and the parts are separated from the sprue

Investment Casting



Investment Casting



Investment Casting



Investment Casting



Investment Casting

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Steps Involved

1. A die for casting the wax is made. (Metal)

2. Wax patterns and gating systems are produced form the

metal dies by injection.3. Precoating ( Slurry- 325 mesh silica flour suspended in

ethyl silicate solution)

4. The coated assembly is placed in the mold and pouring

the investment molding mixture around the pattern.5. Dewaxing and Preheating

6. Pouring

7. Cleaning operations follow cooling of the casting

Investment Casting








• Advantages of investment Moulding

– Parts of great complexity and intricacy can be mold

– Close dimensional control and good surface finish

– Wax can usually be recovered for reuse

– Additional machining is not normally required - this is anet shape process

• Disadvantages

– Many processing steps are required

– Relatively expensive process



Refractory Coating MaterialsSilica (67%) + Ethyl Alcohol (33%)

Silica + Alumina + Sodium silicate + Polyvinyl Alcohol

Plaster of paris (60%) + Finer silica (25%) + Talc (15%) +

Water

Investment Molding Mixture:

Sand (95%) + Alumina Cement (5%) + Water (27-31 %)

Wax

Blends of Beeswax, Carnauba, Ceresin, Acrawax, Paraffin

and Resins

Investment Casting



Permanent Mold Casting Processes

• In permanent mold casting, the mold is reused

many times

• The processes include:

– Basic permanent mold casting

– Die casting

– Centrifugal casting



The Basic Permanent Mold Process

Uses a metal mold constructed of two sections designed

for easy, precise opening and closing

• Molds used for casting lower melting point alloys are

commonly made of steel or cast iron• Molds used for casting steel must be made of

refractory material, due to the very high pouring

temperatures



Permanent Mold Casting

Steps in permanent mold casting:

(1) mold is preheated and coated



Permanent Mold Casting

Steps in permanent mold casting:

(2) cores (if used) are inserted and mold is closed

(3) molten metal is poured into the mold, where it solidifies.



Advantages and Limitations

• Advantages of permanent mold casting: – Good dimensional control and surface finish

– More rapid solidification caused by the coldmetal mold results in a finer grain structure,

so castings are stronger

• Limitations:

– Generally limited to metals of lower meltingpoint

– Simpler part geometries compared to sandcasting because of need to open the mold

– High cost of mold



Applications of Permanent Mold Casting

• Due to high mold cost, process is best suited to high

volume production and can be automated accordingly

• Typical parts: automotive pistons, pump bodies, and

certain castings for aircraft and missiles• Metals commonly cast: aluminum, magnesium,

copper-base alloys, and cast iron



Die Casting

A permanent mold casting process in which molten

metal is injected into mold cavity under high pressure

• Pressure is maintained during solidification, then

mold is opened and part is removed• Molds in this casting operation are called dies; hence

the name die casting

• Use of high pressure to force metal into die cavity is

what distinguishes this from other permanent mold

processes



Die Casting Machines

• Designed to hold and accurately close two mold

halves and keep them closed while liquid metal is

forced into cavity

• Two main types: 1. Hot-chamber machine

2. Cold-chamber machine

H Ch b Di C i



Hot-Chamber Die Casting

Metal is melted in a container, and a piston injects liquid

metal under high pressure into the die

• High production rates - 500 parts per hour not

uncommon• Applications limited to low melting-point metals that

do not chemically attack plunger and other

mechanical components

• Casting metals: zinc, tin, lead, and magnesium

H Ch b Di C i




Cycle in hot-chamber casting:

(1) The die is closed and the piston rises, opening the port and

allowing molten metal to fill the cylinder.




Cycle in hot-chamber casting:

• (2) The plunger moves down and seals the port pushing the moltenmetal through the gooseneck and nozzle into the die cavity, where it

is held under pressure until it solidifies.

H t Ch b Di C ti



• 1. The die is closed and thepiston rises, opening the portand allowing molten metal tofill the cylinder.

• 2. The plunger moves downand seals the port pushing the

molten metal through thegooseneck and nozzle into thedie cavity, where it is heldunder pressure until itsolidifies.


Hot Chamber Die Casting



3. The die opens and the cores, if

any, retract. The casting remains inonly one die, the ejector side. The

plunger returns, allowing residual

molten metal to flow back through

the nozzle and gooseneck.

