Department of Applied Sciences, BVCOE New Delhi
ES-119 UNIT -1
MANUFACTURING PROCESS
Manufacturing Process
Department of Applied Sciences, BVCOE New Delhi
Importance of Manufacturing�towards Socioeconomic Developments
Department of Applied Sciences, BVCOE New Delhi
Classification
Depending on their primary objective to use; and hence, three classes arise, as discussed below.
Primary manufacturing processes�
Secondary manufacturing processes
�
Advanced manufacturing processes�
WHY STUDY�MANUFACTURING PROCESSES?
Three Phases Of The Manufacturing Process
Engineering Materials: Classification
Manufacturing Materials
Fall into three general categories:
Metals are classified as:
Inorganic Materials Include:
PROPERTIES OF MATERIALS
Classification of material property
MECHANICAL PROPERTIES
STRENGTH
The load carrying capacity of the product is known as Strength.
Department of Applied Sciences, BVCOE New Delhi
DUCTILITY
Malleability
Comparision of ductility and malleability
compressive quality.
RESILIENCE
Toughness
HARDNESS
BRITTLENESS
STIFFNESS
MEASUREMENT OF PROPERTIES
Universal Testing Machine
-Do-
(BHN)
CASTING PROCESS
Features of a Sand Mold
Schematic illustration of a sand mold, showing various features.
PARTS PRODUCED BY CASTING PROCESS
Most of the IC Engine Parts
Machine Tool Parts
Very Large Components for Hydraulic Turbines
Frying Pans
When Casting Process is �Inevitable
SAND CASTING PROCESS
Steps involved in Casting Process
PATTERN MAKING
Pattern is a mould forming tool
A single pattern may be used for making many mould cavity.
Pattern making involves
Pattern
Required casting
Core box
Sand mould in two-piece flask
Cavity
Sand
Core
Gating
system
PATTERN MATERIAL
Patterns can be made from- wood, metals, alloys, plaster, rubber, wax, etc.
The selection of a pattern material depends on
the size and shape of the casting
dimensional accuracy,
the quantity of casting required and the molding process.
PROPERTIES OF PATTERN MATERIAL
Easily shaped, worked, machined and joined.
Resistant to wear and corrosion.
Resistant to chemical action.
Dimensionally stable and unaffected by variations in temperature and humidity.
Availability and Economical.
WOOD
Easily available.
Low weight.
Low cost.
It absorbs moisture and hence dimensions will change.
Lower life.
Suitable for small quantity production and very large size castings.
METAL
Used for mass production
For maintaining closer dimensional tolerances on casting.
More life as compared to wooden patterns
Few of the material used include Al, Fe, Brass etc.
Aluminum is widely used.
PLASTIC
Low weight
Easier formability
Do not absorb moisture
Good corrosion resistance
POLYSTYRENE
Easy to make pattern as it is soft.
Polystyrene changes gaseous state on heating.
Used for prototype (single piece) castings.
Also known as Disposable patterns.
TYPES OF PATTERNS
SOLID PATTERN
SPLIT PATTERN
Most widely used type of pattern for intricate castings.
Used When
The pattern is split into two parts.
The two halves of the pattern should be aligned by making use of the dowel pins.
Loose Piece Pattern
Used when
The obstructing part of the contour is held as loose piece by a wire.
After the molding -the main part is removed - loose pieces are recovered through the gap
TYPES OF PATTERNS
Loose pieces
Part 2
Part 1
Dowel pin
(c) Loose piece pattern
(b) Split pattern
(a) Solid pattern
Match Plate Pattern
The cope and drag halves - along with the gating system and riser - are mounted on a match plate on either side.
When removed after moulding, a complete mould with gating system is obtained.
Used for rapid production of small and accurate casting.
The match plate may be of wood, steel, magnesium or aluminum.
Match Plate Pattern
Runner
Pattern
Gate
Pattern
Match plate
(a) Gated pattern for 8 castings
(b) Match plate pattern
Gated Pattern
It is used for producing small sized cavities in one mould.
The gating and runner system are integral part of the gated pattern.
A single runner is used for feeding
considerable amount of the moulding time is saved.
Removable and Disposable pattern
A removable pattern can be used for producing multiple identical moulds.
For disposable patterns, the pattern is left in the mould instead of being removed from sand.
The pattern material vaporizes when the molten metal is poured
cavity thus created is filled with molten metal. The method is also known as full mould process or cavity less method.
