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ME 1107

Manufacturing Process

Casting

Part-2

Raju Ahammad

Lecturer

Department of Mechanical Engineering

Khulna University of Engineering & Technology (KUET)

Khulna-9203, Bangladesh

Email: rajuahammad@me.kuet.ac.bd

Website: www.kuet.ac.bd/me/rajuahammad/

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Properties of Good Core

Permeability
Gas generation
Collapsibility
Thermal stability
Strength
Friability
Refractoriness

Explain each term!

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Moulding Sand

Principal constituent:

Silica-86-90%

Aluminium- 4-8%

Iron oxide-2-5%

Smaller amount of Ti, Mn, Ca some alkaline component.

According to the nature moulding sand may be classified as:-

Natural sand- Natural resources like lake, river.
Synthetic sand- Artificial sand by mixing Clay free sand + binder+ other materials as required properties can be controlled easily.

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Moulding Sand

According to the use sand is classified as:-

i) Green sand -18-30% clay + 6-8% water.

ii) Dry sand -No moisture.

iii) Loam sand -30-50% clay + 18-20 water.

iv) Facing sand -Finely divided bituminous coal -2-8%.

v) Backing sand -Used sand.

vi) Parting sand - Parting surface. clay-free silica

vii) Core sand - Making core, binder (linseed Oil)

Viii) System sand - Machine moulding.

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Moulding Sand

Properties of moulding sand:-

Permeability - grain size, grain shape, and moisture and clay contents

Cohesiveness - sand grain particles interact and attract each other within the molding sand

Adhesiveness - get stick or adhere with foreign material

Plasticity - get compacted and behave like a fluid.

Refractoriness - withstand high temperatures

Chemical resistivity - should not chemically react with the metal

Dry strength - dry sand layer must have sufficient strength

Green strength - must have sufficient strength

Collapsibility - must be collapsible

Flowability- It is the ability of sand to take up the desired shape

Explain each term!

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Properties of Moulding Sand

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Sand Testing

WHY SAND TESTING IS NECESSARY??

Sand testing is necessary for moulding sand for a number of reasons. Properties of moulding

sand changes due to the following factors:

a) Contamination of foreign matters.

b) Washing actions.

c) Gradual change and distortion of grain.

d) High temperature.

So periodic tests of moulding sand is necessary to determine the essential qualities of

foundry sand.

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Sand Testing

Properties which are tested are as below:

Permeability Test
Grain fineness Test
Sand mold strength
Moisture Content
Clay Contest
Hardness Test
Refractoriness

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Permeability Test

Permeability is a condition of porosity and thus is related to the passage of gaseous materials through the sand. It is expressed as the volume of air in cubic centimetres that will pass per minute under a pressure of 10 kg/m2 thorough a specimen of sand 1 square centimetre of cross-sectional area and one centimetres in height.

There are four conditions of permeability:

a) Base permeability: It is the permeability measured in a specimen of packed dry sharp sand.

b) Green permeability: It is the permeability measured in a specimen made of moist moulding sand.

c) Dry permeability: It is the permeability measured in a specimen made of moulding sand and dried at about 100 to 110 degree celcius.

d) Baked permeability: It is the permeability measured in a specimen made of sand with thermo-setting binder and backed at some temperature above 105 degree celcius.

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Permeability Number

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Permeability Meter

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Permeability Meter

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Permeability Meter

Permeability test is carried out by using a permeability meter consisting of an aluminium casting in the form of a water tank and a base. A balanced tank floats inside the water tank. A specimen tube extends down to the specimen and opens into the air space. The sand specimen is placed at the base and sealed with marcury. Lowering of the floating tank makes air to pass through the sand specimen. Air is passed through a nozzle to adjust the flow rate. For the sand, flow rate should be slow.

Permeability test is conducted with the specimen usually of 20.26 cm2d cross-sectional area and 5.08 cm height, placed in the instrument cup, which provides a mercury seal, and a predetermined amount of air is forced through the specimen under controlled conditions. The permeability reading is taken by noting the time in which 2000 c.c. of air is passed through the specimen at constant pressure. Then permeability number is obtained by putting the time in seconds in the above mentioned equation.

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Permeability Meter

S.No	Type of sand	Permeability number
1	Loam (15% moisture)	<5
2	Moulding mixture for Cast Iron	0 to 80
3	Moulding mixture for Bronze	35
4	Moulding mixture for Aluminium	20 to 40
5	Steel – dry sand	60 to 100
6	Steel – green sand	150 to 300

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Types of Casting

According to the moulds, patterns and cores are permanent or expandable:

1. Expandable mould casting------- Sand

2. Permanent mould Casting------- Cast Iron

3. Semi-Permanent mould Casting--Graphite

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Expandable Molds

Made of sand, plaster, ceramics & similar materials.
These are generally mixed with various bonding agents or binders.
These materials are refractories.
Capable of withstanding the high temperatures of molten metal.
After the casting has solidified the mold is broken up to remove the casting.