4. Ejector pins push the casting

out of the ejector die. As the

plunger uncovers the filling hole,

molten metal flows through the

inlet to refill the gooseneck, as in

step (1).




Cold-Chamber Die Casting Machine

Molten metal is poured into unheated chamber fromexternal melting container, and a piston injects metalunder high pressure into die cavity

• High production but not usually as fast as hot-chambermachines because of pouring step

•

Casting metals: aluminum, brass, and magnesium alloys

• Advantages of hot-chamber process favor its use on lowmelting-point alloys (zinc, tin, lead)



Cold-Chamber Die Casting

Cycle in cold-chamber casting:

(1) With die closed and ram withdrawn, molten metal ispoured into the chamber

C ld Ch b i C i



Cold-Chamber Die Casting

Cycle in cold-chamber casting:

(2) ram forces metal to flow into die, maintaining pressure duringcooling and solidification.

Die casting



Die casting

- a type of permanent mold casting

- common uses: components for

rice cookers, stoves, fans, washing-, drying

machines,

fridges, motors, toys, hand-tools, car wheels, …

HOT CHAMBER: (low mp e.g. Zn, Pb; non-alloying)

(i) die is closed, gooseneck cylinder is filled with molten metal

(ii) plunger pushes molten metal through gooseneck into

cavity

(iii) metal is held under pressure until it solidifies

(iv) die opens, cores retracted; plunger returns

(v) ejector pins push casting out of ejector die

COLD CHAMBER: (high mp e.g. Cu, Al)

(i) die closed, molten metal is ladled into cylinder

(ii) plunger pushes molten metal into die cavity

(iii) metal is held under high pressure until it solidifies

(iv) die opens, plunger pushes solidified slug from the cylinder

(v) cores retracted

(iv) ejector pins push casting off ejector die

M ld f Di C ti



Molds for Die Casting

• Usually made of tool steel, mold steel

• Tungsten and molybdenum (good refractory qualities)

used to die cast steel and cast iron

• Ejector pins required to remove part from die when it

opens

• Lubricants must be sprayed into cavities to prevent

sticking

Ad d Li i i



Advantages and Limitations

•

Advantages of die casting: – Economical for large production quantities

– Good accuracy and surface finish

– Thin sections are possible

– Rapid cooling provides small grain size and goodstrength to casting

• Disadvantages:

– Generally limited to metals with low metal points

– Part geometry must allow removal from die



Centrifugal Casting

A family of casting processes in which the mold is

rotated at high speed so centrifugal force distributes

molten metal to outer regions of die cavity

• The group includes:

– True centrifugal casting

– Semicentrifugal casting

– Centrifuge casting



True Centrifugal Casting

Molten metal is poured into rotating mold to produce a

tubular part

In some operations, mold rotation commences after pouring

rather than before

Parts: pipes, tubes, bushings, and rings

Outside shape of casting can be round, octagonal, hexagonal,

etc , but inside shape is (theoretically) perfectly round, due to

radially symmetric forces

True Centrifugal Casting



True Centrifugal CastingSetup for true centrifugal casting.



Semicentrifugal Casting

Centrifugal force is used to produce solid castings ratherthan tubular parts

• Molds are designed with risers at center to supply feed

metal• Density of metal in final casting is greater in outer

sections than at center of rotation

• Often used on parts in which center of casting is

machined away, thus eliminating the portion wherequality is lowest

• Examples: wheels and pulleys



Semicentrifugal Casting



Centrifuge Casting

Mold is designed with part cavities located away from

axis of rotation, so that molten metal poured into mold

is distributed to these cavities by centrifugal force

• Used for smaller parts

• Radial symmetry of part is not required as in other centrifugal

casting methods



Centrifuge Casting

PRODUCTMOULD

LIQUID

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Centrifuge Casting


http://www.themetalmachine.com/index.html



Advantages and DisadvantagesAdvantages

High rate of production

Better quality and surface finish of castings.

In most castings, a core is not needed. Directional solidification of

metal is promoted that gives good strength and elongation.

Thin and intricate shapes can be easily cast.

No runners and risers are needed.

Mechanical and physical properties of castings are better.

Non-metallic inclusions and gases are minimized.

Foreign inclusions and defects generally exist on the surface and thus

inspection of castings is simplified.

Directional ro erties and stren th is so ood.





Disadvantages

Cost of installing and maintaining spinning equipment is high.