Disposable pattern
Polystyrene Pattern
Hot metal
Sand
(b) Hot metal replacing
disposable pattern
(a) Disposable pattern
in sand mould
Sand
Pattern Allowances
Types of Allowances
Shrinkage Allowance
Machining Allowance
Draft or Taper Allowance
Distortion Allowance
Rapping or Shake Allowance
Shrinkage Allowance
Provided to compensate for shrinkage of metal during solid contraction.
Pattern is made slightly bigger.
Amount of allowance depends upon type of the material used, its composition, pouring temperature etc.
Note: The contraction of the metals is always volumetric, but the shrinkage allowances are always expressed as linear measures.
Shrinkage Allowance for different metals
Metal Allowance mm/m
Cast Iron (Grey) 10.5
Steel 21.0
Brass 16.0
Aluminum 16.0
Zinc 24.0
Lead 24.0
Tin 7.00
Silver 10.0
Machining Allowance
Provided to compensate for machining on casting.
Pattern is made slightly bigger in size.
Amount of allowance depends on -
Draft or Taper Allowance
It is given on all the vertical surfaces to facilitate easy withdrawal of the pattern.
The factors influencing this allowance are the design of the pattern, its vertical height and the method of molding.
It can be expressed either in degrees or in terms of linear measures.
Typically it ranges from 1 degree to 3 degree for wooden patterns.
Draft or Taper Allowance
(b) Pattern with draft
(a) Pattern with zero (no) draft
(Not to scale)
Distortion Allowance
Casting which has an irregular shape and some such design that the contraction is not uniform throughout will distort during cooling.
To eliminate this defect an opposite distortion is provided in the pattern so that the effect is neutralized and the correct casting is obtained.
This can be done in trial and error basis to get the distortion amount.
Distortion Allowance
(c)
Pattern with distortion
allowance
(b)
Casting produced when no
distortion allowance is provided
(a)
Required shape of
casting
Rapping or Shake Allowance
When a pattern is to be withdrawn from the mould, it is first rapped or shaken, by striking over it from side to side, so that its surface may be free of the adjoining sand wall of the mould.
As a result of this the size of the mould cavity increases a little and a negative allowance is to be provided in the pattern to compensate it.
It is negligible in small and medium sized castings.
SAND CASTING PROCESS
Steps involved in Casting Process
Types of Moulding Sand
Natural foundry sand
Synthetic (or Artificial) foundry sand
Common Moulding Sand
Green Sand:
18-30% clay, 4-8% water.
Collected from natural resources.
Maintains moisture content for long time.
Dry sand:
It is green sand dried and baked.
Yields porosity absent castings, as there is no moisture.
Suitable for very large size castings.
Loam sand:
Clay and silica are mixed in equal proportions.
Parting sand:
Used to permit easy withdrawal of the pattern from mould cavity.
Core sand:
Core oil is mixed with silica sand to get Core sand.
It should be stronger than moulding sand.
Properties of Moulding Sand
Cohesiveness or Strength
Chemical resistivity
Permeability
Flowability
Adhesiveness
Refractoriness
Collapsibility
Cohesiveness or Strength
It is the property of sand particles to stick together firmly so that the pattern is withdrawn from the mould without damaging the mould surfaces and edges.
Moisture and clay content determine the strength of the moulding sand.
Chemical resistivity
The sand used for moulding should be inert and should not react chemically with the metal/alloy being poured into it.
Special care has to be taken for removing reactive metals like magnesium and titanium alloy while preparing moulding sand for casting.
Permeability
Property of the sand which allows gases and steam to escape through the sand mould.
Large amount gases and steam is formed due to heating of moisture, additives and other materials.
Gases must escape out, otherwise this will result unsound casting or blast the mould.
It largely depends on the sand grain size, shape, moisture content and clay content.
A soft ramming will increase permeability
and hard ramming will decrease permeability
Flowability
Flowablity of moulding sand refers to its ability to behave like fluid to flow to different corners and intricate details on pattern without much special efforts when it is rammed.
It is more significant in the machine moulding.
Adhesiveness
The sand particles adhere to the mould box surfaces by the property called adhesiveness.
This property helps the sand to retain the mould cavity and stay in the box.
Refractoriness
Property of the moulding sand which enables it to withstand high temperature of the molten metal without fusing,
Depends upon the metal which is to be cast.
