casting ppt

Saurabh srivastava

Shane alam

Srikant kumar

Sujeet kumar thakur

Vinod rajbhar

Vishvesh kumar pandey

The process by which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process.

- Can be used to create complex internal and external part geometries

- Some casting processes can produce parts to net shape (no further manufacturing operations are required)

- Can produce very large parts (cast parts weighing over 100 tons have been made like hydroelectric plants)

- Can be used with any metal that can be heated to its liquid phase.

- Some types of casting are suited to mass production

C-clamps formed by casting.

Making of a bullet

It is used to make large parts(typically Iron but also Bronze, Brass, aluminum).

There are basic steps in making sand castings.

Pattern making

Coremaking

Moulding

Melting and pouring

Cleaning

In pattern making, a physical model of casting, i.e. a pattern is used to make the mold. The mold is made by packing some readily formed aggregated materials, like molding sand, around the pattern. After the pattern is withdrawn, its replica leaves the mould cavity that is ultimately filled with metal to become the casting.

In core making, cores are formed, (usually of sand) that are placed into a mold cavity to form the interior surface of the casting.

Moulding is a process that consists of different operations essential to develop a mold for receiving molten metal.

Melting is a process of preparing the molten material for casting.

It is generally done in a specifically designated part of foundry, and the molten metal is transported to the pouring area where in the molds are filled.

• The casting is separated from the mold and transported to the cleaning department.

• Excess metal is removed (Fins, wires, parting line fins, and gates).

• Subsequently the casting can be upgraded using welding or other such as procedures.

• Final testing and inspection to check for any defects

Advantages Disadvantages RecommendedApplication

Least Expensive in small

quantities (less than 100)

Ferrous and non -

ferrous metals may be

cast

Possible to cast very

large parts.

Least expensive tooling

Dimensional accuracy

inferior to other

processes, requires

larger tolerances

Castings usually exceed

calculated weight

Surface finish of ferrous

castings usually exceeds

125 Root Mean Square.

Use when

strength/weight ratio

permits

Tolerances, surface

finish and low

machining cost does

not warrant a more

expensive process

It may be defined as “a replica of the object to be cast, used to prepare the cavity into which molten material will be poured during the casting process”. The making of patterns, called patternmaking.

Commonly used pattern materials are

Wood

Metals

Plastics

Plaster of paris

Wax

It is cheap and easily available.

Can be shaped easily.

Light in weight.

Good surface finish.

Can be preserved for long time by applying varnish to it.

Strong and durable.

Have good surface finish.

Do not deform on storage.

Maintains dimensional tolerances.

They have low weights.

High strength.

High resistance to wear.

Smooth surfaces.

Durable and cheaper.

Plaster of paris is usually used in making master dies and molds, as it gains hardness quickly, with a lot of flexibility when in the setting stage.

Variety of patterns are used in casting and the choice depends on the configuration of casting and number of casting required Single-piece pattern Split pattern Follow board pattern Cope and drag pattern Match plate pattern Loose-piece pattern Sweep pattern Skeleton pattern

(a)Split pattern

(b) Follow-board

(c) Match Plate

(d) Loose-piece

(e) Sweep

(f) Skeleton

pattern

Moulding sand, also known as foundry sand, is sand that when moistened or oiled tends to pack well and hold its shape. It is used in the process of sand casting.

Types of moulding sand:-

Natural sand i.e. contains sufficient amount of binding clay.

Synthetic sand i.e. they are clay free high silica sand, suitable binders are added to make them usable.

Green sand:-

Mixture of silica, clay and water, it contains moisture also.

Moulds prepared by this sand does not require any baking before pouring the molten metal.

Dry sand:-

it is moisture free.

It possess greater strength than green sand.

1. Strength:

Strength of the moulding sand depends on:

Grain size and shape

Moisture content

Density of sand after ramming

Thermal stability:

The sand adjacent to the metal is suddenly heated and undergoes expansion.

If the mould wall is not stable under rapid heating, cracks, buckling and flacking off sand may occur.