Handling of machine requires skilled labour.

Corrosion and cracking of permanent moulds occur so often.

In production, molten metal is needed in small intervals that usually

creates trouble in controlling the quality and chemical composition.

Continuous Casting



Continuous CastingContinuous casting is a casting process in which the operation of

pouring, solidification and withdrawal of casting from an open

mould are carried out continuously.

Continuous Casting




Working Principle of Continuous Casting

The molten metal is continuously supplied from the ladle to the

intermediate ladle called ‘tundish’ from where it is continuously

poured into the mould at a controllable rate, keeping the level at a

constant position.

The mould usually made of copper or graphite is open at the

bottom and is water cooled so as to extract the heat of the metal

causing its solidification. The shape of the mould corresponds to

the shape of the desired casting.

The process is started placing a dummy bar at the bottom of the

mould upon which the first liquid metal falls.

Continuous Casting

Continuous Casting



Working Principle of Continuous Casting

The molten metal from the tundish enters the mould and takes the

shape of the mould. The water cooled mould controls the cooling

rate of the metal, so that it solidifies before it leaves the mould.

The metal after coming out of the mould is further cooled by

direct water spray (or water with air) to complete solidification.

The solidified metal is continuously extracted (along with the

dummy bar) by ‘pinch rolls’, bent and fed horizontally and finally

cut to the desired length.

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Advantages

In Continuous Casting Sprue, runner, riser, etc. are notused. Hence, no waste metal this leads to 100% casting

yield.

Continuous Casting process is automatic.

Continuous Casting Product has good consistent

soundness.

Continuous Casting products Mechanical Properties are

high and very reproducible.

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Disadvantages

Continuous casting is not suitable for small quantity

production.

Continuous and efficient cooling of moulds is required,

else, center-line shrinkage develops.

Continuous Casting process requires large floor space.

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Moulding



gAccording to the method of removal of CastingExpendable moulding

Multiuse moulding

According to the material usedSand moulding

Investment or Precision mouldingCeramic moulding

Plaster moulding

Metallic mouling

According to the method usedHand moulding

Machine moulding

Types of Sand Mold



yp

• Green-sand molds

Mixture of sand, clay, and water; – “Green" means mold contains moisture at time of pouring

• Dry-sand molds

Organic binders rather than clay – And mold is baked to improve strength

• Skin-dried

Mold Drying mold cavity surface of a green-sand

mold to a depth of 10 to 25 mm, using torches or

heating lamps

Types of Sand Mold



yp

• Core-Sand molds

Mixture of sand, binders, water, and additives.

• Shell molds

Moulding process in which the mold is a thin shell of sand held

together by thermosetting resin binder

• Miscellaneous Molds Pit & Floor molds

Loam molds

Hot box molds

CO2 process mold

Vacuum molds

Carbon Dioxide Mould Casting (CO2 Mould Casting)



Carbon dioxide molding also known as sodium silicate

process is one of the widely used process for preparing

moulds and cores. This process also called Sodium silicate

process.

Molds are strengthen by the application of Carbon DioxideGas instead of baking or any other process.

Sand Mixture = Silica sand+ Binder (Sodium Silicate)

+ Water

+ CO2

Steps involved in making carbon dioxide mould



Step 1

Suitable proportion of silica sand and sodium silicate binder (3-5% based

on sand weight) are mixed together to prepare the sand mixture.

Additives like aluminum oxide; molasses etc. are added to impart

favorable properties and to improve collapsibility of the sand.

Step 2

The pattern is placed on a flat surface with drag box enclosing it. Parting

sand is sprinkled on the pattern surface to avoid sand mixture sticking to

the pattern.

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Step 3

The drag box is filled with the sand mixture and rammed manually till itstop surface. Rest of the operations like placing sprue and riser pin and

ramming the cope box are similar to that of green sand moulding

process. At this stage, the carbon dioxide gas is passed through the vent

holes for a few seconds.

Step 4

Sodium silicate reacts with carbon dioxide gas to from silica gel that

binds the sand particles together. The chemical reaction is given by:

Na2Sio3+CO2 à Na2CO3+SiO2

(Sodium Silicate) (Silica gel)

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Step 5

The sprue, riser and the pattern are withdrawn from the mould, and gates

are cut in the usual manner. The mould cavity is finished and made readyfor pouring.