If the sand lacks this property, it will fuse on while coming into contact with the molten metal,
Collapsibility
Property due to which the sand mould easily collapses after solidification of the casting to allow a free contraction of the metal.
Absence of this property, the contraction of the metal will be hindered by the mould and results in tears and cracks in the casting.
Moulding
It is the process of making a mould with desired cavity in a suitable material like sand to pour the molten metal.
Types of Moulds
Green Sand Mould
Dry sand Mould
Loam Sand
Metal Mould
Tools used in Moulding Process
Tools used in Moulding Process
Shovel: It is used for mixing moulding sand and filling moulding sand.
Riddle: It is used for removing foreign materials from the moulding sand.
Rammer: This is used for packing or ramming the sand into the mould.
Trowel: It is used for smoothening the surfaces of the mould.
Tools used in Moulding Process
Sprue Pin: conical wooden pin used for making an opening to pour molten material into the cavity.
Vent Rod: Used for making small holes to permit gases to escape when the molten material is poured.
Draw Spike: Used for drawing pattern from the sand.
Moulding Boxes (Flasks): Rigid frames made of iron or wood to hold sand.They are usually made of two parts. The top flask is called cope and bottom flask is called drag.
Procedure for mould making
Gate
Cavity
Sand
(c)
Pouring
basin
CORE
A core may be defined as a sand shape or form that makes the contour of a casting for which no provision has been made in the pattern.
An obstruction placed and positioned in the mould.
Placed in the mould in specially created cavities called core print.
Made from sand, metal or ceramics.
Get holes or internal cavities.
Cores
Core Properties
Strong to retain the shape while handling.
Resist erosion by molten metal.
Permeable to gases.
High refractoriness.
Good surface finish to replicate it on to the casting.
STEPS IN CORE MAKING
Cores are made of clay free silica sand which is thoroughly mixed with suitable binders, water and other ingredients to produce a core mix.
The core mix is packed into a core box that contains a cavity of desired shape.
Core making has following operations:
Core sand preparation.
Core making
Core baking.
Core Sand Preparation
The core sand with additives is mixed homogenously to obtain uniform strength.
Core Making
The core sand is filled in the core box and rammed. Then it is removed from the core box. Cores can be made manually or using machines. Core box is essential for production of cores.
CORE BAKING
Carried out to remove moisture and to impart strength to the core.
Under-baked cores release gases and may cause many defects.
Over-baked cores may collapse too early and may break before solidification of the metal.
GATING SYSTEM
It refers to the passageway through which molten metal passes to enter mould cavity.
Pouring Cup
Sprue
Sprue base
Runner
Casting
Riser
Gate
POURING CUP
The molten metal is not directly poured into the mould cavity because it may cause mould erosion.
It acts as a reservoir from which liquid metal moves smoothly into the sprue.
SPRUE
Metal pouring cup
Corners
Metal pulling
down
Low pressure
zone
Gate
Gate
Sprue
Liquid metal
(a) Straight sprue
(b) Tapered sprue
SPRUE
Sprue helps in feeding metal to the runner, which in turn reaches the cavity through gates.
The sprue may have either straight or taper shape.
In straight sprue, metal contracts inwards and is pulled away from sprue walls. This causes aspiration of gases and air from mould.
In a tapered sprue, the liquid metal flows down firmly in contact with walls and this reduces turbulence and eliminates aspiration.
SPRUE BASE
This is a reservoir for metal at the bottom of the sprue to reduce momentum of the molten metal.
The molten metal as it moves down the sprue gains in velocity, some of which is lost in the sprue base well by which mould erosion is reduced.
This molten metal changes direction and flows into the runners in a more uniform way.
RUNNER
It is used to take molten metal from the spure base and distribute it to the several gate passageways around the cavity.
In case of a single gate, the runner may not be required.
RISER
A riser is a passage of the sand in the cope to permit molten metal to rise above the highest point in casting after mould cavity is filled up.
It provides many advantages as follows:
1. At start it allows the air, steam and gases to go out of the mould.
2. It ensures that the mould cavity is completely filled.
3. Acts as a reservoir to feed the molten metal to the casting to compensate during solidification.
GATES
The gate is a channel which connects runner with the mould cavity and through which molten metal flows to fill the mould cavity.
Depending on the casting size and gating design, various types of gates are used in the moulds.
They are classified as Top gate, Bottom gate and Parting gate.