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Permanent Molds

Made of metals that maintain their strength at high temperatures.
They are used repeatedly
They are designed in such a way that casting can be easily removed.
Metal molds are better heat conductors than expandable nonmetallic molds
The solidifying casting is subject to a higher rate of cooling.

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Composite Molds

Made of two or more different materials such as sand, graphite metal.
They are used in casting to-

1. improve mold strength

2. control the cooling rates

3. optimize the overall economics of the process.

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Classification of Casting Processes

1. Conventional Moulding Process:

a. Green Sand Moulding

b. Dry Sand Moulding

c. Flaskless Moulding

2. Chemical Sand Moulding Process:

a. Shell Moulding

b. Sodium Silicate Moulding

C. No Bake Moulding

3. Permanent Mould Casting:

a. Gravity Die Casting

b. Low and High Pressure Die Casting

4. Special Casting Process:

a. Lost Wax

b. Centrifugal

c. Continuous

d. Ceramic Shell Moulding

e. Evaporative Pattern Casting

f. Vacuum Sealed Moulding

Casting Process can be classified into following four categories:

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OUR CONCENTRATION

1. Green Sand Moulding

2. Shell Mould Casting

3. Gravity Die Casting

4. Low and High Pressure Die Casting

5. Lost Wax Casting (Investment Casting)

6. Centrifugal Casting

7. Continuous Casting

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Shell Mould Casting

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Shell Mould Casting

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Shell Mould Casting

Shell mold casting or shell molding is a metal casting process in manufacturing industry in which the mold is a thin hardened shell of sand and thermosetting resin binder backed up by some other material.

There are some steps of shell molding which are below:

1^st step 2^nd step 3^rd step

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Shell Mould Casting

4^th step 5^th step

6^th step 7^th step

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Shell Mould Casting

Step 1: The fine grained sand is mixed with a thermosetting resin binder. A special metal pattern is coated with a parting agent, (typically silicone), which will latter facilitate in the removal of the shell. The metal pattern is then heated to a temperature of 350F-700F degrees, (175C-370C).

Step-2: The hot casting pattern is then poured the sand mixture. Due to the reaction of the thermosetting resin with the hot metal pattern a thin shell forms on the surface of the pattern. The desired thickness of the shell is dependent upon the strength requirements of the mold.

Step-3: The excess "loose" sand is then removed leaving the shell and pattern.

Step-4: The shell and pattern are then placed in an oven for a short period of time which causes the shell to harden onto the casting pattern.

Step-5: After completing the hardening process, shell is separated from the casting pattern by way of ejector pins built into the pattern.

Step-6: Two of these hardened shells, each representing half the mold for the casting are assembled together either by gluing or clamping.

Step-7: The manufacture of the shell mold is now complete and ready for the pouring of the metal casting.

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Shell Mould Casting

Advantages

Greater surface finish and dimensional accuracy. Tolerance about ± 0.03 to ± 0.13 mm.
Machining and Cleaning cost is negligible.
Less floor space and greater production rate.
Less skilled labor needed.
Mould can be stored.
Very small amount of sand needs to use.

Disadvantages

Patterns are very expensive.
The size of the casting is limited < 450 Kg.
Highly complicated shape can not be obtained.
More sophisticated equipments are needed.
Resin binder is expensive.
Minimum thickness can be cast is 4 mm.

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Applications Shell Mould Casting

Pipe Fittings, Valves etc.

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Lost Wax Casting

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Lost wax casting

Investment casting is one of the oldest manufacturing processes in which molten metal is poured into an expendable ceramic mould.

The mould is formed by using a wax pattern - a disposable piece in the shape of the desired part. The pattern is surrounded, or "invested", into ceramic slurry that hardens into the mould.

Investment casting is often referred to as "lost-wax casting" because the wax pattern is melted out of the mould after it has been formed.

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Lost wax casting

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Lost wax casting/Precision casting

Materials for Slurry:

Plaster of Paris or Gypsum products for non-ferrous castings

Ethyl silicate, Sodium silicate and Phosphoric acid for steel castings

Sometimes fine grain silica sand with binders

6-15 mm

Hot vapour of Trichloro-ethylene

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Lost wax casting

Advantages of Lost wax casting:

Very smooth surface and high dimensional accuracy.
Reproduction of surface details and dimensions with precision.
No cores and loose pieces.
Very thin sections can be cast.
Complex shape is easily obtained.
No or little machining is needed.

Disadvantages of Lost wax casting:

Limited by the size and mass of the casting.
Expensive process due to labour and cost of materials.
Long time process.
Large floor space needed.