Refractoriness:

Refractoriness is the property of withstanding the high temperature condition moulding sand with low refractoriness may burn on to the casting.

The refractoriness of the Silica sand is the highest.

Flowability:

Flowability or plasticity is the property of the sand to respond to the moulding process so that when rammed it will flow all around the pattern and take the desired mould shape.

Adhesiveness:

It is the property of the sand due to which it is capable of adhering(fixing) to the surfaces of the other materials.

Various elements connected with a gating system:-

Pouring basin/cup.

Spruce.

Spruce base runner.

Runner.

Runner extension.

Ingate.

Riser.

Pouring basin : This is otherwise called as bush or cup. It is circular or rectangular in shape. It collects the molten metal, which is poured, from the ladle.Spruce : It is circular in cross section. It leads the molten metal from the pouring basin to the spruce well.Spruce Well : It changes the direction of flow of the molten metal to right angle and passes it to the runner.Runner : The runner takes the molten metal from spruce to the casting. Ingate: This is the final stage where the molten metal moves from the runner to the mold cavity.Slag trap : It filters the slag when the molten metal moves from the runner and ingate. It is also placed in the runner.

Pure metals solidifies at a constant temp. equal to its freezing point, which same as its melting point.

The process of solidification starts with nucleation, the formation of stable solid particles within the liquid metal. Nuclei of solid phase, generally it start appearing at a temperature below the freezing temperature. The temp. around this goes down and is called super cooling or undercooking.

In pure metals super cooling is around 20% of the freezing temp.

In case of pure metals fine grains are formed near the wall of the mould.

Dendrite formation:-

In alloys, such as Fe-C, freezing and solidification occurs overa wide range of temp. There is no fine line of demarcationexists between the solid and liquid metal.

Here, ‘start of freezing’ implies that grain formation whileprogressing towards the center does not solidify the metalcompletely but leaves behind the islands of liquid metals inbetween grains which freeze later and there ismultidirectional tree like growth.

Defects may occur due to one or more of the following reasons: Fault in design of casting pattern Fault in design on mold and core Fault in design of gating system and riser Improper choice of moulding sand Improper metal composition Inadequate melting temperature and rate of pouring

Surface Defects:-

These are due to poor design and quality of sand molds and general cause is poor ramming.

Blow is produced by gases which displace molten metal from convex surface.

Scar is shallow blow generally occurring on a flat surface.

A scar covered with a thin layer of metal is called blister. These are due to improper permeability or venting..

Drop is an irregularly-shaped projection surface caused by dropping of sand.

A scab when an up heaved sand gets separated from the mould surface and the molten metal flows between the displaced sand and the mold.

Penetration occurs when the molten metal flows between the sand particles in the mould.

Buckle is a V-shaped depression on the surface of a flat casting caused by expansion of a thin layer of sand at the mould face.

These defects also occur when excessive moisture or excessive gas forming materials are used for mould making.

Blow holes are large spherical shaped gas bubbles.Porosity indicates a large number of uniformly distributed tiny holes.

Pin holes are tiny blow holes appearing just below the casting surface.

Inclusions are the non-metallic particles in the metal matrix, Lighter impurities appearing the casting surface are dross.

Control of all casting stages is essential to maintaining good quality

Castings can be inspected visually or optically for surface defects

In destructive testing, specimens are determined for the presence, location, and distribution of porosity and defects

Pressure tightness of cast components is determined by sealing the openings in the casting and pressurizing it with water, oil, or air.

Cupola Furnaces:-

A typical cupola furnace consists of a water-cooled vertical cylinder which is lined with refractory material. The process is as follows:

The charge, consisting of metal, alloying ingredients, limestone, and coal coke for fuel and carbonization (8–16% of the metal charge), is fed in alternating layers through an opening in the cylinder.

Air enters the bottom through tuyeres extending a short distance into the interior of the cylinder. The air inflow often contains enhanced oxygen levels.

Coke is consumed. The hot exhaust gases rise up through the charge, preheating it. This increases the energy efficiency of the furnace.

Although air is fed into the furnace, the environment is a reducing one.

