Metal Casting

Department of Metallurgical Engineering

Gandhi Institute of Engineering and Technology, Gunupur - 765022

SupervisorDr. Ajaya Kumar Pradhan

Presented By Hariballabha Mahanta

Introduction: Metal Casting is one of the oldest

materials shaping methods known. Casting means pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify. The solidified object is called the casting.

The mold, into which the metal is poured, is made of some heat resisting material. Sand is most often used as it resists the high temperature of the molten metal. Permanent molds of metal can also be used to cast products.

Steps involved in making a casting:1. Make the pattern out of Wood , Metal or

Plastic.2. Prepare the necessary sand mixtures for

mould and core making.3. Prepare the Mould and necessary Cores.4. Melt the metal/alloy to be cast.5. Pour the molten metal/alloy into mould and

remove the casting from the mould after the metal solidifies.

6. Clean and finish the casting.7. Test and inspect the casting.8. Remove the defects, if any.9. Relieve the casting stresses by Heat

Treatment.10.Again inspect the casting.11.The casting is ready for shipping.

Casting steps (Schematic)

Schematic diagram of casting mould:

Classification of casting Processes:

Casting processes can be classified into following FOUR categories:

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

2. Chemical Sand Molding Processesc. Shell Moldingd. Sodium Silicate Moldinge. No-Bake Molding

3. Permanent Mold Processesf. Gravity Die castingg. Low and High Pressure Die Casting

4. Special Casting Processesh. Lost Wax i. Ceramics Shell Moldingj. Evaporative Pattern Casting k. Vacuum Sealed Moldingl. Centrifugal Casting

1. Green Sand Molding: Green sand is the most diversified molding method used

in metal casting operations. The process utilizes a mold made of

compressed or compacted moist sand. The term "green" denotes

the presence of moisture in the molding sand. The mold material

consists of silica sand mixed with a suitable bonding agent

(usually clay) and moisture.

Advantages: Most metals can be cast by this method. Pattern costs and material costs are relatively low. No Limitation with respect to size of casting and type

of metal or alloy used

Disadvantages: Surface Finish of the castings obtained by this

process is not good and machining is often required to achieve the finished product.

Few important casting processes

2. Dry Sand Molding:When it is desired that the gas forming materials are

lowered in the molds, air-dried molds are sometimes preferred to green sand molds. Two types of drying of molds are often required.

Skin drying and Complete mold drying.

In skin drying a firm mold face is produced. Shakeout of the mold is almost as good as that obtained with green sand molding. The most common method of drying the refractory mold coating uses hot air, gas or oil flame. Skin drying of the mold can be accomplished with the aid of torches, directed at the mold surface.

3. Shell Molding Process: It is a process in which, the sand mixed with a thermosetting

resin is allowed to come in contact with a heated pattern plate (200 oC), this causes a skin (Shell) of about 3.5 mm of sand/plastic mixture to adhere to the pattern..

Then the shell is removed from the pattern. The cope and drag shells are kept in a flask with necessary backup material and the molten metal is poured into the mold.

This process can produce complex parts with good surface finish 1.25 µm to 3.75 µm, and dimensional tolerance of 0.5 %.

A good surface finish and good size tolerance reduce the need for machining. The process overall is quite cost effective due to reduced machining and cleanup costs.

The materials that can be used with this process are cast irons, and aluminum and copper alloys.

4. Sodium Silicate Molding ProcessIn this process, the refractory material is coated with

a sodium silicate-based binder. For molds, the sand mixture can be compacted manually, jolted or squeezed around the pattern in the flask.

After compaction, CO2 gas is passed through the core or mold. The CO2 chemically reacts with the sodium silicate to cure, or harden, the binder. This cured binder then holds the refractory in place around the pattern. After curing, the pattern is withdrawn from the mold.

The sodium silicate process is one of the most environmentally acceptable of the chemical processes available.

The major disadvantage of the process is that the binder is very hygroscopic and readily absorbs water, which causes a porosity in the castings.. Also, because the binder creates such a hard, rigid mold wall, shakeout and collapsibility characteristics can slow down production.

Some of the advantages of the process are: A hard, rigid core and mold are typical of the

process, which gives the casting good dimensional tolerances;

good casting surface finishes are readily obtainable;

Advantages of metal casting:

Casting is one of the most versatile manufacturing process.

Casting provides the greatest freedom of design in terms of shape, size and the product quantity.

Casting imparts uniform directional properties and better vibration capacity to the cast parts.

Casting produces machinable parts.Shapes difficult and uneconomic to obtain

otherwise may be achieved through casting process.

( Contd..)

A product may be cast as one piece, there by eliminating the need of metal joining processes.

Very heavy and bulky parts which are otherwise difficult to get fabricated may be cast.

Metals (like cast iron) difficult to be shaped by other manufacturing processes may be cast.

Casting can be designed for equal distribution of loads and for minimum stress concentration in order to achieve more strength and increased service life.

Casting process can be mechanized and usefully employed for mass production of components.

Limitations of metal casting:

Dimensional accuracy and surface finish of the castings made by sand casting processes are a limitation to this technique.

Many new casting processes have been developed which can take into consideration the aspects of dimensional accuracy and surface finish.

Some of these processes are die casting process, investment casting process, vacuum-sealed molding process, and shell molding process.

The metal casting process is a labor intensive process

Applications of Casting:Transportation vehiclesTurbine vanesPower generatorsRailway crossingsAgricultural partsAircraft jet engine partsSanitary fittingsCommunication, Construction and

Atomic Energy applications, etc..

Different Sections in Foundry:1) Pattern making2) Sand mixing & preparation3) Mould and core making4) Mould assembly & handling5) Melting6) Pouring7) Shaking out8) Fettling & finishing9) Heat treatment10)Inspection & testing

Reference:1. A text book of Production Technology Vol. I /

O.P. Khanna / Dhanpat Rai Publications 2. A text book of Production Technology

(Manufacturing Processes) / P. C. Sharma / S. Chand & Company Ltd

3. Manufacturing Technology (Second Edition) / P N Rao / Tata McGraw-Hill Publishing Company Ltd

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http://nptel.iitm.ac.in/