SPECIAL CASTING METHODS

SPECIAL CASTING METHODS

SPECIAL CASTING METHODS

1. Die casting 2. Centrifugal casting 3. Investment casting

Two types of die casting machines are

1. Hot chamber die casting machine

2. Cold chamber die casting machine

ADVANTAGES OF DIECASTING

1. Large quantities of identical parts can be produced rapidly and economically

2. The parts produced are having smooth surface and close dimensional tolerances.

3. Thin and complex shapes can be produced accurately and easily

4. They require less floor area

5. Sound casting are produced with less defects

6. Rapid cooling rate produces, high strength and quality in many alloys.

DISADVANTAGES:

1. Cost of equipment and die is high

2. Only some nonferrous alloys can be economically die cost

3. Die castings may contain some porosity due to the entrapped air

4. Not economical for small runs

5. Heavy castings can not be cast

-> Centrifugal castings are classified into three types

1. True centrifugal casting

2. Semi centrifugal casting

3. Centrifuging

ADVANTAGES:

1. Casting produced are sound

2. Separate gates and rises are totally eliminated

3. Percentage of rejects are very low

4. Thin suctions and intricate can be easily cost

5. Castings are free from porosity defects

6. Castings have dense metal with very good mechanical properties.

DISADVANTAGES:

1. All shapes cannot be cast

2. Initial investment is high

3. Maintenance is quick expensive.

INVESTMENT CASTING:

ADVANTAGES:-

1. High dimensional accuracy with very closed tolerances.

2. Thin sections, to the extent of 0.75mm can be cost.

3. This process can be adopted for all types of metals and alloys that can be melted and poured.

4. Complex shaped parts can be casted.

5. Suitable for mass production of small sized castings.

6. Sound and defect free castings can be produced.

DISADVANTAGES:-

1. Unsuitable for castings more than 5kgs.

2. The raw materials, special tooling, equipment and technology is costly.

3. Precise control is required at all states of production.

DEFECTS IN CASTING AND CAUSES , REMEDIES:-

1. SHIFTS: this is an external defects due to mismatching of top and bottom parts of the casting and miss alignments of flashes. Make alignment of flashes and matching should be done.

2. WARPAGE: This will happen during or after solidification. Stresses developed at different at different rates of solidification.

3. SWELL: enlargement of mould cavity by metal pressure. Sand should be rammed perfectly.

4. FIN: a thin projection of metal not a part of the casting. They may occur at the parting line.

Incorrect assemble of will cause tins and improper weight on top.

5. DROPS: it occurs when the upper surface of the mould cracks and sand pieces of sand fall into the molten metal.

6. FIN HOLES: small holes of dia less than 2mm appear on the top surface of the casting.

These are caused due to absorption of hydrogen or carbon monoxide when the moisture content of sound is more.

7. SHRINKAGE CAVITY: this is due to high temperature of pouring metal.

8. SAABS: these are rough, irregular projections on the surface of a casting due to too fine sand having low permeability as uneven mould ramming.

9. Metal penetration and rough surface due to high permeability and large grain size.

10. COLD SHUT AS MIS RUNS: it is discontinuity of casting due to imperfect fusion of two streams of metal in the mould cavity, this defect is due to two much thin sections, damaged patterns and intermitting powering etc.

11. HOT TEARS: these are internal dragged discontinuities on the costly surface due to poor design, abrupt change in sections, no proper fillets and corner radii are provided properly.

12. POWERED SHORT: when the metal is not completely filled at one pouring. When the metal is not completely filled at one powering.

DEFECTS CASTING:-

1. SHIFTS:

Causes: core misplacement, miss matching of top and bottom parts of the casting miss alignment of flasks.

Remedies: to provide large areas for uniform solidification add sufficient rib like shapes, to provide equal cooling rates. Sand should be rammed properly.

2. WARPAGE:

Causes: stresses developed at different suctions of a casting, due to different rates of solidification.

Remedies: same as above

3. SWELL:

Causes: enlargement of mould cavity by metal pressure. Improper ramming of the mould.

Remedies: same as above

4. FIN:

Causes: a thin projection of metal not intended as a part of the casting occurs at parting of the mould. Cores in correctly assembled insufficient weighing of moulds improper damping of flashes.

Remedies: same as above

5. BLOWHOLES:

Causes: due to untrapped bubbles of gapes with smooth walls due to excessive moisture in sand per mobility of sand is low sand grains are too fine ram is may be hard venting is sufficient.

Remedies: same as above

6. DIRT:

Causes: crushing of the mould due to improper handling, sand wash presence of slag particles in the molten metal.

Remedies: may be prevented from entering the mould cavity by proper fluxing and the use of direct traps.

7. DROPS:

Causes: due to crakes on the upper part of the mould cracks. Sand falls into the molten metal.

Remedies: same as above

8. PINHOLES:

Causes: small dia holes of less than 2mm dia visible on the surface of the casting due to absorption of hydro general.

Carbon monoxide when the moisture content is high. When steel is powered how set laddles sufficiently