Burning of coke under reducing conditions raises the carbon content of the metal charge to the casting specifications.

As the material is consumed, additional charges can be added to the furnace.

A hole higher than the tap allows slag to be drawn off.

Various numbers of chemical reactions take place in different zones of cupola.

Well:-

The space between the bottom of the tuyeres and the sand bed inside the cylindrical shell of the cupola is called as well of the cupola

Combustion zone :-

The combustion zone of Cupola is also called as oxidizing zone. It is located between the upper of the tuyeres and a theoretical level above it. The total height of this zone is normally from 15 cm. to 30 cm.

The heat generated in this zone is sufficient enough to meet the requirements of other zones of cupola.

A temperature of about 1540°C to 1870°C is achieved in this zone. Few exothermic reactions takes place in this zone these are represented as:

C + O2 → CO2 + Heat

Si + O2 → SiO2 + Heat

2Mn + O2 → 2MnO + Heat

Reducing zone :- Reducing zone of Cupola is also known as the protective zone which is located

between the upper level of the combustion zone and the upper level of the coke bed. In this zone, CO2 is changed to CO as a result of which the temperature falls from combustion zone temperature to about 1200°C at the top of this zone. The important chemical reaction takes place in this zone which is given as under.

CO2 + C (coke) → 2CO + Heat Melting zone:- The lower layer of metal charge above the lower layer of coke bed is termed as

melting zone of Cupola. The metal charge starts melting in this zone and gets collected in the well.

the chemical reaction given as under.

3Fe + 2CO → Fe3C + CO2

Preheating zone:-

Preheating zone starts from the upper end of the melting zone and continues up to the bottom level of the charging door. This zone contains a number of alternate layers of coke bed, flux and metal charge.

The main objective of this zone is to preheat the charges from room temperature to about 1090°C before entering the metal charge to the melting zone.

It is simple and economical to operate

It can refine the metal charge, removing impurities out of the slag.

It can be used to reuse foundry by-products and to destroy other pollutants such as VOC from the core-making area.

High melt rates

Chemical composition control

Efficiency of cupola varies from 30 to 50%.

Less floor space requirements comparing with those furnaces with same capacity.

Since molten iron and coke are in contact with each other, certain elements like Si, Mn are lost and others like sulphur are picked up. This changes the final analysis of molten metal.

Close temperature control is difficult to maintain.

Plaster-Mold Casting:- Mold is made of plaster

Mixed with water and additives and poured over a pattern

After plaster sets, pattern is removed and the mold is dried at 120 C

Have low permeability – gases can not escape

Patterns are made of: Al alloys, Thermosetting plastics Brass or Zinc alloys

Have fine details and good surface finish

Form of precision casting

Developed in the 1940’s

Produces close dimensional tolerances

Good surface finish

Low cost process

Common methods of making shell molds.

(a) Pattern rotated and clamped

(b) Pattern and dump box rotated

(c) Pattern dump box in position for the investment

(d) Pattern and shell removed from dump box

There is very high dimensional accuracy and surface finish.

Process is suitable for both ferrous and non-ferrous precision pieces.

Allows flexibility of design.

Cores are typically eliminated.

Can be used for precision castings of ferrous and non-ferrous metals of any size.

Coremaking is eliminated.

Binders or other additives and related mixing processes are eliminated.

Multiple castings can be combined in one mould to increase pouring efficiency.

Dies can sustain very high production rates

High design flexibility and complexity allows products to be manufactured from a single casting instead of from an assembly of cast components.

Good accuracy, consistency and surface finish are possible, with high metal yields.

Cleaning, machining, finishing and fabrication costs are low.

Inertial forces due to spinning distribute the molten metal into the mold cavity Dry-sand, graphite or metal mold can be rotated

horizontally or vertically Exterior profile of final product is normally round

Gun barrels, pipes, tubes

Interior of the casting is round or cylindrical If the mold is rotated vertically, the inner surfaces will

be parabolic Final product has a strong, dense exterior with all of the

lighter impurities on the inner surface.