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Advantages

Instantaneous strength development. The development of strength

takes place immediately after carbon dioxide gassing is completed.

Since the process uses relatively safe carbon dioxide gas, it does not

present sand disposal problems while mixing and pouring. Hence,

the process is safe to human operators.

Very little gas evolution during pouring of molten metal.

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.Disadvantages

Poor collapsibility of mould is a major disadvantage of this process.

Although some additives are used to improve this property for ferrous

metal castings, these additives cannot be used for non-ferrous

applications.

The sand mixture has the tendency to stick to the pattern and has

relatively poor flow ability.

There is a significant loss in the strength and hardness of mouldswhich have been stored for extended periods of time.

Over gassing and under gassing adversely affects the properties of

cured sand.

Plaster Mold



Similar to sand casting except mold is made of plaster of Paris (gypsum - CaSO4-2H 2O)

•

In mold-making, plaster and water mixture is pouredover plastic or metal pattern and allowed to set – Wood patterns not generally used due to extended contact with

water

• Plaster mixture readily flows around pattern,capturing its fine details and good surface finish





•Advantages of plaster mold – Good accuracy and surface finish

– Capability to make thin cross-sections

• Disadvantages:

– Mold must be baked to remove moisture, which can

cause problems in casting

–

Mold strength is lost if over-baked – Plaster molds cannot stand high temperatures, so

limited to lower melting point alloys

Ceramic Mold Casting



Similar to plaster mold casting except that mold is made of

refractory ceramic material that can withstand higher temperatures than plaster

• Can be used to cast steels, cast irons, and other

high-temperature alloys

• Applications similar to those of plaster mold castingexcept for the metals cast

• Advantages (good accuracy and finish) also similar

Different Moulding Processes



Process Advantages Disadvantages Examples

Sand many metals, sizes, shapes, cheap poor finish & tolerance engine blocks,

cylinder heads

Shell mold better accuracy, finish, higher

production rate

limited part size connecting rods, gear

housings

Expendable

pattern

Wide range of metals, sizes,

shapes

patterns have low

strength

cylinder heads, brake

components

Plaster mold complex shapes, good surfacefinish

non-ferrous metals, lowproduction rate

prototypes of mechanical parts

Ceramic mold complex shapes, high accuracy,

good finish

small sizes impellers, injection

mold tooling

Investment complex shapes, excellent finish small parts, expensive jewellery

Permanentmold

Good finish, Suitable for smallsizes, High production rate

Costly mold, simplershapes only

gears, gear housings

Die Excellent dimensional accuracy,

high production rate

costly dies, small parts,

non-ferrous metals

gears, camera bodies,

car wheels

Centrifugal Large cylindrical parts, good

quality

Expensive, few shapes pipes, boilers,

flywheels

Desirable Mold Properties



• Green Strength

To maintain shape and easy handling of mold is possible

• Dry Strength

Strength to resist erosion and metallostatic pressure

• Hot Strength

Strength of the sand at some elevated temperature. Inadequate

strength cause mold enlargement, and may break or get cracked

• Permeability

To allow hot air and gases to pass through voids in sand.

• Thermal stability

To resist cracking on contact with molten metal

Desirable Mould Properties



• Collapsibility

Ability to give way and allow casting to shrink withoutcracking the casting

• Reusability

Can sand from broken mold be reused to make other molds

• Refractoriness

Capacity of sand to resist the higher temperature of molten metal.

• Flowability

Ability of sand to acquire a predetermined shape under pressure and

retain the same when the pressure is removed.

Expendable Mold Casting



p g

- Styrofoam pattern

- dipped in refractory slurry dried

- sand (support)

- pour liquid metal

- foam evaporates, metal fills the shell

- cool, solidify

- break shell part

Plaster-mold, Ceramic-mold casting



Plaster mold, Ceramic mold casting

Plaster-mold slurry: plaster of paris (CaSO4), talc, silica flour

Ceramic-mold slurry: silica, powdered Zircon (ZrSiO4)

- The slurry forms a shell over the pattern

- Dried in a low temperature oven

- Remove pattern- Backed by clay (strength), baked (burn-off volatiles)

- cast the metal

- break mold part

Plaster-mold: good finish (Why ?) plaster: low conductivity => low warpage, residual stress

low mp metal (Zn, Al, Cu, Mg)

Ceramic-mold: good finish

Documents

Module 3 Part II & Module 4 Part I