GATES
(c) Parting gate
Mould
Cavity
Drag
Cope
Mould
Cavity
Cope
Drag
(b) Bottom gate
Drag
Cope
Mould
cavity
(a) Top gate
Strainer core
Pouring cup
TOP GATE
In this case, the molten metal flows down directly into the mould cavity.
Therefore, the hottest metal comes to rest at the top of the casting. As a result, proper temperature gradients ensures directional solidification.
The disadvantage of the top gating system is the erosion of the mould by the falling metal.
BOTTOM GATE
In this case, the molten metal flows down to the bottom of the mould cavity in the drag.
The main advantage is that the turbulence of the metal is kept at a minimum while pouring and mould erosion is prevented.
It is used for heavy castings.
The metal loose its heat as it rises in the mould cavity. So the directional solidification is difficult to achieve.
PARTING GATE
In this type of gate, the metal enters the mould cavity at the parting plane.
It is very simple to construct and very fast to make.
It produces very satisfactory result when the drag is not deep.
This is most widely used gate in sand casting.
Directional Solidification
Stages of Contraction: The contraction of metal or volumetric shrinkage takes place in three stages: liquid, solidification and solid contraction.
Liquid Contraction: It occurs when the molten metal cools down from the pouring temperature to the temperature at which solidification starts.
Directional Solidification
Solidification Contraction:
It is that shrinkage which occurs when the change of state of metal takes place from liquid to solid.
Solid Contraction:
It is that shrinkage which occurs when the temperature falls from the end of the solidification to the room temperature.
Advantages of Casting
No restriction on type of metal or alloy.
No restriction on size of the component that can be casted.
Economically suitable for batch and mass production.
Advantages of Casting
High energy consuming process.
Highly labor intensive.
Raw material requirement is quite high.
For producing 1 ton of steel casting about 2.2 tons of metal, 0.3 tons of facing sand and 4 tons of baking sand are needed apart from many other materials.
More time is involved.
High environmental pollution.
Selection of Casting Process
Quantity to be produced.
Requirement of the product in terms of surface finish, accuracy, complexity etc.
Physical properties of the material.
Process capability to meet the requirement of point 2 and 3.
Initial investment required and operational costs.
Other factors such as environmental pollution, availability of skilled operator (if required), possibility of automation.
Shell Casting
Department of Applied Sciences, BVCOE New Delhi
Shell Casting
Department of Applied Sciences, BVCOE New Delhi
Shell Casting
Department of Applied Sciences, BVCOE New Delhi
Application of shell casting�
Department of Applied Sciences, BVCOE New Delhi
Department of Applied Sciences, BVCOE New Delhi
Investment Casting (Lost Wax Process)
*not in revised curriculum
A pattern made of wax is coated with a refractory material to make mold, after which wax is melted away prior to pouring molten metal
It is a precision casting process - capable of castings of high accuracy and intricate detail
Steps
1. wax patterns are produced
2. several patterns are attached to a sprue to form a pattern tree
Steps :
(3) the pattern tree is coated with a thin layer of refractory material
(4) the full mold is formed by covering the coated tree with sufficient refractory material to make it rigid
Step 5 :
the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity
Steps:
(6) the mold is preheated to a high temperature, which ensures that all contaminants are eliminated from the mold; it also permits the liquid metal to flow more easily into the detailed cavity; the molten metal is poured; it solidifies
Steps :
(7) the mold is broken away from the finished casting -
parts are separated from the sprue
Advantages and Disadvantages of Investment Casting
Advantages:
Parts of great complexity and intricacy can be cast
Close dimensional control and good surface finish
Wax can usually be recovered for reuse
Additional machining is not normally required ‑ this is a net shape process
No parting line
Advantages and Disadvantages of Investment Casting
Disadvantages:
Many processing steps are required
Relatively expensive process
Size limitation, use for less than 0.5 kg
Slow process
Applications of Investment Casting
Permanent Mold Casting/ Gravity die casting
Uses a mould which is permanent i.e. mould can be reused many times before it is rebuilt.
mold constructed of two sections designed for easy, precise opening and closing.
Molten metal poured under gravity action.
Mould made up of dense, fine grained and heat resistant metals.
Steps in permanent mold casting:
(1) mold is preheated and coated
Steps in permanent mold casting:
(2) cores (if used) are inserted and mold is closed
Steps in permanent mold casting:
(3) molten metal is poured into the mold
Advantages and Limitations of �Permanent Mold Casting
Advantages:
Good dimensional control and surface finish
High production rate, so use in mass production.
mold results in a finer grain structure, so stronger castings are produced
Limitations:
Generally limited to metals of lower melting point
Complicated shapes can’t be produced.