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Lost wax casting

Applications:

Artifacts, Jewellery and surgical equipments.
Vanes and blades for gas turbine.
Bolts and triggers for fire arms.
Impeller for turbo chargers.

Jewellery

Gas turbine blades

Impeller

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Centrifugal casting

Book: P. N. Rao/ Rajender Singh

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Centrifugal Casting

In this casting molten metal is solidified while mould is revolving.

The metal solidifies under the pressure of centrifugal force which cause the metal to take up the impression of the mould cavity.

Metals towards the periphery and gases and other materials towards the axis of rotation.

Three centrifugal casting processes:

True centrifugal casting
Semi-centrifugal casting
Centrifuged or Pressure casting

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Centrifugal Casting

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True Centrifugal Casting

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The wall thickness of cylindrical is governed by the quantity of metal that is introduced into the spinning mould.

In this process, the casting are made in a hollow, cylindrical mold rotate about an axis common to both casting and mold.

Rotating axis may be horizontal, vertical or inclined. The wall thickness of cylindrical hole is governed by the quantity of metal that is introduced into the spinning mould.

This process are used for cast iron pipes, liners, bushes and cylindrical barrel etc to be cast.

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True Centrifugal Casting

Rotating axis may be horizontal, vertical or inclined.

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Semi Centrifugal Casting

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Same as true centrifugal casting but only difference is that a central core is placed to form inner surface.
Centrifugal force aids in proper feeding of mould cavities.
Flywheel, Gears etc.
Axis of spinning is always vertical.

Rotational speed is less than true centrifugal casting.

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Semi Centrifugal Casting

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Centrifuged or Pressure Casting

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When casting shape is not axisymetrical.

A number of small jobs are joined together on the revolving table.

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Centrifugal Casting

Advantages:

Castings produced are sounder with dense structure, cleaner and the foreign inclusions are eliminated completely.
Mass production is possible with less rejections.
Use of runners, risers and cores is eliminated.
Mechanical and physical properties are improved.
Low machining cost.
Thinner sections can be cast.
Any metal can be cast by this process.

Disadvantages:

Limited to only cylindrical and annular parts with a limited range of sizes.
It involves high initial cost
Too high speed may result in surface cracks.

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Die

Casting

Book: Rajender Singh

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Permanent Mould Casting

This process is commonly known as permanent mold casting in U.S.A and gravity die casting in England. A permanent mold casting makes use of a mold or metallic die which is permanent.

Molten metal is poured into the mold under gravity only and no external pressure is applied to force the liquid metal into the mold cavity. The metallic mold can be reused many times before it is discarded or rebuilt. These molds are made of dense, fine grained, heat resistant cast iron, steel, bronze, anodized aluminum, graphite or other suitable refractoriness.

The mold is made in two halves in order to facilitate the removal of casting from the mold.

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Permanent Mould Casting

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Permanent Mould Casting

Advantages:

Fine and dense grained structure is achieved in the casting.

(ii) No blow holes exist in castings produced by this method.

(iii) The process is economical for mass production.

(iv) Because of rapid rate of cooling, the castings possess fine grain structure.

(v) Close dimensional tolerance or job accuracy is possible to achieve on the cast product.

(vi) Good surface finish and surface details are obtained.

(vii) Casting defects observed in sand castings are eliminated.

(viii) Fast rate of production can be attained.

(ix) The process requires less labor.

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Permanent Mould Casting

Disadvantages

The cost of metallic mold is higher than the sand mold. The process is impractical for large castings.
The surface of casting becomes hard due to chilling effect.
Refractoriness of the high melting point alloys.

Applications

This method is suitable for small and medium sized casting such as carburetor bodies, oil pump bodies, connecting rods, pistons etc.
(ii) It is widely suitable for non-ferrous casting

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Die Casting

Die Casting:

Die casting is a process in which molten metals are forced by pressure into a permanent metal mold is known as a die. The pressure is maintained until the liquid metal solidifies. It is also called pressure die casting. Die consists of two parts : a) Stationary, b) Movable or Ejector

Types of die casting :

There are mainly two types of die casting------

1)Hot chamber die casting

Holding furnace is integral to machine

Used for lead, tin, zinc etc low melting point alloys.

Hot chamber machine has two types of arrangements:

a) Direct Air Pressure Type

b) Submerged Plunger Type

2)Cold chamber die casting

Metal is melted in a separate furnace and then poured into machine.

Used for aluminum, manganese etc High melting point alloy.

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Die casting, Hot Chamber, Submerged plunger Type

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Die casting, Hot Chamber, Submerged plunger Type

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Die casting, Hot Chamber, Direct Air Pressure Type

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Die casting, Cold Chamber

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Die casting, Cold Chamber

The metal in this case is melted in a separate furnace and the required quantity of metal is ladled to the machine. A plunger operated hydraulically forces the metal into the die. A metal shot chamber (cold-chamber) is located at the entrance of the mold. A piston is connected to this chamber which in turn is connected to a power cylinder.