High cost of mold
Not suitable for all metals.
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/ Pressure die casting
Molten metal is forced into a permanent metal mold or die under certain 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
STEPS
Closing of two halves of die.
Forcing the molten metal under pressure into die.
Holding the two halves together during pouring and solidification.
Opening the two halves and ejecting the casting.
Die Casting Machines
Two main types:
Hot-Chamber Die Casting
Metal is melted in a container, and a piston injects liquid metal under high pressure into the die
Machine is operated by a hydraulic plunger.
Applications limited to low melting‑point metals that do not chemically attack plunger and other mechanical components
Casting metals: zinc, tin, lead, and magnesium
Cycle in hot‑chamber casting:
(1) with die closed and plunger withdrawn, molten metal flows into the chamber
(2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification
Cold‑Chamber Die Casting Machine
Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity
High production but not usually as fast as hot‑chamber machines because of pouring step
Casting metals: aluminum, brass, and magnesium alloys
Advantageous for high melting‑point alloys
Cycle in cold‑chamber casting:
is poured into the chamber
(2) ram forces metal to flow into die, maintaining pressure during cooling and solidification
Advantages and Limitations of �Die Casting
Advantages:
Economical for large production quantities
Good dimensional accuracy and surface finish
Thin sections are possible
Rapid cooling provides small grain size and good strength to casting
Disadvantages:
Generally limited to metals with low metal points
Part geometry must allow removal from die cavity
Every metal and alloy can not be cast.
Equipments are costly
Centrifugal Casting
A group 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
Centrifuging casting
True Centrifugal Casting
Molten metal is poured into rotating mold to produce a tubular part
Parts: pipes, tubes, bushings, and rings
Rotational axes can be either horizontal or vertical
Outside shape of casting can be round, octagonal, hexagonal, etc , but inside shape is (theoretically) perfectly round, due to radially symmetric forces
Semicentrifugal Casting
Centrifugal force is used to produce solid castings rather than 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 where quality is lowest
Examples: wheels and pulleys
Semicentrifuging Casting
Schematic of a semicentrifugal casting process.
Centrifuge Casting
Centrifuging Casting
Metal is poured into the central pouring sprue and spun into the various mold cavities.
Casting Quality
There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the product
The defects can be classified as follows:
Defects common to all casting processes
Defects related to sand casting process
Misrun
A casting that has solidified before completely filling mold cavity
Some common defects in castings: (a) misrun
Cold Shut
Two portions of metal flow together but there is a lack of fusion due to premature freezing
Some common defects in castings: (b) cold shut
Cold Shot
Metal splatters during pouring and solid globules form and become entrapped in casting
Some common defects in castings: (c) cold shot
Shrinkage Cavity
Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze
Some common defects in castings: (d) shrinkage cavity
Sand Blow
Balloon‑shaped gas cavity caused by release of mold gases during pouring
Common defects in sand castings: (a) sand blow
Pin Holes
Formation of many small gas cavities at or slightly below surface of casting
Common defects in sand castings: (b) pin holes
Penetration
When fluidity of liquid metal is high, it may penetrate into sand mold or sand core, causing casting surface to consist of a mixture of sand grains and metal
Common defects in sand castings: (e) penetration
Mold Shift
A step in cast product at parting line caused by sidewise relative displacement of cope and drag
Common defects in sand castings: (f) mold shift
Defects in Casting
Blow Holes
Appears as small round voids opened to the casting surface.
Caused by hard ramming and low permeability sands
Defects in Casting
Shrinkage Defects
Caused by inadequate feeding of molten metal.
Defects in Casting
HOT TEARS
Appears as external cracks or discontinuities on casting surface.
Caused by hard ramming, too much of shrinkage of molten metal and poor design of casting.
Defects in Casting
MISRUNS
A misrun casting is one that remains incomplete due to the failure of the metal to fill the entire mould cavity.
This can happen when the dimensions of a casting is very less or the metal temperature is too cold, so that the entire section is not filled before the metal solidifies.
Defects in Casting
COLD SHUT
Imperfect fusion of molten metal in the mould cavity.
POUR SHORT
Mould cavity is not completely filled for the want of molten material.
INCLUSIONS
Foreign material present within the metal of a casting. An inclusion may be oxides, slag, etc