At the start of the manufacturing cycle the correct amount of molten material for a single shot is poured into the shot chamber from an external source holding the material for the metal casting.
The power cylinder forces the piston forward in the chamber cutting off the intake port. The power cylinder moves the piston forward which forces the molten material into the casting mold with great pressure. The pressure causes the liquid metal to fill in even thin sections of the metal casting and press the mold walls for great surface detail. The pressure is maintained even after the injection phase of casting manufacture.

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Die casting, Hot Chamber, Direct Air Pressure Type

Advantages:

Less floor space
Precision manufacturing and less machining cost
Thin sections of the order of 0.5 mm is possible
Improved surface finish
Less rejection, strong and dense metal structure
High production rate (800 castings per hour)

Disadvantages:

Cost of Die and equipments is high
Life of Die decreases rapidly due to high temperature
Only non-ferrous alloys are cast
Size is limited
Special skill required for die maintenance

See more from Reference BOOKS!!

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Die casting vs Permanent Mould Casting

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Hot Chamber vs Cold Chamber Die Casting

Cold chamber die casting process differs from hot chamber die casting in following respects.

Melting unit is generally not an integral part of the cold chamber die casting machine. Molten metal is brought and poured into die casting machine with help of ladles.
Molten metal poured into the cold chamber casting machine is generally at lower temperature as compared to that poured in hot chamber die casting machine.
For this reasoning, a cold chamber die casting process has to be made use of pressure much higher (of the order of 200 to 2000 kgf/cm2) than those applied in
High pressure tends to increase the fluidity of molten metal possessing relatively lower temperature.
Lower temperature of molten metal accompanied with higher injection pressure with produce castings of dense structure sustained dimensional accuracy and free from blow-holes.
Die components experience less thermal stresses due to lower temperature of molten metal. However, the dies are often required to be made stronger in order to bear higher pressures.

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Die casting, Hot Chamber, Direct Air Pressure Type

ADVANTAGES OF DIE CASTING OVER SAND CASTING

Die casting requires less floor space in comparison to sand casting.
It helps in providing precision dimensional control with a subsequent reduction in machining cost.
It provides greater improved surface finish.
Thin section of complex shape can be produced in die casting.
More true shape can be produced with close tolerance in die casting.
6. Castings produced by die casting are usually less defective.
It produces more sound casting than sand casting.
It is very quick process.
Its rate of production is high as much as 800 casting / hour

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Continuous Casting

What is continuous casting? Describe every types of continuous casting with neat sketch? Write down the advantages, disadvantages and applications of continuous casting.

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Casting Defects

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Casting Defects

Casting defects refer to those characteristics which create a deficiency or imperfection exceeding quality limits imposed by design and service conditions. Casting defects are mainly divided into three catagories.

Major or most severe defects (Metal penetration, rough surfaces that interfere machining and finishing operation)
Intermediate defects (High cost of repair)
Minor defects (Easily and economically repair)

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Casting Defects

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Casting Defects

Blow holes
Shrinkage
Porosity
Misruns
Hot Tears
Metal Penetration
Cold shuts
Cuts and washes
Inclusions
Fusion

Drops
Shot Metal
Shift
Crushes
Rat-tails or Buckles
Swells
Hard Spot
Run out, Fins and Flash
Spongings
Warpage

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Casting Defects

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Casting Defects

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Surface roughness – caused due to too coarse moulding sand or high pouring temperature. In steel casting, iron is oxidized which reacts with silica to form rough compounds
Scabs or buckles – due to sand shearing from cope surface. Occurs due to too fine sand, uneven ramming, high moisture, low running of molten metal.

Blow holes – Internal voids as a result of excessive gaseous materials. Caused by hard ramming, excessive moisture, low permeability, improper venting, excessive gas producing ingredients.

Blow Holes

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Casting Defects

Pinholes – Surface reactions cause surface porosity or pinholes. Magnesium reacts with water vapour and forms H₂.

Blow Holes

Pin Holes

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Casting Defects

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Hot tears – Cracks during solidification. Caused by discontinuity of casting, excessive mould hardness, improper metallurgical and pouring temperature control etc.
Cold-shots – two metal stream meeting together are too cold to fuse properly.

Caused by slow pouring, improper gate design.

Hot tears

Cold shots

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Casting Defects

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Run outs – Drainage of metal from cavity. Caused by too large pattern, inadequate mould weights and excessive pouring pressure.
Fins – Usually occurs at the parting of mould and core section. Caused by run out of metals.
Internal air pockets – Caused by pouring boiling metals.
Misruns – Some portion is not filled with metals. Caused due to low pouring temperature, lack of fluidity of metals, too small gate etc.

Run outs/ Fins

Mis run

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